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Speed Improvement/Lesson Learned

hevansrv7a

hevansrv7a

Well Known Member
My -7A was built - in my mind - to go a little faster than "stock". I used SJ cowl, plenum and all three wheel fairings, omitted the steps and have only com and transponder external antennas. Until now I was disappointed. My max speed at altitude was 174 kts (200 mph) and that was with more than 2700 rpm on my Prince prop.This was a good increase from 155-56 without the fairings.

Craig Catto and Sam James said I should be going faster. Van's said they don't believe in the SJ stuff and that I was doing OK.

Long story short - I realigned one of the main wheel fairings which was out a couple of inches as measured about 2' aft of the rudder.

Today told the tale. I gained about 6 or more knots. No other changes were made. The TAS on the GRT is calibrated and is within 1 kt IMHO.
Effective pitch before varied from about 72 to 75 over a large number of observations. Today it averaged 78.These three readings in ktas are at density altitudes 3560-3840.
2500 159 67%
2700 174 86%
2770 180 94%
The best it would do in similar air (cold!) a few weeks ago was 6 or even 8 knots less.
Based on prior experience, the numbers will be a little better at 8000 to 9500. I'll be anxious to see what my re-pitched Catto 3-blade will do.
 
Holy Cow...

That's a big difference, Howard. I'd love to see your follow up when you retest. It's amazing how the wheel fairing made that much drag.
 
flytoboat said:
H Evan,
Could you explain "Effective Pitch" and what the numbers you cite represent? :confused:
Thanks
Click to expand...
Effective pitch as I understand it is the number of inches the airplane moves forward per revolution. It's the real-life equivalent of the pitch number that the prop. mfr. gives you. The mfr. usually is giving you a geometric measurement at a specific station on the blade. Because everyone does it differently and props are never equally effective, the numbers used by the mfr. are only comparable within his own product line. The "effective" version is just measuring the result in the same terms. Therefore on a given airframe when drag is reduced, one result may be an increase in effective pitch which I think means less "slip" in non-engineering terms. The effective pitch is thus also useful as a comparison between props of different make and design if there are no airframe changes.

All experts (I am not one) are welcome to gently straighten me out on this.

=C64*5280*12/60/B64*1.15 where c64 is knots and b64 is RPM. The 5280 is feet per mile and the 1.15 converts knots to mph. The 12 and the 60 correct for hours to minutes and feet to inches.
 
Light bulb on

AHHHH. I was thinking pitch as in nose up or nose down. Thanks for your very good explanation.
 
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hevansrv7a said:
Today told the tale. I gained about 6 or more knots. No other changes were made. The TAS on the GRT is calibrated and is within 1 kt IMHO.
Effective pitch before varied from about 72 to 75 over a large number of observations. Today it averaged 78.These three readings in ktas are at density altitudes 3560-3840.
2500 159 67%
2700 174 86%
2770 180 94%
The best it would do in similar air (cold!) a few weeks ago was 6 or even 8 knots less.
Click to expand...
This is certainly encouraging, but on any given flight, there could be anomalies that cause measured performance to vary. We shouldn't jump to big conclusions based on data from one flight. It will be very interesting to see if this new speed is repeatable on several subsequent flights.
 
Confusing

I think I am having trouble with my IAS/CAS readings and am consulting with GRT about them. They are suddenly reading much slower than ever before even though my RPM's are as expected. A very rough GPS observation today gave a little over 170 kts at 9000 approx. Not good enough, but only a quickie two-way run in the approx. direction of the wind. Meanwhile, my Catto 3-blade will arrive this week and we'll go back to testing that and the airframe at the same time. Bottom line - I just don't know yet. Thanks for asking.
 
hevansrv7a said:
I think I am having trouble with my IAS/CAS readings and am consulting with GRT about them. They are suddenly reading much slower than ever before even though my RPM's are as expected.
Click to expand...
Howard,
Have you done the basic troubleshooting steps like pitot and static leak checks, confirmed the pitot tube is still pointing in the right direction, and that nothing at all has changed in the area of the static ports?
 
Not yet, but that's certainly on the list

Kevin Horton said:
Howard,
Have you done the basic troubleshooting steps like pitot and static leak checks, confirmed the pitot tube is still pointing in the right direction, and that nothing at all has changed in the area of the static ports?
Click to expand...
Oddly, the readings seem OK at lower speeds. A leak at higher pressures could maybe explain it. However, the GRT self-calibration function for TAS should have caught that and didn't, so that's why I'm looking first at the electronics. It's a work in progress.

Kevin, welcome back - 14% - does that work for you?
 
Some progress

I confirmed that there is no leak in the pitot pressure hose. Static ports look OK. Pitot was angled to one side a few degrees - quite noticeable. I don't know either how it happened nor when. Did not get a chance to flight test it today. I also deleted the entire TAS calibration table from the GRT and will do it over when I next am able. Haven't heard from GRT about that yet.
 
