Angle of attack dihedral wing

[+TS]

Why has the lower wing of a dihedral wing a greater angle of attack ?

Can't seem to get the picture

[Prof2MDA]

It's not the "lower wing", it's the sideslip angle. The air moves from an outboard section to inboard section, and if there is dihedral that results in a higher AoA. Nice explanation in Flightwise,Volume 2, Aircraft Stability and Control, page 146.

[Vmu]

To get the picture, use a book or a magazine as a "wing". Open the book with a "dihedral" and look at it from the front (in the direction of the relative wind). Then turn the book slightly to simulate a sideslip angle. You will then see that the sideslip causes the "upwind" wing to get a higher AoA.

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"Recovery was marginal..."

[scroggs]

There are two aspects to this question. You need to first consider the effect of roll on the wing, and then the subsequent effect of sideslip.
A dihedral wing in S&L flight will produce equal lift on both wings as the aoa and effective relative airflow (ERA) is the same on both sides. Introduce a balanced roll, and the ERA on the downgoing wing appears to come more from below, increasing the angle of attack. The upgoing wing has a correspondingly reduced angle of attack. If the roll is not enforced with more or continued aileron input, this effect (true of any wing) will tend to resist the roll. A dihedral wing will, if the roll is stopped at a small bank angle, still have a greater amount of effective lift from the into-roll wing than the outer wing due to its lift vector being closer to the vertical, and will tend to return to S&L flight.
Going back to the rolling scenario, as the roll is introduced, a component of the weight acts sideways and introduces a sideslip element to the equation. This means that in a RH roll, the RH wing leads the LH wing. Therefore the ERA meets the wing at a point closer to the RH wing tip and leaves the wing at a point closer to the LH wing tip. If you look at a diagram of this dihedral wing from behind, you can see that the effect is that the RH wing has a greater aoa than the left.
This effect is only relevent following a disturbance in roll, and is referred to as lateral ststic stability. Once in a balanced (ie no sideslip) turn, with a fixed angle of bank, it is irrelevent whether or not the wing has dihedral.
Edited to remove bullet from foot and take the JCB away!

[This message has been edited by scroggs (edited 16 November 2000).]

[Vmu]

Scroggs

You're right, except on one detail:

Quote:
"A dihedral wing will, if the roll is stopped at a small bank angle, still have a greater amount of effective lift from the into-roll wing than the outer wing due to its lift vector being closer to the vertical, and will tend to return to S&L flight."

This is a common misconception. I've even seen it in some (not so good) textbooks on the subject.

The truth is this:
As long as the amount of lift from each wing is the same and the lift distribution is equal (ie no sideslip), the aircraft will not return to S&L flight. The angle between the lift vectors and the vertical is irrelevant. A restoring moment will not be produced by the lower wing's lift vector being closer to the vertical. This is because the moment produced by the lower wing around the CG is exactly balanced by the moment produced by the upper wing(same amount of lift acting at the same distance from the CG). This is true regardless of the amount of dihedral and bank. To return the aircraft to S&L flight a sideslip or a control input is needed.


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"Recovery was marginal..."

[This message has been edited by Vmu (edited 17 November 2000).]

[scroggs]

Vmu,
you're absolutely right, of course. I should have thought it through a little more instead of relying on hazy memories of CFS teaching!

[Ignition Override]

Pardon my question and I mean no disrespect to any students or line pilots, but unless one is studying aero engineering, just how does this knowledge help us become better pilots? If it helps, then is it during stall or unusual attitude recovery?

I'm just curious and have always known that the regulatory authorities are not interested in line pilots' input.

[This message has been edited by Ignition Override (edited 21 November 2000).]

[scroggs]

IO,
just my opinion, but in any profession worthy of the name, background knowledge is always valuable. You may never need to know that the aoa on a downgoing wing increases - until you're in a situation of having to recover a wing drop close to stalling aoa in your 747 or Airbus. Even then, you don't need to know why the wing drop will worsen when you attempt to recover using opposite aileron. You may have time, before you spear in to the ground (avoiding schools, hospitals and houses, like a good little pilot), to wonder whether using a judicious amount of rudder might have been better. But you don't need to know that, do you? Surely it's all just a plot to feather the nests of the CAA/FAA/JAA and the flight schools, isn't it?
If you want to be taken seriously as a pilot, and you want the profession to have the gravitas that justifies its relatively highly paid status, do you not think that it needs qualifications worthy of that status? Of course, if all you want is for flying to be regarded as a manual trade like, say, bus driving, then let's have fewer, easier and cheaper exams. That'd be good - everyone could be an airline pilot then! But do you really want that? Bus drivers don't get paid much and, last time I checked, I didn't get the impression that their occupation was held in high regard by the rest of the world. No disrespect to bus drivers (who do a fairly rigorous driving examination as well), but I'd rather leave things as they are.
Of course, you could always stick to MS Flight Simulator - no exams, no pay, and no pain when you screw up!

[scroggs]

Sorry IO, I've just re-read both your post and mine and I think I may have gone a bit OTT! I'll put the soap boax away for a more suitable occasion.
Cheers!
Scroggs.

[Ignition Override]

Scroggs: Your points are interesting, and I don't disagree, considering that we study information and procedures which go far beyond what might be required for other 'transport industry' jobs, and there is much more to be aware of. At my last oral (on a 'steam-gauge' turbofan), the Check Airman did not ask us any deep systems questions. As for the potential question regarding whether continuous ignition uses 4 joules or 20 joules, we're fortunate that such rote memorization has no connection with 'real-world' pilot judgement or procedural compliance.

Hopefully, there is enough in our AOM, FOM and COM (along with the liability placed on us, which deflects a lot of it from the company) which could justify our wanting industry-level pay and benefits.

What I can't figure out is why in many countries, pilots apparently seem to study how clearways are designed, how weather forecasting formulas are done, along with other theoretical knowledge. One of our FOs here came from Cathay Pacific, and he could not believe the level of theory which is required to convert from US ratings. It was my impression that aside from that, there is still plenty for us to study and be aware of which separates our profession from others.