Void's Forums

Okie Dokey, here's a change, perhaps you can help.

In a Gas Turbine engine (turbojet) the shaft connecting your tubine stages and the compressor effectively gives you a positive feedback system, with higher airspeed giving higher compression. My questions is this, what is to stop the pressure being increased to such an extent that:

A) Engine shakes itself apart
or
B) You are combusting an ultra lean mixture and melt the turbine with exhaust

Is it something as simple as a blowoff valve in a (turbo)supercharger?
I'm hoping perhaps void can help me here, given his past.

The thing is, there is a maximum RPM on the turbojet engine just like there is on a piston engine. On turbojets the guages read in percent (%) of maximum RPM rather than RPM (revolutions per minute). I can't even remember what the maximum RPM actually was on any of the jets I worked because it was always referred to as "%". If I had to guess I would think the max RPM is somewhere between 20,000 and 30,000 RPM, probably low to mid 20s. The jets I used to work on never actually reach 100% but optimal maximum on the turbine was around 98% and on the turbofan engines you also had a fan speed indicator that was also displayed %. The fan speed (front set of blades that drive the bypass airflow) was always a lot lower than the turbine speed. I recall somewhere around 70% but I could be wrong on this as it's been 13 years since I worked jets. The F-16 used a turbojet and the A-10 (which I also worked) used a turbofan. So because there is a maximum RPM that also means there is a maximum amount of airflow through the engine. The engine isn't going to go any faster than it can push the airplane through the air so there is a finite limit on the amount of air that goes through it. As far as keeping the fuel air mixture right there are mechanical fuel pumps and then there usually is a fuel control system controlled by computer (similar in principle to fuel controls on modern cars). Likewise the computer will prevent an overspeed condition like if you put it into a full thottle dive which is probably what you have in mind.

Regarding shaking themselves apart, every part is very carefully balanced. Every fan blade is weighed and marked with their weight in grams. Opposing blades have to weigh the same amount. I used to run engines on the airplanes on a tie down cable and we had vibration sensors that we hooked up to the engines at key locations and they would hook up to a test computer on the ground. At key RPM settings there were vibration limits at each point on the engine that you must be within. If the vibration tests fail you have to take whatever action necessary (rebuild the motor). Of course on a piston engine if things aren't properly balanced and you run it at too high of an RPM it will shake itself apart as well. I don't know if any of that answers your question and I can't even guarantee any of it is correct. :)

It's funny you asked this because we just walked in the door coming back from watching an IMAX film at the Omnimax called "Fighter Pilot". The film was filmed at Red Flag at Nellis AFB, NV. I was stationed there for 3 years working the F-16 and they had a lot of shots of the flightline and the base and I spent most of my time right there. I was a crew chief and they had a lot of film of a lot of the things I used to do like launch/recover airplanes, change engines, run them in the test cell, etc. It brought back a lot of memories and it probably gave you about the closest feeling to flying in a fighter jet that you could get without actually doing it. I got a ride in an F-4 when we were in Spain and it was the most awesome experience of my life. I had to take my kids to see this movie.

Yeh that kinda answers it, thanks for taking the time. So is the max. RPM, due to friction or by design? (or both )

It's funny you asked this because we just walked in the door coming back from watching an IMAX film at the Omnimax called "Fighter Pilot". The film was filmed at Red Flag at Nellis AFB, NV. I was stationed there for 3 years working the F-16 and they had a lot of shots of the flightline and the base and I spent most of my time right there. I was a crew chief and They had a lot of film of a lot of the things I used to do like launch/recover airplanes, change engines, run them in the test cell, etc. It brought back a lot of memories and it probably gave you about the closest feeling to flying in a fighter jet that you could get without actually doing it. I got a ride in an F-4 when we were in Spain and it was the most awesome experience of my life. I had to take my kids to see this movie.

Cool, I'd love to be a fighter pilot, but i'd be too short
An IMAX film about fighters sounds good fun , they should make more films like that - and I don't mean another Top-Gun because that was more about homo-eroticism than planes
Did you get a ride in the F-4 as some sort of civilian fun ride or whilst you were still serving?


PS. The reason I asked this plane stuff is because I am writing a Physics report about future air propulsion. I am doing a section on turbojet/prop/fans as a prulude to the main event which will be about ramjets, scramjets, beamed propulsion, and anything else that looks interesting

For your level of research I would suggest you not go by anything I say but by doing some good google searches on how jet engines work. To be honest I don't know if they were "governed" to a maximum RPM or if there were physical limitations that restricted them to a specific RPM. I know for the turbofan engines it is a physical limitation on the fan itself because we try to get the highest % RPM out of them as possible and there were minimum limits on the maximum RPM. I think I was wrong in my last post because I believe we had to be able to get the fan to turn at least 97% and to be honest I don't remember what that is a percent "of" at the moment.

