[Bob Axsom]

My semi-flat fiberglass wingtips with rounded edges were a bust and I wave been advised that sharp edges are necessary to get the desired speed increase. This is different than a tip plate in that the tip does not extend beyond the wing surface. The trick as I see it is achieving that sharp edge and not have any air gaps between the upper and lower wing surfaces and the flat metal tips.

I believe I have thought this through enough that I can do it. What I now plan is cutting a 1/2" wide strip off of the edge of the 6' sheets of 0.016" 2024T3 to be used for an inner flange. The sequence is critical to doing the job right I think.

1 - "C" clamp the spliced strip inside the wing skin, hard against the inside of the leading edge, outboard edge precisely aligned with the wing skin outboard edge.

2 - Start at the top front of the wing and carefully drill a #30 hole in the flange strip through the center of the forward wing tip mounting hole which is already sized and dimpled for a #8 flathead screw (that's what I mount my existing 8 wing tips with) and 1/8" clecoe it in place. Then move to the next hole aft and repeat until the last one is done then do the bottom in the same way. Mark for wing tip and orientation and remove the flange strip.

3 - Saw 120 1" brackets from 6061T6 sharp corner 1/16" thick 3/4"x3/4" angle (break sharp edges, etc., etc.) and "C" clamp them to the inside of the flange strip with the outboard bracket vertical surface precisely aligned with the flange outboard edge and drill a #30 hole in the horizontal surface using the flange hole as a drill guide. Mark every bracket and the flange with a corresponding number so every bracket has a specific installation location.

4 - Using a non-floating plate nut as a drill guide, clecoe the bracket, flange and platenut (oriented for&aft) together with a 1/8" clecoe the drill the first platenut mounting hole with a #40 drill. Clecoe the stack with a 3/32" clecoe and drill the other mounting hole. Repeat the process until all of the holes are drilled then disassemble for cleanup, deburring etc.

5 - Enlarge the screw holes in the flange and brackets to clear the wing skin dimples (1/4" to 3/8") dimple the platenut mounting holes in the thin flange and countersink the mating 3/32" platenut mounting holes in the brackets for the 3/32" rivets. Take apart debur the enlarged holes.

6 - Rivet the FLOATING platenuts to the stacked flange and brackets.

7 - Clecoe the flange assemblies back inside the wing.

8 - Cut oversize tips from the large sheets of 0.016" 2024T3 aluminum ...

9 - At this point the bracket present a fairly large target but I need to refine the process for drilling holes through the tips into the brackets and clecoeing as I go. There are several ways to do this but I have to work it out yet.

10 - Remove the tip and flange assembly from the wing, dimple and countersink and debur the tip to flange bracket mounting holes and rivet the tips to the flange assembly.

11 - Use RTV to seal the inside of the interface between the flange and the tip.

12 - Reinstall and progressively mark, cut, try and repeat until the flat internally sealed sharp edged tip exactly matches the surface of the wing skin.

Bob Axsom

 

[Tom Martin]

Alternate method
1. fix fluted/notched angle to wing first. Use 3/4 by 3/4 by .032. You might have to take one side down to 5/8" depending on how much wing skin over hang you have.
2. place end piece over end of wing, make it larger then required, and drill into flange
3. rivet flange to end piece
4. install and trim to exact size.

 

[Bob Axsom]

Thanks Tom

I have some unstated requirements that are driving my approach especially the ones that require complete interchangeability with all of my other wing tips and no alteration of the of the painted wing. I haven't been able to get out to the hangar to measure the distance from the last rib to the edge of the wing skin - I'm expecting the need to cut off the inboard edge of the brackets as you mentioned. I have all of the material available so I will go with that but I agree thinner brackets would be better. I also didn't mention something that I'm sure you assumed - I am going to reinstall my end plugs in the ailerons and only extend the flat tip back far enough to cover the outboard end of the wing and aileron interface.

Bob Axsom

 

[jsharkey]

It's not impossible...

...to mold or otherwise form sharp corners on composite tips.

 

[rvmills]

Bob,

Couple thoughts, as I've been considering this project as well. These are the thoughts of a rookie of course, so caveat emptor!

I've been thinking along the lines of Tom's suggestion, and you could probably put floating nutplates in a that angle AL flange, so you could press in and tighten them up to make the tip flush along the entire span. Not sure if there's a rub there, in that with a continuous flange there might be some binding and some resulting gaps between the wing skin and tip material. Your independent flanges might do better, though it is a much more complex project.

Mike Thompson has an alternative approach. I looked at Mike's tips at the Taylor 150 and asked him about the flat plate tip-to-wing surface interface, and how he mounts his tips to the wing.

