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        Discussion Plane CG... where should it be?

#1 Dcope17 Feb 02, 2013 12:19 AM

Plane CG... where should it be?
 
SO, I acquired this little plane and know nothing about it. I have a .40 size trainer I built like 10 years ago and a 3D foamie ( which required no building )
I noticed a weight in the cowl but I have no idea what size motor and battery pack were used to need this weight.
I like this little plane and don't want its first flight ( in my possession ) to be its last.

Here's what I have...

http://i613.photobucket.com/albums/t...ps0e071663.jpg

http://i613.photobucket.com/albums/t...psbba43009.jpg

http://i613.photobucket.com/albums/t...ps62eef3e5.jpg

#2 ggcrandall1 Feb 02, 2013 01:04 AM

A good starting point. Measure the chord at the wing root. Then go about 25% of the root chord back from the leading edge of the wing.

Glen

#3 wizard of odd Feb 02, 2013 01:05 AM

FWIW, that looks like a Herr Engineering AT6.

sirzeppu has just maidened his here http://www.rcgroups.com/forums/showthread.php?t=1820734, maybe ask him.

#4 derpron Feb 02, 2013 06:20 AM

general rule is 35% back from the leading edge of the wing. but because its a warbird, its more like 25% back.

#5 scirocco Feb 02, 2013 06:43 AM

Quote:

Originally Posted by colj00 (Post 24009224)
general rule is 35% back from the leading edge of the wing. but because its a warbird, its more like 25% back.

And just what's your source for that? I don't think I've ever seen as far back as 35% quoted as a rule of thumb guide - maybe 33% max, more typically 25-33%.

After detailed calculation, it may well end up further back than that, but to throw 35% out as a "general rule" is I'm afraid just BS.

If you want to learn something about cg, try reading this: http://www.theampeer.org/cg/cg.html

For the OP, if it is indeed the Herr AT-6, apparently the recommended cg is at the rear of the spar: http://www.rcgroups.com/forums/showp...5&postcount=70 & http://www.rcgroups.com/forums/showp...19&postcount=5

#6 Teddy Ong Feb 02, 2013 07:05 AM

it should be about 1/4 to 1/3 of wing root

#7 flypaper 2 Feb 02, 2013 07:24 AM

I'd go with the 1/3 of the chord at the root because the swept back leading edge would put the CG a little futher back.Keep the speed up when you land it. The AT6/ Harvard, is a known tipstaller.

Gord.

#8 derpron Feb 02, 2013 07:36 AM

Quote:

Originally Posted by scirocco (Post 24009305)
And just what's your source for that? I don't think I've ever seen as far back as 35% quoted as a rule of thumb guide - maybe 33% max, more typically 25-33%.

After detailed calculation, it may well end up further back than that, but to throw 35% out as a "general rule" is I'm afraid just BS.

If you want to learn something about cg, try reading this: http://www.theampeer.org/cg/cg.html

For the OP, if it is indeed the Herr AT-6, apparently the recommended cg is at the rear of the spar: http://www.rcgroups.com/forums/showp...5&postcount=70 & http://www.rcgroups.com/forums/showp...19&postcount=5

oh ive read a fair amount of times that 35% is a good place to start.

#9 Azarr Feb 02, 2013 09:15 AM

Quote:

Originally Posted by colj00 (Post 24009525)
oh ive read a fair amount of times that 35% is a good place to start.

I think you may be confusing the actual CG measurement with something I see often on RCG (it makes me cringe) and that is some people provide the CG measurement from the wing root without explaining that the calculation is done on the Mean Aerodynamic Chord (MAC) and not the root chord. Quite often you'll see this on Delta planforms where the OP say "the CG is at 50% from the leading edge" when the CG is actually 20% of the MAC (even though the physical point on the aircraft is the same)

I've actually seen someone try to balance a conventional plane at 50% from the leading edge "because my delta flew fine that way" The results, of course, were disastrous.

Read the article that scirocco linked to and take a look at a CG calculator. None will recommend an actual CG starting point back as far as 35%. (realize that there are exceptions for everything and some 3D pilots will run a rearward CG because they think it makes hovering easier, but may adversely affect all other aspects of flight)

Azarr

#10 Dcope17 Feb 02, 2013 09:24 AM

Thanks everyone... I'll PM sirzeppu about his setup and how well it worked.

Also, thanks for that link showing how to set the CG. I'll be giving that a read this evening when I get home and settle in.

Last, its maiden will likely be at the local football field with a track around it. I figured it won't fly as slow as my trainer but with that long of a runway... I'll bring it in hot and take the whole length of track to set it down if need be.

#11 Evan D Feb 02, 2013 10:37 AM

If it was mine I would balance it on the spar even with where your aileron horns are. As noted since it has a sweep to the wings the normal 25-30% back from the LE doesn't work.

Also as noted some of these old Harvards are difficult to fly especially slow.

#12 Teddy Ong Feb 02, 2013 10:54 AM

Use a CG calculator. Much better. Measure and key in the numbers.

#13 Sparky Paul Feb 02, 2013 12:22 PM

4 Attachment(s)
The similar sized House of Balsa T-6 would be unflyable with the c.g. further aft than as shown!
The T-6 is an AWFUL airplane. Sometimes fun to fly, but very sensitive to control throws and will tip-stall at every opportunity!
There was a lot of activity years back racing these things with OS 15 motors. Getting them off the ground at the start was always amusing, directional control on the ground was poor. Switch to the OS25 motor got rid of that problem.
This is my last one, switched to electric when a sufficiently powerful and light motor became available. Still had the too-heavy Nicad battery. With a Lipo, it's a lot better.

#14 daughtry50 Feb 02, 2013 04:25 PM

Quote:

Originally Posted by Sparky Paul (Post 24011643)
The similar sized House of Balsa T-6 would be unflyable with the c.g. further aft than as shown!
The T-6 is an AWFUL airplane. Sometimes fun to fly, but very sensitive to control throws and will tip-stall at every opportunity!
There was a lot of activity years back racing these things with OS 15 motors. Getting them off the ground at the start was always amusing, directional control on the ground was poor. Switch to the OS25 motor got rid of that problem.
This is my last one, switched to electric when a sufficiently powerful and light motor became available. Still had the too-heavy Nicad battery. With a Lipo, it's a lot better.

Please, AWFUL to fly, all of them. I don't think so. I've owned built up Texans and foamie Texans and they flew just fine. The reason, my wing loading on them was fairly light. If you have a Texan that flys the way you describe then it is probably heavy and also set up tail heavy. A Texan will not fly like a trainer but, set up properly they fly nicely.

#15 DustBen Feb 02, 2013 05:52 PM

Quote:

Originally Posted by scirocco (Post 24009305)
And just what's your source for that? I don't think I've ever seen as far back as 35% quoted as a rule of thumb guide - maybe 33% max, more typically 25-33%.

After detailed calculation, it may well end up further back than that, but to throw 35% out as a "general rule" is I'm afraid just BS.

If you want to learn something about cg, try reading this: http://www.theampeer.org/cg/cg.html

For the OP, if it is indeed the Herr AT-6, apparently the recommended cg is at the rear of the spar: http://www.rcgroups.com/forums/showp...5&postcount=70 & http://www.rcgroups.com/forums/showp...19&postcount=5

I'm a little concerned by the 35% figure also.
I've flown 100's of models and cannot recall any model with such an aft CG location listed as the "starting point".
25% is a nice starting point and can slowly be moved back.
At 35%, nothing is going to happen slowly.


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