Aerodynamics question

[Tim Kearsley]

Hi all,

Being fairly new to this fantastic hobby, I've got a very basic, theoretical question. When you apply a bit of aileron to commence a turn, if you then centre the Tx stick again does the aeroplane more or less keeps the bank on and hence keep turning? My experience suggests that it does, but I'm puzzled as to how this works with a model that has dihedral. I thought that the dihedral tended to make the model recover from changes in bank, due to the differential lift on each wing when they are not level?

I've not had much real stick time yet but I've done quite a bit on a simulator and I've noticed that the bank does seem to slowly "evaporate" on some models as the turn progresses. Is this the reason why?

Comments very welcome!

Cheers,

Tim.

[Alistair Taylor]

Good question.

Dihedral angle is part of the answer, as is whether the model has a high or low wing (or the pendulum effect), and sweep back.

In brief;

1 - Dihedral tends to make a plane want to roll back to level if it's banked over

2 - A high wing model will usually want to roll back to level if banked over, as the center of gravity is below the center of lift - the plane acts a bit like a pendulum (hence pendulum effect). Conversely a high-wing plane will usually want to continue rolling.

3 - Sweep back also gives stability, and a plane with sweepback will usually want to roll back level if it's banked over.

See this more detailed explanation;

http://selair.selkirk.bc.ca/aerodynamics1/Stability/Page5.html

Generally speaking, you want a model plane to roll back to level from banked flight in order to make it easier to fly. So with a low winged plane, you would add enough dihedral to ensure that this happened.

If a model is neutrally stable (i.e. once banked it stays banked) it requires a lot more concentration to fly it, although this may be a desirable characteristic with aerobatic planes that you want to stay where put.

If a model is negatively stable, you're asking for trouble, as it will want to diverge from straight and level flight ALL the time.


AlistairT

[Tim Kearsley]

Thanks for confirming my own thoughts Alistair. I hadn't considered how much the wings are swept back as a factor - that's most interesting. I guess the innate stability of a high-wing model with some dihedral is useful when you are learning, though I must admit that, even though I'm still a novice, I do find occasionally that the way a trainer "unwinds" the bank during a turn can be a nuisance!

Many thanks for your input.

Cheers,

Tim.

[Peter Burry]

It's not really true that dihedral corrects banking. What it does is to stop side-slip. If a model banks without turning then it will side-slip. If the model has dihedral then the effect of the side-slip is to give a greater angle of incidence to one wing than to the other, and this rolls the model back, correcting the bank.

But if the model turns without banking (as in rudder-only models) then it will side-slip outwards. Now the effect of the dihedral is to give a greater angle of incidence to the wing on the outside of the turn, making the model bank towards the turn. So dihedral can either reduce or increase the angle of bank; either way, it does it to reduce side-slip.

Sweep-back on wings has a similar effect. If a model with sweep-back side-slips, then the wing facing into the slip will generate more lift than the wing facing away from the slip, giving the same restoring effect as dihedral.