Today's Progress

I aligned the pitot with the seam in the wing parallel to the chord, confirmed there was no leak in the pressure side and went flying. In general the improvement is confirmed.

The caveat in all this is that I don't absolutely know that the pitot was OK until a few days ago. But..

I first erased all the TAS corrections in the table in the GRT and recalibrated for around 170 kts and got the same -4 correction as I used to have. Calibrating the GRT involves flying upwind and then downwind with constant altitude and headings within 5 degrees of track and letting the instrument solve the problem using GPS for true speed.
Ca
Then I did some speed trials. At 8000', 8500 DAlt I got 178 kts. At 7000 I got 182 kts. At 7800' or 7500 DAlt on 67% I got 166 kts which is a lot better than I used to see on trips. I noticed that at first the 7000 test was also getting only 178, but after a little while it cilmbed up those last 4, so I'm not sure that the lower altitude got me anything, especially because it was the same RPM. Mixture? On the step? I'm just not that good a test pilot to know. My best speeds before the fairing alignment were 172-174.

These speeds are at the same 2760-2770 rpm's as before. The effective pitch of the prop is up to 79" now, which is exactly what Lonnie Prince said it was on the sticker on the prop.

Also see my posting on ram air effect. http://www.vansairforce.com/community/showthread.php?t=27921

Now I am putting on the Catto prop which came back yesteday. Next week I should have some comparisons to report.
 
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The only way I measure speed runs is with a GPS going 4 compass directions, add all speeds divide by 4. Then only variable is altitude & temperature.

A constant speed prop would greatly improve your speed at higher altitude, with a ton less RPM.
 
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I don't think so

Geico266 said:
The only way to accurately test speed runs is with GPS going 4 compass directions, add all speeds divide by 4.

A constant speed prop would greatly improce your speed at higher altitude, with a ton less RPM.
Click to expand...

If you fly directly upwind and directly downwind and average them you get your speed. But more to the point, I'm only trying to compare to my own airplane anyhow. Without the GRT you could not do this accurately enough but with it I believe the method is good enough for this purpose. Once calibrated, the GRT demonstrates its accuracy by computing enroute HW and XW in real time. Whenever I check it against the GPS, using just the HW component, it's within one knot as compared to TAS. I'be been observing this for 130 hours, more or less.

Assuming you could fly upwind and downwind perfectly, assume 180 TAS with a 30 kt wind. 150 + 210 = 360 which is exactly twice 180. The GRT has the smarts to fix small errors in this scheme because it knows both heading and track during the test runs.

I don't dispute that in theory the 3 leg or 4 leg method is more accurate but in realistic terms, an unskilled test pilot such as myself can probably get better results with the GRT given the amount of precision flying needed to get 4 legs.

That said, I don't believe you are accurately describing the 4 leg method. if you do the math you describe you will get the wrong answer. See this link, please. Thanks to Kevin Horton for that one. Also this one. The reason just averaging them gives the wrong answer is the trignometry of the cross wind vectors. Thought experiment: An airplane flys 1000 flights of equal duration with random winds all of the same velocity. In reality the net effect of all this will be a headwind because even some of the near 90 degree quartering tailwinds will have the effect of reducing forward track speed.

As to getting higher speed with less RPM at high altitude, let's remember that engine HP is equal to torque times rpm divided by 5252. If my throttle is already wide open above 8000' for example, then a bigger bite at a slower rpm will not produce as much HP. For such a configuration to produce more speed it would have to be getting it from blade design, not blade pitch. If there is less HP then the airplane will not go as fast, assuming no change in prop efficiency.

A C/S prop has the advantage you describe only when there is surplus air density. There are combinations of MAP and RPM that are equivalent and the most efficient is usually the one with WOT. I do find that at lower altitudes the FP limits performance. Van's tests indicated the Sensenich was at least as fast as almost any C/S save, perhaps, the Hartzell BA and it was not about RPM rather about blade design. Are Elippse's props C/S? RPM's in races are much higher than 2700 and there has to be a reason.
 
I do

Paul Eastham said:
So do you still believe realigning your fairing gave you 6 knots, or not?
Click to expand...
It gave me at least 4 and as much as 8 knots to a level of certainty that both makes sense and seems true to me. I think there are a couple of knots left, too, since the fairings are still not perfect and the nose fairing is not perfectly aligned with the tire.

I used to plan 150 kts at 67% for XC trips. It would usually indicate (TAS) under 155 and never more than 160. Yesterday's test got 166 at the same power setting but at a higher altitude than my wife lets me fly. There's a genuine improvement there and it's (as we IT guys say) non-trivial.

