Johnny C!

Affirmative!

Thanks for a great thread!

Johnny C!

FLYBOY

My Feedback: (11)

You also have to remember, gyroscopic is one of the 4. Each plays a part and all add up to the left turning tendancies.

Matt_McCarty

My Feedback: (2)

Gyroscopic precession is only a factor when yawing or pitching. A yaw moment will cause pitch, and a pitching moment will cause a yaw.
Examples:
Piper cub lifting tail when accelerating for takeoff (Pitch down) will cause plane to yaw left. (yaw left)
Cessna 172 rotating from takeoff (Pitching up) will cause plane to yaw right. (yaw right)
(Yaw left)-(Pitch up)
(Yaw right)-(Pitch down)

Johnny C!

Matt,
I had thought about this too.
I guess that is an indictation of
how little precession effects an
aircraft in relation to the other
left turning factors. We all know
about right rudder on take off, but
I don't recall ever needing any left
rudder at rotation, even in a tail
dragger (cross-winds not included,
of course).

Johnny C!

DKjens

My Feedback: (50)

So, the way P-Factor is explained in the learning books is wrong, because it is explained as a yaw force during a fairly high alpha, low speed, high power climb, but P-Factor really only has an effect during the changes in the plane's attitude, not once it's reached it.

DKjens

William Robison

My Feedback: (3)

DK:

Backwards again. High power combined with high alpha gives p-factor effects. When you change attitude the pitch/yaw force is from precession.

Bill.

PLANE JIM

My Feedback: (109)

something about the ascending blade produces more than the descending blade or vice versa and

Dirtnaper

My Feedback: (11)

The descending blade will have a greater angle of attack as opposed to the ascending. Go get kershners books they will go in depth about it.

Ya know what, DONT worry about it just fly jets and everything will be ok

drone pilot

Does the process get reversed if you fly a pusher?????

William Robison

My Feedback: (3)

Drone:

Pusher or tractor doesn't matter, it's the prop's direction of rotation. Fly any of the aerobatic planes using the Vendenyev M-14 engine and you need left rudder on TO.

If the top blade of the prop goes to the right, as seen from the pilot's seat, right rudder. Conversely, if the upper blade goes to the left, left rudder is needed.
Bill.

tralalala

Sorry to revive a dead topic, but I found this thread through google.

I had the exact same question as DK, in that why doesn't the asymmetric lift on the CW prop disk cause it to pitch up rather than left.

To go back to DK's helicopter example; in forward flight, a helicopter requires forward cyclic pressure to combat flapback tendencies. (ignore all the disymetry flapping mechanisms that are engineered to subdue flapback for this example). If the rotor disk were CCW viewing from the top, the advancing blades are on the right, and retreating blades on the left. So in forward flight, the asymmetric lift exists as the right side of the rotor disc creates more lift than that of the left side (just like the prop pfactor). Now if the explanation of pfactor were applied, the helicopter would want to roll left, yet it doesn't. Due to gyro precession, the helicopter wants to pitch up instead.

DK, instead of your experiment grab one of these. http://www.rudystoys.com/images/products/107_big.jpg
As you spin it up between your palms and let it fly away, pitch it forward and out so it flies away from your body. The prop toy will fly forward and create asymmetric lift across itself. But you'll see that it does not roll left even though the right advancing blades are creating more lift than the left. The toy pitches back instead and discontinues to fly forward.

I also understand Bill's point to make a clear distinction between Pfactor and gyro precession. However, when explaining gyro precession, the books only tell you that any pitch or yaw input from the tail surfaces cause an undue resulting yaw or pitch of the prop disc 90degs ahead. Why isn't the yaw force caused by pfactor considered to create gyro precession?

MajorTomski

I love new life to this!

Back in 23 from Matt:


Each of these in turn

-P factor. A yawing of the aircraft due to a combination of changes in relative velocity and angle of attack across the face of the propeller blade.

FOUND ONLY when the propeller is at some positive or negative angle that is any different from perpendicular to the on coming airflow. NOT present (or extremely small) when most aircraft are in cruise.


-Torque A force that causes rolling of the airframe opposite the turning of the propeller. Present whenever there is power turning the propeller.

