B

Bob Axsom

Well Known Member
Most have read the performance report by the RV-10 pilot where he stated that the maximum speed was far below max RPM. Recently I was flying at 7500ft, max throttle, leaned for best power, trimmed for hands off level flight, Pictorial Pilot on with a specific course, Altrak holding altitude, and I increased the RPM in increments of 10 and recorded the TAS. I recognize some of the pitfalls of the test method, not retrimming after each change, etc. but still the results are food for thought:

RPM Kts
2450 171
2460 171
2470 170
2480 171
2490 171
2500 171
2510 171
2520 171
2510 171
2520 171
2530 170
2540 171
2550 171
2560 171
2570 172
2580 173
2590 174
2600 174
2610 174
2620 173
2630 172

Bob Axsom
 
This is the kind of data we need. Thanks for posting this. Can we trouble you for fuel flow at these rpms next time you are flying? This is very interesting.
 
The airplane isn't instrumented for that data

The airplane has no fuel flow instrumentation. At 2450 RPM where I run it all the time the fuel flow is reliably 10gal/hr. Sorry I don't have it at the other RPM settings.

Bob Axsom
 
Bob-
Since your data indicates flat 171 kts speed between 2450 and ~2565 rpm, it would be interesting to see how much lower the rpms could fall and still maintain approx the same 171 kts speed (possibly improve your mileage a little)...
 
Would also be interesting to know the manifold pressures at each RPM/speed index

Mike
 
Did you re-lean?

Did you re-lean at every RPM change? Shouldn't make a huge difference, but it does make a difference. You might find that the speed peaks at peak RPM if you do that.

Also, how exactly are you leaning? Leaning to peak indicated airspeed? Leaning to some specific value ROP? Just curious.

FWIW, when doing performance/speed testing, I lean to peak IAS after each RPM change.

)_( Dan
RV-7 N714D
http://www.rvproject.com
 
Requested info

I have some digital EI gauges (tach for example) but the manifold pressure and the EGT are analog. For leaning my procedure is to run it up to peak and back off until I hit the first indication of smoothness (sweet spot) it is usually about 50 degrees rich of peak. So my answers to the questions are:

1 - I did not change the mixture at all during the test.
2 - Mixture 50 degrees rich of peak EGT.
3 - Manifold Pressure 22.7 in (It was constant through the full range of tests - I did record it but it was not a digital readout.)

Bob Axsom
 
Last edited:
Try it

Try leaning to peak IAS, and readjust as necessary after each RPM or power change. You may see a few more MPH.

Keep an eye on your CHTs. This is an all-out performance test, not necessarily a representation of a "normal mode" of cruising.

)_( Dan
RV-7 N714D
http://www.rvproject.com
 
For a given Manifold Pressure MP, assuming equal proportions of fuel and air, more rpm will mean more power. There can be no doubt of this because rpm is exactly proportional to the volume of fuel-air mix pumped (with equal MP). The experiment needs to be repeated with equal mixture or, as suggested, leaning for speed. In effect, you can get this kind of result even with a fixed pitch prop by leaning to peak and then advancing the throttle a little.
 
"Exactly" gives me pause

Often in my academic days instructors would say something like "Ignoring the effects of ... this is true." I think that is a convenient way to make the math models work. There are many variables that need to be dealt with in conducting an experiment of this kind and I don't think the variables can be controlled adequately and objectively enough to produce the result you expect without biasing the experiment. In other words if you are convinced that something is true you tend to repeat the experiment and tweak the variables until the expected result is observed. My test equipment is certainly far to primitive to to give conclusive results but my test indicates to me that under some circumstances the maximum RPM achievable with a constant speed prop on a specific airplane does not provide the greatest aircraft system velocity. The question I guess is power to thrust conversion efficiency. If the power is greater for a given RPM then the pitch is greater and in a theoretical revolution of equal efficiency the higher pitched prop will travel farther. Is there an optimum prop blade angle of attack beyond which it starts loosing efficiency - especially in the case of the downward traveling blade. There is a node in my data centered on 2600 RPM beyond which there is a definite drop off in TAS - that is difficult for me to ignore.

Bob Axsom
 
Bob - you are right that "exactly" is in a theoretical context. For example, there is the torque curve in every piston engine. That said, the increments are so small and the rev's so low that I would guess that the torque curve is not the issue. Lycoming says peak HP is at 2700. Superior says the same. I'm not sure, but I think Continental is the same (as in my C-150). The difference in prop efficiency is an interesting idea, but I'm sceptical of that because a FP prop in Van's testing is almost always best at 2700. I have more faith than you do in the C/S prop designers. If you try - as others suggested - leaning for each RPM, I think you will get slightly different results, tending towards peak speed and peak HP at 2700. In my C-150, even on the ground, I can easily find a mixture setting that will give me lower RPM at increased throttle. That's because the mixture setting is restricting the total flow of fuel to the engine and thus increasing the throttle opening is making it go lean of perfect mixture. I'm not suggesting your test is insufficient, only that you use a different technique for the sake of getting more definitive information. Thanks.
 
Higher RPM more peed

When you increase RPM you increase HP. Every 100rpm is worth about 3HP.

As long as your tip speeds are below supersonic than your prop should not be
less efficient. There is some loss as you approach supersonic and some find the 72" faster than 74" dia. The higher HP offsets the loss in prop efficency nearing supersonic tip speeds. Therefore the higher the RPM the faster you go.

Measuring 1 mph is a hard thing to do.

ALL racers OVER Rev and the more the better. Some of the Reno formula racers are turning very high RPMS, like the O-200 Cont's turning +4000 RPM!

Dave Anders Revs to 2,900 RPM but he has the money to overhaul his prop often and he also goes thru the engine more often than you may want. The Hartzell is rated to a max RPM of 2,900. Lycoming is red lines at 2,700 RPM, but allows a slight OVER Rev, BUT if it is too much they recommend a tear down. I don't have the cutoff value of RPM that requires inspection. Racers of course ignore this.


Bottom line is more RPM more speed. If you want to fly your plane everyday with your wife and kids in the other seat, I would worry about damage to engine. I would not over Rev above say 2750 RPM or 2,800 RPM. Also check the tach is accurate.

I did this on my RACE RV-4 by adjusting the prop gov. I has a Hartzell HC-C2YK. It was also polished and the leading edge smoothed. Rough prop leading edge and blade surface cost speed. If the back paint is chipped it should be refinished.

If you have fixed pitch you get what you get. Many fixed ptich props over rev the engine at WOT. The reason many Fixed pitch props are faster than some C/S props is the fact they are over Revving and making more HP.

George
 
Last edited:
Retest of sorts

Well I took it back up this afternoon (it is difficult to find time when you are still in the unpack boxes phase of moving) and tried adjusting the mixture after each RPM change and I tried retrimming after each RPM change. Wife just called me for dinner so have to rush - the test was MUCH more difficult but the bottom line was I was able to get 175 kts at 2630 with adjusting the mixture for max speed as several of you wonderful folks suggested. Thanks - the work continues.

Bob Axsom
 
More Info

Adjusting the trim and working with the other variables just didn't work. I did a little of this and it seemed to have little effect but it was messing up my concentration on the Tach, mixture and TAS. I finally hit the ALT button and locked in the altitude with my Altrak at 8500 ft. The previoust test was at 7500 ft and I started out the test today at that altitude but the scattered clouds at this altitude to the East caused me to do too much dodging so I turned west and climbed up to 8500 after a couple of tests. I found that mixture was very critical at peak speed - in other words I could run the EGT over a span of 200 deg. with the mixture vernier and at some point in there I would get max speed but it was elusive. Having unknowingly seen the maximum speed I would try to fine tune for more and I would lose it in a decay of around 5 kts to a TAS of around170 kts. Then I would try to reestablish the previously seen high and I could get it but I had to work for it. The EGT gauge became just a rough guide and only the mixture control and the airspeed indicator were useful in finding (achieving) the maximum speed for a given RPM. From my point of view there was a peak speed at some optimum speed mixture with a sharp drop off in speed to a plateau about 5kts slower on either side of the optimum speed mixture. The OAT was 2C and the Manifold Pressure was 21.8 (analog gauge interpretation here) for the test at 8500ft MSL (altimeter setting 30.13). The maximum RPM I could get on either of my tests was 2630 and at that RPM I was able to get my maximum speed of 175kts. This is also the speed I was able to achieve at 2600 RPM at this altitude. One fellow in this thread mentioned adjusting his governor to allow a higher RPM - I will have to study this and consider the possibility realizing that the limit is there for a reason.

Bob Axsom
 
The prop is a wing.

