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A Q on gyroscopic effect of propellers

10 Posts

Here

The mass of the two bladed propeller is many times greater in the direction of the blades than at right angles to them. Thus the gyroscopic resistance to a change in direction is large when perpendicular to the maneuver and small when the blades are in the plane of the maneuver.

Engines with three bladed propellers have their mass distributed more evenly and offer the same resistance to change in direction at any blade position.

Can anybody explain this? I am sure it's wrong.

Administrator
Shoreham EGKA, United Kingdom

I am sure it's wrong.

Not wrong, but explained weirdly... I'm afraid I can't do much better myself (at least not in English and not without pen and paper).

But try this text here (it is about model aircraft propellers, but physics is physics!), the explanation given is a bit more precise: http://www.supercoolprops.com/articles/gyrovibes.php Start reading from the paragraph beginning with "Back to reality"!

EDDS - Stuttgart

What I think they try to say is something like this.

"With a two-bladed prop there comes a time (2x2700 times a minute to be precise, at max RPM) when the blades are horizontal, momentarily. At this precise moment you can pitch the aircraft up or down with no gyroscopic effects. Similarly, when a two-bladed prop is exactly vertical you can yaw the aircraft with no gyroscopic effects. (Or actually the other way around, I don't know exactly.)

With a prop with three or more blades there is always a blade that is positioned so that if you yaw or pitch the aircraft, that blade will resist that yaw or pitch because of gyroscopic effects."

But I agree the text is rather hopeless. It starts going wrong in the first half sentence. "The mass of the two bladed propeller is many times greater in the direction of the..." Mass is a constant (except when close to light speed) and does not alter with the direction. If they would have used "momentum" or "impulse" or something like that, at least it would have been scientifically correct. Not more readable, just not scientifically wrong anymore.

I would agree that a 2B prop would have a smaller gyro effect than a 3B prop, but only because there is less metal in it.

But these articles say something quite different.

The fact that a 2B prop spends a bit of its time in a position where its inertia around the plane's yaw axis is nil, is IMHO irrelevant because the yaw rate is orders of magnitude slower than the prop rotation.

If you were a really fast worker and, with the 2B prop, you waited till it is in just the right angular orientation, and jumped on the pedals and yawed the plane through the required angle in those few milliseconds, then a 2B prop would indeed have the stated advantage.

Otherwise, i.e. in any practical situation, I just can't see how this works.

Administrator
Shoreham EGKA, United Kingdom

Think of it a different way - when you fly a two bladed Bell 206 helicopter, there are times when your control input has no immediate affect, as the blades are on the axis of the desired motion of the helicopter - so you gotta wait 'till they get around to that point (plus precession, but that's beyond the scope of this simple analogy). For the unwary, you have put in even more control by then, and are over controlling, but that's a different story.

Then you have a three or more bladed rotor, control input, reaction - no waiting...

That said, other than for the most agile aerobatic aircraft, I doubt that this is much of a concern...

Home runway, in central Ontario, Canada

I would agree that a 2B prop would have a smaller gyro effect than a 3B prop, but only because there is less metal in it.

The 3-blade prop is likely also smaller in diameter, and IIRC the gyroscopic moment goes as square of the disk diameter.

The Sky Ranch web page seems like an out of context cut and paste.

What about the old rule that the physics of a spinning prop are the physics of a spinning disc?

That would be my view, which is why the number of blades appears irrelevant to aircraft handling - for a given moment of inertia of the disk.

I also think what I wrote above, about yawing the aircraft at the precise moment the 2B prop is in the optimal orientation, would be true if the prop actually briefly stopped rotating at that point, which it clearly doesn't.

So I think both of the websites that talk about the gyroscopic effects being directly related to the number of blades are incorrect.

Administrator
Shoreham EGKA, United Kingdom

Maybe instead of number of blades they actually refer to the diameter of the prop?

If there was such an influence, I think a 2 blade prop would shake the aircraft much more than a 3 blade prop which it does not do (provided the masses are balanced).

Regarding the helicopter, the blades are much longer and most importantly they spin slowly compared to a prop so I would assume that you see more effects that are not within the physics of a spinning disc.

I suggest that another way of looking at this issue is the practicalities of the time taken to effect a pitch or yaw change as a result of a control input, relevant to the prop position.

Considering just a 2-bladed prop, the blades changing position from the vertical (minimised effect on e.g. yaw changes) to the horizontal position (maximum effect on e.g. yaw changes) is 90 degrees - or 1/4 of a turn. At 2700RPM, the angular velocity in Hz of the prop is 2700/60=45Hz or 45 full rotations of the prop in a second. So the number of 90 degree position changes would therefore be 4x45=180 changes in one second. 1/180th of a second or 0.0055 seconds

If you make a control input at T=0, the time taken to make a control surface position change, the changed airflow to apply a force to the control surface, the effect of that force to overcome the intertial effects of the airframe and the resulting positional change in airframe rotation about its centre of mass; is all going to take quite a few 1/180ths of a second IMO.

So I don't think (for a SEP operating at typical engine speeds with 2, 3 or even 4-bladed props), that this effect is even a factor, and that a better thing to do would be to consider the rotating prop simply as a solid disc with the associated moments of intertia. A heli may be slightly different because of the lower rotational speeds, and a larger 'disc' of course.

EuropaBoy
EGBW
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