Static versus Dynamic Weight

[Jag01]

Hi Guys,

Ok so here the thing, I am wondering, does a rotating prop actually add to the static weight on an aircraft when considering weight distribution for CG purposes.

For example let say I have an aircraft that flys tail heavy, so simple answer put more weight up the nose end. OK so if I place a heavier prop or spinner for that matter, which would increase the static weight distribution, would that actually have the desired effect of increasing nose weight when the engine is running and the aircraft is flying.

Pretty simple question however I somehow dont think the answer is quite so simple. I look forward to your ideas.

Cheers,

Jeff

[Bill Clark]

i always think about things like this by going to the extreme. my brain tells me if you put a 2000 pound prop on it would definetly make a difference. the gyroscopic effect is probabley not going to do you any good though if your into agressive 3d. how about one of those fat steel spinner nuts?

[Bam Bam]

I am no expert, but i use a carbon spinner for 3d that is 140gr lighter than the true turn i use for imac. i can definatly tell the diffrence in the c.g. change.

[aviti]

Good question...I've wondered this myself.

[Jag01]

Yes it is a good question and everyone that I have spoken too has a different theory. I really would like to get to the bottom of this one.

I can totally understand the whole gyroscopic effect of a heavier rotating mass, however I am yet to be convinced about the variation in affect on actual CG of the aircraft and whether (if any) the effect is linear by comparison to the weight.

I would really like more input and ideas on this one.

cheers,

jeff

[Mithrandir]

the center of gravity of the prop remains unchanged when it is turning (or when it isn't turning).... the CG is the same regardless of the props RPMs...

But... a tractor configuration (prop in the front) has elements of Pitch and yaw instability whereas a pusher is stable... or tends to be stable'izing.....

of course... this is assuming newtonian physics.... as we all know... the mass of an object does increase with increasing velocity...

[ombry]

The short answer is that the center of mass will move just as if the weight it static (no rotation). You could balance the plane with the heavier prop and the motor stopped, and then start the engine and acheive the same balance location (provided the thrust force didn't influence the balance - perhaps better to perform this experiment in a vacuum). The flying inverted trim test for CG would produce the same result since this is more or less a steady state test (meaning the flight conditions aren't dynamic through the observation period and thus gyroscopic effects are minimal).

However, this does not mean that the dynamic handling qualities we typically associate with a forward CG will be identical between the rotating and non-rotating mass. A larger rotating mass will induce greater gyroscopic stabilization about the pitch and yaw axes at the expense of greater gyroscopic coupling between the two. This might produce a feeling of being even more nose heavy as the plane might feel less responsive in pitch and yaw.

[Ian]

	Quote: Originally Posted by Mithrandir
	of course... this is assuming newtonian physics.... as we all know... the mass of an object does increase with increasing velocity...

And the plane gets narrower, and time slows down. does this mean wing loading goes up and you get less time out of a tank?

Maybe I should stop flying at 0.99999C

[sherman89]

Any weight added forward of the datum will move the CG forward, any weight added behind the datum will move the CG reward. Gyroscopic procession is a separate issue that will influence flight characteristics. A spinner will have less effect than a weighted tipped prop because it's mass is centered closer to the point of rotation. A weighted prop nut will have less effect than a spinner. A battery pak on the firewall will have no effect.

[Mr.Willow]

I don't know about the effect heavier spinner has on dynamic weight, but you can get the engine to spool better with lighter spinner. I would rather add weight to motor box than add a heavy spinner...

[bodywerks]

Yes a heavier prop or spinner will change the cG of your plane, even when the prop and spinner are turning. But the effects of gyroscopic precession, for example, become more pronounced, and there will be more of a lag when you accelerate or decelerate the throttle.

[BobPhx]

	Quote: Originally Posted by Ian
	And the plane gets narrower, and time slows down. does this mean wing loading goes up and you get less time out of a tank? Maybe I should stop flying at 0.99999C

From YOUR perspective you would get MORE time out of a tank. But from the TANK's perspective.. it would be the same....