Asymetrical Thrust and Gyroscopic precession

[ProudPilot]

Ok, so here's one that just came up in MEI. Why isn't P-factor subject to gyroscopic precession?

If thrust is a force, felt by the propeller, when we talk about P-factor why isn't the force felt by the top and bottom of the blade?

 

[tgrayson]

If thrust is a force, felt by the propeller, when we talk about P-factor why isn't the force felt by the top and bottom of the blade?

Precession happens when the axis of rotation is changed. The idea that a force is "felt" 90 degrees from the point of application is really just a model to aid in thinking about which way the gyroscope is going to precess.

As long as you control the yawing force with another yawing force, the axis of rotation of the propellers will not change and you will have no precession.

 

[ProudPilot]

Ok, I've had some basic physics but my definition is incorrect on precession so I think I need the crash course.

Precession is due to a torque force. So as an airplane would pitch, a new outside force would torque the propeller blade at top and bottom and be felt 90 degrees in the direction of rotation. However, if there is uneven lift, due to a differential AOA on left and right side of a propller arc, wouldn't that also cause an uneven lift, and therefore blade torque along the rotational disk causing precession?

The science and definition is escaping my grasp. Anyway to dumb it down a bit would help.

Thank you!

 

[tgrayson]

Anyway to dumb it down a bit would help.

Unfortunately, no. Gyroscopes are not intuitive at all, and every explanation I've seen of their behavior relied on rotational vectors. For instance:

P-factor wants to yaw the airplane counter-clockwise when viewed from above. The torque vector is determined using the right hand rule; curl your fingers in the direction of rotation and the resulting vector lies along your thumb. In this case, it points up. Now, the momentum vector of the propeller is determined in the same way, which creates a momentum vector that lies along the axis of rotation pointing ahead of the airplane. If you combine the momentum vector and torque vector, you get a new momentum vector that points slightly upwards, which implies the pitch up you expect.

However, when you use right rudder, you create a torque vector that lies equal and opposite of the p-factor vector, and so they cancel, and the momentum vector of the propeller does not change its orientation.

I realize that's a terribly unsatisfying explanation.

 

[ProudPilot]

Yeah... I'm going to have to defer to someone smarter than I to decipher that. I'll bring it along for my MEI class, however. I appreciate the technically correct definition because now I feel like a private pilot finding out the Va is an angle and not an airspeed.

 

[meritflyer]

Wouldn't every V speed technically be an angle?

 

[tgrayson]

Wouldn't every V speed technically be an angle?

Angle? Angle of Attack? If so, absolutely.

 

[juxtapilot]

Yeah... I'm going to have to defer to someone smarter than I to decipher that. I'll bring it along for my MEI class, however. I appreciate the technically correct definition because now I feel like a private pilot finding out the Va is an angle and not an airspeed.

I asked Zeidlik to dumb it down a bit once and he just laughed and said "It's PFM." I think thats true.

 

[Realms09]

Unfortunately, no. Gyroscopes are not intuitive at all, and every explanation I've seen of their behavior relied on rotational vectors.

I realize that's a terribly unsatisfying explanation.

Here is my attempt at explaining the effect without reference to rotational vectors. It may still in fact be unsatisfying

The apparently odd behavior of gyroscopes (such as a spinning aircraft propeller) arises from a simplification our mind naturally makes when we look at these sorts of things. What does a spinning propeller look like? To our eyes it looks like a solid stationary disc and should therefore behave like a solid stationary disc. But the reality is it is not a stationary disc - it is a large group of moving particles of mass. Thankfully, the tools of basic physics tell us how particles of mass react when they are acted upon by forces.

Basic Physics

Concept 1: A moving particle of mass has momentum, which is equal to its mass times its velocity. This quantity has a direction, meaning 5 units of momentum up is different than 5 units of momentum to the left, down, or right.

Concept 2: Change in momentum is caused by forces. Specifically, the time rate of change of momentum is equal to force (F = mass X acceleration). If the a force acts on an object, that object's momentum will adjust to be more in the direction of the force.

Let's apply these basic concepts to a simplified propeller.

