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Bob Axsom

Well Known Member
I've been wrestling with the west bound AirVenture cup race endurance requirements and I've decided to make another set of 3" wing tips to mate with the outboard end of my tip tanks. I talked to Mike Thompson at the Tennessee Valley Air Race on Saturday and learned that he gained 3 to 4 kts based on information gained in multiple test flights with his cut off flat plate capped tips. This reduced his span by about 1.5 feet. When I take off my stock tips and 9" wide tip tanks and install my 3" tips the span is reduced by 3 feet. I measured a 3 kt gain in speed with this configuration. My conclusion is the stock Hoerner tip design is the main source of drag and whatever flight regime it provides benefit to is unimportant to me and the tank extension is not a big drag contributor. We shall see - I just ordered the cloth.

Bob Axsom
 
It'll be interesting...

I look forward to see picts of the build and the #'s of the speed gain/loss.
 
Benchmark

For purposes of a reference for judging performance change I will subtract 3 kts from the best current configuration speed of 184.4 kts to establish a 181.4 kt baseline. Anything over that will be a gain over the tip tank and stock tip configuration.

Bob Axsom
 
Go tipless?

Bob Axsom said:
I've been wrestling with the west bound AirVenture cup race endurance requirements and I've decided to make another set of 3" wing tips to mate with the outboard end of my tip tanks. I talked to Mike Thompson at the Tennessee Valley Air Race on Saturday and learned that he gained 3 to 4 kts based on information gained in multiple test flights with his cut off flat plate capped tips. This reduced his span by about 1.5 feet. When I take off my stock tips and 9" wide tip tanks and install my 3" tips the span is reduced by 3 feet. I measured a 3 kt gain in speed with this configuration. My conclusion is the stock Hoerner tip design is the main source of drag and whatever flight regime it provides benefit to is unimportant to me and the tank extension is not a big drag contributor. We shall see - I just ordered the cloth.

Bob Axsom
Click to expand...

Bob,

Have you considered just taping off the rib holes and test flying with no tips installed? My personal theory is that wing tip design is largely cosmetic, whereas decreased span is well-documented to reduce drag.

Congratulations on doing well in Courtland; I had hoped to participate but couldn't make the schedule work out.
 
No I hadn't considered taped off end ribs

No I hadn't considered taped off end ribs. I think the sharp edges would cause drag related to the air disturbance at its interface with the end of the wing. Mine extend out 3" at each end but the top and bottom are rounded to the upper and lower wing surface so the frontal cross section is less than a 3" extension of the wing (6 inches for both tips). The profile seen in the top view of the tips is the same as the bottom of the airfoil so it should cause a slow compression and relief in flight. I think that is as good as I can do. If someone does try it I would certainly like to know their findings.

Sorry you couldn't work in Courtland - it was a good race.

Bob Axsom
 
Bob,
I was also thinking of taping the end of my wings once I am flying. I will cut and shape a piece of foam then place it inside the end of the wing. After that a will use 200 mph tape to hold the foam in place. I looked at the F1 Rocket Evo wing and it looks like they have a flat plate. The test will be easy, cheap and provide another data point. However, that won't happen for a long time. Maybe you can take a look at the said configuration. The results may surprise us.
 
Can't do it

It won't pass my current ugly acceptance test. I know it shows a flaw in my devotion to speed but I just can't do that to my beautiful little bird. Tom Martin has done something like what you describe to his EVO Rocket and it is the fastest one I know of so it evidently works but I have to approach it a little differently. I'll wait and see how it works on yours. If it is fast enough I'm sure as form follows function, it will become beautiful in my eyes as well.

Bob Axsom
 
Hoerner not the issue here

Bob, the Hoerner tip has a long history and isn't known to be a draggy design.

Your post indicates that you took span away, modified the tip shape, and noted a small increase in speed. The big deal here is that you reduced wing area. At high dynamic pressure (ie: low and fast), wing drag is dominated by parasite factors, and much less by induced drag. So, cutting the span down and flying the "worm burner" profile yielded a small gain.

To isolate the influence of the wing tip, keep all other factors equal and test variation in tip design only.

Today, with the advent of 3D CFD analysis, alterations in tip planform are more commonly used for drag reduction.

The last pure tip design I did was a couple years ago, for an air racing L39. We took the tip tanks off, and fitted a racing tip to the plane. Designed for approximately 3G flight, that tip worked like magic. It posted the fastest lap time for an L39 ever, but crashed while lapping the field.

I can help you with a better tip shape if you like...
 