1. Going faster is not as simple as putting more power into the prop.
2. The prop is nothing more than a rotating airfoil.
3. All airfoils have a best L/D at a certain Angle of Attack. Above or below that Angle of Attack, the L/D deteriorates.
4. The ratio of engine power transferred to the prop as Thrust (Lift) compared to that lost to the prop as (Drag) is determined by L/D of the prop blades. (Let?s ignore blade tip Mach Drag here.)
5. The best (prop blade) L/D Ratio Angle of Attack must give the maximum prop thrust (and the least drag) for a given engine power.
6. Now, the blades on a CS (Constant Speed ) prop are set by the governor to maintain the RPM, not Angle of Attack.
7. The power from the engine is absorbed as both Lift and Drag on the prop blades. The total of these will be such that they together absorb all the engine power in a steady state condition. NO more no less; otherwise the RPM would be varying.
8. However, it DOES NOT follow that the prop blades will necessarily be at there best L/D angle of attack for a given set of conditions. They are simply moved to the position where the total vector of LIFT and DRAG equals the torque the engine is producing. (It is TORQUE that determines the LOAD on the prop, but contrary to the currently popular view, it is POWER that determines the THRUST on a prop NOT Torque. But as POWER = TORQUE x RPM, if the RPM is constant, as it effectively is in aero engine, then TORQUE is Proportional to POWER. See http://en.wikipedia.org/wiki/Torque)
9. If MR. Hartzell and Herr MT have done their sums correctly, then the prop they recommend for your RV will be near its? BEST L/D angle of attack under typical cruise conditions. If not, for example, if the blades were way to small, then the prop would need to operate at an excessively high angle of attack.
10. So, if a particular prop was optimised (excuse the Australian spelling) for 2450RPM and 170KT and if you were stupid enough to adjust your prop so that it would run at 3000RPM, clearly the drag increase would be huge and the amount of total torque needed just to rotate the prop would leave a lot less for the LIFT component.And the blades pitch would have flatten right off and not be at the best L/D angle of attack.
11. So clearly every CS prop MUST have a SWEET spot where it is at best LD Angle of Attack. Increasing RPM above that may initially bring a slight increase in speed due to an increase in power (at the expense of fuel burn), but eventually the increase in power will be offset by the increase in drag as the prop moves away from the best L/D angle of attack.
12. The other factor which determines a sweet spot is the fact that across the length of the propeller blade the section is rotating at a different speed and so to obtain a constant Angle of Attack the blades are twisted. Clearly, the change in twist can ONLY be optimised for one RPM at a given TAS.
13. This is all born out by the test result of various propellers on an RV-6 where the Maximum speed obtained was not necessarily at the maximum RPM See: http://www.lessdrag.com/lycomingpropeller.html
Pete.
 
Thank you, Bob. That is very interesting. I have the blended airfoil prop and will setup to do the same 'not terribly scientific' test myself. Several around me fly in the 2400 rpm range and I'd be very interested in the comparison.

Again, thanks for the test.

Best,
Doug
 
Just to throw another confusing variable into this soup, I've only recently realised that the tip speed does not increase that much from take-off to max cruise. So I find it interesting that less than max rpm may give the best cruise speed while no-one advocates taking off & climbing at less than full rpm. For example, assuming a 72" dia prop,

At 2700 rpm static the tip speed is approx 500kt (actually 509 kt)
At a climb speed of 90kt, the tip speed is approx 517 kt
At a cruise speed of 170 kt the tip speed is approx 537kt

There is no doubt that the experimental data shows that maximum speed is obtainable at less than max rpm. That means for a 6% increase in propeller blade airspeed, the most efficient rpm decreases substantially - a surprising result. Unless the best climb rpm is close to the best cruise figure? Does anyone have any max speed or climb data at lower altitudes?

I find this stuff fascinating - if I had a c/s prop I would be contributing data rather than asking questions!

Pete
 
It is very facinating

I think it is great that these good thoughts on all sides are coming out. In normal cruise I will set the mixture 50 degrees rich of peak and a little more as necessary to get a smooth "feel" from the engine. Rightly or wrongly I don't care what the gauge says specifically, I look for the leanest smooth running setting on the rich side of peak with my "rich" Airmotive mod kit in the Carburetor. The prop is set to 2450 RPM and the throttle is wide open. The manifold pressure is what it is. With this setup I expect to cruise at 170 to 173 knots. This is the empirical approach to performance prediction. I don't expect to change the airfoil(s) on my propeller so they are what they are and if the designer made the best choices great if the designer did not and there is something better out there I would consider going to it. In the constant speed world for my airplane I have seen nothing that tops Hartzell. Reliable reports have convinced me that the blended (I believe this is a multiple airfoil approach applied over the length of the prop blade to optimize the overall prop performance) airfoil prop performs a little better than my Hartzell prop. Either one will do the family cruise function just fine but if you want maximum speed and are not at all concerned with efficiency I believe the rules change. I know that an aircraft with an unrestricted constant speed prop will reach maximum aircraft speed before the prop reaches maximum RPM (anyone that has experienced a run-away prop knows this) but is this true in the range controlled by a properly functioning system. In my system, the tests I ran today indicate that it is not. It appears that if I am willing to pour the fuel through the engine at an alarming rate and subject the system to greater stress of the highest RPM the system will allow, the plane can be made to fly at its best speed. This is just my current view and I do not hold this out as fact.

Bob Axsom
 
Last edited:
Mach Drag

Regarding Penguins comment on the abrupt rise in drag for a 6% increase in tip speed, it is all about Mach Drag, which sets in around 0.8 Mach (80% the speed on sound). For example the Boeing 747 will cruise economically around Mach 0.82, burn a little more fuel around Mach 0.85 and absolutely guzzle the fuel at Mach 0.88.
Aircraft like the DC-9 and B737 with less sweep struggled even at Mach 0.78. The more sweep the better for lower Mach drag; which is what I suspect the 'Blended Airfoil' is all about.

Pete.
Down Under.
 
Prop theory

The title of the thread was RPM for max speed with a C/S prop. To be clear max speed = max RPM, Max efficency (best MPG) = low RPM.

The reason for higher MPG at low RPM has to do with the big picture, airframe drag and best engine RPM/MAP for fuel efficency. The prop is only one factor. All other posts are interesting but the proof is in the speed.

The prop is a wing analogy comes up a little short since the prop is rotating, which is a big difference.

Airfoil shape, area, diameter, chord vs. span, blade thickness, number of blades, twist distribution all come into play. The Hartzell Blended uses newer theory on prop design but is only a few MPH faster than the good old HC-C2YK. That is how much difference the new theory makes. There is only so much the air beaters can do.

The Hartzell Blended is the fastest prop on the (I)O-360 (180HP) as tested by Van, so they must have done something right, but also shows how we have reached the limits with prop design.

YOU CAN ADJUST the angle of attack by adjusting the RPM and that is the key to the efficiency of c/s props over fixed prop. You can't adjust the twist distribution which is designed into the prop and a function to the speed you want the operate at. We are not stuck with one "angle of attack" for any condition. Meaning you can control the angle of attack for the power/altitude/speed you are at by setting the ideal RPM.

However for pure speed the faster you turn the faster you will go, although it may not be as efficient. My comment was how do you go the fastest. Higher RPM does mean more power with the Lycoming engine, which means more speed. Again racers are all turning well over red line. To do anything less means going slower.

As far as regular cruise efficiency, best MPG I agree with most of the above. For Best efficiency you will turn the engine slower for a given MAP to reduce noise and internal engine friction. The efficiency of the prop is not the big of a factor in my opinion, it is more putting the engine at optimal RPM/MAP.

The max speed and max efficiency issue is apples and oranges. Max MPG achieved more by engine efficency not prop efficency. Throttling back and running slight over-square, leaned as much as the manufacture allows for best efficency (lean of peak ops is another debate).

George
 
Last edited:
Speed variation with RPM

Pete,
I gather you don't accept my 'rotating airfoil' theory, even though I am sure the air molecules just see an aerofoil section go by and don't know that it is rotating.
And maybe I have got some or all or it wrong, but after LOOKING AT THE DATA six months ago, when I was researching props fro my RV-7, it was apparent that there was something going on. I have thought long and hard about it an dicussed it with many high time pilots and engineers. And we must not ignore data.

There is not always a direct relationship between RPM and TAS for CS Props

Here it is again, for a Hartzell Two Blade HC-C2YK-ABF/F7666A-4 on an RV-6A from http://www.lessdrag.com/lycomingpropeller.html

RPM 2700 2500 2400 2300
2500' 205 205 198 193 Equal at 2700 & 2500
5000' 201 202 200 198 Slower at 2700 than 2500
7500' 200 197 196 194 Faster at 2700 than 2500
10000' 194 196 194 193 NO FASTER THAN AT 2400
12500' 185 191 190 187 NOT AS FAST AS 2300

Now this prop/engine combination is only faster at 2700 RPM than at 2500 RPM AT ONE ALTITUDE.....7,500'.

