DKjens

My Feedback: (50)

I would like to hear inputs about and discuss the resulting action from the P-Factor. Lets for clarity assume that we have a single engine piston powered plane with a CC rotating prop, so the blade on the left will be the down moving blade.
As far as I know, all manuals and teaching books for full scale aviation explains the P-Factor something like this: When the airplane is in a high power, low speed, high alpha climb, the down moving blade of the prop, in this case the left blade, will have a higher AOA to the prevailing wind, than the up moving blade. This means that the down moving blade creates more lift, or higher pull force, and this causes the plane to yaw to the right.
My argument is, that the resulting action from this schenario is a force that pitches the plane up, not yaws it to the side. Even fixed wing pilots and instructors are familliar with "gyroscopic precession", and it's mentioned multiple times in the teaching books prior to the arrival of this subject. Then when P-Factor is brought up and explained, it seems that everything about gyroscopic precession is forgotten. I raised this question, and I was just adviced not to bring that up with any inspector. I have a feeling, that the old, and in my opinnion wrong, explanation has been used for so many years to explain this yawing phenomenon, which is caused by the effect of the swirling prop wash among other things, that they (whomever they are) have decided just to stick with that.
In short terms, gyroscopic precession is: "Gyroscopic precession is the resulting action occurring 90 degrees from the applied force." http://www.tpub.com/air/10-3.htm
This doesn't matter if it's a helicopter rotor, or a spinning prop on an airplane. On a helicopter, to pitch the rotor disc forward, the AOA is raised on the receding blade, causing a lifting effect at the tail end of the rotor disc. Well, how come this is completely ignored when explaining the P-Factor.
Please help me understand, or agree with me that something is wrong here.

DKjens

Robinaire

My Feedback: (23)

Hi DK; History of me: 60 years of flying; 5000+ hrs, some 60 makes & models flown from homebuilts to Lockheed 10. Former FBO, CFI, floor sweeper, john -cleaner and whatever is needed in a dealership & FBO!! Your problem is trying to lump everything under "P-factor"!! P-factor is P-factor; gyroscopic precession is just that; swirl effect is swirl effect. Don't try to lump all under the one heading. I grew up in the '30s, and all engines in general use were relatively low HP and didn't develop the torque of an R-2800! I was taught ALL of the forces, and that the real problem is when they gang up on you!! And that is what they do every time you push that throttle ahead! On the ground, and considering taildraggers only, (tri gears don't sit at high aoa until you lift off. In a taildragger, you keep the tail on the ground until you reach full throttle, and then when you lift the tail, p-factor disappears until you lift off to climb. By the same token, if there is no P-factor, there is no precession, as forces on the prop disc are equal. Again, increase the AOA and P-factor comes back to produce the precession! They are all lying in wait for the unwary, so think of how they all interact and act accordingly!! Lee Robinson; W. Palm Beach, FL.

MajorTomski

Well, for starters let's get the definition of P factor down right first:

P-factor is a differential thrust across the face of the blade due to localized changes in relative velocity and angle of attack. Sadly in most literature being published, only the angle of attack portion is being taught. First pardon my old school English measurements; I never made the transition to metric ;-)

A propeller spins at a certain RPM, for our discussions let us only look at the tip of the propeller. For a given RPM the propeller tip is moving a specific velocity in feet per minute. For example a 4-foot propeller the tip is 2 feet from the hub. The distance the prop tip is traveling is the circumference of the circle or PI x D; (3.1416 x 4 = 12.58 feet) Now if the engine is turning a typical 2400 rpm on a Cessna type aircraft, the tip speed works out to 12.58 feet (one revolution) x 2400 revolutions per minute or 30,200 feet per minute or for smaller units (dividing the minutes by 60) 503 feet per second. (88 feet per second equals 60 miles per hour) so the prop tip is doing 340 miles per hour.

Lift is a function of the following equation: Lift (or in the case of a propeller Thrust (T)
T= ½ CL R V2 A
CL is the Coefficient of lift. The CL is a function of the airfoil shape and its angle of attack; this is determined by wind tunnel work.
R RHO the density of the air
V2 the velocity through the air squared
A area of the wing. In the case of a propeller it gets more complex because there is an integration of the different angles of attack and speeds along the length of the blade. If we confine our discussion to just the tip, we only have to work with one value. Note that the Velocity is a squared value in the equation so a little a small change in velocity causes the most change in lift.

