smartt1982
Topic Author
Posts: 226
Joined: Sat Nov 24, 2007 11:17 pm

Jets Versus Prop And Open Rotor

Wed May 30, 2012 10:38 pm

What is the main reason that jets are better than props at high alt/high speed?.

Is it purely down to the fact that the props will encounter issues with the tips of the blades becoming inefficient at higher speed or more to do with the extra power required at the shaft as TAS increases.(power required = Drag x TAS), why does this not affect jets?.

My understanding is that jets produce a constant thrust and so at a constant IAS thrust required is the same.

With props the thrust is being produced from shaft power so even at a constant IAS, the TAS is increasing so more power is required at the shaft to produce the same thrust at the prop, why does not affect jets?

How are these issues overcome considering that some companies are trying to push open rotor technology, is there not the same issue to be resolved?
 
tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

RE: Jets Versus Prop And Open Rotor

Thu May 31, 2012 1:07 am

Quoting smartt1982 (Thread starter):

What is the main reason that jets are better than props at high alt/high speed?

Jets are ducted so they can generate a higher pressure rise across the engine.

Quoting smartt1982 (Thread starter):
Is it purely down to the fact that the props will encounter issues with the tips of the blades becoming inefficient at higher speed or more to do with the extra power required at the shaft as TAS increases.(power required = Drag x TAS), why does this not affect jets?.

The tip loses do affect jets but not nearly as badly (since the fan case shrouds the fan). Jets have exactly the same power requirements (drag x TAS for steady flight).

Quoting smartt1982 (Thread starter):
My understanding is that jets produce a constant thrust and so at a constant IAS thrust required is the same.

They do not produce the same thrust; jet thrust falls off with altitude just like props due to the decreasing density of the air. They're not as sensitive to thrust as a function of airspeed though.

Quoting smartt1982 (Thread starter):
With props the thrust is being produced from shaft power so even at a constant IAS, the TAS is increasing so more power is required at the shaft to produce the same thrust at the prop, why does not affect jets?

It does. Modern high bypass turbofans produce most of their thrust from shaft power to the fan. It's usually better to think of a modern engine as a ducted turbofan rather than an actual jet.

Quoting smartt1982 (Thread starter):
How are these issues overcome considering that some companies are trying to push open rotor technology, is there not the same issue to be resolved?

The same issues are there; open rotors use much higher solidity fans and much fancier blade shapes to try to mitigate the problems.

Tom.

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