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  #1  
Old 09-01-2012, 01:21 PM
Bob Axsom Bob Axsom is offline
 
Join Date: Mar 2005
Posts: 5,685
Default Aileron Mount Drag

I have previously made two "safe" attempts at addressing these seemly draggy locations and both failed. One was to cover the top hole with a thin aluminum plate that moved up with the aileron at the wing tip and the other was to flush plug the recesses in the ends of the flaps and ailerons. I have rationalized a cause for the failures similar to holding a bottomless cup into the wind out of a car window compared to the same cup with a bottom in it. In both cases the inlet was unaltered but the outlet was closed or choked down. There is rational thought that says they should have worked but experiments and testing showed otherwise.

I don't know how much can be gained in this area but I'm convinced if there is anything it is on the inlet and shape of the structure on the bottom of the wing. This is a worrisome area because the constantly moving roll control surface is right there adjacent to the possible drag area. I have seen this addressed with fiberglass fairings on a very fast RV-8 and the well known owner who shall remain nameless told me they weren't worth much. Still, I don't have anything better to look at at the moment and I have three weeks until the next race. Yesterday, I got under the right wing especially the outboard end of the aileron with some 0.016" 2024-T3 aluminum and some tools and fashioned a square edged fairing that would cover the back slanting mount and the width of the opening and could be mounted with screws to make it serviceable. It looked neat but it occurred to me that the resulting ramp would be several times wider than the mount. I could rivet a rounded solid leading edge to this and I ordered a couple of feet of bar stock just in case I decide to go this way. There is a more wedge shaped fairing possibility but this could change the direction of the air flow and actually force more of it into the gaps for control movement - vertical parallel side walls seem best. I can still put on the modeling clay and give fiberglass a shot as well. Ice and improperly mounted fairings in this area could cause serious problems.

Just thinking at this point.

Bob Axsom

Last edited by Bob Axsom : 09-01-2012 at 06:05 PM. Reason: Typo
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  #2  
Old 09-02-2012, 08:34 AM
gereed75 gereed75 is offline
 
Join Date: Sep 2007
Location: pittsburgh pa
Posts: 554
Default

Bob, I just spent a bunch of hours in this area blanking off the flap, aileron and wingtip ends and mounting Vic's hinge fairings. (Vic's fairngs are excellently crafted pieces and effectively close the large opening in the hinge area).

I tuft tested the upper side of the inboard aileron hinge area before and after mounting the fairing. Tuft testing showed a significantly smoother flow on the top side of the aileron hinge area with the fairings.

However, I did not see a measurable increase in speed, either as a result of the end closures or the fairings. If there is a speed increase, as intuition says there must be, it seems lost in the clutter.

I would suggest that figuring a way to close the bottom to top opening in the hinge area would be beneficial. The additional frontal area produced by the fairing looks to the eye to be significant, and if it could be avoided while still closing the opening, you might be ahead.

I seem to be very stuck at the 181 TAS region (3 way NTPS method at 6000' PA corrected for temp). I too am considering more HP to break that barrier. Good luck
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Last edited by gereed75 : 09-02-2012 at 08:39 AM.
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  #3  
Old 09-02-2012, 11:51 AM
Bob Axsom Bob Axsom is offline
 
Join Date: Mar 2005
Posts: 5,685
Default Excelent input

Quote:
Originally Posted by gereed75 View Post
Bob, I just spent a bunch of hours in this area blanking off the flap, aileron and wingtip ends and mounting Vic's hinge fairings. (Vic's fairngs are excellently crafted pieces and effectively close the large opening in the hinge area).

I tuft tested the upper side of the inboard aileron hinge area before and after mounting the fairing. Tuft testing showed a significantly smoother flow on the top side of the aileron hinge area with the fairings.

However, I did not see a measurable increase in speed, either as a result of the end closures or the fairings. If there is a speed increase, as intuition says there must be, it seems lost in the clutter.

I would suggest that figuring a way to close the bottom to top opening in the hinge area would be beneficial. The additional frontal area produced by the fairing looks to the eye to be significant, and if it could be avoided while still closing the opening, you might be ahead.

I seem to be very stuck at the 181 TAS region (3 way NTPS method at 6000' PA corrected for temp). I too am considering more HP to break that barrier. Good luck
Your test results with the closed recesses at the ends of the ailerons and flaps reinforce what I have found so far - no increase in speed. In fact, my tests showed a decrease in speed using the same test method as you describe. I'm starting to recognize this as a science project with very little hope of an increase in speed.

