Puzzling engine behaviour

Today I finally got the maiden flight in with my Henri Stooff's Blue Pants but interestingly it has an identical quirk to my pal's Blue Pants. When I fly a half loop from upright to inverted or inverted to upright, the engine almost cuts then picks up again. It's disconcerting to say the least until you get used to the fact that the engine will not cut out.

Mt pal's plane has a rectangular tank as per plan and is powered by an AM 0.15 diesel with an 8 x 6 prop.

My plane has a PAW 0.15 TBR diesel with an 8 x 6 prop and a wedge stunt tank with the pick up at the same level as the needle valve.

I thought that my plane might not suffer from the same problem as my pal's because if anything mine is much lighter, got a more powerful engine and has a wedge tank. Clearly I thought wrongly

Any ideas folks?

Not being familiar with the plane, is the engine upright, inverted, or side mounted, and how lean are you setting the engine? Any engine will lean out in a tight maneuver, and, if set too lean from the get go, it could "burp" as you suggest.

Hi Leonard,

The engines are mounted upright and are certainly not set to run on the lean side if anything they are likely to be just a tad on the rich side. Diesels are prone the eject their innards slowly through the exhaust if run too lean or over compressed.

The planes both run OK inverted and the right way up. The half loops were certainly open enough as the flights were proving flights and I didn't want to push things too far as far as manouevres go.

You can see what a Blue Pants looks like (not mine) at:

http://freespace.virgin.net/j.hatch/bp.htm

cheers

If it were outsides only, it would sound like a "throwing too much fuel to the head" problem. But, since it happens both ways, it sounds like a fuel draw problem. How far behind the engine is the tank and what size venturi are you using (and what size needle? I need the choke area.)

It has been said that diesels do not suck fuel well. This is not true of ST G20-15's, ED Racers, or ED Hunters. Both the AM and the PAW have fairly restricted intakes, so I don't think that is the problem.

It is also said that diesels run best side mounted. Tigres run fine anyway you mount them. The original Blue Pants had an upright ED Racer and was World Champion in 1954, so I guess Racers run OK upright. My Hunter runs fine upright, as well.

Looking at reviews of similar engines (early model AM 25, and plain-bearing PAW), I get the impression that both AM 25 and PAW might run better on an 8 x 4 than on an 8 x 6.

This sounds like a problem a uniflow tank would help solve. It may also be a symptom of running a little undercompressed compensated for by a little richer needle. If the fuel flow is reduced in the loop, the engine might become momentarily more undercompressed and thus burp.

Hi, Vernon....This is Chuck Potts, down in Mobile, AL. My experience with the PAW's tells me that you might check several things: First, check that backplate to see if it is loose or coming unscrewed. Next, check to see if your venturi is ALL the way into the case, and properly tightened; also, check to see that the O-ring between the venturi and case is not stretched or loose; next, take a look down the venturi, and see if the holes are visible in the spraybar. If they are, loosen the needle valve body and turn it until no hole is visible. Another trick that might come in handy in some applications, is to go back to silicon tubing for fuel line, and use a wire twist-clamp to make it secure. By the way, I use a Master Airscrew 9-4 on my .15Br version, and it gets over 11,000 on the ground, so your smaller prop must really be turning up!
Let us know what you find that works out your problem.

Chuck, in Alabama.

Hi Chuck,

The backplate is OK. The venturi is an intergral part of the casting so there's no chance of it not seating proprtly and the spray bar is orientated correctly. I was going to try an 8 x 4 and a 9 x 4 today but there was a lot of turbulence and I didn't want to risk losing the plane during one of its high altitude 'burps' My CS Oliver Tiger replica powered Warlord combat plane was blown around a lot but interestingly showed the same 'burping problem' when turned very sharply. A clue to the behaviour was finding that the engine was running over compressed a tad. Reducing the compression reduced the symptoms considerable.

I have not got around to measuring the bore of the venturi and the spray bar diameter to get an input from Leonard as I can not find my vernier calipers. A week's holiday from home cleared my mind of the location of my treasured mesuring device.

Hever heres a picture of my Bluepants minus the engine cowl.

[photo not recovered: bluepants.jpg]

I've since replaced the Tornado 8 x 6 prop with a rigid nylon 8 x 4

I flew the Blue Pants at the weekend with an 8 x 4 prop and the burping all but disappeared. Certainly it is no longer a heart stopping event. I also noticed that the elevator movement is perhaps excessive at 45 degrees up and 35 degrees down. Can't acount for the asemmetric movement. How do I limit elvator movement without major surgery? Sadly the flying session was brought to an end by a broken prop when the plane nosed over on landing.

Vernon,
You do not have to alter the plane at all. Space the lines closer at the handle. It is better if you do not hit the limits of travel anyway. When you hit the limits, all of the line tension is on the line being pulled instead of being distributed between both lines.
That's one of the reasons why adjustable handles are used.
Good luck,
George