If my explaination is incorrect then I would love to know why this propeller will produce 2 less mph at 2700RPM than at 2300RPM at 12,500'. (A Lycoming produces a lot more power at 2700 RPM than at @300 RPM.)

Go at look at the site. The figures for ALL the props tested show that there is not always a direct relationship between RPM and TAS for CS Props.
Pete.
 
The Altitude Affect is Darned Interesting

Well like you say you can't ignore the data. The numbers at 7500 are so similar to what I saw that I have to pay attention but darn it I have to stew on this one a while. My intuition does not see the mechanism for the low 2700 RPM numbers. Mach 1 changes with altitude in an increasing manner and it does not change direction to my knowledge. If the numbers are one time tests I have to suspect the mixture sensitivity is getting into the act and the numbers are not in fact the peak possible TAS numbers. One other thing I can think of is the angle of attack of the airplane required to sustain level flight changes with altitude because the gravity force is constant (f=ma)
mass x(32ft/sec/sec) at all altitudes while the air density is decreasing so a greater wing angle of attack is required to provide enough lift force to balance it. The aircraft angle of attack increase changes the total prop disk angle of attack and the prop blade angle of attack would seem to be a function of the equally offset blade angles within the disk and the individual blade offset from the direction of flight. When the blades of a two bladed prop are vertical there seems to be no difference to my way of thinking between the blades but when they are horizontal the downward arcing blade seems to be getting a better bite and greater speed (kind of like Doppler effect) than the upward arcing (retreating blade). Now, following your earlier assertion that there is an optimum angle of attack for an airfoil it seems to me that at least one of these blades is not at that angle at the higher altitude greater angle of attack required for level flight. At first thought it seems like this effect would be the same for all RPM settings but for lower RPMs the prop blade to disk angle is greater and the aircraft angle of attack is greater to produce the required lift that at lower speeds. However, your data show a lower TAS at the highest RPM so the high RPM aircraft AOA must be higher to sustain level flight and the prop pitch must be lower to maintain the high RPM. H-m-m-m, maybe if the RPM were lowered the pitch would increase and its effect in the thinner air would increase - hard to say without a test because the higher RPM would be easier to maintain in the less dense air so its pitch would certainly be higher than at low altitudes. At high altitude which would produce the most thrust - the higher angle slower turning prop or the lower angle faster turning prop. Well you certainly make a guy stop and think. For now I have to fall back on my data that show I can get "maximum" TAS at maximum RPM in the 7 to 9 thousand feet range with very precise leaning which I think does not conflict with a limited set of your data. I will have to think about higher altitude tests some day but in west bound cross country air race events in the US midwest high altitudes is not very important.

Bob Axsom
 
So many variables.

Wow. This is geating heavy.
You have come up with more variables that had not occured to me.
Certainly, the Wing angle of attack at hight altitude IS going to be higher to produce the desired life as the IAS will be lower, consequently producing all the effects you mention. How significant they are only Mr. Hartzell and Herr MT can tell us.

I still believe that the blade twist can only be optimised for ONE helix and so at any TAS only ONE RPM will produce the optimised helix. If you take it to the extreme, the actual Helix prescribed can never be right at the beginning of the Take-off roll with zero TAS. It must then be better matched to the design helix as the TAS increses and should reach the optimise helix as it approaches the designed cruise TAS.

Got got to go an align my U/C fairings or I will have a slow RV.

Pete.
 
Prop Efficiency

Further to my last comments I found the following on the thread http://www.vansairforce.com/community/showthread.php?t=3787 by Kevin Horton who has some very detailed performance data.
One explanation could be that the prop efficiency improves quite a bit as the speed increases, so the power available increases (power available = power produced by the engine times the prop efficiency). The data that I've seen for my prop shows a significant variation in prop efficiency with speed in the low speed range - 50% prop efficiency at 50 kt, and 70% prop efficiency at 100 kt, for example.

Seems the prop efficiency varies even more than I thought.
Pete.
 
even with supercomputers, the best props are no more than 85% efficient. all props are compromises, as is the "blended". all prop designs (and turbine fans) are optimized for a single airspeed/rpm/power combination.

the problem is that airframes are rarely matched perfectly with propellers

the short answer to Bob's speed paradox is that if you were able to superimpose the plots of "drag v. speed", "power vs. RPM" and "prop efficiency vs. RPM" for each of the major components (RV airframe, Lyco, Hartzell) you'll find that the peaks don't align perfectly, i.e. max airspeed is achieved at less than max power/rpm.

at 174 kts, Bob's RV has hit it's "drag wall" AND above 2600 rpm, the prop efficiency drop-off is steeper than the power curve, thus cancelling (and eventually defeating) any further increase in power.

the variables in each configuration are more than enough to account for the "max speed below max rpm."

assuming the engine is operating properly, for Bob's particular combination, the hartzell is "saying" it is most efficient at higher angle of attack/lower rpm at 174kts.
 
penguin said:
At 2700 rpm static the tip speed is approx 500kt (actually 509 kt)
At a climb speed of 90kt, the tip speed is approx 517 kt
At a cruise speed of 170 kt the tip speed is approx 537kt
Pete
Click to expand...

just curious: how did you arrive at the non-static tip speeds? the total tip speed is the sum of rotational velocity and forward velocity, i.e. 670kts @ 170kt cruise.....this is supersonic at 7500' msl (644kts) hence extremely loud and at the ragged edge of efficiency....which jives with drop in airspeed above 2600 rpm
 
Data Scatter

fodrv7 said:
There is not always a direct relationship between RPM and TAS for CS Props

Here it is again, for a Hartzell Two Blade HC-C2YK-ABF/F7666A-4 on an RV-6A from http://www.lessdrag.com/lycomingpropeller.html

RPM 2700 2500 2400 2300
2500' 205 205 198 193 Equal at 2700 & 2500
5000' 201 202 200 198 Slower at 2700 than 2500
7500' 200 197 196 194 Faster at 2700 than 2500
10000' 194 196 194 193 NO FASTER THAN AT 2400
12500' 185 191 190 187 NOT AS FAST AS 2300

Now this prop/engine combination is only faster at 2700 RPM than at 2500 RPM AT ONE ALTITUDE.....7,500'.
Click to expand...
I hate to be a negative but if this data is from LESS DRAG it is useless.



I plotted all data on a spread sheet. The data is all over the place and it just does not pass the "makes sense" criteria. I am sure Jim is a nice guy but his testing method is............. lets just say it makes no sense, the data is useless. The curves should not cross back and forth between altitudes, the inflections and slopes (shape of curves) are not consistent. You can draw NO conclusion from the data. There should be some pattern.

RV's ARE NO WHERE NEAR THE "EDGE" OF MACH TIP SPEED: If you want Helical Tip Speed and efficency you need between
Mach .88 to .92. At 205 MPH, 72" prop at 2,700 RPM, the tip speed is 899.9 ft/sec. At 8,000 feet mach(1) = 1085.3 ft/sec, so we are at Mach 0.82, way way way below critical. Unless you are swinging a 80" prop on a Harmon Rocket going 250 mph at 8,000 feet you are not even close to Mach .92. Tip speed is not a big issue for RV pilots. G
 
Last edited:
ship said:
just curious: how did you arrive at the non-static tip speeds? the total tip speed is the sum of rotational velocity and forward velocity, i.e. 670kts @ 170kt cruise.....this is supersonic at 7500' msl (644kts) hence extremely loud and at the ragged edge of efficiency....which jives with drop in airspeed above 2600 rpm
Click to expand...
Actually, it's the vector sum of the two velocities.

They are at right angles to each other so the sum will be the Squareroot(509^2 +170^2) = ~537
 
I'd take the flight data any time over conjecture. I see nothing much wrong with the method here. Similar results were posted recently on a Canard forum with a fellow using an MT C/S prop. Best speed was achieved at around 2600 rpm not 2700 and this varied considerably with altitude as this chap flies up to 18,000 feet often.

Certainly as I climb at a given hp (same MP and rpm) I have to increase pitch (twist actually) on my IVO prop to maintain the same rpm due to lower density and slightly higher TAS at a given IAS. With a C/S, the govenor is doing this for you so you don't notice it.

The data posted here indicates that Hartzell probably did a pretty good job designing the prop as there are only a few knots variation in speed at quite widely varied rpms. For me, I'd set the rpm much lower and suffer a few knots loss for the lower noise level and fuel consumption.
 
mlw450802 said:
Actually, it's the vector sum of the two velocities.

