Profile fuselage tank position

I've often read that the outlet of the tank on a profile fuselage should be above the NVA by about 1/4 inch. Why is this? What's the technical reason for it as when inverted it will be 1/4 inch below!
Some help required!!
Adrian

The "technical" reason is that this is what works!

Typically, if you center the fuel tank, you will notice that you need to raise the tank about 1/8" to 1/4" to get the engine to run identically (or similarly) both upright and inverted. Try it!

That should tell you something about the effects of the height of the fuel "head" on fuel pressure. Pretty important to the "stunt run".

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"We're having something a little different this year for Thanksgiving. Instead of a turkey, we're having a swan. You get more stuffing." -George Carlin

LAST EDITED ON Dec-07-03 AT 02:25 PM (CST)
 
It doesn't matter where the fuel tube comes out of the tank, although the pick up inside the tank usually should be on the center line of the tank. The center line of the fuel tank should be about 1/8" to 1/4" above the fuel outlet in the spraybar. From there you flight test and check if the airspeed is the same upright or inverted. If it's slower (rich) inverted, raise the tank. If it speeds up (lean) when inverted lower the tank.

Giffy

Thanks Larry & Giffy. OK I understand the 'effect' - what I can't understand is 'why'. Our profile planes are (except for the fin) mirror images flying normal or inverted. If they are essentially identical in either direction why isn't the tank centred? What am I missing. Larry I know it works - just want to understand it!
Adrian

While your plane might be a mirror image upright to inverted, your engine isn,t. One way the exhaust will be up and the bypass down and the other way, just the reverse. IF you suscribe to the theory that the center of crankcase volume is the point that you wish to center your fuel pickup on, then on some engines it will check out. I don't suscribe to that line of thinking, but the late George Aldrich did to a certain extent.
I tend to think that it has something to do with the bypass flow and that you may need a little leaner or richer mixture during inverted flight.

Bigiron

The principle at work in our engines was described by Mr. Bernoulli and is known as the Bernoulli Theorem for steady flow of a continuous stream of fluid: The sum of the pressure energy, kinetic (velocity) energy and potential (elevation) energy at any point in a stream is equal to the sum of the energies at any other point along the same stream. Fuel flows in a circuitous path out of the tank, around the engine, through the needle valve, down the venturi, through the crankshaft, around the crankcase, up the bypass, and into the cylinder. Along the way it moves slower and faster, changes volume, increases in temperature and miraculously shows up just in time to explode.

Changing the height of the tank changes the initial condition of the Pressure energy available at the very start of the process. The initial pressure energy's relation to the energy delivered to the engine varies with the height of the tank in relation to the engine. Because tanks vary and tank mounts vary and plumbing varies, it is easier to use trial and error and adjustment to make sure that engine runs are as consistent as possible (or at least very predictable).

Why? Well, when you are flying inside loops and outside loops it would be best if the model turns the same speed both ways. It makes it easier to fly perfect figures since your timing and control is the same in each turn of the maneuver.

If you are running a uniflow tank, it is the position of the uniflow outlet in the tank that is important. The scientific explaination is that the engine thinks it is sucking fuel from the uniflow outlet and does not know where the pickup is.

I have never ever positioned a tank on a profile model with the uniflow outlet positioned (vertically) anywhere but on the centerline of the engine. Well, I may have had to tip a tank up or down maybe 1/32 in., but that probably is because in slight inaccuracy in my soldering.

Having said that, I see others with high mounted tanks who get good runs. So it works for many, just not for me. My advice is to make the tank position height adjustible. Start out with the tank somewhere and move it as needed to get a run that suits you.

I agree with you Jim. Try it, if it don't work, moveit.
Bigiron

. My
>advice is to make the tank position height adjustible.(sic)
>Start out with the tank somewhere and move it as needed to
>get a run that suits you.

Ron B.
F4Fguy

Absolutely correct!Make it totally adjustable ,both vertically and horizontally in both fore and aft and spanwise directions.For initial flights,mount it against the fuselage,about an inch behind the engine,and on the engine c.l.
Doing this will guarantee you'll never have to touch it again!

Duh!! Just what did you say there Bruce!!! I guess it's right since I didn't compute.

Bigiron

Bigiron13

Thanks guys - I think that helps. I can see now the engine is not 'mirror image' - got to work out an easy adjustment for my tanks.

All the previous info is right on. I noticed Bruce indicated that manuever speed equality is the end determining factor. Upright and inverted equal speed is the interim point. Adjust from there so inside and outside manuevers are equal.

Also, for analysis, remember that up and down aren't relative to horizontal earth plane when the plane is flying in a hemisphere. G forces cause up and down referene and fuel surface to "rotate" to approximately 60 degrees from horizontal (airframe) refernce.

This coupled to the assymetry of the engines causes the effect.

