Inboard / Outboard Location of Tanks on Profiles

(Pre-Question Note: Isky's board had a pyramid scheme listed until 6:00 PM today. We don't have that here. Thanks, Leonard!)

We have had several threads on vertical tank location. What about horizontal? That is, if you need to move the tank out 1/4" on a profile to clear a gear clip, would you anticipate any problems?

I am assuming "not" in two cases.

Jim --

Thanks. Makes sense. Yeah, uniflow with muffler pressure.

I ended up building the first of several profile front ends that I've assembled in the last twenty years that had a "tank pocket" sunk into the space between the motor bearers to accept a 1" tall tank with a little leeway to move it as much as 3/32" either way, if not more, should the tank need adjusting. It seemed "natural" to do it with the Excalibur II that I was building at the time.

But since then I have heard of a few complaints against "wide" fuel tanks on profiles that I think a pocket of the type I've been using would have eliminated. Leonard dislikes the removal of so much material that could be stiffening the motor bearers, but I use a pair of hardwood crossbars making a sort of ladder out of the bearers and bars that is extremely rigid.

K.

_

I never liked the cut outs, I never liked the plastic tank, and I could never understand mounting the engine inboard. Engines will run different upright and inverted. On an inverted installation the engine will tend to run richer on inside loops and leaner on outside loops. The difference lies in the centrifrugal force on insides throwing the fuel towards the head and on outsides resisting the flow. You will sometimes need to make a slight tank height adjustment to compensate even though your upright and inverted lap times may be the same. But you can, generally, compensate for it. On a side mounted engine it doesn't make any difference whether the head is aimed in or out. The fuel flow forces will remain constant, with minor variations (very minor), if any, caused by increasing or decreasing line tension. We are not talking going from a positive ten g's to a negative ten g's (or worse). This is why my FP 40 profile runs fine when mounted sideways, but develops a slight "burp" in the intersections when inverted or upright.

Mounting the tank inboard with the engine outboard as some have tried on short nose airplanes, can, as noted, cause fuel flow problems and make setting the needle more difficult on the ground. Just remember, the fuel "head" on a normal installation is the distance between the farthest inboard location of the fuel (inside wall of the tank) and the pick up tube location (outside wall of the tank). This is the pressure head that will vary in force from the beginning to the end of the run and is being compensated for in a uniflow tank.

If you mount the tank inboard, however, the pressure head is now measured from the inside wall of the tank to the needle valve of the engine. You have as much as doubled the distance, increased the pressure head, and increased your problems. So it is not only the setting of the needle on the ground that is affected, but the actual run in the air as well. And if you have trim problems so that the line tension increases and decreases throughout the flight, so will the increased pressure head, changing how the engine runs.

I don't have any "science" to explain, but this is my experience: Especially Fox 35's with outboard tanks seem to lean out more during the run (straight suction feed). And I have some new fox 35 powered profile ships about to launch with outboard uniflow tanks. Then maybe I'll have something to compare. But I've never had any negative experience (other than strange fuel line routings, stock mufflers getting in the way of flipping the prop, and the fact that I get the "ugly" view from inboard the circle) with plastic clunk tanks, mounted inboard. OS40's do OK with muffler pressure but the Fox 35's seem to do slightly better without.

With the same tank and engine, mounted profile outboard, the fuel head changes from + 1/2 in to - 1 1/2 in. Moving the tank into the fuselage 1/2 inch should duplicate the upright situation.

Say the fuselage is 1/2 in thick, and the tank is inboard. Head goes from + 3 in to + 1 in.

In all cases the change in head is 2 in, the width of the tank. With the upright and outboard profile this is a 100% change in head, but with the inboard tank, the total head is 3 in, and the change is only 2 in, 1.e. 2/3 of the total head. Is that right?

Does this mean that an inboard open vented tank gives less change between beginning and end of the flight?

Hooray for uniflow.

Tanks are tricky, though. Just when you think you have it all figured out, something comes along to change things. (Now if we could just come up with something to make the engine pick up going into the wind and slow down going down wind...)

You might be able to accomplish something like you mentioned by changing the venting on the tank. I've never tried this myself, but I seem to remember sketches on the old Perfect fuel tank boxes showing fuel tank vents pointed forward (INTO) the wind, and vents pointed backwards (DOWNWIND). The latter case produces suction on the vent. Now, when the plane goes into the wind, the engine leans out and runs faster, causing speed to increase. Going downwind causes less suction, making the engine run richer and slow down. You could probably make this work with uniflow. But it seems that it might be tricky to get the needle set properly and consistently. All of this works well during level flight on windy days, but during maneuvers, it gives the opposite effect from what you want. In maneuvers, you'd like to have the plane slow down on the downward side of a loop. With the backward-pointing vents, when the plane speeds up, the engine runs leaner, which causes it to run even faster...positive feedback! Forward-pointing vents would work best here, since the engine would run richer and slow down when the airspeed tries to increase.

Bob Balsie

The reasoning was clearly explained. Frank Williams was having a problem with the ST60 in a sidewinder mount where fuel was condensing and pooling at the top of the bypass, and then spilling over under maneuvering. This was particularly true with very rich settings. This made it run very inconsistently. Mounting it inboard eliminates the pooling by scavenging any raw fuel back into the crankcase due to centrifugal force, and the runs smoothed out. There's a full explanation of this in Frank's profile Bearcat article in a mid-80's MA.

If you don't have this problem, mounting it inboard won't help anything.

Note also that Frank also started out trying to fix the Fox "burp" on outsides the same way, theorizing the same problem. This was an incorrect hypothesis, as the engine then started burping on insides! The correct solution is of course to pack the bypass.

Thanks to all. Since the post I ran across Robert L. Angel's article in the 1960 Air Trails Annual where he outlines the basic theory of uniflow. I had a bad picture, and now I got it (I think).

The submerged inlet does a little more than I thought. Theoretically, you could mount the tank in the outboard wingtip with a backward facing pinhole inlet (non-siphoning) and have a steady run (if you could find a ground needle valve setting that would allow you to get up to speed).

The fundamental I did not understand was that a uniflow tank is in effect a pressure regulator, and the head calculations we would normally do are largely irrelevant.

Advantages of uniflow I didn't get are the differences between 6 g straight and level vs 5 g overhead.

I think the reference here is that anything overhead is going to have gravity fighting it. So, if you have 3 gs straight and level, you would have 2 gs overhead if your tension, otherwise, remains the same. 6 gs level, would mean 5 gs overhead (that's a lot of pull!).

The main thing with the uniflow is that it maintains a more constant fuel pressure from full to empty tank.

Well, not quite. The fuel pressure at the tank outlet is constant. What happens afterwards is still subject to the same pressure head issues. In the case of the tank in the outboard wingtip, the pressure drop from trying to climb "uphill" from the wingtip to the needle valve would still exist. This would be reasonably constant, but very low. Opposite problem with a tank on the inboard tip - you'd get very high pressure.

That's why the horizontal and vertical position of the tank still matters, which was the basis of the thread to begin with.

Yes. That is what I thought I said. You say things a little better than I do.