Dumb question of the day...

Why I'll leave up to the physics experts. This sounds like a good Phd. thesis paper subject, if anybody cared. NASA might have it all figured out already. The problem is similar to keeping the fuel feeding in a rocket when it goes through gyrations in space.

Phil,

Good insight here. Many of the little thrust motors on spacecraft are "ullage motors". Their purpose is to provide slight accelerations of the vehicle to get the fuel to slosh in the direction of the fuel pickup, usually at the back of the tank. "Ullage" is the engineer's word for "empty space in a fuel tank."

This is a subtle one for sure. One possible answer is that in addition to the purely radial centripetal acceleration, there might be a slight tangential acceleration component, for reasons I don't see. Or a Coreolous (sp?!) term. But I think that fuel moves to the back of the tank in Australia too. I'll have to ponder this one...

Andy

>AH! "coriolis force" could be "the biggie". I'll have to ponder this

Nah, the guys in Australie fly left-handed and clockwise.

A simple answer to the above question is, "Because".

Well, all of the engineers might attempt to explain it, but it seems that the fuel does end up in the back of the tank and at the outside. However, if you are concerned about sloshing or some such, and Matt had a plane that did just that, you can build a tank where the outside wedge angles out towards the back. (Narrower in the front, wider in the rear) Al's been doing that on some of the tanks he builds. In Matt's case Brett made a tank for him like that and cured the problem. But the problem here is, even if the fuel does slosh forwards and backwards (due to yaw or whatever) you still need to put the pick up where the fuel is most of the time. And unless you have a real "squirrely" type of clunk tank, even that wouldn't solve the problem. The clunk tends to stay in the rear.

But, if you are a skeptic, build a plane with the pick-up in the front and then report back--after you pick up the pieces.

Yes, well, a simple suggestion that maybe someone on the board will be du--er, um--- curious enough to try:

Remember filling a bucket with water, and swinging it around and over your head, and watching the water stay put in the bottom? Then stopping the bucket's swing, to see what would happen? Yeah, we all did it! Only took me a couple of tries to figure out why at the top was not a real good idea.

Why not construct a BFT (BIG Flippin' Tank) with the inner side open, tethered by a similarity to the lines on a C/L plane. (Handle will be optional) Fill it like the bucket,then swing the sucker around, and you should be able to observe the action of the liquid in the tank. Perhaps it would answer the question one of the other members poses: "Why is the world spinning around?" Don't try this right after lunch! (Don't ask)

Yeah, I dunno. At the end of the day, the sum of all the accelerations must be pushing the fuel aft and out board (Empirical Evidence). But, I just can't visualize it. Centrifugal force should push the fuel outboard and forward if the tank is angled front out relative to the tangent of the circle (due to acft yaw angle), Yaw rate around the Y axis should push the fuel forward, Pitch rate around the Z axis should push the fuel forward, deceleration entering maneuver should push the fuel forward (acceleration exiting the maneuver would push it aft). Gravity is always toward the center of the earth (duh ). What major factors have I missed?

...like a famous person on this forum said "inquiring minds want to know"...

Like so many things that are "different" about our cl airplanes, the answer is the result of flying them in a tethered environment. Our airplane and everything attached or contained in it wants to follow all of Newtons laws. First and foremost, like any mass in motion it wants to fly straight ahead until acted on by some outside force.

Well, in reality, our airplanes operate in a constant environment of "outside forces". The tether, our control lines, force them to fly in a constant arc. Were it not for the tether under steady state flight the fuel would simply sit in the bottom of the tank due to the forces of gravity.

Any outside force which affects Newton's intense desire to keep the mass...er, uh...stunter going in a straight line will "accelerate" the mass in the direction of the force applied. When the steady state condition is thus altered all of the mass which was in a steady state wants to remain where it was...or where it was going. If fluid in nature (like fuel...or air for that matter) the mass that is subject to that acceleration it will move (actually, of course, it is attempting to "stay" where it was or where it was going in our case).

Thus, if you try to slide the beer mug down the bar like in a Clint Eastwood western, you'd better do so carefully because although the mug will go. the beer will try to stay where it was and (in slow motion) will pile up against the trailing side of the glass until it starts to spill out over the top. Accelerate the mug fast enough and you can pretty much empty it without ever tilting the glass!

Same thing happens inside our tanks.

The fuel wants to go in a straight line like Newton intended. Instead it finds itself gettting accelerated forward and to the left (in clockwise level flight) and, like the beer in the mug, the fuel appears to move in a direction opposite the acceleration. (It is, of course, the tank that is moving and the fuel is merely trying to stay put ... Newton at work again). Thus the fuel will always end up "puddling" at the back outward corner of the tank with the "elevation" of the surface dependent on the amount of pitch acceleration (g forces in maneuvers) and the effect of gravity.

Bob Reeves in flight film of this is all the proof you need that Newton was a pretty smart guy. I love to watch it and have numerous times run the wingover sequence frame by frame to observe in a few seconds what I took way too many words to express. The old saw about pictures and words comes to mind.

By the way, lest anyone think I'm some sort of guru who just knows all this stuff, let me straighten you out.

I had asked much the same question as the originator of this thread many years ago and actually built a tank with a "vertical wedge" with the outermost corner about 1/3 of the way aft. My thoughts were exactly the same. Hey, I can shorten the fuel draw by more than three inches and centrifugal force (hmmmm...no, I won't go there) will keep the fuel right out in that wedge. Worked great right to about the figure eights at which time the engine would quit due to fuel starvation. Oi vey. Back to the drawing board!

