[Nuisance]

To my fellow fanatics

Recently, there has been a thread about cooling drag on the Lancair email list. I have obtained the permission of the author, Fred Moreno, to post it on my website.

I think that once you 'freeze' the airframe (by that I mean use an existing design, like a Van's kitplane), the things you can do to go faster (be more efficient) are few. A straight airplane, a general cleanup, and then the main area you can work on is engine cooling drag.

It is lots of work though!

Anyway, go here

http://www.lazy8.net/speedmods.htm

and scroll down to "An Engineer's Perspective".

Have fun!

 

[Bob Axsom]

I printed it

Well actually I am printing it as I write this message. As always - good stuff! Will study more completely. I have pulled the lower forward baffle and extensions out of my plane and will test again when I get the magneto reinstalled. My records during the evolution of my lower cowl mods show that that is the fastest configuration I have come up with so far. I'm hoping to see that 173 kt range come back. I am keeping all of the rear baffling and the horizontal baffles along the cylinders in the lower cowl and I will try the cooling air inlet plugs with that configuration.

Bob Axsom

 

[Kevin Horton]

I have pulled the lower forward baffle and extensions out of my plane and will test again when I get the magneto reinstalled. My records during the evolution of my lower cowl mods show that that is the fastest configuration I have come up with so far. I'm hoping to see that 173 kt range come back.

Bob - Do you do multiple flights to test speed with each configuration, or just one? There are a lot of reasons why results from any one flight might be off, so if you are trying to measure small changes you really need to get results from multiple flights.

If you hope to see the effect of small mods, you also need a technique that gives results that are independent of the wind speed. Simply averaging the GPS ground speeds on different headings will give a result that depends on the wind. The wind induced error can easily be greater than the speed difference from any small mod.

 

[Bob Axsom]

Oh So True

I have finally come to recognize what you and John Huft have been saying all along. I used the US Air Race Inc. Handicap procedure for over 20 flights (I'm very stubborn) before I tried the obvious two-way straight into the wind and reverse and I also tried random two way and 4-way all using the USAR procedure for establishing the flights at 6,000 ft density altitude. My observation based on this limited sample way the multi-directional methods that were not 2-way directly into the wind and reverse yeilded a compromise speed. I find myself sneaking a peek at the TAS window of the airspeed indicator to get a clue of how I'm doing independent of the GPS techniques. The problem with the 2-way method is it is difficult to get a wind of constant direction and velocity over the Ozarks where I do my testing (or anywhere else I suspect). After trying this method a couple of times I gave up and went back to the USAR method. In the one flight where I made two random 2-way (two opposite directions), a random 4-way (two 2-ways of opposite directions perpendicular to each other), and the USAR method, the USAR yeilded the lowest number and one of the 2-ways provided the highest number.

Another error I think I have introduced into my test method is using the TruTrak Pictorial Pilot and Altrak for constant direction and altitude. My thought is I am burning true airspeed in fighting cross wind and vertical air movements and what I am recording is a resulting track speed.

In the case of multiple flights on different days with the same configuration using the USAR method I found my speeds for the tests fell across a 4 kt range.

Still I get a sense of what is working and what is not to the extent that something that I have spent many months developing and looks really good can be scrapped with a clear conscience. I wish there was a wind tunnel that I could work with to "tune" these changes under strictly controlled conditions but it continues to be a crude cut and try relatively gross change approach that is very lucky to hit the optimum.

I'm sorry for the long winded response the short answer is I do not make multiple flights to test new mods. You are right of course if I want an accurate evaluation multiple flights are required. I tend to say, well that worked or it didn't and usually it didn't.

Bob Axsom

 

[bumblebee]

Another error I think I have introduced into my test method is using the TruTrak Pictorial Pilot and Altrak for constant direction and altitude. My thought is I am burning true airspeed in fighting cross wind and vertical air movements and what I am recording is a resulting track speed.

