Regardless of bank angle, there is still the same amount of air passing under and over the wings; they do not know if they are horizontal or not.

I'm a mechanical engineering student. I haven't taken fluid dynamics yet (next semester ), but I believe that the dihedral issue is more based on simple mechanics.

Imagine, for an exaggerated example, that the dihedral was 90 degrees, so that when the plane is flying level, the wings are each at a 45 degree angle. If you bank to the right so that the right wing is now horizontal (45 degree bank right) you will have an unbalanced wing with the weight of the plane completely at the left end. Gravity will pull down on the plane's CG, causing it to roll back to the left, until the moment force of gravity is balanced on the left side by the left wing generating some vertical lift.

Now imagine the same plane with no dihedral in a 45 degree bank. Gravity will have a much harder time 'righting' the plane because there is vertical lift being generated by the left wing, and less vertical lift being generated by the right wing.

Unfortunately I haven't flown a model plane yet! This theory would be proved if the dihedral had less self-righting characteristics at higher speeds, which means that as the lift force of the wings increases (in the direction perpendicular to the surface of the wing), the effect of gravity becomes negligible. If you could fly really, really fast in a plane with dihedral, it should have almost no self-righting characteristics at all (if my postulate was correct).

Let me know what you think!

You have it --
What many aerodynamic buffs fail to see-is that the basics of making something which holds together - is first n formost in the design of a plane
also the angle of attack -if zero - negates all the other carefully shaped angled stuff

In real world there IS some AOA- and power is ALWAYS insufficient.
so all of it is just a big compromise to keep it from stalling and crashing.

Rule #1
If it its light enough none of the other stuf matters
#2 If it is too heavy NOTHING matters
It is all a big compromise for a specific task
The Starloaders/ C5A etcc. may work under the same basic idea of lift n power as a flat winged foamie but the dynamics create far different interpretations of these basics.

Granted that's right. But it has no bearing on how dihedral works regardless to which "theory" you subscribe.

Airflow has very little to do with mechanics. Also I'm not sure just what "mechanics" you're reffering to. Is it that diagram that Campgems posted up?

If so I have no argument with that diagram being true. However once you fly some rudder controlled models that are equipped with dihedral you'll soon come to realize that the effect you get is far more than that simple geometric difference can provide. However this isn't an "either-or" situation. Both the geometric and lift coefficient factors are working together at the same time so they are additive. However the geometric factor only works when the model is able to fly in a bank but not side slip. As I mentioned before that just never happens for long and only in a transitory way as the model accelerates sideways into a side slip. To actually achieve that odd manner of flying in a straight line in a steady manner while banked you'd need to be using enough rudder to semi knife edge off the side of the fuselage.

Again, that's true but it won't stay in a steady state bank like that. It'll accelerate into a side slip immediately because there's no sideways lift to counter the tendency to do that. However I'll grant you that if you COULD do this then yes it would correct itself from the geometric lift shifting. And certainly in the brief moment while it is in the early stages of accelerating sideways the totality shifting to majority shifting to semi major part of the corrective torque would be due to your extreme pigeon wings...

Lets look at a more typical example. For this I'll use my Mini Reb rudder only model. No ailerons to foul the situation. Span is 36 inches and dihedral is 1.5 inches at the last rib at 14 inches from the root. That's a 6 degree angle. Oh, it's also a cabin model with a high wing so there's some pendulum action at work helping to stabilize things as well.

So let's bank the model by 6 degrees. SInce I've got the CAD fired up it's easier to just measure stuff than calculate it but I'm sure that trigonometry will bear out the numbers. With the low wing flat I find that the upper wing is spot on 1/2 inch shorter. That's a .5/18 = 2.7 % difference in lifting area. In level flight with the model weighing (and therefore lifing) 19 oz that's a corrective torque of only 0.5 oz. Actually that's is pretty close to how much it would take to correct the flight when flying.

However the model never flies like that for any appreciable length of time. Any model that is banked WILL sideslip unless you hold "top" rudder and force it to knife edge to some degree. But without 4 functions you can't hold a rudder elevator model IN a knife edge flight since any rudder elevator model has to have dihedral so the yaw can produce side slip and the side slip work with the dihedral to roll the model. So if I'm stuck with the geometric effect I can never get more than .5 oz of torque to roll my model. And trust me, when it's snapping itself around the sky there's FAR more than just .5 oz of torque moving it.

So let's look at a sideslip situation. I know from looking at my model that full rudder produces roughly 4 to 5 degrees of yaw angle. This being observed just before the model snaps harsly to the side in a snap roll. Lets look at the angles induced by that yaw angle. Now a 4 degree yaw angle induces a gain of 0.4 degrees on the leading wing and reduction on the trailing wing of obviously the same amount. So now I dash over to NASA's Foilsim ( a superb little tool by the way) and figure out what my difference in lift will be..... (Jeopardy music plays)......OK, with my semi span lifting 10 oz I'm flying with an angle of attack of only .2 degrees at 30 mph. If I increase the angle to .6 degrees as would happen when the wing yaws by 4 degrees the lift increases to .702 lbs or 11.2 oz while the other wing would obviously reduce by the same 1.2 oz. So now I've got 2.4 oz of torque to roll my model..... which is less than I thought but then I don't think this thing flies at 30 mph for much of the flight.

Let's look at 40 mph. The new angle of attack is now -1.12 degrees (it's a Clark Y style wing so it still has lift with small negative angles of attack) Adding that same gain of .4 degree brings the one side to -0.8 and a lift of 11.8 oz and a loss on the other side of the same or close to it. So now I've got 3.2'ish oz of torque to roll my 20 oz model.

The numbers are actually smaller than I expected based on the performance I've witnessed from flying my model. However of course the weight is largely centered on the fuselage so I guess that helps a lot. The wingtip also has beveled up shape with the bottom angling up to the arc of the upper surface over a 2 inch run so those act like small polyhedral surfaces and that may be adding that little extra kick.

My bad. There's inexperience for ya. I was looking at a little mock-up I made, but definitely didn't take into account side slipping or anything like that. The dynamics and geometry just made so much sense to me I got excited

I'll go back to my beginner forum now

I saw a quote on one of the list that applies here.

In therory, theory and practice are the same.
In Practice, they are not.

I kept trying to keep the discussion at the basic theory level, but the realist in the group keep bringing it back to the Practice level. If you want to apply it to your model, go with the "practice" application. There is more involved in just the geometry of the wings.

Don

PS. I'm on jury duty for an unknow time so it may be a while before I see my flying buddy to get his input on this thread. In the mean time, there has been a tremendous amount of good info presented here. I guess that's what these forums are about though.

There is definitely some good info here; it's taking me a while to sort through it all, but it's starting to sink in. I'm definitely interested and curious about fluid and aero (and thermo) dynamics, so this is great.