sillyness

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I was sitting here putting a fuel tank together and got to thinking... how are our gas engines getting their fuel during long down-lines? There is no way the clunk can come to the front of the tank with any fuel tubing or clunk I can find. Do they just accept/pass an air bubble in the fuel line? Sould we run a small header tank to make sure they are getting gas at all times (like I used to run on helicopters)?

Thanks

A320driver

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Ever riden a roller coaster and felt what happens to your body when you drop down a vertical? You get pushed back into your seat, same priciple with the your fuel in the tank.

Stick Jammer

While I've never found any conclusive evidence as to what exactly takes place in the fuel tank during down-lines, this scenario is doubtful. The plane would have to be accelerating faster than the acceleration rate of free-fall for the fuel to get pushed to the rear of the tank. Typically the throttle is at idle during a down-line which means the plane is traveling at much less than free-fall speed due to air resistance. Gravity would therefore pull the fuel to the front of the tank. There is typically more than enough fuel in the carb and line to keep the engine running during down-lines. A header tank is not needed.

Tired Old Man

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If you want to believe anything, believe that the plane will experience negative "g's" in the push over, resulting in the fuel "floating" to the high end of the tank. The same thing occurs in full scale light aircraft, making them prohibited from inverted flight unless they have a special fuel and oil system.

If you have ever flown in a light plane when the pilot pushed over rather hard, you would have seen everything loose in the cockpit floating free from whatever it had been resting on. I kept a pencil and route chart suspended in mid air for about 5 seconds once. What a hoot! The passenger wasn't too pleased, though.

A320driver

My Feedback: (1)

The inertial forces acting the airplane and thus on the fuel keep it to the rear of the tank. That is why you have a heavy clunk attached to the end of the fuel pick up, to make sure it ends up in the same place as the fuel. If you sucked air through the fuel line during down lines the engine would certainly run rough for a few seconds every time you went back to level flight, which it doesn't.
By the way most GA aircraft are not certified for aerobatics because of structural limitations not fuel feed. I have flow plenty of gravity feed aerobatic airplanes with no problems. If you really want to show off your piloting skills to your passenger with the floating pencil trick, put it up on the glare shield then get it to float up and in to a position were you can catch it with your teeth!

sillyness

My Feedback: (25)

I'm familiar with the acceleration (G) dynamics occuring in the tank and the forces that are felt as I fly for a living and have degrees in this sort of thing. In a hard pushover the fuel and clunk go to the top of the tank and partially to the back, depending on the airfoil and the required negative AOA to effect a negative G pushover. In the initial acceleration on a down-line, fuel would go to the back of the tank. After stabilization in the down-line (constant speed) the fuel will be sitting in the front of the tank. Of course, we are all so good on the sticks that we fly all of our patterns at constant speed, so it will skip going to the back of the tank.

Any time anything is moving at a constant speed, no matter what direction, the object feels "God's G" as the only force. G forces are just another way of saying acceleration and anything with a constant speed is obviously not accelerating, thus the only force felt will be gravity (if in proximity to earth... if in empty space no force will be felt). This means that a dowl-line at a constant speed (and SLOW speed with the high drag aerobatic ships!) is the same as standing the plane on its nose on the ground.

So... we know the clunk is out of the fuel. I guess what I was really asking is: does the carb "pass" the air bubbles very quickly (faster than fuel) to get to the next gas in the line? Does this cause any lean conditions in the run? I'm a decent physicist but I'm a mechanical moron with these fancy Walbro carbs.

Tired Old Man

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It was on the glare shield, but I was laughing too hard to catch anything. Agreed, structural is the primary limiting factor in aerobatics, but aircraft not certified for aerobatics, such as the 150/152, 172 and 182 will also suffer from fuel starvation in some the more extreme angles/attitudes. A 150 can also lose the mags if negative "g's" are excessive. Don't ask me how I know. Different day, though.

Ken Bryant

My Feedback: (10)

So what type of fuel line do you use as a clunk line in a Gassers fuel tank? I know with standard glow you use a lighter silicon line which gives freedom of movment to the clunk. SInce Gassers use a different fuel line what goes in the tank?

sillyness

My Feedback: (25)

The most flexible lines I have found are Aerotrend Easyflex and Stihl Chainsaw fuel line (most lawnmower/chainsaw shops should have it). The tygon F-4040-A 1/8" (small) diameter line I got from my hobby shop is fairly flexible, but it's the small diameter. In the tanks I'm using, the 3/32" Stihl line yields the same results as the 1/8" Tygon, except you obviously get better draw.

