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Wheel fairings...+16MPH?

pierre smith

pierre smith

Well Known Member
Mornin' guys/gals,
Our -6A (Sojourner) now has 36.5 hours of the required 40 flown off and for the last few weeks have not had the main fairings/pants on.Last week I did a square (North, west,south, east) pattern at 7500' and WOT or 75% and managed 180MPH with our 0-360 and 3 bladed Catto prop, which Craig pitched for max cruise....turning around 2650 RPM.
Then we installed the main gear fairings and wheel pants coupled with Bob Snedaker's intersection fairings and WOW!!! At 7500', the RPMs climbed to 2725 and the 4 way average went to 196.5MPH on the GPS...way more than I thought we'd get. The temperature was about the same as the earlier test and I'm really happy so far, with a few insects on the leading edges and no paint. I repeated the square pattern at 7500 feet and 2500RPM and still averaged 182MPH......nice. The airplane has the later style 'sheared' wingtips and weighs 1046. Are these numbers pretty much what the rest of you have found?
Regards,
Pierre Smith

ps The climb rate also increased by approximately 250FPM! Pleased, yes.
 
pierre smith said:
... and the 4 way average went to 196.5MPH on the GPS...way more than I thought we'd get.
Click to expand...

Don't shoot the messenger, but a 4 way average of ground speeds is not equal to the TAS. You can see the errors if you sit down and look at the wind triangles.

For example, imagine an aircraft with a TAS of 150 kt. The wind is 270 degrees at 30 kt. This is not an unusual wind speed at altitude, where you often need to fly these tests in order to get good air conditions. The ground speed (GS) will be 120 kt on the W run, and 180 kt on the E run. These average to 150 kt - so far so good. But, on the N run, the wind is exactly off the wing tip, and the wind triangle is such that the GS is equal to 153 kt. The GS heading south is also 153 kt. The average of the four GS is 151.5 kt, which is a 1% error. The error is approximately the same no matter what the wind direction. If the wind speed is higher, the error is higher. A 60 kt wind (not unknown - I had 70 kt at 5,000 ft one day last month) with a 150 kt TAS gives an average GS of 156 kt, or a 4% error.

There is info on how to accurately calculate TAS from GPS data in this thread.
 
Thats ALL?

I flew the same square and I was indicating 475mph in my RV8 with a WW302(twice as fast as a WW 151) prop and Cummings turbo deisel. Time to change engines Pierrre!

Don't shoot Kevin, we still need to let George chime in before we decide who to shoot!

All BS aside, Pierre and Larry have a beautiful airplane. Can't wait until it gets paint.

BTW Congrats to Canada for voting in the conservative crooks!

RK
 
Kevin is correct in his statements about 4-way speed tests. However, the numbers you are seeing (16 MPH) increase seem about right. I had a similar increase with the fairings in my O360/ RV6 (and I used Kevin's spreadsheets to calculate!)
 
Yes + 16 mph is very reasonable

To actually answer your question rather than question your answer YES.. I did the same thing with a RV-6 180 HP that had a climb prop. With no fairings or pants it would barely go 186 mph WOT at 2750 RPM and 200 density altitude after installing the old style fairings and pants it would then indicate right at 200 mph. 2 years later I went with the fibergalss fairings, very tight fitting pressure recovery pants and swoopy intersection fairings top and bottom then I could indicate 204 mph!!!
I was so thrilled with that I started minimizing drag everywhere, fuel vents, tightened the baffleing, fine tuned the incidence of the HS, Lightspped ignition etc, ultimately I got to where I could indicate 210 mph at 2850 rpm! Boy was it ever sweet at that rpm!. Of course I may get blasted for exceeding the red line by folks who know better than I but what the heck, its experimental and I had great fun with it.
 
Airspeed

Taking a quote from Bob Kromer, former enineering test pilot at Mooney Aircraft, "How about real world cruise performance? We configured the airplane for the most efficient cruise power setting and looked at GPS derived groundspeeds with that power setting flying four cardinal headings (N, S, E, W). That's the way you find out what your airplane will really do in level cruise. Forget the information off the airspeed indicator--these numbers are subject to all kinds of errors. To find out how fast your airplane really is, for a specific altitude set the power setting you want to test, hold altitude and heading very precisely and note GPS stabilized groundspeeds while flying N, S, E and W on the DG. Don't correct for any wind, just fly stabilized headings and altitude. Write the stabilized groundspeed from the GPS for each stabilized heading. Add those four groundspeeds up and find the average. The result is your aircraft's true airspeed for the altitude and power setting you are flying. And it's very accurate."
 
