Easy way to determine tank height position?

[Andrew Tinsley]

Hello everyone,
                     Went to the field early this morning to try and trim out a plane. First thing was to get the tank height correct. However I was on my own and thought it would take a lot of work, running my stooge, trying to work a stopwatch and count laps!
  I thought I might shorten the process, by simply warming up the engine and checking rpm upright and inverted. I have a suitable plane mount on my field box and a tachometer, so away I went and within 5 minutes I had the upright and inverted rpm identical to the limit of the tachometer accuracy.
  Right, now to fly and check the upright and inverted lap times..........yes both were the same as far as my stop watch fumbling could tell me.
  My question is "was I just lucky"? I am fully aware that the dynamic flight situation is very different to the static one. What do the experts think?

Regards,

Andrew.

[RC Storick]

Did it work? Enough said!

[Alan Hahn]

I am sure it is good first approximation, and in your case a final one too!

The main difference is how the fuel is pooling in the tank. When you fly it is on the outer edge of the tank. At rest it is on the top or bottom. I guess if the tank is filled enough so that the uniflow vent (if that is what you have) is still submerged in the fuel, your method should be pretty good. If you are normally plumbed, I think the tank height is a little less sensitive and you method should still be fine.

[Andrew Tinsley]

Alan and Robert,
                      Thanks for your replies. If it is as simple as that, then why do I see people making a lot of flights with adjustments in between? The last time I saw this happen, it took about an hour to get a final tank position. I have never heard of doing it with a tachometer and checking upright to inverted rpm.
  I really thought that there must be SOME snag, it just seemed too simple!

Regards,

Andrew.

[Mark Scarborough]

IMHO, obtaining equal lap times upright and inverted is the first step. The actual performance under a load is the final and most critical factor. The only way to know that is correct is to actually fly inside and outside manuevers. For some reason, I have one plane that when set for equal lap times in level flight, loads differently and will speed up in the outside manuevers, so I had to adjust, slightly, the position of the tank to obtain equal speed in the manuevers and ignore the speed in level flight.

[Alan Hahn]

Alan and Robert,
                      Thanks for your replies. If it is as simple as that, then why do I see people making a lot of flights with adjustments in between? The last time I saw this happen, it took about an hour to get a final tank position. I have never heard of doing it with a tachometer and checking upright to inverted rpm.
  I really thought that there must be SOME snag, it just seemed too simple!

Regards,

Andrew.

Mark is right that what happens in maneuvers may need you to change tank height. However what you have done will probably work most of the time to get pretty close.

However, I will note that my Electric Nobler flies slightly faster inverted (about 0.1 s less per lap)  than it does upright--moving the battery up and down makes very little change  .   Also it draws slightly less power while doing those inverted laps. So in this case, the airplane is slightly more aerodynamically efficient inverted than upright. You would most likely not notice this with a glow setup, because we typically adjust tank height to even out the runs. This would be one thing your "static" method wouldn't fix.

So you can see why you can get to be more complicated. There is more going on than simply engine rpm, although that is the biggest factor to be sure.

[RC Storick]

Mark is right that what happens in maneuvers may need you to change tank height. However what you have done will probably work most of the time to get pretty close.

Tank height will give same lap speeds. Maneuver speeds are set with uniflow location. At least thats how I do it.

[John Stiles]

I'm glad this thread came up......I am flying a McCoy .29 type 21 engine, and a home made tank with rudimentary venting.........I have the tank centerlined with the engine.........why is it crapping out while inverted, but flying excellent otherwise? When I start into a loop, the engine goes into power band, and after the maneuver it gos back very slightly rich....which I like for a ringed engine. I'll sit on my hands while I wait for enlightenment! P/S I'm totally a Rube when it comes to all this technical jargon....my special field of training came in the service: Sniper...so you see I have very little aviation experience except when the pilot says "Get Out Now"

[Howard Rush]

I'd go along with Mark. Pick a prop that runs the engine at the 2-4 break, so you can hear the difference between inside and outside loops if the tank height (uniflow location) is wrong.  Do horizontal eights dead downwind and overhead eights.  That's the best way I can think of to match speed in insides and outsides.  Who cares whether your upright and inverted lap times match? 

