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As you already have the washout, don't do anything.
Tip stalls come from a couple of sources:
Too much elevator.
Aft c.g.
..
Don't yank the plane off the ground on takeoffs if it's underpowered.
Keeping the airspeed up prevents tip stalls when landing.
If these conditions are considered and fixed as appropriate, tip stalls are highly over rated as problems.
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ps. Drooping the ailerons -removes- the benefit of washout!