They are at right angles to each other so the sum will be the Squareroot(509^2 +170^2) = ~537
Click to expand...

D'oh!! you're right. i'm a bonehead. :eek: 7 years of engineering school down the drain. (paraphrasing belushi in animal house)
 
ship said:
D'oh!! you're right. i'm a bonehead. :eek: 7 years of engineering school down the drain. (paraphrasing belushi in animal house)
Click to expand...
Ah, an aficionado of the classics!
A man after my own heart! :)

-mike
 
Now we are getting somewhere

Delighted we now have some excellent minds bending to the wheel.
GMCJETPILOT (ironic call-sign to be discussing props) must admire your data plotting. It is 35 years since engineering at Uni for me, but rather than see the data as all over the place, there is a real pattern to the plot. All the curves have the same shape; an increse in TAS with RPM and then a decrease, EXCEPT the 7,500' (where TAS increase constantly with RPM) which is near Full Throttle Altitude. Curious. Seems the plot is telling us something and if the data was all over the place the errors would be random. I don't see a random nature to the curves.
And Ship, don't worry too much about the vectors; I knew there was something wrong with the numbers, but couldn't be bother applying myseelf to figuring it out. I don't think MACH Drag is the answer to Bob's question. Besides, I had to drag myself away to go and do some more fiber glass work on the gear fairings, or it will never fly.
Pete.
PS. I would really appreciate some more 'Critique' on my previous post "The Prop is a Wing."
 
Critical Mach

The critical Mach for the Fabulous DC-9 was 0.75. The cruise Mach was 0.76. Just above the critical mach. But of course there isn't an instantaneous increase in drag, but it has begun to set in. A few times I have seen a faint shadow across the wing at its' THICKEST section which is where the flow over the wing is highest; where the flow was supersonic. The DC-9 wing didn't have a lot of sweep, but it had more than any prop.
It's all soooooo interesting.
Pete
 
Drag & sweep has a lot to do with wing section. For one military program that I worked on we were able to use less sweep by adapting an airliner section, that meant more weapons could be carried as the outboard portions of wing were not too far aft of the cg.

I buy your prop is a wing theory. I don't buy the Mach rise drag theory. The prop tip is at about 0.8M max, the whole deal about the blended prop is that the section changes across the blade span, so you would hope that a somewhat supercritical section is used at the tip.

I don't buy efficiency improves as speed goes up - the airspeed the prop is seeing does not change very much as rpm dominates. If you argue that the prop is most efficient at a very narrow airspeed, and that reduced rpm at cruise brings the prop speed down to near to static speed, I might begin to listen if there were any data to support (But I don't think there is so is probably inaccurate speculation).

I find it hard to believe that props are so speed sensitive for best efficiency - but again, no data to support.

Pete
 
RVator Mixture for Max Power

Just a comment about the mixture information in the latest RVator. The 120 degree rich of peak max power setting is the area where I found the narrow band and elusive maximum speed (175kts) with my standard Hartzell Constant Speed prop. This was not something I was looking for during my testing to comply with a request to repeat my test but vary the mixture for maximum speed (TAS) after each RPM increase. I did look at it and noted that it was about 120 degrees rich of peak which was much richer that I normally operate even in racing. That won't happen again. I found the RVator to be much better than usual from an operational standpoint this time even though I totally disagree with Van's perspective that extra tanks are ok for Canadians and Australians but unsafe for Americans.

Bob Axsom
 
More RPM = More Speed

penguin said:
I buy your prop is a wing theory. I don't buy the Mach rise drag theory. The prop tip is at about 0.8M max.

I don't buy efficiency improves as speed goes up - the airspeed the prop is seeing does not change very much as rpm dominates. Pete
Click to expand...
Pete you are right any thing under Mach .92 you are fine. We don't go that fast. Also the big issue is the efficiency of our piston engines. RPM, friction and HP (specific fuel consumption). I still contend if you want to go faster (ie MAX SPEED) than MAX RPM. MAX SPEED IS MAX RPM.

If you want to get max air-miles per gallons than a lower RPM is better. I have no idea why people are measuring a slight speed decrease going from 2,500 RPM to 2,700 RPM. I always found a slight speed increase of 1-2 MPH. Bottom line MORE RPM = MORE HP = MORE SPEED = MORE FUEL BURN. Look that is why many fixed pitch props that are a little under pitched go so fast when they are turning 2,800 RPM. It is not the physics of the Prop but plan old HP.

George
 
Last edited:
There has been testing done on this on several different airframes and powerplants on several different forums I post on. It is not always the case that max rpm=max speed.

This is dependent on the propeller airfoil, pitch and twist as well as the engine's VE curves and friction hp curves. I can tell you as someone who has built racing engines for 25 years that max rpm DOES NOT always equate to max hp.

In the case of Lycoming and Continental engines which have relatively lame camshafts, poor head flow and restrictive induction systems, there will be a finite point where hp begins to fall off as VE and frictional losses conspire against increased airflow through more rpms. This is the same on any internal combustion engine whether it's an O-360 at 2700 rpm or an F1 engine at 19,000.

Testing on some canard designs with the same airframe and engine but diffrent props revealed quite an rpm range where max speed was obtained in flight testing. As max hp would always be attained at the same rpm, this only leaves the propeller as a very important part of the equation.

To deny the flight data from several experienced pilots (some professional test pilots) and make a blanket statement that max rpm always = max speed would seem ill-advised.
 
Well if you say so

rv6ejguy: There has been testing done on this on several different airframes and power plants on several different forums I post on. It is not always the case that max rpm=max speed.

Too complicated for me :D. You talk about canard planes, Formula race cars and I think fixed pitch props. I don't know about dem-dare-things. My experience is with Lyc O320's and O360's and the Hartzell C2YK with F7666 blades and RV's, which is a typical engine/prop combo in RV's. What engines and props are you talking about (auto engine conversions?) and fixed props. It has been my experience that you DON'T slow down with higher RPM, that is all. You know more than I do about engines, but my eye balls have shown me a different story with in my experience.

This is dependent on the propeller airfoil, pitch and twist as well as the engine's VE curves and friction hp curves. I can tell you as someone who has built racing engines for 25 years that max rpm DOES NOT always equate to max hp.

Regarding PROPS, I find it hard to think a prop (properly designed) would reach max efficiency at 2500 RPM when mounted on an engine rated for 2700 RPM. That tells me the prop is not a good match with a Lycoming, the airframe or both. Twist distribution and the airspeed are related. Granted a C2YK/7666 is made for a Mooney or Comanche going 190MPH, not ideal for a RV as the new Blended prop. However 30 years of technology only got about 3 MPH more (Blended to C2YK/7666). So the old C2YK ain't that bad. All do respect I have "tested" this many times and going from 2500 to 2700RPM makes you go faster. How much? All most can't measure ir but about 1-3MPH.

I suppose other engine/prop combos are more efficient at a RPM lower than max. Some turbo props are turning props at 1900 RPM and going 290 knots while the engine is at 90%. I am keeping the discussion to Lycoming and Hartzell in RV's, so my comments only apply to this combo.

In the case of Lycoming and Continental engines which have relatively lame camshafts, poor head flow and restrictive induction systems, there will be a finite point where hp begins to fall off as VE and frictional losses conspire against increased airflow through more rpms. This is the same on any internal combustion engine whether it's an O-360 at 2700 rpm or an F1 engine at 19,000.

I am well aware of RPM/HP/Torque curves of engines. Lyc publishes RPM/HP/MAP/ALT nomograms and charts for their engines. Max HP is found at max rated RPM. Your comment's may be true but academic, because Lycoming shows RPM increases HP (up to rated RPM). If you want to talk about F1 racer and frictional loss, great that is cool stuff, but the data does not support what I think you are trying to imply. Lycomings DO make more HP with RPM right up to and beyond 2700 RPM. Fact. Of course at some point HP peaks and than drops with increased RPM but this is not the case here.

I know you fly the Subaru and may be we are talking apples and oranges. I have no idea what the most efficient RPM is on an alternative engine or what RPM other props work best at. I do know Lycomings/Hartzell. They will "PULL" well past 2,700 RPM and make more than rated HP (with in reasonable limit). The Hartzell props where also so very happy to take the extra RPM (HP) and go faster. BTW the Hartzell C2YK/7666 type certification approval or rated to 2900 RPM and 250HP.

Lycoming's 2,700 RPM limit is a structural (certification) limit, not a limit of the engines ability to "pump" or breath. However I would never turn a Lycoming over 5% faster than the rated RPM, unless you are racing the airplane only but that is just me. If I fly anyone else in a plane, I am going to be more conservative with it, than if it was just me in the plane, ie race plane only.

