Additional set of racing wing tips

Bob Axsom · #1 06-16-2009, 10:46 AM

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

klovning · #2 06-16-2009, 11:55 AM

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

Bob Axsom · #3 06-16-2009, 02:03 PM

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

Alan Carroll · #4 06-16-2009, 02:55 PM

Quote:

Originally Posted by Bob Axsom

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

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.

Bob Axsom · #5 06-16-2009, 04:26 PM

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

AX-O · #6 06-16-2009, 05:13 PM

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.

Bob Axsom · #7 06-16-2009, 07:22 PM

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

Bill Wightman · #8 06-17-2009, 12:02 PM

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...

Tom Martin · #9 06-17-2009, 01:52 PM

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.

Bill Wightman · #10 06-17-2009, 02:01 PM

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