Wing tips?

[Simon Chaddock]

Whilst there are pros and cons for various wing sections are there any 'rule of thumb' guides for the shape of a wing tip?

I note that a form of 'crescent' tip is now common on some props and they do seem to a bit quieter than square cut or rounded tip blades.

Of course quiet does not necessarily mean more efficient and most wings are travelling rather slower than a prop tip.

The winglets now fitted to most passenger jets are only supposed to be worth their weight on long flights and at very specific speeds yet some full size high performance gliders now have them as well.

So are the benefits of special wing tip shapes on models fact or fiction?

[Biggles' Elder Brother - Moderator]

Basically all wings would be far better off if they didn't have any tips! Wings work best (in terms of lift)when effectively infinitely long.

There is no such thing as the "perfect wing tip" if there was all planes would have it! Every wing tip shape is a compromise.

Square tips offer benign stall characteristics - the whole wing more or less stalls at the same time - no tip stalling tendency. But they have very strong tip vortices so are very drag prone.

Pointy wing tips have weak tip vortices - so a lot less drag. But they are very prone to tip stalling and planes with pointy wings can be a right handful.

Delta wings sort of have the best of both - low drag in normal configuration with good low speed manners. But they are not good lift providers for the area generally.

Rounded wing tips are a "halfway house" and attempt to avoid tip stalling behaviour whilst at the same time reducing drag.

You pays your money and takes your choice! It depends a lot on the duty the aircraft will be expected to do. Fighters need speed and the designer can usually assume that the pilot is competant so a low drag "tricky" wing is OK. Small private aircraft are often flown by relatively inexperienced pilots and speed isn't really an issue, so a nice well behaved (if somewhat draggy) wing is in order. And so on.

BEB

[Erfolg]

In some respects, the emphasis on wing tip shape can be misplaced.

If the model or aircraft is flying fast, with very little incidence, the losses from the wing tips will proportionably low compared to those from wing profile, body etc.

I have tried winglets (other than on a flying wing) the once, did it revolutionise the model performance. I cannot tell you one way or the other. It did help with handling as a rudder elevator glider, in that a turn would be held. The downside, was that normally a glider is tipped out of lift, so I would hold some rudder in. With winglets, the thermal may have died, yet the circling feed back was not there.

So it seems it is particularly that if wing tip shape does really matter, it is of low speed handling when landing, or a single airspeed.

Many elliptical and crescent wing tips, particularly where there is a lot of aper from root to tip, benefit from washout.

The real issue for modellers is that we seldom know, one way or the other.