Surface Warrior said:
Taking a quote from Bob Kromer, former enineering test pilot at Mooney Aircraft, "To find out how fast your airplane really is, for a specific altitude set the power setting you want to test, hold altitude and heading very precisely and note GPS stabilized groundspeeds while flying N, S, E and W on the DG. Don't correct for any wind, just fly stabilized headings and altitude. Write the stabilized groundspeed from the GPS for each stabilized heading. Add those four groundspeeds up and find the average. The result is your aircraft's true airspeed for the altitude and power setting you are flying. And it's very accurate."
Click to expand...

Ask Bob Kromer to do the math. The method that he supposedly proposed is not accurate, as demonstrated in my original response. It is only accurate if the wind is zero. Arguing otherwise is like quoting some expert how says the world is flat. Wrong is wrong, no matter who says it is right.

Edit: fixed a spelling error.
 
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153k ??

How do you get 153k when the wind is 90 deg to the aircraft? Not tring to be smart but I have a hard time understanding where the extra 3 knots comes from?
 
Your numbers are comparable with mine

Pierre I never ever even considered flying my plane without every fairing in place so I can't give you an unfinished airplane speed but your numbers after putting on the fairings are comparable with mine. I made my own fairings going the modeling clay and lay up the first layer in place route. I have the Horner tips. In miles per hour my TAS every day converts to 196.8 to 202.5 mph. The low number is at 2450 rpm with a non-blended Hartzell C/S being turned by a O-360-A1A with LASAR ignition, the "rich" Airmotive Carb mod. installed, leaned to around 75 deg. rich of peak. As I learned from a couple of wise participants in this forum (and yes I know you know all of this stuff - I've read your experience posts and they are very impressive) to get maximum speed leaning is extremely critical so full throttle, max rpm (system limited to 2620-2630 rpm for now), and leaned for max speed (about 120 degrees rich of peak but this literally has to be vernier dialed in for peak speed on my plane) is where I get the high number. Today I was flying back from Little Rock in the nice clear cold air at 8,500 feet I was seeing an unusually high 209.4 mph TAS but I had to work like a lab rat to hold that peak.

Your speed looks right on the money to me.

Bob Axsom
 
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airspeed and altitude calibration

Try the national test pilot school at www.ntps.edu. Look for the information tab, downloads tab and the several files for GPS PEC. The spread sheet has a 3 leg, 4 leg, and atmospheric calculation tabs. Put in the data for 4 legs and it will give you TAS, wind, std dev, etc. It uses ground track, GPS speed for 3 or 4 legs and calculates the rest. Then take that data and put it in the last spread sheet and it will calculate indicated airspeed error. Very nice program. You just have to pick a heading, let the ias, gps speed, and heading settle down for 10-15 seconds and take data. If you std dev is too great, do another 4 leg run.
 
4 leg average

For the first post, taking the average does have some error. However, taking the difference of the averages for fairings and without will probably give you a pretty good number. 16 mph is probably close.
 
[email protected] said:
How do you get 153k when the wind is 90 deg to the aircraft? Not tring to be smart but I have a hard time understanding where the extra 3 knots comes from?
Click to expand...

It's all in the wind triangle. Let's imagine our aircraft has a heading of 360, with a TAS of 150 kt. The wind is 270 at 30 kt. If we draw out the wind triangle, we have a line straight up with a length of 150 kt. This line starts at a point we will call A, and stops at a point we will call B. The wind line is drawn from the top of the first line, going from point B straight to the right, ending at point C. The ground speed (and track) is represented by the line that connects points A and C. This line is longer than the one between points A and B. Because the heading and wind are at 90 degrees to each other, we can use some of our long lost geometry skills to know that the ground speed is equal to the square root of the sum of the squares of the other two values. The square of 150 is 22,500, and the square of 30 is 900, giving a sum of 23,400. The ground speed is equal to the square root of 23,400 = 152.97, or 153 if we round it off.

Note - this bit about using the square root of the sum of the squares is only valid when the wind is exactly off the wing tip.

This would be a lot easier if we were talking in a bar and you could see my hands moving :)

More on wind triangles here.

More on right triangles and how the length of the hypotenuse is equal to the square root of the sum of the squares of the other two sides here.
 
All good stuff

Yes wheel pants are important apparently. The value sounds high but think it is possible. The exact value is hard to measure but +16 mph or what ever is not unrealistic in the 180-200 mph range. Thanks for the info Kevin.


