Why are wings tilted up from horizontal?
 

Looking at a jet head-on, the wings appear to be tilted up a few degrees in wide "V" shape. To me, this would appear to create a slight lift component in a horizontal direction toward the other wing which would cancel out with the horizontal component from the other wing, and waste lift that could go toward keeping the jet aloft. I'm sure there is a good reason for this ... I just can't see it. Thank you for considering my question.

It improves stability and helps to keep the aircraft right side up, but a thorough technical explanation is beyond me at this time of day.

It's called 'dihedral' and is one of a number of design features that can increase lateral stability.

Lateral stability refers to the tendency of the a/c to return to an 'upright' attitude after being disturbed ie is now in a one wing low attitude. It is also involved with how much force must be applied by the control surfaces to cause the a/c to roll.

Some a/c may be too stable as a result of other effects stemming from their design. These may include 'anhedral' to reduce the a/c's lateral stability.

Anhedral is the opposite of dihedral, and refers to the wing tips being lower than the wing root.

'Anhedral' - like the 146? Thought it was just the weight of all those engines!! ;)

Improvement of stability:-

As the aircraft banks, the lower wing reaches horizontal, whilst the upper wing is above its normal "straight and level" position. The lower wing therefore produces more of a vertical component of its lift than the other. There is thus a tendency for the aircraft to try to roll back to its "wings level" attitude.

Yes, also a good example of 'Asymmetric Anhedral' !! ;)

Thanks Captain Stable - you explain in one paragraph what takes Trevor Thom several pages in his pilot manuals!

...so is the slight anhedral on the 146 a direct result of the high wing? I assume the relativley low centre of gravity makes the aircraft over stable?

Is the high wing also responsible for the steep pitch angle required during decent, or is this more to do with the large size of the flaps decreasing the angle of attack of the wing by a large amount thus requiring more nose down pitch? Or is it just that the drag produced by the flaps is acting well above the horizontal plane of the rest of the a/c?

See, thats all the thanks you get - answer one question and it leads to two more;) (studying aerodynamic effects for a PPL at the moment and actually finding it quite interesting:eek: )

As an uniformed PAX (and not studying anything!) I wonder if the 146 anhedral design, as well as the approach angle etc. that you ask about - was simply the natural result of the design brief?

They wanted to make a short haul pure jet that could operate on short fields in awkward areas. In that they succeeded. However, the market soon cought up with them in the form of Embraer and Canadair. They also found that the number of very small fields with tricky approaches was limited, thus cutting their advantages of first into this size and type.

I sit to be corrected. ;)

Captain Stable

Unfortunately the mechanics of your explanation doesn't quite work. To create a rolling moment you need one wing to create more lift than the other, not just a different vertical component.

The key to dihedral is that it's a link between sideslip and rolling moment. Without sideslip, there's no rolling moment, whatever the angle of bank. For an aircraft with dihedral, the angle of attack of the wing in the direction of the sideslip is increased and vice versa, hence it rolls in the direction opposite the slip.

[Which may explain why it took Trevor Thom a little longer to explain :)]

knobbygb

That's close. The position of the centre of gravity doesn't influence the roll stability. The pattern of airflow around the fuselage in a sideslip causes low wing aircraft to be naturally unstable, and high wing aircraft to be naturally stable. Thus high wing aircraft often have some anhedral to reduce the stability, which might have some undesirable side effects.

Bookworm, I know what you're trying to get at. Unfortunately, the way you put it, it's a load of dog's innards.

There can be plenty of rolling moment with no sideslip whatsoever. If you don't understand this, you never did any aerobatics at all! ;)

However, as I say, I know what you're trying to get at, and you're correct in that my explanation was over-simplified. However, given the question, I was deliberately trying to keep it simple!

I have to admit that I never quite understood anhedral at all... so many high-wing designs have it ( BAe 146, An types etc etc...) but according to my antediluvian brain, it shouldn't work!

Waiter! Another bottle of the '81 please! <hic>

I'll drink to that, Stable!

Damn, where did that horizon go... :)

Perhaps the most known anhedral sporting planes are the Tupolevs. Here is a pic of a Tu-134 from airliners.net:

http://www.airliners.net/open.file/220376/M/

http://www.airliners.net/photos/small/6/7/3/220376.jpg

It's unusual in a low wing, but if you look at the sweep back on the Tu134 in another photo it's clear that they need something to avoid having too much roll-slip coupling. The sweep itself contributes strongly to stability.