An airplane in a steady state climb, at a constant angle of attack (say 5 degrees and a constant airspeed (say 120 mph or 176 fps) causes the propeller to strike the on coming are at the same angle of attack the as the wing. From vector analysis that on coming airflow has a component that acts perpendicular to the front of the propeller and a vector that acts in the plane of the propellers rotation. If our airplane has a 5 degree angle of attack then from trig, the component of the in coming 120 mph air is 10.4 mph acting in the plane of the rotation, and 119.4 MPH perpendicular to the plane of rotation. We only need to focus on the 10.4 mph component.

As the propeller rotates in this incoming air it sees that 10.4 mph component reverse direction. At 12 and 6 oclock it is zero, at 3 oclock its PLUS+10.4 mph, at 9 oclock its MINUS-10.4 mph.

Now add these two values to the propeller tip speed from the above discussion. At 3 oclock the prop tip now has a speed of 350.4 MPH (340+ 10.4) and at 9 oclock the tip now has a speed of 330.6 MPH (350-10.4) if we plug just these two values into our equation above

T= ½ CL R V2 A

And use an arbitrary 0.1 for CL, 0.01 for RHO and 10 for area we get
T @3:00=(.5)(0.1)(0.01)(350.4)(350.4)(10)=1,227 pounds of thrust (not a real number because of using the ones just something to work with)

But at the 9:00 position
T @9:00=(.5)(0.1)(0.01)(330.6)(330.6)(10)=1093 pounds of thrust

A difference of 134 pounds of thrust pulling harder at the right propeller tip causing a turn to the left.

And that is only due to the difference in rotational velocity. Buried in the CL term is the angle of attack. It to changes as the prop spins by +/- the 5 degree incoming air. Its effect are additive to the rotational values but they are linear not squared so they have much lower effect on the thrust.

That is where P- factor comes from.

Now to slipstream spiral. About 5 years ago I began a study as to where spiraling slipstream came from, and why no one ever discusses the other effects that it should have on an airframe. that the slipstream should also affect the wings and stabilizer the same way that it does the fin, and the result should be a mild roll to the right, but this is NEVER discussed or resolved in written media. This drove me to do a study. I have the fortune of working at a facility that has a huge old aviation library. My study of the slipstream spiral, at least in the 17 or so textbooks I looked at, didn’t exist before the publishing of Stick and Rudder. I came to the conclusion that spiraling slipstream is one of the rare aviation phenomena that has not been mathematically quantified. This combined with the lack of a photograph of a smoke stream actually wrapping around a fuselage lead me to believe that the spiraling slipstream is an aviation myth grounded in Stick and Rudder. However, I recently was directed to a 1930s NACA paper that DOES actually quantify the angle of the slipstream. It says that this twist is at MOST 3 degrees off the centerline of the aircraft. Not the huge force indicated in Stick and Rudder and I'm still working on a summary on this one.

Lastly Gyroscopic precession is just whayt Robinare said.

T

DKjens

My Feedback: (50)

Major Tomski,
My point is, that gyroscopic preseccion is, a force applied acts 90 degrees later. This tells me, that the force resulting from more thrust created from the right tip and less thrust created from the left tip, which is a force yawing left, takes effect 90 degrees later, and should cause a pitch up. I really don't see how this is different from a helicopters rotor disk, except here, the different AOA of the tips are controlled with the swash plate, but to make the heli pitch forward, you increase the AOA of the receding blade, and decrease the AOA of the advancing blade.

I can certainly see how a tail dragger on the ground with engine running WOT will want to yaw like crazy, but I think that's more due to the swirling slip stream hitting the rudder and fuse, than to what's described as the P-Factor. And the reason this is so much worse on a tail dragger than a tri gear, is that a trigear has the steering wheel firmly planted on the tarmac until lift off, whereas the tail dragger has the tail flagging in the air, very succeptical to any air stream.

