Speed improvement from fairings/wheelpants

At data performed at 11,500' DA WOT. RV-6A, O-360, 9.5 pistons, carb.
Four course speed runs using IAS. As long as all the IAS were close I considered it a valid test.

1) No fairings or wheelpants on. Gear leg stiffener on back of nose gear.
Baseline for speed changes (IAS speeds: 156, 156, 155, 155 mph)

2) Added main gear leg fairings, intersection fairings and wheelpants. Wheelpants primed and basically smooth but not painted. All other parts painted. (IAS speeds: 173, 171, 172, 172 mph)

Speed increase over baseline 16 mph. RPM increased by about 80 as well.

3) Added painted nose gear leg fairing, intersection fairing (to bottom of cowl) and unfinished (not even primer) nose wheel pant. (IAS Speeds: 175, 174, 175, 173 mph)

Speed increase about 3 mph.

Total about 19 mph. It may have been higher if the baseline did not have the nose gear leg stiffener which may have acted a bit like a fairing.

Typical for an RV...

I got 15 MPH increase on my RV-9A. The first flight was the only one I flew with the gear legs bare and no wheel fairings. I was flying at 2,300 RPM and 23" manifold pressure both times.

Good data

Ron Lee said:

At data performed at 11,500' DA WOT. RV-6A, O-360, 9.5 pistons, carb.
Four course speed runs using IAS.

1) No fairings or wheelpants on. Gear leg stiffener on back of nose gear.
Baseline for speed changes

2) Added main gear leg fairings, intersection fairings and wheelpants. Wheelpants primed and basically smooth but not painted. All other parts painted.

Speed increase over baseline 16 mph

3) Added painted nose gear leg fairing, intersection fairing (to bottom of cowl) and unfinished (not even primer) nose wheel pant.

Speed increase about 3 mph.

Total about 19 mph. It may have been higher if the baseline did not have the nose gear leg stiffener which may have acted a bit like a fairing.

Click to expand...

Nice Nice Nice data, thanks for taking the time to measure this before after.

19 kts

RV-7A, Superior IO-360 180 hp, James cowl, plenum & wheel fairings. At least 19 kts.

16 MPH

We made 186 MPH with the nosegear leg fairing and upper intersection fairing on at 7500 DA. It shot up to 202 MPH or better with everything on, plus around 200 FPM more climb...we were very pleasantly surprised. A whole different airplane and even harder to slow down. Craigs prop also gained a bunch of RPM, redlining at 2700, just as he said it would at WOT.

Regards,

Hi Pierre,
I agree - there was a huge increase when we added all the goodies!
Slightly off topic but:
Can you or someone else tell me why the RV6 has got a nose leg stiffener?
I assume it is the same as on the 7A main gear?

The reason for asking is that I am considering adding a stiffener to NOSE gear of the 7A we are currently building.
Thanks.

Janekom said:

Hi Pierre,
I agree - there was a huge increase when we added all the goodies!
Slightly off topic but:
Can you or someone else tell me why the RV6 has got a nose leg stiffener?
I assume it is the same as on the 7A main gear?

The reason for asking is that I am considering adding a stiffener to NOSE gear of the 7A we are currently building.
Thanks.

Click to expand...

The stiffener is an option now. In the "old days", RV6A kits came with two piece aluminum fairings for the mains, and you had to make a smooth fiberglass wrap around wood stiffeners for the nose gear. No pre-made fiberglass fairings existed.

As to preferences in regards to being better or worse (except for ease of building), I don't have the answer.

L.Adamson

Hi, Ron! 'Sorry to rain on your parade, but the rpm increase is not consistent with the speed increase. Typically you should experience 100 rpm increase for each 6 mph to 10 mph speed increase, the latter with a higher pitch prop. Your TAS from IAS would be 184.7, 205.0, and 207.4; that would be a 20.3 mph TAS increase from the first to the second! That should have given you a 280 rpm increase for a 76" EFFECTIVE pitch prop. Consider this: if your prop has an EFFECTIVE pitch of 76", that means you will travel 76" forward for each revolution. 100 rpm would give 100X60X76/(12X5280)=7.2 mph. It is possible that the fairings changed the air flow at the static ports to give an IAS error. This shows the fallacy of using IAS even for relative testing! The only true check of performance increase is to measure two-way GPS groundspeed.

Other builders have reported around a 20 mph TAS increase when adding wheel pants and gear leg fairings, so the speed increase reported here appears credible.

