A friend of mine has an LA46 that runs beautifully after switching from a RoJett .40 muffler to an old OS 762(?) muffler.  Like a lot of guys, he doesn't like the stock E-3030.  With the old set-up he had a lot of pre-ignition.

The only problem with the run now it that it cuts out and then restarts in the vertical leg between loops 2 and 3 of the clover (obviously not a great place for an engine to cut out).  After the clover he gets ~18 laps before the engine quits, so the tank is not near empty in the clover.  The tank is a standard 1 x 2" metal wedge with open uniflow vent terminating about 1/4" in front of the pickup.  The tank is mounted with the rear end about 1/4" outboard of the front to eliminate the stop/start/stop/start oscillation he was getting previously in the last couple laps.  I vaguely recall hearing about this problem some time in the past, but don't recall the solution.  Anyone know what it is?  Thanks in advance.   

Jim,
My best guess is that you may be on the right track.  Possibly the roller coaster momentum built up in loop 2 slings the fuel upward in the climb so that it decelerates more slowly than the plane and rises above the pickup just long enough to give the pilot coronary arrythmia.

It's obviously not a situation he will want to live with. 

Jim,
My best guess is that you may be on the right track.  Possibly the roller coaster momentum built up in loop 2 slings the fuel upward in the climb so that it decelerates more slowly than the plane and rises above the pickup just long enough to give the pilot coronary arrythmia.

It's obviously not a situation he will want to live with. 

   Make the peak of the wedge a more acute angle, and tilt the back end of the tank a little (or build it with a wedge).  Even a little bit of change makes a big difference. The standard Veco T-21 style is just a little to shallow and tends to do exactly that - the acceleration draws the fuel towards the bottom of the tank, AND, it yaws nose-out which tends to move it further towards the front.

    Brett

What I don't get with the above explanation is that the tank has 18 laps left in it after the last  maneuver ........... 18!

So if it is a standard tank it must be truly massive for all of that to be true, and if the fuel is being thrown around enough then perhaps a smaller tank is in order?

What I don't get with the above explanation is that the tank has 18 laps left in it after the last  maneuver ........... 18!

So if it is a standard tank it must be truly massive for all of that to be true, and if the fuel is being thrown around enough then perhaps a smaller tank is in order?

   The issue is that the fuel us getting pulled down to the bottom and front of the tank as a result of the acceleration of maneuvering. It doesn't go to the equilibrium fuel surface because it's the acceleration changes too quickly but it goes in the expected direction. Tilting or tapering the tank makes it hang around the rear of the tank longer. Increasing the wedge angle makes it hang around the peak of the wedge longer.

    I have experimented with this pretty carefully. I built some tanks for the PA61 that just had dimensions that came out even, including the wedge angle. They had the same issue described here. Not quite quitting but going lean in both inside loops and then running a fair number of laps after. Reducing the fuel load to eliminate the excess laps led to it quitting. I made two more tanks with the wedge sharper by one click of "grid" on my CAD program, and then I could run much closer to the end without any anomalies. One more click of "grid" on the next tank, with a more acute angle, could run to within a few laps. My current tanks can be run with as little as 2-3 laps after the pattern and they do not change AT ALL in the overhead or 4-leaf.

    This is all actually very critical in FAI  - you don't want the engine running funny in the 4-leaf but you can't afford to run excess fuel just to mitigate it because of the possibility of running over the 7-minute time limit, particularly at low lap speeds.

    Note that in some cases you have have worse issues in the overhead than the 4-leaf since the outward acceleration is lower.

     I would also point out that the trim and overall fuel load of the airplane makes a big difference. Nose-out yaw trim makes the fuel want to run to the front of the tank more than it would otherwise. Lots of fuel means lots of CG shift, particularly on light airplanes, and that makes the yaw angle more nose-in as the flight goes on. There are a lot of variations on this theme but that's why a good tilt or a lot of wedge obviates having to consider the fuel load based on the trim.

     Brett
   

Well, I learned something here today. My Magician/McCoy 40 Red Head has this same problem. If I cut it just a little close on fuel it will quit in the first or second loop of the clover. It will restart after I have leveled off, and run several laps afterwords. I will make a new tank with more wedge at the rear and see if that helps. It does not happen very often. Usually if I fill the tank and it sets for a while before I fly, It pukes a little fuel out the venture and looses just enough fuel to make it quit in the clover. Thanks for the info Brett.

    Try a 1/8-1/4" of shim underneath the back end of the tank to tilt the back end outboard.

    BTW, the problem is in the first loop of the clover. It might quit in the second but that's only because of the empty space in the line from the first loop. Most of the time - NOT EVERY TIME - if it gets through the first loop of the clover, it will make it through the other 3. The combination of the altitude of the first loop reducing the outboard acceleration and the nose-out motion of the airplane in the loop make it much more prone to flow away from the pickup during the first loop. It does something similar in the first part of the 4th loop but you end up with the fuel flowing to the back of the tank rapidly as you complete it and more outboard acceleration, so it USUALLY - NOT EVERY TIME - keep going even with slightly less fuel.

     Brett

Guys,

Its me Kimerski is talking about. This is on my P-40 Shark prifile ship. I think the pre ignition was actually the rubber extention I had on the RO tube muffler. Don't know. I have three head shims in to stop the preignition but I don't think that is it. I'm also using an OS A-3 which is kind of short IMO or so it seems to me. I now have my tank mounted with 1/4" of tilt and that solved the uneven run at the end of the run. I used a 12.25X3.75 APC and it flew ok, but that prop makes it nose heavy. It weighs 2-15 (47 oz.) I'm going to try a bolly 12+ X4 CF prop and see if that works OK. I also want to try a Vess 12X4.........But that belch has to go away first!
I told Kim the perimeters so I'll repeat the here; 6mm venturi (LA 25), std. OS NVA, three head shims (Yes, I know-I'll take 1-2 out) and an OS 762 muffler. That is the one that came with the FP 35 as I understand it. It has a very small outlet and gives very good gas mileage. Also cuts the noise level a BUNCH!!!! And 10-22, 50/50 fuel.

Jerry

Randy, tank to engine is 5/8", profile as Jerry mentioned.  Jerry, per Chris' last post, was the belch happening before the muffler change (also, was it happening before I suggested the 1/4" tank tilt?  I think you made both those changes around the same time(?)  Brett, your info about the angle of the wedge on T-21 style tanks is totally new to me, and very helpful.  Excellent advice as always.  Much appreciated!

Chicken hopper tanks are made for that specific purpose. On overhead sharp turns in slow combat, if one does not use a "clunker" tank, a chicken hopper tank needs to be used. Some uniflow stunt tanks were made with an internal baffle inside, so the fuel would not "fall" from the pick up line if the centripetal force was not enough to keep the fuel at the wedge or from flowing forward.

 

I think it was the problem and see you friday. 

Culprit found! It was indeed the back plate screw! With the exception of me hitting the ground  on Sunday, the engine ran just as it always had, great!

Oh, me hitting the ground? Well, I needed about 4" more of space. Uh.... broke my $30-40 bolly 2 blade CF prop but that was the only damage 'cept to my pride  (and a bent NV which I straightened)

Jerry