Aerobatics - why biplanes?

[Justiciar]

having ailerons farther away from fuselage, what do you mean by more leverage

I would imagine this is a reference to the moment of inertia of the aircraft, i.e the rotational inertia and the torque applied to the aircraft, which is a product of the force exerted x the distance from the axis of rotation. So, the same force applied at a greater distance from the axis of rotation produces a greater torque and therefore a faster role rate. Looked at another way, the same force applied close in will produce less torque than the same force applied further out, meaning that the rate of change of angular momentum (role rate) will be less the closer in the ailerons are located. This ignores the effect of prop slipstream, which will of course be greater one side of the aircraft than the other.

The combination of wider wings and longer ailerons will generally produce a faster role rate than the same aileron on a shorter wing, all other factors being the same.

[mmgreve]

So, the same force applied at a greater distance from the axis of rotation produces a greater torque and therefore a faster role rate

Why would more torque in your mind make the roll faster? It just makes it stronger in my book. The closer you move to the fuselage, the same amount of aileron deflection would amount in a quicker roll (though less torgue, so might not happen due to inertia)

Since most aerobatic machines are pretty light, I would assume inertia not to be an issue.

[Zulu Alpha]

inertia is an issue. Getting a 1000lb aircraft to rotate and then stop quickly requires quite a high torque.

If you have ailerons near the fuselage they will create drag but lower rotation force (torque) on the fuselage. If you have them further away they generate the same drag but higher rotational force.

There is also the strength issue. the forces on the aileron can be quite large so you need a strong wing to mount them on. Pitts acheive this with short wire braced wings . Carbon monoplanes achieve it with carbon fibre spars.

So you would need bigger ailerons on a Pitts which would require a stronger wing and would create more drag.

How does Pitts S1T compare with Laser200 and Edge 360 in terms of weight, roll rate and wing span

Pitts is about 850lbs and Laser/Edges are about 1025lbs

Pitts S1T and Lasers are about 300-360 degrees per second. Edge is about 420 degrees per second. These are all speed dependent so these are for about 130/140kts

Not sure about a Pitts wingspan but the Laser and Edge are about 25 ft.

[stiknruda]

S1T wingspan is 18' (+/- a couple of inches!!)

S2 wingspan is 20'


Stik

[Happyloop]

Biplane looks graceful

Biplanes are draggy, rather slow, light and strong.
Biplane are quite good at knife edge flight and rotations on the 3 axes.
Even a vintage biplane with rather slow roll rate and low power is a great aerobatics performer due to his natural grace...
Enjoy the Bucker Jungmeister (50% scale)
https://www.youtube.com/watch?v=tnB7L-O2VJo

The Buckerholic

[photofly]

Originally Posted by Zulu Alpha

Roll rate is a consequence of the size of the aileron not the short or long wingspan. Having aileron further away from the fuselage gives more "leverage" but aileron near the fuselage gives faster roll at low speed when its in the prop slipstream.

Moment of inertia scales with distance squared; rolling torque scales as the distance. That suggests that with equal aileron area an aircraft with two short wings will roll faster than an aircraft with one long wing, by a factor of 2.

Also you'd have to take into account how the mass to maintain sufficient strength goes up with the wingspan. Long wings will have higher bending moments so need to be stiffer, and therefore heavier, with higher rolling moment. Again that mitigates against one long wing and in favour of two short wings, for good roll rate. That's before you take into account the extra stiffness of a braced truss arrangement available to a biplace and the reduction on weight that allows.

The ailerons on a Pitts are still a long way outboard of the prop.