Bob...with all due respect, you are seriously overthinking this whole process and making it needlessly difficult on yourself.

The most accurate method for testing drag mods is to "Fly the square".

Fly a timed square (30 sec legs + turns) on the cardinals (0, 90, 180, 270) at defined power (2500/WOT) and defined DA (e.g. 8000') using shallow banks in the turns (15* or standard rate turns).

use a recording GPS and/or EFIS if available
fly 3 laps per test (minimum)
fly 3 tests per mod (ideal)
start with same conditions (full tanks, engine temps stabilized, etc.)
average all legs for GPS and TAS
calculate standard deviation (R-squared)

Using A/P and Altrak is HIGHLY recommended to ensure precision and consistency....you simply cannot hand-fly as accurately as a/p+altrak

Your results using this method should be within +/- 1% .... FAR more than enough for anything except scientific (aka divorce-worthy) tinkering.

At RV speeds, a 1% error is effectively impossible to measure or notice.

In the case of multiple flights on different days with the same configuration using the USAR method I found my speeds for the tests fell across a 4 kt range.

You are doing well if you consistently achieve a 4kt range. Remember, a 4 kt RANGE = a 2 kt MARGIN (up or down)...this is very close to the minimum measurable variation.....don't get crazy trying to measure beyond this....it ain't worth it and you'll never know for sure if you measured an "improvement" or just a change in the wind. The "last knot" will be the most expensive and the least accurate.

After this, the only thing left is to bribe Paul Dye to get you into the NASA wind tunnel....and that's big bucks on both counts

 

[Bob Axsom]

All three of you are consistent

I can't defend the approach I took as the "right" approach but I do have to maintain a healthy measure of independence and skepticism. I believe that the "square" method is a reasonable compromise method that reveals maximum speed only on dead calm days and it matters whether you fly constant heading or constant track to obtain your data. I have flown repeated cycles in the same flight and the speeds derived from individual cycles have been within one knot of each other. The four knot variations occurred when the tests were conducted on different days with the same configuration. My airplane is fairly clean and I have found it extremely difficult to gain any speed regardless of the mental, physical ot spiritual effort and knowing from personal experience that the speed I record may be off the actual maximum by many times the amount gained or lost by the modification causes the evaluation process to drift in a subjective direction. Having a disciplined approach like the square method or the triangular method or the 2-way method makes the testing more objective and I have found for me personnally that is VERY important. My autopilot will not fly a constant heading and its inflight corrections for real time variations in cross wind will always cause speed measurements to be less than the actual true air speed. The same is true of or the altitude hold function over hilly country with a rising and falling air mass. Test flights early in the mornings of calm days provide the most reliable results for me but when I'm ready and able to test, I test, grudgingly accept the results, try to rationalize what they mean and decide what to do in the future. For the professional pilot or the everyday pleasure flier you take what you get and that is that but if you are modifying your own plane with a desire to find the best configuration for racing speed and you have a demonstrated four knot error range you can't just throw the data on the table and walk away. You have to eventually decide whether the mod stays as is, gets modified or removed. It is not easy! This is the kind of problem that leads the test organizations to develop the "fly the squares and average the results" methods I'm sure. Testing costs money and schedule so procedures are essential to control this imperfect activity. ARGH! my head hurts I'm going back to bed.

Bob Axsom

 

[Kevin Horton]

Bob...with all due respect, you are seriously overthinking this whole process and making it needlessly difficult on yourself.

The most accurate method for testing drag mods is to "Fly the square".

