Single Engine Turning Forces

M

meritflyer

Well-Known Member
When we pitch a single engine aircraft up, we use a right rudder input to counteract the left turning forces and maintain coordinated flight (g-procession, torque, p-factor, and slipstream).

So why do we use a left rudder input to maintain coordinated flight when we pitch down?
 
meritflyer said:
When we pitch a single engine aircraft up, we use a right rudder input to counteract the left turning forces and maintain coordinated flight (g-procession, torque, p-factor, and slipstream).

So why do we use a left rudder input to maintain coordinated flight when we pitch down?
Click to expand...

no because p- factor and torque are caused by the engine spinning one way, thats why you apply right rudder durring the takeoff roll also.
 
meritflyer said:
When we pitch a single engine aircraft up, we use a right rudder input to counteract the left turning forces and maintain coordinated flight (g-procession, torque, p-factor, and slipstream).

So why do we use a left rudder input to maintain coordinated flight when we pitch down?
Click to expand...

Gyroscopic precession is NOT a left turning force during pitch-up. Look in the books...all the examples talk about is a tail-wheel type airplane lifting the tail. If you don't believe me, go get a gyro toy and spin it, then pitch up and see which way it turns. I had to prove this to my CFI instructor because he didn't believe me.

As far as using left rudder in the descent, do you have continuous power in during the descent? If not, you just took off torque, lowered P-factor, and cut down on slipstream.
 
I agree that gyroscpic precession is not a left turning force in a climb. However, it is a left turning force in a descent. I would think that gyro-p would be the main reason for the need for left rudder in a descent.

And yes, to answer your question, there is power in the descent (although it may be less) which causes the need for left rudder. You didnt eliminate torque unless you are in a zero thrust condition. P-factor is lowered along with slipstream - so I agree there.

However, I think dont think there has been a clear cut explanation why this occurs. Lets keep hashin' it out and see what we get to.
 
While the effects of P-factor are lowered due to reduced power in a descent, it is still there and acting opposite as when the aircraft is in a nose-up attitude. Now as the aircraft descends, the ascending blade is taking a larger 'bite' of air, causing asymmetrical thrust and the need for left rudder to overcome it.
 
CamYZ125 said:
While the effects of P-factor are lowered due to reduced power in a descent, it is still there and acting opposite as when the aircraft is in a nose-up attitude. Now as the aircraft descends, the ascending blade is taking a larger 'bite' of air, causing asymmetrical thrust and the need for left rudder to overcome it.
Click to expand...

HUH??? :insane:

The descending blade will always be taking the bigger bite and creating a left turning tendancy. Regardless of Attitude.
 
desertdog71 said:
HUH??? :insane:

The descending blade will always be taking the bigger bite and creating a left turning tendancy. Regardless of Attitude.
Click to expand...

Gotta agree with D Dog on this one with a few minor corrections. P-factor is predominantly a factor associated with a high angle of attack. Blade loading or thrust is equal in level flight. The center of thrust is "down the pipe" in level flight. When we transition into a nose high attitude, the descending blade has a higher resultant velocity than the ascending blade which acutally does result in asymmetric thrust or loading on the prop which in turn "pulls" the nose of the aircraft to the left.

By stating the descending blade produces a bigger bite of air regardless of attitude isnt accurate for the reasons I mentioned above.
 
meritflyer said:
Gotta agree with D Dog on this one with a few minor corrections. P-factor is predominantly a factor associated with a high angle of attack. Blade loading or thrust is equal in level flight. The center of thrust is "down the pipe" in level flight. When we transition into a nose high attitude, the descending blade has a higher resultant velocity than the ascending blade which acutally does result in asymmetric thrust or loading on the prop which in turn "pulls" the nose of the aircraft to the left.

By stating the descending blade produces a bigger bite of air regardless of attitude isnt accurate for the reasons I mentioned above.
Click to expand...

Correct in level flight the forces are equal however in pitch up or down the descending blade has an increased angle of attack.
 
desertdog71 said:
Adverse yaw created by the increased induced drag on the ascending blade while decelerating or in pitch down attitudes. :)
Click to expand...

Ummm... I really dont know. Maybe?? Normally adverse yaw has to deal with differential aileron inputs. When we turn to the left, the left aileron goes up and the right aileron goes down. This tends to destroy lift on the left wing while increasing lift on the right. Adverse yaw says that the aileron deflected down produces more lift which in turn creates more drag and would cause the nose to turn opposite the direction of the turn.
 
desertdog71 said:
HUH??? :insane:

The descending blade will always be taking the bigger bite and creating a left turning tendancy. Regardless of Attitude.
Click to expand...

Maybe I'm being retarded here... I agree, and can definitely see that when in a nose-up pitch attitude, the descending blade has a higher AOA in relation to the relative wind than does the ascending blade.

But it just seems to me (And I'm sitting here drawing pictures to help understand it) that if the airplane is pitched down and the relative wind changes, then it's the ascending blade that is going to have the higher AOA.