Of course, now I'm going to use a Catto 3-blade with a much higher pitch, so the numbers will change. At least now I have a reasonable basis for comparing the props.
 
Catto Performance

Can't wait to hear how the Catto compares to the current prop!!

Glad to hear of the performance improvements!

Cheers,

db
 
Please explain the misalignment

2" misalignment is so gross that I think I must not be interpreting your description correctly. That suggests that you just put them on and let them fall where they may. Even at that, it is surprising that there is enough slop in the design to get a 2" misalignment in the main wheel fairing installation. Anything that sticks out in the air has got to be aligned properly to maximize speed. I have an O-360-A1A powered RV-6A with a Hartzell C/S 72" dia. 7666 blades prop and steps. My baseline speed at 6000 ft density altitude was 170.6 kts. To measure increases in performance the test method must be constant! At first I used the www.us-airrace.org handicap procedure and I ran a LOT of tests using their 3-way average procedure on tracks of 000, 120 and 240 then at the urging of Keven Horton and John Huft I reran the numbers in the National Test Pilot School spread sheet to mathematically eliminate the wind and learned that at best average method results were equal to the NTPS results and with increased wind the average method results are ALWAYS slower than the NTPS results. Like you I am very interested in speed given the basic configuration I have selected (especially steps and external antennas). With many modifications made and most discarded I have managed to get the speed up to 177.8 kts. The big items were the complex installation of simple concept baffling in the lower cowl which added 4 kts and removal of my tip tanks and stock tips with 3" streamlined tips for racing only which reduced the wing span by 3 feet and increased the speed by three kts. I worked with the inlets and outlets and the only effects I got were higher CHTs and slower speed. I am currently developing subfairings for the stock wheel fairings (only the nose gear is complete) and I believe at this point I have another 1 kt gain. I will not know for sure until I reconfigure the wing tips for the Taylor 125 air race in a couple of weeks and compare with the 177.8 kt before subfairing race configuration speed.

I like what you are doing because it shows an interest in performance instead of just repeating the build process. I have been doing that for the past four years and it is both educational and satisfying. As far as the prop is concerned, I believe there is great potential for speed increases. I have heard nothing but good things about Catto props. I have communicated with Hartzell and I am pretty well convinced that a fixed pitch prop will give the greatest speed in a given environment. Curves that I have on the hartzell props show that the efficiency of the 7666 blades drop of dramatically at higher airspeeds and the earlier 7496 blended airfoil blades are more efficient in this speed range than the current 7497 blended airfoil blades. This is a VERY expensive change that I will not be able to incorporate until the blades need replacing. The Hartzell engineer that I talked to said that for all out racing with no consideration for other airplane performance parameters it would probably be a good idea to replug the crankshaft and go with a speed optimized fixed pitch design. The numbers I saw with Paul Lipps original tests (on an RV-7 I believe) were not sufficient for me to go that way. I will be interested to see the results of your tests with the Catto prop.

Bob Axsom
 
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Bob,

Note the explanation "measured 2' aft of the rudder". It appears heavensrv7a was using the stretched/doubled string method of checking for thrust line alignment of his fairings. This 2" misalignment "aft of the rudder" would be a small fraction right at the aft end of the fairing.

Cheers,

db
 
db1yg said:
Bob,

Note the explanation "measured 2' aft of the rudder". It appears heavensrv7a was using the stretched/doubled string method of checking for thrust line alignment of his fairings. This 2" misalignment "aft of the rudder" would be a small fraction right at the aft end of the fairing.

Cheers,

db
Click to expand...

Bob Axom and db1yg, db1yg is correct that the 2" is at quite a distance from the axle. I did that merely for precision. I propped up a 2 x 4 horizontally behind the airplane on which to make marks. I took a sight line from my rear tie down ring to the nose gear and called that the centerline. I propped up a piece of plywood at the front of the fairing with the edge aligned with the center of the front of the fairing. For the rear of the fairing I used two methods - string and laser. I didn't use a double string here, but that is how I did the leg fairings.

I also measured the gross distances between the fronts and rears of the wheel fairings, but that method is centerless and thus less useful. It is telling me that I'm not done yet. That said, the airplane flies straight now, at least at around 130 KIAS. I haven't checked it again at higher speeds, but it was dropping the right wing before. Sans fairings it flew straight. I have no rudder tab and don't use my wing trim.

