Fuel tank behind CG
 

Hi all, I have only just started trying my hand at this form of flying, started with a modeltech 3Demon which has allowed me to do some basics ,hovering , spins not quite flat but close for this plane etc.
I have just brought a 26cc sukhoi 26m & have heard that putting the fuel tank behind the CG point allows it to be more tail heavy but makes landing easier after the tank has been run down.
Do many people do this with their 3D birds ? are there any real advantages in doing this ?
I guess what i'm asking is is it worth doing.
cheers Steffan.

RE: Fuel tank behind CG
 

If you have the option, put it on the CG. Then it will fly the same way (predictable) all the time.

RE: Fuel tank behind CG
 

What he said. Mount it as close to the cg point as possible. Most planes you can get close to the cg but not perfectly so there will be somewhat of a change as the fuel burns off. The point is to minimize this effect

RE: Fuel tank behind CG
 

I understand what you guys are saying about getting the tank as close to the CG so theres minimum change during the flight.
In the article I read I think the theory of putting it behind was that you could have a tail heavy plane at the begining of your flight for
doing your 3D stunts etc ,& it was easier to land at the end of your flight.
Seems alittle extreme to me ?.

RE: Fuel tank behind CG
 

I can't speak for 26cc planes but my 50cc AeroWorks Extra 260 has the rear end of the fuel tank touching the wing tube which puts the tank about an inch and a half in front of the CG. I cannot even feel the difference from full to empty. Also, my AeroWorks Extra 260 60-90 size is set up exactly the same and I can't feel the fuel burn on it either.

RE: Fuel tank behind CG
 

I haven't tried that but I may sometime. Ihave an Aeroworks 1.20 size YAK 54, the tank is ahead of the C.G behind the firewall, I don't like it. If I use an aft CG for 3D stuff, it only really goes tail heavy the last half of the flight. with the starting out C.G. in the manual I can't even tell the difference betwen the beginning and the end of the flight.

My 50cc Airwild has the tank mounted right on the C.G. it is set up tail heavy, and can be difficult to land in a head wind. I guess the low wing loading and the Center of Gravity makes the nose point up, When I land it in a descent head wind I have to hold down elevator to land it. very un-natural to me. Still better than trying to land it even with minimal tail wind.

RE: Fuel tank behind CG
 

Not a terrible idea, but make the first flight with a 1/4 full tank, the next with a 1/2 full tank, and so on. It would make me nervous to try it with a full tank.

David

RE: Fuel tank behind CG
 

Do you have any info on this article? Website, newsletter, publication? I've always put the tank on the CG whenever I can. If you have a 32oz. tank(big plane where its the tank/CG is more common), that's alot of fuel.. why would you want to compensate for anything? I know you said it was in an article.. so please share it. Thanks. :)

RE: Fuel tank behind CG
 

Unfortunately I dont have the info on the article at the moment , hence the question on this forum , but I will keep looking & if I do locate it again I'll post the details.
Cheers Steff.

RE: Fuel tank behind CG
 

Thanks Steff... no problem.. I was just curious.. You did a good job in sharing the principles of that info.. and it might stand to reason for certain apps too.. I'm mostly curious in the app it was used..

I'm now wondering how a mechanical "weight-shifter" might alter performance... I mean.. if one is going to simply use the fuel to alter the CG for easier landings.. What if someone came up with a device/design that could drastically shift weight.. and what that open up in manuevers.. 3D type manuevers.. or ... hmm.. what kind of tumbling/flipping/etc.. would be possible?

The top-level pilots flying high-end aerobatic planes do have ways to alter CG for sure... its usually a compartment located in the aft section of the airplane.. and they add weight, or detract weight depending on what type of pattern/competion they're flying.. to get certain performance traits, based on the CG.. hmm... if it was mechanical and in flight it would be like dual-rates... only you'd have adjustable CG.. which is essentially how this thread got started.. only you have to fly a whole tank for adjustment... but what if you could change it at will?? Could someone achieve some very "Mad" 3D stuff??
Theres a bit of expansion of the original concept. :)

RE: Fuel tank behind CG
 

A small electric motor or stepper motor with the weight/ballast moving up & down a spider gear would be one mechanical way , but of course that is also more weight.
Steff.