DKjens

William Robison

My Feedback: (3)

DK:

Quote:

...but I think that's more due to the swirling slip stream hitting the rudder and fuse, than to what's described as the P-Factor. And the reason this is so much worse on a tail dragger than a tri gear, is that a trigear has the steering wheel firmly planted on the tarmac until lift off, whereas the tail dragger has the tail flagging in the air, very succeptical to any air stream.

This, sir, is just plain wrong.

Lee and Tom have tried to explain it, simply put P-factor is caused by the off-center angle of air entry into the prop disc.

Here's a note I wrote several years ago, it explains it in yet another way that might help you see whnat it really is.
------------------------------------------
P-Factor myths exploded (post # 1)

Gentlemen, and twin builders too:

There are many misconceptions about “P-factor,� and the term is misused more often than not.

First, what it is.

When the airflow enters the propeller disc at any angle other than 90 degrees, we have P-factor effects.

For purposes of this discussion we will say our taildragger airplane sits on the ground with the nose pointed up at 15 degrees. We will also say the propeller pitch is 15 degrees, just to simplify things. Now, with the nose up 15 degrees, the prop shaft is also pointing up 15 degrees. Now, when the propeller turns, and the airplane starts its takeoff roll, the rising blade has the 15 degree pitch cancelled by the 15 degree up angle of the prop shaft, and the descending blade has that same 15 degree shaft angle added to its pitch. So, in effect, the descending blade has a 30 degree pitch, and the rising blade has zero pitch. The majority of the pulling power is developed by the descending blade, giving off center thrust, and that off center thrust is “P-factor.� The effect is zero at zero airspeed, and the effect builds until the tail wheel comes off he ground. This is why you have to gradually add right rudder as the airplane accelerates, and neutralize it when the tail wheel lifts, disregarding torque.

When the tail wheel comes off the ground and the airplane assumes a level position continuing the take off run, the air flow into the propeller disc is then on center, P-factor no longer has any effect, because it just isn’t there anymore.

With tricycle gear, and the plane sitting level at rest, there is NO affect on the plane from P-factor. It does not exist. If the plane sits slightly nose down or nose up, there is a small amount, but it’s so small it can be ignored. This is one of the reasons why a trike is so popular for training. Both in full scale and R/C. They are just easier on take off.

In normal flight P-factor will never affect the airplane, as the airflow, in relation to the airplane, never gets more than one or two degrees off axis. Key word here is “Normal� flight. Most aerobatics are done in a normal controlled flight regime.

When doing aerobatics that depart from normal flight, 3d, gyroscopic maneuvers, harriers, and so forth, p-factor can rear its ugly head.

But 99% of what people call P-factor in normal flight is truly nothing but torque reaction, and that’s another story for another time.

I welcome any and all comments pointing out my errors in this, amplifying my statements, or adding something I have forgotten.

Bill.
--------------------------------------------
This note, and several others you might find of interest are located at
http://www.rcuniverse.com/forum/tm.asp?m=1063414

Bill.

William Robison

My Feedback: (3)

A side note -

Wolfgang Langschweisse's book "Stick and Rudder" is, even though it's about 70 years since first publication, still 100% valid. The laws of physics concerning flight have not been changed, no matter how much the politicians might like to. He didn't know HSI, ADI, so forth, but he might have known "Riding the beams." He definitely knew the "Needle and Ball," but your CFI probably called it a "Turn Coordinator."

No matter what names are used for this and that, the book is highly recommended. Study it and you'll be miles ahead on getting your PPL.

And it's still in print. That alone should tell you something.

Bill.

Taildragger726

Very interesting thread, but, really ,, P-factor is a condition that tends to shorten or add discomfort to cross country flying. A contributing factor is usually too much coffee.

You knew it was coming,,,

William Robison

My Feedback: (3)

N7:

Reminds me of an old girl friend, who wanted to experience aerobatics.

I borrowed a friend's UPf-7 and took her for a ride.

Barrel rolls, a loop or two, an Immelmen and a split S, I culd hear her yelling, thought she was having a great time. Then in a spin she let go, barfed over the side.

