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Old Jun 12, 2012, 01:41 PM
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Old Blighty
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Fuel's affect on plane's CG; where will the fuel predominantly reside?

I'm looking for some advice on a unique plane I'm building, so I hope someone can help. I've never posted in this part of RCGroups before

I'm reluctant to give too much away about the plane's design, but it's fuel powered and due to its inherent design, very pitch sensitive.

I've built two 400ml fuel tanks which I'll embedded in the wings exactly on the plane's CG line, so that (in theory) when either full or empty the plane's CG should remain unchanged. I have concerns about the affect of the fuel sloshing round, so I've put a baffle plate down the CG line of each tank. I would love it if there was some sort of 'fuel gel' or 'fuel sponge', but to the best of my knowledge, this does exist.

Anyway, on to my specific question...

An experienced flyer down my local field said that even though the plane will be doing all sorts of maneuvers and experiencing all sorts of accelerations, when all said and done, most the time the fuel will tend to move/sit at the back of the tank - is this true?

I have just also seen the below webpage, which 'seems' to also corroborate this?
http://www.rc-airplane-world.com/bal...airplanes.html

"By the way, your airplane must be 'flight ready' when you balance it i.e. battery pack in place or fuel tank empty. With the latter scenario, an IC plane is best balanced with an empty fuel tank otherwise the plane will become tail heavy as the tank empties during the flight."
Many thanks
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Old Jun 12, 2012, 02:50 PM
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Yes the fuel tends to stay at the rear or back of the fuel tank most of the time. As the Gee Forces of the plane flying around tend to push the fuel back. I had once thought that maybe when a plane does certain stall manuevers such as a stall and spin, that the fuel might go to the front, but when the plane is in free fall, the fuel still tends to wind up at the rear of the tank.

Years ago, when I was big into pylon racing as well as high perf 1/2a planes too, we used to just use a fixed fuel pickup going to the rear of the fuel tank. Sometimes with the tube bent more over to the right side (for the left turns). Anyway, the planes could do everything except fly inverted (you used that to kill the engine when done flying).

Yes you balance the plane with an empty fuel tank, unless you really really know what you are doing.
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Old Jun 12, 2012, 03:04 PM
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Old Blighty
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Many thanks indeed for your feedback - excellent - thank you

So, if you balance your plane with an empty tank, then this would still mean that it will become tail heavy during flight - and especially as the contents of the tank empty? Is this something that can be 'felt' in most planes?

In your high perf 1/2a planes days, why would you not have a fixed pipe going to the rear, with a small flexi-pipe and clunk on the end so you could fly inverted?

(For a bit on my background - at the moment I fly electric planes and gas turbine helicopters - I've never flown a fuel plane before)



UPDATE: I've just Googl'd - I had no idea 1/2A planes were so small. That would explain the lack of clunk
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Old Jun 12, 2012, 03:43 PM
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Back in the 50's when pulse jets were quite common, the solution was to fill the tank with very coarse steel wool (well rinsed and cleaned first) as these pulse jets would not stand any fuel sloshing at all. This seemed to cure the cut out problems so many had. That might work in your case.
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Old Jun 12, 2012, 03:58 PM
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Old Blighty
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Thank you - that sounds like a very interesting idea! I guess the weight of the steel wool would need to be considered and how much volume of fuel it would take up, but it's a great idea I'll definitely consider. Thank you.
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Old Jun 12, 2012, 06:44 PM
Fueled by Arabica Beans
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Planes are balanced with an empty tank so that it won't become tail heavy during flight. It only becomes more stable with a full tank. The effect is not terribly noticeable as long as the fuel tank isn't ridiculously large. Fuel slosh is not a major concern. The only thing you don't want to do is to mount the fuel tank center behind the center of gravity. Even with "pitch sensitive" designs, it should not hurt flight performance much by having a forward center of gravity ("nose heavy") on take off.
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Old Jun 12, 2012, 08:04 PM
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There is a "foam" that is inside the fuel cells of some race cars.
I don't know what it specifically is but you might try to get that.
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Old Jun 12, 2012, 09:59 PM
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There was a video posted on another site that was filmed in, I think Germany about two years ago. A camera was mounted to the outboard wing pointed at the fuse that had a tank hard mounted. It was a split screen so you saw what the plane was doing and what was going on with the tank. I was much surprised to see that the fuel did not stay at the bottom back of the tank like I thought it would. Just doing a simple loop made the fuel flow from the rear to the front and back again. It sloshed around in a spin. The video was so people could understand fuel movement in the tank during flight and it really does move around, it even leaves the clunk exposed in a dive.
It also showed how well the anti foaming agents used in fuel today worked. The tank was hard mounted and there was zero foaming.
I had the video in my favorites to show students but it quit working last year but it was a big eye opener.
I mount my tanks over the CGs center and notice no CG shift, however this is in stunt and IMAC planes that are very stable.
The steel wool in the tank and a good filter sounds pretty good? Worth a try!
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Old Jun 12, 2012, 10:12 PM
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Originally Posted by Ian Davidson View Post
Many thanks indeed for your feedback - excellent - thank you
In your high perf 1/2a planes days, why would you not have a fixed pipe going to the rear, with a small flexi-pipe and clunk on the end so you could fly inverted?