Perhaps the tach errors are different at the two different rpms. If so, perhaps the real rpm increase was more than 80 rpm.

OK, so my RPM increase may be wrong.....

It was what I recorded. If my speed improvement is consistent with what others got then using IAS was "close enough." I have done GPS runs and it is too time consuming for my needs.

My philosophy is to try to get the best data possible knowing that there may be errors. To the extent that others inputs and suggestions can improve what I do...thanks.

GPS runs are the easiest to do if you know how. First off, the effect of wind is completely and totally exaggerated. At 200 mph TAS, a 20 mph wind at 90 deg to your flight path will only cause an error of 1.0 mph; anything less than 90 deg. will give even less error. So if you get the forecast winds aloft, and fly a GPS ground track into the wind and with the wind and average the results, your error will be lost in your ability to maintain your course and altitude! That three-way stuff works, but it won't give you better data than this if the forcast winds aloft are less than 30 mph and you fly with and into the forecast direction, corrected from true to magnetic. My testing with several RVs shows significant errors in IAS due to the placement-of and the type-of the static port(s). But Hey! If you like the results you got, who am I to tell you any different! If you are actually getting 207.4 mph TAS at 11,500' dalt, you need to share the magic you have done on your 6A with fellows like Bob Axsom who have sweated out all kinds of mods to get his TAS to 204 mph at 6000' dalt!

elippse said:

GPS runs are the easiest to do if you know how. First off, the effect of wind is completely and totally exaggerated. At 200 mph TAS, a 20 mph wind at 90 deg to your flight path will only cause an error of 1.0 mph; anything less than 90 deg. will give even less error. So if you get the forecast winds aloft, and fly a GPS ground track into the wind and with the wind and average the results, your error will be lost in your ability to maintain your course and altitude! That three-way stuff works, but it won't give you better data than this if the forcast winds aloft are less than 30 mph and you fly with and into the forecast direction, corrected from true to magnetic.

Click to expand...

This works as advertised, if the upper winds are actually as forecast. Sometimes they are, and sometimes they aren't. The biggest advantage of some other test methods is that their accuracy does not depend on the accuracy of the upper wind forecast.

But, if you do any speed test on several different flights, then you should be able to see whether you have a consistent result. I'd avoid making any decisions based on the results of performance tests done on one flight. The results from any given flight might be off for any number of reasons.

elippse said:

My testing with several RVs shows significant errors in IAS due to the placement-of and the type-of the static port(s).

Click to expand...

Did all these aircraft have Van's recommended pop rivet static ports, or did some of them have something else? Was there a pattern to the results, and if so what was it (i.e. did they all have airspeed systems that read high)? Had the ASIs in these aircraft been checked for ASI instrument error? Had the accuracy of the OAT probes installation been checked (many builders put the OAT probe in the NACA scoop on the side of the fuselage, and these installations often read too high).

Vertical "wind"

I have not heard it mentioned here but I find the vertical movement of the airmass that testing is done in to be important. Fronts and big lows are obvious but mountain waves less so. Some days it is so pronounced I cannot keep it out of the red at full throttle. Even a change in pitch attitude for level flight is noticeable on the most extreme days. Some say you need to be at ridgetop level but I have rode a 500 fpm wave over 60 miles at 7500 feet along the east slope of the rockies. It is usually pretty apparent at that point (a rodeo actually) but other times very smooth and subtle.

This phenomenon may be a source of some inconsistency in day to day testing.

Over the years I have made many changes in my aircraft in an attempt to gain speed. To test these modifications I have used various test proceedures. The three leg gps system works quite well, and is repeatable, but engine power can be significantly different depending on density altitude and outside air temperature. This means that todays testing at 24 squared and 10C can be quite bit different than tomorrows run at 0C. I had done weeks of testing various changes when I realized that my fixed power setting was actually giving me different power for any runs that had variations in temperatures. A couple of test runs just do not tell you the whole story and you really need to approach this type of testing in a professional manner. Personally I do not have the time, nor money to spend on fuel, that is required to get real results.
Hands down the best test method is to fly beside another aircraft at powersettings that will let you fly side by side. After your modification fly beside the chase plane at the same power settings and see what happens. It is important that both aircraft are loaded similiar to last test. A passenger, for example, in a tandem aircraft can change the tail loading and effect top speed.

