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CG Question

Old 04-19-2011, 05:37 AM
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LostMyPlane
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Default CG Question

I was wondering if the CG of a wing stays the same if used in a different plane configuration. I have a straight wing off a low wing stick type plane and would like to build a fuse going from the trailing edge of the wing to about 9” in front of the leading edge to convert it to a flying wing. The wing’s CG is about 4 1/2" from the leading edge. Will this CG stay the same if used in a flying wing configuration?

I made a simple pic of what I’m thinking.

Thanks

Maximillion
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Old 04-19-2011, 05:55 AM
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Default RE: CG Question

Hi!
Ofcourse!
But ..remember that a flying wing needs a reflex airfoil to work ok!
Old 04-19-2011, 06:09 AM
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LostMyPlane
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Default RE: CG Question

Thanks Jaka!!
Old 04-19-2011, 06:10 AM
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MinnFlyer
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Default RE: CG Question

In a word, yes.

It is the wing that does the "flying". The tail is only there to guide the wing and the fuse is only there to hold the tail... of course, sometimes it also holds the engine and pilot
Old 04-19-2011, 08:07 AM
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Default RE: CG Question

Careful... some of the terminology is being misapplied here....

NO the C.G. ( Center of Gravity ) does NOT "stay the same" once the wing is on a plane.... or used in a different plane.

Rather it is affected by what you have in the fuselage, etc.



The CENTER OF LIFT which is what you really are concerned about, stays in the same position on the wing... however for the plane to fly the CENTER OF GRAVITY of the PLANE must stay in a workable postion relative to the CENTER OF LIFT.

That relative position may NOT necessarily be at the 4 1/2" mark at all!

Don't forget that a flat wing elevator versus an airfoil elevator on a plane will affect where the C.G. must occur for the plane to be stable.

Also in flying wing planes you must shift the C.G. substantially forward to aid stability to counteract the lack of the tail forces.
Old 04-19-2011, 08:13 AM
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Default RE: CG Question

Wow, alot of great info....thanks a ton guys and keep it coming!!
Old 04-19-2011, 09:47 AM
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Default RE: CG Question

Jaka hit on an issue that is likely to sideline the project, namely the type of airfoil needed on a flying wing. The trailing edge has to reflex upward to counteract the down pitching moment of the airfoil. Normally the tail does that for you, but without a tail it has to be done with the wing itself. Reflexing the ailerons upward might be enough, but generally that reflex is built in a gradual sweeping shape as opposed to the abrupt angle that moving the ailerons will give you.
Old 04-19-2011, 09:50 AM
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Default RE: CG Question

LMP,

How are you going to compensate for the missing weight of the tail?

Read these threads:

http://www.rcuniverse.com/forum/m_62...tm.htm#6231164

http://www.rcuniverse.com/forum/fb.asp?m=9462075

http://www.rcuniverse.com/forum/m_17...tm.htm#1775052
Old 04-19-2011, 09:58 AM
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Default RE: CG Question

At the very least given that it's a straight wing, he'll need booms or some other mechanism to provide some counterbalance... and of course some vertical surfaces for stability.... then he'll need a nose to move the engine and C.G. forward.

IMHO too much re-invention of the wheel.

I'd just fashion another fuse and tail...

I have one large Edge 540 plane where I trashed the fuse and tail, but the wings are pristine.

I've started working on a profile fuselage and grabbed a tail from another crashed plane of similiar size.

I know this will fly.
Old 04-19-2011, 09:59 AM
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Default RE: CG Question

ORIGINAL: jaka

But ..remember that a flying wing needs a reflex airfoil to work ok!
Nonsense. I have built many flying wings and they have all been symmetrical
Old 04-19-2011, 12:56 PM
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Default RE: CG Question

I think the understanding of CG gets confused many times, for some newbies and even some experienced guys, because when we try to ask or answer questions regarding it, the explanation is difficult.

Every plane has a center of gravity (CG). It is the "actual" point at which the plane sets level. If you add weight to the front or the back the "actual" center of gravity for that plane changes. This is not to be confused with where the center of gravity "should be".

The "should be" or "recommended" CG is a particular spot that you want the plane to set level. This spot is determined in a couple of ways. The manufacturer or designers suggestion or, when that is not given, it is figured out using a formula to find that spot. You then have to add or adjust weight to get the "actual" center of gravity to match the "should be" center of gravity.

Tom

Old 04-19-2011, 01:03 PM
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Default RE: CG Question


ORIGINAL: tacx

I think the understanding of CG gets confused many times, for some newbies and even some experienced guys, because when we try to ask or answer questions regarding it, the explanation is difficult.

Every plane has a center of gravity (CG). It is the ''actual'' point at which the plane sets level. If you add weight to the front or the back the ''actual'' center of gravity for that plane changes. This is not to be confused with where the center of gravity ''should be''.

The ''should be'' or ''recommended'' CG is a particular spot that you want the plane to set level. This spot is determined in a couple of ways. The manufacturer or designers suggestion or, when that is not given, it is figured out using a formula to find that spot. You then have to add or adjust weight to get the ''actual'' center of gravity to match the ''should be'' center of gravity.
Bingo, give the man a cee-gar!

Not only that, but quite often newbies think that the C.G. is a fixed point when as you've pointed out they are talking about the RECOMMENDED LOCATION instead.

However this is a bit off from what the OP is seeking... He really means "Center of Lift"... as he is confusing CG for CoL.


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