UPDATE: I've just Googl'd - I had no idea 1/2A planes were so small. That would explain the lack of clunk
Well in racing at the time, we weren't using throttles as throttles. you had full throttle or kill the engine, no idling and no in between speeds. it was all go fast and turn left. No aerobatics. This was for pretty much all pylon racing planes too, not just the little ones. My racing planes with .40 engines were setup with a metal tube for a fuel pickup too. My Quarter Midget .15 planes the same way.

The 1/2a planes tended to get quite small, down to 20 to 24 inch wingspan planes with Cox TeeDee .049 or .051 engines on them. At that time after you killed the engine, the plane had a glide ratio of a Space Shuttle. But they would sure go fast when the engine was running. But as we kept going faster and faster, we started using bladder fuel tanks to pressurize it good for all out performance. Then you only filled the fuel tank with enough fuel to wait 60 seconds for the start and four laps around the course with a little extra just in case. The RC systems of that era were not small or light enough yet to allow for things like throttles, fuel shutoff devices, etc.

But at first we didn't have RC carbs on the engines. So to kill the engine after a race heat, you flip the plane over inverted and it dies, then you land dead stick. We did this will all the racing planes at the time. Later they wanted something like a throttle to kill the engines, so some guys used RC carbs, but this affected the performance a little bit, so most everyone used something like a servo controlled free flight engine shutoff device and no RC carb. The modern all out pylon engines still tend to not use RC carbs but use a modified free flight fuel shutoff or floodoff device controlled by the servo. Some all out pylon racing engines have big bore carbs on them, but the carb is only good for full throttle and kill the engine, as the port timing in the engine won't let it idle below 4,000 rpms anyway. A lot of us started using bladder tanks, which pressurizes the fuel supply a whole lot. So you then had to have a fuel shutoff device or a carb that let you kill the engine.
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Old Jun 12, 2012, 10:22 PM
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Yeah it is going to be tricky, if you are going to go on the hairy edge of CG balancing towards the rear of the plane. I would suggest starting with the CG a little farther forward and slowly move it back a little bit at a time. If the plane doesn't feel right, land and move the CG forward again.
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Old Jun 13, 2012, 01:55 AM
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Old Blighty
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Thanks again to everyone Ė really great feedback!

I didn't want to reveal what I was building, simply because there are others on RCGroups I didn't want to find out yet. However, I don't think they ever venture into this section, so I'll risk it...

Ok, basically I'm building a jet powered flying wing. 54 inch wingspan. 770 square inches of wing area. I believe none one has ever attempted anything similar, but please correct me if I'm wrong. I've already progressed a long way but I've always had concerns about the movement of the fuel.

The CG of the fuel tanks currently sit EXACTLY on the CG line of the wing and it's really too late for me to change this as I've cut out the foam. However, I'm now going to have to try and removed them and to de-solder one lid to change the location of the clunk so it's at the rear - behind the baffle plate (see the 4th picture). This is going to be a pain, but thanks to your feedback, I can see necessary.

I like the steel wool idea a lot to prevent fuel slosh, but I'll need to see how much fuel space this occupies.

Below are a few snaps - I would really appreciate if these didnít leave this thread - thanks.

(The first 2 photos are very early - before the design of the tanks was completed)









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Old Jun 13, 2012, 08:18 AM
Fueled by Arabica Beans
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Wow. That is totally not what I was envisioning. lol I think you've got a good plan with the fuel tanks as long as you do move the clunks to the back. Are you considering any flight stabilization system?
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Old Jun 13, 2012, 08:32 AM
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Wow, that is pretty cool. Very unique too. I think you are on the right track. Do let us know how it goes, you have a bunch of us interested in your plane and progress.
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Old Jun 13, 2012, 08:36 AM
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Old Blighty
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Quote:
Originally Posted by ChillPhatCat View Post
Wow. That is totally not what I was envisioning. lol I think you've got a good plan with the fuel tanks as long as you do move the clunks to the back. Are you considering any flight stabilization system?
Yes, I am. I didn't intend to at the start, but I think it's going to be the only way to counter-act the fore and aft fuel movement. Electronic stabilization shouldn't be needed for the role, but since the same unit does both I might as well wire that in too.

Removing those tanks and de-soldering the lid to get to the clung, is not going to be easy though
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Old Jun 13, 2012, 08:47 AM
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Originally Posted by earlwb View Post
Wow, that is pretty cool. Very unique too. I think you are on the right track. Do let us know how it goes, you have a bunch of us interested in your plane and progress.
I will do, thank you.

I did a lot of experimenting with the tank design focusing on the rate at which the fuel in both dropped, as I was surprised at the size (diameter) of the feed you needed between them to ensure that when you sucked fuel from one side, the other side dropped an equal amount.

In the end I had to settle on a 10mm brass tube, as when I just linked the two with a fuel pipe, the tank you sucked the fuel from would be half empty before the other side started to drop.
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