On the A-10 a lot of smoke comes out of the nose of the airplane when they shoot the big 30mm cannon and that smoke goes into the engines. It collects on the fan blades and the more they do this the lower the maximum RPM on the engines become (dirt on blades = drag). We used to have to wash the front fan blades and then hook hoses up to the engines and run soap/water into them while we ran them to clean the internal blades. We had to be able to get the RPM up to a certain level or the engine would have to be rebuilt. There has to be some good information on all of this on google somewhere. I might have to dig up my old tech school books and read over the engine books which I know would help refresh my memory (at least for all of the numbers I tried to mention). I think I saw my A-10 manuals laying around somewhere a couple of months back.

When I got my F-4 ride I was a crew chief on F-4E and F-4G models, stationed in Germany. We used to go to Spain a lot for training (along with several other countries). Because all of the F-4s have two seats there is a much higher chance of getting an "incentive ride" which is what they are called in the Air Force. It is a way of being recognized for doing a good job. I was also supposed to get a ride in an F-16 when I worked them right before I left Nellis but some Congressman took my spot! It's much harder to get a ride in an F-16 because we only had 3 on the entire base that had 2 seats (trainer models). Most F-16s only have one seat. All active A-10s only have one seat so there is no chance of getting a ride in one of those unless you steal one and fly it yourself. Not recommended.

I worked on some of the first F-16s (1977-1979 models). At that time they used the Pratt & Whitney F100 engine. Shortly after that they started running more efficient and more powerful GE engines. I'm not sure what the newest ones run. Here is a link on the F100 with basic principles:

http://www.pilotfriend.com/aircraft%20p ... ngine1.htm

That link is kinda weak though because they don't even mention the section of the engine that provides a very significant portion of thrust called the "afterburner" section. The afterburner is bascically nothing more than dumping raw fuel into the nozzle section right after the turbine. At full throttle without afterburner the nozzle at the back is closed, then when the afterburner kicks in the nozzle opens up (or the engine would probably explode).

Yeh, that's cool. As I say this is only an auxiliary secion of my research report, it's just something that struck me whilst doing my research and never seemed to be adressed

On the A-10 a lot of smoke comes out of the nose of the airplane when they shoot the big 30mm cannon and that smoke goes into the engines. It collects on the fan blades and the more they do this the lower the maximum RPM on the engines become (dirt on blades = drag). We used to have to wash the front fan blades and then hook hoses up to the engines and run soap/water into them while we ran them to clean the internal blades. We had to be able to get the RPM up to a certain level or the engine would have to be rebuilt.

As far as I know (and you're the expert remember), the smoke from the cannon is depleted of oxygen and could theoretically cause the engine to flame-out if it sucked in too much of it. They designed the plane to route this smoke away from the pods as much as they could, and they only fire the cannon in short bursts anyway.
It's interesting that enough of it went through the engines to actually hit the performance as you described.

and the A-10's cannon.....

oh what a cannon!!

Yeah, the Army guys really liked our A-10s because we provided close air support for them. As far as the cannon, there was a "rounds limiter" that was normally set to 150. What this means is that for each pull of the trigger the most you could shoot was 150 rounds. At 4000 rounds per minute (low speed setting) that is a little over 2 seconds of trigger pull. What struck me more than the smoke bogging the engine was reports from the pilots saying that they could actually feel the airplane slow down when they pull the trigger. :) Yeah, the gun is pretty impressive. The gun and canister (which holds 1150 rounds if I recall correctly) is about the size of a Volkswagon Beetle. I believe they also use the 30mm GAU-8 in the C-130 gunship.

hehe, he said "rpm"!

Perhaps this should be moved to the RedHat forum?

Void Main wrote:Yeah, the Army guys really liked our A-10s because we provided close air support for them. As far as the cannon, there was a "rounds limiter" that was normally set to 150. What this means is that for each pull of the trigger the most you could shoot was 150 rounds. At 4000 rounds per minute (low speed setting) that is a little over 2 seconds of trigger pull. What struck me more than the smoke bogging the engine was reports from the pilots saying that they could actually feel the airplane slow down when they pull the trigger. Yeah, the gun is pretty impressive. The gun and canister (which holds 1150 rounds if I recall correctly) is about the size of a Volkswagon Beetle. I believe they also use the 30mm GAU-8 in the C-130 gunship.

It sure is badass, I believe the engines are cranked to full thrust when the trigger is pulled to help it maintain speed - Newton's third Law of motion etc.