Mike's tips are 3" in span, and have a removable flat plate tip. Think of it as a wing-shaped cyllinder on the end of his wings, with the flat plate covering the outside of the cylinder. You've probably seen them, but thought I'd throw it out here. His design does add the 3" to the span, but you could probably make them 1" or 2" and acheive good results.This design does offer a couple nice features.

First, his 3" tips go on and off just like most normal tips (same flange design), and just like yours and mine...so that makes them fully interchangeable with your other tips. The flange on the inner part of the tip would probably be easy(er) to make as well.

Second, his flat plates on the ends are interchangeable as well, and Mike has tried some different designs, including some that had mini-fences. He attaches the flat plates to the tips with (I believe) 4 screws (maybe more). However, in looking at them, the plates were tight and flush to the wing...no air gaps that I could see.

The benefit of the 1", 2" or 3" exrtension is that the flat plates are easier to swap in and out, or so it appeared. You might give Mike a shout to get more detail. I know he's your RV Blue adversary, but I'm sure he'll give you the straight scoop!

I'll be watching you develop this with great interest, as I've been trying to sleuth out how to do some variable flat plate designs as well.

Best of luck with the design and development!

Cheers,
Bob

 

[Bob Axsom]

Well, Well

I got to spend a little time at the hangar yesterday working on a better solution for the fuel drain fairings. With the half hour left over I decided to remove the right wingtip and take a look at what I have to deal with to implement this "simple" conceptual design. The problems are several. The skin extension past the last rib is not consistent; the tip screw mounting holes are too close to the end of the wing skin to install the platenuts on the angle brackets without relief; the end of the wing skins may not be parallel and in line with the direction of flight etc. I had mentally worked out solutions to all of the observed problems but it requires a lot of detail work. Thinking of how to get that done before the race in Plainview, Texas on June 5th and deal with my responsibilities at home I came up with a new idea: install the previous flop semi-flat tips which are already interfaced with the wings, mark the desired cut lines, cut off everything outboard of the mount interface straight line, install the flat metal tips on the old residual mount with angles & rivets between platenuts and seal the interface on the inside leaving the sharp edge flat tip on the end of the 21' wing. I plan to fly with one new metal flat tip and one currently demonstrated fastest 3" streamlined tip. The yaw result should indicate the tip configuration with the least drag.

Bob Axsom

 

[Bob Axsom]

5-17-10 Progress

I removed the right 3" streamlined racing wing tip and checked the end of skin alignment with respect to the leading edge using a large square. It is very close to square with the leading edge slightly longer than the trailing edge - at this point I and calling it close enough to perfect that it is not to be messed with.

I installed the right semi-flat tip on the wing and marked where the skin ends on it. The I removed it and applied masking tape as a marker using the edge as the cutoff point. I carefully cut off the the portion of the tip that extended beyond the wing skin using a die grinder and cutoff wheel. Then I reinstalled it on the wing to assure that I had good alignment.

I decided to make brackets for attaching the flat metal plate to this fiberglass flange out of 1/16x1/2x1/2 6061T6 aluminum angle. I cut these using a metal blade in my small Craftsman bandsaw (wear safety glasses). These generally go between the platenuts on the flange and the spacing I used for those platenuts depended on the demands of the curvature of the airfoil. Some of these are very close together and some are widely spaced so the bracket sizes vary widely - some are only wide enough for one rivet and some have to share the platenut mounting rivet (I try to avoid drilling out the existing platenut mounting rivets so as not to affect the tip to wing mounting hole pattern even slightly but I have resorted to it once so far). This part of the job is very critical and vulnerable so I am methodically working through it on bracket at a time. So far I have 16 done and more than 30 to go. It makes a good activity to replace eating and reading when I wake up in the middle of the night.

I plan to do just this one and fly it with the 3" streamlined racing tip on the other wing to verify that it is worth building the other flat metal tip with sharp edges.

The approach looks very good. More later.

Bob Axsom

 

[Bob Axsom]

5-20-10 Status Right Wing Tip

I plan on starting the work to attach the 0.016" tip to the tabs on the fiberglass flange tomorrow.

Bob Axsom

 

[Bob Axsom]

Test flew this evening

I test flew this evening with the new flat metal tip on the right wing and the 3" streamlined racing tip on the left wing. The results were inconclusive. I expected a yaw to the left, which I had on the climb out as usual but was it more than usual - well maybe but it was not significantly more if any at all. When I leveled out the ball stayed pretty much centered with no effort from me. I ran a speed test but I was rushed by the coming sundown and my unlighted plane in racing configuration so I did not have max RPM until the second leg and it was not leaned properly until late in the run so the slow 177.9 KTAS is not meaningful. The work time on this "simple wing tip" is much more than I expected. The wing tip fits very well and it is trimmed perfectly flush with the wing skin's outer surface. There is a 2024T3 0.090"x1"x8" stiffiner riveted inside the tail of the tip. There is a balsa wood plug in the end of the aileron. Conclusions: If there is any gain in speed it will probably be small. I will not complete the other one before the race in Plainview, Texas on June 5, 2010. I will build the other tip and carefully test both on the airplane in the future. Below are the two wing tips in today's flight test.