Ted

p.s. oops, forgot. I wanted to expand on my early comment about "air". A great experiment to illustrate accelerative effects requires two pieces of equipment (well, three if you count the air). A car (not a convertible and a long van is terrific) and a helium filled balloon. Put the floating balloon inside the van, crank it up (the van, not the balloon) and drive away. You might expect that floating balloon to move to the back of the van because it's got mass, right! I mean the van is accelerating and the floating balloon should want to stay in place thanks to Newton.

Instead, the balloon moves forward, blocks you vision and nearly causes an accident. (don't try this at home). What happened is that the air within the van also has mass (we don't often think of it that way but, like any fluid, it does). Like the fuel in our tanks when its container accelerates the air wants to stay where it was. Because it is contained within the van it gets dragged kicking and screaming along with the van but "piling up" in the back of the van just like the fuel in our tank.

the physical result of that piling up is that the atmospheric pressure (and mass) is greater the further aft you go in the van. Because the balloon is lighter it wants to go where the pressure is lower, ergo the air moves aft and the balloon moves forward. (exactly the same physics at work as when you let it go outside, it floats away, your little girl gets mad at you and you have to buy another one).

The faster you accelerate the more rapidly the balloon moves forward. Mr. Wizard just loved this s...tuff.

pps. Sometimes I just can't shut up, sorry. Another interesting reality is what would happen to the fuel in the tank if there was no top to the tank and the tether acceleration caused it to spill out the top. We might assume it will fly "away from" the circle due to centrifugal force. Huh-uh. Once again, Sir Isaac prevails. Once the mass becomes "untethered" it also becomes "unaccelerated" and wants to continue in the direction it was going at the point it lost that acceleration. Therefore, it flies away at a tangent to the circle, not away from it.

this is also true of when your muffler falls off (or your cowl, or whatever), don't look for it straight out from where it exited. It'll be forward and away from the circle.

"Beer is proof that God wants us to be happy"
Benjamin Franklin

Randy Powell

Ted,

Good Show, pretty much what I wanted to say, having pondered this much of the day. Only one small quibble: I claim the fuel is accelerated forward only while the plane is accelerating from rest. Once the plane is moving at a constant speed, the only two acclerations present are gravity downward and the centripetal acceleration radially inward toward the center of the circle. So the fuel definitely sloshes to the back outward corner as the model accelerates up to speed, but o

Edit: Good grief! This thing ate the rest of my post! I had previewed it too!

To finish: but once level flight at a constant speed is reached, there is no component of acceleration tangent to the circle. The fuel is neutral at the back of the tank. The rear tank wall doesn't have to push the fuel along because both are moving at the same speed and not accelerating. If say, a sudden gust of wind should slow the model, then the fuel outruns the rear tank wall, and sloshes forward. A sudden increase in speed and the fuel pushes against the back tank wall, as the back tank wall pushes on the fuel to speed it up.

As I said, a minor quibble. Your use of Newton's laws is the right way to understand the problem. If you're not a professional physics type, you certainly paid attention in class. You're certainly way ahead of many a hapless undergraduate!

Andy

Yes, we were accellerating, and then things levelled out, and the position of the pickup became moot, (or, so we thought) This thing just CAN'T Keep accellerating...

According to that well known expert *me* , you have it right. In level flight at a constant speed, I can't come up with any component of acceleration tangent to the circle to *push* the fuel back. I conclude it is in a neutral condition fore and aft (see post further on). You've done a good job of noting how various maneuvers temporarily introduce components of acceleration tangent to the circle to cause forward or aft slosh. Seems correct to me too.

What I take away from all this is that the fuel tube probably comes un-dipped frequently during stunt runs, but the engines are set up to tolerate this. The charge of fuel/air stored in the crankcase helps damp out the effects of momentary fuel loss at the intake due to a bubble in the fuel line.

Why is it that the "dumb" questions are often the hardest to answer, and the most interesting?

Andy

Sloshing can happen, but, perhaps, not in the way you have described. Oh, and the "charge of the fuel/air stored in the crancase" doesn't exist for more than one stroke of the engine. That is why the needle valve is so important. It regulates the amount of charge entering the crankcase which is then expelled on the first down stroke. After that, what remains is so weak the engine would mis-fire (quit) on the stroke following if there were no fuel to enter on the upstroke. You can hear this is some engines at the end of the run where they nearly shut off and then start again. But this is also evidence of that sloshing that you have mentioned.

During the maneuvers there is very little going on to even allow the fuel to move forward--at least to any degree to uncover the pickup tube. If the engine slows down going up hill, gravity is pulling the fuel back as well (Don't read this Igor). When the plane aims down, there is still some acceleration in spite of the fact we are tryhing to slow it down. When the plane slows in a corner, well, it is a corner, and that, again, throws the fuel to the back (and bottom/top) of the tank. So, the only thing left is that slight yaw of the nose leaning out, which, at times, puts the front of the tank farther outboard of the circle.

So, we now have the plane constantly turning left and pulling the fuel to the back (well, not actually "pulling", but you get the drift) and we have centrifrugal force trying to keep the fuel going in a tangent away from the center of the circle and we have the nose yawing out (because of the way it is trimmed) with centrifrugal force wanting to push the fuel to the farthest outside edge of the tank. Thus, at the very end of the flight we can have the remnants of fuel in the tank "sloshing" to the front and uncovering the pickup tube. The engine sputters from starvation, there is suddenly less yaw and the fuel flows to the back and the engine restarts. This can happen several times until there is not enough fuel remaining to restart the engine even when the remainder flows to the rear.

Yes, that can happen. But if we put the pickup tube in the front to get that clean shutoff at the end, we will find it cutting out in the vertical eight or hourglass because the fuel is still being thrown to the back during these maneuvers. Better to suffer the inglorious burp shutoff at the end than to have the plane fall to our feet during one of these overhead maneuvers.