dihedral - any good explanations?

[agonyairfo]

The idea that dihedral causes the lower wing to have a higher "vertical" component of lift is incorrect. It has to do with the airplane's movement through the air...angle of attack.

requoted from above from the Pilot Handbook ..."If a momentary gust of wind forces one wing to rise and the other to lower, the aircraft banks. When the aircraft is banked without turning, the tendency to sideslip or slide downward toward the lowered wing occurs. Since the wings have dihedral, the air strikes the lower wing at a much greater AOA than the higher wing."

I know on paper it looks like the lowered wing is now perpindicular to the horizon so it has more lift pulling it up, but lift is only relative to the angle of attack/oncoming air. Nothing helps keeps the "wings level" other than an autopilot. The only way dihedral helps keep the wings level is once you put the airplane there, it will tend to stay there.

 

[pilotyip]

only know

The idea that dihedral causes the lower wing to have a higher "vertical" component of lift is incorrect. It has to do with the airplane's movement through the air...angle of attack.

requoted from above from the Pilot Handbook ..."If a momentary gust of wind forces one wing to rise and the other to lower, the aircraft banks. When the aircraft is banked without turning, the tendency to sideslip or slide downward toward the lowered wing occurs. Since the wings have dihedral, the air strikes the lower wing at a much greater AOA than the higher wing."

I know on paper it looks like the lowered wing is now perpindicular to the horizon so it has more lift pulling it up, but lift is only relative to the angle of attack/oncoming air. Nothing helps keeps the "wings level" other than an autopilot. The only way dihedral helps keep the wings level is once you put the airplane there, it will tend to stay there.

Only know what I was taught in aerodynamics, maybe I got it wrong, but I got an A anyway.

 

[Amish RakeFight]

The idea that dihedral causes the lower wing to have a higher "vertical" component of lift is incorrect. It has to do with the airplane's movement through the air...angle of attack.

requoted from above from the Pilot Handbook ..."If a momentary gust of wind forces one wing to rise and the other to lower, the aircraft banks. When the aircraft is banked without turning, the tendency to sideslip or slide downward toward the lowered wing occurs. Since the wings have dihedral, the air strikes the lower wing at a much greater AOA than the higher wing."

I know on paper it looks like the lowered wing is now perpindicular to the horizon so it has more lift pulling it up, but lift is only relative to the angle of attack/oncoming air. Nothing helps keeps the "wings level" other than an autopilot. The only way dihedral helps keep the wings level is once you put the airplane there, it will tend to stay there.

Perhaps I'm misreading your post, but I think you may be contradicting yourself. If the AOA is greater on one side, then the vertical component is also greater for that side, relative to the raised wing which now has a lowered AOA and thusly, less lift or a decreased component of lift. It may be that the overall vertical component of lift (the sum of lift from both wings) remains relatively the same, but their is still an imbalance in lift being generated, so it's fair to say that the vertical component of lift is greater on one side monetarily until both wings have achieved a level attitude.

When a gust raises one wing and consequently causes the lowering of the other, a restoring force (dihedral) helps provide positive static and dynamic lateral stability. This is exhibited through decreasing oscillations towards the horizontal plane. With each cycle dampening the roll more and more.

The AOA on the lowered wing is greater than the raised wing and produces more lift than the raised wing. The lift being generated on the lowered wing is greater than the lift on the raised. The sideslip towards the lowered wing is what creates an effectively greater AOA. The positive angle (V-shaped wings) is what allows the AOA to increase when lowered.


Not to muddy things any more, but yaw also plays a roll in this restoring force.

As the lowered wing begins to produce an increased AOA and more lift than the raised wing, a yawing moment will take place in the form of adverse yaw which helps restore lateral balance. As the lowered wing begins to produce more lift, it incurs a drag penalty and yaws the nose towards the lowered wing which begins to rise. As this yaw towards the lowered wing (albeit rising) takes place, it assists in lifting the raised wing (which is now lowering) to continue the cycle of oscillation. This is due to the acceleration of the raised wing as it begins to lower so that it can rise abd roll back towards the originally lowered wing. Both wings will experience this until the oscillations completely dampen out.

 

[nosehair]

As the lowered wing begins to produce an increased AOA and more lift than the raised wing, a yawing moment will take place in the form of adverse yaw which helps restore lateral balance. As the lowered wing begins to produce more lift, it incurs a drag penalty and yaws the nose towards the lowered wing which begins to rise. As this yaw towards the lowered wing (albeit rising) takes place, it assists in lifting the raised wing (which is now lowering) to continue the cycle of oscillation. This is due to the acceleration of the raised wing as it begins to lower so that it can rise abd roll back towards the originally lowered wing. Both wings will experience this until the oscillations completely dampen out.

BAM! ..never thought of that before. Great stuff!

 

[nosehair]

requoted from above from the Pilot Handbook ..."If a momentary gust of wind forces one wing to rise and the other to lower, the aircraft banks. When the aircraft is banked without turning, the tendency to sideslip or slide downward toward the lowered wing occurs. Since the wings have dihedral, the air strikes the lower wing at a much greater AOA than the higher wing."

The Pilot Handbook quote isn't about the vertical/horizontal component of lift. It is the poorly worded explanation of the increasing AoA as the lowering wing descends. If the last sentence said "strikes the lowering wing at a much greater AoA than the rising wing", it would be more technically accurate - and understandable.

But, also, I do think that the vertical to horizontal component of lift also factors in.

 

[VW Pilot]

If anyone has ever flown an Piper Warrior, Archer or Arrow, you have experienced this. Now that I read the posts it makes even more sense. These planes have some serious dihedral. I also noticed when flying the C-172, it rolls much faster than the warrior...because it has little to no dihedral but still it makes a stable airplane.