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Fuel tank question - 3/19/2012 8:36 PM   
mranga


 

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How does a fuel tank allow fuel draw when a model airplane is heading straight down (for example, during an arobatic maneuver) ? The clunk tube is probably not going to allow the clunk down to where the fuel (not likely to be flexible enough to bend in a "U" shape - or may be I am using the wrong kind of tubing). An auxilliary fuel reservoir to hold a little fuel would do the trick but I have not heard of anybody using one.

(Note: Cross posted to the 3-D forum as well but this is really more of a beginners question.)


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RE: Fuel tank question - 3/19/2012 8:50 PM   
BarracudaHockey



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Status: offline 		Hopper or header tanks aren't unheard of and as you guessed they are bubble traps. Jets and choppers use them a lot.

The quick answer to your question is that most people don't fly straight down long enough for a clunk to be the problem, at that point, the ground is more of an issue.

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RE: Fuel tank question - 3/19/2012 8:58 PM   
combatpigg



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Status: offline 		The fuel that is already in the line will take a certain amount of time to get gobbled up by the engine. Hopefully the amount of fuel already in the line will outlast the length of the dive. All it takes is a split second for the pickup tube to reassemble a full load of fuel back into the fuel line once it gets resubmerged.

The fuel line pickup can be made "short" on purpose to help keep the fuel pickup wet during long dives....but then it comes back to bite you during prolonged verticals.

I run a lot of engines wide open with a high pressure latex bladder "fuel tank" and this allows you to fly the plane to any extreme without a loss of fuel flow. You would need a fuel line pinch to kill the engine with this set up, though.


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RE: Fuel tank question - 3/19/2012 10:07 PM   
Gray Beard


 

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Status: offline 		Header tanks are used quite often in model planes. I have one stunt plane I had to install a 2 ounce header tank. Originally the main fuel tank was/is too far back to allow me proper fuel draw. I tried pumps but they were more work then function. By installing the header tank it cured all my problems. You see them a lot on choppers more then in planes. I was going to try a pressure tank like the Pig uses if the header didn't work. There are a couple of draw backs to the bubbles pressure tanks but they do work great.

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RE: Fuel tank question - 3/19/2012 10:28 PM   
CGRetired



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Status: offline 		Yeah, as Baracuda stated, once "over the top" and headed straight down, the big factor is not fuel but the ground.

You would not normally accelerate headed straight down, although, it's not unheard of. Normal operation is to head down at idle after making, for instance a "hammerhead" or a "stall turn" maneuver. The pilot reduces the throttle at the top of the maneuver as the engine cannot produce enough thrust to keep the plane moving, the throttle is pulled to idle, the rudder is pushed either right or left to facilitate the maneuver, then the plane will "fall to the side of the rudder turn.

At that point, no more throttle is added until the bottom of the maneuver which happens pretty quickly. There is still plenty of fuel available to do this.

If timed properly, this is a really pretty maneuver.

My all time favorite, though, is the slow roll. But that's not the point of this discussion.

CGr.

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RE: Fuel tank question - 3/20/2012 2:18 AM   
SeamusG



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Status: offline 		Every maneuver has its "proper" throttle management. As CGr. stated that down lines are normally at idle and then the throttle is advanced when you pull out as part of your next maneuver.  Most of us sport pilots ignore them.But like already stated - full throttle straight down ain't gonna last very long. Either you have ample fuel in the line when you pull out or - well - you bury it.


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RE: Fuel tank question - 3/20/2012 4:00 AM   
Gray Beard


 

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quote:

ORIGINAL: SeamusG

Every maneuver has its ''proper'' throttle management. As CGr. stated that down lines are normally at idle and then the throttle is advanced when you pull out as part of your next maneuver.  Most of us sport pilots ignore them.But like already stated - full throttle straight down ain't gonna last very long. Either you have ample fuel in the line when you pull out or - well - you bury it.


Most the time when I see someone gassing the plane on the down line then when it's time to pull out of the dive the wings tend to come apart. Watched a guy do it on his first flight of a 1/4 scale plane that just installed a new radial engine. I always wanted a round engine, not that one though!! The pilot blamed it on the extra wing loading due to the round engine?? Go figure.

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RE: Fuel tank question - 3/20/2012 12:48 PM   
CGRetired



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Status: offline 		Yeah, that too!! Most planes are not made for those sort of stresses. Wings can and will fold up. Then it's straight down and in.

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RE: Fuel tank question - 3/20/2012 1:11 PM   
BarracudaHockey



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Status: offline 		We do it with 3D planes but again, not for nearly long enough to fuel starve an engine.

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RE: Fuel tank question - 3/20/2012 2:08 PM   
jester_s1


 

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Status: offline 		If you're diving with the power on you're accelerating and the fuel is at the back of the tank. At idle, our engines can run quite a while with the clunk out of the fuel. If you don't believe it run your engine dry at full throttle at the end of a flying day then turn it down to idle and start it again. Mine will usually idle for 10-15 seconds even with bubbles coming through the line, which is longer than any downline I've ever done.

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RE: Fuel tank question - 3/20/2012 6:16 PM   
opjose



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quote:

ORIGINAL: jester_s1

If you're diving with the power on you're accelerating and the fuel is at the back of the tank.



Eh, not true. For some reason this "myth" keeps getting repeated.

Our planes simply cannot sustain the 1.5+G acceleration rate required to make that work.

Even an RC turbine jet can't do this.

The math doesn't bare it out, and the videos disproviing the myth don't either.

quote:

ORIGINAL: jester_s1

At idle, our engines can run quite a while with the clunk out of the fuel. If you don't believe it run your engine dry at full throttle at the end of a flying day then turn it down to idle and start it again. Mine will usually idle for 10-15 seconds even with bubbles coming through the line, which is longer than any downline I've ever done.


That however is true to an extent. My Funtana likes to differ though my other planes do not.





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