Ed,

    What you are describing as a "standing wave" appears to be a separate and different matter than "fuel spitback" as described below.

My understanding of spitback as described below is both intake and exhaust ports being open at once and communicating, allowing high pressure exhaust to outflow through the intake slightly before being sucked in by the intake stroke, and this "intake outflow" causing the spray of fuel.
 
The spitback phenom, as described in recips with valves, seems improbable in rotaries as long as there is not overlap to the porting that allows the same chamber to be open to exhaust AND intake. Likewise, the spitback prob appears to be a carb issue, not likely to be reproduced with fuel injection, if the injection of fuel is timed to occur optimally during the intake portion of the combustion cycle.
 
As for a standing wave phenom, that would be interesting if that comes into play here... I am all ears with regards to finding out what is causing Dave L's probs. I hope he can resolve it to his wife's satisfaction such that she will fly with him again in the RV.
 
Thanks for passing on the info, regardless of how immediately pertinent it is... makes me think and figure things out.

Dave Staten

Ed Anderson wrote:

Hi Dave,

 

    Here is something to consider.  With certain lengths of intake combined with certain engine RPMs the  pulses generated by the rotors in the intake can actually cause a condition referred to as "Standoff".  This is where the pulses actually cause a "Standing wave" of fuel particles ejected by the pulses  at the entrance to the air intake. 

 

There is some information on the web about "Fuel Standoff" or "Fuel spitback".  Here are a few examples.

 

 We noticed a phenomena, after a race the engine cover was wet with fuel, the phenomena is called fuel standoff or spitback, during a lecture in the States, Allen Lockheed(son of the founder of Lockheed aerospace) alluded to the phenomena saying that this is caused because the exhaust is out of tune and the pulses were pushing the fuel out of the intake.

 

Posted a question a while back about my 1986 175 Merc spitting out gas from thr carbs. I'm in the process of reviving this old boat so I've not fooled with it much since i posted last (been working on other stuff). Some suggestions were bad reed valves, gummed up reed valves, stuck floats or cranks seals. Well this evening I had her running with the carb air box cover off. It's not spitting gas, its actually blowing the gas out the throat of the carb. The middle and bottom carbs are the worse. One side of the top carb is doing it a little. It is blowing so much out, the gas pools in the throat and runs down the air box. I also noticed once while cranking it over the middle and bottom carbs puff out white smoke a time two like it was exhausting thru carb.

 

Subject: velocity stacks and stand-off

While we were dynoing this 3/4 race engine (280 degree duration cam) installing the velocity stacks in every case made the fuel "stand-off" disappear. Without the velocity stacks there was a cloud of fuel in front of the carb inlets that you could feel the wetness on your hand up to over 1 ft away. The fuel seemed to be in constant motion into and out of the carb throat. the fuel did not seem to "blow away" into the dyno room. When the velocity stacks were installed, the cloud was no longer there, and the engine made slightly more power from the point where it came on the cam, ie. about 3200 rpm up to around 4500 rpm. Above that rpm the power was always less with the stacks than without them.

There is however a mechanical downside to the Weber 4BIDA, and that is that they are somewhat hard to tune. The carburetor's fidgety nature has been known to produce a potentially-fiery-phenomenon known as "Fuel standoff," which can transform a race car into a smoldering pile of ash in short order.

 

 

.....is correct on both valves being open at the same time (even if just a little), the longer duration ,usually the more overlap. This is actually used to create a slight intake "siphon". The longer the overlap and the more combustion gas will enter the intake tract. In engines that have short intake tracts with multiple carbs (like Webers) you will actually get a fuel "standoff" above (or beside if they mount crossways)the carbs.

 

 

 

 

So, Don't know if that could be part of your problem or not, but thought you might want to know.  It would probably only happen at certain rpms. 

 

Ed Anderson

  

 

----- Original Message -----

From: DaveLeonard

To: Rotary motors in aircraft

Sent: Thursday, October 28, 2004 12:40 AM

Subject: [FlyRotary] There I was, half way to Vegas...

So as you know I have converted to a normally aspirated state.  I went flying to vegas this weekend, and actually won $300 net even after my wife lost $100.

 

The engine ran as well as coulld be expected considering my sea level WOT MAP is only 28".  Max n.a. static is 46-4700 RPM.  Plenty enough to take off well over Van's recomended gross and fly at 130 KTAS (using only 8 gal/hr).  Better than a typical cessna - but less tan I had with the turbo.  Anyway,

 

The point of this e-mail is to discuss injector backflow...  I have always had a problem with a faint fuel smell in the cocktpit.  As part of the n.a. conversion I removed the plenum over the intake (see pic.).  Every flight since removing that plenum has included a rather strong smell of fuel coming in theough the passenger air vent.  This smell is only evident after take off.  It turns out that my TWM secondaries, mounted on a downhill part of the intakes, freely leak fuel down and out of the bellmouths even during WOT operations.  This leaked fuel then seeps out the cracks in my cowl and goes into the passenger air vent.  It also loostly covers everything on that side of the engine, and the side of the fues around the pax air intake with 2-stroke oil.  This was enouth to make me uncomfortable.  The wife, who is still reeling from the emergency landing after the turbo broke, now states that she will only fly cessnas from now on - due to the very disconcerting smell of fuel the whole trip - and I can't blamer her.

 

While in vegas I tried to program the 'B' computer to use the secondarys only above 27" MAP but I guess the adjustment did not take because we still had the same condition on return.  I have checked and re-checked the fuel system far any evidence of a leak.  It is coming out the back end of the intake for sure.

 

Something to think about if you are installing the TWM injectors near the air inlet end.

 

Dave Leonard