Fly a timed square (30 sec legs + turns) on the cardinals (0, 90, 180, 270) at defined power (2500/WOT) and defined DA (e.g. 8000') using shallow banks in the turns (15* or standard rate turns).

use a recording GPS and/or EFIS if available
fly 3 laps per test (minimum)
fly 3 tests per mod (ideal)
start with same conditions (full tanks, engine temps stabilized, etc.)
average all legs for GPS and TAS
calculate standard deviation (R-squared)

Simply averaging the GPS ground speeds does not fully account for wind. For example, if our TAS is 160 kt, and the wind is 045 degrees at 25 kt, our ground speeds will be:

hdg GS
000 143
090 143
180 179
360 179

average = 161

if the wind is 50 kt (not at all uncommon at altitude), the average ground is up to 164.5, with a TAS of 160 kt. This isn't nearly accurate enough to see the effect of a small mod.

If you are going to go to the trouble to fly the tests, you might as well use a calculation technique that gives an answer that is independent of wind. I highly recommend the NTPS Spreadsheet, using the four leg option.

 

[Tom Martin]

Over the years I have done a lot of "testing" with my rockets. Getting accurate consistant data is very difficult. I use a three leg gps system that I borrowed from Kevin Horton's web site. It is pretty good but I have found that there are huge differences in HP based on temperature so that really needs to be calculated as well. As Bob Axom has stated "it makes my head hurt"! The best most accurate testing for drag that you can do is fly beside a friend who has not made any changes. Pick power settings where you are flying exactly the same speed. Make your modification and fly the same power settings again. This takes a whole bunch of variables out of the equation and has proven to be most effective for me when I am trying to show performance changes. Last fall I flew beside my friend and got some base numbers, then changed exhaust systems, and flew again. There was no difference, nada, nothing. However a few changes to my cowling outlet area and I could show a speed increase related to the other aircraft. For me this has proven to be the simplest and most accruate way to test for speed changes.

 

[captainron]

Cooling drag

I'm starting to wonder if a different thought pattern is required here. Looking at my fuselage that has yet to have the engine mount installed makes me think about the large frontal area here that needs to be covered with something. Sure, it's going to get an engine mount and then a very flat engine. And then a huge cowling just to blend in with the firewall pattern.
Thinking about all the twins I have flown and their extremely tightly cowled engines, engines which were much larger (in displacement) than what we're working with here, has me thinking that maybe a different approach is needed. Take a look at an Aerostar or a C-310 and notice that the cowling or nacelle is not much bigger than the width and height of the engine itself. Then, think about the fact that there may be TWO turbochargers, an intercooler, the rest of the exhaust system, and every other accessory found on any engine packed in there!
Our firewalls look like they have room for a radial engine to be mounted in front of them!
Don't know what the answer is except to move the engine forward enough so that the cowling can be reduced in height and volume.
How have the planes that have their engines mounted where the fuselage has to be fat anyway (E-Z's, Velocity, etc.) fared in speed compared to the Vans planes with the same HP/weight ratios?

 

[David-aviator]

I'm starting to wonder if a different thought pattern is required here. Looking at my fuselage that has yet to have the engine mount installed makes me think about the large frontal area here that needs to be covered with something. Sure, it's going to get an engine mount and then a very flat engine. And then a huge cowling just to blend in with the firewall pattern.....
.....How have the planes that have their engines mounted where the fuselage has to be fat anyway (E-Z's, Velocity, etc.) fared in speed compared to the Vans planes with the same HP/weight ratios?

Except for one or two highly modifid airplanes, the pushers are not much better. I built a LEZ and Cozy and felt there was much turbulence across the prop that defeated some of the reduced drag of the configuration. I know there was much experimentation with positioning the prop aft of the engine to minumize this issue.

Thanks much to the guy who started this thread. I believe I have some serious drag issues to address with a modified cowling to accomodate the Subaru cooling needs and this is will help sorting it all out.

 

[pierre smith]

People.....

Our firewalls look like they have room for a radial engine to be mounted in front of them!

Hi Ron,
The occupants are the reason for the wide firewall....look at the skinny -4, no wider than a guys shoulders. In reality though, when you look at the 6's, 7's and 9's from the side, the cowl is very pointy if you ignore the carb intake. Much more aero than a Spamhawk or Mooney.

