Aerodynamics - Yawing, Slipstream
 

Hi there.

I am trying to get a grip of the aerodynamics. I understand what happens but not in detail why.

For example when I apply rudder and yaws a small aircraft I get a roll. Everywhere people speak about the outer wing goes faster and creates more lift. Ok. But how can that be, the wings are attached to the aircraft. They must go with exactly the same speed??

Also the slipstream. Everyone says it rotates around and hits the fin on the left side pushing the aircraft to the left. But if it is a rotating spiral. How come it won't rotate and someday hit it on the right side? Why only on the left?

Thanks!

The "outer wing goes faster" explanation of yaw-roll coupling is not the whole explanation (so you are right about that). Dihedral is more important - the wing that goes forward when the aircraft yaws has a higher angle of attack and so creates more lift, which causes the roll. You might need a model to look at if you can't easily visualise what I mean.

The spiral slipstream does always hit the fin on the same side. It goes round the fuselage like a screw thread. Again you need to visualise it carefully. Of course if the fin was below the fuselage it would push on the other side. And some engine/propeller combinations rotate the other way round so the spiral slip stream is the other way.

ok.
you apply rudder and the aeroplane rolls. this is because during the yaw one wing is moving faster than the other until the yaw ceases. there after the aircraft will be at an angle to the direction of flight and both wings will be flying at the same speed.

the slipstream rotates around and hits the fin....
no it doesn't.
in reality the slipstream moves straight back past the aeroplane which is why fairings are aligned straight back and work.
of the rear edge of the propeller is a vortex sheet that curls at the prop tips. it is this vortex sheet off the prop that spirals around the fuselage and hits the tail. if you put a little piece of tape on the fuselage, in flight it will stream straight aft for most of the time but as the vortex sheet from the prop passes it will flick around in line with the vortex sheet. on a normal 2 bladed prop you will see 2 flicks per revolution of the prop.
where this impinges on the fuselage depends on the relative speed of the aircraft, the rpm of the prop and the fuselage length.

keep at the theory. it is what makes mastering it all possible.

But that explanation is what has confused the OP - he has noticed that the rolling force is still there once the yawing movement has stopped and the wings are again travelling at the same speed. Its dihedral that causes the yaw-roll coupling.

the leading wing has it's full length in the airstream. the trailing wing is partially blanketed by the fuselage. there is a difference in the lift as a result.

Yes that's true as well...

Will there not also be a rolling force due to the airflow striking the horizontal stabiliser above the cofg, which in a slip (aka sustained yaw) is presented sideways to the airflow?

immeasurably insignificant force from the edge of the horizontal stab.

Ok, interesting.

Someone spoke about dihedral making the roll when applying rudder.
But in for example a PA28 aircraft. There is not that much of dihedral or back swept wings. Still the rolling moment continues when holding the rudder. ??

When you speak about the wing goes faster you mean the wing goes faster during the yaw? But when the yaw is finished and you just hold the yaw in place with rudder.What keeps the roll to continue now? When both wings move equally fast?

Why would it have a higher angle of attack?

Side slip....

The rudder is a distance from the longitudinal centre-line of the aircraft, just as the ailerons are. The ailerons apply a rotational (moment of) force because they are a distance from the centre-line. When you apply rudder, you are similarly applying a rotational force. I suspect that if there were a fin and rudder below and as well as above the centre-line, that the instantaneous secondary roll would occur, but the continued rolling motion would not.

Hi Krallu, in the model glider world, some gliders do not have ailerons but use dihedral wings and the rudder for turning... It works something like this...
Apply right rudder and the glider yaws to the right, pushing the tip of the left wing towards the incoming air. As the tip of the wing is at a high dihedral angle the air gets underneath it and gives it a lift. The right wing tip similarly gets pushed downwards by the airstream.
This method of control, which only uses the rudder, relies upon the wing having sufficient dihedral (say 15 degrees.) for it to work well. In a full size Cessna or Piper the dihedral is much less, so the effect is not so noticeable.

Prop vortex or not, surely a 'yaw string' cannot be used on a piston single precisely because it will not measure the yaw angle between the aeroplane and the relative wind, but will show the distorted airflow behind the prop (i.e. it will never stream aft in balanced flight as long as the engine is running). The (slightly - at cruise speed) rotating air mass behind the prop will impinge on the entire fuselage, wings, and fin. The net effect is to yaw the aeroplane and provide a slight anti-engine-torque roll effect as well.

I wish my engine was powerful enough to give me that torque roll effect on take off, i sit longingly waiting with my foot hovering over the right rudder but alas my old lycoming just keeps the aircraft pointing straight ahead down the strip.

if you hold a model stationary and "yaw" it one wing will move forward the other backwards, so they are NOT going exactly the same speed, especially at the wingtips there is a speed to be added/subtracted from the aircraft overall speed.

If the yaw is finished there is no more roll - if you are "holding the yaw with rudder" you are still yawing!:rolleyes:

No. The aircraft is yawed ie not travelling in the direction it is pointing, but it is not yawing further (which it would have to be doing for one wing to be flying faster than the other). And the yawed aircraft experiences the rolling force that the OP is trying to understand. Go and try it sometime - you end up flying straight and level with crossed controls (side-slipping, as has been said).

The rolling force comes from a number of effects as has been explained, of which the dihedral effect is the greatest in a light aircraft - even though the amount of dihedral is only a few degrees.

Piperboy, you are getting a torque roll effect, which is offset by the rotating airflow effect. With a weedy Lycoming there (should be) a slight yaw to the left on take off (or in any high power / low speed scenario). More yet on a soft field take off, as the torque-induced roll will put more pressure on the left main wheel which will drag in the mud a bit more than the right one.

Indeed, this torque-roll was the only way I could get the Yak52 to steer at a slippery, muddy, winter Barton. Differential brake had no effect whatever, but whacking power on and off sharply would make it turn left or right quite effectively as the appropriate wheel dug in.

Probably 'cause the Maule family put the fin on with an angle to the fuselage.

Add to the theme, what happens if flying a 150/2 or 172, and you open the left cabin door about 8 inches or so. Left door open, plane turns _____? Why?

True, but I have flown both the fixed pitch and the CS version of the exact same model and the difference is quite noticeable, I guess that extra 150 to 200rpm the CS puts out on TO does make a bit of difference on the torque

Left, coz the door acts as a camber on the fuselage?