Smaller Wing Tips for Speed

[Pilottonny]

Bob,

It looks like it is hollow now? If so, buy a can of builders foam, fill it and your done!

Regards, Tonny.

[Pilottonny]

Bob, I am definitely not an expert at this, but will the flat tip not create a lot of turbulence and cause more bad than good, this way?

Last weekend I installed the flap and aileron on my right wing and was really annoyed to see these gaping holes between the aileron and the wing and flap, where the hinges and the pushrod are. I bet you could gain a couple of knots, if you find a good way to close these off (or at least make the holes smaller).

Than there is these ugly flap hinges sticking out into the breeze: six of them as well! If you can make a good fairing for those, you win 6 times!!

BTW, I will be watching your progress, so that I can copy it . (Don't worry, I am not interested in racing).

PS: can you do the mods on the Ailerons and Flaps first, I am going to paint mine soon!

Regards, Tonny.

[roee]

Tonny,

I could be wrong, but I think the six ugly hinge brackets out in the breeze are unique to the flaps on the RV-9 (and maybe the -10 and -12?). The RV-6/7/8 flaps use a piano hinge with nothing out in the breeze.

The ailerons, if I'm not mistaken, are deliberately designed to allow airflow from bottom to top over the leading edge of the aileron. I think this is meant to help keep the flow attached to the aileron at high alpha.

Originally Posted by Pilottonny

Bob, I am definitely not an expert at this, but will the flat tip not create a lot of turbulence and cause more bad than good, this way?

Last weekend I installed the flap and aileron on my right wing and was really annoyed to see these gaping holes between the aileron and the wing and flap, where the hinges and the pushrod are. I bet you could gain a couple of knots, if you find a good way to close these off (or at least make the holes smaller).

Than there is these ugly flap hinges sticking out into the breeze: six of them as well! If you can make a good fairing for those, you win 6 times!!

BTW, I will be watching your progress, so that I can copy it . (Don't worry, I am not interested in racing).

PS: can you do the mods on the Ailerons and Flaps first, I am going to paint mine soon!

Regards, Tonny.

[roee]

Here's a question for Bob A., Bob M., and hopefully some aero guys to chime in:

If you go with a flat wing tip like this with no significant span, why even extend it all the way back to the trailing edge? Does that serve any purpose aerodynamically? Why not just end the wing tip at the rear spar, and make a matching end cap for the aileron, so they're flush with each other?

[Bob Axsom]

This morning I went to the airport, installed the battery and tested the new semi-flat tips. The resulting speed was 181.5 kts. After the flight it took me 2 hours to replace the semi-flat tips with my 3" span streamlined tips and refruel.

I reflew the test and the resulting speed was 182.3 kts so I do not intend to pursue the flat tips any further.

Bob Axsom

[rvmills]

Originally Posted by roee

Here's a question for Bob A., Bob M., and hopefully some aero guys to chime in:

If you go with a flat wing tip like this with no significant span, why even extend it all the way back to the trailing edge? Does that serve any purpose aerodynamically? Why not just end the wing tip at the rear spar, and make a matching end cap for the aileron, so they're flush with each other?

roee,

Had that discussion with Wayne (the F1 Rocket pilot that did flat tips last year, and felt he gained 3 knots). He has the same clipped wing I have, and we discussed doing just as you describe. In fact, from what I understand and from our discussion, that is how the EVO wing is constructed (flat tip that stops at the wing trailing edge, flat aileron cap, separate but all lined up). Tom or other EVO drivers will correct me if I'm off the mark on that.

Doing so may very well be a good idea from an aerodynamic perspective (I imagine its cleaner that way), but since I'd only use the flat tips during races, then go back to my standard tips, I don't want to drill holes in my ailerons and add the cap, then have the holes in my aileron when I go back to the normal tip. I guess one could make a flush insert with nutplates that could go in when the normal tip goes back on...but I looked at the ailerons on mine, and there's not a lot of "meat" to work with there. Drillin' & fillin' moveable control surfaces is a bit beyond what I want to play with as well.

Given this is fun racing (sorry for the sacriledge Bob!), the level of complexity that I'm willing to take on is somewhat limited, and I want to be able to have everything reversible.

Originally Posted by Bob Axsom

This morning I went to the airport, installed the battery and tested the new semi-flat tips. The resulting speed was 181.5 kts. After the flight it took me 2 hours to replace the semi-flat tips with my 3" span streamlined tips and refruel.

I reflew the test and the resulting speed was 182.3 kts so I do not intend to pursue the flat tips any further.

Bob Axsom

Wow Bob, not what I expected at all...and you put a lot of work into them! I'll pursue it and see what I get. If its a bust, we'll know more. If it works as it appears to have for Wayne, perhaps more testing would then be warranted (that'd be your call of course). Wayne's wing and mine are already clipped (3.5" per side), so perhaps there's more to it than just going shorter, or there's a place where it starts to have a greater effect. That's a bit counterintuitive to me though, as you would think that shortening a longer wing would have a bigger effect, and at some point diminishing returns would kick in...somewhere before the "this wing won't fly any more" point. Of course, there are a lot of things that go counter to intuition in this aerodynamic game, as you know and have taught us from your work! And heck, it could be that your 3" tips are just a really clean design, and going flat created more drag than lopping off 3" from them gained in span reduction. Hmmmm...do you still have those molds and templates?

