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Author Topic: Stunt Plane Mass Distribution Peculiarities  (Read 3987 times)

Online Howard Rush

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Stunt Plane Mass Distribution Peculiarities
« on: June 19, 2015, 06:46:30 PM »
Has anyone noticed anything interesting when going from mass in the nose centered on the airplane's X-Z plane to mass in the nose off the X-Z plane?  An example might be switching between a cylinder-down engine mounting to a side mount.  I'd guess that having mass in the nose off the X-Z plane might make the airplane wiggle in corners in roll or yaw.  
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Offline Brett Buck

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Re: Stunt Plane Mass Distribution Peculiarities
« Reply #1 on: June 19, 2015, 10:50:43 PM »
Quote from: Howard Rush on June 19, 2015, 06:46:30 PM
Has anyone noticed anything interesting when going from mass in the nose centered on the airplane's X-Z plane to mass in the nose off the X-Z plane?  An example might be switching between a cylinder-down engine mounting to a side mount.  I'd guess that having mass in the nose off the X-Z plane might make the airplane wiggle in corners in roll or yaw.  

   I did some experiments with weights in various spots to intentionally create products of inertia, and it largely worked the way I expected. That wasn't with the engine in different orientations, but with weights at the tips (near the LE on one side and near the TE on the other).

    Brett
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Offline Chris Wilson

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Re: Stunt Plane Mass Distribution Peculiarities
« Reply #2 on: June 20, 2015, 08:11:59 PM »
What were your expectations Brett?
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Offline Brett Buck

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Re: Stunt Plane Mass Distribution Peculiarities
« Reply #3 on: June 20, 2015, 08:50:03 PM »
Quote from: Chris Wilson on June 20, 2015, 08:11:59 PM
What were your expectations Brett?

   I expected that as I shifted the principle axes from the geometric axes that it would result in (in this case) yaw torque (from masses at the LE and TE rotating the inertial roll and pitch axes about yaw). Essentially, it would try to rotate around the principle axes instead of the geometric axes. Interestingly. I got the least overall motion with a small intentional shift, which suggested to me that the principle axes of the unaltered airplane was already off a bit. I fiddled around for a long time, then decided to build in the weights. This was with the #2 Infinity that crashed in 2005. I haven't tried it with the current airplane.

      Brett
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Offline Chris Wilson

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Re: Stunt Plane Mass Distribution Peculiarities
« Reply #4 on: June 20, 2015, 10:08:47 PM »
Thanks Brett, what was the intentional shift that resulted little change?
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MAAA AUS 73427

You have enemies? Good. That means you've stood up for something, sometime in your life.
 Nothing in life is so exhilarating as to be shot at without result.  It's not enough that we do our best; sometimes we have to do what's required

Offline Brett Buck

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Re: Stunt Plane Mass Distribution Peculiarities
« Reply #5 on: June 20, 2015, 10:18:15 PM »
Quote from: Chris Wilson on June 20, 2015, 10:08:47 PM
Thanks Brett, what was the intentional shift that resulted little change?


   Intentional shift of the apparent principle axes - essentially, it flew a little better with about 1/4 ounce of weight on the inboard LE and the outboard TE, shifting the principle axes a bit about +Z.

    This was all discussed at length on SSW about 10 years ago, in the context of where you wanted to place the tipweight box.

    Brett
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Online Howard Rush

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Re: Stunt Plane Mass Distribution Peculiarities
« Reply #6 on: June 21, 2015, 12:01:01 AM »
Thanks, Brett.  That was my suspicion.  Electrickery gives one some latitude about where to put the "fuel", so I'll take some advantage of it.  
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Offline Brett Buck

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Re: Stunt Plane Mass Distribution Peculiarities
« Reply #7 on: June 21, 2015, 02:11:49 PM »
Quote from: Howard Rush on June 21, 2015, 12:01:01 AM
Thanks, Brett.  That was my suspicion.  Electrickery gives one some latitude about where to put the "fuel", so I'll take some advantage of it.  

     The one thing I didn't expect was that it appeared the principle axes were skewed noticeably before I started. I might have expected some of that in the other axes, but not about pitch. There *were* a lot of structural repairs by the time I did it, so, who knows.

     A possible alternative explanation was that I was compensating for kinematics effects as discussed in the SSW thread on the topic.

    Brett
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Offline Igor Burger

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Re: Stunt Plane Mass Distribution Peculiarities
« Reply #8 on: June 22, 2015, 05:12:45 AM »
Quote from: Howard Rush on June 19, 2015, 06:46:30 PM
Has anyone noticed anything interesting when going from mass in the nose centered on the airplane's X-Z plane to mass in the nose off the X-Z plane?  An example might be switching between a cylinder-down engine mounting to a side mount.  I'd guess that having mass in the nose off the X-Z plane might make the airplane wiggle in corners in roll or yaw.  

Not only yaw and roll but also asymetric AoA :- ))  I think its effect is trimmable by control surfaces ... or did I miss something?


My model has battery on top of fuselage in line with motor, so that the CG is aproximately 15mm over the wing (and also LO on purpose). All works as I expected.
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Offline Lauri Malila

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Re: Stunt Plane Mass Distribution Peculiarities
« Reply #9 on: June 22, 2015, 03:29:45 PM »
So, how big problems can I expect when I turned the cylinder 30 degrees more down in my new model? Old is -10 degrees from horizontal, new -40.
To compensate, I glued tipweight to wing's top skin and moved lead-outs down. I hope it works like that but the model will be light, about 55oz. so some extra weight to balance it is not out of question..

Lauri
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Offline Tim Wescott

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Re: Stunt Plane Mass Distribution Peculiarities
« Reply #10 on: June 22, 2015, 03:42:11 PM »
Quote from: Lauri Malila on June 22, 2015, 03:29:45 PM
So, how big problems can I expect when I turned the cylinder 30 degrees more down in my new model? Old is -10 degrees from horizontal, new -40.
To compensate, I glued tipweight to wing's top skin and moved lead-outs down. I hope it works like that but the model will be light, about 55oz. so some extra weight to balance it is not out of question..

If you're going to be messing around with the motor in a way that changes the vertical CG, it might be a good idea to make the leadout guide as a separate piece that can either be adjusted up and down, or that can be replaced with one that puts the leadout at a different location WRT the wing.

The leadout guide isn't stressed much as far as I know, so you should be able to keep it light.
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Offline Lauri Malila

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Re: Stunt Plane Mass Distribution Peculiarities
« Reply #11 on: June 22, 2015, 03:52:56 PM »
I'm secretly hoping that someone who knows the maths could give a rough estimate how much the cg shift is.
I think it's not a catastrophe, my engine is very light. L
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Offline Igor Burger

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Re: Stunt Plane Mass Distribution Peculiarities
« Reply #12 on: June 22, 2015, 10:47:00 PM »
Hang your model on leadouts and you will see :- ))
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