Kevin is the wind triangle man, and agree but will still say flight test and speed measurement is full of other errors, small, but error never the less. Not only does the GPS have errors (depending on satellite positions relative to your position at that time) but other errors. The biggest is the non uniformity of air or atmosphere, vertical wind and horizontal wind variations. The only guard is to fly in air that is as stable as we can get, such as in early morning for example. The next error maker is pilot technique and recording errors. Auto pilots and altitude holds are a real plus here. Also repeating the flight test over several sessions at differnt times can help. With recording EFIS to take data for later download also helps minimize recording errors.

In the spirit of the conversation the 3 wind triangle (constant track) method probable gets the error as small for all conditions. The four leg could be dead on depending on winds or could be off a few percent? We should strive to achieve the min error where we can.

FLying at 8000 feet presents some problems, primarily the stability of the air mass at that altitude. I find flying low, 500 feet or less if there are no persons or objects on the ground, over known land marks or road, two 180 degree run over that same course can produce the best results. Although a little sporty (with risks), knowing actual temp, pressure and winds by a ground observer over a know distance to be a very accurate cross check to GPS and airspeed indicator. Finding a place to do this under conditions when the winds are almost calm and thermals nil (early morning) is important and hard to find sometime, depending where you live. Since you are low to the ground and you can absolutely measure the winds you have a way to measure true speed. Vertical wind components should be small or Nil over flat land in early morning, ie no thermal activity. Add the fact you can do timed runs over fixed distance land marks, you have another way, along with the airspeed indicator and of course GPS to calculate speed or winds. Of course timing errors if not done properly of only a few seconds can be a significant error on short run lengths. The longer the ground run the better. I suppose you could do passes over a runway with a guy recording your speed with a radar gun. (You know the, Your speed is, radar signs police leave at the side of the road, you could buzz one of those . :rolleyes: )

Low level top speed or sea level speed is relative to speeds at all altitudes, especially for a non turbo plane. Since speed at altitude is relative, near sea level 100% power speed runs are a good measure of absolute performance. Flying at 8,000 has some disadvantages, mostly it is easier to do in any part of the country and admittedly less risky. The down side is you really don't know what the winds are doing up there. If winds are gusty, inconsistent over time or distance, with vertical components at 8000ft, it can add error. Doing the flight test over different days or times can help average that out. Also temp and pressure corrections may cause some small error in knowing density altitude.

The down side of low flying is finding a safe place to do this and bird strikes to name a few items. Of course ground thermals need to be minimized by flying early in the morning over flat land in calm stable air mass condtions. Flying low lets you know what the winds are before you start. Key is flying low which I do not recommend or condone, so do it at your own risk, as my Lawyers Dewy-Cheat'em and Howe told me to say. Of course adherence to FAR's are absolute.


In the end what are you trying to do. I personaly find the best speed number I can use is block speeds. Flying many trips over known distances to get average block speeds corrected for winds from take off to touch down. That is the most useful to me.


George
 
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John C said:
Try the national test pilot school at www.ntps.edu. Look for the information tab, downloads tab and the several files for GPS PEC. The spread sheet has a 3 leg, 4 leg, and atmospheric calculation tabs. Put in the data for 4 legs and it will give you TAS, wind, std dev, etc.
Click to expand...

The NTPS spreadsheet uses Doug Gray's method to go from GPS data to TAS, and is a very good way to go. I really like the way they use the data from four sides of a box to allow a bad data point to be discovered.

gmcjetpilot said:
FLying at 8000 feet presents some problems, primarily the stability of the air mass at that altitude. I find flying low, 500 feet or less if there are no persons or objects on the ground, over known land marks or road, two 180 degree run over that same course can produce the best results.
Click to expand...

George makes some good points. There are some advantages to low altitude testing, if you can find times with smooth air. Low altitude speed course tests is a very well accepted flight test method for aircraft in our speed range. I'm sure George has this figured out, but anyone else doing this should spend some time thinking about the effects of wind. If you simply fly between two points, crabbing into wind, the average ground speed will be less than the TAS. You need to very carefully design a test and data analysis approach that will properly account for the wind.

If you are in a part of North America that is covered with grid roads, you can use those to your benefit. Fly on a heading that is exactly 90 degrees to the grid roads - don't attempt to crab into wind. Have an observer in the other seat run the stop watch and keep an eye open for traffic, birds, etc. The pilot does his best to hold an exact altitude and speed. Record the time to go between grid roads (these grid road patterns go for miles and miles, so you can use timing points several miles apart if you want). The wind component that is at 90 degrees to your heading has no effect on the time recorded between the grid roads. The wind component parallel to your heading will have an effect, but if you calculate a ground speed for each run, and average the two groundspeeds, that will equal the TAS. Don't simply average the times for the two runs.
 
