Hmmmmmmmmmm...

MaxPete

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Most of the small stationary GT that were used pump and generator application used a regular old automotive starter and run surprisingly low shaft rpms in the 3500-5500 range. Most big stationary GT's for power generation run in the 4000-5000 SRPM range.

Yup - I think I read that a big turbofan on a 747 class aircraft has a max speed of about 7500 RPM - which, given the diameter is pretty frickin' fast.
 

RustiePyles

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Yup - I think I read that a big turbofan on a 747 class aircraft has a max speed of about 7500 RPM - which, given the diameter is pretty frickin' fast.
747A has a fan diameter of 111.1" so at 7500RPM thats 218144.340 SFPM at the fan OD :yikes:
 

Jim

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Yup - I think I read that a big turbofan on a 747 class aircraft has a max speed of about 7500 RPM - which, given the diameter is pretty frickin' fast.
Not sure what the revs are Pete, but it's much less than that. Like a prop, fan blade tips have to stay subsonic. You could take the fan diameter and 1000 fps as sonic.... and work backwards from there. My guess is less than 2500 rpm.
 

RustiePyles

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Not sure what the revs are Pete, but it's much less than that. Like a prop, fan blade tips have to stay subsonic. You could take the fan diameter and 1000 fps as sonic.... and work backwards from there. My guess is less than 2500 rpm.
@1000 FPS with a 111.1" fan your at 2065.854 RPM
 

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Soon as I saw that tractor with the apu in it, some crazy memories came back. I used to be a mechanic at an airline that had a couple of Boeing apu's mounted in the back of Ford pickups. They were used as air starts, both on the 747s and one on the 2 and 3 engine aircraft. Inspections and maintenance were done regularly by us mechanics, and it was fun to do a run up for a nozzle pressure test. If the cycle wasn't done right, it would not light up, and excess fuel would be sitting in the turbine. When it did finally fire up, the canopy covering the engine would almost lift off the back of the truck. Back in those days I did a lot of afternoon shifts, means working in the dark. The rampies would have both hooked up to a 747 and do a couple of false starts on one or both. Get a call on the radio, hussle up to the ramp and see what's going on. Then the shit hits the fan. Time is everything with aircraft departure off the gate. Pull the airstarts off the plane and well away, everyone stand back, and then go through the cycle and hope like hell it lights up. Sometimes it didn't. But if it did, very impressive, and scary, heart pounding moments. Some of us have seen wet starts on jet engines, kind of neat to see when you are not on the aircraft. The little Ford 150 would be almost engulfed in a fire ball as the Boeing turbine fired. After a few minutes it would get moved back under the 747, nozzles connected and hope they have enough pressure to turn its engines. If it did, all was good and time to go home. If not, then I am on the hook for a delay, and got some splainin to do.
 

MaxPete

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You are quite correct Jim - I think I was quoted that number for the high pressure shaft. The LP shaft (with the fan at the front) does indeed turn much slower - likely 2-3000 RPM.
 

Jim

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Off the top of my head... Pi of 111" is about 350" ... about 30 ft per rev. div by 1000... times 60 sec.... about 2000 rpm.
 

Jim

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Today's brain teaser:
Standard day speed of sound (sos) is about 760mph.
Your typical jet exhaust comes out the tailpipe at around 1000-1200mph.... yet it's subsonic... no boom.
Why?
 

RustiePyles

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Today's brain teaser:
Standard day speed of sound (sos) is about 760mph.
Your typical jet exhaust comes out the tailpipe at around 1000-1200mph.... yet it's subsonic... no boom.
Why?
hotter air is less dense so it raises the airspeed at which a sonic boom is possible?
 

jetmechmarty

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Today's brain teaser:
Standard day speed of sound (sos) is about 760mph.
Your typical jet exhaust comes out the tailpipe at around 1000-1200mph.... yet it's subsonic... no boom.
Why?
I always thought a sonic boom was caused by an object moving through the air at faster than the speed of sound. That exhaust doesn't involve an object stacking the sound waves.
 

Jim

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I always thought a sonic boom was caused by an object moving through the air at faster than the speed of sound. That exhaust doesn't involve an object stacking the sound waves.

An afterburner emits sonic booms. Hence the crackling...
 

Jim

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I always thought a sonic boom was caused by an object moving through the air at faster than the speed of sound. That exhaust doesn't involve an object stacking the sound waves.
The exhaust is a dense hot gas. In that respect it can be looked at as an object that will displace air.
 

Jim

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hotter air is less dense so it raises the airspeed at which a sonic boom is possible?
Ding ding ding..... close enough.... :smoke:
The speed of sound in air (or exhaust gas in this instance) is dependent on temperature. The higher the temp, the higher the sos. Jet exhaust is hot enough that 1000mph is subsonic. Add an afterburner and velocity increases to about 1500mph with only a slight increase in temp.... sos is exceeded.... we get that intermittent crackling sound and can see the shock waves (diamonds) in the exhaust plume.
 

59Tebo

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Speed of sound as defined: 744.4 MPH @ sea level @ 32* F. So, yeah, temperature is going to affect sos. Wouldn't that fall somewhere under the Combined Gas Laws (Combining Boyle's Law, Charles Law, and Gay-Lussac's Law) ?
 

Jim

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Speed of sound as defined: 744.4 MPH @ sea level @ 32* F. So, yeah, temperature is going to affect sos.
In air, the temp is the only variable. So saying "sea level" is immaterial... it doesn't matter or play into the equasion. If the temp is 32°, the sos will be the same regardless of altitude.

800px-Comparison_US_standard_atmosphere_1962.svg.png
 
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