Noseweight

[Curare]

Hey guys, just another stupid question from me.

I snuck out with my electric imitation and pushed it though sequence yesterday, and it was pretty had to control exits, rounds were too high, and corners would bobble. I decided that noseweight was the remedy, and fitted a reasonably heavy plastic spinner. This morning the flight was much more locked in, but I did noticed that I'm getting a lot of yaw on the squares. I'm now wondering whether having a heavy spinner (reciprocating weight) would be adding to the gyroscopic precession, which is giving me more yaw.

I'm now thinking that fixed weight under the engine would have been a better idea, so as to reduce my reciprocating weight.

Thoughts?

[Tim Wescott]

First, the obligatory engineering language lesson:

You're thinking of rotating mass.  "Reciprocating" means something going back and forth.  To soar to the heights of picky, "weight" is how hard something pulls on your arm when you pick it up (i.e., it's a force), while "mass" is the amount of "stuff" in the thing.  Mass is invariant on this slow ol' earth, while weight can vary depending on how the object in question is being accelerated.

Second, to actually try to answer your question:

Gyroscopic precession is proportional to the moment* of inertia of the rotating mass.  The contribution of each particle in a rotating mass to the whole is proportional to the square of the distance from the center.  That means that a gram of stuff at the tip of a 12" propeller has 36 times as much effect as a gram of stuff on the very edge of the spinner.

So I don't think that's the answer.

I suspect that what happened is that before you put on the tip weight the flight was so wild that you just didn't notice the yaw.  Now you've calmed down the wingwazzles from a too-rearward center of gravity, you're at leisure to notice the next worse thing -- which is the yaw.

That was the easy part.  Now the hard part: is it really yaw you're seeing, and what to do about it?  Make sure it's not roll because of incorrect tip weight.  Make sure it's not a combination of tip weight problems and leadout position that's turning the roll into yaw.  Make sure that your wing and elevator are both level at the same time.  Make sure you don't have any significant tweak in the rudder.

Uh, what else?  Oh -- ask someone smarter than me what may be causing yaw.

It certainly won't hurt anything to put weight somewhere that's not spinning, and the yaw could be from something that is spinning -- I just don't think that the spinner is your problem.

* There's that word again!

[Curare]

Tim, you're right on all counts, I've been reading too many engine building forums, where reciprocating mass means something, (completely different from what I was talking about).

As for your comment that it might have always been doing it but bigger issues were present is probably pretty valid, I can't honesty say whether it was doing it before or not.

Might have a fiddle with a bit of tipweight next time, and pay a bit more attention to what's going on where.

[Mark Scarborough]

If you moved the CG forward,, ( which is what you did), it is possible that the leadouts need some adjustment since that position is relative to speed and center of gravity,, I believe if the leadouts were to far aft you could see some funky business,,

Tim, lots of words well put in place,,,, you have done this writing stuff before,, 

[Steve Helmick]

You moved the CG without moving the LO's forward the same amount? Maybe that's it? Also, remember that an IC plane converted to electrons needs the CG moved forward by a fair amount, because of the lack of fuel weight...er, MASS.  Steve

[Randy Cuberly]

Greg,
Locate the new CG and set the center of the leadouts about 1/2 inch behind that to start.
snap some reasonably hard corners and watch the wing.  If the outboard tip drops hard, reduce the tip weight about 1/8 ounce at a time until it doesn't.  Check your line tension above 45 degrees if it is reduced by a lot then move the leadouts forward about 1/8 inch at a time until it improves.  When you get the leadouts wher the line tension above 45 degrees is OK then you may be able to add a little tip weight back to improve tension further.

Keep in mind that sensitivity may be relative to control deflection and reducing you handle spread may be in order also.

Unless you have something else like warps or rudder deflection that should also reduce the "Yaw" you saw.  I assume the "Yaw" was an oscillating motion lateral to the wing.  That usually indicates a leadout position that is too far aft.  Like Tim I seriously doubt that it's caused by the spinner weight.

I do agree that it's better to add "dead weight" than on the rotating mass, but I've seen it done successfully many times.

Randy Cuberly

[Brett Buck]

This morning the flight was much more locked in, but I did noticed that I'm getting a lot of yaw on the squares. I'm now wondering whether having a heavy spinner (reciprocating weight) would be adding to the gyroscopic precession, which is giving me more yaw.

I'm now thinking that fixed weight under the engine would have been a better idea, so as to reduce my reciprocating weight.

   Rotating, as noted. But the answer is almost certainly no. Precession is a relatively small effect compared to almost any other of the possible/likely trim errors that cause the same issue.

     Brett