Got it

I get it, Fly A to B but end up at C which is a greater distance then A to B there for you travel faster over the ground to get to C in the same time it should have taken to get to B. When your done you are now in the wrong place.

I like the last part.

"This would be a lot easier if we were talking in a bar and you could see my hands moving "

Love to join you some day.
 
Speed

When I'm flying an airplane, I'm not concerned with TAS or IAS. My concern is how fast I'm traveling across the "ground" for flight planning purposes. If my averaged four cardinal heading (N, E, W, S) GPS ground speed is 160 knots for the airplane I'm flying, then I flight plan 160 knots, which is the average cruise ground speed of the airplane in a no wind environment. When using the internet flight planning tools, based upon the time of the flight, the program will factor in wind speed and direction for the flight, and I'll have a pretty good indication of ground speed and ETE for a particular flight.
 
Surface Warrior said:
When I'm flying an airplane, I'm not concerned with TAS or IAS. My concern is how fast I'm traveling across the "ground" for flight planning purposes. If my averaged four cardinal heading (N, E, W, S) GPS ground speed is 160 knots for the airplane I'm flying, then I flight plan 160 knots, which is the average cruise ground speed of the airplane in a no wind environment. When using the internet flight planning tools, based upon the time of the flight, the program will factor in wind speed and direction for the flight, and I'll have a pretty good indication of ground speed and ETE for a particular flight.
Click to expand...

Averaging those four GPS ground speeds is accurate enough for your purposes then, as it doesn't much matter whether the result is off by a few knots. But if you have made a modification to your aircraft (as the original poster had), and want to know whether the speed has increased, and if so, by how much, then you need to use an accurate flight test method. Otherwise it is garbage in, garbage out.
 
No need to follow the grid

There is no need to follow a grid, worry about crab angles, or your compass calibration card. Life is simple with a GPS. With the NTPS GPS PEC program, all you have to do is fly 4 legs. Just pick a heading or track generally north, generally east, ... ect. The heading or track does not need to be precise at all, just steady. Keep the same altitude, prop rpm (fixed), MP, IAS. Let the GPS speed and GPS TRACK settle for a few seconds. The data you need are - for each altitude - pressure altitude; OAT; MP; IAS; compass heading Write it down. Then for each leg - GPS TRACK; GPS Ground Speed; Write it down. Enter the track and speed into the 4-leg program, you will get TAS, wind. Put the pressure alt, temp, TAS, IAS data in the PEC tab and it will calculate a bunch of stuff including IAS error.

After a few trials, you can get the std dev down to about 1-2 knots.

That is very good for those of us who don't have the test pilot skills especially in our new airplanes.

Do a number of speed points at 4,000 feet for example. From slow to full throttle, you can try one with flaps down. Then just do full throttle at 8,000; and 12,000. If your engine is still running good, and you really feel the need, go down to 2,000 agl. There is no need, in my opinion, to go any lower. I have all the data I need at 4,000 feet (3,700 agl).

My FAA FSDO inspector encouraged me to take a "test engineer" along for the data taking. He did not want any of us having our heads down playing with a computer. I just used a knee board and a test card.
 
John C said:
However, taking the difference of the averages for fairings and without will probably give you a pretty good number. 16 mph is probably close.
Click to expand...

Not to be argumentitive ;-) but define "pretty good".

The 'delta average' method you suggest doesn't work since the wind (and other conditions) will likely be different on different days. If you are testing at altitude winds could be big on one day small on the next. Not too big a deal if it makes you think that wheel pants are good for +15mph instead of +13, but it wouldn't be too cool if it made you think that your VG's made you go faster by 2mph.

When you factor in all of the possible errors (turbulent air mass, humidity, temperature, day to day airframe drag (bugs/wax) etc) it is amazing if we get within a 1 or 2 percent, which is 3.6mph of uncertainty at 180mph. Problem is, many of the speed mods make less difference than this. If you made a mod that makes a 1mph difference and your measurement error is 4mph you have a problem...

If you bite the bullet and figure out how to use the NTPS spreadsheet you will eliminate one source of error.

