A few jet engine questions (jet pilots/mechanics please step in)

[Electric]

Have a few questions about jet engine operation. If someone could answer any of these questions or point me to a resource where I can look up the answers it would be greatly appreciated.

1. How is power controlled in the older jet engines used in the 50s (the first reliable engines such as in F-86, Mig-15) before they started using computers to regulate fuel flow and stator blade pitch? Did it require any special precautions, like limiting the rate at which power lever is advanced and retarded? If so how would a pilot know the fastest rate at witch the power can be moved forward?

1. a. Is it possible to "over rev" an old jet engine?

2. As far as I know, some jet engines do not operate reliably at very high angles of attack, at a very high altitude or at a very high speed. My understanding is that it's caused by a compressor stall, or because the flame is somehow extinguished in the ignition chambers. Is that really the case?

2.a How are these problems mitigated on the fighter jets that see extremes in the angle of attack, altitude and speed on daily basis?

3. It seems like it is harder to airstart a jet engine? Is that really the case, if so why?

4. Everything else being equal, centrifugal flow engines are more reliable than axial flow. Is that still the case if you were to compare a modern axial flow turbofan with a centrifugal turbojet made by Rolls Royce or by Russians (in Mig-15) from the 50s. (Obviously we are not comparing fuel economy or power output, just reliability.)

Thanks in advance!

 

[X3 Skier]

1. The older models had hydro mechanical computers involving cams, levers and other gizmos.

1a. Most had a flyball governor as part of the control system that limited speed.

2. The problems of high angle of attack and high speed operation is because of inlet distortion and the resulting variation of the angle of attack on the compressor and or fan blades cause them to stall and the result is they no longer "pump". Then the high pressure in the combustor blows the fire back out the front with a hell of a bang. High altitude problems are because combustion cannot be sustained at the low pressure and oxygen levels.

2a. By better stall margins in the engines and inlet design.

3. Not really once you get the speed up and/or lower altitude with denser air.

4. Not really. Modern axial flow engines are much more reliable than older engines of any type. The reason centrifugal engines were the first used was they were derived from centrifugal superchargers used in WWII fighters and bombers. Once GE, led by Gerhard Nuemann, developed variable geometry for axial compressors, their inherent performance advantages led to their dominance for aviation powerplants. Centrifugal engines dominate ground power units since they are simpler and more rugged and the weight penalty is not a consideration.

Cheers

 

[flyingmoose]

Have a few questions about jet engine operation. If someone could answer any of these questions or point me to a resource where I can look up the answers it would be greatly appreciated.

Ill give it a shot!

1. How is power controlled in the older jet engines used in the 50s (the first reliable engines such as in F-86, Mig-15) before they started using computers to regulate fuel flow and stator blade pitch? Did it require any special precautions, like limiting the rate at which power lever is advanced and retarded? If so how would a pilot know the fastest rate at witch the power can be moved forward?

Some of them used mechanical type fuel controls. If I remember correctly the older stuff used compressor bleed ports that would open to prevent compressor stalls and stuff that variable stators fix now.

1. a. Is it possible to "over rev" an old jet engine?

Not sure, never seen it happen.

2. As far as I know, some jet engines do not operate reliably at very high angles of attack, at a very high altitude or at a very high speed. My understanding is that it's caused by a compressor stall, or because the flame is somehow extinguished in the ignition chambers. Is that really the case?

Where do you get this information from? Some engines do all of those things quite well. High angles of attack could have been a problem with the older stuff, but most newer fan engines don't care much.

2.a How are these problems mitigated on the fighter jets that see extremes in the angle of attack, altitude and speed on daily basis?

SOme of the fighters use variable blocker doors or inlet cones to change the speed of the intake air.

3. It seems like it is harder to airstart a jet engine? Is that really the case, if so why?

By airstart I assume you mean inflight without the use of a starter. If you have the proper airspeed its not really an issue. Using a starter is always easier.

4. Everything else being equal, centrifugal flow engines are more reliable than axial flow. Is that still the case if you were to compare a modern axial flow turbofan with a centrifugal turbojet made by Rolls Royce or by Russians (in Mig-15) from the 50s. (Obviously we are not comparing fuel economy or power output, just reliability.)

Thanks in advance!

I wouldn't say so.

 

[Electric]

Thanks alot for the answers. My initial information is from reading here and there, nothing formal.

Another question, I forgot to include this into my original list. I heard somewhere that normally igniters (spark plug equivalent?) are turned off, but need to be turned on during critical phases of flight or for high angle of attack maneuvering? Is that true? Is the flame more likely to be extinguished during such operations or it is for pure redundancy to reduce the risk of engine failure where there is no time to troubleshoot/restart?

 

[Ted]

Igniters are usually left on during areas when it is more probable to have a flame-out, such as when flying through rain or ice.

 

[X3 Skier]

As an aside, one of the more spectacular failures I was involved as a propulsion engineer with the USAF was on a TF30 where the supervisory electronic control failed and since the hydro mechanical control had no flyball governor in that version, the engine "ran away" and exploded on the test stand.

A similar failure happened later in my career on an F100 in an F-15 and blew up on the runway. The pilot broke his ankle jumping over the side. The F100 was the first to have a completely electronic control except for the fuel valve(s).

Cheers

 

[flyingmoose]

Yup, in the Caravan (PT6) you put them on in precip t/o and landing etc... Newer a/c have auto igniters that will come on if they sense certain things.

 

[Ted]

Yup, in the Caravan (PT6) you put them on in precip t/o and landing etc... Newer a/c have auto igniters that will come on if they sense certain things.

And on the Commander the boss said "DON'T TURN THOSE ON! THEY'RE EXPENSIVE!"