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Author Topic: Liquid level during rotation.  (Read 2414 times)
Chris Wilson
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« on: February 20, 2012, 04:47:46 PM »

Again, not sure if this question is best posed here in the Engineering forum and its 'resolution of forces thinking' or elsewhere but anyway ........

Does the direction of rotation make any difference to the level of the liquid contained in a fuel tank?
(Please assume constant speed and acceleration and the conditions of simple level flight.)

What I am trying to determine is does the level favour the forward direction that it is headed in or not.
Or does it just assume a completely tangential path despite having a distinct direction ?

Thanks.

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Tim Wescott
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« Reply #1 on: February 20, 2012, 05:53:07 PM »

Well, since you've gone and criticized the engineers for trying to sum the forces to zero, even though that's what every good mechanical engineer is trained to do, I'll just have to avoid doing that.

The liquid inside the tank has no clue what direction the tank is rotating, only how it is being accelerated.  So when you come to the resolution of the accelerations (ha ha!) the liquid will not respond to the rotation direction.

If you're wondering why tanks should be canted out a bit at the back end, that's either because it's just good housekeeping to make the sump deepest at the pickup point, or because due to the fact that the fuselage nose is ahead of the center of gravity it is actually canted out at the front with respect to the circle that the fuel is traveling in -- so you need to cant the tank out a bit at the back to resolve those accelerations and make everything work out right.
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« Reply #2 on: February 20, 2012, 06:03:04 PM »

Hi Tim and thanks for the reply mate.

So the actual direction of travel is of no consequence but the fact that the tank's physical shape is ahead of the CG and therefore its front is angled outwards slightly gives the impression of the liquid within that container has shifted.

Got it! (Had that problem rattling around inside my head for the longest time and do you think that I could resolve it?)  Undecided
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« Reply #3 on: February 21, 2012, 01:32:30 PM »

Hence both front and rear feed tanks are o.k. in racing.
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« Reply #4 on: February 21, 2012, 03:01:34 PM »

Hence both front and rear feed tanks are o.k. in racing.
Hi Ian,
            had a long talk about about this last night with our mutual friend Herb Hanna and he put me straight on a lot of the practical thinking about this.

The tank is not that far ahead of the CG in a conventional stunter so the angle it assumes relative to its projected point of tangent on the flight circle is not that much and of greater importance is the angle of yaw the model assumes.

The yaw angle dictates the use of angled or wedge shape rear feed stunt tanks.

In racing a front feed can see the model accelerating from a stand still with a dry feed pipe unless it is completely covered, and speed models, well do they even entertain the notion of yaw in the first place?

Science can make your head hurt at times. y1
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« Reply #5 on: March 16, 2012, 08:25:54 PM »

There was video from someone in the Tulsa Gluedobbers of a Sullivan SS tank on a profile.  It pretty clearly showed the fuel sloshing around mainly towards the back of the tank, whether the plane was level with the ground or in a wingover.

As a corollary, building a forward feed tank requires a very large forward angle to the tank.  The front has to be at least twice as deep spanwise as the rear in order to feed all the fuel.
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« Reply #6 on: March 18, 2012, 01:44:39 AM »

fuel sloshing around mainly towards the back of the tank

yes, centrifugal force in maneuver does it
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Rafael Gonzalez
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« Reply #7 on: March 18, 2012, 08:38:58 AM »

Vector forces at 90 deg (perpendicular) have no effect on each other. As constant acceleration is experienced by the fuel (centripetal force), the fuel sees the outside edge of the tank as the bottom. It also stays at the bottom of the tank due to gravity. As we change the angle of the aircraft to point nose up, the fuel sees our gravity and, if sustained long enough, it will all go to the back of the tank following the wedge.
Now, when we nose dive, in order for the fuel to stay at the back of the tank, we have to ACCELERATE  towards the ground and match at least the force of gravity (9.8 M/s(s)) for the fuel to stay motionless. It will experience no gravity. . The fuel will go to the front eventually as this can be detrimental to the pretty model if substained for an extended period.  n1

These set of laws causes many gyrations for tank design. A baffled tank, chicken hopper, hose clunker tanks, etc. can be explored and altered for better fuel draw. A heavy clunker that can follow the fuel to the front of the tank, is an ideal solution. Many R/C modelers have a clunker tank but do not allow the clunker to flex to the front of the tank. Same in C/L. I've seen hundreds of flame outs in R/C while the aircraft free falls. The solution is to get a tank and a heavy clunker. For a solid tank, a baffle ~ 1/4 in front of the pick up with a hole the size of the tubing and almost to the inside edge of the tank (towards the pilot), will hold the fuel longer than is needed for verticals. It will also reduce the sloshing.