The SJ fairings attach at a point and a line. The outboard point is the axle nut AN4 bolt. The inboard line is the top of the single flange that is bolted to the axle flange and bent inboard and then up to allow the fairing to clear the brake. I had a tool and die shop bend the 1/10" aluminum stock for the bracket and did not originally check it. That's how the misalignment happened, at least in part. My adjustment method was the only one left: I clamped a bar on the top of the flange and bent the flange in the horizontal plane with manually applied leverage. It's still not perfect and I haven't done the other yet.
----------------------------------------------------
Bob, I truly admire all the painstaking research and experimentation you've done. That said, I have no such ambitions. I was and am just trying to get the airplane to perform as I believe it should. My speed measurements are not up to the precision standards that some use and that's OK. I know that with my GRT properly calibrated I can quickly and easily get valid, useful comparisons from one day to the next. That it is very probably within one knot of the truth is frosting on the cake and useful for knowing when to say "enough". One of these days I will validate it all with the NTPS method or this one. Meanwhile, I can get useful information very quickly in this digital age.

Bob, I've been meaning to ask you: did you verify that you gained speed by opening up the baffles in the wheel fairings aft of the tire? Mine are solid and my tire opening clearances are tight.
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I'm not expecting higher speeds with the Catto, although I could get a happy surprise. The Catto went back for adjustment and the idea is to get the RPM's down. Since the Prince is turning 2770 (I've seen 2800 on other days) there is just more HP there. I'm trying to get, with the Catto, good climb and smooth, quiet cruise well below 8000'. I'll take what I can get in the way of speed at those lower altitudes.

I originally wanted to cruise the airplane at 8000' or above but it turns out that my wife is very uncomfortable for a day or so after any flight that goes over about 5000'. That's why I now have the goal of best cruise at less than ideal altitudes. With the Prince I could only get to about 66% without hitting redline 2700. I could have re-pitched the Prince and still might, but for now I thought the Catto might do better. It won't be an apples to apples comparison except in terms of speed and fuel flow.
 
2" measured at the rudder is something a bit less than 1 degree. Very hard to imagine a measurable airspeed drop with that. That is about .150" at the end of the fairing, give or take?

Here is an interesting question - how much slower would an RV fly if we yaw the entire airplane 1 degree? It would be quite difficult to discern only one degree, but it would be interesting to try. For example, how far would the ball be off with one degree of yaw booted in?

The nice thing about this experiment is that it could be tried on the same flight, fly straight and measure, yawed and measure, repeat three times.
 
The speed returned with bulkheads out

I noted from other peoples comments that some wheel fairings came with bulkheads in them. Mine did not. On the thought that this was a direct source of drag, I decided to make 3-piece floam bulkheads covered with fiberglass in mine which conformed to the outside diameter of the tires +~1 inch. In the case of the nose gear there is no room in front of the tire so only the rear of the tire was closed off in the fairing. I test flew the plane and it flew slower. Since I had a race coming up in the near future I cut out the bulkheads and the speed returned to the previous level. I theorized that my specific configuration was generating a new air flow pattern and turbulance below the fairings. Maybe flat bulkheads would have been better. Note: The wooden object at the rear of the bench is the mold for one of my other mods - 3" span wing tips, a mod that worked very well.

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Bob Axsom
 
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Conundrum

Alex, your math looks OK. Keep in mind, though, that the 2" is out of a possible 40" approximately, to the center.

More to the point, the observations seem valid to me and I made no other changes. While it seems like a big effect, it's the only reasonable explanation. So we are left trying to figure out why a small angular change in wheel pants would have such a big effect. Maybe the pants themselves are not straight. SJ pants are very flat sided, unlike Van's. I know that it seemed like a lot more than one degree both visually and when I applied the bending forces.

Two schools of thought will arise. First, there will be those who simply doubt my numbers. OK, but they are what they are. I'm telling the truth, but it's possible I'm missing something. Second will be those who, like me, think that there are some extra effects from the way air flows around the pants. That's a reasonable hypothesis but almost impossible to test outside a wind tunnel. If you are in my school of thought, the rudder test will be interesting but not necessarily on point.
 
AlexPeterson said:
snip

Here is an interesting question - how much slower would an RV fly if we yaw the entire airplane 1 degree? It would be quite difficult to discern only one degree, but it would be interesting to try. For example, how far would the ball be off with one degree of yaw booted in?

The nice thing about this experiment is that it could be tried on the same flight, fly straight and measure, yawed and measure, repeat three times.
Click to expand...

Pete Howell suggested putting a piece of yarn on the wing with some tick marks towards the aft end, to be able to determine how much yaw is induced. Sounds like fun.

I guess this would be on point, if yawing the entire aircraft as much as the wheel fairing was "yawed" only caused a small loss in speed. Need to get the data. The data would be quite solid, since the test would be done on the same flight.
 
I'm Interested

I am pretty sure I have a main gear fairing alignment issue, so I am very interested here. The key for me is, the speed diff will need to be pretty significant if I am to go back into Fiberglass ach-eee-double hockey stick.

I'm sure Alex and I will be forced to get some data on this:D.

I have a rudder wedge and needed to raise an aileron to correct a heavy wing. This will be fun to EXPERIMENT with.
 
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