As far as the Sube installations, the guys in Canada building the 10 with a belly mounted scoop so far looks the best....very P-51 like with gentle curves into and out of the radiator. My buddy has an Egg 4 cylinder in his 7 and can't get more than 162 MPH at 34" and 4800 engine RPM....feels like he's dragging a drogue 'chute!

Regards,

 

[pierre smith]

Another freebie

The CAFE Foundation also tested an unwaxed Mooney, then waxed it and gained 2.8 MPH or put into horsepower, would need an additional 8.3 HP to do the same! Here:

http://cafefoundation.org/v2/pdf_cafe_reports/Wax Job with PICT.pdf

Pierre

 

[rv6ejguy]

As someone who has done a fair amount of flight testing with regards to cooling drag and engine performance, my view is that it is very difficult to see or measure changes consistently smaller than 2% in an RV. The side by side in the same air seems to be the best way to gauge performance as both aircraft are subject to the same atmospheric conditions..

As someone previously mentioned, temperature and even humidity do affect hp. Unless you correct for that, the results are skewed even more as more than one variable has changed.

 

[rv7boy]

The Wright Brothers

This thread makes me appreciate Orville and Wilbur even more!
Don

 

[captainron]

Hi Ron,
The occupants are the reason for the wide firewall....look at the skinny -4, no wider than a guys shoulders. In reality though, when you look at the 6's, 7's and 9's from the side, the cowl is very pointy if you ignore the carb intake. Much more aero than a Spamhawk or Mooney.

As far as the Sube installations, the guys in Canada building the 10 with a belly mounted scoop so far looks the best....very P-51 like with gentle curves into and out of the radiator. My buddy has an Egg 4 cylinder in his 7 and can't get more than 162 MPH at 34" and 4800 engine RPM....feels like he's dragging a drogue 'chute!

Regards,

Hi Pierre, doesn't an -8 firewall measure out to about the same width as a -7?

 

[bumblebee]

Simply averaging the GPS ground speeds does not fully account for wind. For example, if our TAS is 160 kt, and the wind is 045 degrees at 25 kt, our ground speeds will be:

hdg GS
000 143
090 143
180 179
360 179

average = 161

if the wind is 50 kt (not at all uncommon at altitude), the average ground is up to 164.5, with a TAS of 160 kt. This isn't nearly accurate enough to see the effect of a small mod.

If you are going to go to the trouble to fly the tests, you might as well use a calculation technique that gives an answer that is independent of wind. I highly recommend the NTPS Spreadsheet, using the four leg option.

This is true....but at the risk of splitting hairs, it's folly to chase 1-2kts in a 180kt airplane using instrumentation with a cumulative margin of error greater than 1-2kts.

As all good testers know, when operating within the margin of error, every data point has a confidence level of zero.

Using GPS or TAS is close enough, unless, as you say, the test is being performed in winds above a meaningful percentage of TAS e.g. 50 kts. I would argue that testing should not be performed in such conditions if accuracy is required.

 

[osxuser]

Sometimes I wonder how much of this stuff I can apply to the Cessna Cardinal before I get intro trouble. At what point does a 'fiberglass repair' become a modification? Maybe I just really suck at making fiberglass shaped like the original ...

 

[Bob Axsom]

... it's folly to chase 1-2kts in a 180kt airplane using instrumentation with a cumulative margin of error greater than 1-2kts.

Ah yes but we continue to try. Last year in the AirVenture Cup if I had averaged 1 more knot I would have beaten the slowest RV-8 in my RV-6A with a 1.5 feet longer than standard wingspan (tip tanks). It's hard but not impossible to squeeze out improvements and even harder to measure them but accepting defeat is an option that has its own drawbacks.

Bob Axsom

 

[frankh]

I was thinking sir Geroge Carey

This thread makes me appreciate Orville and Wilbur even more!
Don

Who flew heavier than air in like 1850 or something...Of course i'm slightly biased...

