PPRuNe Forums - High wing anhedral

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High wing anhedral

Hey guys..

Recently attended an interview with an airline and got the following Q which I could not answer. Maybe you could help?

Why has the Pa28 Dihedral?
-For stability purposes
Ok, the C172 have high wings, why does it not have dihedral?
-Because its naturally stable. It has CG underneath..
Ok, then, but the BAE 146, its got anhedral angle, why does it so?
-Because they wanted to reduce some of the stability, thus anhedral angle
Ok, but look at the C172, it has not got anhedral angle, why does it not??

Now, here I could not continue.
The best thing I can come up with now after the interview is:
They wanted it to be stable, haha...
No but seriously.

Why?

Hi,

I think becaues Bae 146 have a swept angle which help to damp out the bank
which high wing provide the similar effect too but also enhanced the unpleasant
dutch roll character. So we make it adhedral to avoid the dutchroll character
which can upset the passenger.

On the other hand, the C172 didn't have the swept wing so the roll damping is
not too much to cause the severe dutchroll.

Best regards

tough question

you should have countered with...does all your excellent training of aviation minutiae make up for your poor pay?

First of all, when the 172 question came up i would of countered with "but it does have a dihedral, albeit less than a low-wing" (assuming you accurately phrased the question u got), then added that while, as you correctly stated, highwing aircraft are more stable, the 172 also has struts that produce drag, which make up for some of the stability thus requiring a bit less dihedral.

The BAE 146 has an anhedral because a highwing & sweptwing combo produces an aircraft that is too stable. Oh and of course because it looks better... :)

I was under the impression that C172 has got dihedral - though, not much. :8

Hinted at in above posts... Wing sweep back is strongly stable in roll, so high wing types need anhedral to reduce - it most clearly seen in the Harrier.

For the 146 however, it is so the engineers can reach the engines :D

Link

Quote:

SweepbackWing sweepback also increases dihedral effect. This is one reason for anhedral configuration on aircraft with high sweep angle, as well as on some airliners, even on low-wing aircraft such as Tu-134 and Tu-154.

[edit] Vertical position of the center of massThe center of mass, usually called the center of gravity or "CG", is the balance point of an aircraft. If suspended at this point and allowed to rotate, a body (aircraft) will be balanced. The front-to-back location of the CG is of primary importance for the general stability of the aircraft, but the vertical location has important effects as well.

The vertical location of the CG changes the amount of dihedral effect. As the "vertical CG" moves lower, dihedral effect increases. This is caused by the center of lift and drag being further above the CG and having a longer moment arm. So, the same forces (lift and drag) that change as sideslip changes produce a larger moment about the CG of the aircraft. This is sometimes referred to as the pendulum effect.[note 4]

An extreme example of the effect of vertical CG on dihedral effect is a paraglider. The dihedral effect created by the very low vertical CG more than compensates for the negative dihedral effect created by the strong anhedral[note 5] of the necessarily strongly downward curving wing.

[edit] Effects of too much dihedral effectA side effect of too much lateral stability, caused by excessive dihedral among other things, can be yaw-roll coupling (a tendency for an aircraft to dutch roll). This can be unpleasant to experience, or in extreme conditions it can lead to loss of control or can overstress an aircraft.

Quote:

Originally Posted by NOD

so high wing types need anhedral to reduce - it most clearly seen in the Harrier.

- well you learn something EVERY day - I always thought it was so the outriggers would reach the ground.................

If the 172 had no dihedral, where would you mount the fuel pickup ports in the wing tanks? You would have a high unusable fuel quantity.
The 172 does have dihedral and that level of stability enhances its general mission capability.

Interesting question. But I'm not sure there is any need to produce a correct answer. In my experience of setting and marking exams there is a class of question where the marking sheet says 'any reasonable answer'. The examiner has to see whether the candidate is being honest and reasonable or is given to making wild guesses.

Hey all.

As always, great answers guys. appreciate it!
Now its more clear. Its funny how fast you loose your knowledge if your not refreshing it constantly...

Thanks

This sort of question is like asking "How long is a piece of string?" The whole of the aircraft design has to be considered and how the effects of the parts interact. Some high wing types have anhedral, some dihedral and some are neutral. It depends on how all the factors interact to affect the stability and if there are other considerations (outriggers for example) that must be allowed for.

Some factors that can affect things include low CG, high keel surface, sweepback, desired amount of stability (or even instability), fin area and lots of others I could probably recall if I thought longer.

Mike-wsm's answer has merit and we hope is what the interviewers had in mind. But why hasn't anyone here mentioned wingflex? Apparent anhedral on the ground whilst dihedral in the air.

Hi folks.

here 's a simple but tricky question.

how does dihedral works? the key word is : HOW?

Everybody has his own explanation and I am confused.

thank you.

PFM. Pure Bleeping Magic.

The BAe146 anhedral is a misnomer. The wings are pre-stressed downwards when on the ground due to the structural and stressing design such that under level flight load there is no or minimal anhedral.

It’s not always the achievement of an answer that matters. It’s the way you deduce and present the result of the arguments.

I must be seeing some kind of weird optical illusion in flight then, considering that the 146 anhedral appears very much the same whether parked, rotating, climbing, cruising or approaching...

One thinks you are confusing it with perhaps the B-52 with its much greater span and larger amount of wing flex...

To understand how dihedral works, imagine a dihedral pair of wings banked 10 degrees. The effective width of each wing is now different - one wing is closer to horizontal than the other. The more horizontal wing has a greater effective width and so a greater vertical lift vector than the one pointing upwards, and this unbalanced force rolls the aircraft back until each wing has the same effective width and the lift forces become equal. Thus the dihedral gives rise to an automatic stability acting to keep the aircraft level.

Sorry safteypee, you are sooo wrong about the 146!

Anhedral and dihedral have no effect unless the aircraft is slipping. Whether one wing appears more or less parallel to the horizon is irrelevant. Lift is a function of angle of attack and relative airflow, and has nothing to do with earth reference.

If an aircraft is slipping, anhedral or dihedral will cause the wings to have different (asymmetric) angles of attack. Therefore there will be asymmetric lift across the wingspan, which will generate a rolling moment to bank into the slip (anhedral) or away from the slip (dihedral). Hence it is called lateral stability (stability in roll).

(That's an oversimplification. In practice, as someone has already said, many factors affect stability.)