Testing on some canard designs with the same airframe and engine but different props revealed quite an rpm range where max speed was obtained in flight testing. As max hp would always be attained at the same rpm, this only leaves the propeller as a very important part of the equation.

Well if you say so :D I assume these are fixed pitch props you are talking about, which is apples and oranges to constant speed props with variable angle of attack (prop disk loading). Its impossible to comment on the prop test you mention, with out data, but I was talking about c/s props where you can control RPM, which is the subject of the thread. Fixed pitch props are a different animal, by their very nature are efficient at only one RPM and forward speed. However if you want to race a Lycoming you want higher RPMs (with in limits) regardless of prop. No doubt the test you talk about showed some props allowed higher RPMs and others where more efficient at lower RPMs (or would only achieve lower RPMs). The key here is to match to engine, prop and the airframe, but again fixed and c/s props are apples and oranges.

All I know is I raced those little darn EZ's (w/ fixed props) and they beat my RV-4 (c/s prop) in a 150-160HP (320 cu-in) class. They where turning their O320's well over 3200 RPM and making more than 160HP, despite their displacement (RPM=HP). They also must have picked a good fixed prop because they where WAY faster. Even though there engines may have been modified, it is well known by the racers even stock Lycoming's make 3-5 more HP per 100 Revs, granted up to a limit. I adjusted my gov to 2800 RPM, and YES thru flight test I went faster than I did at 2700 (may be 1 mph which is like 2.5 HP worth of speed). I only had a stock O320 150HP engine and beat all the 160HP RV's in the class, except one. The one that finished in front was a 160HP RV with a fixed prop, and he was way "over-turning", I think about 2900, so he had 10 HP on me from the get go (engine compression), plus another 3HP or 5HP for the higher 100 RPM. The other 3 RV's in the class had 160HP and constant speed props (at 2,700 RPM or less no doubt). I passed them. Did the extra 100 RPM help me, yea sure a little. My point RPM makes you go faster in the context I am talking about. If you had two matched RV's with Lycs and Hartzell's race, one was limited to 2500 RPM and the other 2700RPM, the RV running the higher RPM will win. Want to race. I know you know racing and you know RPM is power. (Power = Torque x RPM, where 1 hp is defined as 550 lb-ft / s = 33,000 lb-ft / min)


To deny the flight data from several experienced pilots (some professional test pilots) and make a blanket statement that max rpm always = max speed would seem ill-advised.

Fair enough I guess I am ill-advice, probably since I am advising myself. :eek: My comments are made in the context of 2400-2700 RPM with a Lycoming (I)O320 or (I)O360 and Hartzell. In this context I stand by it, ill-advised I guess, again, as usual , Doha! FLIGHT TEST NOTE: To measure 1 or 2 MPH is very hard to do. The tolerance is at least +/- 1-2 MPH. It is hard to say with the data I have seen, such as Bob's post that started this thread if a conclusion can be drawn. He did a great job but we are limited and the atmosphire, playing havoc on data. Any test pilot will tell you this.

I don't know what Pro test pilot you are talking about. No offense to any Pro Pilots out there. I am a "Pro Pilot" and fly large jets, on occasion flight test them as part of my work and have an engineering degree, but frankly that does not mean I know what I am talking about with little prop planes. :D It's my experience, in RVs and 15-25 different piston prop planes I have flown, higher RPM's don't slow you down. I don't understand how (why or should) a prop hit wall wall at 2700 RPM, at least with in the normal operating range of the engine and airframe of a RV. The engine is not running out of steam at 2700 RPM, in my opinion, so as you say it must be the Prop, bad prop, bad prop come here bad prop, shame on you. A prop should be optimized up to max engine RPM and normal altitudes as I think the Hartzell is.

I go up like all RV'ers and push/pull knobs and pull on the stick, writing down what I see on the GPS or AS thingy (along with PA & temp). All I know is my RV with a stock Lycoming 360 and Hartzell HC-C2YK/7666's, goes just a little faster with more RPM , under normal conditions, from sea level to 10,000 feet. A few 100 RPM always got me a few MPH's, of course at the cost of noise and fuel burn. I think the better question is what is the most efficient RPM, air-miles per gallon (zero wind standard day). G

PS (Note altitude effect lowers or flattens the efficiency curve across all RPMs not just at 2700 RPM. Of course by the time you are that high the engine is making much less HP anyway and you are going slower. Unless there is a tailwind advantage and you have O2 most RV's work best down below 12,000 feet. I do notice that at high altitudes there is less speed change vs RPM increase. However we have been talking about 1 or 2 MPH all along anyway, not a big deal of speed anyway.)
 
Last edited:
Nope. In my side by side with my friend's 6A Lyc, Hartzell C/S he found highest speed was at 2630 rpm, not 2700. About 3 knots faster. I was right off his wing, we both had our GPSs on and I could watch him pull away slightly as he brought the rpm back.

Similar results to Bob's posted numbers.

Second case was a canard with a Lyc and MT C/S prop. Best speed was had at 2550 rpm in this case.

Third case was a Mustang II Lyc Hartzell C/S 2600 was the magic rpm on this combo.

I flew a PA31P a few times and I don't remember full fine giving top speed on this aircraft, just some really fine noise. That was a good sounding aircraft. Someone who has flown one more recently can correct me if I'm wrong on this on.
 
Wrong again (me?)

rv6ejguy said:
Nope. friend's 6A Lyc, Hartzell C/S he found highest speed was at 2630 rpm, not 2700. About 3 knots faster. Second case was a canard with a Lyc and MT C/S prop. Best speed was had at 2550 rpm in this case.

Third case was a Mustang II Lyc Hartzell C/S 2600 was the magic rpm on this combo.
Click to expand...
Hummm well I always learn from your post's and I will research it, I'll get back to you. I could be wrong for the first time (I mean again Doha! :p ) Just for fun I have plotted the data that started this discourse; notice the trend lines and deviation; not buying it yet, no offense to the Pro Test pilots, but we shall see. I'll call Hartzell and ask an expert prop man, what, why, how, if; I know the Lyc is making more Ponies at 2,700rpm. Given normal assumptions (at least mine from observation) more HP means more speed, not withstanding the current debate.)

I find it hard to grasp 30 RPM raised the the speed 3 MPH! It does not pass the reasonable test (Yaw Sure U betcha) That is one hell of sweet spot. I don't believe a constant speed prop sits on a bubble 20 RPM wide.

Average:171.6190476; AvgDev:1.015873016; StdDev: 1.244033379; MEDIAN 171.00

The solid black saw tooth line is raw data, other lines are best fits showing trends. You have to get to a 6th power polynomial to get any fit? Oh Really? All you RV'ers w/ constant speed props get out and flight test this theory. I have never heard of this in 20 year and 10,000 hour CFI/ATP, but I am willing to be convinced.

Signed, Doubtful but interested (aka George) :D

PS The fastest RV in the WORLD! :eek: is Dave Anders RV-4, over 250MPH. He turns this Lyc IO-360?HC-C2YK/F7666 2900 RPM. I still think RPM is speed, but we shall see. Also Dave's RV-4 is faster then the Harmon Rocket (IO-540) and winner of the Cafe Foundation Triaviathon (climb, top and min speed formula performance contest).
 
Last edited:
I have no doubt that some Hartzell/ Lyc C/S combinations do achieve max speed at max rpm and agree that other combinations that do not, are not optimized possibly for this flight regime.

MT for instance is said to match their props for best cruise performance at a specified altitude and this may be one reason why they often seem a few knots off a Hartzell and others in a max speed contest. If I'm not racing, I'm fine with this approach as I spend more time in a cruise configuration than any other and I'm not bending the throttle all the time.

If I was racing, I'd likely have a different prop altogether just like the aircraft running at Reno do. These are special props, running special governers for special aircraft. They don't bear much resemblance to run of the mill stuff on most of our aircraft.

All I'm saying is that since none of us have flown every aircraft type or even every RV/ prop/ engine combination out there, nobody can say that max rpm = max speed in every case. The basic statement need qualification.

Altitude comes into this equation as well and C/S users sometimes don't appreciate that the pitch is changing as airspeed and altitude changes. Just the rpm stays the same. The MT canard user found max cruise speed was at a much reduced rpm and therefore much higher pitch at very high altitudes. Prop slip and prop airfoil L/D at different speeds and air densities presents another set of variables.

I do believe that Hartzell does an excellent job designing props for the RVs because they perform very well. I welcome any others who have done extensive flight testing in this vein to share their results.
 
Some Info , answers and more questions

rv6ejguy said:
[1]I have no doubt that some Hartzell/ Lyc C/S combinations do achieve max speed at max rpm ...