-Spiraling slip stream- the theory that the propeller induces a spiral of air around the fuselage that strikes the fin/rudder as some angle of attack that causes a yawing force. Said to be cancelled if there is a sub rudder. Or if the rudder is placed outside the slipstream as on an Erocoupe. Supposedly present all the time. Not to be confused with the turbulent spiral that is visible off a propeller tip in humid air, which flows the wrong way to support the theory. Flaws in the theory the same airflow should cause forces on the wing roots and horizontal stabs that would also cause a significant roll towards the direction of rotation, opposite the roll caused by torque. But this second effect is never discussed in any aviation textbook.
My personal developing theory is that the yaw that is supposedly due to this spiral is actually due to a higher airflow down the side of one side of the fuselage due to the additional thrust of the P factor on that side. This would have the same effect in causing a yaw by the rudder but eliminate the rolling issue.

-Gyroscopic precession- if a force is applied to the edge of a spinning disc, the disc reacts by moving in plane 90 degrees later in rotation. In all the examples discussed here, in a high angle climb the force on the propeller from the maximum thrust at 3-oclock results in a pitching up precession force at the 6-oclock positions. This force is over come by a little down elevator. Now, what has crept in to this discussion is a false assumption that this force is only present when the aircraft is in a transition state. In reality the disc will continually try to pitch up whenever the P factor thrust is present, such in a steady state climb. It is just being over come by the aerodynamic forces at work. Now there are transitory precession forces whenever there is a pitching or yawing of the aircraft. This form of force diminishes when the aircraft returns to straight and level flight. Pull the nose up, a force has been induced on the prop at 12 and 6 oclock, this will result in a momentary yaw to the left during the transition from the first AOA to the second. Do an un coordinated stomp on the rudder to the left and the nose should pitch down. To the right and the nose will pitch up. This was the much commented on bad characteristic of the WW-I Sopwith Camel.

To answer your question
Why isn't the yaw force caused by p-factor considered to create gyro precession?

It is present, and constant but cancelled out by application of the elevator in the climb.

HTH
MTC
YMMV
Tom

tralalala

Tom,

I agree with everything you have said in your reply including the bit on transitory precession and the constant pitch up result in a steady state climb. In closing you agreed that the pitch up component exists but is canceled out by down elevator. My question then becomes; since the 3 o' clock max thrust force precesses into the 6 o' clock pitch up force, how does left yaw tendency still remain on the aircraft? The force cannot remain at the 3 o'clock position as it will precess to 6 o'clock? And it surely shouldn't exist at both 3 o'clock and 6 o'clock simultaneously?

MajorTomski

Two different forces. The thrust ( blast of wind) at 3-0clock, at high angle of attack remains constant, and causes yaw because of the higher speed air on that side of the airplane. The resultant force from the prop acting across the face of the propeller ends up at the 6-oclock from precession. Thrust at 3, equal and opposite displaced reaction at 6.

Make sense?

tralalala

I follow your explanation Tom.

Sorry, this leads to another question. Does this "thrust component at 3 o'clock" exist on the heli in forward flight example? The heli example had advancing blades on the right which in essence creates more lift/downwash, so following your explanation the helicopter should experience left roll (thrust) in addition to pitch up (reactant force) should it not?

William Robison

My Feedback: (3)

Precessive forces in a fixed wing airplane, in normal operations, can safely be ignored. It's the mass ratio - the prop is too small a perentage of total airframe weight for them to be anything more than a minor perturbation.

A rotary winged airplane is an entirely different matter.

Bill.

MajorTomski

From my old understanding 12 oclock the nose 6 oclock the tail in a chopper. CCW rotation

The difference between the two systems, as discovered by Cierva I believe, is the rotor is not ridgier on the shaft. A helicopter can only move forward because the cyclic increases the lift at the 9 oclock position which results in a nose down pitching moment of the disc and thus forward flight.

The advancing blade also lags a considerable amount, lowering the swept area on the right side of the copter. The motions that you ask us to ignore in your first post, lead-lag, pitch and flap, are the same motions that cancel out all the disc/rotation induced problems so that the heli can fly.

And that's from a text book read about 40 years ago. I'd have to get into a library to get anymore concise.

mnowell129

Interesting discussion that I happened upon by luck.

Not entirely. A lomcevak is performed by yawing the aircraft at near zero airspeed and full throttle, allowing the precession to cause a nose down tumble. It is a significant force. I grant you that this is not "normal operations" but does indicate that the magnitude of precessive forces can be significant at low airspeed and high throttle. If this needs some reinforcement, hold your airplane by the canopy and at full throttle yaw it back and forth while not restraining it in pitch. The nose up and nose down pitching moments are surprising.