On the first couple of flights of my first Impact, the line tension in insides was way different than the line tension in outsides.  I suspected that the wings weren't level, but the problem was speed difference because the tank height was wrong.  I don't remember hearing the difference. 

[Andrew Tinsley]

Alan, I am surprised that moving the battery up and down has no effect on inverted to upright run times. Surely the electrons flow downhill, faster than uphill? I would expect to see a difference!!!!!!!!!!!!!!!

[Alan Hahn]

Alan, I am surprised that moving the battery up and down has no effect on inverted to upright run times. Surely the electrons flow downhill, faster than uphill? I would expect to see a difference!!!!!!!!!!!!!!!

Andrew,
It's actually more dependent where the positive terminal is coming out of the battery. That needs to be 1/4" above the mean line of the prop shaft. Since the electrons move from the negative terminal, the positive one allows the spent and tired electrons (after they have turned the motor) to flow back in nice and evenly, keeping the whole battery at a constant  pressure.

[John Stiles]

Alan, I am surprised that moving the battery up and down has no effect on inverted to upright run times. Surely the electrons flow downhill, faster than uphill? I would expect to see a difference!!!!!!!!!!!!!!!

That must be why they say to be careful to not get a kink in the return wire

[Larry Fulwider]

 . . .  I thought I might shorten the process, by simply warming up the engine and checking rpm upright and inverted. I have a suitable plane mount on my field box and a tachometer, so away I went and within 5 minutes I had the upright and inverted rpm identical to the limit of the tachometer accuracy. . . .
. . .
. . .  My question is "was I just lucky"? . . .

Lou Crane will love you! Lou has been promoting the idea of pre-setting tank height by holding the plane at 45 degrees upright and inverted as long as I can remember there being forums.

The caveat is that the method is a "get it in the ball park" test, not a final answer. Inverted and upright speeds is the next test (the final test, for many.) Top flyers (and crummy tank builders) check inside and outside speeds for the optimum tank height, as the top flyers above already mentioned.

Larry Fulwider

[Mark Scarborough]

Alan,
I have found on my electric that its actually the position of the ESC that more closly regulates the speed,, the battery is not  a very reliable method for adjusting the lap times,, The battery gives false indications during vertical transitions,, 

[Scott B. Riese]

Robert ....WHAT is uniflow location?

[Jim Thomerson]

Not Robert, but what the heck?  By uniflow location, we mean the position of the uniflow opening in the tank.  Where is it in a vertical sense?  I start out with it on a horizontal line with the hole in the spraybar; and, if needs be, move it a hair up or down.  One can move the whole tank, or pop the back off and resolder the uniflow.  I have some old tanks which have a vertical baffle in them.  I put the uniflow on the inside of the baffle, as it is too much trouble to route it to the outside by the fuel pickup.  I've put uniflow outlets from the front of the tank, like a Palmer tank, to back by the fuel pickup in the usual position.  All work well.  The position of the fuel pickup is not important, as the engine thinks it is sucking fuel from the end of the uniflow, and does not know where the pickup is. 

Sometimes you can tell when the uniflow comes out of the fuel near the end of the flight.  The engine will go rich, then lean out as it runs out of fuel.  With muffler pressure, you may not be able to tell. 

The uniflow was invented by Edme Mariotte in the 1600's.  Google Mariotte syphon for more info. 

[Howard Rush]

Jim beat me to the definition.  Sorry I didn't explain earlier. I haven't done the ciphering, but I'd assume it affects the balance between insides and outsides.  Hence the remark about crummy tank builders, of which I am one. 

I figure that an airplane doing a rulebook loop is operating at 9 G's (8.3 at the top; 10 at the bottom).  This amplifies the tank height effect, plus the height difference due to sin (angle of attack) * distance of the tank behind the engine fuel intake.  The difference in run between insides and outsides may also be a function of flap-elevator rigging.  Because of the factor-of-nine amplification in round loops-- more in square corners-- timing the airplane upside down and right-side up is, I think, a waste of time, but I'd be willing to see some calculations.  Holding your airplane in different positions on the ground is even more of a waste of time, because you aren't getting level-flight practice while you're doing it.  What you can do on the ground is to get the tank height level with the engine fuel intake.

Unfortunately, until somebody builds us an automated judging system, there's no good way to measure speed in maneuvers.  You gotta go by sound and feel, which may suffice.