Wing drop at the stall
I have a problem, and would be grateful if anybody can suggest a solution.
I'm in the middle of a certification test programme on a largely benign lightweight 2-seat, high wing, tractor monoplane with a mid-tail. Fairly torquey engine, which requires about 2/3 right rudder on t/o and a couple of pounds continuous rudder input in the cruise. 10° sweep, no dihedral to speak off, half span flaps.
At idle, the stall is marked when you run out of nose-up pitch authority. As we increase throttle however it shows an increasing tendency to drop a wing - reaching about 60° right wing drop at 60% MCP. It also rolls right about 60° from a 30° left turning stall with PLF and 30° from a 30° right turning stall with PLF. This all happens around min.weight, near MTOW it's just within certification limits.
Standard fixes for this are obvious: -
- droop the flaps
- reflex the ailerons
- increase washout.
All of which add up to roughly the same thing. But with 21° (take-off) flap, the stall characteristics are still only slightly more benign and still nowhere near the certification limit of 20°. To add to the fun, the aircraft is right on the certification limit for Vso, so although technically I can do so quite easily, increasing the washout is likely to increase Vso and stop the thing being certificated at-all.
I've considered Toblerone stips, which should help the lack of natural stall warning, but don't know if it's likely to help the wing drop much.
I've considered turbulators, but it's not a laminar flow wing, so I can't see that they're going to do much - but I could be wrong. Anybody know better?
Would defender style wing fences help?, I know they did on the Islander which is similarly marginal but are a bit of a ****** to fit.
I'm stumped. Help. Has anybody got any solutions that have worked for them. Sadly, I'm in charge, so haven't anybody else to dump the problem on.
G