WASH IN AND WASH OUT

[John Hitchcock]

Hi chaps

can someone bring me up to speed on how to build in wash in and wash out or twist in after covering?
also what are the benefits to adding above

[Mike Rolls]

John
It's always best to build in wash-in or out at the construction stage, especially if the structure is a rigid one - geodetic open structure, D-box, or the like. Easiest way is simply to put appropriate packing under the LE or TE so that it is built in from the outset.
As for adding it after covering - a lot will depend on the rigidity or otherwise of the structure and what you use for covering. Tissue and dope will normally respond to holding the structure in th ejet of steam from a kettle and twisting to achieve the amount of warp required - in fact going a little past what is needed will often be necessary with the structure 'springing back' a bit. With film covering simply substitute the covering iron or hot air gun for the steam - in this instance two pairs of hands are a definite help.
As for benefits - washout means the the panel affected will have a lower angle of attack that the rest of the wing and thus the stall at the tip is delayed - the same amount of washout at both tips is often used both in models and full size a/c for this purpose.
The other effect of both wash-in and wash-out is to roll the a/c. Wash-in will raise the tip at which it is applied, wash-out will depress it.
Wash-in will have the effect of stalling the tip before the reat of the wing and is normally to be avoided. The exception is in trimming free flight models were you will often find models with polyhedral or tip dihedral having a small amount of wash-in on the inner panef of th ewing which will be on the inside of the trimmed turn, often combined with a small amount of wash-out on the tips. This reduces the likelihood of the trimmed turn becoming a spiral dive should the model be disturbed in flight - I once heard somebody describe the technique as giving the model 'Something to lean on' whihc is a good analogy.
The other use of wash-in in trimming relies upon the fact that it both increases the lift and the drag of that panel and that this effect vasries according to speed. When a model is flying relatively fast it is at a low angle of attack and the extra lift (wash-in)predominates. When flying slowly the extra drag predominates. This at speed wash-in on (say) the right hand panel will tend to produce a left hand turn; on th eglide, trimmed near to tha stall, the extra drag will tend to turn the model to the right.

HTH

Mike

[John Hitchcock]

Thanks for that Mike , I'v only just glanced at it so far , ill print it off and
study intently during morning ablutions!

cheers my friend

JH

[Mike Rolls]

John
You are very welcome
Mike