HobbyKing.com New Products Flash Sale
Reply
Thread Tools
Old Jan 21, 2013, 06:13 PM
Registered User
Joined Jun 2003
676 Posts
Discussion
Adverse yaw?

I recently built two high wing electric models that are shaped similar to a Cessna 180.

I was stunned when these planes would not respond to aileron inputs the way all of my other models have done.

When I add in aileron the plane basically rolls over on its side like a knife edge. Adding in up elevator in an attempt to pull the plane around, it would try to roll back level. Using a stick full of rudder was the only way to get it to turn! I do not have differential ailerons mixing or rudder mixed it.

I have never needed it before on any planes. All of the planes I have built in this size have been stick type planes with both high and low wings. These planes easily fly fast and slow using only elevator and ailerons.


Below are the links to my builds. The plane with rudder only and dihedral turns well however its not the best in wind. The two Extras do not need differential ailerons or rudder mixing.

Why does the high wing plane behaves so differently? I tried this same design with a symmetrical wing and it still had adverse yaw issues.

I understand how adverse yaw works I just don't understand why the high wing has major issues with it while a stick plane has none that are noticeable. The one major difference is fuselage side area. The high wing definitely has a lot more side area than a Stick type fuselage. I don't see how that would affect it.


http://www.rcgroups.com/forums/showthread.php?t=1812942
http://www.rcgroups.com/forums/showthread.php?t=1798911
http://www.rcgroups.com/forums/showthread.php?t=1795569
grant31781 is offline Find More Posts by grant31781
Reply With Quote
Sign up now
to remove ads between posts
Old Jan 21, 2013, 06:44 PM
Ascended Master
Sparky Paul's Avatar
Palmdale, CA
Joined Oct 2000
13,499 Posts
Adverse yaw typically comes from the drag of the downgoing aileron pulling the nose outside the commanded turn direction. The wing goes up, but the nose goes the wrong way.
Differential aileron is about the only cure, without doing something fancy to the aileron, like changing to a Frise style hinge.
Sparky Paul is offline Find More Posts by Sparky Paul
Reply With Quote
Old Jan 21, 2013, 07:19 PM
"...certainty is absurd."
kcaldwel's Avatar
Joined Jan 2007
3,632 Posts
Does your aileron version with the problems still have the dihedral in it? It isn't clear from your links.

A high wing airplane is more stable in roll than a low or mid wing airplane, because the CG is below the centre of lift. This roll stability will make it want to roll to level by itself, particularly when the G load is increased form up elevator in a bank. If you have a bunch of dihedral as well, that makes it even more roll stable by further raising the lift centre above the CG and adding the dihedral roll stability. It will try to level it self out.

That really isn't an adverse roll problem. And despite the common belief among RC pilots, adverse yaw isn't really usually caused by a difference in drag between the up and down going ailerons:

http://www.rcgroups.com/forums/showp...0&postcount=31

Aileron differential is usually pretty ineffective at really solving adverse yaw. It will cause the nose to drop from lowering the overall wing AoA and Cl with aileron input. This gives the airplane extra airspeed when rolling into a turn, which can improve the control response.

Kevin
kcaldwel is offline Find More Posts by kcaldwel
Reply With Quote
Old Jan 21, 2013, 07:47 PM
Ascended Master
Sparky Paul's Avatar
Palmdale, CA
Joined Oct 2000
13,499 Posts
My GP J-3 Cub is delightful to fly because of the adverse yaw of the scale ailerons.
Without putting in rudder, as is required in the full-scale, the turns are sloppy with the nose outside the turn.
Sparky Paul is offline Find More Posts by Sparky Paul
Reply With Quote
Old Jan 21, 2013, 08:04 PM
"...certainty is absurd."
kcaldwel's Avatar
Joined Jan 2007
3,632 Posts
Yeah, but it is because of the rotating lift vectors, not differences in the aileron drag. The effects are the same at full-scale.

Kevin
kcaldwel is offline Find More Posts by kcaldwel
Last edited by kcaldwel; Jan 21, 2013 at 08:29 PM.
Reply With Quote
Old Jan 21, 2013, 09:01 PM
Registered User
Joined Jun 2003
676 Posts
I built the aileron wing without dihedral and with dihedral. Without it was better but not by a lot.