[2]MT for instance is said to match their props for best cruise performance at a specified altitude and this may be one reason why they often seem a few knots off a Hartzell ...

[3]If I was racing, I'd likely have a different prop altogether just like the aircraft running at Reno do. These are special props, running special governors for special aircraft. They don't bear much resemblance to run of the mill stuff on most of our aircraft....

[4]All I'm saying is that since none of us have flown every aircraft type or even every RV/ prop/ engine combination out there, nobody can say that max rpm = max speed in every case. The basic statement need qualification.

[5]Altitude comes into this equation as well and C/S users sometimes don't appreciate that the pitch is changing as airspeed and altitude changes. Just the rpm stays the same. The MT canard user found max cruise speed was at a much reduced rpm and therefore much higher pitch at very high altitudes. Prop slip and prop airfoil L/D at different speeds and air densities presents another set of variables.

[6]I do believe that Hartzell does an excellent job designing props for the RVs because they perform very well. I welcome any others who have done extensive flight testing in this vein to share their results.
Click to expand...
[1] I guess I still need to research this, as I have reached my level of incompetence.

[2] MT may be slower because it's optimized for cruise, BUT the other reason's are 3-blades (slower, ref current RVator, don't kill the messenger) and wood/composite blades are thicker than metal. Both of these are not as efficient.

[3]Reno is a whole different story. The fast prop movers are still the WWII iron and as far as I know there are only a few props that will fit a Merlin or 2,480hp two-row 18 cylinder radial (I drool when I think of it). The Sport guys run all kind of props but I think they tend to be Hartzells or McCauley, highly polished with very sharp leading edges. A rough prop , chips in the finish or leading edge costs several MPH.

[4] Agreed, I think :confused: I think, explanation below (advance factor J). Higher angle of attack (lower RPM) is more efficient at high speeds, but this still does not fully explain why RPM increases decreases speed.

[5] This may explain many of the questions and observations. I found a good explanation on the web, excerpt below:
--------------------------------------------------------------------------------------------------------------
FIRST let's talk about about piston engine efficiency real quick (NOT RELATED TO MAX ABSOLUTE SPEED)

Piston Engine Efficiency
Piston engines are remarkably consistent in their efficiency, compared to turbine engines. In other words efficiency does not change much with air temperature or rpm.

The only significant factor affecting the efficiency of the piston engine is throttle setting. When the throttle is retarded (making the manifold pressure lower than surrounding air pressure) the engine looses efficiency. (thus best to fly at altitude allowing WOT at or below 75% so you can lean.)

Pilots should keep in mind that a given amount of power can be produced by an infinite number of manifold pressure (MP) and rpm combinations. For example the following MP x rpm combinations all produce equal amounts of power:
? 22" x 2400
? 23" x 2300
? 24" x 2200

The engine will be more efficient if the pilot choose the higher MP and lower rpm combination. (Our discussion is max speed not efficiency, but if you want efficiency you would pick a RPM 2500 or less.)

It is critical to note that none of the above theory will do any good at all unless the pilot leans the mixture to the maximum economy setting, as specified in the Pilot Operating Handbook.

Turbo-chargers allow a piston aircraft to fly faster and higher. However, the turbo-charger also tends to make the engine run hot, because it heats the air as it compresses it. Therefore, above some critical altitude the engine will overheat unless the mixture is richened to help cool it. As soon as this is happens the overall efficiency of the engine begins to decline. (nice to know and why turbos for RV's are not a panacea.)

Based on above, any power setting will have an optimum altitude(s) for the piston engine. These will be the altitude(s) at which full throttle produces the desired amount of power, with the mixture set for maximum economy.

Just as with the Turbo-prop engine, the BHP of the piston engine is independent of velocity. (interesting observation, pilots overrate the RAM (rise) in their air induction, sometimes by a factor of 3 to 5 due misunderstanding, measurement error and assumptions.)

In a normally aspirated piston engine BHP decreases with altitude. A turbocharger will maintain the power with altitude until the critical altitude is reached. Then power will decrease with altitude.

NOW FOR PROPS
Fig 1 - Propeller Efficiency

Any given wing will have a certain angle of attack at which it is most efficient. This is true for the propeller as well. It is after all just a wing flying in a helix pattern.

Since the angle of attack of the propeller depends on both rpm, diameter and TAS, the propeller efficiency will vary according to the ratio of these factors. With a fixed pitch propeller, getting the ideal advance ratio while also keeping the aircraft's wing at the ideal angle of attack for best range is almost impossible.

The ratio Velocity/rpm x diameter is called the Advance Ratio. The formal definition of advance ratio is:

J= V / (r * D): where J is Advance ratio, r is rpm and D is propeller diameter, V is TAS. (Fig 1) (NOTE: the only thing we can control directly is RPM) The propeller efficiency of a fixed pitch propeller will be a maximum at only one advance ratio, as shown in the diagram shown in FIG 1, So:
J < less efficiency
J > more efficiency (to a limit)

(THIS IS THE SMOKING GUN, HOWEVER LOWER BLADE AOA = HIGHER RPM = LOWER EFFICENCY? BUT! ARE WE AT THE APEX OF THE EFFECINCY PROP CURVE, WHERE A SMALL INCREASE IN "J" EQUALS A LARGE DROP IN SPEED *OR* ARE WE SOME WHERE ALONG THE CURVE WHERE A SMALL INCREASE IN "J" IS AN INCREASE IN SPEED. KEEPING IN MIND WE HAVE MORE Brake Horsepower (BHP) at higher RPM. From the data Bob presented, speed goes up and than "DROP OFF" with LOWER "J" (higher RPM), but if we are going down in "J" (increasing RPM) how are going up in speed and than "FALLING OFF THE BACK SIDE"? Does not totally compute.)



Figure 2-Constant Speed Prop
The most efficient J depends upon the propeller blade angle. Course propellers (large blade angles) will be more efficient at larger advance ratios (high speed). Fine pitch propellers will be more efficient at small advance ratios.

That is why constant speed propellers are desirable for cross-country airplanes vs. fixed.

The diagram Fig 2 shows how the efficiency varies with different propeller blade angles.

With a constant speed propeller the blade angle will vary from a small angle to a large angle (limited by low and high pitch stops). This allows the propeller to be efficient (i.e. operate at the optimum angle of attack) at a variety of advance ratios.

Fig 3 - Available Thrust Horsepower (THP)

We defined propeller efficiency in (Fig 3): (click link below for slide show)
http://142.26.194.131/aerodynamics1/Performance/Graphics/apx.gif

Prop efficiency (n) = THP / BHP
Thrust Horsepower - THP
Brake Horsepower - BHP
(Often props are assumed 80% efficient (n) +/- a few %)

Therefore: THP = h x BHP
This results in a THP available curve as shown in (Fig 3).

It is worth noting that BHP is a constant the THP curve will have the same shape as the propeller efficiency curves examined earlier. These in turn are the same shape as the L/D vs. CL curve.

Ref: ( http://142.26.194.131/aerodynamics1/Performance/Page8.html)
--------------------------------------------------------------------------------------------------------------

[6] Granted and I think the Blended prop takes advantage of some new thinking in prop design. However the more I learn, the more I think prop design is part physics, science, part magic and talent of the designer balancing aerodynamics, strength, weight, efficiency and manufacturing cost.

There is an optimum AOA for the prop for a given condition, but the factors are many: "J" factors, Prop blade airfoil selection (CL - coefficient of lift = airfoil profile, number of blades, blade area, shape, thickness and twist distribution), aircraft drag, HP available and altitude effect (greater angle of attack for same thrust), all go into the mix.

