Compressor stall or fuel starvation?

[Ironhand]

Hi all.

 

I was playing around trying to find novel ways to break my Su-25T. At the bottom of an outside loop (while I was upside down) both engines quit. I had to roll upright and restart them. I don't know about you but I think that is cool as hell. I've never had a military flight sim model this type of thing.

 

Now what I'm wondering is this: Is this a compressor stall or fuel starvation? I'm leaning toward the the latter explanation, since it's only happened to me while inverted. Here's a track if anyone would like to take a peek: Compressor Stall or Fuel Starvation .

 

Rich

[Maulkin]

I performed a high speed, high alt, dive with an Su-25 (in LOMAC 1.02) just to see if I could get the wings ripped off, and I had the engines quit. Is that a compressor stall?

[EvilBivol-1]

Rich, lol, did you catch my comment about this in the Bugs sub-forum->Su-25T AFM thread? I was hinting at this exact situation. I believe it is a fuel starvation problem. I think the plane is fitted with pumps to feed fuel under zero/negative G, but only for a short period of time. So there ya go... :) Gotta love it.

 

Although, I could be wrong. The above is based on a vague memory of a discussion in the Russian forums.

[169th_TweetPuke]

This is a fairly common feature of US military aircraft. I assume it iscommon to most military aircraft but I don't really know. Typically, flight at less than zero G is limited to a minute or less. The fuel pumps in the feed tank for each engine are surrounded by a bell shaped chamber. There is a "donkey dick" fuel pickup that can swing around under negative G to the "top" of the chamber to pick up whatever amount of fuel is trapped there. When the fuel is consumed from the chamber it gets very, very quiet and the pilot gets demoted to test pilot so he can verify the crowbar-like gliding qualities of high wing loaded jets. Engine oil systems (in older jets anyway since I don't know about more current ones) often have similar, or less, negative G capability as well.

 

This isn't really too restricting as there is very little use for negative G flight beyond a short duration "bunt" unless the aircraft is used for display purposes. It can then be modified to meet those requirements. It's all about the real reason airplanes fly: $$$$$$

[britgliderpilot]

I performed a high speed, high alt, dive with an Su-25 (in LOMAC 1.02) just to see if I could get the wings ripped off, and I had the engines quit. Is that a compressor stall?

 

Nope, that was a bug :P

No thrust above Mach 0.8 in the Su25 and A10.

 

 

The air coming onto the blades of a jet engine compressor is effectively coming in at an angle of attack - the angle between the forward velocity vector of the incoming air, and the velocity vector of the compressor blade as it spins.

Compressor stall occurs when the angle between those two velocity vectors becomes excessive, and the flow separates from the compressor blade - just as a wing will stall at too high an angle of attack. Usually happens when you accelerate shaft RPM quickly while at low speed. Technically it CAN happen when decelerating shaft RPM rapidly at high speed, but that usually isn't as big a problem.

 

Modern jets don't have much of a problem with this, but early F14s had big troubles with it - so that'd be slam the throttle forward at low speed, big RPM increase . . . . . and the compressor stalls and buggers up the airflow through the engine. The BIG problem is when it stays stalled . . . . and suddenly the pressure in the combustion chamber is greater than that coming through the front of the engine. See where I'm going? ;)

 

I suppose you could get shock stalls in a compressor at high Mach numbers, especially in a jet designed without variable intakes to modify the airflow.

[SUBS17]

Another good one is flooding the engine on the ground, another realife feature that was reported as a bug.(tried to start engines with throttle open)

[Sealpup]

Modern jets don't have much of a problem with this, but early F14s had big troubles with it - so that'd be slam the throttle forward at low speed, big RPM increase . . . . . and the compressor stalls and buggers up the airflow through the engine. The BIG problem is when it stays stalled . . . . and suddenly the pressure in the combustion chamber is greater than that coming through the front of the engine. See where I'm going? ;)

 

Then you get a loud BANG and flames shooting out both ends. And the firetrucks rolling and the airplane's owner wanting to strangle you because he thinks you just tried to blow his plane up (or, atleast thats in the civie world anyways. And this isn't from personal experience, I swear! Really!)

[Ironhand]

This is a fairly common feature of US military aircraft. I assume it iscommon to most military aircraft but I don't really know. Typically, flight at less than zero G is limited to a minute or less. The fuel pumps in the feed tank for each engine are surrounded by a bell shaped chamber. There is a "donkey dick" fuel pickup that can swing around under negative G to the "top" of the chamber to pick up whatever amount of fuel is trapped there. When the fuel is consumed from the chamber it gets very, very quiet ....(Remainder snipped)

 

Thanks for that very complete answer. There's always something new to learn. :) And, thank you, EvilBivol-1, as well. So it seems to be a fuel starvation issue. Love the little realism touches in this sim.

 

Maulkin and Britgliderpilot,

 

While I've only tried this a few times trying to break the airplane, I've never had the engines quit in a dive. I have reached a point, usually just before a wing shears off, where acceleration slowed to a crawl. But the engines were still producing full thrust. Was that what happened in your case, Mauklin? Or did your engines spool down? And, Britgliderpilot, are we sure the slowing or lack of acceleration after a critical speed is a bug?

 

Rich

[britgliderpilot]

Maulkin and Britgliderpilot,

 

While I've only tried this a few times trying to break the airplane, I've never had the engines quit in a dive. I have reached a point, usually just before a wing shears off, where acceleration slowed to a crawl. But the engines were still producing full thrust. Was that what happened in your case, Mauklin? Or did your engines spool down? And, Britgliderpilot, are we sure the slowing or lack of acceleration after a critical speed is a bug?

 

Rich

 

Well I'm still in v1.02, where wings don't break off - ah, how wonderful it must be to have the AFM . . . . ;)

 

 

In the readme for v1.02, it is proudly announced (as a bugfix for v1.01) that ED have "Corrected zero thrust phenomena in A-10 when flying at M>0.8".

 

I tried this a while ago, and I think it's a bug that crept back in somehow. I'm pretty sure v1.02 didn't have an advanced enough failure model to deal with stuff like that.

[Ironhand]

Well I'm still in v1.02, where wings don't break off - ah, how wonderful it must be to have the AFM . . . . ;)

 

 

In the readme for v1.02, it is proudly announced (as a bugfix for v1.01) that ED have "Corrected zero thrust phenomena in A-10 when flying at M>0.8".

 

I tried this a while ago, and I think it's a bug that crept back in somehow. I'm pretty sure v1.02 didn't have an advanced enough failure model to deal with stuff like that.

 

Yes. All kinds of interesting things happening in 1.1.

 

Here's what seems to be happening in a dive. I haven't tested extensively but... You reach a point where only steepening the angle of the dive will increase your airspeed. Starting out at 5000 m altitude in a naked bird, I reach a TAS of 1100 km/hr (more or less and with all airplane parts still attached) just before impacting the sea. Don't know what Mach number or fraction that might be and too weary to figure it out.

 

For those who experience engine loss, the problem is fuel starvation. As the stresses on the airframe increase, the dive tends to steepen, unless you are actively pulling back on the stick. If you let the dive steepen on it's own, you start pulling negative Gs as indicated on the AOA Indicator. And shortly thereafter, your engines die.

 

Rich