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Electric Stunt => Gettin all AMP'ed up! => Topic started by: FLOYD CARTER on May 31, 2019, 01:01:05 PM
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What's with all I hear about electric stunters balancing aft of normal? How does the plane know what's pulling it around? Scientific minds want scientific answers. Please, no "first-hand" accounts.
But, for the record (first hand account), my electrics are all bench-trimmed, and CG is where I think it should be. So far, I don't remember having to put lead in the tail.
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But, for the record (first hand account), my electrics are all bench-trimmed, and CG is where I think it should be. So far, I don't remember having to put lead in the tail.
Sounds like a plan. Given your bench trimming I'd bet all are safely flyable, so fly them and let the airplane tell you it it is nose heavy or tail heavy - then adjust handle spacing and/or ballast to suit.
My electrics seem to run the gamut from a little nose heavy to a little tail heavy. Have been mostly successful with changing out components to achieve balance. None of mine have nose ballast (but some have oversize motors) and on a few I have resorted to "nose lightener" (ballast on the tail) based on how they fly.
Just like with slimers..!
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FWIW I think it is the other way around, some are balancing their electrics more nose heavy than they would an IC powered model.
And also with the leadouts farther back.
Some guy with the initials PW for example:
https://stunthanger.com/smf/gettin-all-amp'ed-up!/putting-things-in-strange-places/msg554562/#msg554562
I would also like to hear some sort of theory about why this seems to work .
Pat MacKenzie
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Electric Impacts want a farther forward and higher CG. I don’t know why. I suspect the leadout position has to do with the orientation of the principal inertial axes.
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I Thought it had something to do with shift change in CG with wet fuel usage. Electrics don't have a CG shift during flight. ???
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I Thought it had something to do with shift change in CG with wet fuel usage. Electrics don't have a CG shift during flight. ???
It’s way more than the increment from fuel.
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What's with all I hear about electric stunters balancing aft of normal? How does the plane know what's pulling it around? Scientific minds want scientific answers. Please, no "first-hand" accounts.
You have it backwards, they are trimmed with much further *forward* CGs. I can only guess at why.
It definitely does not know what it pulling it around, but it does know what the speed does in the corners.
Brett
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Electric Impacts want a farther forward and higher CG. I don’t know why. I suspect the leadout position has to do with the orientation of the principal inertial axes.
You and Paul must be envisioning something different than I can. I can see the magnitude going up, but not a significant shift in the orientation. Everything is still down the centerline of the airplane, more or less. Shifting the principle axes significantly requires something to be asymmetrical.
Brett
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I'm thinking as a layman, I have no training in this field, only intuition from playing with my toys. So here's one of my thoughts on the subject:
I wonder if the vibration of an IC engine affects the turbulence of the layer of air over the top of the wing(is that what's called "boundary layer?"), thereby reducing lift in a certain area. Could a particular chordwise location of the maximum(or minimum) point of vibration induced turbulence cause the plane's turning axis to shift fore or aft and necessitate compensating with a leadout shift?
Btw, I'm strictly an IC flyer. I've learned a lot on our Hangout show lately as the discussions have been increasingly e-Powered. And we have some very knowledgeable e-Folks teaching us about it.
Rusty
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OK then. Judging from your comments, perhaps this electric CG question is just a "popular fable".
I do what most do; bench trim the CG "about right", then fly it to see how it handles, and adjust CG to suit.
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I do what most do; bench trim the CG "about right", then fly it to see how it handles, and adjust CG to suit.
That's the truth of the matter! #^
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That's the truth of the matter! #^
Yeah, I get that, and that's the way I would work the problem too. The only reason I conjured such an idea was to feed the technical thinkers among us.
Rusty
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The way I see it is both systems are similar in weight (with electric slightly heavier 1-2oz) but in electric the bulk of the weight (battery) is closer to the CG while the lighter component (motor) is farther from the CG. A purpose built electric should have a lighter nose construction so there is less weight in the nose. So over all you should expect to have an aft CG with electric if you keep everything the same. Another detail here is that electric CG and weight does not change during flight. IC cg changes so if you balance the model normal with 0 fuel at the beginning of the flight you are flying 5-7oz in the nose in addition to the weight you originally have.
If you convert a IC to electric I think you may break even with the CG or have a slightly more nose heavy bird.
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Electric Impacts want a farther forward and higher CG. I don’t know why. I suspect the leadout position has to do with the orientation of the principal inertial axes.
The question is compared to what? If I compare my models converted from IC to electric (actually I had only one ;D ) I did not see any CG shift (except for mentioned fuel effect). HOwever when I used light prop with smaller moment of inertia, I really needed move CG forward, because it has much smaller stabilizing effect. So that could be reason - if you compare Impact with large heavy prop for piped engine and smaller hollowed light prop for electric, it can make that difference.
Another thing is tail size. I went up with tail %. I remember Ted's rule for CG which had to be % of tail to wing area. I found that it well worked for tails up to 25%. However my Max Bee had tail volume from 30% to 33% (actually one had 33% and others had 30%). I found that the rule goes just opposite, that large tail autorithy allows CG more front and still cornerring well. So it looks like small tail up to 20% needs CG front, larger tail up to 25% allows CG little back to 25% and large tail up to 33% again allows CG shift back front. Combined with logaritmic flaps with limited feedback forces I can say that it can well fly also at 10% - that was CG position on my first model with such flaps as I used it on my first WCh in Sebnitz 2002 where it finished 10th. I had identical model with classic flaps and that model had CG at 19% giving similar feeling in maneuvers. But later I went with CG back to make it sharper in corners and Max Bee as I fly it now has CG somewhere at 17%. My older IC models with classic flaps were somewhere at 22% without fuel.
http://www.netax.sk/hexoft/stunt/the_max_ii.htm
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The question is compared to what? If I compare my models converted from IC to electric (actually I had only one ;D ) I did not see any CG shift (except for mentioned fuel effect). HOwever when I used light prop with smaller moment of inertia, I really needed move CG forward, because it has much smaller stabilizing effect. So that could be reason - if you compare Impact with large heavy prop for piped engine and smaller hollowed light prop for electric, it can make that difference.
Another thing is tail size. I went up with tail %. I remember Ted's rule for CG which had to be % of tail to wing area. I found that it well worked for tails up to 25%. However my Max Bee had tail volume from 30% to 33% (actually one had 33% and others had 30%). I found that the rule goes just opposite, that large tail autorithy allows CG more front and still cornerring well. So it looks like small tail up to 20% needs CG front, larger tail up to 25% allows CG little back to 25% and large tail up to 33% again allows CG shift back front. Combined with logaritmic flaps with limited feedback forces I can say that it can well fly also at 10% - that was CG position on my first model with such flaps as I used it on my first WCh in Sebnitz 2002 where it finished 10th. I had identical model with classic flaps and that model had CG at 19% giving similar feeling in maneuvers. But later I went with CG back to make it sharper in corners and Max Bee as I fly it now has CG somewhere at 17%. My older IC models with classic flaps were somewhere at 22% without fuel.
http://www.netax.sk/hexoft/stunt/the_max_ii.htm
Thank you Igor for answering a question I had not even asked yet. I am updating a 2000 design to today's movements and sizes and I redesigned and built the stab to 25% based on the plans stated wing area of 710sq". Later, when I measured the plans and did the math, the actual area came out at 650. That bumped my Stab area to 27%. I thought that may be excessive but I can see from your success that it is not.
Thanks - Ken
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More is better. Forward cg is determined by maneuvering capability, aft cg by stability. A bigger tail allows a wider cg range in both directions.
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Does direction of prop rotation figure into this anywhere?