Movable Tip Weight

[Ken Culbertson]

This is what happens to your brain when you can't fly or build on a beautiful Saturday with light winds.   You get brain farts like this one but, sometimes....

When I started using my battery location to fine tune my fore-aft CG I started wondering about other weight concentrations and what could be the result of making them adjustable (Matt-stop laughing).  The actual CG is the spot on the plane where it will balance when placed on a "pin" including lines and all that jazz.  Almost impossible to locate since the plane will be flying.  So what happens to the real CG when we move the tip weight forward or back.  The side to side location would stay the same while the fore aft would change. It seems that having the tip weight movable, say through a tube running front to back might give you an easy way to quickly fine trim the fore aft CG without changing the weight of the plane.   Another candidate might be the wheel location on fuselage mounted gear.  I think I read where Ted Fancher tried movable ballast.  Same concept.  Adding an ounce or so of doo-dad devices to the outboard tip should not be a problem.  I have rarely carried less than an ounce in tip weight.

I am disposed to give it a try on "Trifecta" nearing completion but if someone else has tried it and it is a stupid idea, pull me back from the cliff.

Ken

[Tim Wescott]

Well, yes, but.

It probably doesn't matter for what you're proposing, but balance gets weird when you're talking about rotating bodies (like your plane when you're doing a corner).

For a body that has an effectively spherical weight distribution, the rotation of the body is exactly parallel to the torque moment applied to it.  For a body that doesn't have an effectively spherical weight distribution, like, say, a toy airplane, the rotation of the body is only parallel to the torque moment if that torque moment happens to be around one of the bodies natural axes of rotation (there's three of them, all at right angles to each other -- if you want a deep dive, they're the principal axes of the moment of inertia tensor).  It was this little physics detail that led to the infamous inertial roll coupling incidents that crashed a bunch of long skinny jet fighters in the 1950's and '60's (long & skinny = not effectively a spherical weight distribution).

For a nice symmetrical plane, the natural axes of rotation should be along the fuselage, along the wing, and straight up and down.  For a plane that's skewed a bit, by, say, having a tip weight way in the back of the wingtip, then those axes of rotation won't be congruent with the wings and fuselage -- they'll be rotated a bit.  That means that as you're flying a corner, the torque needed to rotate the airplane in pitch will also rotate it a bit in yaw.

Now, I think that the amount of difference a one ounce tip weight is going to have on a 50 ounce plane is going to be minimal.  If you're absolutely world class you may notice it, but if you're merely pretty good, then I don't think you will.

None the less, when I've considered tip weight anywhere significantly forward or aft of the plane's CG, I've said "nah", and left it in the "usual" spot.

[PJ Rowland]

Tim, you have such a clear aerodynamic understanding, and that explanation is really sweet considering how "short" it was..

When I played around with Warts I was very close to adding the acronym Wing Anti Roll Tensor on it for a laugh, but assumed noone would get it.

For what its worth.. the effect ( If I understand it correctly) what is the result of having a "fixed" amount of wingtip weight and moving it forward and back?

I have a weight box and it CAN be more complex due to having a cube for a tip box. There is an argument to be had that WHENEVER YOU move the actual location of the tipweight ( I have the weight wrapped in tape so bits dont move themselves ) any time the weight is moved this has an effect on the weight distribution.

Tim is correct in that its not a simple answer to say "it does this " because the basis of that explanation involves weight.and vector distribution that is constantly under change.

But the best I can answer is.. when I move × amount of fixed weight from the front of the box to the back of the box ( 1 " change) I notice the equivalent of "less tip weight" 

If I raise or lower it vertically within the box I cant say I notice anything.. maybe I could talk myself into feeling a vertical CG shift under some manouvers... but id be kinda guessing..

Whereas For/aft seems somewhat noticeable, I dont even use it as a trim function. As forward as I can go and as much as is needed to give a clean corner.

[Ken Culbertson]

Whereas For/aft seems somewhat noticeable, I dont even use it as a trim function. As forward as I can go and as much as is needed to give a clean corner.

I am responding to both you and Tim.  Thank you for the explanation.  I actually understood it.  What both of you said in different words was that a shift in the location of the tip weight would affect trim.  But what I also read was that it would probably make it worse considering that I already put my tip weight at or slightly forward of the CG and that is really close to the tip LE.  So the only way to go would be back and that would make it worse.  Thanks guys, I am moving back from the cliff.

Ken