I'm building a GP Ultimate Bipe. The ailerons are full span and end at the flat wingtip. I think that these would be much more suceptible to flutter because of interaction with the aileron and the wingtip vortex. I'm planning to cut about 1" off the aileron stock and glue it in place at the wing tip. Tell me why I'm right or wrong or whether it even makes a difference. I am an AE so don't be afraid to be technical.

I lost an original Jensen Ugly Stick when I did just the opposite of what you are planning. Fluttered the wing in two 40 seconds in to the first flight. Go for it

Probably a good modification. On some of the Joe Bridi Kaos series pattern planes
he recommended this same modification when using large engines and if the plane
would be flown at higher speeds. Must be something to it.

tommy s

I don't fly pylon, but every pylon racer I've looked at was set up like that. Sounds like a good idea. Depending on how big your non-moving portion is, you will loose some aileron effectiveness at speed, but it might not matter. (I once cut about 2" off the tip of each aileron as a field repair (long story), the reduction in roll rate was higher than I expected.

The aileron is actually pretty ineffective when extended out to the tip (the tip portion is ineffective) because the lift near the tip goes to zero. So, the portion of the aileron near the tip is unable to modify the flow in a significant way to roll the craft.

Unless the GP is not up to their usual standards, you have nothing to fear.
My SIG Ultimate uses 4 ailerons, full-span, the scale plane full span, but bottom only.
Keep the control system stiff and tight.
Servos part-way out the span connected directly, not torque rods.
Chip Hyde's LARGE Ultimate uses full-span.
A light wing structure and a heavy aileron, or a long-chord aileron can join together into flutter, but the Ultimate, most likely not.

While you're certainly correct, my question is, HOW near the tip? And how does that interact with the lever-arm effects that cause an aileron near the tip of the wing to be more effective at producing roll rate than one near the root?

(I went back and found a picture of the plane where I cut the ailerons off at the tips. It was actually quite a bit more than 2" from the tips, more than I remembered doing. Which could easily result in a large drop in roll rate with out conflicting with the very tips being ineffective).

I would say that 10 to 15% (of the half-span) away from the tips is sufficient. You want the ailerons out as far as possible to produce a good rolling moment, but no so close that they are just dead weight (there is likely added weight at the tip due to needing a stronger structure there to accomodate the aileron).

One reason to extend the ailerons all the way out to the tips is simply to make the design of the craft simpler. Consequently, the construction is simpler. So... there are arguments both ways.

Thinling on this, the TOC planes for example are required to be scale.
This means to-the-tip ailerons.
Now going to the full-scale world, the TOC originals compete for the big bucks!
If there were an aerodynamic (1st place money) advantage to truncating the aileron span, it would show up there.
OTOH, on planes where manuvering is the last thing on anyone's mind; big transports, outboard ailerons are locked out once the flaps go up.
The well-known torsional effect of an outboard aileron causing reversal in the roll direction could be countered by building a stiffer wing.
The MBA in Accounting that has the final design approval no doubt says... "Nope! Stiff costs money. Lock those ailerons."
Many of the more modern Goodyear racers I've seen use inboard ailerons for the same reason. All the plane has to do is fly fast, turn left. And not have a tip-stall while doing it. The more advanced planforms on Nemesis, Madder Maxx, even "No it's not a Shoestring" (Ray Cote was quite upset when I asked. ) feature inboard ailerons.

Inboard ailerons work well on fast planes and are effective enough to get the job done. Good reasons for inboard ailerons on fast models are aerodynamic (reduces tip-stall tendency) and structural/mechanical as they are less likely to flutter if the aileron is fairly short and isn't attached to a long torque rod. Aileron flutter becomes a big problem in fast, thin-winged models. I've gone so far as to tape the ends of my ailerons so they can't move. It works fine on pylon racers to just twist the ailerons. An Ultimate probably wouldn't have such problems though. Make the ailerons light and stiff and you should have no troubles.

-David

I think that the bigger issue in full size aircraft is that stiff is heavy. I think the structures/aerodynamics guys would get hold of the aircraft design before the MBAs, and require that the weight be minimized

Regarding the full span ailerons, I think it depends on how you want to fly your plane. If you want a fast "pattern" type plane, then cutting them short probably makes sense. In this case you'll probably be using a high-ish pitch prop, and be doing lots of "horizontal" manuvers.

If you want a slower aircraft with more vertical manuvers, then full span ailerons and a low pitch prop are probably the way to go.

I think it all depends on what kind of flying you want to do with it.

Tom

That's right, different ailerons for different airplanes..
You won't see you Forumla 1 racer doing this..
.
(Tony F's electric powered Funtana S)

Let the ailerons go to the tip... this is an aerobatic plane not a racer... I don't agree that the outboard part of the aileron doesn't do anything. The roll induced angle of attack will be the greatest at the tip and any fixed trailing edge there will induce a stall which will decrease roll rate. The aileron at the tip may not be helping to roll the airplane but it is getting out of the way to prevent the tip from stalling (aileron deflection reduces the effective angle of attack). If you want to get rid of some useless aileron start at the root where it has no moment to work with. The only reason to have ailerons at the root is for hovering.

Ty

P.S. the airplane in the picture behind me is my Pitts S1S with an early version of the Ultimate wing... this version only had half span ailerons and used tip plates. It rolls just as fast as the ones with full span ailerons.