 Hoff
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Igor Burger
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« Reply #8 on: March 18, 2012, 10:37:35 AM »

Beside centrifugal force out of the circle, we have also centrifugal force out of the loop (prependicular to out of circle) ... combined with positive AoA makes the fuel travel back, just look to that video, you will see clearly fuler on back of the tank. That why we need pick up tube on back wall of the tank.
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« Reply #9 on: April 05, 2012, 09:16:17 PM »

<snip>
Vector forces at 90 deg (perpendicular) have no effect on each other. As constant acceleration is experienced by the fuel (centripetal force), the fuel sees the outside edge of the tank as the bottom. It also stays at the bottom of the tank due to gravity.

This is why I chose a round clunk tank for my stunter. No corners for the clunk to hang up in, fuel/ clunk follows a 180 deg arc. (approx.)
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« Reply #10 on: April 08, 2012, 09:36:57 AM »

I have made and flown uniflow tanks with the uniflow at the front of the tank, like the Palmer tank; and uniflows here and there along the side back to the usual, a bit in front of the pickup.  I have not been able to see any difference in how these various tanks run. 
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« Reply #11 on: April 09, 2012, 06:04:58 AM »

 Same thing here. I have made a test tank with 3 uniflow tubes. One 25mm from front wall, the other in halfway and the 3rd 10mm in front of pick-up.
 The only difference I managed to find, is that needle setting is slightly more difficult when the rearmost uniflow tube was in use. I guess there was some interference from bubbles to the pickup tube.
 But nothing really dramatical. I feel a bit more confident with the u/f more in front, especially in places where I must fly without a practise flight first. L
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Chris Wilson
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« Reply #12 on: April 10, 2012, 05:28:35 PM »

I would think that distancing the feed pipe from any vent pipes (uniflow or not) that lie along the outboard wall will be severely influenced by the amount of yaw the model assumes in flight.

Canting a uniflow tank out to combat model yaw assumes that all pipes will terminate within that canted reservoir surely.

Or have I got this wrong?
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« Reply #13 on: April 17, 2012, 05:58:02 PM »

I just (finally) scanned this thread (quickly,) and if there is any mention of fore/aft accelerations caused by drag increases when maneuvering, and drag reductions when maneuvering lift ends, I didn't catch it...

In the textbooks, Induced Drag Coefficient varies as the square of the Lift Coefficient. The other factors remain unchanged, as the 'reference area' is still the wing planform area, since Induced Drag is a consequence of lift.

Whatever the low, level, 1g loaded cruising flight Induced Drag is, in a 10g turn it becomes 100 times as great. In a 25g turn it rises to 625 times as much. Are you trying to convince me this does not affect airspeed? Suddenly and violently in a square turn; strongly and quickly in rounds, but still occurs. I feel fairly sure that applied drag loads-on more quickly than engine thrust can completely overcome. (Line angle also affects the initial Induced Drag condition.)

When such a drag rise is applied, the model must slow at least some: fuel will slosh forward, of course. When the loads unload, engine power governs the acceleration back to "flank speed" and causes a rearward slosh.

As Igor pointed out, the directions and quantities of forces through maneuvers requires considering several 'vectors' as well as the flow of accelerations along the path flown.

At least, IMHO...
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Howard Rush
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« Reply #14 on: April 17, 2012, 10:54:13 PM »

That's interesting.  It looks like if the tangent of the angle of attack is greater than drag/lift, then the fuel will slosh to the front of the tank.  If the tangent of the angle of attack is less than drag/lift, then the fuel will slosh to the back of the tank.  Did I get that right?  Draw them vectors.   
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« Reply #15 on: April 18, 2012, 05:13:11 PM »

Howard,
It's been many decades since I had any formal study of the vectors involved, and then not at a college-math level...

IF the relationship between the lift and drag components stays the same, then their amplitudes change as the increase in lift for the path desired. If a greater lift component, then a greater drag component. Plots of CsubL v Alpha apparently stay linear over quite a bit of the range of AoA. So, at least initially, the model should slow significantly, no?