Frank the brit...

 

[rv7boy]

Sir George

Frank, I believe you mean Sir George Cayley.

See http://www.centennialofflight.gov/essay/Prehistory/Cayley/PH2.htm

I was thinking of all the painstaking calculations of lift and drag Wilbur and Orville performed from data obtained in their wind tunnel. But I certainly do admire Sir George Cayley for all his contributions to the science of flight. (My name derives from Britain, too, even though there are about 20 generations dating back to circa 1629 when my Hull ancestor came to this continent.)

Tally ho, ol' chap!

 

[bumblebee]

Ah yes but we continue to try. Last year in the AirVenture Cup if I had averaged 1 more knot I would have beaten the slowest RV-8 in my RV-6A with a 1.5 feet longer than standard wingspan (tip tanks). It's hard but not impossible to squeeze out improvements and even harder to measure them but accepting defeat is an option that has its own drawbacks.

Bob Axsom

(Do I dare mention that merely adding length to a wing might create additional drag? )....

I race, therefore I empathize. I could sneeze and lose a knot over the course of a 100 mile race. If you REALLY want to lose sleep, start racing sailplanes.....you'll be up at 3am cooking up secret waxes and coatings all in the name of finding that elusive knot while your (soon to be former) wife calls in the guys with the white jackets.

Not to give you more ulcers, but prop efficiency is another m-a-j-o-r contributor to speed variability. Due to the immense drag rise at the tips, running a prop at 2700 RPM can create MORE drag than running at 2500 RPM. This large multiplier means it is critical to keep RPM exactly on the money for every test run. You then must correct for density altitude (temp and humidity) variability. Prop management is easily worth a knot or two all by itself over the length of a race.....

Just discussing it, I can already feel the obsession creeping back....that's why most of us don't have hair (or wives).

Keep trying! But make a consistent test program and stick with it. Consistency counts for more than exotic techniques.

 

[Nuisance]

It is true there are many variables...living in the rockies, I find the vertical movement of the atmosphere the biggest challange, bumblebee has named others.

Still, by use of a little vector arithmetic, and 3 equations in 3 unknowns, we can eliminate winds aloft from the list of variables. And, better yet, by simply downloading the spreadsheet provided by the National Test Pilot School at
NTPS Spreadsheet
we can just fly the squares, and go home and type the numbers in, and we have the basis of a consistant test method.

For each square, we get four sets of three legs. The spreadsheet calculates wind velocity, wind direction, and our true airspeed (the 3 unknowns) for 4 sets of three in each square. It also calculates the standard deviation of the resulting TAS, which provides a measure of the quality of the square that was flown. If there was vertical movement of the air mass, the pilot did a less than perfect job holding altitude, the winds aloft varied over the distance of the square, etc., the std. dev. will be large, and we know to ignore the test. If the std. dev. is small, we can have some confidence in the data.

I have found that a std. dev. of less than 1.0 means a fairly good test, you can set your own criteria.

So, why not?

 

[rv6ejguy]

Lots of great theories here. For solo work, I forget about TAS/ GPS. Find some smooth air, same altitude and close to same OAT. Nail the VSI and altimeter for 3-4 minutes and read the ASI. Can get +/- 2 knot accuracy if you are good.

 

[Kevin Horton]

Lots of great theories here. For solo work, I forget about TAS/ GPS. Find some smooth air, same altitude and close to same OAT. Nail the VSI and altimeter for 3-4 minutes and read the ASI. Can get +/- 2 knot accuracy if you are good.

Yes, I was planning on posting a message suggesting just this. The GPS based methods are great if you really need to know TAS. But, if all you care about is how the speed with the latest mod compares to some other configuration, then you only need to know the change in speed. You don't need to know the absolute speed. So, just get a stabilized ASI reading, pressure altitude and OAT. Convert the ASI reading to TAS. This isn't the real TAS, as we haven't corrected for ASI instrument error, static source position error or errors in the OAT. But, these three errors will be about the same on all flights (assuming you don't make any changes that would affect them), so they cancel out if we are just looking for a difference in speed.

