Hi guys, let me try and answer you without naming any manufacturer unnecessarily:
Originally Posted by
jason
can we have a bit more info on the model? Also what engine? what bypass? What gap is around the engine and what is the distance between the engine and tail pipe? what configuration is the tailpipe? (twin walled? does the inner slide on the bypass or does the outer wall slide on?) Assuming it's a twin walled pipe how much does the outer pipe extend beyond the inner at the exit?
Jason
Jason,
The gap around the engine is 12.5mm, the distance between the engine and the bell mouth/tailpipe is 20mm. I have a full bell mouth fitted onto the back of the bypass. The pipe is double walled with the inner pipe pulled over the end of the bell mouth. The outer pipe extend 15mm passed the inner pipe at exit and the outer pipe is inside the fuselage by 10mm.
Originally Posted by
Craig B.
Any chance you have a restricted inlet? Sounds like engine might be starved for air on ground and is pulling it from back of engine around the case. Too small a tailpipe can aggravate this too or engine position not correct wrt pipe.. Is the engine a 180rx?
Craig,
You may be onto something. However the tailpipe and bypass is of P-200 spec (it is not a P-200) and as I've indicated earlier I have two more setups exactly like this. Therefore I believe there can only be three issues: first, like you say, a problem with the intake duct (which is not collapsing, but may be causing turbulence before the air get to the engine intake); second, due to the shape of the fuselage exit (the exit turns back into the fuse) the air may get reverse trusted into the fuse, but I'm not sure if that also causes a back pressure into the pipe itself; finally, I may have the back of the engine a little bit too far away from the bell mouth and may have to push it in a touch more.
Originally Posted by
Kevin_W
Could the inlet duct be collapsing when the turbine is at high power, then popping back into shape once the vacuum subsides?
Kevin,
The intake is not collapsing, but I am going to do some tests without the top cover of the bypass. My thinking, as per you and Craig, is that the most likely problem being a lack of air on the engine intake side (for whatever reason). Should the setup work fine without the top cover, then I will open up the bypass at the front with some narcs ducts cut into it and try to get more air into the front of the engine. If the tests without the top cover is unsuccessful I will then make a modification to the rear exit of the fuselage to see if the 'lip' on the end may be causing some sort of 'return' of air into the pipe.
Guys,
I hasten to add, that the model and engine are both well respected brands and widely used. The model inlet ducting design is good and not radical at all. I did cut two narcs ducts into to inlet ducting to allow air into the fuselage to make sure that air will get around the outside of the bypass and thrust pipe. I do this on all my bypass installs otherwise you don't get any air around it when the gear doors are closed.
Thank you for your input.
Jan