Later, when she was once agsin rational she asked why I had ignored her yelling at me through the "Gosport" tube? (She was a big fan of early aviation stories) When I explained that it waz not a Gosport (speaking) tube but rather a relief tube she said she had thought it smelled funny.

Bill.

DKjens

My Feedback: (50)

OK, I've made a (poorly drawn) illustration of why I disaggree. This illustrates two rotor/propellor discs.

The disc to the right illustrates a helicopter rotor in hover with a constant pitch (AOA) of +6 deg, and the air hitting the rotor at 90 degrees. A change in pitch is applied through the cyclic. This input adds 5 deg of pitch at 12 o'clock and reduces 5 deg of pitch at 6 o'clock. If gyroscopic precession did not occur, the disc would start tilting towards us, but because of GP, the applied force takes effect 90 deg later, and actually tilts the disc to the left.

The disc to the left is a propellor, with 6 deg of pitch (AOA) at the tips. If the oncoming air direction is changed is illustrated, would it not have the same effect on the AOA that the tips meets the air, as the rotor disc to the right. The books say that this causes the disc to tilt away from us, but if GP is applied, just like on the other disc, the disc on the left would tilt to the right, which would mean applying a pitch up force on the airplane.

I am sorry if I seem/am thick headed, but I really don't see a difference in these two scenarios.

DKjens

William Robison

My Feedback: (3)

DK:

In a helicopter, with its large diameter rotor at a low rpm, precessive forces do play a large part in its operation. A co9nventional propellor with its smaller diameter, even though the rpm (in full scale) is two to three times the rotor speed, precession plays a much smaller role than the AOA of the air coming into the prop disc.

Bill.

DKjens

My Feedback: (50)

Bill,
I understand your argument, I just don't understand why a force isn't a force, whether it acts on a heli rotor disc or a prop rotor disc. I will try to set up an experiment at home as illustrated on the attached sketch. I will mount an electric motor on a long dowel, supported at the top so it can move tilt freely in all directions, like a pendulum. The end of the dowel will be suspended over a reference point. When the motor and prop is turning, there should be no torque forces making the dowel move, since it's kept from spinning. If I take an air blower and blow air from above the rotor disk at an angle, or I could use a large tube around the prop disc and tilt it, I can simmulate the conditions described with real planes (changing the angle the disc meets the air. If I am correct, the disc should want to tilt away from the angle of the incoming air, actually increasing the angle between the disc and the air. If I'm wrong, the disc should want to tilt to the side, perpendicular to the change of angle of the incoming air.

I know this is just a small experiment, but will it not give an indication af the effect of P-Factor.

DKjens

William Robison

My Feedback: (3)

DK:

Sounds like a good experiment. Try to have the incoming air at a high velocity to get as close to the real world as you can.

Let us know the results.

The presessive forces definitely exist, it's just that in a conventional prop they are much smaller than the aerodynamic. In high speed high power aerobatics precessive forces do become a factor to consider, but not in take off or normal climbs.

Bill.

DKjens

My Feedback: (50)

Bill,
Yes, I know that the gyroscopic forces and thereby follows gyroscopic precession are very large in aerobatic planes, but that's due to the changing attitude of the prop disc, right. On a plane either on the tarmac or in a steep climb, the attitude of the prop disc is more or less constant. I want to be sure I am right here. When we talk P-Factor we're just talking the effect the angle of relative air to the prop disc has on the plane, not the whole yawing thing, i.e. the swirling air and other possible factors. If the P-Factor is limited to just this, then I think the force of this is very minimal, and the yaw is really caused by the air stream.

DKjens

William Robison

My Feedback: (3)

DK:

Quote:

...the yaw is really caused by the air stream.

\
Exactly. The air stream entering the prop disc at an angle to the prop's axis of rotation generates the effect we call P-factor.

Bill.