Yes, OAT and humidity will also affect the results. But, if we get a bunch of data from multiple flights at different OATs in any one configuration at the same pressure altitude, weight, CG, rpm, MP, leaning technique, etc, we can plot all the TASs vs OAT. If we get enough data we should be able to see the trend of TAS vs OAT. Once we know how much the TAS changes per degree OAT change, we have a way to correct all those TAS values to what would be achieved on a standard day. That correction should be relative unaffected by typical mods, so once we have found it, we can use it in the future.

I need to find some historical upper air data showing dew point and temperature. I have an old NACA report that shows the effect of humidity on engine power. I hope to figure out roughly how much the power would be affected by the normally seen humidity values at altitude.

 

[airguy]

You guys are giving me ulcers. Ya'll need to go log about a dozen hours in a flight school rental 172, and then go back to your own plane. You'll never complain about that "elusive knot" again....

 

[Nuisance]

Lots of great theories here. For solo work, I forget about TAS/ GPS. Find some smooth air, same altitude and close to same OAT. Nail the VSI and altimeter for 3-4 minutes and read the ASI. Can get +/- 2 knot accuracy if you are good.

It must be nice to have air that smooth, without up and down drafts.

 

[rv6ejguy]

I'm based right beside the Rockies and it often is not smooth except in the winter months- so I test fly a lot during these times. Occasionally, I can find a smooth morning or evening. Test flying for speed measurements is essentially a waste of fuel in gusty, windy or turbulent conditions anyway. Just gotta be patient for a good day or like I said before, get your RV buddy and fly formation.

 

[rv969wf]

Oklahoma winds

I'm based right beside the Rockies and it often is not smooth except in the winter months- so I test fly a lot during these times. Occasionally, I can find a smooth morning or evening. Test flying for speed measurements is essentially a waste of fuel in gusty, windy or turbulent conditions anyway. Just gotta be patient for a good day or like I said before, get your RV buddy and fly formation.

I understand your thoughts. Trying to do any flight testing where I live in the panhandle of Oklahoma is not good. I think we have maybe 10 days out of the year that are nice. Well OK maybe 20 days. The summers here are hot, windy, rough etc. A normal day to fly where I'm at is 20-25mph gusting to 35mph +. Ugh.... We do have some nice days but not very many. I still have not been able to get apples and apples testing with any of the mods that I've done. Some days its so rough even at alltitude I can't even get my RV out of the yellow zone on the airspeed indicator to do any airspeed tests. I'm a little jealous of the RV fliers that live in nice areas without all the bumps and rough air. Ohwell, I'll quit complaining as I chose to live where I'm at for now.

 

[captainron]

I understand your thoughts. Trying to do any flight testing where I live in the panhandle of Oklahoma is not good. I think we have maybe 10 days out of the year that are nice. Well OK maybe 20 days. The summers here are hot, windy, rough etc. A normal day to fly where I'm at is 20-25mph gusting to 35mph +. Ugh.... We do have some nice days but not very many. I still have not been able to get apples and apples testing with any of the mods that I've done. Some days its so rough even at alltitude I can't even get my RV out of the yellow zone on the airspeed indicator to do any airspeed tests. I'm a little jealous of the RV fliers that live in nice areas without all the bumps and rough air. Ohwell, I'll quit complaining as I chose to live where I'm at for now.

All that, and no women to be found! Alan, maybe you should consider moving!