Kevin: Even if the winds are somewhat off from forecast you will still have outstanding results if the winds are less than 30 or 40 mph at a TAS of 200 mph. If your plane's speed is even higher, you can tolerate even stronger winds with little error. If your plane has a TAS of 200 mph and you have a 40 mph wind 30 deg off your groundtrack, you will go 233.6 mph GS one way and 164.4 mph GS the other. Average? 199 mph! The 3 RV's I tested used the rivet and had typical IAS errors of 8-12 mph at about 185 mph TAS. They also had about 5-6 deg OAT error which typically had the sensor installed in the cabin duct and got stagnation heating with the attendant dalt error and TAS error. When my prop design was installed the OAT was almost constant up to 10,000' on a-6 and a -9A! I think that's because my prop design has so much more flow into the engine cooling inlets that air goes in, gets heated, emerges from the inlet, then flows down the side of the fuselage and into the cabin! I recommended to Jim Smith ('likely name - Smith!) that he install his OAT sensor in his tail cone and with that his OAT showed forecast temps vs altitude plus a fixed error. Jim cut a 1/16" thick washer in half and mounted it cut-side forward immediately behind and surrounding his port and got his IAS error down to about 1-2 mph.
Duane: I have a Rec. Vehicle level bubble on the side of my canopy so I can see the fuselage angle in flight. I usually fly with my Altitude-Hold on, which holds altitude rock-steady, and after trimming my flap reflex, my nose angle is about +0.5 deg. and my IAS is what it usually is. Several times when flying near hills I will see my rpm and IAS go down and notice on the bubble that my nose is somewhat higher, then later my rpm and IAS will increase and my nose will be lower. I am fortunate in having a nice flat area near Santa Maria Ca where I do my testing and I get very consistent results. I've done a significant amount of testing of my Lancair and the airplanes of others with my prop installed and so I have developed a test protocol which allows me to determine the installed power and parasite drag area of their planes which I then use to design their props. I totally disregard their OAT numbers and use only forecast temps. Since my equations solve for induced loss, it is only necessary to do the testing at their preferred design density altitude. Low aspect-ratio planes such as most of the RVs show a much larger change of speed with altitude than do those with higher ARs. Jim is now making a set of wingtips I designed for his -6 which will increase his wing area from 110 to 115 and increase his AR from 4.8 to 5.8. He's an avid experimenter, somewhat along the lines of Bob Axsom, and we'll see if his higher altitude speed goes up with these tips!

elippse, I never claimed 207.4 mph TAS. I used IAS with whatever errors exist with it as a quick way to determine relative speed improvements.

elippse said:

Kevin: Even if the winds are somewhat off from forecast you will still have outstanding results if the winds are less than 30 or 40 mph at a TAS of 200 mph. If your plane's speed is even higher, you can tolerate even stronger winds with little error. If your plane has a TAS of 200 mph and you have a 40 mph wind 30 deg off your groundtrack, you will go 233.6 mph GS one way and 164.4 mph GS the other. Average? 199 mph!

Click to expand...

I was going to gripe about compass error affecting your approach, but now I note that you use GPS track. I will agree that with most typical winds, your approach would yield reasonably low errors. I still like the four leg GPS method that the NTPS uses, as it has you collect redundant data, which allows the quality of the test data to be checked. But, that is personal preference.

elippse said:

The 3 RV's I tested used the rivet and had typical IAS errors of 8-12 mph at about 185 mph TAS.

Click to expand...

Interesting. I don't think very many RV builders do rigourous airspeed error tests, so perhaps the basic design isn't as good as I believed. I have seen some builders claim their testing showed the recommended system was accurate, but I don't know the details of their test protocol, so I don't know how credible their results were. I'm looking forward to doing several flights worth of static system error checks on my aircraft, once I get flying.

elippse said:

They also had about 5-6 deg OAT error which typically had the sensor installed in the cabin duct and got stagnation heating with the attendant dalt error and TAS error. When my prop design was installed the OAT was almost constant up to 10,000' on a-6 and a -9A! I think that's because my prop design has so much more flow into the engine cooling inlets that air goes in, gets heated, emerges from the inlet, then flows down the side of the fuselage and into the cabin! I recommended to Jim Smith ('likely name - Smith!) that he install his OAT sensor in his tail cone and with that his OAT showed forecast temps vs altitude plus a fixed error.

Click to expand...

