Dutch Roll/adverse yaw, difference? (Full Version)

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FrankC29 -> Dutch Roll/adverse yaw, difference? (2/9/2004 8:33 AM)

I have read many technical descriptions of both phenomenon, and they seem to sound somewhat alike, or maybe I'm just dense. Can someone describe the difference between the two? What cures or helps cure these if they show up?



beepee -> RE: Dutch Roll/adverse yaw, difference? (2/9/2004 12:10 PM)

Frank,

Can't be a whole lot of help, but a dutch roll is an oscillation event where the wings begin to oscillate up and down in combination with yaw. I have never seen it happen in RC. It is something that full scale pilots will deal with. Adverse yaw is a steady state event where greater drag occurs in the outside (high) wing after roll initiation causing the plan to yaw in the opposite direction.

That is as far as I can go without digging up some references. Hope it helps.

Bedford



MajorTomski -> RE: Dutch Roll/adverse yaw, difference? (2/9/2004 2:17 PM)

Frank, Dutch roll is yaw that appears to be a roll. It is most common in swept wing aircraft. The KC-135 has a nasty DR if not handled properly by keeping the yaw dampener on all the time.

In that aircraft, the event starts with a little yaw in one direction, but because of the sweep-back of the wings, it is perceived in the cockpit as a roll in to the original direction. An un-commanded or uncoordinated yaw to the left results in an un commanded roll to the left. This is caused by the skid to the left causing a momentary increase exposed area of the right wing and a decrease in the area of the left. Take a swept wing model and look at it nose on. Now yaw it to one direction. See how the wing that moved forward now looks larger than the wing that went aft?

The pilot applies Right aileron to stop the perceived left roll. But he's putting the aileron in just as the natural stability of the aircraft is also stopping the roll to the left and it is naturally going back to the right too. So the pilot just added a little super charger to the roll back to the right. Which usually results in overshooting and the aircraft goes into a bank to the right. You get in to the pilot chasing these rolls at a frequency that makes the situation worse not better. If not properly corrected the aircraft is over on its back in four cycles. The correct input at the start of the problem is rudder control to maintain heading, to eliminate the uncoordinated yaw that is the source of the problem.

Adverse yaw is the other way around. In the above case a yaw causes an uncommanded roll. In Adverse yaw a Roll causes a momentary uncommanded yaw or turn in the wrong direction. In an aerobatic aircraft we want aileron input to cause roll and only roll. An aircraft with adverse roll will at first yaw in the opposite direction of the way you want to roll, then pause and chase the ailerons around in the correct direction. A Cessna 150 has this designed into it to teach the student to step on the rudder to get the turn going in the correct direction. You can also have aircraft with pro-verse roll, the aircraft actually turns too fast in the direction of a roll.

The two phenomena are alike in that you get an unwanted response for a particular input. They are opposite in what they do yaw input for a dutch roll causes rolling. A roll input in the case of adverse yaw causes a yawing motion.



Dsegal -> RE: Dutch Roll/adverse yaw, difference? (2/9/2004 3:23 PM)

Dutch Roll is a problem usually encountered in freeflight models. It is defined as "out of phase oscillation in pitch and yaw." Got that? (G) In other words, the tail end of the model wags about in a clumsy circle. It is not caused by a control input such as one aileron dragging as in an RC model. The cure in freeflight models is larger tail surface area. Not an issue for RC flying.



Flypaper 2 -> RE: Dutch Roll/adverse yaw, difference? (2/9/2004 4:40 PM)

Used to have an RC model bipe back in the 60s called a Wizard. First take off got into a dutch roll. Keept getting worse till it rolled in. Second flight did the same thing till it rolled upside down and went straight in demolishing it. Found out later, small a rudder is blanketed in by the fuse area in a climb. As Dave says, needed more rudder,fin area.



BMatthews -> RE: Dutch Roll/adverse yaw, difference? (2/9/2004 6:35 PM)

Both these conditions are caused by the same thing, too small a vertical fin area compared to the wing.

In the case of dutch roll I believe it's the lack of yaw stability combined with the effects of dihedral in the usuall free flight or rudder+elevator models that causes the back and forth rolling effects. In models with little or no dihedral the result is more just a wagging of the tail or a visible adverse yaw when aileron is applied. I've also noticed that models with too small a vertical fin area tend to hang tail low in turns and appear to be "skidding" around the turn.