Bob Axsom

 

[elippse]

Sharp-edged tips

Boy!, You are some experimenter! You didn't say at what altitude you did these tests, but the effectiveness of these tips, which has to do with the Oswald efficiency factor, relates to the effective span of the wing which has to do mostly with the induced drag. As such, you will have a positive benefit from these tips at all altitudes and speeds, but the higher or slower you fly, the more it will show up. So maybe in comparing these with the rounded tips you'll see a slightly lower stall speed and a little more speed at higher density altitudes.

 

[Bob Axsom]

6,000 ft density altitude

6,000 ft density altitude is where I ran this test. I have decided that I need to do testing at lower altitudes for some possible benefits.

Bob Axsom

 

[Bob Axsom]

Tom I appreciate your wisdom

Alternate method
1. fix fluted/notched angle to wing first. Use 3/4 by 3/4 by .032. You might have to take one side down to 5/8" depending on how much wing skin over hang you have.
2. place end piece over end of wing, make it larger then required, and drill into flange
3. rivet flange to end piece
4. install and trim to exact size.

You would not believe the hours I spent getting my approach worked out just right - well maybe you would.

Bob Axsom

 

[Tom Martin]

Bob
Based on your early results we might conclude that the major speed gains that come with smaller tips, on RV wings, is the initial reduction in size. And that after a certain frontal area has been removed further gains are of decreasing benefit. I suspect you will see more of an improvement at lower altitudes. I like the one wing test for wing tips, and unless you can see some yaw differences at lower altitudes, I would not think it worth your time to do the other tip. Good work Bob.

 

[rvmills]

Boy!, You are some experimenter! You didn't say at what altitude you did these tests, but the effectiveness of these tips, which has to do with the Oswald efficiency factor, relates to the effective span of the wing which has to do mostly with the induced drag. As such, you will have a positive benefit from these tips at all altitudes and speeds, but the higher or slower you fly, the more it will show up. So maybe in comparing these with the rounded tips you'll see a slightly lower stall speed and a little more speed at higher density altitudes.

Bob
Based on your early results we might conclude that the major speed gains that come with smaller tips, on RV wings, is the initial reduction in size. And that after a certain frontal area has been removed further gains are of decreasing benefit. I suspect you will see more of an improvement at lower altitudes. I like the one wing test for wing tips, and unless you can see some yaw differences at lower altitudes, I would not think it worth your time to do the other tip. Good work Bob.

Bob, really nice work, and it does look like a lot of time and effort. The metal flat tip looks great. I'm watching and reading the posts with great interest, and the discussion is bringing up a number of comments and questions.

1. I agree with Tom that after a certain point of span reduction, further benefit probably decreases or goes away. Changing from my 1" flat fiberglass tip to Bob's metal flat cover plate probably would fall into that catefory. However, my flat tip goes back all the way to the aileron tip, so the gap there and the open end of the aileron still exists. Bob's new design stops at the back of the wing and fills the aileron end, like the EVO wing. Do you guys think that would be a marked improvement over my tip (and Wayne Hadath's similar design)? It would be interesting to see how close our wing cross section is Bob...if they were the same, I'd send you my flat tip for a side-by-side test! Likelihood of our screw holes matching up is probably low though.

2. The one tip (differential tips) testing is interesting. Not trying to rebut the method, as you guys have done far more testing than I...but the rudder application to keep the ball centered seems such a subjective and somewhat variable thing. Perhaps with lots of time in the saddle you get to know just how much rudder it takes, but in testing small differences in yaw would seem to be hard to detect. And I'd be hard pressed to be able to tell how much yaw/rudder pressure change from standard would show a 1, 2, or 3 knot gain from a new tip. Would like to pick your brains on that...what are you looking for, and what do you think would indicate a design worth keeping to test further? I do appreciate the time element, and the reason for doing one tip at a time (honest). However, I would think the only way to fully evaluate tip design changes would be to run the tests with both (full) sets of tips back to back, in very similar conditions of alt, temp, wind, bumpiness, etc. Not sure I'd throw out this design (or any) after one test. Your guys' thoughts?