A320dirver:
I'm convinced after a lot of testing that the clunk does not go to the front of the tank with the fuel (at least in the 16oz and 24oz tanks I have on my DA and ZDZ, and the HUGE clunks I have in them). The fuel does not sit in the back of the tank either. In a constant speed down-line the fuel sits in the front of the tank... there are NO intertial forces acting at that time. Not until there is acceleration in some direction. Trust me on this one, go ask your old high school physics teacher, or ask any free-fall parachutist. After they reach terminal velocity (constant speed) they feel no more acceleration sensations (pit of your stomach). The force of drag equalling the force of gravity results in them feeling 1 G in the vertical direction... gravity.... just like they were laying on their faces in the dirt.

Aerobatics:

I flew a plane once that had inverted flight tanks. They were only fed fuel when the plane was upright, so when inverted you had a time limit. They said that after 30 seconds you would hear the sickly sound of motors spooling down.

Most GA planes are restricted structurally, but there are exceptions. The plane I'm in now is restricted to positive G manuevers for several reasons... none of them structural. The airframe is rated to negative G's, but you could run into hydraulic control boost and engine oil system cavitiation if you push very hard negative (and I know this to be a fact). It leads you fly high speed ridge crossings at 200 feet pretty carefully. Another pasenger favorite if they aren't tossing their cookies yet.

famousdave

My Feedback: (61)

Quote From Sillyness :

"The fuel does not sit in the back of the tank either. In a constant speed down-line the fuel sits in the front of the tank... there are NO intertial forces acting at that time. Not until there is acceleration in some direction. Trust me on this one, go ask your old high school physics teacher, or ask any free-fall parachutist. After they reach terminal velocity (constant speed) they feel no more acceleration sensations (pit of your stomach). The force of drag equalling the force of gravity results in them feeling 1 G in the vertical direction... gravity.... just like they were laying on their faces in the dirt. "



Sillyness, I have done studies on fuel distribution in military fighter aircraft and on attack helicopters during hardovers or other gravity breaking maneuvers. We probably all agree that when a plane is accelerating, in a turn or in an upline there is some degree of force keeping the fuel in the back of the tank. Even lateral acceleration will push the fuel rearward due to its fluidity. But on the downline it unfolds like this:

Lets say we are in upline and have reached our peak, now we do a rudder turn and start our downline. All the fuel is in the rear of tank already as the upline decays, and gravity pulls it aft. For a very brief second, we are in zero G, but that does not matter as the fuel cannot change position in zero G unless there is some other source of acceleration. We make the turn, which again, forces the fuel outward due to centripetal force (in this case to the rear of the tank) then we accelerate downward due to gravity and some push from the spinning prop (its not spinning backward so it is not braking the initial fall to terminal velocity, its aiding it) During that acceleration, all the fuel is still forced into the rear of the tank either from centripetal force from the turn or longitudinal acceleration from the downline.

During the downline we are accelerating toward an equilibrium (where drag cancels out acceleration). This is known as terminal velocity - the velocity at which acceleration terminates due to drag (simple terms). Terminal Velocity varies based on coefficient of drag, air density, STP, amongst other things. Theoretically at some point in time the force of drag will equal the force of gravity and yes, conditions in the tank would allow fuel to shift forward. While theoretically, this can happen, it would take about one mile in a downline for it to occur and absolutley NO change in engine speed. We all know that is not realistic. Even in our longest downlines there is still some acceleration, either longitudinal or lateral (fron rudder inputs for instance to hold the line) any of these will cause the fuel to move in the tank. Even the slightest acceleration is capable of moving the fuel aft. I will guarantee you that even on graceful downlines, the airplane is accelerating, even though it looks like it is slowing or holding constant speed. We have verified this with forward looking radar and on board laser speed measuring devices installed in UAVs.

The slightest amount of acceleration (in the order of cm/s^2) and undetectable by the human eye, will hold the fuel aft.

