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CG Setup and fuel level?

Old 09-27-2010, 03:39 AM
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Packdaddy
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Default CG Setup and fuel level?

Hey guys

I'm still setting up my latest addition to the hangar, a Sebart Su29 50e. I've converted it to nitro with a Saito 91 and it's just about ready to go - I'm just installing the radio gear, onboard glow and some other electronics i got for it.

I'm trying to mount batteries etc so as to get the CG right without using balast, but am slightly confused.

I've mounted the fuel tank in front of the wing tube, right up against it. This means it's in front of the recomended CG position. SO.... should I be measuring the CG with a full tank, half tank, empty tank or what? I guess the most important thing would be that, tank full or tank empty, the CG is still within certain limits, but what limits?! that would also mean you have less room to move the CG, as each limit is approached when the tank is empty or full... i hope this makes sense, maybe I've over thought it?!

The plans say they recommend CG at 125mm from the LE at wing root, 120mm for pattern flying or 130mm for 3D. Should I just use them as limits?!

HELP!

Mark
Old 09-27-2010, 04:53 AM
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David Bathe
 
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Default RE: CG Setup and fuel level?

If your not using a fuel pump, you'll need to mount the tank directly behind the firewall.Balance without fuel...Yeap adding filling the tank will effect the CG, making the plane slightly nose heavy.For general sports flying this shouldn't effect the pleasure.Set it at the 125 mark and see how it goes.
Old 09-27-2010, 05:20 AM
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Packdaddy
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Default RE: CG Setup and fuel level?

Thanks for the reply Dave. I've never mounted a tank right up against the fire wall before, why do you say that? I had to modify the fuse in order to get the tank low enough in relation to the carby

The front of the tank is currently about 4" from the firewall
Old 09-27-2010, 06:09 AM
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David Bathe
 
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Default RE: CG Setup and fuel level?

What can happen is that the engine may begin to suffer from fuel starvation if the tank is to far way... the pressure from the muffler just isn't enough to force the fuel that extra distance from the tank to the carb during vertical flight for example. A pump or pressure regulator can solve that problem but... keep it simple as they say.I'd set the tank closer to the firewall for now, then try moving it back some as you go along.You're going to get slight trim changes because of fuel usage, just wouldn't bother yourself about that at this stage or certainly if you're only planning on using the model for sports fun.If it starts driving you crazy, mount the tank on the CG and use a pump.What I've done in the past is to set a couple of simple flight conditions on the tranny, flick the switch to get a slightly different trim condition as the fuel is used... reached a conclusion...what's the point!To much work... and the trim change isn't that much, IC sports aircraft have been set-up that way for decades, hasn't ruined anyones funJust fly and enjoy.
Old 09-27-2010, 06:19 AM
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da Rock
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Default RE: CG Setup and fuel level?


ORIGINAL: Packdaddy

Thanks for the reply Dave. I've never mounted a tank right up against the fire wall before, why do you say that? I had to modify the fuse in order to get the tank low enough in relation to the carby

The front of the tank is currently about 4'' from the firewall

Fuel tanks are mounted against the firewalls because engines often can't draw from farther away dependably. We also rely on muffler pressure to help make the fuel flow even more dependable for tanks mounted close. Some engines draw better than others, but model designers have no clue which you will use, so the recommendation is always to mount it as close as possible.

Everyone also works out CG location with an empty tank because it works well for everyone to do so. Think of the confusion and extra effort that would be caused if we didn't. You would have to use a certain size tank with a required amount of fuel, or have specific requirements for load and placement. If you didn't follow the recommended load, you'd then need to compensate for what you did differently. As it is, most modelers can't work out where the CG should be when there is a simple no-fuel standard.

Find and adjust your CG with no fuel. Look at all the models flying next time you're at the model flying field. Most of them were setup that way, at least the ones the modeler actually checked and adjusted the CGs, anyway. It works great, doesn't it.
Old 09-27-2010, 07:04 AM
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Default RE: CG Setup and fuel level?



An Aircraft is always balanced for the worst condition.
If the tank is ahead of the Centre of Pressure then the tank should be empty.
If the Tank is behind the Centre of PRessure, then the tank should be full.

