Aerodynamics & Aileron chord

[ChrisE]

I have always believed that with a full strip aileron the part near the fuselage did little more than act as a flap and increase drag.

I notice however that some models, including low power ones such as Tod Long's Tiny X, use ailerons that are widest near the fuselage and then become narrower as they taper toward the tip. These low power models are those where I would have thought extra unnecessary drag the most undesitable.

I recognise styling as a driving force & I guess that you could possibly argue something about the strength of the aileron as the drive for the aileron is located near the fuselage - although I don't believe it.

Is there a good reason for this practice I believe it to be the exact reverse and have the narrowest aileron near the centre & the widest near the tip! The ribs would of course need to be designed to give the correct overall profile.

I recognise the desirability of strip aileron is more about ease of construction than anything else.

[Ollie]

If one divides a full span aileron equally between a root half and a tip half, the tip half will produce about three times the rolling moment as the root half just because the tip half of the aileron has an average moment arm from the center line that is about three times longer.

The drag doesn't increase much for deflection angles under roughly 4 degrees but goes up sharply for deflections much greater than about 4 degrees. If the rolling moment of inertia is kept small with light wing tips, less deflection will be required, adverse yaw will decrease, roll rate will increase and maneuvering drag will be reduced.

[ChrisE]

OK that is what I understand & as far as I can tell it doesn't explain the logic of having a SMALLER aileron chord at the tip than at the fuselage. This is particularily true in a model where very large control surface deflections - and hence very high drag caused by those deflected control surfaces - are normal. A lot of free & apparantly useless drag doesn't appear to be helpful.

[Ollie]

I don't think of designs as logical or not. I think of them as the resolution of conflicting objectives for higher purposes which the designer is free to prioritize. The goodness of a design can only be judged in the context of the purpose and priorities of the designer. For example, contest rules can serve to unify the purpose of various designers while still leaving them free to set some of the priorities. Lacking some set of unifying purpose and priorities it is quite difficult to judge designs objectively.

Some designs are done with humor, art or novelty as the controlling purpose and priority. Where is the logic?

If you really want to know, ask the designer what he had in mind.

[Sparky Paul]

Long thin surfaces actuated at one end, such as the typical strip aileron are prime candidates for aerodynamic/structural flutter.
(Goldberg Tiger II is well-known for this.)
Tapering the surface reduces the unsupported mass out along the span, lessening the possibility.
Another reason would be when the wing shape itself is tapered. The taper of the aileron will generally be a percentage of the chord, so it tapers also.
Note on the flying wings styled ala ZAGI™ the elevon's chord increases towards the tip. I've tried them the other way, and the wider tip works better.
And washout can be sanded into the surface, tapering it.

[ChrisE]

Ollie,
Thanks for the philosophy. I am just trying to understand where function ends and aesthetics begin. The best balance between them is a matter for the individual. Science is not.

Sparky Paul,
Tapering the aileron and its effects on flutter is something that I hadn't considered.
Funnily enough the tapered wings that I have built over the years have never had tapered ailerons but I can see your logic. I still wonder if the matching aileron taper is to keep it "looking right" more than it is to do with the effectiveness of the total wing system.

I feel an experiment coming on.

[Karl B²]

On a plane designed for prop-hanging aerobatics, aileron area within the propwash is the only part controlling roll when hovering. While not as much of a factor for a boom fuse like the Tiny, minimizing the gap between the aileron root and the fuselage is another step to increase the effectiveness of the surface. Larger gaps, just like hinge gaps, bleed the pressure through more easily.

Karl

[ChrisE]

Ah now that really does seem to make sense. The Tiny X flies so well that I hoped there was a functional explanation!