 

[Bob Axsom]

Well Mr. Horton & Mr. Huft I have seen the light

I had gotten back on the stay off the website wagon but I had to log back on and thank you two guys for not giving up on me. I knew very well that the U. S. Air Race, Inc. Handicap Procedure was underestimating the TAS in the presence of wind even if it is steady. I had determined that flying directly into the wind and reverse was the only way to directly eliminate a steady state wind from observation averages in determining TAS. I played around with trig and resolved better a estimate for one of my past tests but it was a lot of work and I had not gone through the work necessary to establish a procedure. Well, your persistent nudging over the past year or so, bothered me a lot. I knew you were telling me the right thing but I just wanted to do it my way even when I knew my way was underestimating the actual TAS. A few minutes ago I finally went to the reference spread sheet and plugged in the numbers from a particularly disappointing test flown two days ago. I had pulled out the lower forward baffle and the extensions that seal to the bottom of the cowl outlet and decreased the spark plug gap to the minumum 0.016". I expected to see a speed of around 172kts - I got 168.6kts. The excel spread sheet with the imbedded functions immediately told me the wind was 190.4 degrees at 36.6kts and the TAS was 170.6kts. Since all of my historic data are derived from 3-way testing I will use that page for entering the past test data and get more realistic TAS numbers for my past experiments. It may give people a head ache to read this stuff but try to experimentally increase the performance of your airplane and measure the results with a procedure that provides inconsistent and inaccurate results if you want to feel real pain. I'm sure the buddy system works for a lot of folks but when you are working alone as I am, this is a wonderful tool. I have opened the spark plug gap to the maximum 0.021" and want to fly a test again tomorrow before I make anymore mods. Thanks guys, this looks great.

Bob Axsom

 

[gmcjetpilot]

Well not quite

(Do I dare mention that merely adding length to a wing might create additional drag? )....

but prop efficiency is another m-a-j-o-r contributor to speed variability. Due to the immense drag rise at the tips, running a prop at 2700 RPM can create MORE drag than running at 2500 RPM. This large multiplier means it is critical to keep RPM exactly on the money for every test run. You then must correct for density altitude (temp and humidity) variability. Prop management is easily worth a knot or two all by itself over the length of a race.....

Just discussing it, I can already feel the obsession creeping back....that's why most of us don't have hair (or wives).

Keep trying! But make a consistent test program and stick with it. Consistency counts for more than exotic techniques.

disagree and agree. There is a difference between efficiency and max speed. In general you will go faster at 2,700 rpm than 2,500 rpm.

Yes prop tip speed is important (vector of fwd speed and angular speed). You are RIGHT your prop is NOT as efficient with higher tip speeds (sometimes at high cruise speed, climb is different), meaning HP into the Prop produces less thrust per HP. You will hear about 80% efficiency for example. So from you theory or comment smaller dia props are more efficient? Well yes kind of sort of, depending on fwd speed and HP loading.

The formulas and math for props are pretty complicated, but the prop guys have it all tabulated (from theoretical and flight test). I have some of that data for the Hartzell using the older prop with the F7666 blades. I can actually calculate efficiency with RPM, prop dia, air density (altitude), applied HP (disk load) and slip coefficient, which involves airspeed and airframe drag. So PROP calculations can not be made in a vacumn, the plane is part of the system. Like I said the math involved has all kinds of interrelated factors. Even with the data and a spread sheet it's a hassle. (Note: The Hartzell BA prop goes 3.5 mph faster than the old version that looks the same and made is made the same way. Why so fast? The BA prop was optimized for the RV and the speed/drag/pwr and altitudes we fly at.)

To be fair, flight is the best way to determine what is fastest. Efficiency is harder to tell in flight because you might be talking about a fraction of one percent by changing altitude or airspeed or RPM. Bottom line if you want max efficiency you run the RPM at very low RPM's. This is of course the advantage of constant speed props, RPM control separate from power (Manifold pressure). However low RPM is not top speed.

In general more RPM means more speed. Its no accident Reno racers are turning 3,000 or more RPM. (Note: at 3,000 rpm Lycoming requires an engine tear down per a service bulletin, just saying, not recommending over speeding engine.)