FWIW

Adrian,

Coming into this one a bit late, and you have a lot of good comment already.

One thing that works for me is to check tank height on the ground before the first flight.

Try this:
Start the engine and set it moderately rich.
Hold the model fuselage centerline horizontal, outboard wing straight down. (This is just a reference and 'rest' position.)
Rotate the wing (in "roll") to about 45º cockpit UP, while keeping the fuselage centerline horizontal. Note any RPM change. If it isn't easy to hear, have a buddy check it with a tach.
Roll it to 45º cockpit DOWN (keep fuselage centerline level.) Check RPM by ear or tach in this position.
If there's an RPM or firing mode change (say, from 4- to 2-) between the "rolled" attitudes, the tank is too high to the side it went richer.

Shut down the engine. Easiest and safest way is to roll the outboard wing straight up, aim the nose down, and close the vent inlet with a finger. Fuel pickup goes into air space, not fuel, and blocking venting stops fuel feed quicker.

Make a first try at the indicated adjustment: Lower the tank relative to the richer rolled position. Fire it up and run through the same check. With luck, you may get RPM in each rolled attitude almost identical with the first try. It might take several, but that's better than flaming out rich or lean in the first flight.

This doesn't explain WHY one engine likes the tank in one position, and a different make or size prefers a different height. Bruce Hunt, I think, in the thread, mentioned some of the complexities in our engines. I'm glad a simple pre-flight check can cover most of the very complex true science involved.

What this wig-wag test gives us is a simulation of the ANGLES of "weight" felt by the fuel in the tank while the model flies upright (cockpit UP attitude) and inverted (cockpit DOWN attitude.) The FORCES (from ordinary weight and centrifugal force) are greater in level flight. We can't simulate that, but their ANGLES do well enough to show where the happy medium tank height is. In high-g maneuvers, FORCES are even greater, so any difference in level either way is exaggerated.

There may still be a slight tweak needed if a model flies nose-up or nose-down, or severely banked, in level flight. The wig-wag process should keep the setting change small enough to survive the flight and tune further.

That's really helpful Lou. I'll try just that. I have a new tacho on its way to try and get these set up problems right. It's interesting how in thios hobby one thing works for one and something else for another - hence trying to get some theory on board. Your practical method sounds A1

Many thanks Adrian

Snip, snipped a bit...

Why? Well, when you are flying inside loops and outside loops it would be best if the model turns the same speed both ways. It makes it easier to fly perfect figures since your timing and control is the same in each turn of the maneuver.

Bruce Hunt
Salem, OR

Bruce,

Does this mean that in 2004 you and I will not be discussing why your Shark roars around in outsides, that I will (finally) be spared your pathetic defense, this going clear back to the days when--in outside maneuvers--your Banshee let you down a couple times?

As Bruce has pointed out, there are just a few variables involved here, too many for us weenie modelers to calculate in coming up with a perfect--or even real close--tank height for any given powerplant.

For example, lots of people suggest mounting the tank high; certainly works for my Fox 35s. However, both of my ARF FSs--O.S. 25FP for power, GRW 3.25-ounce chicken hopper tanks--ended up with tank almost 1/8" *lower* than centered. Nearly a full 1/2" difference with the only variable of significance being the motor.

Is the answer Fuel tank pickup height in relation to Bypass height, not Spray-bar/shaft height?

Not for me.

Is the answer Fuel tank pickup height in relation to Bypass height, not Spray-bar/shaft height?
Chris

No.

It is fuel tank height in relation to what kind of run you want in upright and inverted level flight. And then, possibly, it is fuel tank height in relation to what kind of run you want in the maneuvers.

I would not be overly surprised to be told there is a formulaic answer to the problem. I would be tremendously surprised to find anyone on this site has given it any serious thought. Not counting Ted, who is supposed to be working on his new PA model.

LAST EDITED ON Dec-09-03 AT 08:58 PM (CST)
 
I thought I had this figured out. On my Ukey, with the OS Max S, the tank needed to be 1/4" above centerline for equal engine runs. With the Brodak 40 on the same airplane, the tank needed to be 1/8" below centerline. So - on the baffle engines, above the C/L, on the Schnurle(sp?) engines, on the centerline or below.

With this newly acquired knowledge, I replaced the Fox on my Ringmaster with a TT36 Pro. So I went from 3/16" above C/L to right on the C/L. Everything worked right - not! The TT for some reason wants the tank even higher above the C/L than the Fox!

If anyone has anything other than experimental data and testing each engine, I'd like to see it. Until then, it looks like try until you get something that works for the particular engine.
Steve

For what it's worth - Fox Combat Specials needed the tank mounted high, too, back when we were flying Slow Combat. About 3/16", IIRC.

Where do rear exhaust want their tanks?