Tuck Under

[FuelFlow]

Can anyone tell me what causes Tuck Under.Would I be correct in saying that it is due to the rearward movement of the centre of pressure, and a decrease in the downwash angle over the tailplane?

Another question is about root spoilers. Can someone please confirm that they cause the inboard on the wing to stall first,causing a normal nose pitch down rather than a tip stall,causing a pitch up.

Thanks FuelFlow

[ft]

When they first ran into the tuck under problem with the P-38 it was concluded that it was caused by the airfoil reaching Mcr, i e small regions of supersonic flow formed on the upper surface of the wing. The corresponding shockwaves made the airflow separate far ahead of where it normally would, causing a drastical reduction of lift and downwash. And, as you said, this gave the elevator less negative AoA and more (positive) lift, causing the plane to pitch down even more. They eventually solved it by adding flaps IIRC.

As for how a stall affect the location of the center of pressure, I really should know but I don't. It should move forward in a normal stall as you lose lift on the rearward portion of the upper surface first. I guess it is entirely possible that the opposite happens in a tuck-under situation with the airflow separated further ahead on the airfoil. If I'm wrong I'm sure someone will correct me here. :)

Not sure what you mean by "root spoilers". Simply spoilers mounted near the root of the wing?

Cheers,
/ft

[Checkboard]

Qestions answered in:

Mach Tuck ?? (http://www.pprune.org/ubb/NonCGI/Forum3/HTML/000215.html), and
Center of Pressure (http://www.pprune.org/ubb/NonCGI/Forum3/HTML/000637.html)

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Tech Log forum moderator

[A Very Civil Pilot]

I thought that root spoilers, causing inboard stall before the outer section, was to allow the ailerons to stall as late as possible.