Help on dihedral effect on low wing aircraft 2

[puaero]

I know dihedral is added for stability and control but I'm having difficulty visualizing (and therefore explaining) the effect of a DIFFERENCE in dihedral on one side or the other. For instance; hypothetically speaking if the dihedral on a model is 10 degrees on the left side and 0 degrees on the right, which wing is producing more lift and therefore; which way would the aircraft roll???
Thanks for any help on this!

 

[rb1957]

the wing at 0deg will create more lift than the one at 10deg, so there is a "righting" moment restoring the airplane to a symmetrical balance.

 

[puaero]

Thanks rb1957. Could you elaborate just a little bit more. For instance, if the wing with the lower dihedral is producing more lift, is it because of the resultant sideslip and normalized freestream air over the wing???

 

[MiketheEngineer]

Sometimes it helps to exagerate the scenario. Image left wing with 45 degrees of dihedral and the right with none. NOW which way is it going to roll --- LEFT. The vertical component of the lift vector is much less.

Does that help??

The reason mfg's put in dihedral is to make the plane self-stabilizing. Left wing drops - right wing loses some lift - plane rights itself - vice versa.

If you are a visual rules pilot and get caught in a cloud -most mfg's say "take hand off of controls - chop power a bit and trim for say a 500 fpm descent." Plane will fly itself out of the clouds perfectly level. I had to do it ONCE!!

Actaully - most piston planes have more right dihedral or less left dihedral to help counter the engine torque.

 

[puaero]

Thanks Miketheengineer! That's the analogy that I was looking for!

 

[rb1957]

"stick free" stability is a wonderful thing ! ... brings to mind the joke about the pilot, the flight computer (and all it's switches) and the dog (dog bites pilot if he tries to touch anything!)

as to the question, maybe this helps ... for the inclined wing, you project it down onto the horizontal plane to determine it's effective lift

 

[jetmaker]

One way to look at this is that both wings produce the same amount of "lift", that is, the amount of force normal to the wing's surface.

Now if you draw a vector diagram, you will see that the vector on the left wing has 2 components: upwards and inwards; whereas the right wing only has an upwards component.

Evaluate the equilibrium of the system, and you will likely see that the roll moment is unbalanced, as is the lateral force. The airplane will then roll to re-establish equilibrium.

jetmaker

 

[Dan320]

I would say we are forgetting dynamics. The horizontal lift component from the inclined wing will make the a/c turn, as in any normal aileron turn. An aircraft will not behave like a pendulum. It is difficult to percieve an aircraft with unequal dihedral, it will probably fly with both wings pointing upwards the same amount, but with the fuselage and tail canted.
Dan

 

[rb1957]

dan, i think the unequal examples are illustrations of a perturbed plane (the one wing at 10deg, is an airplane with both wings with 5deg dihedral and the airplane is rolled 5deg to one side

 

[Dan320]

The dihedral effect only works to right the a/c if the fin and rudder are small enough to allow sideslip. If they are bigger than that, the a/c will enter a spiral. If the fin and rudder are too small, the a/c will start to dutch-roll at some angle of attack. This is the classical trade-off which sizes the vertical tail. Again, the a/c is not a pendulum and the idea of an a/c flying straight with one wing pointing up is not realistic (unless the pilot uses control input to make it do so).

 

[crysta1c1ear]

As a child I saw pictures of planes with negative dihedral and wondered how they could be stable if dihedral was supposed to be added for stabilization.

After a little thought, I imaged real world objects that I understood better and where the dihedral is more extreme: a shuttlecock for playin badminton, and a typical old style parachute.

If it is the dihedral effect that is stabilizing the shuttlecock, then why doesn't something like a parachute come down upside down?

I decided that basically the pendulum idea above does provide an intuitive explanation. For example aircraft with negative dihedral usually appear to be suspended underneath the wings, whereas aircraft with normal dihedral can have the body appear to sit on the wings, more like the shuttlecock arrangement.

From that,we can see that if both positve and neatve dihedral can work, then no dihedral is required, and some older planes just have horizontal wings. That isn't really a problem.

I think it just boils down to having the centre of mass below the centre of lift. It is the reason people hang underneath hanggliders and paragliders. Trying to ride on top of a hangglider would not be a good idea.

=

So one wing at 10 degrees and the other straight?

As somebody said, that amounts to having the body on the plane at an angle of 5 degrees. So you'd be sitting in a sloping cabin.

But then the tailplane would be on crooked too. In conventional planes (which are a stupid design) the tailplane pulls down and the main wing has to overcome the weight of the plane plus the downward drag of the rear wing. If that were on crooked, it would be pulling sideways too.

So I think your plane would be flying round in circles.
Eg if the tailplane were pulling down and 5 degrees right after the front wings had more or less levelled, then I think you'd be going round and round in left hand circles.

Since you'd have lost a little of the downward lift from the tailplane, I think the nose would be pitched downard too. You'd have to fly faster to achieve level flight.

 

[rb1957]

i don't think the proposal was to build a plane with one wing horizontal and the other at 10deg. i think the question was "how would a plane in this configuration behave?" physically it'd be a plane with 5deg dihderal thats rolled 5deg. so it'd roll back, as the wing at 0deg has more lift than the one at 10deg

 

[Dan320]

Here's how to see the light and how dihedral works: Take a rectangular piece of paper and make a dihedral break in the middle to form a crude "wing". Now look at it from in front to see the leading edge. If you turn the wing as if the rudder was making it turn what will you see? The wing closer to you will show its lower side and the wing away from you will show its upper side.
Now think of your eyes as the airstream flowing over the wing. The wing near you will have increased angle of attack compared to the one away from you. This is by the way how single-channel RC models steer and roll.
If you put the wing on top of the cabin, the fuselage will add dihedral effect as will the fin. Some high-wing a/c have negative dihedral on the ground so that when the wing flexes upward in flight the dihedral effect will not be too large.

Dan