Ivalej1

Ok the way i was taught was. that the clunk stayed at the back of the tank. and moved up and down if the plane was upside down or right side up. ok one of the guys at the club say. that it has to move to the front too to get fuel in the engine in a dive. im like no capilary action will leave the fuel in the back of a tank in a dive. the helie guys have the clunk going to the front but when ur in a hover i dont think there any gees on the tank forcing the fuel to one side. ohh well. what do u guys think? thanks.

MinnFlyer

My Feedback: (4)

The guy in your club is an idiot... but that's obvoius, he's from New Jersey
(It's ok, I'm an ex-Jersey boy myself)

But seriously, you do NOT want your clunk to go forward in the tank. In fact, I usually place a piece of stiff tubing in the clunk line to PREVENT that from happening should you come to an abrupt stop.

âûñøåå îäíî

that was cold.... and I know why you left.....

yea, you want the klunk to stay in the back

Ed Smith

In addition to the previous, do not have the clunk up against the back of the tank. It needs to be 1/4 to 3/8 away from the back of the tank.

Ed S.

Stick Jammer

Either that guy doesn't fly airplanes or he has a lot of deadsticks. As mentioned, you want the clunk to remain at the rear of the tank.

Richard L.

My Feedback: (24)

That guy was smoking something. In a dive, fuel doesn't drop any faster than the tank or the plane itself. In fact, the prop will still pull the plane down, and fuel will still stay at the back of the tank.

doggscube

My Feedback: (1)

I know I'm new to the hobby so I might not be aware of another factor, but I would think that the fuel would collect at the front of the tank if your dive acceleration is less than 32 ft/second squared (gravity) due to throttle setting and airframe drag.

This is not to say that the clunk should be able to go to the front, just thinking through the physics.

-Jeff

waagbuck

My Feedback: (1)

An airplane with so much drag that it accelerates in a dive slower than gravity is something I would like to see. Perhaps the pilot is capable of a nose-down hover?

Richard L.

My Feedback: (24)

In this case, it wouldn't be a dive. If you shut off the engine and point the nose of the plane straight down, then it would fall at 1g and airframe drag would be negligible.

Red B.

No, I'm afraid not.

Assuming that the aircraft is in a vertical dive with the engine at idle it will speed up (accelerate) until the force of gravity equals the drag force. From that time on the aircraft will fall towards earth with constant velocity, i.e. with zero acceleration. When this happens fuel will accumulate at the front part of the tank due to the force of gravity. Remember that the fuel itself is in a sealed compartment and not subject to air resistance.
As an analogy consider yourself in an elevator going downward at constant speed. Pour some water out of a bottle and observe where it will end up. Presumably on the floor. This is true for any speed of the elevator as long as it is constant.

Interestingly enough the fuel will accumulate in the front of the tank even during the first phase of the vertical dive, when the speed is increasing. This is because air drag causes the acceleration of the aircraft to be LESS than the earths gravity, i.e. 1g (32 ft. / second squared).
The fuel in the tank however is free to move about at will relative to the surrounding aircraft and since it is not subject to drag forces from the air it will fall towards earth with an acceleration equal to 1g. The front of the tank will of course stop the fuel from falling out of the plane :-) and hence this is where it will end up.

The only way to get the fuel to collect in the rear of the tank during a vertical dive is if the thrust of the engine is maintained so that it is greater than the drag forces acting on the aircraft.

Since the stiffness of the fuel tubing prevents the clunk from falling forward towards the front of the tank, the reason the engine doesn't die during the dive is that it survives on the fuel available in the fuel tubing between the clunk and carburettor.

IMHO, the main reason that the engine doesn't stop during the dive is that the dive isn't maintained long enough for the fuel in the fuel tubing to be used up.

/Red B.

britbrat

Red B -- you have it absolutely dead right.

MinnFlyer

My Feedback: (4)

Besides, if you're flying so long that you almost drain your tank, you're probably pissing off someone who's waiting to fly.

Stick Jammer

I would tend to agree with most of post #10 but one thing doesn't fit with this scenario. If the clunk is indeed out of the fuel in a long dive, there should be a sizable length of tubing that is filled with air by the time the plane levels out and throttles up, which in turn should cause a noticeable blurp in engine performance. I have never found this to be the case with any of my planes no matter how long the dive is.

DBCherry

My Feedback: (4)

There wouldn't be a "noticeable blurp" because a bubble of air in the line will simply cause the engine to go lean for a second, assuming that the fuel in the line is replenished soon enough. If not the engine would go lean, then quit.

Red's right though, the main reason the engine doesn't quit is because it's very difficult to maintain a dive long enough to use up all that fuel in the line. If the engine is throttled up in the dive then the fuel will remain at the back of the tank, if the engine is at idle, then the consumption is very low.

Keep the clunk at the back of the tank.
Dennis-

DMcQuinn

My Feedback: (13)

Red B is correct. The fuel does go to the front of the tank pretty quickly. If you are high enough, you can kill the engine this way. But as others have said, you generally are not totally nose down with throttle up high enough to drain the line. And it is surprising how quickly air in the line is sucked through.

Ivalej1

hey thanks guys. not all nj guys are dumb. (me is that guy lol) today i was down at the field and it was another debate with him. i said if thats such a problem. just put a hopper tank in the plane that way. it will always be full and not matter what hapens your engine will get fuel thanks guys

26C4U

IMO, the only "abrupt stop" with enough force to get your (unstiffened) clunk up front while flying is called a crash.

Ivalej1

lol u got a point there

Stick Jammer

Then explain why an engine running on the ground that for one reason or another is getting air bubbles in the fuel line runs noticeably erratic. At any rate, I would consider "going lean for a second" very noticeable.

Roby

Very interesting reading indeed.

It seems to me , from what I've seen over the years, typically
at the altitudes we're flying at, (2 to 3 hundred ft). If you have
your plane nose down long enough to run out of fuel.

Your in the ground. Yes/No ??

Red B.

Stick Jammer wrote: The difference between the two situations is that when you get a stream of small air bubbles in the fuel tubing from a leak or fuel foaming in the tank you are effectively leaning out the engine over a longer period of time. This tend to alter the RPM of the engine which in turn will have an effect on the two-phase flow (air and fuel mixed together) in the fuel tubing which again will have an effect on the RPM. The engine will run erratically and be very hard to tune.

When in a dive one air bubble will enter the the fuel line. The size of the bubble will depend on engine RPM and for how long the dive is maintained. The air bubble will get out of the system very quickly, especially at high RPM.
Even at low RPM (engine at idle when diving) you will probably not notice anything special because the air bubble will be a lot smaller (less fuel consumed at low RPM) and also the inertia of the rotating propeller together with the windmill effect on the propeller will tend to keep the engine running during the brief time it takes for the air bubble to be 'sucked' through the carburettor.

/Red B.

Ivalej1

thanks for the help guys. he understaznds now. keep shooting away this is interesting.

DBCherry

My Feedback: (4)

Guess that depends on your definition of a crash.

I've had my clunk wind up at the front of the tank after hitting the tall grass at the end of the runway when I landed too fast.
Dennis-

DBCherry

My Feedback: (4)

Oh, and I've been using that piece of stiff tubing for a number of years now and have never had a problem since.
Dennis-

Ivalej1

well i never used it. but im going to from now on