Scale Soaring UK

As a general rule when do you consider adding differential to the aileron settings on lower aspect ratio wings as might be used on PSS models? Is there a ratio/wing camber/section where the need for the differential should or may be added or indeed not be used?

It will be good to hear a few replies or is the answer set in stone?
M

I generally program 2:1 differential as a starting point.

After the maiden flight I will try a few rolls and watch how the model reacts. If there is adverse yaw I generally increase the differential and if there is proverse(?) yaw I decrease iy until I am comfortable with the way it flies.

Martin.

I agree with Martin, works for me. I find that the width of the aileron very much controls the required differential along with the flying speed of coarse.

For a low aspect ratio model such as PSS you will not need nearly as much differential as on a typical glider with high aspect ratio. In fact on ultra-low aspect ratio such as a PSS jet you will likely be at 1:1 or maybe 1.1:1. Too much diff on that sort of model is going to make the wing dig in and the nose drop in a turn.

Simon

Thanks for the input, It may be the nose down attitude I’m looking at and not stall on sharp turns? I’ll try backing off on the differential. The article link above also suggests adding wing tips which I’ll try.
Cheers
M

With a low aspect ratio glider, the effect of too much differential causing the wing to dig in has two effects - firstly the nose drops causing a natural reaction to add up elevator. Secondly, differential is added to counter adverse yaw - in this case the excess differential will cause a yaw in the direction of the inside of the turn. This combination of up elevator and into turn yaw are perfect conditions to cause an incipient spin! Even if you don’t get that extreme result, when the wing digs in and the nose drops the natural application of up elevator will bring the nose back up as the turn continues, and rather than a circular, level turn, you will get more of a roller-coaster turn. In effect, excess differential will give a secondary pitch effect. This is made worse if the pss model does not have rudder (or you don’t use rudder).

Simon

Not sure which radio you are using, but I usually put aileron differential on an adjuster so I can fiddle with, sorry optimise aileron differential in flight, if you can't do that another option would be to use flight modes/phases and have a different differential on each flight phase so you can see what effect changing differential has.

There is no rudder activation on this model, just ail and ele. That’s a good idea about flight modes, I’ll program that in. Now is my local slope ‘local?’

M

I remember test flying a PSS Phantom Jet for someone at Ringstead. It was a good blow, but boy was it hard work keeping it flying. Not sure whose kit it was, but it had a heavy plastic fuselage and veneered foam wings. It had a drag:lift ratio rather than a lift:drag ratio! But equally I have seen many PSS models fly well.

Simon

I’ve been talking to Alan at SAS, he says don’t use any differential as it has an adverse yaw effect. But to use the flaperons and ele to quicken the turn.

Any rate, for now I’ve deleted the differential.

A club member has a built up Hawk which flies in 15+knots but nothing less and has a rebuild after every (non) flight, another member turned up with a foamie Hawk minus its ducted fan and produced a perfect flight causing a few smiles on the slope!