Simplified Propeller

1. Spinning undisturbed.

I've provided an ugly sketch of the idea in the file Precession 1.jpg. In this example two particles of mass are connected to one another with a rod (whose mass is ignored). The masses and rod spin together on a pivot point that is located in the middle of the rod. The velocities of the two masses are the same in magnitude but opposite in direction. Three different views of the same setup are shown to describe the situation: a top view, a front view, and a side view. One mass hides the other mass from view in the side projection. Notice that here the masses are spinning completely in the horizontal plane (which also means the velocity vectors lie completely in the horizontal plane).

2. A torque is applied.

Let's move on to the top of Precession 2.jpg. Here we apply a torque to the system by pushing one particle up and pushing the other particle down by the same amount. The forces are indicated by gray arrows. Notice that a torque is apparent in the front view and top view, but that no torque is apparent in the side view.

3. Effects of the torque.

For simplicity we apply these forces really fast - so fast that they change the momentum of the masses in an instant impulse (like hitting a ball with a bat). What effects do the forces have? To answer this question we look closer at the side view. Farther down in the picture I've un-eclipsed the masses from the side view to look at what happens at them individually. The first mass is initially traveling horizontally, but the force delivers a vertical impulse that changes its velocity to be more upward in direction. Similarly, the second mass is initially traveling horizontally, but the force delivers a downward impulse that changes its velocity to be more downward in direction. The trajectory of one is angled upward and the other is angled downward. Putting the particles back on top of each other in the side view, the conclusion is upon us. The plane of rotation, which was initially horizontal in the side view, is now angled away from the horizontal. The propeller tilts in the side plane.

Conclusion
The weird nature of this result is that a torque is apparent in the top view and the front view, but the change in propeller orientation is apparent in the side view. This is the source of the "90 deg. in the direction of rotation rule." What I've tried to show, however, is that there is nothing weird about it.

The masses just go where they are pushed to go.

 

[seagull]

I'll take a stab at an even easier explanation of it, in terms that a kid can understand (used it on my own!). I have demonstrated and drawn it a few times, so this is explanation without pictures is more challenging.

Take a ball attached to a rope, and have it swinging around fast enough until the rope is out straight. The rope is attached to a fixed point in the center (could be your hand just spinning it in one spot, try it with a string and a weight attached to the end). It's simplest to spin it horizontally (plane of rotation is parallel to the ground).

Now, at any point on it, while it is rotating, give it a whack straight down.

It is still attached by the string to the same point. So, what does it do? Well, it is still going around, so we have that. It is going to start moving downwards as well, and it will continue to do that until it reaches a point (depending on the combination of rotational speed and downward speed you gave it, so depending on how hard you hit it), until it reaches a point where, due to being attached to the string, it has to start back up again.

Now, the plane of rotation has changed. If you try this, or just think about it, that change HAS to be 90 degrees from where you first hit it (applied the force).

So, what does this have to do with a prop, or a spinning wheel? A disk is just a string with an infinite amount of weights on it all spinning around the same point. A prop, of course, is just 2 or what ever amount of blades it has. It all works out the same, and the concept is easiest to understand by looking at a single point.

Probably could have done that better, but kids woke me up after not enough sleep, so, for those that see what I'm saying here, feel free to clarify it for the rest!

 

[Gumps]

"As long as you control the yawing force with another yawing force, the axis of rotation of the propellers will not change and you will have no precession." -tgrayson

I think what the O.P. is asking is why do we have left turning tendencies associated with p-factor instead of pitch up tendencies? Your anwser assumes coordinated flight at all times.

Personaly I believe the concept of precession is over emphasised with easy-to-go-to training aids like spinning bicycle wheels and weights on a string. The actual amount of precesson felt on training airplanes (at least) seems quite small compaired to the mass of the airplane. Otherwise EVERY pitch and yaw input we use would be 90 degrees out of phase! There probably is a small amount of pfactor induce pitch via precession but we adjust for it. (when we're un-coordinated)

On the other hand you could just be a smarta$$ and say,"When I apply power the nose pitches up, 'must be p-factor induced pitch via precession!"- And who knows maybe that's part of it!?? :dunno:

 

[tgrayson]

I think what the O.P. is asking is why do we have left turning tendencies associated with p-factor instead of pitch up tendencies? Your anwser assumes coordinated flight at all times.

I did assume the OP wanted to know why we needed right rudder, which is what I answered. If you assume no rudder application, then the question becomes moot, because we *do* get a pitch up in that situation.