Why is it that the high wing model suffers this phenomenon while the stick type planes and low wings do not suffer it? How would the rotating lift vector change?

Rudder only with dihedral the plane turns like crazy and will do barrel rolls. Ailerons on the plane, It does not want to turn. Actually will not turn with just ailerons. Just lays over into a sort of knife edge. Add in some rudder and it turns.
grant31781 is offline Find More Posts by grant31781
Reply With Quote
Old Jan 21, 2013, 09:04 PM
Don't take your guns to town
vespa's Avatar
Thousand Oaks, CA
Joined Mar 2004
2,608 Posts
With a high wing, lots of dihedral, and a small vertical tail, the plane won't stay banked unless you're holding aileron (or rudder), so unlike your Extra, you get the adverse yaw effects of the ailerons throughout the turn instead of just during the roll. And the more you try to roll, the more it yaws in the opposite direction -- where the dihedral causes an opposing roll force. The reason it happens so dramatically in this design is because the high wing and small fin both contribute to the dihedral effect.

So just as a Rud-Ele plane doesn't work well with insufficient dihedral, an Ail-Ele plane doesn't work well with excessive dihedral. Easiest solution is lots of differential and a bigger fin. Better solution is to add a rudder (mixed to ail) and enlarge the fin. Neutral handling solution is much less dihedral, mixed rudder, and same size fin.
vespa is online now Find More Posts by vespa
Reply With Quote
Old Jan 21, 2013, 09:21 PM
Registered User
Joined Jun 2003
676 Posts
How much fin is needed? Using the tail volume cofficient of .03 the fin is way too small. I had the last plane spin out of control .

Right now I have the Vertical fin enlarged to 9% of the wing area. Rudder is included in that figure. Without alierons it seems adquate.

Most model designs for trainers I have seen suggested the plane to be about 4 times as long as the wing chord and have the H stab 18 to 20% of the wing area and the V stab to be 1/3 ro 1/2 of the H stabs size.
grant31781 is offline Find More Posts by grant31781
Reply With Quote
Old Jan 22, 2013, 10:45 AM
Ascended Master
Sparky Paul's Avatar
Palmdale, CA
Joined Oct 2000
13,499 Posts
My first plane with ailerons,..... maybe 1962 was a 10-channel reed shoulder wing, which had flown well with rudder-elevator-motor. The wing had enough dihedral. Just adding ailerons, all it would do is a very sloppy loop when the ailerons were used. Dropping the wing dihedral to almost flat fixed that.
My Eindecker with a flat wing with ailerons and a flying rudder would do the oddest manuver in a rudder only turn. The nose would point out of the turn and the wing inside the turn would rise, until the plane was "flying" sideways.
My CR Renegade, a high end professionally made 60" sloper with full-span ailerons did the best example of adverse yaw ever, when I programmed flaperons.
With full flaperons, all the way down, commanding a turn in one direction resulted in a skidding yaw in the opposite direction, very eerie to experience. The aileron that would go down in a turn was already as far down as it could get. The other aileron would rise, trying to roll that side down, but at the same time seriously unbalancing the drag force from it and the other side, which had all the drag, and the resultant moment creating the yaw.
Crow, with the ailerons popped up, and inboard flaps down, is a more controllable and useful configuration.
Flaperons with outboard ailerons only is tip-stall city.
Sparky Paul is offline Find More Posts by Sparky Paul
Reply With Quote
Old Jan 22, 2013, 12:48 PM
Don't take your guns to town
vespa's Avatar
Thousand Oaks, CA
Joined Mar 2004
2,608 Posts
You can estimate an appropriate tail volume with some simple sizing equations explained here by Mark Drela but this is quite simplified and does not account for the very low Reynolds numbers and thus low effectiveness of your small tail, nor does it include all of the destabilizing effects of your large fuselage and tractor propeller. The benefits of a large vertical are so great and drawbacks so small that it's almost impossible to make the tail too large. One notable exception is free-flight models which use grossly undersized verticals to improve spiral stability.

