Mesae,

I want to respond to your 2/2/06 critique of my earlier post. I don’t mind criticism and challenge, but let’s lay some ground rules. Let’s ignore the rotation of the earth, relativity theory (special or general), quantum mechanics, and alternative dimensions or universes, except in case of emergency. Let’s stick to Newtonian physics. Flawed though it is, it is still useful for predicting the behavior of everyday sized physical objects at common speeds.

First of all, let me restate my main point, which no one has commented on, so far, maybe because it is a little off the subject. Before I read this thread, I believed like most other pilots that spiraling slipstream was a significant force causing an airplane to want to yaw left under various conditions. When I stumbled onto this thread, it was already four pages long, but I liked the inquisitive tone. It made me think, and I began to question some of my previously held beliefs as to why an airplane does what it does.

Actual physical evidence for the spiraling slipstream seems to be hard to find, but I think we all agree that it must be there. Also, I am not questioning the validity of any of the common explanations of P Factor found in most flight manuals.

However, while I was pondering the earlier discussions, it just occurred to me, out of nowhere, that there could be another cause for the left yaw at positive AOA. This is not directly related to spiraling slipstream, but I thought I would toss the idea out there, and see if anyone had any thoughts on it. I am not doubting the existence of the usual suspects, but I am just proposing that there may be another force that has not been discussed, so far as I know. I am suggesting that there should be a yawing force on the airframe that is not a result of forces at the prop hub.

What I am proposing is that there should be a Bernoulli effect. Bernoulli’s law states, “When the speed of a fluid increases, the pressure in the fluid decreases.” If you blow over the top of a soda straw in a glass of liquid, the fluid level in the straw rises. The asymmetrical prop loading at positive AOA causes an asymmetrical acceleration of the air. The air should be moving faster on the right, therefore, the air pressure on that side of the fuselage should be lower, from the nose to the tail. Is there something wrong with my reasoning here? Does this make sense to anyone besides me? Maybe this belongs in another discussion.

Now, back to the gyroscopic effects, briefly, one more time, and then you and Dick Hanson can call it gibberish if you want, but LouW and other readers may want to think about it some more. A spinning propeller would act just like a gyroscope, but only in a vacuum. When you put it in air, aerodynamic forces, such as asymmetrical prop loading, can come into play. There can be no asymmetrical prop loading on a simple gyroscope, but a propeller is a device with both aerodynamic and gyroscopic properties. A gyroscope is just a gyroscope. They are not equivalent devices.

This is your quote from FAA Pilot Handbook. "The rotating propeller of an airplane makes a very good gyroscope and thus has similar properties. Any time a force is applied to deflect the propeller out of its plane of rotation, the resulting force is 90̊ ahead of and in the direction of rotation and in the direction of application, causing a pitching moment, a yawing moment, or a combination of the two depending upon the point at which the force was applied."

This looks like a perfectly good definition to me. “Any time” a force is applied to deflect the plane of a gyro in one direction (such as asymmetrical prop loading would do), the result is a force in another direction. Isn’t that what it says? A gyroscope is a device that redirects force. I do not see where it says anything about motion causing force. I repeat, according to Newton, force results in motion, not the other way around. This may not be as obvious to some as it is to others, but that’s my story, and I am sticking to it.

I will admit that it is not all that obvious. There are web forums devoted to this subject. There are Ph.D.’s who know all the mathematical formulas and can do the calculations, but don’t really get it.

Forget about motion causing force. If you think about it the other way around and reread my posts, they might make more sense.

Maybe this will help. There are no perfect gyroscopes in nature, just like there are no perfectly rigid bodies, and no perfect circles. These are just concepts, but they have proven themselves to be useful. (I wax philosophical, but we did not say no metaphysics allowed.)

Enough on gyroscopes.

Let’s get back to the slipstream.