In both conditions this will result in a Centre of gravity (CofG, CoG, CG)that is ahead of the centre of pressure, resulting in a stable airplane.

Now wrt to the tank being mounting with its butt end against the wing tube, no problem as long as you have a tank that is pressurised by exhaust back pressure. AS Long as there is a positive pressure in the tank and a negative pressure (lower than 1Atm) in the carb then fuel will be drawn toward the carb. For 3D type airplanes it would be wise to consider a pump or sorts.

Old 09-27-2010, 12:42 PM
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Default RE: CG Setup and fuel level?

Hi!
4" from the firewall will not work!!!! You must mount it against the firewall! And mount it according to the tank mounting rule.
Most engines use silencer pressure but that still isn't enough for coping with the various tank pressure situations. As for the C of G! This is allways dialed in using an empty tank as most airplanes are in the tractor configuration (Engine up front).
Old 09-27-2010, 01:29 PM
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Gray Beard
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Default RE: CG Setup and fuel level?

How far you can mount your tank away from the engine/fire wall also depends on the engine. I have a few that are mounted several inches away from the fire wall and work just fine. Some engines and mufflers draw fuel and make pressure better then others, as close to the fire wall is still always a good idea. I also have several planes with the tank mounted more then a foot away from the engine and over the CG but these are all engines with some sort of pumping system like YS or OS pumper engines. A photo showing one of my grand mistakes. The plane is a plans built little pattern plane. I was talked into using the stock mounting rails instead of a normal mount. That left me no room for a fuel tank behind the engine due to the rails. You can see how far back I had to mount the tank and it is over the CG area. It didn't work!! As soon as 1/4 of the fuel was used up there wasn't enough pressure to help the engine draw and it would go dead stick. I'm just waiting to get another YS engine for this plane. They don't care where you mount the tank. I did try after market pumps but they didn't work all that well. They worked but I couldn't get them regulated correctly, again, my bad. Always CG with the tank dry!!
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Old 09-27-2010, 02:58 PM
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Jetdesign
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Default RE: CG Setup and fuel level?

One of the major reasons I'm investing in electric power is to avoid the CG change that occurs with fuel usage. I tried pumps but found them to be more hassle than they are worth.

I have a Yak with my 2 stroke OS, tank is probably 3" from the firewall. Engine probably runs a little lean during vertical maneuvers/climbs due to gravity working directly against the fuel draw of the engine.

Balance the plane with the tank empty, and use the rearmost balance point suggested in the manual. You generally will never reach that point in the air due to the fuel off-setting that balance point, but if you do run out you're still within controllable limits.

Balance point is just a recommendation anyway; start off safe and adjust to your liking.
Old 09-29-2010, 04:04 AM
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Packdaddy
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Default RE: CG Setup and fuel level?

Thanks for the input guys.

As Gray Beard mentioned I reckon it's going to be dependant on the engine. It doesn't seem that far back to me, it's not even right over the CG. So I think I'll try it as is and see how she goes. I'll just be carefull for the first few flights and make sure i'm in gilde range of the field to deadstick if needed - which is a good idea anyway!

If it gets lean I'll buy a perry pump. I reckon a more consistent CG is more important than a simple setup. Pretty sure most pattern guys wouldn't mount their tanks up front!

Thanks for feedback on CG calcs too, makes sense!
Old 09-29-2010, 06:11 AM
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TimBle
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Default RE: CG Setup and fuel level?

If its any consolation, the tank in my Escapade is as far aft from the firewall as it will go, around 1 inch. I use complicated plumbing to get to the carb (don't ask) and I have no issues with fuel feed simply because the back pressure is sufficient to always work with the vacuum in the crank case at bdc. It always draw fuel even in the vertical after 15min of flying (running on the wiff of an oil rag)

Old 09-29-2010, 11:20 AM
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Default RE: CG Setup and fuel level?

Many (possibly most) pattern guys run YS engines with fuel pumps. They can have the tank as close as possible to the CG point because they have a pressurized and regulated fuel system.

As you wrote, you can try what you have and make adjustments if needed.

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