The Benefits of Anhedral in the Horizontal Stabilizer

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Lendo

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Looking at the new Boeing T-X Jet Trainer and some LSA like Black Shape Prime and it's predecessors, I'm wondering what the benefits are of the Anhedral of the HS Or are they trying to minimize some other effect like Anhedral in the wing?
TIA.
George Lendich
 

Topaz

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Haven't seen the Boeing T-X submission yet, but it's usually to get the tail down out of the wing wake, especially at high angles of attack. Sometimes there are geometric issues such as storing on a carrier.
 

Victor Bravo

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In conjunction with a small rounded rear fuselage, anhedral in the tail can slightly reduce the intersection drag where the tails meet the fuselage. Increasing the angle of intersection beyond 90 degrees makes a slight reduction in drag. Demonstrated on the Grove GR-7 racer "Bummer's Bullet".

Same principle used on upright and inverted V-tailed aircraft in an attempt to lower drag, seen on numerous gliders notably the Schreder HP series.

As mentioned by Topaz, intersection drag may not be the primary or only reason for V-tails, anhedral, etc.
 

Aesquire

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Usually, it's to move the HS out of the wash of flaps etc. It could be used to compensate for excessive wing dihedral, but that seems a really stupid thing to do. It should be easier to change the wing, and not deal with the change at the tail.

The F-4 Phantom II was given dihedral in the wings to solve a spiral mode stability issue. It was easier to crank the wing outboard since the titanium main spar/center section wasn't easy to quickly change.

In the F-4 Phantom's case tail Anhedral was for...(Per Wikipedia, the slow blog, not to be used for confirmed reference ) "The all-moving tailplane was given 23° of anhedral to improve control at high angles of attack while still keeping the tailplane clear of the engine exhaust." IIRC on the F-4 Phantom it introduced a spin recovery problem.
 

StarJar

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I think it also improves stability. Since lift is negative on the tailplane, anhedral would now act like dihedral. The side becoming more horizontal has more force adding to gravity's (because it's force is negative) so it would help level the plane (in a small way).
I've always figured this same principal creates a destabilizing characteristic to V-tails. Relatively small in magnitude though, because of their short roll moment.
 
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fly2kads

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Usually, it's to move the HS out of the wash of flaps etc.
This would make sense. Since turbulent air from the wing wake reduces the dynamic pressure seen by the tail, and this effect would increase as the lift coefficient increases (stronger wake).

IIRC on the F-4 Phantom it introduced a spin recovery problem.
Darroll Stinton mentions that in one of his books. I recall an illustration showing how the arrangement shed pro-spin vortices at the high angles of sideslip found in a spin.
 

bmcj

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One of my Air Force aero classes looked at the F4 as a study of trade offs and cascading effects. High HS mount to clear the jet blast, HS anhedral to get part of the stab clear of wing wake, dihedral on the outer wing panels to counter the negative stability issues created by the HS anhedral, and long chordwise slis cut in the HS trailing edges to absorb some of the twisting moments caused by the HS crossing multiple wake patterns.
 
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