3. Finally, on Paul's comments on the design. Paul, the part that confuses me is when you said that these tips (and I paraphrase) would improve performance at all altitudes, and be especially effective at higher altitudes and higher angles of attack (maybe even yielding an decreased stall speed, due to Oswald Factor. I may be thinking of two different parameters (decreased span and tip edge design) and mixing them inappropriately...I'll try to explain:

I'm assuming that span reduction will decrease parasite (frontal area) drag, and that will show the most improvement in the low-fast (high air density, low density altitude, high q regime). However, both Wayne and I noted that along with increased speed down low, our flat, sharp-edged tips gave us an increase in stall speed. I'm assuming that is from decrease wing area (lifting body area). When it comes to edge design (sharp or rounded), then Oswald Factor kicks in, and sharp is better than rounded, because it keeps the vortex attached longer and decreases induced drag. Perhaps what you're saying is that sharp edges would have less induced drag than rounded edges on wings with identical spans. I do get that, but what Bob (and me and others) are trying to do is make a tip that is optimized for speed down low. Do you feel short, flat and sharp-edged is the right path? You and I have corresponded on triangular tips, which I think would be optimized for high and fast, such as in the Airventure Cup, due to its combination of decreased wing area and increased effective span. But for down low, it seems flat is best...concur?

I was looking at Rare Bear Friday, and its wings are really, really short, but have somewhat symmetrical rounded tips, much like Bob's 3" rounded tips. Makes me wonder about them as well. I'd love to see a side-by-side test of your rounded 3" tips versus the flat tips Bob...and wish I was closer to help you with the work, or do some tip-swapping between us and testing!

Lots of musing here, sorry to go long...oh, and just to be sure...I'm not questioning anyone's methods or thoughts at all...just trying to learn from the masters!!

Good luck Bob, and again, nice work!!

Cheers,
Bob

 

[Bob Axsom]

I think I agree with all that have responded

I will build the other tip as I did this last flat tip and do a real test. It takes about two hours to change tips (as long as I am not switching in or out my tip tanks) but conditions change and a repeat using the same test method for both tip designs under as "identical" conditions as possible is going to be my objective.

Bob Mills, there is no chance that our hole pattern is alike so we will have to pass on that idea.

This was my first asymetrical tip test so my calibration is nonexistant. A 1 kt gain would be tremendous so I will pursue the flat tip testing to the end and see what it brings.

It may very well be that the shorter flat tipped wing is better than the rounded tip down low and I intend to test in that environment as well as my standard 6,000 ft d alt. This will give a 2x2 matrix of comparable speeds with altitude and tip design variables.

Bob Axsom

 

[elippse]

3. Finally, on Paul's comments on the design. Paul, the part that confuses me is when you said that these tips (and I paraphrase) would improve performance at all altitudes, and be especially effective at higher altitudes and higher angles of attack (maybe even yielding an decreased stall speed, due to Oswald Factor. I may be thinking of two different parameters (decreased span and tip edge design) and mixing them inappropriately...I'll try to explain:

I'm assuming that span reduction will decrease parasite (frontal area) drag, and that will show the most improvement in the low-fast (high air density, low density altitude, high q regime). However, both Wayne and I noted that along with increased speed down low, our flat, sharp-edged tips gave us an increase in stall speed. I'm assuming that is from decrease wing area (lifting body area). When it comes to edge design (sharp or rounded), then Oswald Factor kicks in, and sharp is better than rounded, because it keeps the vortex attached longer and decreases induced drag. Perhaps what you're saying is that sharp edges would have less induced drag than rounded edges on wings with identical spans. I do get that, but what Bob (and me and others) are trying to do is make a tip that is optimized for speed down low. Do you feel short, flat and sharp-edged is the right path? You and I have corresponded on triangular tips, which I think would be optimized for high and fast, such as in the Airventure Cup, due to its combination of decreased wing area and increased effective span. But for down low, it seems flat is best...concur?

I was looking at Rare Bear Friday, and its wings are really, really short, but have somewhat symmetrical rounded tips, much like Bob's 3" rounded tips. Makes me wonder about them as well. I'd love to see a side-by-side test of your rounded 3" tips versus the flat tips Bob...and wish I was closer to help you with the work, or do some tip-swapping between us and testing!

Lots of musing here, sorry to go long...oh, and just to be sure...I'm not questioning anyone's methods or thoughts at all...just trying to learn from the masters!!

Good luck Bob, and again, nice work!!

Cheers,
Bob

What I was trying to get across is that in a comparison of Bob's rounded tip to his flat tip that the flat tip will always be better, as the rounded tip increases induced drag at all flight regimes. To go fast down low, flat out with minimal turns, reduce the span. At Reno, however, with its typical density altitude of about 7000' with lots of turns, maximize span and keep area to the minimum allowed. That induced drag at high G around a turn kills-off speed rapidly with short, low-aspect-ratio wings with round (planform) tips as on a Pitts. You can actually observe how much they slow down. The Sport that is the one to watch, if Mike ever really opens it up, is the Questair Venture with its 10:1 AR. That Egg is a really great design! 'Wish dentist Anders would get his flying again! You guys are on the right path; all you need now is a high efficiency prop turning about 3200 rpm!

