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Dihedral & Anhedral

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Dihedral & Anhedral

Old 29th Aug 2002, 12:20
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Dihedral & Anhedral

Last year I was at the Shoreham air show, eyeing up the harrier. Now, forgive me if I get this the wrong way round, but on say the 737, the wings viewed from the front have pronounced dihedral I.e. they point upwards, which I believe promotes roll stability. The Harrier on the other hand, has pronounced anhedral, with the wings pointed downwards. Also, the Harrier has the bulk of it's hull beneath the wing, whilst the 737 has it's above the wing root.

Why is this? I fly paragliders, which also have heavy anhedral & my body which represents the hull is a long way below the wing. I'm told that the anhedral is the only way which will allow my aircraft with it's pendular stability to turn. Is that right?

Incidentally, at the show, I asked a harrier pilot the same question & he didn't know either!
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Old 29th Aug 2002, 13:30
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Anhedral is evident on many modern fighter aircraft. It is done to purposely make them unstable and thus giving much faster rates of roll and therefore rapid manoeuvrability.
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Old 29th Aug 2002, 16:11
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Wings with a lot of sweep need less dihedral because the sweep contributes to roll stability (don't as me how, I don't know). If you have too much roll stability the plane can't turn fast so they reduce stability to a satisfactory level by giving swept wings anhedral.

Paragliders have anhedral for a different reason - to help keep the wing open I believe.
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Old 29th Aug 2002, 17:08
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cwatters,

Swept wings improve directional stability.
For example, yawing to the right causes the left wing to be positioned more perpendicular to the incoming airflow. This in turn increases the drag of the left wing, causing the a/c to yaw back to the left.

Erik.
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Old 29th Aug 2002, 18:39
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Parapunter

The above replies are correct, but overlook the specific problems facing the harrier designers. In order to produce efficient vertical thrust, the gas flowing from the engine nozzles requires a reasonably clear path vertically downwards. This required the use of high wings. But high wings combined with a fairly high wing sweep angle would produce unacceptably strong lateral stability. The addition of anhedral both on the wings and tailplane, reduced the larteral stability, thereby restoring a reasonable degree of roll response.
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Old 29th Aug 2002, 21:04
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Keith

I have grave reservations about sticking my nose in on this one, but for the first time ever I could find myself at odds with something you have posted.

Your last sentence was “The addition of anhedral both on the wings and tailplane, reduced the lateral stability, thereby restoring a reasonable degree of roll response.”

I can only agree with this if the roll response you quote is that due to a lateral gust. If it is the roll response due to aileron deflection that you mean, then I disagree.

The rate of roll that happens following aileron deflection on any aircraft has much more to do with the roll damping of its wing rather than any lateral stability it may possess. Static lateral stability of the sort produced by dihedral ONLY produces a stable righting moment (that opposes ailerons) if sideslip also develops from the side of the down going wing.

It is the need to obtain a reasonable (as opposed to huge) response to a lateral gust that leads people to employ anhedral. Without it an aircraft with a high wing and low CG, especially if the wing is also swept, will have a terrible (huge) tendency to roll when hit by a lateral gust. So whether it is a 146 (where you are concerned about passenger comfort as well as controllability on a gusty approach) or a Harrier (where aiming steadiness and low level ride are the considerations) the answer is to include a rolling moment due to sideslip that has the opposite arithmetic sign to that coming from sweep plus high wing and low CG. This means anhedral.

You can even use huge lateral stabilty to generate high roll rates. Nearly 40 years ago as a very new tp just posted to Aero Flight I was given the SB5 (Lightning look alike with a tiny donk and a fixed gear) to fly on an open day. All of the experienced guys having declined to be seen dead in the device picked rather more manoeuvrable mounts (like the FD2 or the HP115) On my first flight I found why: the stick forces were huge, the rate of roll from aileron was negligible (thanks to them being unpowered, very close to the fuselage and being almost too heavy to deflect). But the monster had a sweep of 60deg so this huge lateral stability enabled a modest facsimile of a twinkle roll to be produced by kicking the rudder and making deliberate use of the huge rolling moment due to sideslip.

Regards

John
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Old 30th Aug 2002, 11:45
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Cheers chaps. The mists clear. Elegantly enough, the Shoreham bash is on again this w/e. see you there.

The Punter.
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Old 30th Aug 2002, 20:11
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John, you are of course completely correct.

I should perhaps have said that, by reducing lateral static stability, anhedral reduces the effort required to hold the aircraft in a sideslip, and also reduces its tendency to roll away from lateral gusts and sideslip.
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