Wing Dihedral..
 

Hey guys, my instructor asked me the other day, what is wing dihedral for? I researched about it and read some good material on the phak and afm, but for some reason I cannot come up with a good answer. I just don't want to memorize a few lines and resit it to him. I want to understand it before I go back to school.

Any good insight would be well appreciated.

Not sure if I can explain it in text ... but I'll take a stab at it.

When an airplane banks and turns, the lift component is no longer perpindicular to the ground. There becomes a horizontal component of the lift. This introduces a sideslip in the turn.

For an aircraft that has a dihedral wing (that is a V design as apposed to a ^ design), during a turn, the wing that is lowered in the turn has a higher angle of attack due to the sideslip which I mentioned above. Because the lower wing has a higher angle of attack, it creates a rolling moment back in the opposite direction (a roll opposite of the banked turn). Therefore, this creates stability in the roll axis.

The short answer is that it comes in handy for lateral stability.

A simple analogy is to think of dihedral compared to the hull of a boat. The hull of a boat does the same thing that dihedral does for an airplane. When a boat is hit sideways by a single wave, it returns back to it's original position due to the shape of it's hull.

For example, if an airplane is hit by a gust of wind forcing it to bank, but not turn, the result is a sideslip. This means the relative wind is now coming toward the airplane from the low wing side of the airplane. With dihedral, the lower wing has a greater angle of attack than the higher wing due to the sideslip (remember where the relative wind is coming from). Due to this higher angle of attack on the lower wing, the low wing generates more lift than the high wing allowing the low wing to return to it's original position.

I agree with KC: Except the reason for the sideslip. Too complicated. Research adverse yaw, that causes the sideslip. Then read KC's second paragraph. That should complete your picture of the scenario, IMO.

Edit: Thinking some more about this and rereading KC's second paragraph I can see where it might be difficult to picture this. So when you read his paragraph pick up a piece of paper and fold it into the "V" shape KC speaks of. Now hold it up and with your pointer finger from your other hand point directly at the nose of that paper (your aircraft wing). Now bank the paper slightly while keeping your finger oriented the same, lets say a slight left bank. Your finger represents relative wind by the way. Now input a slip to this bank, from adverse yaw. The slip is fairly simple, take the wing and apply some right rudder, so the nose swings up higher. Now look at the AOA this would form on the lower (left) wing, remember a left turn, and compare it to the AOA formed on the right. Notice the difference, and thus your difference in lift.

Multipilot: Unless I am reading this wrong, "the result is a sideslip toward the lower wing" is a skid. You are referring to the nose being inward and the tail outward? The opposite of what is actually happening in an aircraft, though it is what happens in a boat. I don't know that I would use the boat scenario for this explanation. A boat skids when a turn is entered, then banks. Similar to proverse roll in an aircraft, stomp on the rudder, outside wing moves faster thus more lift, and rolls in the direction of the yaw.

Poor choice of wording in that particular sentence on my part I suppose. Tail inside the bank and the nose outside. Should have ended it at sideslip and left out the part about "toward the lower wing". My brain was ahead of my fingers as I was already thinking about where the relative wind was coming from as I was typing that.

Clarification of my boat analogy: The boat is not turning, just slapped on the side by a wave. The boat's initial reaction would be to return to it's original upright position.

That ^ would be referred to as anhedral, like the horizontal stabilizer of the old F-4 Phantom.

X

I am not sure an examiner wants any more specifics than what is in the PHAN however this is how it makes sense to me. - This was a concept I struggled to grasp so I welcome any feedback on my current understanding. I am not going to explain why the aircraft enters a sideslip when banked assuming that part is understood. I want to explain how the dihedral angle basically takes advantage of sideslip to correct the bank.

When an aircraft enters a sideslip it is essentially flying slightly sideways with the low wing projecting into the flightpath. (keyword slightly). With such a small sideslip, it can be difficult to understand how dihedral corrects the bank so let's exaggarate the sideslip.

Imagine an aircraft flying almost completely sideways towards you with the low wing leading the way. (In a sideslip, the low wing projects forward) This type of flight is not reasonably possible but let's just imagine it for illustration purposes. The dihedral angle slopes upward so think of the tips of that low wing that is coming towards you now sloping upward due to the dihedral. The dihedral angle is usually very small however you can again exaggarate the dihedral to get the point. Try to think of that low wing angle sloping so much that you can begin to see the bottom of the low wing from your vantage point. This would not increase the angle of attack of the leading edge but rather increases the AOA of the side of the wing - exactly what you would expect for an airplane flying sideways. Now let's think about the high wing which is projecting away from the relative wind on our sideways flying aircraft. The high wing also has an upward angle due to dihedral however since it is on the outside of the sideslip, it doesn't increase the AOA like the other wing - rather the dihedral angle of that wing further exposes the top of the wing to the relative wing (decreasing AOA). The higher AOA on the low projecting wing increases lift and goes back to wings level.

Let me know what you all think??

Just take note, in a coordinated turn, dihedral does nothing. Otherwise, nice explanation, it is a daunting task to explain this without pictures/diagrams.

Your right- I should have said "disturbed from level flight" rather than "banked".

Try this website:
Aerospaceweb.org | Ask Us - Wing Twist and Dihedral