Turbofan Engines
Joined: Mar 2002
Posts: 4,569
Likes: 1
From: Florida
Design the mechanical parts of an engine , put it together and test it on a test stand measuring the thrust vs RPM on a nice cool temp day, all the time monitoring turbine temps and pressure to ensure that it will run for at least a reasonable overhaul cycle.
Now tweak the control so that it just makes an advertised thrust rating on a hot day and call whatever rpm it takes 100 %
After that your thrust/RPM may vary (your fuel consumption certainly will
)
Now tweak the control so that it just makes an advertised thrust rating on a hot day and call whatever rpm it takes 100 %
After that your thrust/RPM may vary (your fuel consumption certainly will
)
Joined: Oct 2000
Posts: 436
Likes: 0
From: N. Europe
The RPM corresponding to 100% is rather arbitrarily chosen. At one point in the design process, for one particular model/generation of that engine it probably was the maximum RPM to run the engine at during normal operations and thus, they labelled it 100% RPM.
Then, things changed. The engine got better turbine cooling, things were tested and found to be able to take more than the initial calculations showed or whatever. Thus, the maximum usable RPM increased. Now, the maximum RPM is higher than what you decided to call 100% earlier on.
Easiest way out is allowing >100% RPM. Think of it as "104% of a given reference RPM" rather than "104% of what this particular engine is capable of".
Could just as well be a simple number, but 26,470 RPM is a bit harder to remember and verify than 104%.
Cheers,
Fred
Then, things changed. The engine got better turbine cooling, things were tested and found to be able to take more than the initial calculations showed or whatever. Thus, the maximum usable RPM increased. Now, the maximum RPM is higher than what you decided to call 100% earlier on.
Easiest way out is allowing >100% RPM. Think of it as "104% of a given reference RPM" rather than "104% of what this particular engine is capable of".
Could just as well be a simple number, but 26,470 RPM is a bit harder to remember and verify than 104%.
Cheers,
Fred
