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"Pop-Stall" Common On Harriers?

Sat Oct 16, 2004 9:47 pm

I was watching the Battle of the X-planes on TV today, and there was a bit when the JSF "flamed-out" just before landing.
The experts say its because of hot gases building up, and flaming out, ive never heard of this happening (or seen it), so does it happen often and like the JSF guy was saying, is it a common occurance with Harrier operations.
Also hows it occur, and does it do any damage to the airframe or engines?
The term he used was "pop-stall" if i heard it right.
Its not the fall that kills u, its the sudden stop at the end..
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RE: "Pop-Stall" Common On Harriers?

Sun Oct 17, 2004 2:47 pm

I'm no expert, but if I remember correctly it is caused by the VTOL vehicle ingesting hot air (exhaust) back into the engine. I can't explain exactly 'why' that is a problem, but I know it was a problem with the early Harriers and Kestrels, and was a problem on the X-32 or 35 during the testing. But ingesting already superheated exhaust back into the engine intake is bad, and that was the problem.
Here Here for Severe Clear!
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RE: "Pop-Stall" Common On Harriers?

Sun Oct 17, 2004 5:12 pm

My understanding of it could best be summed up as the ultimate application of instantinous density altitude.

As you know density altude robs engines of power, the higher the density altiude, the lower the density of the air, the less power is made.

So what you have happen on these vtol aircraft is that the engine is sucking in cold dense air and producing lots of power. All of a sudden it gets a shot of exhaust gas, very hot, not dense because of the heat, and not as much O2 in it because it is exhaust. That causes the engine, which is still set (Fuel flow) aproppriately for the colder air, to stall out, there isn't enough air in the engine to burn all the fuel efficently.
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RE: "Pop-Stall" Common On Harriers?

Sun Oct 17, 2004 5:16 pm

You know thinking about it, I dont' really like that explanation of what is happening.

Ever fly?

Ever use the carb heat?

If you have you should have noticed a drop of RPM as the heat is added. This is because when the heat is added, the air in the carb becomes less dense. Now in this case the inlet is just heated to the amount of oxygen in the air remains the same as when it was off.

Ok now what is happening in the jet.

Think Carb heat on anabolic steroids.

You hare adding hot, less dense air, several times hotter then in a carb heat aircraft, add to the fact that it is exhaust air, that has allready been burned once, less air, and the fact that there will be a lag in the ablity for the fuel system in the jet to catch up, and you have all the makings for a hell of a compressor stall.

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