Aerodynamics - Yawing, Slipstream
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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!
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!
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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.
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.
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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.
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.
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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
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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.
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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?
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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?
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?
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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.
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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.
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.
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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.
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.
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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.
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But how can that be, the wings are attached to the aircraft. They must go with exactly the same speed??
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?
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If the yaw is finished there is no more roll - if you are "holding the yaw
with rudder" you are still yawing
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.
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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.
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.
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but alas my old lycoming just keeps the aircraft pointing straight ahead down the strip.
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?
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Probably 'cause the Maule family put the fin on with an angle to the fuselage.
Left door open, plane turns _____? Why?
Last edited by piperboy84; 20th Sep 2013 at 20:27.