I know you are very methodical in your testing...do you think one flight with each is enough to make the call? I know that's easy to say, as its your time and gas money (and I respect that). Just wondering how I'll approach it, especially if I see the same result (which will lead to many ribbings from the Stead peanut gallery!)

One last Q...did you note any bending, twisting or vibrating in those trailing edge endcaps?

Sorry that did not go better Bob...we'll keep digging in for speed!!

Cheers,
Bob

[Bob Axsom]

I saw and felt no vibration or other adverse effect from the flat tips. I cut down the spanwise gap between the aileron and the tip from 5/16" to ~ 1/8". The take off roll seemed longer with the flat tips and the plane required what seemed like a lot more right rudder to center the ball - like almost standing on it in the climbout and quite a bit in level flight. It was not at all difficult to fly just the ball wanted to rest off to the right more than usual. On the first leg I was feeling pretty good looking at 206.4 kts ground speed tracking 360. Even after I turned to 120 the ground speed was 175.4 kts (5 consecutive 20 sec. interval average). Then when I turned to 240 it dropped to 159.8 kts so I knew it was not going to be a big breakthrough modification. Later at home when I plugged the numbers into the NTPS spreadsheet the result was 181.5. There was some minor evidence that the rear edge of the right tip contacted the aileron (1/16" paint chip on the aileron and some epoxy resin missing from the tip). When I took off the ATIS gave the surface winds as calm but when I returned after the test flight they were 170 at 7. The calculated wind speed at 6,000 ft density altitude was 201.1 at 26.9.

After I went through the change to my streamlined 3" tips the surface wind on ATIS was 170 at 10. When I returned it was 170 at 10 gusting to 18. The calculated wind at 6,000 ft d alt. was 197.6 at 36.9. The leg speeds with the same 360, 120, 240 GPS ground tracks were 217.2, 170.8 and 153.4 respectively.

It is obvious that the wind was building and the most significant change may have occurred during the first test flight. There can certainly be a case made for retesting but my gut, the inflight feel and the NTPS numbers tell me not to waste any more time on this (I can sense John Huft cringing). Time and money resources have to be respected when you believe the mod is a lost cause. I have no doubt that it is faster than the stock tip but what I developed before is better I think.

If you would like to do the short tip and aileron plug mod that may not be as much trouble as you think. I made balsawood plugs for mine in an earlier unsuccessful test that worked very well. I installed the plug into the recess, drilled a pilot hole through it and the end aileron rib, installed it with some automotive silicon sealant, a dimple washer and a flathead sheetmetal screw. It was solid and I was able to dig it out without too much trouble after the test showed a speed loss instead of a gain. I think the speed loss was due to the opening on the high pressure lower side of the wing being left open while the exit low pressure side was being blocked off. I've got an idea for a closure fairing around the aileron mount after which the plug and another upper surface closure mod may go back in. Just thinking at this point.

Bob Axsom

[rvmills]

Roger all Bob. I'll do some similar testing, and see what I get. I'll carry some of the load with ya!

IIRC, Wayne's tips have small extensions (or drop downs, if you will) that match the outer aileron hinge, giving it a bit of a fairing effect...that's the intent anyway. Trying to decide if I'll do that as well.

Will be interested to see the closure fairings and end caps as you sleuth it out. Will keep you posted on my progress.

Cheers,
Bob

[Pilottonny]

Bob,

Speed reduction with flat tip: what I expected!. I would try a flat plate, slightly bigger than the wing, extending an inch or so around the wing and see what that does. Doesn't take a lot of time to make. That way you will reduce the turbulance (air flowing from the bottom to the the top of the wing, over the tip).

Flap hinges: I didn?t know the 7 had a different flap design. I bet that for the 9, a knot or two may be gained with a good fairing around those hinges.

Ailerons: I don?t mean the opening at the top and bottom, along the length of the aileron. It?s those openings where the hinges and pushrod are. Maybe this is a different design on the 7 as well? I was going to close the sides of the flap and the aileron, like I have done with the elevator and the rudder, but I guess it?s no use with these openings that are way more draggy.

Regards, Tonny.

[elippse]

As you know, the equivalent parasite drag area for an RV-6 is about 2.2, and for a -6A is about 2.32. If you increase your wing area, since the 23013 airfoil has about a 0.006 CD, your parasite drag area will increase by about 0.006 X area increase. If you increase the span, the induced drag will decrease by 1/span^2. So if you want to go faster at low altitude where induced drag is minimal, decrease your wing area. If you want to go faster above 7000', increase your wing span. If you cut your span back, say 1' or 2' on each side, and then add triangular tips to end up with a little less area and a little more span, you'll get the best of both! The reason why is that the area, parasite drag, only increases half as fast as does the span, induced drag, with a triangular-planform tip. You will also see a decrease in stall and landing speed and roll-rate, and a better take-off and climb rate. But the tip shape is very important as it determines the Oswald efficiency factor which relates to the effective span of the wing which is where the tip vortex forms affecting the induced drag. A flat tip with sharp edges top and bottom trips the flow around the tip from bottom to top and keeps the vortex near the tip. But if the tip has rounded edges top and bottom, the air flowing up and around will stay attached due to Coanda effect and cause the vortex to form farther inboard, increasing induced drag. The slashed or sharp-edged tip has an Oswald efficiency factor of 0.81, whereas the rounded tip, in planform or as viewed from the front, is down more toward 0.75, and since this factor is in the denominator of the equation for CDI, lower values are worse. Have any of you seen Jim Smith's tips on his -6?