Look carefully for leaks.

In a bizarre case, you may have had a "magic leak" in your original configuration that you fixed.  We'll hope and pray that wasn't the case.  It's far more likely that you introduced something.

For later, a 7" pitch seems an awful lot for that engine, unless it's been significantly retimed -- once you get things sorted out you may want to see if you can find a combination of venturi and needle setting that works well with a 4" pitch prop.

First flight after change: launch RPM 10,000 on 11x7 APC prop. but when the plane was launched, the RPM immediately dropped to the level barely maintaining the level flight. After boring 6 minutes landed safely.

Second flight after change: launch RPM 10,300 - the same behavior. It looked like the engine in the air started running extremely rich.

   I would suspect that you are not getting adequate fuel suction, and the massive pitch is causing a very large unload after you launch it.

    As everyone else says, you need to get rid of the 7" pitch prop. Get a 12-4 and crank it up enough to get a decent in-flight speed. That will greatly improve the fuel draw, at least. Then see.  You might need as much as 5:, but over that, and you are losing tremendous performance. 

   This high pitch just isn't used in stunt, aside from special cases (4-strokes).

    Brett

Except -- the engine is slowing at launch, not speeding up.

 Oy vey!  It's *going rich* precisely *because* it is unloading. It needs to either be revved up more (good) or the venturi needs to be smaller (with might make it run reliably, but leave it with an uncompetitive setup).

    Brett

     

  " but I thought that engines "run away" because as they unload in the air they effectively go lean -- what am I missing? "

Not correct that they go lean, normally when they unload they go rich , you can even hear this on perectly setup engines, after launch within 1/2 to 1 lap, you can hear the engine unload and go richer

Randy

..so "Pete says so" isn't good enough? (answer not required) 

....Says the guy that doesn't know which direction to mount an engine in a Smoothie. 

Chris wrote:"inherent rich running behavior of an inboard mounted tank".

Why is this "inherent behavior"? The suction point of the fuel line (clunk) is shifted towards the inside of the circle w/r to the spray bar therefore the centrifugal force acting on a fuel particle entering the clunk is smaller then in the case of the tank mounted normally. Now...this shift is about 2" and the radius of the circle in my case is 66' ( 792") therefore the shift constitutes 2/792 = 0.25%. This means that the centrifugal force acting on a fuel particle entering the clunk is 0.25% smaller.

I feel this effect can be safely ignored in comparison to the other effects managing now the flow of fuel.

These other effects include ( as mentioned by Chris): long and bent fuel line, fuel flow reducers and uniflow ram pressure.

Please see the attachment.

The Uniflow Inlet Extension (UIE) positioned the way it is shown fixed the problem of running rich and dropping RPM.

I have made three flights yesterday: the first and third were with UIE, the second - without it. The first and third flight showed very stable RPM that did not change after launch and stayed the same in normal, inverted flight and while climbing or diving. The second flight showed the RPM drop immediately after launch but this drop was almost acceptable.

By writing "almost acceptable" I mean the plane slowed and there was almost zero tension overhead but more experienced pilot could have probably flown the entire pattern with this speed. The normal and inverted RPM in the second flight were the same. More....the engine seemed to be in the 2-4-2 break mode while flying normal level and inverted level and showed some indication of increasing the RPM while climbing and decreasing while diving.

My conclusion: I know the entire fuel delivery system I had to construct is against the rules, is complicated and hard to understand. At the same time, I have found a way to fly this setup in my comfort zone and at my level.

The question regarding the needle redundancy still remains unanswered though.

Thank you all for your thoughts and comments.
Regards,
Matt

 

 

Chris wrote:"inherent rich running behavior of an inboard mounted tank".

Why is this "inherent behavior"? The suction point of the fuel line (clunk) is shifted towards the inside of the circle w/r to the spray bar therefore the centrifugal force acting on a fuel particle entering the clunk is smaller then in the case of the tank mounted normally. Now...this shift is about 2" and the radius of the circle in my case is 66' ( 792") therefore the shift constitutes 2/792 = 0.25%. This means that the centrifugal force acting on a fuel particle entering the clunk is 0.25% smaller.

You are probably correct with the above but you fail to recognise that the tank is up hill in relation to the jet and a fluid will run downhill regardless of how little or how large the G forces involved.

The behavior is inherent due to physics and to view this another way simply tilt the models outboard tip down about 60 degrees to give you what 'down' would be in flight, is the tank now above the jet?


P.S. you reckon that you have tank issues, check out the pic!

Hi Chris,
Rotating the front half of the crankshaft housing 90 degrees is an interesting idea and technically possible for this engine.

I failed to understand how the outboard wing tip can go down 60 degrees in the C/L flight above the flat surface.

Regards,
M

Chris,
Now I agree with your definition of "Down" in flight.

In flight, the fuel free surface in the tank is always perpendicular to the vector defined as the vector sum of the gravity and centrifugal forces.

Because my tank is shifted about 2" to the inside of the circle w/r to the spray bar, the fuel free surface in the tank is about 2" higher along the negative "Down" direction in flight than the hole in the spray bar.

Because negative "Down" is actually "Up", I believe we can use the term "up hill" introduced by you.

In my never ending quest to understand the true nature of things ( my wife says that I produce problems out of nothing only to spend weeks trying to solve them), I have found the acceptable solution. It is my Uniflow Inlet Extension (UIE) that allows the engine and "up the hill" fuel tank to work ok.

I am not saying my solution is perfect for competitive stunt flying because it is not but it is good enough for me to practice with very stable RPM everywhere in the hemisphere, having good lines tension and decent lap time.

The needle redundancy in my current setup still remains a mystery waiting to be explained in a convincing manner.

Best Regards,

M