I used to do advanced studies on fuel movement and airframe parameters on aircraft in severe aerobatic and zero-g conditions. We used radio control planes with accelerometers and strain gauges. They had clear tanks and dyed fuel with on board cameras to watch what happens in basic and radical maneuvers. After all the research, we found that fuel starvation was rarely a concern, especially on multi-tank aircraft. On our R/C planes, even a 1/16" bubble can cause a burp or even a flame out. If the fuel were indeed being drawn away from the clunk, we would see a lot more flame outs in downlines.

I can't remember the last time I saw I flame out in a down line.... can you?
Something is holding the fuel aft 'cuz that clunk sure 'aint moving forward!!

That force is longitudinal or lateral acceleration.
DP

mstroh3961

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It would be a neet experiment to use a small onboard camera and put theory to the test!

A320driver

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Thank you Desertpig. Goes back to what I said on my first post but in far more detail than I would care to type.

mAvRiCk-inactive

When usng smoke the smoke is still pumping on the downlines so this proves desertpigs theory correct. I never use throttle on te downlines, but I remmebr in my old plane I used about a 1/4 throttle for the last half of a loop with smoke on even sometimes coming out of a hammerhand/stall turn!!

Dan

Stick Jammer

Not the case. Once the plane is nose down it would have to be accelerating faster than the rate of free-fall to keep the fuel at the rear of the tank. Free-fall acceleration is 32 feet per second at the end of the first second, 64 feet per second at the end of 2 seconds, 96 feet per second at the end of 3 seconds and so on. This rate just continues to multiply as long as the object is falling. This would be in an environment without air resistance. With the engine at idle, the plane is accelerating much slower than that. The fuel is in an enclosed environment within the tank therefore it does not experience any resistance as the plane drops to the ground. Gravity would pull the fuel to the front of the tank as soon as the plane starts it's downward motion because the fuel is in a free-fall situation at that point. Even though in some cases there may be slight inertia on the fuel at the time the plane is pushed over, it doesn't take gravity very long to overcome this.

sillyness

My Feedback: (25)

Desertpig,
Thanks for the time it took to write that. I'll think for a while and get back to you.

So... next scenario... you are a beginner and accidentally start the down-line under power, such that your airspeed is exceeding terminal velocity. Then you cut the motor to idle, generating a deceleration on the down-line. My guess is that the motor will cough, but the high airspeed and prop braking keep the motor turning and it re-lights after fuel makes its way back into the carb. This would explain why jets need headers and gassers don't... the jets can't re-light themselves.

Stick Jammer

sillyness,

Terminal velocity and free-fall are two different animals. The acceleration rate at which any given object will reach terminal velocity is slower than the acceleration rate of free-fall. Terminal velocity is different for every object depending on mass and drag. Free-fall acceleration is constant no matter what the object is. The airframe is attempting to reach terminal velocity in a down-line with the engine at idle. The fuel within the tank is accelerating downward at the rate of free-fall. Which is faster?

sillyness

My Feedback: (25)

OK... thought about it.

I agree with Stick Jammer.

Picture this... you are in an airplane flying level. You push over to 1/2 G. The plane is accelerating downward (at 16 ft/sec^2 initially), but the there is still 1/2 positive G on the airplane... it keeps everything on the floor and you in your seat, though everything weights 1/2 as much as normal, including fuel. You always have to add in gravity when dealing with vertical accelerations. In the vertical plane, as soon as ANY drag is felt, acceleration of the airframe becomes less than free-fall acceleration, and everything inside it starts feeling weight again. The fuel will move to the front of the tank long before terminal velocity.

If the plane has been falling for a bit, is experiencing a little drag, and is now accelerating at 30ft/sec^2 (still very fast), the innards of the plane (and fuel) will feel a downward force of (2 m/s^2 * weight of objects). Granted, your stomach will still feel light, but that's just cause it's lighter than it's used to feeling. But, the plane is accelerating at 30 ft/sec^2, and the fuel which feels no drag other than the front of the tank, is trying to accelerate at 32 ft/sec^2.

Another example... you are on a slide at the park. You are accelerating down the slide, but not at 32ft/sec^2, so you stick firmly to the slide. Your accel is less than free-fall accel, so you feel the difference.

sillyness

My Feedback: (25)

Stick Jammer,

Sorry dude.. I edited my post before I saw yours... I realized my thinking was in error. I gotcha. Those four years of physics are slowly coming back

Stick Jammer

As I said before, I've never found any conclusive proof that this is what takes place, but the laws of Physics surely point to this conclusion.

famousdave

My Feedback: (61)

Cameras don't lie.. unfortunately, my work was classified so I cannot post the results or any of the vids. It was pretty conclusive and would clear up this discussion. Trust me, the fuel stays in the back of the tank. Same goes for smoke oil. If you have flown a downline in a full scale edge, extra, cap or any other sport plane capable of extreme aerobatics you will notice that during the downline you were still pressed rearward INTO your seat. You are pressed INTO your seat because you are accelerating. If you are in your seat, the fuel is aft.

