Bypass problem.
 

I always use a bypass with all my installations (currently I have three, exactly the same) but I am having a problem that I have never come across before.

Here is what happens; the start goes perfect but when I go to full power after start I end up with fire inside the bypass. When it happens, it sounds like something is getting sucked into the engine, but there is nothing. Afterwards, the engine is perfect, only everything is black and scorched from the residue left after the fire on the inside of the bypass and on the engine. Everytime I have to use my Co2 FireX to put out the fire.

Today I had a perfect start and flight with the specific jet, but the above happened when I wanted to have another flight. Previously the same thing had happened on the same jet - then I changed the bypass because it burned badly, I changed the engine and it's ECU.....but obviously I have not found the problem.:mad:

Could it be that some sort of back pressure forms and forces all the trust back into the front of the bypass?

Your thoughts will be appreciate.

Jan

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

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?

Could the inlet duct be collapsing when the turbine is at high power, then popping back into shape once the vacuum subsides?

Hi guys, let me try and answer you without naming any manufacturer unnecessarily:

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.

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.

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

Hello Jan,

Is it your Hawk?. Is it doing the same thing as what happened in Toowoomba?.
When it was "Popping" it sounded more like gas igniting rather than change of engine noise from air restriction.
You might try removing the gas lines and valve and do a air pressure test including any festo fittings.
There may be a small leak there.
Just an thought from what I saw.

Bruce

Bruce,

There was no gas connected.

Cheers,

Jan

More than likely it has nothing to do with the bypass at all. There is something going on with the turbine, the pump, the RPM sensor and the ecu. My advice would be to take the turbine out of the jet and put it on a bench and see if you can get it to do the same thing.

I believe you are getting a situation where the ECU thinks the turbine is spinning at the high RPM of full power when it's not. As a result you get too much fuel flow in the turbine for the actual RPM's present and you are flooding the turbine causing the fire. The black soot is kero that isn't burning all the way.

So I would check your RPM sensor. It may be giving a bad reading to the ECU. Also, I have seen stuck starters slow down the turbine while the pump thinks it needs more fuel. Same thing will happen.

The sucking sound is probably kero catching on fire inside the turbine.

Just my guess.

Andy

Andy,

Good advice, thank you. Only one point; after it happened the first time, I did change out the turbine, ECU and temperature probe (as the lead was badly burnt). I did not change the pump or the solenoid.

I agree with your black soot statement. The sound as you say, may very well be the kero catching fire - when the fire start it's like a 'rich' IC engine trying to burn off an over supply of fuel. After the last time I had the feeling that with flames also coming out the tail pipe at the back of the jet, that it was almost like an afterburner effect - in other words, burning fuel behind the turbine. (Once had the same situation with diesel pumped at idle with as make system).

I will run some tests and see if I can reproduce the situation under a prepared (controlled) situation, but I guess these will all be without the bypass. So if I can't get it to do it like that, I'll be back to the bypass theory. Should this happen I will then take one of my other turbines (I have 5 of the same engines) and install it completely into this model - and try again.

Cheers,

Jan

Yes Jan
Move the engine back. The engine exhaust/tailpipe/bellmouth is a venturi airpump. If it works too well the mass flow through the bypass will cause a pressure drop ahead of the intake. Compressor stalls, airflow reduces and excess fuel burns in tailpipe. The flow reverses as power drops and you have what you have.
Curiously most bypass systems are designed wrong. NASA research in early days of turbines showed that a "ejector tube" (bypass) can produce up to 40% more thrust.
But the bypass and inlet volume must be adequate for the increased airflow. Volume immediately ahead of the intake must be as large as possible.
Andre

Andre,

Trust you are well.

If what you say is true, then it will explain why it is doing this with two different engines, ECU's and temp probes. BUT, why is it then not doing this every time?

Cheers,

Jan

Jan,
Turbines and their associated dynamics sometimes seem unpredictable. More than likely differences in temperature, spool-up times etc could trigger this.
I have seen something similar in the bypass system of the L-39 from Germany.
I am currently very involved in a CSIR/ Stellenbosch University effort to breed new young Msc and PHd aeromotive engineers.
We are developing new microturbine tech as a learning and research tool.
More than likely they are going to end up in the USA or Europe.
Too late for me to escape.
Cheers
Andre

Andre, all that being said, how can you then be sure it is only a case of moving the turbine back?

Jan,
Its seems the most logical thing to try under the circumstances. You have pretty much eliminated the engine as the culprit. Too close and you may see an EGT rise/thrust loss.
I would move the engine 10% of the current gap at a time. It may very well be something else since I do not know the rest of your system.
But keep in mind the basic effects as I explained them. You may hit the sweet spot first time. If the inlet ducting is too small in cross section area it may remain an issue.
At the other extreme if the engine is so far forward so as to limit bypass air it may also improve.
Hope it helps.
Andre

Andre,

The inlet is about 4.5 times the size of the turbine inlet, what would you consider enough intake area?

I will actually measure my other two installations to compare the exact turbine outlet to bell mouth distance. All the installations use the same turbine and bypass/ bell mouth ducting. So the inlet and outlet side of the bypass is also a constant.

Thanks for your help.

Jan

Jan,
For the turbine intake that is OK no doubt but the bypass air may be of such a high flow that it may cause a large pressure drop ahead of the turbine.
It may sound daft but a sensitive vacuum gage ahead of the intake may tell the story.
It is not far fetched to assume the bypass air being more than that for the turbine itself.
The engine then sees itself operating at the equivalent of a very high altitude.
Andre

Andre,

So my earlier assumption of having to make some narcs openings at the front of the bypass duct might also be a way to resolve my issue. Your thoughts please?

Jan

Jan, the way you described the outlet at rear of fuselage also sounds like it will do a good job of drawing air out of the fuselage volume, good for keeping things cool.
If your issues is on the ground only the Naca ducts will help to let more air into the ducts.
Depending on how well the fuselage is sealed off in flight its difficult to say which way the air is going to go.
The issues with the AW L39 went away when not using the top cover, at least on the ground.
In this case it was compressor stall on spoolup and down.
Andre

This can be proven by taking the turbine out and running it on a bench.

Andy has my vote also. ( Hate admitting that) If it runs outside then it is in airflow set up. If it does not, which I'll bet it doesn't, then its in the engine. Make sure you don't do your test run in bright sunlight. It may mask some flame you won't see. The most likely way you get flames out the back is too much fuel in the combustion chamber. Un-burnt and wet fuel into exhaust. RPM sensor, Fuel pump advance settings, would be my best guess.
Interesting and keep us up to date. Good Luck

Mate, I am open to all suggestions - just bear in mind that I have used more than one engine, ECU and temp probe in this situation. I have 3 of exactly the same engines installed with exactly the same bypasses in three different airframes, BUT only in this installation do I have the problem. I might also add that all the ECU's have exactly the same setups. So can it really be the engine on it's own, without the bypass setup inducing the result?

From all the advice I'm getting it seem to me that Andy and Andre, both have very good insight. Both of them are pretty much saying the same thing. Compressor stall - either way the turbine is being over fuelled.

I am just waiting for my fireX to be refilled before I run some tests, will let you know.

Thanks for helping.

Jan

You are saying that more than one engine does the same thing in the Jet? You can also run it without the top of the bypass. This will give it more airflow. I would be ready with a fire extinguisher if you do that though in case it catches fire again.

Sorry Andy, I was editing at the same time as your post. Yes, as per my original post - it has happened with two different engines (both being the same brand and model).

Jan

Can you send a picture of your bypass installation? Show the intakes to.

Jan do this first before changing anything else, exactly what I had with my Hawk.