 

[CraigC]

NACA performed similar tests and published them in the 30's and 40's. They found a flat extended plate a certain ratio of the chord performed the best in reducing tip drag and improving speed. I cannot find the book right now. The plate was not flush with the wing but protruded a certain amount above and below the wing reducing turbulent flow at the tip. A precursor to the winglet I guess.

 

[AX-O]

Bob,
I appreciate all your work and follow it. Once flying I plan to experiment with wing tips as well. However, my designs will be a little different than yours. During my Sub Sonic class at Test Pilot School they gave us the document attached to this post. It shows that although the flat plate tip is highly efficient, a round tip (like a fuel tank) or other shapes are even more efficient. I know some of the tips shown would be ugly and difficult to make however, it will give me something to do after the plane is flying.

If you look at figure 31 form c, the tip efficiency is quoted at 1.52 vice the flat plate ones shown on figure 30.

Figure 31(form c) looks elegant and may be made using the same process followed when making the fairing between the wing and the fuselage (like on rockets, pics below).

Once again thanks for your work.

http://docs.google.com/fileview?id=0BxZA8yrZXBYdY2ZlNzM3ZGQtNWMwZC00ODZlLWI2YTctMmIzOTY1NDk0NzFi&hl=en

 

[Bob Axsom]

Good feedback

Bob Mills, I could ship the streamlined 3" tip molds to you if you like and you could make a set if fiberglass tips for your RV-Super 6 if you like. You need to put mold release wax on them when you layup the glass but I'm sure you are aware of that from your flat tip work. They are heavy as they are made of solid wood (2"x4" pine and 3/4" plywood). The plan view profile is the same as the curvature of the lower airfoil surface and the top and bottom are curved to a chord line from the leading to trailing edge.

Craig, After finish the flat tips and test them it would be easy to modify them to accept add on plates which extend beyond the airfoil. I'll think about that for a while but it sounds like a reasonable thing to do.

Axel, I printed the sheet and will study it for future work. Good luck with your airplane.

Bob Axsom

 

[rvmills]

Bob Mills, I could ship the streamlined 3" tip molds to you if you like and you could make a set if fiberglass tips for your RV-Super 6 if you like. You need to put mold release wax on them when you layup the glass but I'm sure you are aware of that from your flat tip work. They are heavy as they are made of solid wood (2"x4" pine and 3/4" plywood). The plan view profile is the same as the curvature of the lower airfoil surface and the top and bottom are curved to a chord line from the leading to trailing edge.

Bob,

That's quite an offer!...but if we do that, please let me pick up the shipping both directions (I'd insist! ) I'd like to see how your flat tips do in comparison to the 3" tips...if flat wins, I may stick with flat and (like you) may work on cover plates, per Craig's discussion. If we go that route, we should talk to Mike Thompson, who's done some of that work, so we can avoid dead ends he's found.

Craig, After finish the flat tips and test them it would be easy to modify them to accept add on plates which extend beyond the airfoil. I'll think about that for a while but it sounds like a reasonable thing to do.

Agreed...could be some gems hidden here...will look at the refs when home. Interesting thing to look at will be if the benefit shown in the NACA studies exist at high q/low alt regimes (parasite drag reduction) or at higher altitude/higher alpha regimes (induced drag reduction)...or both. IMHO, we're looking for the former...though if we can reduce parasite drag and a little induced drag simultaneously, that would be a big win. Probably a holy grail kinda thing, like harnessing fusion!

Axel, I printed the sheet and will study it for future work. Good luck with your airplane. Bob Axsom

Interesting Stuff Axel...will also look at that. I'm wondering how the rounded tips compare to Bob's 3" tips.

Fun discussion and experiments!!

Cheers,
Bob

 

[CraigC]

My post was just a general observation. I love aerodynamics and why things work. In the late '80s John Roncz wrote a series for EAA that I saved. I did the spread sheets on my 5.25 inch media! But please, find the NACA book. I will look up the name this evening. A lot of their work was in early wind tunnels. Although in the book they did show pictures of plates on actual aircraft. The plates grew into the winglets we see today. They are more productive at higher airspeeds.

 

[RV8RIVETER]

But please, find the NACA book. I will look up the name this evening. .

Could these be what you are referring to?

http://www.nap.edu/openbook.php?record_id=11839&page=18

http://ntrs.nasa.gov/search.jsp?R=4...=all&Ntx=mode+matchall&Ns=HarvestDate%7c1&N=0

http://ntrs.nasa.gov/search.jsp?R=1...=all&Ntx=mode+matchall&Ns=HarvestDate%7c1&N=0

 

[CraigC]

No,

The book was a complete study of wing sections.