To agree, airplanes need right rudder at various times. However, the p-factor explanation has several holes that are never explained adequately.
1) No one ever bothers to mention that the prop disk affects the air ahead of it and behind it, thus the real effective airflow velocity and angle are not as simple as the relative angle of attack of the aircraft. A prop draw air into itself creating it's own relative wind. This would be dominant at zero airspeed.
2) The gyroscopic effects cannot be ignored. Any extra thrust from one blade on one side will be applied to the airframe through precession. Any of the arguments about how much extra force is created have to be applied to a nose up pitching moment, not a yaw.
3) Precession only lags by 90 degrees in a system with parts are that free to move. In a system with a rigidly fixed part, like a rigid rotor or propeller the lag angle may be much less than 90 degrees, perhaps 60 or 70 degrees. Thus an inclined rigid prop disk may experience both pitch and yaw forces as the resultant displacement is somewhere off vertical.

Precession is often poorly explained and then misquoted. In a rotating system if a force is applied the rotating component tries to displace (move) (maybe) 90 degrees later. If the system is free to move it will do so 90 degrees later. If the displacement is resisted by some other component then it is with a force equal and opposite to
the applied force but rotated 90 degrees later in the rotation.

The real answer is that p-factor is likely real, the effect is likely that that increase on thrust on one blade vs. the other results in a predominantly nose up and a less powerful nose left moment due to the gyroscopic translation of the thrust. The nose up part is probably ignored because with the tail wheel on the ground, who would notice?

zope_pope

I should take the time to say that what most people call P-Factor is just the 3 left turning tendancies put together.

It's a pet peeve of mine, but I manage to keep my trap shut most of the time. :P

DKjens

My Feedback: (50)

Wow, I really started something here. I can see that more and more chime in, who have the same train of thought as I do. As I recall, in the teaching books, the P-Factor was described solely as the result of the higher AOA on the downwards moving blade. Other turn tendencies were called their respective names, i.e. the P-Factor was not described as an akkumulation of all the turning tendencies.

It is also interesting to figure the swash timing on a common shaft twin counter rotating disk heli, now the input is given 45 deg prior to resulting force.

DKjens

zope_pope

Hmm, let me see if I can remember my Ground School... I just need to clarify what I said to make sure no one misunderstands me.

The left turning tendancies are:
P-Factor
Torque
Spiraling Slipstream

They are three different forces that all make you turn left. They are most prevalent at High power settings, high aoa, slow speed.
In the "Discovery of Flight" series by Jeppesen, the PPL book has very good diagrams of each of these tendancies. I'll see if I can find some diagrams and post them. Like they say. A picture is worth a thousand words. When i get a chance. I will try to type up explainations in plain english so that we may clear up things for non full scale pilots, and we all don't simply refer to the above as P-Factor.

Matt_McCarty

My Feedback: (2)

4 Type of turning Tendencies----->Post #23

P-Factor---> AOA of prop blade, most present in High power setting climbs, Descending blade has higher AOA
Torque---> For every action there is an equal and opposite reaction
Spiraling Slipstream--->Air stream that hits the left side of rudder causing a yaw
Gyroscopic Precession Described---->Post #28


Matt McCarty 2757018 CFII Exp 12/06 CFII(Certified Flight Instructor Instrument)
2757018 AGI AGI(Advanced Ground Instructor)



^FAA certificates........to make this post more reputable

zope_pope

Ooops. Youre right. I am only a low time PPL, so forgive me. :P
Nicely said though.

Adam Rondeau 3034583 PPL

mnowell129

Due to the rigid rotor configuration.

mnowell129

The explanation of this is never satisfactory in engineering terms. The descending blade does have a higher
AOA, however the increased thrust of this blade is applied to a rotating system an thus applies forces to
the airframe later in the props rotation. Helicopter control is firmly grounded in this physics, a prop obeys
the same physics. Because a prop is rigid the angle through which the thrust takes effect is less than 90 degrees, thus there
will be some yaw forces created, there are also some pitch forces as well.
The effect of p-factor is real but it is rotated through some angle between 45 and 90 degrees. The common description
is incomplete in that it ignores the precessive advancement of the force due to being applied to a rotating component.
The increased AOA is the root cause of p-factor, but is not the entire result.
Vibration theory completely explains this phenomena, but it is way out of scope for a piloting handbook and doesn't make
anyone a better pilot.