But the bottom line is that you don't have enough differential and/or rudder mix. This is required for all planes, regardless of dihedral, airfoil, fin size, etc. but is especially critical for your design.
vespa is online now Find More Posts by vespa
Reply With Quote
Old Jan 22, 2013, 02:18 PM
Registered User
The Willamette Valley, Oregon
Joined Dec 2008
1,171 Posts
A note on adverse yaw

People like to say that adverse yaw is due to mainly rotated lift vectors due to rolling motion. I totally understand the concept (hinted at here http://www.av8n.com/how/htm/yaw.html#sec-adverse-yaw) , and I'm sure the effect is not trivial, but this line of argument fails to address the following situation--

Pilot giving hard right aileron, plane rolls to a modest right bank angle but also adverse-yaws severely to the left, so that the airflow hits the right side of the fuselage creating a left sideforce that totally negates the right-pointing horizontal force vector from the right bank. Net horizontal force or turning force is zero and as pilot continues to hold strong right aileron, the plane flies in a straight line, constant heading, slight right bank, heavy left yaw angle (i.e. a yaw string streams severely to the left). The bank angle remains constant despite the pilot's heavy right aileron input, because the sideways airflow is interacting with dihedral (or the "effective dihedral" conferred by a high wing placement) to create a strong left roll torque, so the net roll torque is zero.

Here roll rate is zero so the adverse yaw torque MUST be being created by the fact that the lowered left aileron is creating more drag than the raised right aileron.

It sounds like this is what the poster is describing. His earlier aircraft perhaps had much less side area so he did not experience this before?

I have personally experienced this sort of situation in the following full-scale aircraft--

Challenger Ultralight
Ka-6 sailplane
Some others I can't think of at the moment-- maybe the Libelle sailplane, I could check my notes

A small vertical fin and a slab-sided fuselage with ample side area both play into this situation. Also a high-mounted wing plus some dihedral, for a high degree of positive "effective dihedral". The Challenger especially has these physical characteristics.

Another version of the situation is almost the same except the sideforce from the airflow hitting the side of the fuselage is strong enough to actually make the flight path curve slowly to the left-- i.e. the aircraft is slowly turning AGAINST the direction of bank, and AGAINST the pilot's continued right stick input. I experienced this in the Challenger ultralight also.

One cure is to use the rudder along with the ailerons to keep the yaw string centered, like you are supposed to. Then the aircraft will roll just fine in response to the pilot's aileron inputs.

By the way I don't agree that the dihedral-like, stabilizing effect conferred by a high wing placement, and/or a CG located well below the center of side area, is more pronounced when the G-load is high. On the other hand, the effect is clearly most pronounced when the slip angle is high (yaw string streaming sideways), and the effect is zero when the slip angle is zero (yaw string streaming straight).

Steve

Quote:
Originally Posted by kcaldwel View Post
Does your aileron version with the problems still have the dihedral in it? It isn't clear from your links.

A high wing airplane is more stable in roll than a low or mid wing airplane, because the CG is below the centre of lift. This roll stability will make it want to roll to level by itself, particularly when the G load is increased form up elevator in a bank. If you have a bunch of dihedral as well, that makes it even more roll stable by further raising the lift centre above the CG and adding the dihedral roll stability. It will try to level it self out.

That really isn't an adverse roll problem. And despite the common belief among RC pilots, adverse yaw isn't really usually caused by a difference in drag between the up and down going ailerons:

http://www.rcgroups.com/forums/showp...0&postcount=31

Aileron differential is usually pretty ineffective at really solving adverse yaw. It will cause the nose to drop from lowering the overall wing AoA and Cl with aileron input. This gives the airplane extra airspeed when rolling into a turn, which can improve the control response.

Kevin
aeronaut999 is offline Find More Posts by aeronaut999
Last edited by aeronaut999; Jan 22, 2013 at 06:54 PM.
Reply With Quote
Old Jan 22, 2013, 02:44 PM
Don't take your guns to town
vespa's Avatar
Thousand Oaks, CA
Joined Mar 2004
2,608 Posts
aero, for well designed planes and pilots, the dominant contributor to adverse yaw is typically lift vector rotation. There are several other contributors (e.g. induced drag, profile drag, separation, etc.) and these are significant and often dominant, just not usually. The sideslip maneuver you describe is a perfect example of uncoordination due to the aileron deflection because there is no roll rate and thus no lift vector rotation, and this I believe is what the OP is experiencing.
vespa is online now Find More Posts by vespa
Reply With Quote
Old Jan 22, 2013, 02:54 PM
Registered User
Joined Jun 2003
676 Posts
Aileron differential and rudder mixing have my plane flying fine. It is just weird that Tiger stick type planes have so little adverse yaw that I have never needed to mix rudder or have differential ailerons. Some of the spad coroplast planes call for Elevator and aileron only with a fixed V stab. I have flown a few of them and they are bank and yank planes. Roll over and pull back and the plane tracks like it on rails.