What do you use as "truth data" when checking the accuracy of OAT installations? If you've got a good approach here, I'd love to learn about it. OAT accuracy is one area where I have not yet identified a satisfactory reference to test against. I don't have access to a quality, calibrated test OAT installation, nor do I trust the predicted temperatures at altitude.

Level

Paul

I like the idea of a level. I fly east to west over the front range soon after takeoff fairly often. Upon entering the mountains, I am sensitive to any sort of indication that could be construed as a mechanical issue. The level would tell me I am only climbing in a sinking airmass!

I am interested to hear how the wingtips turn out!

I was VERY pleased with the difference I experienced with my partially faired RV-7A. My records are back at the hangar, but I seem to recall that it was about 11 knots TAS increase from bare naked to what I have now which is the upper intersection fairings and the leg fairings on all three. I will add the wheel pants and lower fairings when I have time, but it is too much fun flying to take her out of service for the rest. I also should add that my upper intersection fairings are still dirty, as I did the initial install but still haven't got around to breaking out the filler and such to fine tune the upper intersection fairings (they aren't comletely flush with the bottom of the fuselage). What is truely amazing is flying with the wheel pants fairing brackets on with no wheel pants. Those bad boys look like two giant speed brakes hanging out there. As for testing, I do a 4-way gps run and record the gps ground speed as well as the TAS off the Dynon, with the auto pilot flying the plane. I am hoping for another 10 knots or so of TAS increase with the lower half of the fairings installed. That would give me a little more than 20 knots TAS total increase, which is phenominal in my opinion!

Kevin Horton said:

What do you use as "truth data" when checking the accuracy of OAT installations? If you've got a good approach here, I'd love to learn about it. OAT accuracy is one area where I have not yet identified a satisfactory reference to test against. I don't have access to a quality, calibrated test OAT installation, nor do I trust the predicted temperatures at altitude.

Click to expand...

Well, first of all, you can do a ground check of your OAT sensor, comparing it with something else, using ice and hot water, or whatever. In flight, to see stagnation temperature rise for your installation, do this: get to the altitude where you are going to test, and fly the plane as slowly as you can and note the temperature; make sure you allow enough time for the temp. to stabilize. Once you have done that, go to WOT until your speed stabilizes, and note the temp. If it has gone up, your installation is getting stagnation rise. At 200 mph TAS it's about 7.2F maximum, (TAS, mph)^2/5520. I have found that the forecast temps are usually very close to what I see on my two sensors.
As far as static port placement errors, do this: note your runway altitude before takeoff, go around the pattern and do a high speed pass down the runway, and note your xxx have a passenger note your altitude; you need to be flying the plane! At 8000' dalt, 200 mph TAS, 177 mph IAS, the dynamic pressure is 80.4 psf, 1.14" Hg, not correcting for compressibility. A -100 foot error would give about -0.1" Hg static error, giving a 1.24" Hg pressure differential into the airspeed indicator. It would then show 185 mph IAS, an 8 mph IAS error. That would translate into a 9 mph TAS error. I did some testing on a Reno racer that had the static vented inside the cockpit. His IAS showed more speed that his TAS! 'Course, you may not want to correct your error if you are trying to sell the plane to someone and want to show the individual hw fast your plane is. In fact, put a few layers of Magic tape just ahead of the static port and get your IAS way up!

Calculating True Airspeed

The true Airspeed calculators I have seen tell you to fly a heading for the legs. For example, www.reacomp.com/true_airspeed/ says to fly the 4 cardinal headings. Is it valid to fly the 4 cardinal ground tracks instead as would happen if you set the GPS linked autopilot to fly N then E then S then W??

Fin
9A

Finley Atherton said:

The true Airspeed calculators I have seen tell you to fly a heading for the legs. For example, www.reacomp.com/true_airspeed/ says to fly the 4 cardinal headings. Is it valid to fly the 4 cardinal ground tracks instead as would happen if you set the GPS linked autopilot to fly N then E then S then W??

Click to expand...

You need to use the test method that is compatible with the data analysis method you plan to use. The formula in the www.reacomp.com/true_airspeed applet only gives the right answer if you actually flew legs with headings 90 degrees apart. If you use tracks 90 degrees apart, the applet won't give the right answer.

But, I agree that methods that rely on GPS track are better than ones that rely on compass headings, as many aircraft probably have heading errors. I really like the NTPS four leg method, as it uses GPS track, and it has you gather redundant data so you can assess the quality of the test point. More info here.