Tall Paul -> RE: Dutch Roll/adverse yaw, difference? (2/9/2004 6:56 PM)

"Not an issue for RC flying."..
welllllll...
maybe.
ZAGI™'s have an annoying "head shaking" when returning to level flight. A suggested cure is vertical surface below the wing at the tips.
Have done the mod, but the wind is not cooperating.
My Evans "Slo-mo-shun" (straight leading edge, swept forward trailing edge and dihedral) had a lateral instability at high speeds, which I fixed by -cutting- the vertical area.
Back in the days of mechanical instead of electronic stability augmentation, the Boings used a bob-weight yaw damper to lessen, not eliminate, the dutch roll. Watching the ground go by over the wing leading edge, a farm house say would go behind the wing leading edge. A short time later, it would reappear, and then go away.
In the extreme aft of a 747, the yawing motions were quite noticeable! The wrong place to have the bar! :)
Electronic yaw-rate sensors do the job much better nowadays.



FHHuber -> RE: Dutch Roll/adverse yaw, difference? (2/9/2004 8:15 PM)

Dutch roll is the tail wagging side to side, with accompanying roll oscillation due to dihedral.

Adverse yaw is the aileron deflecting downward acting as an airbrake, making the tail turn the direction opposing the aileron defelction.

You can have dutch roll in any size aircraft. The 4*40 has some dutch roll at high power, high angle of attack low airspeed. ( you can watch the tail wag durring a steep climb on take-off)

Dutch roll is normally a speed dependant phenomenon. Actually, almost any airplane will exhibit a little dutch roll at some given airspeed and angle of attack combination. The airplane designers try to make the combination be at a place where you won't normally opperate the airplane.

Adverse yaw is a factor of every aircraft with ailerons... However it will be worse with flat bottom airfoils, and high angle of attack. In some cases, the adverse yaw can completely overpower the rudder.

Yes, there are similarities in that they both normally are aggrivated by low airspeed and high angle of attack. Adverse yaw is aggrivated by low power (less air forced over the rudder) Dutch roll is aggrivated more often by high power (more turbulence along the fuselage and over the tailplanes)

Dutch roll... you can live with easilly. Adverse yaw... can get you in trouble quickly. The natural tendancy when the aileron seems to nt be truning youthe way you want to go is to push the stick harder. That is a mistake as you will probably stall the wing you are trying to make go up... and spin the plane. (lots of beginners crash this way trying to take-off or land)



Flypaper 2 -> RE: Dutch Roll/adverse yaw, difference? (2/9/2004 9:26 PM)

Often wondered why the trainer designers don't design in differential ailerons. Easily done mechanically as well as programable. Would have saved a lot of planes as well as teaching the beginner a bit of aerodynamics.



Tall Paul -> RE: Dutch Roll/adverse yaw, difference? (2/9/2004 9:47 PM)

"Adverse yaw", like its cousins "tip-stall" and "downwind turn" get much more press than it's worth.
A moderate amount of differential aileron; more up than down can fix most encounters with adverse yaw, if it's even noticed!
To see it for real usually requires a long-span scale model.. the J-3 Cub when built properly; barn-door ailerons instead of strips, will have a delightful adverse yaw!
The pilot can opt to mix the rudder with the aileron to keep the nose into the turn, or, better, use his left thumb and fly the nose around the turn, the same as if he were in the real thing. Much better than mixing it out.
The place really bad adverse is commonly encountered is with flaperons. When these are fully down, turning becomes extremely interesting.. as the turn command expects the surface on the outside of the turn to go down, but it's already as far down as it can go, while the other surface can go up, the difference in drag between the fully down side and the up going side pulls the nose to the opposite direction commanded. VERY disorienting, to those of us that fly these things, when first encountered.
Makes flaperons somewhat less attractive... don't need that kind of response when landing! :(



FHHuber -> RE: Dutch Roll/adverse yaw, difference? (2/10/2004 3:39 AM)

yes, the most common cure for adverse yaw is differential aileron throw.

Actually, every plane has a bit of adverse yaw... Its just very small with an IMAC 30% scale Extra or similar planes.

If you are increasing lift.. you are increasing drag (conservation of energy) So when you increase upward lift on one wing, you are adding negative "lift" on the other with the aileron on the Extra 330. Because you have a symetrica airfoil and its the AOA that determines if the airfoil is lifting up or down (relative to the fuselage) the end result is miniscule adverse yaw. (you can easilly ignore it when above stall speed.)

With a flat bottom trainer wing... the adverse yaw is more pronounced.... but due to the overpowering we do... if you aren't flying slow, you may not see it.

With this Triplane... Adverse yaw is so significant that I have BOTH differential aileron throw (50% less downward throw than up mechnically, not computer mixed) AND I have 30% rudder mixed to aileron and I still need to use the rudder in same direction as the aileron for a good coordinated turn. On landing approach adverse yaw can overpower full rudder at 50% aileron throw. (and any more aileron would put it in a spin instantly)



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