 

[Bob Axsom]

Some thoughts about tips

There is a tendency to think of wing work from the perspective of the the cross section of the airfoil - classroom like. The air pressure above the wing is lower than the air pressure below the wing. Skipping around all of the analogical mind games to explain this in terms that can be plugged into formulas that work, etc. At the end of the wing that separation/isolation is no longer perfect. The well contoured square tip promotes flow of high pressure bottom surface air around the tip to the low pressure top surface. A flat tip does not provide the same smooth path and the flow would be expected to be more turbulent but the volume affecting the upper surface flow and the span wise lower surface flow should be less. If that is true the cumulative force on the lower surface should be higher and the cumulative force on the upper surface should be lower than on the same rectangular wing with rounded tips - more lift, thus lower angle of attack to maintain level flight and lower induced drag. Since the flat tip has zero span while the rounded tip has some span there is some parasitic drag associated with the rounded tip due to the cross section of the tip being dragged through the air with no lift contribution. The airflow over the flat tip spills over it in a turbulent manner and that disturbed air causes drag - how does that compare with the parasitic drag of the longer rounded tip? If you add tip plates to the flat tip to minimize the bottom to top surface spillage and maintain more uniform chord wise flow, nearer full lift potential of the wing would be preserved but the two sided surface above and below the wing skin adds more parasitic drag to the airplane. If the tip plates are not perfectly aligned with each other and the direction of flight the parasitic drag increases even more. Since the wing is moving through the air at a high rate of speed the flow of air around the tip is more pronounced toward the rear of the wing where upper flow separation is occurring and a vortex flow results. Perhaps the tip plate only on the top of the wing and only toward the rear would be most effective in drag reduction.

Bob Axsom

 

[C-FAH Q]

tips

Bob,
This sounds to me like another set of wing tips tests for you to do.

 

[Bob Axsom]

How about this?

Here is a thought. You curve the leading edge of the tip back in a line that sustains the leading edge function to a point behind the wing where it is joined by the curving back trailing edge so that the only common point of conflicting pressures is an aft directed point behind the wing - think NXT. Maybe that is what Van was trying for with the "Batwing" tip.

Well, I have to get ready for that West Texas race on the 5th of June at Plainview then make the left flat metal wing tip and test that mod. After that the flat metal tip would be easy to populate with platenuts for attaching various plates, fins, winglets, etc. I think there is something to be gained if patience and thought are maintained.

Bob Axsom

 

[Bill Wightman]

For 1g flight, a tip having a 1/4 ellipse at the leading edge and straight back thereafter works well. Or you can rake the tip profile out a bit aft of the ellipse to promote a clean point of separation for the trailing vortex.

I did a set of tips like this for an L39 and it set the all time course record before crashing in the final heat (wake turbulence). Aspect ratio improvement with the new tips no doubt helped with the speed gain also.

Hey Bob - ever thought about shedding those draggy ailerons for something cleaner? I think there'd be several knots in that.

 

[elippse]

For 1g flight, a tip having a 1/4 ellipse at the leading edge and straight back thereafter works well. Or you can rake the tip profile out a bit aft of the ellipse to promote a clean point of separation for the trailing vortex.
.

There's just something about that word that sends a shiver up my leg!!!!

 

[Bill Wightman]

Now, c'mon Paul - its spelled differently!

 

[Bob Axsom]

Hey Bob - ever thought about shedding those draggy ailerons for something cleaner? I think there'd be several knots in that.

I'm open to information on it. I like the feel and the function of the ailerons. I have given a lot of thought to and some experimentation with the openings and support structure drag but not the aileron design itself.

Bob Axsom

 

[Bill Wightman]

The current ailerons could be replaced with a simple plain hinge type. The plain hinge design can be fully sealed up, hinge axis aligned with the upper surface and a radiused skin on the lower surface (like on a Lancair).

The Friese's problem, from a drag point of view, is the gapped nose. I totally agree they have good feel and with enough size, plenty of authority. But that big gap allows lots of flow over the aileron nose (which it needs for aero balance) but this also makes drag. On the RV6, I think the ailerons are something like 48" long, so that's eight feet of your wing that could be improved upon.

I think it would be really neat for a racer like yourself to have two sets of ailerons, that could be interchanged for sport flying or for racing. I'm just theorizing here a bit, but maybe they could both use the same hinge attach brackets on the wing: the low drag aileron would just move the rotational center to the right point, and you could seal up the gap with speed tape for races. Hmmm....

