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Author Topic: Help! centre of gravity  (Read 1013 times)

Offline phil myers

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Help! centre of gravity
« on: February 04, 2009, 06:43:04 AM »
Hi to all the wise flyers out there, i,m new to c/l, just building my first model (Akromaster), the plans give the point of c.g. as 1-1.1/2 " back from leading edge, no problem, but is that with the engine (0.15+ muffler) , fuel tank, & prop fitted coz if it isn,t the c.g. is going to be way out ? And what is the overall weight supposed to be? There's no mention of that either. phumfph!
P.s. Looking forward to the spring (good weather!) to try my hand at this sport.

Appreciate any advice on this guys and gals

Phil
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Offline Mark Scarborough

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Re: Help! centre of gravity
« Reply #1 on: February 04, 2009, 08:44:05 AM »
Phil
typically, the CG is checked with the airplane ready to fly, everything installed except fuel. Thats just the convention that is used. The fuel load does alter the cg slightly, but in every case I have seen,(except electrics) you establish the CG ready to fly sans fuel.
Hope that helps
welcome to our wonderfull world of toy airplanes on a string,,  ;D
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Offline phil myers

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Re: Help! centre of gravity
« Reply #2 on: February 04, 2009, 09:26:47 AM »
Hi Mark,  H^^    Thanks for the helpful advice. No doubt I'll be popping in to the forum for help and tips over the coming months.
Cheers Phil
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Offline L0U CRANE

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Re: Help! centre of gravity
« Reply #3 on: February 05, 2009, 07:42:43 PM »
Phil,

One final thing - if you are at all handy with math, you could check that the CG is from 15% to about 18% back from the leading edge of the wing.
 The SIG Akro has a straight leading edge, as I recall. If you consider the wing structure plus half the distance the trailing edge tapers as the average wing chord (width, front to back) and find CG at 15% to 18% of that length, you should be nose-heavy enough to survive a first flight, yet far enough back to allow the model to respond pretty well, and to glide a bit to land.

Good model! Find some experienced friends to help you through -mostly- the first flight jitters, and to check that all is right with your model. You'll rarely find better friends than other supportive CL fliers, eager to get you going to enjoy what we have in common - CL flight.

Success!
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\BEST\LOU

Alan Hahn

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Re: Help! centre of gravity
« Reply #4 on: February 05, 2009, 07:48:32 PM »
Quote from: phil myers on February 04, 2009, 06:43:04 AM
Hi to all the wise flyers out there, i,m new to c/l, just building my first model (Akromaster), the plans give the point of c.g. as 1-1.1/2 " back from leading edge, no problem, but is that with the engine (0.15+ muffler) , fuel tank, & prop fitted coz if it isn,t the c.g. is going to be way out ? And what is the overall weight supposed to be? There's no mention of that either. phumfph!
P.s. Looking forward to the spring (good weather!) to try my hand at this sport.

Appreciate any advice on this guys and gals

Phil

Phil,
There have been some other threads about the Akromaster and suggested power and I have always recommended the 10 size engines, just for the reason you mention. The original one was built for the old loop-scavenged 15's--non ball-bearinged and without muffler. The modern 10's (OS 10FP or LA10 or Thundertiger 10) match better.

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Offline phil myers

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Re: Help! centre of gravity
« Reply #5 on: February 07, 2009, 08:19:34 AM »
 My Thanks to Lou and Alan for the tips, I measured the wing cord half way along wing which is 7.25",(tapering flap) so 18% for cg is 1.3". Thats what I'll use. I'll try the .15 engine(rules say I have to use muffler!) but if problems with nose weight will drop to a .10
I'll ask the c/liners at my local club to check her out and fly it before I do. Yikes! At least they can tell me what needs sorting out before I drill it into the deck!! y1 Cheers
Phil
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Offline proparc

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Re: Help! centre of gravity
« Reply #6 on: February 08, 2009, 09:01:57 PM »
This is not going to help your problem but, I was watching a video of the new Eurofighter Typhoon and they weigh the whole airplane to get the C.G. within 50 millimeters!!  A 14,00 lb. fighter with a C.G. within 2 inches!!
« Last Edit: February 12, 2009, 10:28:43 AM by proparc »
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Milton "Proparc" Graham

Offline phil myers

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Re: Help! centre of gravity
« Reply #7 on: February 11, 2009, 06:59:11 AM »
I think my Akromaster will weigh as much as that going by the amount of glue I've used!! HB~>
Going back to engine size, I'm guessing there's a formulae to find out the h.p. needed for a plane size/weight?
If anyone knows, it would be interesting to figure this out. #^ I know the engine companies give kw/hp figures for there
motors, but I'm told these are with some kind of rocket fuel with the engine screaming its lungs out and bursting into flames!
Any advice much appreciated.
Cheers Phil
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