dihedral - any good explanations?

[Checks]

Any good, simple explanations of why dihedral works you guys/gals can share?

Thanks in advance,

Mike

 

[nismo611]

Think about it like a bowl it also kind of has dihedral if you drop a marble in it it stops at the bottom. The same bowl flipped over would allow the marble to roll off a side.

 

[VW Pilot]

In other words it creates more stability along the roll axis of an aircraft. (Piper Cherokees, Beech Bonanzas, Mooneys, Cirrus)

 

[TxPilot0878]

Also done for "Gravity Fueling" of aircraft! Most small aircraft sump from the main tank and this is the best way to assure it gets there! But positive Dynamic stability as well!

 

[pilotyip]

With both wings having a slight upward bend as one wing drops, its competent of vertical lift becomes greater than the vertical lift component of the opposite wing. Thus having a tendency to lift the wing that dropped giving the airplane positive stability. Where is Avbug when you need him?

 

[Amish RakeFight]

From the FAA Pilot Handbook of Aeronautical Kowledge:

Dihedral. The positive acute angle between the lateral axis of an airplane and a line through the center of a wing or horizontal stabilizer. Dihedral contributes to the lateral stability of an airplane.

The most common procedure for producing lateral stability is to build the wings with an angle of one to three degrees above perpendicular to the longitudinal axis. The wings on either side of the aircraft join the fuselage to form a slight V or angle called “dihedral.” The amount of dihedral is measured by the angle made by each wing above a line parallel to the lateral axis.

Dihedral involves a balance of lift created by the wings’ AOA on each side of the aircraft’s longitudinal axis. 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 increased AOA on the lower wing creates more lift than the higher wing. Increased lift causes the lower wing to begin to rise upward. As the wings approach the level position, the AOA on both wings once again are equal, causing the rolling tendency to subside. The effect of dihedral is to produce a rolling tendency to return the aircraft to a laterally balanced flight condition when a sideslip occurs.

The restoring force may move the low wing up too far, so that the opposite wing now goes down. If so, the process is repeated, decreasing with each lateral oscillation until a balance for wings-level flight is finally reached.

Conversely, excessive dihedral has an adverse effect on lateral maneuvering qualities. The aircraft may be so stable laterally that it resists an intentional rolling motion. For this reason, aircraft that require fast roll or banking characteristics usually have less dihedral than those designed for less maneuverability.

 

[nosehair]

Imagine this:

A giant hand is holding your airplane up in the sky, with no forward motion.

The giant hand releases, and your airplane falls straight down. The wings have a 90 degree angle of attack as it falls straight down.

Now imagine that the airplane is moving forward 1 foot for each 1 foot downward movement. That would produce a 45 degree angle of attack.

Continue increasing forward movement to 10 feet forward for each 1 foot drop. That would make a 10 degree angle of attack, (I think!).

Anyway, you see the picture? Each time a wing drops, the dropping action increases angle of attack which increases lift to bring the wing back up.

 

[Donsa320]

Exception to the rule.

Still, the B-25 has mostly negative dihedral and is one of the nicest flying, stable airplanes I've eve flown. Go figure.

 

[jak378]

Still, the B-25 has mostly negative dihedral and is one of the nicest flying, stable airplanes I've eve flown. Go figure.

I'm not sure, but off the top of my head: Does it matter whether the diledral is positive or negative for it to have the desired effect. Never thought about that before.

 

[LR25]

Still, the B-25 has mostly negative dihedral and is one of the nicest flying, stable airplanes I've eve flown. Go figure.

If you let go of the controls, does the b-25 stay in the general direction, or does it right itself?

If you let go of the controls of a champ, it will right itself. Even if the champ wasnt in trim, it will find a point to where it will oscillate between nose up nose down as speed increases and decreases. Dihedral, combined with an asymmetrical airfoil increases stability. It creates dynamic stability, the B-25 probably does not have this property, especially with the anhedral wing.

 

[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.