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Bob Axsom

Well Known Member
Completed the raw subfairing yesterday and I took it out to the airport to make a test flight with it installed. It didn't happen because the fairing was actually hitting the tire on the left side. The angle is so small relative to the vertical plane of the side of the tire that it probably would have worked but I couldn't live with that so I went through a process of filing and trying until I got some clearance then reinstalled it for the flight. I was out of time by then so the test flight was put off until morning.

I took off at 06:00 just after the tower opened and before the sun rose over the hills to the east. The test went well in that there was no damage or unusual behavior of the airplane. I flew a triangular course with tracks of 000, 120 and 240 degrees at 6,000 ft density altitude. The test method is the U. S. Air Race Handicap procedure found at www.usairrace.org and the National Test Pilot School spread sheet was used to mathematically eliminate the wind effect. My benchmark speed with this the "long wing" configuration is 174.3 knots but there are some small speed increase modifications that are not included in that number (cut off exhaust pipes to get the tips out of the slipstream and provide more direct aft flow of exhaust gasses, washers between the prop and aft spinner bulkhead to eliminate contact at maximum pitch and fine wire spark plugs) but the sum effect of these mods is a small fraction of a knot. The speed today was 175.4 knots which is a significant improvement. How much of this will be lost when changing to the faster short wing configuration is unknown but I expect to see 178 knots.

There is still work to be done, the metal alignment strips riveted to the inside of the front part of the new subfairing needed to be replaced with longer ones (done tonight), and the fiberglass has to be filled and painted. All in all it looks like a good racing mod for an RV-6A. It is probably no good for casual operation or landing on unpaved strips.

Bob Axsom
 
I wish I could show you but...

I can't insert images anymore so I will just describ it. I packed modeling clay around the tire filling all of the openings and fairing in from the front of the stock fairing to to the tire 1" above the ground along both sides and finally up to the underside of the old stock fairing. I shaped the clay into a nice streamlined shape below the stock fairing then I coated the stock fairing clay and tire with mold release wax and layed up the fiberglass using EZ-Poxy resin (24 hour cure). I jacked up the plane after the resin cured and pulled the single layer fairing off of the bottom and removed all of the clay. I progressively sanded and layed up fiberglass and sanded for two more layers. I used four existing fairing mounting screws (no my mounting of the nose wheel fairing is not Van's design - it's mine) on the side and a new platenut/dimple washer/#8 flathead SS screw on the underside at the front and back of the subfairing. One the mating configuration was established I diagonally cut the subfairing on both sides from in front of the aft screw on both sides down to the rear of the cutout for the tire to make the fairing removable with three point mounting for both subfairing halfs. Then I added two overlapping fiberglass extensions to the front part of the subfairing to close the gap between the front and rear sections. Finally I riveted a thin aluminum strip from the inside of the front section to overlap the rear section leading edge to secure the alignment. That's where it stands now.

Bob Axsom
 
Bob,

Your quest for maximum efficiency is legendary, yet I find it a bit odd that you overlooked an area that is very easy to address with a simple addition.

I note that your nose wheel pant does not include a fairing that significantly smooths the transition at the circled location. FYI, it is available from Bob Sneedaker at Fairings-Etc.

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I chose a different approach

Rick, That is the first drag location I addressed. I chose a fairing conforming seal instead of a protruding fairing. I will try to at least link to a photo of my solution that was taken on 11-4-08. In a test flight using my consistent test method (U.S. Handicap Procedure at 6,000 ft density altitude and NTPS spreadsheet to mathematically eliminate the wind effect) I found no measurable increase in speed. I left it in because intuitively I believe it has to help. The subfairing on the other hand makes a very measurable improvement. I think the difference at least partially has to do with the low pressure on the upper surface and the high pressure on the lower surface due to the airfoil shape of the stock, old style flat sided non-pressure recovery wheel fairing and the angle of attack of the fairing (greater than the rest of the airplane). It was a cool morning but the rate of climb also seemed better than usual - I held 2000 fpm for several thousand feet on the climbout. That is better than usual on my plane and I pulled the prop back to 2450 rpm shortly after breaking ground for noise abatement.



Bob Axsom
 
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Painted parts

The sub fairing has been painted along with the 3" wing tips.

Bob Axsom



 
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I finally got to test the new subfairing

I finally got the plane in race configuration where the best controlled test speed was 177.8 kts. In the process I also removed the "permanent" tail tie down ring for the first time. The wind was blowing 16 gusts to 22 on the ground so conditions were not good for a test and the speed was so much better that it is a little embarrassing to report as a valid test. I can say that the plane is definitely faster. The test was run at 6,000 ft density altitude using three tracks of 000, 120 and 240 degrees per U.s. Air Race Handicap Procedure. When the results were plugged into the NTPS spreadsheet it told me the speed was 182.6 kts (TAS) or 210.13 MPH. That is rediculous so I'll just say it is faster.

Bob Axsom
 
Final Test Speed 181.3 kts

I had relatively calm surface winds this evening so I went up just before sundown for a speed test. As always I flew the handicap procedure of U.S. Air Race Inc. (www.us-airrace.org) at 6,000 ft density altitude and plugged the numbers into the NTPS spreadsheet and the speed was 181.3 kts or 208.6 mph. Unfortunately, I also removed the permanent tail tiedown ring in the rush to prepare for the Taylor 125 Air Race that ended up being cancelled because of weather. Anyway, that raises the top speed bench mark using this test method 3.5 kts. That is amazing to me.

Bob Axsom
 
Second fastest in Texoma 100

Texoma 100 was flown on Saturday 4-19-08. It was a roughly 100 nautical mile race from Grayson County (GYI) texas, to Antlers, Oklahoma, to Lake Texoma then back to GYI. Seven RVs flew and my wife and I finished second with our fastest race speed to date. I am certain that the new subfairing was a factor in that speed which was an average of 188.36 kts. We were beat by Dave Christman in an RV-8 who averaged 192.88 kts. It looks like I'm going to have to develop subfairings for the main landing gear as well even though I don't believe they will be as effective as the one on the nose gear.

Bob Axsom.
 
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