DIHEDRAL (OR POLYHEDRAL) AND ELEVONS DON’T MIX

by ChandlerRC | March 13, 2017 | (5) Posted in Tips

(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. 

Because if I don’t turn it soon, that plane will end up in the next county!

What are your experiences with di- or poly-hedral wings and elevons?

COMMENTS

Fling'n Chopsticks on May 11, 2017
Very good article. Your testing has given me some more ideas to teach my students about aircraft design and adverse yaw. Knowing 'why' some designs react differently really, really gives them food for thought. Myself included. Thank you for taking the time to write this article.

JD- Taipei
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ChandlerRC on May 11, 2017
JD,

I'm so glad it's a benefit to you and your students! And Rick (the other half of ChandlerRC) and I appreciate the encouragement.

The Flite Test approach to virtually disposable RC planes invites us to try all sorts of experiments and novelties just to see what does and doesn't work. And then to figure out why. Rick and I are like nine year-old little boys when we build and fly, saying Neat! and Cool! more often than any middle-aged men ever should. And we have the curiosity of little boys, too.

I suspect you and your students do, too.

Thanks, again!

-- Mike

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Datum on May 11, 2017
Ahhhh! I built a high-winged plane with dihedral, I failed to understand this problem all the way into a nice tall pine tree. It took a few dozen rocks and some good friends to knock my plane down again.

After I got it flying again I noticed how much more responsive it was via rudder (actually *just* rudder did better than just ailerons haha), but I never connected the dots!

Nice article!
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ChandlerRC on May 11, 2017
Datum,

Doh! I wish I didn't know what that anxiety feels like, when you steer your plane but it turns like a sluggish oil tanker. Right into trouble. Especially when that trouble is 30 feet up!

It feels like double betrayal when ailerons or elevons are supposed to give you so much better control... at least according to everyone's glowing reports. Makes you doubt yourself.

At least it did me. Necessity may be the mother of invention, but frustration with things that should work but don't is the harsh father.

Armed with this knowledge, though, you and I can move confidently between poly- or di-hedral wings and ailerons or elevons, never letting the two meet.

Thanks for the encouragement!

-- Mike

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ChandlerRC on May 11, 2017
At least not to meet without a rudder to coordinate the turns.

[I hate it when I hit Submit Comment before I complete my thought. Doh!]

BTW, Datum, congratulations on your aeronautics degree! Now that you know how the "sausage" is made, I expect it changes how you look at RC flight. Whereas your professors said, "Design it to all these equations and it will fly," you already know, "Heck, put something together and I bet that sucker will fly!"

Funny how a skill that eluded mankind for time out of mind is now so common and easy that children (like you and me!) fly toys just for fun.

-- Mike

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davereap on May 13, 2017
I have two designs that work fine with both ailerons and high dihedral.. have a look on rcgroups and you will find the plans.. they are a PP1 and a Gymball.. the answer being bigger elevons.. the gymball is basically a nutball , like yours, but it has 10 degrees up on each wing half, so 20 degrees total, and the elevons are swept forward, going right out to the tips.. The PP1 is a squarer cut shape . both roll fast and axial..
https://www.rcgroups.com/forums/showthread.php?2105719-My-Super-Simples-PP1-and-Gymball-Easy-Build-Easy-to-Fly-designs
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ChandlerRC on May 16, 2017
Davereap,

Looks like you're using rudder to coordinate your turns. Not a rudderless "bank-and-yank" design as Josh Bixler defines it. Your planes look really nice in the videos! Thanks for sharing.

-- Mike

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nholt on July 26, 2018
Great testing and nice summary. Low aspect ratio wings also have a huge amount of effective dihedral that can depend on the lift coefficient, much like swept wings do. See the M2-F2 lifting body as a good example of what you've shown -- adverse yaw coupling with dihedral effect to cause roll reversal. They used a rudder-aileron interconnect to co-ordinate against this effect. They also solved the adverse yaw problem with M2-F3 by adding a v-stab between the two elevons which allowed them to remove the r-a interconnect.
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ChandlerRC on July 26, 2018
NHolt, thanks for pointing me to the Northrup M2-F2 and F3. Precursors to the STS Shuttle craft. I hadn't known about them.

Frankly, I don't grasp the benefit of the third vertical stabilizer on the F3. It doesn't seem to provide yaw stability, but it might add to that of the other two v-stabs.

Your familiarity with these craft hints at an impressive knowledge of aircraft development. I'd be interested to know what you've explored on models that apply to scale aircraft. I bet there's a story or two to tell. 8o)

-- Mike

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