I could be totally wrong.. maybe the relative wind doesn't really change that much from pitch up to pitch down? Seems like it would to me though... ?
 
meritflyer said:
P-factor is predominantly a factor associated with a high angle of attack. When we transition into a nose high attitude, the descending blade has a higher resultant velocity than the ascending blade which acutally does result in asymmetric thrust or loading on the prop which in turn "pulls" the nose of the aircraft to the left.
Click to expand...

Agreed. Now instead of the aircraft being pitched up (with the relative wind coming from the front and slightly below the longitudinal axis), imagine the plane pitched down. With it pitched down, the relative wind changes from coming from the front and below to coming from the front and slightly above the longitudinal axis. Thus the ascending blade would have the higher AOA and higher "resultant velocity" because it is heading more 'into the relative wind' as opposed to going 'with the relative wind'.

Does that make any sense at all? I could still be completely wrong.. haha. I'll draw some pictures to post when I get home.
 
desertdog71 said:
Adverse yaw created by the increased induced drag on the ascending blade while decelerating or in pitch down attitudes. :)
Click to expand...

Adverse yaw deals with ailerons, as merit pointed out.

And why would the ascending blade have increased induced drag (I assume you mean more than the descending blade?)?? Induced drag is that drag which is a result of the lift created by the airfoil (or propeller in our case). So if you're saying the ascending blade has increased induced drag in a pitch down attitude, then you must agree that it is creating more lift (thrust) than the descending blade??

Sorry 'bout the 3 posts.. should have put them all into one.. ahh well.
 
CamYZ125 said:
But it just seems to me (And I'm sitting here drawing pictures to help understand it) that if the airplane is pitched down and the relative wind changes, then it's the ascending blade that is going to have the higher AOA.
Click to expand...

I can see your point and think you may be on to something. Here are some additional thoughts on the state of aircraft's forces.. Where its taking this discussion? I really dont know.

There are four forces an aircraft experiences in flight - lift, weight, thrust, and drag. These forces are are all happy and equal (lift = weight and thrust = drag) during straight, unaccelerated flight. Now, after we pitch up for instance drag will momentarily be greater than thrust and lift will be greater than weight... remember momentarily. There will be a point when the four forces even in a climb will achieve a state of equilibrium. The same is true for a descent. While there may be a reduction in the angle of attack, it will only be a momentary act. As we pitch down, the angle of attack is decreased and as a result the lift of the wing is reduced. Once we've established a steady state descent, the relative wind is now directly off our nose and we have a new angle of attack along with the four fources being balanced.

So, back to the prop. I think that your theory is correct. I understand what you're saying. I just wish I could find some literature to back it up.
 
Couple of things to consider

The "reverse" P factor idea is somewhat correct, but not nearly as present as in a climb since in a descent you are more aligned with the relative wind (much like you are in cruies).

When are you noticing the large need for left rudder? If you have rudder trim in for a climb and then go imediately to a power off or nearly power off descent, you will need left rudder to compensate for the trim (duh)

Some airplanes have it built into them to have left turning tendancies "cancelled out" but offsetting the prop from center, etc etc.

I noticed these right turning tendancies a lot on the Dash 8. It was "habit" to pull the left engine back more than the right to keep the airplane coordinated. This was more of a trim thing than anything.
 
The reverse p-factor theory is correct. Found it in The Advanced Pilots Flight Manual, 7th Edition, pg. 2-16. In the paragraph it states "a left yaw would mean a slight nose-down tendency and a right yaw a slight nose-up tendency."

The descending blade has a greater resultant velocity and AoA during a climb whereas the ascending blade has a greater AoA during a descent requiring left rudder inputs

Nice work CamYZ125! Post those drawings if you get a chance.
 
meritflyer said:
So why do we use a left rudder input to maintain coordinated flight when we pitch down?
Click to expand...


uh let me scratch at this

From an A&P
Cessna had built some correction for p factor in the engine mounting and in the rudder by tweaking it a few degrees during the decent the turning factors are not a factor so the tweaking is over compensating which requires left rudder during a decent


ah fetch someone has already mentioned this somewhat shouldve read all the posts
 
meritflyer said:
I agree that gyroscpic precession is not a left turning force in a climb. However, it is a left turning force in a descent. I would think that gyro-p would be the main reason for the need for left rudder in a descent.
Click to expand...
Keep in mind that gyroscopic precession is only a factor when you pitch up or down. Once a pitch attitude is established, there is no precession force.
 
D-Dog-
The descending blade will always be taking the bigger bite and creating a left turning tendancy. Regardless of Attitude.
Click to expand...

Merit-
The descending blade has a greater resultant velocity and AoA during a climb whereas the ascending blade has a greater AoA during a descent requiring left rudder inputs
Click to expand...

Methinks D-Dog hath been pwnd. :bandit:


Very interesting topic here.
 
You must log in or register to reply here.
Back
Top