Tailpipe fire ???
 

Hi Guys
Looking for some input on what might be going on.I attended a Jet fly in Montana this weekend,with my Trim F-86,powerby a Ram 750p.The airport elevation is 4800 feet and the temp was around 100 degree,not ideal for flying !My engine was making a howling-moaning noise as it came off idle,and happened once on final when i added powered just before touching down.The next flight,i added power to start my take off roll,the vibrating-moaning started again,i added more throttle and the next thing i saw was a 4' flame shooting out the tailpipe.After put the fire out,ipulled the bypass cover and test ran,sounded ok.Just wonder if the elevation and heat could cause this or should i look deeper ?

Thanks for the help!
Charlie

Moaning and groaning
 

:D :confused: :o

Hi Charlie,

It is not my intention to insult your intelligence but the density altitude in your given scenario, was probably in the vicinity of 9000 feet. Looking in the back of your RAM OPS manual is a chart or paragraph explaining the factor per thousand feet above sea level to be used in reducing the output of the turbine to prevent overspeeding the turbine. Most of the Big Boys attribute your experience to compressor stalls. Hope this helps.

Skeet

Tailpipe fire ???
 

Charlie, I have video of your Rafale at Princeton doing the same thing on take off throttle ups:)

Tailpipe fire ???
 

The sounds you described sound very much like a compressor stall. I have heard several other engines (other manufacturer's engines) make similar noises when installed in a F-86, although I have never seen it result in a full tailpipe fire. My guess is that the high density altitude you were flying at probably made an existing problem even worse.
If your ECU allows, it would be wise for you to slow down the spool-up time of your engine. If you cannot adjust this with the ECU I would suggest that you manually control the spool up time of you engine by only advancing the throttle VERY slowly, especially when you are flying in hot and/or high conditions.

Also check your inlet duct, it might be partially collapsing and then springing back to normal shape once the pressure has equallized.

Tailpipe fire ???
 

Might it also be that the altitude effects caused the mass flow to be too low at the normal idle RPM? This might have the effect of the engine just barely being able to sustain an idle at normal idle RPM due to the thin air, and not be capable (or barely capable) of ramping to a higher throttle setting. From the engine's point of view, it would be the same as asking the engine to try and idle at say 5k RPM less than normal at sea level. The fix for this would be an increased idle RPM while flying at increased density altitude.

Tailpipe fire ???
 

hope your sabre is OK

Tailpipe fire ???
 

Hey Charlie,
It was good to get together with you. I don't know why but 5 years of retirement seems to have erased all knowledge of 30 years of flying jets. Why I didn't see it then I don't know but
Your problem was compressor stall. We were taught that at high density altitude airports to bring the engines up slowly for take off or we would get a compressor stall. On some engines it would mean a taxi back to the ramp for an inspection on others we just monitor the engines for problems and if all ok we could continue. The blowback on the one start you had where we added blower air in the front to get it running was compressor stall. The great F-4 rumbling sound was also compressor stall as well as the flame out the back. These small turbines act a little different because they are not as complex as the big ones but the problem is the same. Wait for a cool day in Spokane and try it again. I'll bet it works fine. It's a great looking , flying F-86.
My problem with the Ram 500 was solved by swaping out the start moter. It seems the bearings or brushes were worn enough that it was dragging down the voltage to the point the glow plug would not ignite the Powermax. Anyway it starts right off now. Lets go flying!!!!!
Galen

Tailpipe fire ???
 

Its not my goal to disagree with experienced flyers, but

Its my opinion, that this is not compressor stall but rather
the combustor running very rich. I have seen and heard
many compressor stalls and they most often occur at high
power settings during rapid acceleration.

In this case the ECU did not know the air was thin and
during acceleration from idle
it pumped in the standard amount of fuel for sea level.
This was way more fuel than the engine could burn in
the can so fuel was burning in the tail pipe.
Fuel burning in the tail pipe can make all kind of
rumblings and vibrations. Then when more power/fuel was
applied the combustion can cooled off and then most of the
fuel was burning in the tail pipe with a great loss in Rpms..

Rocketwuss had a 1000 that during rapid acceleration
only at near full power, it made a big FART sound for less than a sec.

I modified an AMT 280 to accelerate faster and it would go from
%30 power to %100 faster than a AMT 180... Right at the top
there was a loud squeal along with some FARTing for ~200 ms.
Han (the engine designer) was impressed and said that was a minor compressor stall.

The only way to get a compressor stall is to back pressure the
compressor. Its my opinion this does not happen near idle..

Either way the fix is the same. Come back to sea level.

Eddie Weeks

Tailpipe fire ???
 

Hi Eddie,

You are RIGHT ON THE MONEY.
Exactly correct, at least with the AMT.

Have seen and heard this many, many times when testing Pegasus motors - especially the older Mk.3 versions .... but the new HP does't exhibit quite the same tendencies, due to improved flows in the NGV.

Later,

Mike C

Tailpipe fire ???
 

Eddie - I bow to your experience with the minature turbines. There is none greater. However a compressor stall can cause fires, farts and fodder or what ever you want to call it. Nonetheless it's still an improper mixture of fuel and air at whatever setting is being called for.
Regards,
Galen

Too Rich
 

I think Eddie has it right.
For the compressor to stall the flow actually has to reverse direction inside the engine. The highest pressure is right after the air enters the diffuser, which likely is not happening. Ram ECU has no way of knowing the density altitude therefore it will pump fuel at a calibrated flow rate for either RPM on the 1000s or total pressure on the old 750s. At that density altitude the fuel mixture would have been very rich and likely the fuel was not burning completely inside the combustion chamber and igniting after it exited the engine.
Be careful you don't overspeed the engine if it is not RPM governed.
On the 777 which has Trent 892's at 90,000 lbs of thrust at TO sea level the fuel flow will be approx 28,600 lbs / hr. per engine as we roll down the runway . The fuel flow is dropping constantly as the density changes by 9000 feet it will be around 18,000 to 20,000 lbs/ hr. depending on the climb power setting.
The RAM if it has a no **** fuel control unit would automatically sense the pressure, temp, and density and adjust fuel flow (reduce)automatically.
The TF-30 which were in the F-14As were notorious for compressor stalls. They were not optimized for the pressure changes at the face of the compressor due to the long inlets and the blanking out of airflow. The pressure changes caused the flow to reverse directions. The easiest way to clear was pull power to idle and unload the aircraft. Egt would increase always. Too lean a fuel mixture and not enough cooling air to bypass the combustion chambers.

Bags
777 driver , ex Tomcat driver

Tailpipe fire ???
 

That's what I tried to say. In my example, when you try to ramp up the engine from the "too low an idle" situation, it hangs up and burns fuel external to the rear of the turbine wheel. That does nothing to add torque to the shaft.

Eddie, my flatulating RAM was my original 750F, not a 1000. The 750F would poot on rapid throttle up, but was damn reliable.