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When I first flew my EVO I had made some really nice P51 style tips. They looked great and took about three weeks to build. However the kit had come with these plain aluminium flat tips and I just kept looking at them and wondering. So one day I took one of my tips off and replaced it with the flat tip and went for a flight. It was an interesting flight as my made up tip, on the left side, was constantly lagging behind the other wing. The plane would yaw to the left for a while and when things got too far out of wack it would straighten out and the process would repeat itself. Clearly my tip had more drag then the stock flat tips.
Off came the other tip and since then I have been flying with the flat tips. I do not have any definitive numbers but I do know that my glass tips added drag. Although the difference in wing span was slight I did feel that I gave up a little in the stall speed by going with the flat tips. Well I thought to myself ? if it worked for the wing it should work for the tail". And so I cut off all of my glass tips on the tail surfaces and with a bit of foam and glass made the tail tips straight as well. There was absolutely no difference in speed. An engineer friend explained that there is much less drag caused by lift on the tail and so he was not surprised that I did not see a difference with that change.
However now the tail matches the wings and I have a distinct looking rocket.
 
Tom, flat plates seem (surprisingly) effective. I've known other Rv'ers to take the Hoerners off and get good results. But, again, that's taking quite a bit of area off the wing (of the RV). I like the look on your Rocket!

One thing to consider: Van added what I call the "Batmobile" tips to the RV8, which increased span and area from the old design. Guess what... they improved cruise performance.

Bill
 
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ARGH!!!!!

OK, I'll be calm...

The test I am making is a test of the change in tips only. This configuration will have the 9 inch tip tanks at the end of the stock wing and out board of them will be the new tips replacing the stock Hoerner tips.

When I developed the 3 inch span tips that I have raced with for the past two years they were installed directly on the ends of the stock wings and the tip tank (1.5 ft wing extension) and the stock Hoerner tips (2 ft wing extension) were removed for a net reduction in span of 3 ft. The speed increase was 3 kts. Regardless of what aeroheads seem to think I can tell you from many experiments on my specific airplane that 3 kts is NOT A SMALL INCREASE IN SPEED!

Mike Thompson flying his RV-6 initially beat me in the SARL cross country air races because his plane was faster! Since that time we have both modified our planes and since my wing tip modification he has not beaten me (many races). Late last year he modified his plane with short tips and tip plates and reported that several tests had shown an increase of 3 to 4 kts which is more than I claim but the test methods are completely independent and may be an apples to oranges comparison. The point is he appears to have made a speed gain similar to mine with a change limited to the tip envelope. I estimate the span reduction to be on the order of 1.5 ft.

I am quite happy with my tip design but The race next month could end in fuel exhaustion if the western headwinds are higher than normal. By calculation I can make it if I sustain 160 kts ground speed but there is the long taxi in start order behind 30 or 40 airplanes and a 30 second or so interval between releases (if there are no incidents) and at the finish we have to make a very low pass (~50 ft or so) and climb back up and fly several miles to Fond Du Lac for recovery with a temporary tower that is handling all of the racers and all of the AirVenture visitors landing there. In short it is risky.

Based on Mike's results I think it is possible that most of the gain is from getting rid of the stock Hoerner tips with that big up and down swooping outboard edge with respect to the airfoil chord line, the squared off trailing edge with a lot of unnecessary plan view surface area aft of the center of lift (like a big flap). This may not be a great success but my real world observations and fear of running out of gas drive me to give it a try.

No, don't design me a tip - this is already beyond the conceptual stage.

Bob Axsom
 
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Tip design and cruise performance

Bill Wightman said:
One thing to consider: Van added what I call the "Batmobile" tips to the RV8, which increased span and area from the old design. Guess what... they improved cruise performance.

Bill
Click to expand...

Bill - do you know of any testing that verifies this? I've talked to one racer who switched to the newer tips (from the Hoerner tips) and said it made no measurable difference. I don't believe Van's claims any specific benefit?
 
Alan, Vans designed, tested, and implemented the use of the new tips. I did read something about specific testing done on them, in a years past issue of the RVator. Wish I had it right in front of me, but I'm away on a trip today.

I'll take a look when I can, and pm you if I find it ...
 
Depends on the mission

As Bill Wightman said, the main advantage that people are seeing with the cut-ff tips has to do with reduced wetted area. This is because they are racing at low altitude and 1-g flight, where induced drag is insignificant, but wetted area skin friction is key.

You would get the opposite result if you compared max speed at 8500 ft, where induced drag starts to matter more.