George (still researching)
 
Last edited:
Good information so far

Good work so far. The information stimulates some new thought on my part. The effect of TAS on propeller angle of attack is one of the points to ponder. There are things that can be taken incorrectly if not thought out (rationalized) and tested to prove the observers concept. The theoretical propeller in this referenced material could be taken as a constant chord, constant angle (no washout), tipless design. It seems better to apply the information and infinite number of sections comprising the propeller blades with the diameter refering to the positions of these sections from the hub and integrating the result from the edge of the spinner to the tip. This would take into account the different angles of the blades at each station relative to the rotational plane and the variations in airfoil between stations. At one point I saw that a distinction was made between the upward moving blade and the downward moving blade but this was not developed - there has to be a difference in flight. The information takes the high ground on thrust implying that it is all due to lift as in a rotating wing but it is fairly obvious that an airfoil-less inclined plane turning in a fluid medium is going to produce thrust and the low pressure over the upper chamber of the blade airfoil does not produce it all. It is the differential in pressure that makes the airplane go forward I believe. The effects of angle of attack of the blades described are not in conflict with this. So if the pressure infront (on top of the blade airfoil) is reduced by airfoil design the differential is increased but if a flat bottomed blade airfoil is retained the pressure in back would not change. The angle of attack relationship to thrust definition works well to describe the effects of the front and back sides of the prop because they work together to increase the pressure differential in the thrust direction as the blade angle is increased relative to the prop rotational plane up to a point of declining aircraft thrust efficiency. You have certainly given me some new things to think about. Since I started this thread I have been sensitized to the 120 rich of peak maximum power benchmark and I have learned by testing that with my present setup (O-360-A1A, Hartzell C/S) in the 7,000 to 9,000 ft range at that setting I am going to get a maximum TAS of approximately 175 kts. That maximum TAS did not change from 2600 to 2630 RPM on my lean after each RPM increase test run. Only these two data points at the high RPM end of the RPM range were checked on the second run. You may recall that speed is very similar to the speed I achieved in the original test run without releaning after each 10 RPM increase below 2600 RPM. Someday I will explore that further and my gut feel is that I will at least equal my 2600 RPM maximum speed down to some point in the 2500 to 2590 RPM range. Whether the internal friction losses of my engine at some point result in it not being able to produce enough power to sustain high pitch or my propeller thrust production efficiency drops off when the pitch is increased beyond a certain point required to hold the engine RPM back to a selected level, I don't know but I do not believe that the RPM to TAS relationship for my airplane in it's current configuration is a straight line.


Bob Axsom
 
Last edited:
HP on pistons engines is directly affected by intake manifold pressure and air temperature. These are standard corrections applied to every dyno run.

Ram air does affect hp although this effect is only a 1-2 percent at the speeds RVs fly. At higher speeds like at Reno, the atmo engines like on the Thunder Mustang gain a useful amount of hp from ram air.

HP varies proportionally as the absolute density of the air (temperature). There is a big difference in power between -30C and +30C so this is not minor.

WOT is more efficient as this results in the lowest pumping losses.

The unlimiteds at Reno are always experimenting with different props. Strega and Voodoo are not using the original type Hamilton Standard units anymore. Rare Bear was using a cut down, round tipped, 3 blade version of a 4 blade P3 prop a few years ago and is now fitted with a different 4 blade again. In talking with some of the crews, they admit that much testing is done on propellers but don't say more than that!

The Thunder Mustang racer uses a 3 blade Hartzell where the stockers use a 4 blade MT. DG's Lancair has a special high pitched scimitar, 3 blade Hartzell and revised governer and he was doing around 400mph on the straights this year.

Hartzell has learned much from supplying props for the Sport Class. This technology, where applicable, filters down to us, which is a good thing. :)
 
All true kind of (RAM rise is a myth, kind of)

>QUOTE rv6ejguy: HP on pistons engines is directly affected by intake manifold pressure and air temperature. These are standard corrections applied to every dyno run.

Yes, Manifold is the KEY to power for piston engines. Yes, air temp (and humidity) affects power because of air density. Also we all know an atmo engine's power goes down with altitude (air density). (Jet engine efficiency goes up with altitude BTW.) However for the prop discussion we considered a level flight at one altitude (and temp).

The point about power speed affecting HP (increased MAP) is a Pet Peeve. (see: HOW TO MEASURE RAM RISE at bottom of this post). Pilots are under the impression RAM rise to their engine is greater than it actual is. RAM for Jet engines it's a factor, not for piston engines. Folks are delusional about how much RAM their engine gets. First air flow thru the engine is not continuous, it starts and stops, sucks, squeeze, bang blow. Van has measured (accurately) RAM intakes and many provide no gain or a loss. Van's Std filtered airbox gets net to small MP gain; Just NOT getting a loss is a victory, much less getting "RAM AIR". Pilots fool themselves into thinking they are getting higher gains then they really are. There is only about 1" Hg TOTAL dynamic pressure at 175MPH. If you got 30% into your engines intake you would be the man! Most get -.25" to +.25" Hg.)

"BHP output in a piston aircraft is directly related to manifold pressure and rpm? Almost, but not quite. The power output from your piston engine depends on the difference in pressure between the inside of the cylinders and the atmosphere outside the engine. Therefore, a given manifold pressure (e.g. 23 inches) will produce more power at a high altitude than it does at a low altitude, or 23" is more power at 10,000 feet than sea level. Consequently you need higher fuel flow for 23" at 10,000 then at SL. Of course we can't carry 23" at 10,000 unless you have a super or turbo charger. We are happy to get 19.5" at 10,000." (Ref.http://142.26.194.131/aerodynamics1/)


>Ram air does affect hp although this effect is only a 1-2 percent at the speeds RVs fly. At higher speeds like at Reno, the atmo engines like on the Thunder Mustang gain a useful amount of hp from ram air.

Ok, at 400 MPH you have about 5.7" Hg total dynamic pressure available, but us little guys are lucky to have 1" Hg to work with. I don't know about 1-2% power but you are assuming you are actually getting any extra MAP, which for most Lyc powered RV's is very small, if at all, because the intake starts and stops, flow starts, stops , backs-up and starts again, making it a challenge. You can't just take free air-stream at 190MPH and slow it to intake speed with out losses. Most RAM intakes are just spilling air outside the scoop and causing airframe drag. -25% to +25% is a common recover range with 0% typical, IF your lucky.

The problem with the RV's, especially with the forward facing sumps, is there is not a lot of room to develop a good ram air duct. A straight tube is not going to get it (sorry). Look at a P-51 or Hawker Sea Fury. Look at the spinners, they are huge, the blades are airfoil shaped adjacent to the spinner, so the annular scoop below the spinner is getting prop blast. Also in the case of the P-51 a long diverging duct with flow control doors directs the air into the rear engine intake. The Vertical induction on the Lycoming is very good, because it gives ROOM for a diverging airbox (aka Van's FAB). Although the filter is a restriction, it is large the air is slow and it has the side benefit of straightening the airflow reducing turbulence into the throat of carb/throttle body. All good.

Guys getting wood for fwd facing induction think they are going to get 100% RAM recovery. Unlikely and -0% to 20% is realistic. A typical Hartzell or other c/s prop beats the air to death near the spinner (due to blunt blade hubs). You need to get mid-blade-span on the prop to see "good air". Again Van's airbox / cowl scoop puts the intake in the cleaner air vs. dirty air near the spinner. Van's filtered induction for the Lyc fwd face sump has the filter at the cowl entrance (left cowl engine cooling inlet). This is usually not ideal, but the cowl does act to slow the air slightly and gets air away from the spinner and makes room for a diverging duct.

>HP varies proportionally as the absolute density of the air (temperature). There is a big difference in power between -30C and +30C so this is not minor.

When was the last time you took your RV from +30C (86F) to -30C (-22F). By the way standard day temp at 23,000' is -30C. Look temp is important to Jet efficency, but it's not a factor for piston engines. Yes our RV's perform better on a cold crisp (dry) day than a hot wet day. That is because of air density. Less air density engine makes less HP; prop makes less thrust and wing make less lift. Nothing new here, but a pitson engine EFFICENCY is not affected, power yes. Also temp's not an issue in flight test of a Prop vs. RPM at constant Alt conditons.

>WOT is more efficient as this results in the lowest pumping losses.

Agreed, sucking past a partially closed Carb butterfly or throttle body takes work and effort and thus HP used to make power vs. power to use. WOT is most efficient for Carb engines especially because the butter fly biases the fuel to the forward cylinders, so you get uneven mixture to the Jugs. This is the advantage of FI, since fuel is injected into the cylinders intake port directly. In theory it is metered to be exactly the same.

>The unlimited at Reno are always experimenting with different props. Strega and Voodoo are not using the original type Hamilton Standard units anymore. Rare Bear was using a cut down, round tipped, 3-blade version of a 4 blade P3 prop a few years ago and is now fitted with a different 4 blade again. In talking with some of the crews, they admit that much testing is done on propellers but don't say more than that!

Interesting, I know some of the Ham-Std are getting rare and expensive. I do know the clean and polish them to a mirror finish. Leading edges get special treatment.

>The Thunder Mustang racer uses a 3 blade Hartzell where the stockers use a 4 blade MT. DG's Lancair has a special high pitched scimitar, 3 blade Hartzell and revised governor and he was doing around 400mph on the straights this year.

Interesting, 4-blades, what is a "stockers". Do you mean a REAL P-51. If so 2,500HP needs 4-blades. I know the MT dealer guy tried a 4-blade on a Rocket and the speed was way way down. I discussed "J" factor and available thrust HP, I don't think you NEED more than two blades until you get into +300HP and plus 250MPH, however smoothness and noise is a factor for many. The wood 3-blade MT is smoother as I have heard, but slower by 6-10 MPH.