FLYBOY

My Feedback: (11)

I think the thing he is missing is that P-factor happens at a constant angle of attack. There are many factors that cause the left turning tendancies in a climb and as other stated, that is just one of them. The spiraling slip stream is another as discussed. Gyroscopic precession is only a factor if the nose is being moved up or down. In a tailwheel airplane with a normal spinning prop, you lift the nose and it is a left turning tendancy. In a tail dragger, you raise the tail, lowering the nose and it is actually a right turning tendancy due to gyroscopic precession, but the other factors all out weight it and it still needs right rudder. Go back and read all the right turning tendancies in your jeppesen private pilot manual. They explain them pretty well without getting into too much detail so it is easy to understand.

FLYBOY

My Feedback: (11)

http://www.brown.edu/students/Brown_...odynamics2.ppt

This site also explains the left turning tendencies. In your first example, everything would be backwards because you are either thinking of the prop spinning backwards or looking at it from the front. Always think of it from the pilots perspective. the right blade will be moving downward, the left will be moving upward on most single engine planes unless the engine turns backwards.

DKjens

My Feedback: (50)

Flyboy and Bill,
I actually got it now. I kept comparing a spinning prop to the rotor disc of a helicopter, but I forgot that the when cyclic input is given to a heli rotor disc, it changes attitude, and that is why gyroscopic precession is in play al the time there. Once the heli is in forward flight, or the rotor disc is tilted, the cyclic is removed, and the stick is back at neutral.

Flyboy, your explanation about the tail dragger yawing is also absolutely how I percieve it to happen, since the airplane is putting a force to the rotor disc as the tail raises, the resulting force is a yaw, because gyroscopic precession makes the force happen 90 deg later.

Guys, I can't tell you how happy I am that I finally see the light. I see now that I totally forgot about change in attitude of the disc in one scenarion, and no change in the other, and that's the bid deciding factor.

Again, thanks yall.

DKjens

khodges

My Feedback: (1)

Most of the higher math is way over my head, and this is a fascinating subject for me. My train of thought may be way off, but it seems to me that the forward motion of the aircraft introduces an inertial force that whatever combination of gyroscopic precession and P-factor are less able to overcome as the airspeed increases, which would be a reason (among others) that the effects are more apparent during take-off, or at other times when the aircraft is at high-power / low airsped conditions. Anything to that?

FLYBOY

My Feedback: (11)

No! As airspeed increases, P factor isn't present. Think of it as a boat on the water. As you add throttle, the nose comes way up, the prop isn't striking the water the same on both sides, one side has a higher pitch to it. As the boat speeds up, the nose lowers, it gets on the step and the prop is getting the same bite per say on both sides because it is more like 90 degrees to the water going through it. Not an exact way to explain it, but you get the point. The plane does the same thing, as it speeds up, it doesn't move through the air as nose high and the prop blades both bite the same instead of one biting more than the other.

Flyfalcons

Nose up, right rudder. Done.

Taildragger726

Mostly true,, I got to fly a Chipmunk,,,,,the spinny thing on the front was spinning the 'wrong' way.Nose up, left rudder !!

Wayne C

O. K. guys, I'm not a pilot and I'm no expert on aerodynamics. I do know some general physics. Gyroscopic precision happens to any spinning disc all the time and has nothing to do with airflow or angle of attack. It is present in flywheels, automobile wheels, pump impellers and any other round spinning object. Its also why some instruments seem to need frequent adjustment. Its a matter of disc diameter and mass. Considering the mass of the prop as compared to the mass of the rest of the vehicle it is probably a very weak force, easily corrected for. Probably only matters in very extreme movments. It would seem to me to be irrelevent to P-factor except as the forces add to or subtract from each other. just a pssing thought...not to be taken for more than its worth...

Matt_McCarty

My Feedback: (2)

4 types of turning tendencies:

-P-Factor
-Torque
-Spiraling slip stream
-Gyroscopic precession



:-)

Johnny C!

It sounds like all is resolved & all is well here now, BUT!...

I haven't used my physics to this math level that much in the
last 20 years, but I thought the gyroscopic reactionary force was
90 degrees prior in rotation to the input force, not later...

I'm trying to stir the pot, just wanted know if I have
mis-remembered this...

Johnny C!

William Robison

My Feedback: (3)

Johnny C:

Alle-Samee. A force 90 degrees preceeding is equal and opposite to the force 90 degrees trailing. Just depends on how you look at it.

Bill.