 

[Bob Axsom]

Back at it - second tip in work - future thoughts

I'm well into the second tip now the flange has been cut off of the old semi-flat tip and I am mounting the tabs for attaching the flat metal tip. I said I would go ahead and do this even though the differential tip test showed no promise. I do not have unlimited cash or time but some folks say I have a mild case of OCD. I would like to fun a flat tip test at 6,000 d. alt. and a lower (limited by the Ozarks in this area) then switch to the 3" streamlined tips and repeat those tests.

Future thoughts. Install dimpled platenuts for mounting supplemental plates. My initial thought is a plate 2" larger than the flat tip test cut down 1/4" test and repeat until the last 1/4" then after it is tested cut down one last time to 1/8" and test that. The thought is there may be some special size that the effectiveness of the existing wing is greater than the un-tip plated wing and not nullified by the parasitic drag of the tip plate. I don't expect any miracles but if I have the test bed I may as well get the data - who knows I could get lucky.

Current log build and modify hours is 5409.0. THe number of hours in the build log at the time of the first flight was 3877.8 (that's for the complete airplane minus paint).

Bob Axsom

 

[Bob Axsom]

Comparative test completed today

I went through the detailed description of the test method (three leg, stabilized, ground speeds, WOT, Max RPM, 5,500 ft d alt., NTPS spread sheet, etc) but the system threw it out so you'll have to trust me it was a good procedure. Anyway the sharp edged flat metal tip, flush with the wingskin surface gave a speed of 179.5 knots True Air Speed. Then I installed the streamlined 3" tips and repeated the test. The speed with that configuration was 180.4 KTAS. So there will be no more use for the flat metal tips unless I decide to try the tip plates (the $100+ sheet of 0.032" 2024T3 was delivered today from Aircraft Spruce) but I am not very motivated to proceed with this. I have now made ten wing tips (5 pairs) and the 3" streamlined set on the stock wing is my fastest by test.

Bob Axsom

 

[PainterJohn]

Hmmm.. I just stumbled across this thread.. Im very interested in making some tips like this. I do LOTS!! of composite repairs, fabrication, modification and alterations.
Making ANY type of wing tip wouldnt be difficult for me, (or any composite part for that matter) but testing them on an RV would be tough, since I have little more than a pile of aluminum and nothing that actually flies at the moment
I like the part of this where Bob is making a different set of tips that would mount just like the stock ones.. Even to the point it didnt disturb the paint on the wing.
After reading this, I think I have an idea..
Im going to mock up a set in my spare time, and if anyone would like to test them for me, let me know..

And Bob, if you ding your paint, let me know I can help

 

[rvmills]

...the sharp edged flat metal tip, flush with the wingskin surface gave a speed of 179.5 knots True Air Speed. Then I installed the streamlined 3" tips and repeated the test. The speed with that configuration was 180.4 KTAS. So there will be no more use for the flat metal tips unless I decide to try the tip plates (the $100+ sheet of 0.032" 2024T3 was delivered today from Aircraft Spruce) but I am not very motivated to proceed with this. I have now made ten wing tips (5 pairs) and the 3" streamlined set on the stock wing is my fastest by test.

Bob Axsom

Bob,

I know that's a frustrating result, given all the time and effort. But perhaps they'll pay off as a foundation for testing some flat plate "fence/winglet" end caps. Perhaps we can have a good discussion with Mike Thompson and Tom Martin while we're up at the Great Canadian Air Rally, or during AVC/OSH. Keep them in the "lab", and maybe they'll pay off yet!

That being said, your 3" streamlined design may be a winner. Perhaps we can also discuss in WI or CAN your earlier offer to borrow the molds (shipping on my dime!). Probably can't do it before OSH, as its a bit close, and I'm mulling over a few items (fuel being one). But I'd be very interested in duplicating those tips and testing them verus my standard and my flat tips, and getting multiple comparative runs to compare our results. Looking forward to talking that over with you.

Bruce Hammer and I talked tips at the Taylor 150, and he had just made a set. He mentioned that many have tried tip tests, and the results are maddeningly minimal and somewhat unpredictable (I'm paraphrasing). But as you said in an earlier post, a knot here or there becomes a pretty large gain.

We'll keep working it!!

Oh, and Painter John...I'll stand in line behind Bob if he wants to test your design(s). I'd be interested...toughest part is getting them to fit the screw pattern and be a perfect match to the wing. We're all probably close, but slight diferences could be an impact. What type of design are you considering? Might be tough given the distances, but sounds interesting!

Cheers,
Bob

 

[Bob Axsom]

Once again hours of work lost

Once again hours of work lost in a detailed response trashed by this site. Short answer OK and I will not be using any other tips so Bob you are first in line.

Bob Axsom

 

[Ironflight]

Never Give Up!

