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Wily Coyote
2nd Apr 2002, 19:13
Hi all,

I think I understand why many aircraft have positive dihedral and swept wings but I can't figure out why many military fast jets have negative dihedral. Does this not make the aircraft inherently unstable in roll?

Can someone please enlighten me?

Wily

Genghis the Engineer
2nd Apr 2002, 19:19
It is destabilising in roll. But, significant wingsweep (essential in a transonic or supersonic aircraft) is stabilising in roll. It is possible to have too much lateral stability (for a start, it starts rolling all over the place in turbulence, but also it makes it very sluggish in roll), so anhedral is introduced to destabilise. Roll stability will still be positive, but less so.

It also means for aircraft like a Harrier, that the undercarriage legs can be shorter, thus saving weight.

G

Captain104
2nd Apr 2002, 22:22
May I add that wings attached with a negative dihedral not only effects roll characteristics in swept wing fighter aircraft but also directional stability. A fighter is a weapon platform. The 20 mm gatling gun with 6 revolving barrels is still one of the finest tools in strafing. This gun gained highest scores in F-104 fighters due to an
1)extraordinary directional stability (neg. dihedral, swept wing)
2)a remarkable roll stability(high T-tail and a fin underneath the fuselage).
Regards ;)

steamchicken
2nd Apr 2002, 22:35
Some fighters are deliberately unstable; I think the F16 is one (which cannot be flown without the computer). BAE had a testbed Jaguar for control systems that was positively unstable, as they had changed the airframe to make it so

Blacksheep
3rd Apr 2002, 04:42
Er, whats negative dihedral? Is it a bit like positive anhedral?

No flames please, just asking...

**************************
Through difficulties to the cinema

mriya225
3rd Apr 2002, 10:13
Blacksheep,
It's a lot like an unbending polyhedral. ;)
You stinker. :D

Captain104
3rd Apr 2002, 11:00
Blacksheep
negative dihedral=the downward incline of an aircrafts wing or other supporting surface in relation to the horizontal. Esp. the angle thus formed.(New Webster's Dictionary).

Steamchicken
"Some fighters are deliberately unstable; I think the F16 is one (which cannot be flown without the computer)."

Hhmmmm?
I guess I know what you mean: There are several kinds of stability: dynamic stability, static stability a.s.o. Modern fighter AC ( it began with the F-4 Phantom) have no "built-in" dynamic stability and as you write correctly cannot be flown without computers. So far so fine.
This has nothing to do with the unchanged requirement to fly stable when delivering weapons. Just the opposite, modern computers compensate for the missing"natural aerodynamic stability" and provide the pilot with an unmatched stable platform when firing his gun. Even modern stand-off weapons like be fired by a pilot who is sitting in a nice and steady flying machine.
Back to negative dihedral.
As the gentlemen above post quite correct, there are two main points to note:
1) Neg. dihedral is applied to reduce excessive roll stability. The degrees selected are directly related to the sweep-back angle
A rough theoretical formular: 3 sweep-back are related to 1 neg. dihedral. Practically this value is devided by 2.(ratio than 3 to 0.5)
Works nicely on BAe 146 as BiK_116.80 writes: 15 to 3
2) Neg. dihedral helps to reduce a tendency for dutch roll in AC with swept wings and high T-tails as BiK said.

Everybody who had the pleasure to fly B-727 knows what I am talking about.
There are so many real experts in pprune who could give you better infos than me, but hope it helps. ;)

OnTheStep
3rd Apr 2002, 12:33
any ideas why there's 2 anhedral on the iai westwind?

is there anhedral on the mu2? or it may just look it b/c of the underslung tip tanks

Dockjock
3rd Apr 2002, 18:11
Additionally, stability is inversely proportional to manoeverability ie. (as alluded to above) an aircraft with very high roll stability will also have a very low roll rate.

Negative or neutral roll stability is built into fighter aircraft (then compensated for with stability computers for "normal" operations) to facilitate better manoeverability and thus better performance in combat situations.

Captain104
3rd Apr 2002, 18:24
Quite correct. It's a vast area for discussion. Nearly impossible to concentrate on one aerodynamic aspect because all are related and depend from others. ;)

Wily Coyote
3rd Apr 2002, 18:49
Thanks for the response chaps. You've more than answered my questions :)

Wily

OnTheStep
3rd Apr 2002, 23:10
Additionally, stability is inversely proportional to manoeverability....


well! i didin't think anyone listened in celic's class :p

ft
4th Apr 2002, 13:27
Cpt 104,
to correct what I assume was a slip (hmm, or a skid, to open a whole new can of definitional worms ;)), modern fighter aircraft are designed to be statically unstable which is then compensated for by the FBW computers.

Plenty of aircraft are slightly dynamically unstable. Take your hands off and they go into the infamous 'graveyard spiral'... it is quite easy to compensate though, as a pilot. Statical unstability really requires the pilot to be on his toes at all times and correct any attitude deviations. Can be done though, the Wrights did it. :)

Cheers,
/Fred

Captain104
4th Apr 2002, 20:03
ft
my pleasure to see you kidding.:D or skidding?

Best regards.

Checkboard
5th Apr 2002, 05:11
Hmmm - "Statical unstability" would be the same as "Static instability"? :D

3holelover
5th Apr 2002, 16:32
Is it worth noting (or is it even true?) that any aircraft with anhedral is also a high wing aircraft?... I suspect the degree of lateral stability gained by having the fuselage essentially hanging below the wing enables some degree of anhedral to bring the net effect to something closer to neutral... (though undoubtedly still not neutral). But in some of the big cargo birds, perhaps without the anhedral, rolling the things would be quite a chore!

Just a thought....

Groundgripper
5th Apr 2002, 19:17
Not true 3hl - the Tu104 of the 1960s (commercial version of the Tu-16 bomber) was a low wing aircraft with anhedral.
I guess one practical reason it is not much used is that on a large aircraft the wing tips can get perilously near the ground on take off or landing, especially in a crosswind.