DIHEDRAL (OR POLYHEDRAL) AND ELEVONS DON’T MIX

May 10, 2017|ChandlerRC

(Or How Dihedral Can Make Your Plane “Slow-bank and Yank”)

I’ve seen a few questions on this website forums about why elevons don’t work so well on planes with dihedral or polyhedral wings. The answers are competent – don’t mix those two design features – but I haven’t seen an explanation why. So I set out to learn why.

First, like many modelers, I stumbled onto this problem. My second Nutball was a stop sign (original, I know) with the normal Nutball polyhedral. I wanted to learn about elevons, so I split the elevator and added a servo. Fortunately, I kept the rudder.

In flight, using only “bank and yank” without the rudder, the plane rolled into turns, but only reluctantly. Only when I added rudder inputs could I get the snappy turns I’d come to know and love from the Nutball. Of course, if I ignored the elevons and flew the plane like it had just rudder and elevator (both elevons together), it flew just fine.

My next Nutball derivative, a flying Valentine heart, because it was February, had polyhedral adjustable while in flight.

By experimenting with the interaction between polyhedral and the effectiveness of elevons, I think I’ve discovered the problem. It’s adverse yaw.

ADVERSE YAW

Others have described adverse yaw better than me, so here's Samm Sheperd to do just that.

EFFECTS OF ADVERSE YAW ON POLYHEDRAL

When you try to use elevons to roll a polyhedral plane to the right, for instance, the plane rolls right, but due to adverse yaw, the nose yaws left.

In the presence of left yaw, the polyhedral wing tries to roll the plane left. The resulting net roll is the difference between the elevons torquing right and the polyhedral torquing left. The larger, longer-throw, and more effective the elevons, the more pronounced the adverse yaw and therefore the counter-roll. Bigger elevons don't help.

The only remedy is to coordinate the turn using rudder to cancel out the adverse yaw. Then bank turns are snappy again.

Or, remove the polyhedral and you don’t need the rudder. The effectiveness of elevons in bank-and-yank designs is inversely proportional to the amount of polyhedral.

I would expect long, high-lift wings to produce counter roll, too, reducing the effectiveness of elevons. Just from playing around in the air, we can see that even wings with no di- or poly-hedral can successfully be flown with only rudder and elevator. When the rudder induces a yaw, the difference of airspeed over the left and right wings produces a roll in the direction of the yaw. It may not be very responsive, but it does produce roll and bank. The effect is more pronounced on long or high-lift wings, such as those with significant airfoils. So when elevons roll to the right and produce an adverse yaw to the left, the wing fights the elevons.

A second factor is the weaker mechanical leverage of elevons. Elevons, by their nature, are located close to the roll axis of the plane. So their roll torque lacks the mechanical advantage that ailerons produce by their longer leverage arm, being located farther from the plane's roll axis. Less deflection on the ailerons located several inches along the wing produces the same roll torque as larger elevon deflection located closer to the plane's axis. So, ailerons are more effective with di- or poly-hedral wings than elevons are. That seems to agree with observations of aileron wings with dihedral.

Any more, I fly the Valentine plane with a flat wing in bank and yank mode. I only use the rudder to straighten out the take-off roll. Or to coordinate the bank-and-yank to turn quicker, such as when careening downwind on gusty days.