Another example -
Two objects are dropped from a mile up - one a steel sphere, the other a high-drag object like a square wooden block. Initially, both will have IDENTICAL acceleration through free fall, but the wooden block will reach its terminal velocity first due to the effect of drag. The steel sphere will continue to accelerate until it too, reaches its terminal velocity and both will continue to fall a relatively constant descent.

Even though there are significant differences in them, the objects will hit the ground only milliseconds apart from a one mile fall.
If this distance was increased to several miles, we would have a noticeable difference - in the order of several seconds.

Same principal goes for the fuel in the tank. There definitely will be a time when the freefall of the fuel (which has less drag and no wind resistance) will "pass" the airplane and move forward as the plane hits terminal velocity , but that time is measured in many more seconds than our planes are in their downlines. During our downlines, the plane is in freefall and pre-terminal. As long as this is happening, the fuel will be aft.

I don't disagree with the principle of fuel movement in downlines, just the timing of it.
DP

Stick Jammer

desertpig,
You are misusing the term "free-fall" in your examples. Any item dropped from a mile up (or any height) will never experience free-fall due to air resistance. Free-fall can only take place where there is no air resistance. The velocity of an object in free fall has no limit, it increases at a constant rate as long as the object continues toward the earth. The rate of free-fall acceleration is also constant no matter what the object is, a five pound chunk of lead will fall and accelerate exactly the same as a pillow feather will. When an object is falling to earth in the open air, it encounters resistance that steadily increases. Given enough time, the object will no longer accelerate when it reaches its terminal velocity. There is NO object on earth that will fall at a faster rate of acceleration in the open air than the rate of free-fall. I'm not talking about a power dive, I'm talking about nose straight down with the power cut. Bottom line, the plane would have to be accelerating faster than the rate of free-fall to keep the fuel at the rear of the tank. It's never gonna do that with the engine at idle and the nose pointed straight down.

Stick Jammer

The plane doesn't need to reach terminal velocity for this to happen. The instant the acceleration rate of the plane is slower than the acceleration rate of free-fall, the fuel will drop to the front of the tank. From the time the plane is nose down it would have to accelerate beyond 22 MPH within the first second and keep accelerating at a rate of roughly +22 MPH every second (88 MPH in 4 seconds) to hold the fuel at the rear of the tank. This would be possible in a power dive but extremely unlikely at idle.

sillyness

My Feedback: (25)

Stickjammer,

Close... the acceleration of gravity in a no drag environment stays at a constant 9.8 m/s^2. The velocity keeps increasing at a constant rate. Everything else is theoretically correct.

Desertpig,
I have flown downlines in aerobatic aircraft. They dowlines were somewhat short due to the low drag of the jet (even with speekbrakes out). There was a definite force pushing me forward into the straps, even though the aircraft was accelerating.

The same falls true with spin recoveries. As soon as I drop the the nose to vertical in a spin recovery, I'm thrown forward in the straps and the orange I had behind the seat will fall to the front of the cockpit, even though the aircraft is accelerating rapidly (need significant G to keep from overspeeding).

Also... in your steel sphere example... you are comparing 2 different objects with differint drag coefficients and surface areas, but both experience drag. Do the same math with a 100" wide wooden block and something with a drag coefficient of zero (like fuel in a tank or a pilot in a cockpit).

John Murdoch

My Feedback: (11)

With all the physics being throw around, why is it when you're in a car with the windows rolled up, a controlled environment, does that pesky fly that's flying around your nose, doesn't end up on the rear window when you do the 0-60 in 4 seconds?

sillyness

My Feedback: (25)

Because the fly is a very high drag object with very low intertia. The air in your car accelerates at the same rate as the car and helps pull the fly along with it. More importantly, the fly is intentionally positioning itself in the cabin airmass, reducing the effect of what little interia it has. If you thew a dead fly in the air and then stepped on the gas it would move toward the back of the car.