>Hartzell has learned much from supplying props for the Sport Class. This technology, where applicable, filters down to us, which is a good thing. :)

Cool I need to hook into the Reno thing more, never been may be next year.

Cheers George

HOW TO MEASURE RAM RISE: Well first MAP gauges are inaccurate but with that said there are two ways to do it: (This was written by Van and published in the RVator.)

(1)-Read MP gauge before starting engine (on the ramp), observe ambient ATMO air pressure (e.g., 29.92" hg). Go fly and make a pass down the runway right on the deck, WOT, close to the ramp elevation. What is your MAP now? Is it 29.6"hg or 30.2"hg. It will be very very close to 29.92"hg (+/-) 1/10th or two. Compare the two readings and the difference is your RAM. I think you will be enlightened.

(2) - The other method is even more "Sporty". At 8,000' feet run WOT level and read the MAP. Pull up, close throttle, cut off mixture, slow down and stop prop, and Read manifold pressure, Start engine!!!!! Again the difference will be the approx RAM rises.

At low speeds (RV), MP losses will seem greater than at high speeds. When there is a large mass air flow (full throttle) flowing through the induction system, flow losses will reduce the pressure. However, when the airplane is traveling fast (RENO), the energy of the incoming air is converted to pressure to offset, or partially offset these flow losses."

BOTTOM line the intake has LOTS of LOSSES on the way to the cylinder and it is naive to think you can over come all losses and get BIG-OLD "TRUBO" or SUPER CHARGER" RAM air effect, with a simple tube at least. It ain't happening at 190MPH, or at least to the extent that is popularly thought. If you are going 400 MPH you have more total dynamic pressure to use. RV Pilot's fly along and pull the alternate air door and see a 1" drop and go "UREKA!!", BUT not so fast (pun intended). They are really measuring the 1" loss of the alternate air door, NOT the gain of the RAM. I should know because I was the RV pilot.

Van's FAB for Vertical induction and Van's Fwd induction "snorkel" are good and well designed. The standard FAB for vertical induction provides a NET or no loss in MAP, which is pretty good and typical of many unfiltered so called RAM air intakes. NET is about all you can expect if going thru a filter. Much of the gain in "RAM tubes" is more the fact there is no filter than a unique feature of the design, WHICH is offset by extra AIRFRAME DRAG from the extra scoop. Van's "snorkel" set-up for the fwd intake Lyc's may provide a NET to slight loss, but I've not seen data on it. Before you undertake an alternate unfiltered RAM tube under the spinner weigh the possible negligible benefit to ease of installation and that of a good overall simple filtered system. However experiment and test but know what you are measuring. G
 
Last edited:
I'm up here in the Great White North so we see ambients here of +35C to -35. I fly my RV down to -25. There is a huge difference in how the engine and airframe performs at these temperatures. I'm outta right rudder on takeoff.

It's actually not too hard to get 90% pressure recovery on an induction system. We've done extensive testing on race cars and aircraft with sensitive pressure gauges. With a properly shaped plenum even with short ducts. You can see examples of well designed systems on modern bikes and formula cars. Basically you flood the plenum with near-stopped airflow. I did qualify my statement that at RV speeds, you are unliely to ever see more than a 2% hp gain however well designed your system is. I agree that probably without wind tunnel testing or CFA any power gains may be offset by drag penalties of spilling airflow. I imagine the F1 boys have this aspect completely scienced out after thousands of hours of tunnel and dyno work. Love to be a fly on the wall there.

Ram air is relatively inconsequential on turbocharged and supercharged engines at slower speeds although every little bit helps in racing. Strega and others are running up to 130 inches hg boost! Another 4-5 inches at 500mph at the compressor inlet means the supercharger does not have to develop the same pressure ratio which means less stress and lower charge temps.

By stockers, I was referring to the non-racing variety of Thunder Mustangs. They use the MT 4 blade for a more authentic look. One fresh example was absolutely stunning. THE nicest homebuilt aircraft I have ever seen.

As for prop finish, you see some beautiful polished stuff on some of the unlimiteds but in the Sport Class you see Hartzell grey and on a couple fast entries- bead blasted bare aluminum, no decals. When I asked, the guy just smiled. I figure he did not go to the trouble if it did not do anything. Other aircraft have prop covers on so you can't see what they are using.

From many years of porting heads and flow bench testing, I know that mirror finishes are inferior to less fine finishes for airflow. Just as the dimpled golf ball flies further, F1 and Indy car ports now use dimples. I would not be surprised sometime to see dimples or different surface finishes on props at Reno.
 
Cool Stuff

When I was talking temps I meant a 60C spread in the same flight. Sure it gets cold in the hinter lands. No argument air density is key to HP. However the "efficiency"of the engine is not greater, granted greater HP but corresponding greater fuel burn. Piston engines don't GET more efficient with temp. Sure they make more HP but at higher fuel burn. I was talking efficiency. Now Jet engines get WAY more efficient with lower temps and also RAM pressure, but they are continuous cycle engines. Jet engines make more power per unit fuel high up than down low.

As I said you can see lots of Pressure in a scoop or plenum, but what is the manifold pressure compared to ambient. I can put a flat plate out in the wind and put a pressure tap and measure 100% ram pressure. It is not what's in the scoop, it's what's in the engine. I can see that if you have a large volume plenum with static pressure you can "load" the engine with this slug of air when the intake valve opens, but that takes some serious volume with 4 or more jugs sucking at 2700 RPM (19,000 F1).

My point is many builders make straight little tubes and think that it will provide the pressure to the inside of the cylinder, it will not. To be fair we don't have a lot of room for plenums and diffusers. I will take a look at F1 racers and see what they are doing. I don't think we can fit it. I am an IRL fan and followed Danica because she is a great driver, well and she is HOT! :rolleyes:

As far as ram air and turbos, I only mention turbos and super chargers because that is the ONLY way to truly get a manifold BOOST. Real boost. Yes we can get a little of boost over ambient but it is measured in the 0.1"-hg not inches. 90% recovery is good if that is in the cylinder, not that scoop. We play with 1" to 1.6" of HG. To get a 1/4 of that is the real deal, but not common. Most Cessna's and Pipers are running a 1" Loss. I just don't see getting 1.4"-hg more manifold, but I would like to. What do I need to do. :eek:

Yea rough finish is better for "WET" intakes plenums in engines because the fuel stays mixed. Smooth surfaces allow the fuel to collect on the walls of the plenums, which is not good, but for fuel injection I assumed smooth was better since, it is just air. I did not know rough is better for fuel injection also.

What happened to the Thunder Mustang? Did they get the Falcon engine to work properly? It is a pretty piece of machinery and sounds awesome. I know there are not many out there, probably because they cost a butt load. I saw one for sale, at only a quarter Mil. Also the company is defunct I think.

G
 
Last edited:
If you calculate the amount of air flowing through a 3 inch ID tube at 200 mph, you'll see that this is more than adequate to feed a very powerful engine indeed. Something on the order of 550+ hp.

The Thunder Mustang recently set a world speed record for naturally aspirated aircraft and finished a distant second to DG's Lancair. The Ryan Falconer V12 sounds great and seems bulletproof. The 1000hp supercharged version is apparently on hold awaiting a cash infusion from a sponsor. Too bad as someone needs to give serious chase to DG to make it a race again.

I'm glad that there is at least one liquid cooled entry running in the sport class as this makes it somewhat more interesting. Kinda like Danica Patrick in IRL. That girl has the goods.
 
rv6ejguy said:
If you calculate the amount of air flowing through a 3 inch ID tube at 200 mph, you'll see that this is more than adequate to feed a very powerful engine indeed. Something on the order of 550+ hp.
Click to expand...
Thanks, I have do some calculations and get the RV-7 flying so I can experiment. Cheers
 
rv6ejguy said:
From many years of porting heads and flow bench testing, I know that mirror finishes are inferior to less fine finishes for airflow. Just as the dimpled golf ball flies further, F1 and Indy car ports now use dimples. I would not be surprised sometime to see dimples or different surface finishes on props at Reno.
Click to expand...

Check this thread:

http://www.vansairforce.com/community/showthread.php?t=4192

and this link:

http://www.vortelator.com/RV4.html

There is a bit of a discussion going on about adding turbulence to increase speed performance. At first it seemed like BS but your golf ball analogy got me thinking (and a recent article I read about roughness on humpback whales fins being hydronamically superior
http://www.appliedfluids.com/UUST01.pdf)...

Perhaps there is something to adding turbulence to increase speed performance, though it would bother me in a deep way to borrow technology from golfing to make my RV better :).

Chuck
 
You must log in or register to reply here.