Ed_Moorman

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To continue on about Sticks, you can see from the photos that I have built and flown many Stick type planes with anhedral. This is more than enough to prove that it works. Besides, I think it looks cool.

Here is a rehash of an old R/C Report column on dihedral effect with the aero reasons for anhedral in shoulder wing or high wing planes.

Since some of you may be new to RC, here are some definitions. Dihedral is the upward angle of the wings. Virtually all full scale planes, with the exception of jet fighters, have dihedral. Many fighters have the opposite of dihedral, which is called anhedral, the downward slope of the wings.

Dihedral helps level the wings whether you want it to or not. Full scale planes normally have a lot of dihedral so the pilot doesn’t have to work very much to keep the plane upright. A trainer also has a lot of dihedral so it will have a tendency to right itself making it easier for a student to learn how to fly it. Acro planes, on the other hand, don’t need this self-righting effect. What you want in an acro plane is for it to stay where you put it. This might lead you to question whether acro planes need dihedral at all. The answer to this is, “Sometimes,” or “Maybe.” This can depend on a lot of other things that lead to a need for dihedral in an acro plane.

Things start happening to a plane when you put in rudder. We call this roll coupling and it is due to dihedral effect. You have seen roll coupling in a trainer. You put in rudder and the plane rolls just as if you put in aileron. This is great for a three-channel plane, it makes it works, but it isn’t good for an acro plane. When we do knife edge and point rolls or any other maneuver with rudder alone we would like to have pure yaw without the rolling tendency. The big problem is not just dihedral causes this dihedral effect. Several physical attributes of an airplane such as wing location, wing sweep and dihedral can cause roll both in the direction of the rudder and also opposite to it. That’s right, opposite. And since we always have dihedral, sweep and vertical location, every plane is a compromise and a good designer uses knowledge of these effects to make his plane perform as well as possible.

Here are these attributes and what effect they cause when rudder is applied. First here they are grouped by similar attribute:

Here’s dihedral first.
Dihedral Causes roll in the direction of rudder.
Flat Wing Causes little or no roll.
Anhedral Causes roll opposite to the rudder.

Picture the wing as a “V.” When you turn it slightly sideways, air gets under the forward wing panel and on top of the rearward wing panel causing roll. The reverse is true for anhedral. Now I know you’ve probably heard that when you put rudder in one wing panel moves forward increasing speed causing more lift which, in turn, causes the roll. Now I’m not going to say, “Tain’t so,” but I’ve flown mid-wing planes with no dihedral and if you get roll with rudder, it’s in the noise level.


Now here’s sweep:
Sweep Back Causes roll in the direction of rudder
Straight wing Causes very little or no roll.
Sweep Forward Causes roll opposite to the rudder

Picture the plane with swept back wings. Yaw the plane sideways so one wing is cross ways to the air flow. This makes it like a straight wing. The air is coming right across the wing making lots of lift. Now check out the rearward wing. It is angled back so most of the air is going down the wing instead of across. Not much lift. The result is a rolling tendency. A swept forward wing gives you the opposite effect causing opposite roll.
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Finally, here is wing location:
High Wing Location Causes roll in the direction of the rudder.
Mid-Wing Location Causes very little or no roll.
Low Wing Location Causes roll opposite to the rudder. Left rudder, right roll.

Take your high wing plane out and sit it so the fuselage is at about a 45-degree angle to you with the nose closest. Let’s say you are looking at the right side of the plane. Imagine a line down the side of the plane in the center of the fuselage. You may even have a pin stripe there already. When you are flying and put in left rudder, this picture is what the airflow is seeing. To flow around the fuselage, it splits at the center line and part goes over the top of the fuselage and part goes under the fuselage. Some of the air flow that goes over the top hits the underside of the left wing, causing a left roll.

Do the same thing with a low wing plane and you’ll see that a left rudder yaw produces a RIGHT roll. That’s correct, roll opposite to the rudder.

OK, now that I have explained the effects, let me rearrange them another way and look at them grouped by effect:

Rudder input causes roll in the same direction as the rudder for wings with:
Dihedral, sweep back and high wing location.

Rudder input causes no roll on wings that are:
Flat wing, straight wing and mid-wing location.

Rudder input causes roll opposite to the direction of the rudder for wings with:
Anhedral, sweep forward and low wing location.

Very interesting, don’t you think? Take a look at a trainer. It has dihedral and a high wing location. You get a lot of roll when you put in rudder. This is how three-channel planes work. Check out one of the Sticks with a flat wing, but a high wing location. Put in some right rudder and you’ll get right roll. How about a low wing plane? I’m sure there are those of you who think you should take the dihedral out of a low wing plane to make it more aerobatic. Don’t do it. Some time ago, a friend of mine had a low wing giant that was built without dihedral. I asked him to fly it straight and level and put in some rudder, but not correct with the ailerons. He put in right rudder and, sure enough to his surprise, it rolled left!

Here’s what all this means. You would like an aerobatic plane to not go into any rolling gyrations when you put in rudder, especially in knife edge or point rolls. To get rid of the rudder-roll coupling, low wing planes need dihedral or sweep back to compensate for the low wing location. High wing planes need sweep forward or anhedral to compensate for the high wing location. Mid wing planes normally don’t need much of anything.