Your tufting observations are especially interesting. My plan on this particular project was to minimize the flow path from the high pressure lower side of the wing to the low pressure upper surface of the wing through the gaping holes around the aileron mounts and fair in the aileron channels that have the open ends facing forward and attaching to the wing aileron mounts. Your results are at least an indication that with you Vic's fairings you achieved that with no increase in speed. That is what I had been told by another very fast RV-8 builder/racer about the results he achieved with his "canoes" in these four areas. If I continue with this it will be more in the nature of an exercise to expand my knowledge rather than an honest effort to increase speed. Certainly not worth rushing to implement before the race in Pagosa Springs on September 22, 2012 three weeks from now.

My work to develop my cooling air outlet in its present configuration cost me dearly in the earlier races in the SARL Championship Series. Several times I worked right up to the last minute before I had to prepare to travel and my tests showed the decrease in top speed down to less than 180 kts by as much as 2 or 3 knots at some points. I am trying to win the whole thing this year so I had no choice but to make the trip and acquire what points I could (once only 45 for 6th and last place in class). I have my racing methods as sharp as I ever have had them again now after a lot of early season meandering and I am in 2nd place overall with 975 points with 10 races to go (two sets of two on the same day so really only 8 for me). I expect to fall back to 4th at Pagosa Springs but gain on the overall leader who has not entered. Probably not a good time to go experimenting but we will see whether experimental or competitive instincts win out in my near future.

Thanks for your valuable input and it was great to finally meet you at the MERFI race in Urbana, Ohio.


Bob Axsom

Last edited by Bob Axsom : 09-02-2012 at 12:55 PM. Reason: typos
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  #4  
Old 09-02-2012, 09:55 PM
Bob Axsom Bob Axsom is offline
 
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Posts: 5,685
Default The next race is too far away - experimenter winning

I spent some time at the hangar after changing the oil in the truck and dumping it in the collection tank at the airport. I think just closing the majority of the holes flush with the wing skin is the way I will go on this.



Bob Axsom
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  #5  
Old 09-02-2012, 10:01 PM
Bob Axsom Bob Axsom is offline
 
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Posts: 5,685
Default Another possible drag item

Lately while looking at this area I have started wondering about how much drag I'm getting from the counterbalance pipe at the front of the ailerons. I'm thinking I'll fly a two lap three leg speed test at 6000 dalt then plug the ends of the pipes with RTV and repeat. Another interesting test that I probably will not do is plug both ends of only one pipe.



Bob Axsom
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  #6  
Old 09-03-2012, 12:26 PM
David Paule David Paule is offline
 
Join Date: Dec 2009
Location: Boulder, CO
Posts: 5,062
Default

I've plugged a similar hole on my Cessna 180 (the hole for the tailspring support tube) by just covering it with a piece of tape. It's stayed on for more than 15 years now. Simple, light and adequate.

Of course my IAS has generally been under 175 mph and the boundary layer there is probably thicker on my plane. Still worth trying.

For the flat parts of the gaps - one piece of .016 attached to the skin for the flap half of the gap, and maybe another to extend the bottom skin of the aileron leading edge inboard to the fitting. Your aileron deflections in a race are probably low.

Thanks for the photos, these really help understanding what you're doing here.


Dave
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  #7  
Old 09-03-2012, 11:22 PM
Bob Axsom Bob Axsom is offline
 
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Posts: 5,685
Default Well I did some testing today 9-3-12 ...

What a fiasco - I hate when this happens. It was very hot here today - 102F and it was not a real fun day to be out at the airport (BIG SWEAT!). However, my plan was to test plugging the hole in the end of the aileron counterweight pipe today. I didn't want to remove Nav antenna elements, remove the tiedown rings, and install the fresh air vent covers and tape the various landing gear joints so I said to myself (I talk to myself a lot these days) you need to fly two triangular speed tests as is with WOT, max rpm, leaned for best speed (I kind of faking it here The only thing that is sensitive at this point is EGT and I have come to look for 1300 F - MAP is maxed, speed seems to plateau and is very insensitive to small mixture movements - but I try) at 6000 ft. d alt with full fuel tanks; then plug both ends of one pipe, top off the fuel tanks and fly two more; then plug both ends of the other pipe, top off the fuel tanks and fly two more speed test triangles. Well I got the first two sets of test flights in but not the third.

The first flight:

Using the US Air Race Handicap Procedure I climbed to 6000 ft p. alt and the temperature was 27C so I descended to 3,300 ft (Altimeter set at 29.92) for the test run.

First triangle:
Leg 1, 360 deg = 180.2 kts
Leg 2, 120 deg = 196.6 kts
Leg 3, 240 deg = 183.0 kts

NTPS
Calculated wind 10 kts 309.5 deg
Calculated speed = 186.7 kts

Second triangle:
Leg 1, 360 deg = 180.6 kts
Leg 2, 120 deg = 196.2 kts
Leg 3, 240 deg = 179.6 kts

NTPS
Calculated wind 10.6 kts 296.8 deg
Calculated speed = 185.6 kts

Average - 186.15 kts

I know - I have never seen test speeds that high before but of course I had not done the post flight processing to know the speeds were that high at the time. Later after I got home I recognized a 300 ft error in the flight altitude - I should have flown at 3,600 ft. p. alt.

