Wheel fairings...+16MPH?

[pierre smith]

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.

[Kevin Horton]

Originally Posted by pierre smith

... and the 4 way average went to 196.5MPH on the GPS...way more than I thought we'd get.

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.

[N62XS]

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

[sprucemoose]

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!)

[RickS]

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.

[Surface Warrior]

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."

[Kevin Horton]

Originally Posted by Surface Warrior

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."

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.

[collinsj57@adelphia.net]

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?

[Bob Axsom]

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

[John C]

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.