TT.42 quirks

Quirk #1
I'm having a bit of trouble with this motor on outside loops.
It speeds up quite a bit on only this maneuver. It doesn't run faster inverted or overhead, just on the outsides, especially the round loops.

#2 I'm using a uniflow on muffler pressure. The uniflow pipe is about 1/4" from the fuel line and right at the "V" of the tank. It does not stop cleanly. It runs steadily until almost all of the fuel is gone, speeds up nicely to provide a warning, but takes 2 or 3 tries before it finally quits. It'll look like it has quit but then will restart. Is there a quick fix for this problem??

If the tank is on the mounts try putting a shim under the tank and the mounts,1/8"-1/4". It's so hard to make an recomadition because I can't see the plane and what's going on. My All American with a stock McCoy 29 with an upright engine speeds up in the inverted loops ONLY. I can live with that. There are so many factors in stunt, It's fun! scott

How are you running the engine, side mounted on a profile, upright, inverted? It sounds like a slight moving of the tank down, to richen it slightly on outsides, would be your answer. Generally we use lap speeds (upright, inverted) to get a quick check, but then check the loops in the horizontal eight to get the final fix. If the engine goes leaner on one or the other, move the tank just slightly (maybe a business card difference) to even things out.

As to the burping and quitting and restarting at the end, some situations will just do this as the "bubble" breaks on the uniflow tank (just before the end when it is back on standard pressure). It should only last a lap or two. The richer you run the engine probably the more prone it is to doing this. You may try running the engine just a "tad" leaner to see how this works.

You might also try doing a loop (possibly followed by a second real tight loop) down wind just as it starts to do this to get a positive shut off and in the place where you want it. I hesitate suggesting this technique, because if you are not familiar with it, or if the plane is not running fast enough and you shut off at the wrong place in the loop it could lead to disaster. So, since this isn't hurting anything (other than your professional-looking pride) it may be best to just live with this.

>Quirk #1
> I'm having a bit
>of trouble with this motor
>on outside loops.
>It speeds up quite a bit
>on only this maneuver.
>It doesn't run faster inverted
>or overhead, just on the
>outsides, especially the round loops.

Classic schneurle behavior. Acceleration causes the gas flow in the cylinder to be altered, and causes the engine to run faster or slower depending on the direction. It's usually the way you describe for engines mounted with an inverted cylinder. Other mounting angles can make it do the opposite, or nothing. The eights are usually the problem. In severe cases, the second time through the inside, it runs slower than the in level flight. It seems to be much more severe in dense air.

I've returned several reworked motors that ran this way. Shimming the tank to remove this effect creates others, and is not a real fix except in a pinch.

>
>#2 I'm using
>a uniflow on muffler pressure.
> The uniflow pipe is
>about 1/4" from the fuel
>line and right at the
>"V" of the tank.
>It does not stop cleanly.
> It runs steadily until
>almost all of the fuel
>is gone, speeds up nicely
>to provide a warning, but
>takes 2 or 3 tries
>before it finally quits.
>It'll look like it has
>quit but then will restart.
> Is there a quick
>fix for this problem??

I would guess this has nothing to do with uniflow/suction. The fix is easy - tilt the rear end of the tank out about 1/4" so the fuel is forced to the back. I build the tank with this taper built-in, but that's just for convenience.

Trim effects this - if this started after a trim change, like after adding some rudder offset, moving the leadouts aft, or the CG forward, the airplane is flying more yawed out, and making the fuel run forward.

This is also why inside cutoff loops are more effective than outside, and why if you have enough gas to make it through the first leaf of the clover, you can get through the whole thing.

The three thinkers above get it right 99% of the time. Let's see. If we combined their advice we would move the aft end of the tank outboard, move the aft end of the tank up a little relative to the front, and maybe move the whole tank up a milli-smidgeon.

Now that I think about it, sounds like a smart starting setup for profiles! Logic says that raising the aft end of the tank would have a different effect than raising the whole tank in outside maneuvers vs the difference in level inverted flight speed. The additional g forces are what makes the difference between an outside maneuver as compared to flat inverted.

Would the above experts agree or disagree? I am trying to interpret, not propose an answer.

I think that you missed ,"tip the tank" jus a bit to leave the fuel head even on inverted and greater on outside. That woulb be tip the outer edge down .050" leave the mount where it is. This is a good way to see how little makes a diff on some engines and no diff on others. Maybe change the muffler pressure position. ?? Has anyone seen that make a difference??.Maybe the oil is getting in the muff hole on outsides.
MAYBE NOT . .02 MM

Brett, if you move the rear of the tank in, it should quit on the flat while running longer uphill?? MM

I agree with Brett on moving the back end of the tank outboard. Brett does a beautiful job on the tanks he builds where the front end is purposely built narrower than the back. That idea does make the tank empty all the way before the bubble breaks. It can help some situations and will never hurt. I wish there were commercial tanks built in that style (and I have even proposed the idea to John Brodak). For some reason Brett doesn't want to make 10,000 of those tanks a year and sell them at prices that we (cheap) controline flyers would pay.

As to tank height, the only way to find out is to try it. We have made some minor changes that way that solved the difference in the loops of the eight with no apparent change in other areas of the pattern. As I said, it should be just a minor "tweak" that would be needed. I would assume that the opposite could be true as well. In some set ups it may be that it would solve the problem there and cause problems elsewhere.

Try it and see. Then report back. Then we can all benefit.

>Would the above experts agree or
>disagree? I am trying to
>interpret, not propose an answer.

Well, everybody is just guessing, to one degree or other.

My suggestion to fix the cutoff was to yaw the tank. I don't think anybody attributed any value to pitching the tank up or down. For a uniflow tank, the reference is the internal end of the uniflow tube, so tilting it in pitch is no different than moving it up or down and equivalent amount. Rolling it alters the cutoff, and I imagine that putting in a slight roll would have the same effect as having a slightly assymetrical wedge. A positive roll would tend to make it less likely to cut off in insides, for example.

For uniflow tanks, just treat the tank as a point source at end of the vent.

These are absolutely straightforward effects. There is nothing tricky about it. Tanks, despite the lengthy discussions we have about them, are among the most prosaic items of anything in stunt. The tank, other than the simple effects listed above, are almost never a source of problems. As long as they don't leak, they're OK.