Ignorant of the precise results of the first flight, I landed, plugged both ends of the right aileron counterweight pipe, topped the tanks and took off for the first comparative test - flight number two. I climbed to 6000 ft p. alt as before and found the temperature was 26C so I descended to 3700 ft and flew the one pipe plugged test.

The second flight:

First triangle:
Leg 1, 360 deg = 179.6 kts
Leg 2, 120 deg = 190.6 kts
Leg 3, 240 deg = 180.6 kts

NTPS
Calculated wind 7.5 kts 303.6 deg
calculated speed = 183.7 kts

Second triangle:
Leg 1, 360 deg = 179.2 kts
Leg 2, 120 deg = 191.0 kts
Leg 3, 240 deg = 180.0 kts

NTPS
calculated wind 6.9 kts 305.9 deg
calculated KTAS speed = 183.5 kts

KTAS Average with right aileron pipe plugged at both ends - 183.6 kts

Approximately $120 was spent on direct fuel cost for these flights. The end result is, it was technically an invalid test because the baseline flight was flown in denser air than the plugged pipe test flight. However it was not a wasted effort. The test flight with the plugged pipe is in line with my last average from the cooling air outlet mod tests which was 183.0 kts. There was an imperceptibly tiny constant yaw to the left that required slight right rudder to center the ball which is consistent with more drag on the left wing but there was a tiny yaw before that sometimes required a touch of right rudder to center the ball - was it more or less - I don't know.

Tomorrow I plan to plug both ends of the left aileron pipe and re-fly the test triangle. I am not going to remove the plugs and re-fly the "baseline" test. If the speed is higher than 183.6 kts and the ball is more centered that will be an indication that the holy pipe is a drag source.

Bob Axsom

Last edited by Bob Axsom : 09-04-2012 at 04:38 AM.
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  #8  
Old 09-04-2012, 09:04 PM
Bob Axsom Bob Axsom is offline
 
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Posts: 5,685
Default Last aileron counterweight plug test

I plugged the ends of the left aileron counterweight pipe today and re-flew the two triangle speed test under very similar hot (102 F again) windless conditions as yesterday. Temperature at 6,000 p. alt 28 C so the flight was at 3,500 ft with altimeter set at 29.92. The ball was more centered (almost perfect) than during the flight with only the right pipe ends plugged.


The third flight (both aileron counterbalance pipes plugged):

First triangle:
Leg 1, 360 deg = 187.6 kts
Leg 2, 120 deg = 189.3 kts
Leg 3, 240 deg = 183.6 kts

NTPS
Calculated wind 3.5 kts 256.4 deg
Calculated speed = 186.8 kts

Second triangle:
Leg 1, 360 deg = 189.0 kts
Leg 2, 120 deg = 188.6 kts
Leg 3, 240 deg = 179.6 kts

NTPS
Calculated wind 6.2 kts 237.9 deg
calculated KTAS speed = 185.8 kts

KTAS Average with both aileron pipes plugged at both ends - 186.3 kts

Bob Axsom
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  #9  
Old 09-04-2012, 10:07 PM
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Alan Carroll Alan Carroll is offline
 
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Default

Quote:
Originally Posted by Bob Axsom View Post
Average - 186.15 kts

I know - I have never seen test speeds that high before ...
Bob,

This seems like this statement might be the most revealing aspect of your tests? I wonder if perhaps you have uncovered a weakness in the test method you're using?

Based on the Lycoming power chart below, HP varies linearly with pressure altitude, and with the square root of temperature (see note #4). Density altitude on the other hand is nearly linear with both pressure and temperature. Therefore, at a given density altitude (say 6000'), the engine makes more power on a hot day than on a cold one. It would seem that a test method based on comparing speeds at the same density altitude will produce inconsistent results if temperatures differ from standard, because engine power is not being held constant.

There's been a previous thread on why power varies with the square root of temperature, but I don't think it reached a clear consensus.



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  #10  
Old 09-04-2012, 11:01 PM
Bob Axsom Bob Axsom is offline
 
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Default I'll have to spend some time on that

The method has been so consistent before through a good number of years but you are right these exceptionally high speeds for our RV-6A just seem wrong! I absolutely hate the grunt work validating a test method but it seems to me that these test results are not valid even though repeating the test produces consistent results. I suspect my wise fellow racer and professor wants me to find the weakness in my ways. OK - but it will not be fun.

Can't I just say the tests indicate the the plugged pipes are faster than the open pipes in a relative manner?

ARGH!

Bob Axsom
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