Chuck
 
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Kevin Horton said:
For example, imagine an aircraft with a TAS of 150 kt. The wind is 270 degrees at 30 kt. This is not an unusual wind speed at altitude, where you often need to fly these tests in order to get good air conditions. The ground speed (GS) will be 120 kt on the W run, and 180 kt on the E run. These average to 150 kt - so far so good. But, on the N run, the wind is exactly off the wing tip, and the wind triangle is such that the GS is equal to 153 kt. The GS heading south is also 153 kt. The average of the four GS is 151.5 kt, which is a 1% error. The error is approximately the same no matter what the wind direction. If the wind speed is higher, the error is higher. A 60 kt wind (not unknown - I had 70 kt at 5,000 ft one day last month) with a 150 kt TAS gives an average GS of 156 kt, or a 4% error.
.
Click to expand...

For those who don't have excel here is a 'simple' way to think of it (maybe). If you think in terms of wind speed divided by air speed then the errors look like this:


Code: 
Wind/AS     Error       
------------------
10%         .25%     
20%        1.00%   
40%        4.00%


If you have your GPS speeds for the 4runs it's pretty easy to estimate your speed and get a first order approximation of TAS.


Chuck

PS Go 'hawks!
 
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John C said:
There is no need to follow a grid, worry about crab angles, or your compass calibration card. Life is simple with a GPS. With the NTPS GPS PEC program, all you have to do is fly 4 legs.
Click to expand...

I agree 100%. The NTPS spreadsheet is a good way to go. There is one very minor thing to keep in mind if you use the PEC part of that spreadsheet to calculate CAS and you are interested in the most accurate answer: the temperature you enter should be the free stream ambient temperature, which may differ from the reading on your OAT gauge. The OAT indication probably has some ram rise, especially at higher speeds.

The classical equation that covers ram rise in the indicated temperature is:

IOAT/OAT = 1+0.2*K*Mach^2

K is the recovery factor to the probe, which is probably between 0.7 and 0.95. The Indicated Outside Air Temperature (IOAT) and OAT in this formula are in degrees Kelvin or Rankine - i.e. a temperature scale that starts at absolute zero. This formula isn't terribly useful, as not many of us have machmeters in our RVs.

The following formula is mathematically equivalent, but uses TAS instead:

IOAT=OAT+K*(TAS^2)/7592 (temperatures in degrees C, and TAS in knots).

If our TAS is 180 kt, and our probe has a recovery factor of 0.95, the difference between IOAT and OAT is 4 deg C. If we were converting from TAS to CAS to check the accuracy of our airspeed system, and we used IOAT without correcting for the ram rise, the error would be about a knot. Not a huge error, but I hate leaving errors on the table if we don't need to.

How would you determine the recovery factor of your temperature probe if the manufacturer hasn't provided it? You could fly several runs at the same altitude, at different airspeeds from just above the stall to the max speed in level flight. Record IOAT, IAS and altitude. Once you get on the ground, do a best guess TAS calculation for each point. The OAT shouldn't have changed, so see what value of the recovery factor is needed to make the relationship between IOAT and TAS work correctly.
 
Pretty Good

chuck said:
Not to be argumentitive ;-) but define "pretty good".

The 'delta average' method you suggest doesn't work since the wind (and other conditions) will likely be different on different days. If you are testing at altitude winds could be big on one day small on the next. Not too big a deal if it makes you think that wheel pants are good for +15mph instead of +13, but it wouldn't be too cool if it made you think that your VG's made you go faster by 2mph.

When you factor in all of the possible errors (turbulent air mass, humidity, temperature, day to day airframe drag (bugs/wax) etc) it is amazing if we get within a 1 or 2 percent, which is 3.6mph of uncertainty at 180mph. Problem is, many of the speed mods make less difference than this. If you made a mod that makes a 1mph difference and your measurement error is 4mph you have a problem...

If you bite the bullet and figure out how to use the NTPS spreadsheet you will eliminate one source of error.

Chuck
Click to expand...


Chuck, I think that you captured my definition of "pretty good" with the last sentence in your first paragraph. All of your comments about sources of error are right on target. Kevin added a couple and we don't want to get George started or he will have a BIG list. Just kidding George.

I note that most of the errors that you and Kevin cite are appliciable regardless of the method of calculation that you choose. Determining small performance improvements is difficult at best with either method. I do believe the the "delta average" does work if the grids or tracks are well flown. The math error is only 1-2 mph in a 30 mph wind. Of course the GPS PEC allows one to fly less precise tracks which is a big help.

For those of you who have not yet flown your performance runs, try both methods, don't let us discourage you with all the "fine print." Engineers cannot say too much without a bunch of footnotes. Flying to get the performance numbers was a blast. Try it on different days, see if you can tell a difference.

Wait until you try to get the climb and glide data. If you want to see scatter, I'll send you some of my data in a plain envelope. It was ugly and took a long time.

Most important, have fun and keep your eyes outside. Regards, John.
 
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