The spad planes all feature short coupled fuselages, low aspect ratio flat bottom wings, and very little fuselage side area Many of them have rather small tail feathers too. The tail volume coefficients are rather low for them.
grant31781 is offline Find More Posts by grant31781
Reply With Quote
Old Jan 22, 2013, 03:06 PM
Registered User
Joined Jun 2003
676 Posts
Steve,

Are you saying the large flat side area of the fuselage is fighting the wing turning?

I think you are saying when the right aileron is put in the wing banks some and tries to pull the plane around into a turn however now the relative wind is flowing into the right side of the the fuselage. So the force of the wind acting upon the fuselage is fighting the wing so you wind up stuck flying in a straight line banked.

I had the opportunity to fly a Challenger for 2 hours. That plane needs a lot of rudder to turn. A lot more rudder than the Cessna 150 ever needs!
grant31781 is offline Find More Posts by grant31781
Reply With Quote
Old Jan 22, 2013, 04:13 PM
Registered User
The Willamette Valley, Oregon
Joined Dec 2008
1,171 Posts
sideslips

Quote:
Originally Posted by grant31781 View Post
Steve,

Are you saying the large flat side area of the fuselage is fighting the wing turning?

I think you are saying when the right aileron is put in the wing banks some and tries to pull the plane around into a turn however now the relative wind is flowing into the right side of the the fuselage. So the force of the wind acting upon the fuselage is fighting the wing so you wind up stuck flying in a straight line banked.

I had the opportunity to fly a Challenger for 2 hours. That plane needs a lot of rudder to turn. A lot more rudder than the Cessna 150 ever needs!

Yes the Challenger is one of those planes that is a bit whacky and enlightening to fly, because the inherent yaw stability or directional stability is so minimal...especially with the removable doors installed (more surface area ahead of the CG!)

Basically yes... a key point is that as you apply the ailerons the nose swings to the side in the opposite direction as you intended (adverse yaw), and tends to stay there. Then the side of the fuselage is exposed to the airflow and this stops the turn.

This is actually pretty much a description of a standard, cross-controlled sideslip, like you would use for a crosswind landing or to make your descent rate steeper on final. In those situations too, it is the wind (airflow) hitting the side of the fuselage that is preventing there from being any turn (curve in the flight path), even though you are banked. The extra twist in what I'm describing is that adverse yaw is so strong that there is no need for any pilot rudder input toward the high wing, as would normally be required in an intentional constant-heading slip. The adverse yaw from the aileron deflection (stick deflected toward low wing to keep the bank angle constant) is strong enough to keep the nose yawed strongly toward the high wingtip, so much that the resulting sideforce is strong enough to stop the turn and keep the aircraft's nose/heading from swinging toward the low wingtip, even without any high-side rudder at all. And the roll rate is zero, so this adverse yaw is not a byproduct of rolling in any way.

Steve
aeronaut999 is offline Find More Posts by aeronaut999
Last edited by aeronaut999; Jan 22, 2013 at 06:42 PM.
Reply With Quote
Reply


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Help! My HFP does not yaw aok Hoverfly Technologies 7 Mar 02, 2014 04:01 PM
Discussion Unwanted yaw and roll (help plz) 5th Multirotor Talk 16 Feb 12, 2013 03:56 PM
Discussion Bumblebee Yaws Aggressivly teamhairball Multirotor Talk 0 Jan 20, 2013 08:44 AM
Discussion YAW overshoot, altitude loss while yawing, GPS oddities cicadafx Hoverfly Technologies 9 Oct 27, 2011 10:22 AM
Discussion Left Yaw, Right Yaw on my BCX2 jdunn9 Coaxial Helicopters 0 Feb 23, 2007 07:47 PM