You would get even more benefit from added span if you were pylon racing at 3 g's ( also as Bill observed)

Modern research has shown that the best tips are ones that have the maximum span occurring at the trailing edge, and reasonably smooth radiuses on things. But this tip shape is separate from the trade-off between wing span and wetted area.
 
It also depends on all of the existing airplane parameters

There is a tendency for design experts in a field to talk in general design philosophical terms within their comfort zone instead of hard analytical facts about the specific design. Of course it depends on the mission; if I were designing a sail plane I would want long thin tapered wings. I have an existing wing on an existing airplane that has been shown in field modification testing on two similar airplanes (RV-6 and RV-6A) to have a wing tip that is the enemy of maximum level flight speed. Two known airplanes have modified the tips and both increased their speed by 3 to 4 kts based on this change alone. One reduced the existing wetted area by more than 16 feet greater than the other but the increase in speed was similar. This experiment is to reinsert this constant chord, constant thickness and constant airfoil 9" span section back into each wing and measure the results. To do that I have to get my molds back down out of the attic and lay up two more tips because the interface with the stock wing is different (aileron, closure/stiffener and hole pattern) than the interface with the outboard end of the tip tank (no aileron, no closure/stiffener required an different hole pattern).

Last year I flew this specific airplane in the AirVenture Cup Race from Mitchell, SD to Oshkosh at 9,500 ft with one weather required descent to 5,500 ft for several miles followed by a climb back to 9,500 ft and it did very well with an average speed for the race of more than 223 mph. With the east bound race and the ability to land at the airport finish line for fuel (not allowed in this years west bound race), insufficient fuel for an all out maximum power run was not a great risk. The results were good for our RV-6A but the main point is to illustrate that it is not mushing along at a high angle of attack to generate lift with the related induced drag with the 3" non-stock tips at common RV altitudes.

Bob Axsom
 
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Bill Wightman said:
Alan, Vans designed, tested, and implemented the use of the new tips. I did read something about specific testing done on them, in a years past issue of the RVator. Wish I had it right in front of me, but I'm away on a trip today.

I'll take a look when I can, and pm you if I find it ...
Click to expand...

Actually I just found an article in the 1st 2001 RVator. It says that when they tested sheared tips in the blue RV-6A prototype, they couldn't find any "noticeable differences" in stall speed, top speed, or directional stability relative to the Hoerner tips.

The advantages they did mention for the sheared tips included aesthetically more pleasing, molded-in provision for lights, and convex upper surface (more resistant to warping).

I'm not sure if these are the same as the currently supplied tips though, or if there was another article with more precise testing.
 
Climb rate

Bob Axsom said:
Last year I flew this specific airplane in the AirVenture Cup Race from Mitchell, SD to Oshkosh at 9,500 ft with one weather required descent to 5,500 ft for several miles followed by a climb back to 9,500 ft and it did very well with an average speed for the race of more than 223 mph... The results were good for our RV-6A but the main point is to illustrate that it is not mushing along at a high angle of attack to generate lift with the related induced drag with the 3" non-stock tips at common RV altitudes.

Bob Axsom
Click to expand...

Bob - I've been meaning to ask how much climb rate (if any) you gave up with the shorter tips? Can't have been much, based on your race performance.
 
Some I'm Sure

My ROC seems to be around 1,000 fpm (higher at low altitudes) with the short wing. I have never had the stock tips on the airplane without the tip tanks so I can't give a good comparitive response. Once I get the new tips on outboard of the tip tanks then I should be able to give a tip difference only response. With the short wing and the 3" tips I have noticed the takeoff roll is longer (3' shorter wing span) the climb rate seems lower and the roll rate seems higher. As you know I am mainly focused on speed as long as everything else is OK. I tookoff behind you in the Taylor 100 race this year and I was IMPRESSED with the way your RV-8 tookoff and climbed out - much more spirited than our RV-6A.

Bob Axsom
 
First layer done

2009620004.jpg


Bob Axsom
 
Looks nice Bob. That is exactly the same style tips I will make for my RV-4 once it is flying.
 
I think you will be happy with it

I copied the bottom of the wing airfoil for the shape of the tip as seen from above. At the leading edge the tip curves back immediately from the end of the metal wing skin to the maximum span point which of course is co-located with the maximum depth of the lower surface of the airfoil. The trailing edge is very thin (maybe 1/8") and the tip trailing edge span is about 3/4". I made the stiffener out of 0.016 2024T3 aluminum sheet from Aircraft Spruce an I installed it with pop rivets so the mounting flanges could be bent into the tip and not leave a pocket or recess in the tip by the aileron.

I added the second layer of fiberglass to the new tips tonight.

Bob Axsom
 
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