Once again, Bob, I am extremely impressed to see the engineering attention to detail that you are putting into your quest for speed, and the brutal honesty you portray when you find that something doesn't work. That's good engineering, good science, and good ethics. Even though I am not tempted in the same way by the speed demon, I learn a lot from watching you report on your tests.

Once again hours of work lost in a detailed response trashed by this site.

Don't feel bad - even DR himself gave up writing anything of worth WITHIN the editing confines of the forums software....like him, I write anything more than a paragraph in Word, then cut and paste it, so if there is a site burp, I don't lose the work.

Paul

 

[GrayHawk]

Hey Orville (Bob). I think you should build your own wind tunnel.

GH

 

[elippse]

Nice work, Bob! You are a true experimenter and scientist in every sense of these words! 'Sorry it didn't seem to work out as the extant knowledge says!

 

[nucleus]

Copy Your Text Before Pushing "Submit Reply"

Once again hours of work lost in a detailed response trashed by this site.

Bob Axsom

Bob, just before you push the submit button, highlight all your work and copy it. Then if the site burps on you, just restart another post and paste all your work into it, and you are good to go.

Hans

 

[osxuser]

Since there are a couple of us that are still building, I'm going to ask a couple of questions.

If span reduction is what is helping out your top speed, wouldn't we be better off cutting span right off the bat? The Rocket does this, and the increase in wing loading is an improvement in many areas. Of course aileron/flap length ratio is affected. I figured out last week that my -4 wings are beyond the point where I can modify them to be HRII wings, so I'm going to proceed with my heavily modified -4 setup.

If minimum span is the goal with the stock aileron placement, what wing shape would be ideal? I don't recall anything that can't be made out of Carbon

 

[rvmills]

Once again hours of work lost in a detailed response trashed by this site. Short answer OK and I will not be using any other tips so Bob you are first in line.

Bob Axsom

Bob, I surmize that the lost post may have been (at least partially) headed my way...so let me thank you for the time and let ya know I empathize with the pain of seeing it go away, followed by a "no, No, NOOOOO". Been there! I'll scope out the time availability ahead, and see when my glass mentor might be available to watch over me, then get in touch about getting the molds...and thanks much!!

Bob, just before you push the submit button, highlight all your work and copy it. Then if the site burps on you, just restart another post and paste all your work into it, and you are good to go.

Hans

I've do this now, after a few of those "no, No, NOOOO" moments!

Since there are a couple of us that are still building, I'm going to ask a couple of questions.

If span reduction is what is helping out your top speed, wouldn't we be better off cutting span right off the bat? The Rocket does this, and the increase in wing loading is an improvement in many areas. Of course aileron/flap length ratio is affected. I figured out last week that my -4 wings are beyond the point where I can modify them to be HRII wings, so I'm going to proceed with my heavily modified -4 setup.

If minimum span is the goal with the stock aileron placement, what wing shape would be ideal? I don't recall anything that can't be made out of Carbon

Steve,

My -6 wings are clipped like Rocket wings, and I modeled my flat tips after Wayne Hadath's, whose F1 has the same wing as I do (non-EVO). He appears to have gained 4 knots with his flat tips, and my initial tests show a gain of perhaps two. I think it'll take more tests and races to see what the speed delta really is, and at which altitude the flat tips are optimal for speed.

I've been told that the clip of the Rocket wings is to reduce the bending moment to allow for the same g-limits at a higher Rocket weight. There may be more to it as well...perhaps more complex engineering-wise that my simple description. Clipping a normal RV (non Rocket or Super-X) for speed is an interesting thought, and one I'd want to have checked out by pros. But your Q brings up an interesting thought...how short is just right, and how short is too short? Along with the question of what tip shape is the fastest, the optimal span question is one that seems bound in the deep recesses of aerodynamic witchcraft. I've not found any formulae for span to weight ratios, aspect ratio to weight relationships, etc. Perhaps Paul or others have some thoughts on that, and your question on optimal wing shape. And what does your Formula racing experience bring to the table in this area Steve (not sure if wing mods are allowed in Formula, but thought I'd ask).

Cheers,
Bob

 

[Snowflake]

Don't feel bad - even DR himself gave up writing anything of worth WITHIN the editing confines of the forums software....like him, I write anything more than a paragraph in Word...

And you wanted to *decrease* the chances of losing something?

I find the safest thing to do usually is to type out my response here on the forum, and before I hit "submit reply" I do a [Ctrl-A][Ctrl-C]. Select all, Copy, Post. That way, if the post does die for some reason, at least my hard-written verbiage is still in the paste buffer. You can start a new reply, and just paste your previous work in.

At least, that gets around problems with the page timing out or the site getting busy, which seems to be 98% of the problems i've had with this site and others.