Back to normal RPM. You are no where near supersonic at 2,500 rpm or 2,700. However with 200 rpm more you make at least 8 to 12 HP more. This extra HP is way more than the efficiency you lose, therefore more speed. You are less efficient however. Now I am talking about THIN metal props that can run faster. That is why the BA Hartzell is 8 MPH faster than the MT.

To illustrate look at the info below. You see the metal props (thin) are faster than wood props (thick) and the one fix pitch prop, which was ran at a much higher RPM (at 2,730 v. 2,500 or 230 rpm more). It went faster.



Bottom line MAX rpm with in reason makes you go faster simply because the engine makes more power. As I said the prop/engine/speed/hp/rpm/altitude are all related. As you suggest PLAY with RPM? However we don't operate anywhere near critical mach at the prop tip. You get into turbo props and faster planes you are in a different category. Those planes often are geared so the prop turns at 1,900 to make up for the higher fwd speed and larger dia props, which are required to "absorb" the greater HP.

Efficiency wise we (RV'ers) are talking about fractions of a percent changes for RPM changes at high speed cruise, for our HP, speed, altitude and prop diameter. We RV'ers run in the mach 0.80 range and efficiency drop off gradually from 0.88 to 0.92 range. You don't want to go over about 0.90-0.92. The most we might do with a super fast RV with 74" prop @ 2,700 rpm is about 0.85 mach. The RV-10 with a 80" prop might be 0.88 mach.

My point is we don't operate anywhere near the critical mach. Now in climb higher RPM may be more efficient because the fwd speed is lower. Tip speeds that are too slow are also less efficient. There is a peak efficiency and that is based on or set by the blade design.

 

[Kevin Horton]

A few minutes ago I finally went to the reference spread sheet and plugged in the numbers from a particularly disappointing test flown two days ago. I had pulled out the lower forward baffle and the extensions that seal to the bottom of the cowl outlet and decreased the spark plug gap to the minumum 0.016". I expected to see a speed of around 172kts - I got 168.6kts. The excel spread sheet with the imbedded functions immediately told me the wind was 190.4 degrees at 36.6kts and the TAS was 170.6kts. Since all of my historic data are derived from 3-way testing I will use that page for entering the past test data and get more realistic TAS numbers for my past experiments.

Bob - I'm happy to read that you will be using a more reliable test method. This will make it easier to identify worthwhile mods.

When you say "reference spreadsheet", do you mean the NTPS spreadsheet? If so, be aware that is based on GPS track data, not heading data. I thought the handicap procedure you were using was based on flying headings 120 degrees apart.If you don't have GPS track data, then you can't use the NTPS spreadsheet to get TAS. If you also recorded GPS track for each run, then you could recalculate your TAS using the NTPS spreadsheet.

 

[Bob Axsom]

I have the track data.

I am using the NTPS spread sheet and it will be very little work to pull out my records and enter them. I have not personnally validated the results but that should not take much work.

Bob Axsom

 

[rv6ejguy]

This prop thing is all spoken about in general terms. It has been shown with little doubt that MT props are considerably more efficient at lower rpms on several different airframes. Depending on airfoil section, some people in the business state that prop efficiency begins to fall off well before tip speeds reach mach .7.

At Reno in the classes where geared engines are used, almost universally the lowest available gear ratio is used to slow the large props way down to get best prop efficiency.

In the Sport Class, DGs Lancair will reportedly feature a planetary reduction gear to allow the engine to spin up to 4000 rpm for better power and reduce prop rpm down to 2500 for best efficiency.

With direct drive engines like most people use in RVs, you can't do anything about gear ratios easily and you are stuck with spinning the prop fast to make max power. Fortunately Hartzell has done a good job with their designs to make them as efficient as possible at high rpms. They have been highly successful at Reno.

BTW, who is going to Reno this year? This is a great show if you have never been.