The drag component on the instantaneous tangents of the model's curved path in pitch is opposed to the thrust (or thrust plus KE) available to the model. That sound Kosher?

Fuel, being liquid, is restrained only by the tank shell, which is solidly attached to the model and decelerates with it due to rapidly increased maneuvering drag. It is not a steady-state condition, such as occurs in steady flight with no pitch accelerations. Similarly, when unloading from maneuver lift and drag, engine thrust re-accelerates the model. There may be a brief, more-or-less steady conditon in a sustained curving path in pitch - think consecutive rounds - where lift required is basically defined by the figure's radius and tilt, but also varies with the direction and the trig of line elevation regarding gravity's 1g.

Maneuvering-g component, "CF," and gravity's component define an instantaneous "local vertical" that would determine the fuel surface if it endured long enough. These are pretty short duration events; maybe the fuel never stops sloshing, except in low, level flight...?

Wacko? Off the mark? Waiting with 'baited' breath (just had a sardine sandwich...)
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« Reply #16 on: April 18, 2012, 09:04:11 PM »

I should have said drag - thrust, rather than just drag.  

Everything varies with time, but the surface of the fuel (not counting the back-and-forth sloshing (a term I shouldn't have used above) or airplane angular rates) will be perpendicular to the sum of all those forces acting on the airplane.  One could put line tension and gravity, too, but I didn't.  Drag slows the airplane down and causes the surface of the fuel to tilt high in the front and low in the back relative to the direction of flight.  Angle of attack causes the tank shell to tilt high in the front and low in the back relative to the direction of flight.  If (drag-thrust) / lift is greater than the tangent of the angle of attack, the fuel depth will be higher in the front of the tank than in the back.  This is what you were saying, but you also have to consider lift (which is high in maneuvers, too) and angle of attack to see what happens to the fuel level.  I always put my fuel pickup in the back of the tank and never though of why until you got me thinking.  
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« Reply #17 on: April 19, 2012, 01:05:26 AM »

May be I will add some numbers:

drag of lines can be somhere at 2N ... drag of wing in level little over 1N and drag in corner ~3N ... complete drag of model (lines, fuselage, tail, wing, landing gears) in level will be somewhere at 5N and drag in corner somewhere under 10N

model flies in corner at angle up to 10 degrees. If I take extreme of that angle, than tangens is 0.17 so if the prop thrust is 20N then props makes additional thrust up to 3N what can partialy ballance that induced drag ... it is another force pushing fuel back  Grin
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Howard Rush
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« Reply #18 on: April 19, 2012, 12:16:47 PM »

Hence drag-thrust, the net force in the direction the airplane is going relative to the air mass (not counting thrust in other directions).  Actual numbers require effort.
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« Reply #19 on: April 19, 2012, 04:34:31 PM »

Would the fact that the acceleration provided by any pulsed system like that found in IC engines would tend to accelerate the model forward of any liquid mass due to the reluctance of inertia?

Surely this alone would mean that the fuel in the tank tends to lag behind those directional pulses somewhat?
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Howard Rush
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« Reply #20 on: April 19, 2012, 06:33:37 PM »

No, but the vibration might put ripples on the surface.
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« Reply #21 on: April 20, 2012, 04:28:27 PM »

Howard, Thanks!

Much better phrased than my effort. However, I still can't shake the sense that the rapidity of the resolved vector changes in our severe maneuvering is NOT instantly and perfectly matched by the approximate fuel surface at all times. Liquids do have inertia.

Also, thanks for the adjustment to correct for manuevering angle of attack NOT necessarily aligning with the model's instantaneous path. As the photo of Brett's NATS winner, vertical from a low corner, at barely one fuselage length above 'level flight altitude' urges us to consider, the rotation may go well beyond the path of CG until the model 'catches up' to things.

If air molecules required us to use wheels for traction, we'd be looking at a full 4-wheel drift in that photo.  Igor's stop motion image series of several years back also showed clearly that the distances traveled between exposures lessened as the lift load rose to max (or thereabouts.) Pity the exposures couldn't capture sharp images of a specific point on the model a each exposure. Less distance in the same time means slower motion, no? (Presuming that the exposures were at identical intervals, of course. No reason to think otherwise.)

 
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