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Descent and Negative TSFC

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spain
Hi all!

This is just to confirm that what I am seeing and understanding, is correct.

During descent of an aircraft (say A320), with throttle at Idle, you can get negative net thrust due to very low gross thrust but high ram drag at speeds from Mach 0.6 to 0.2. With negative net thrust, you get a negative TSFC with some high magnitudes. Is this correct? For example, some real numbers for the A320:

Altitude: 18,000 ft
Speed: Mach 0.6 (TAS=388 Kn)
RoD: -3200 FT/min
FF: 1636 lbs/hr
FN: -33 lbf
Thus, TSFC: -49.5
 

Roy Holmes

Resource contributor
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1,721
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us-virginia
With a simple straight through jet, you can use one value for intake area because all the air goes down the intake and all the thrust is applied at the back end.
The engine intake requires power from the turbine to compress the intake air
The intake area as far as the core engine is concerned is just that part through which air enters the engine core and is mixed with fuel and burned. Ram drag is calculated using the intake area for the core engine.
With a complex by-pass engine most of the thrust comes from the fan in a similar way to a turboprop. The fan is, essentially, a propeller which is driven by the turbine.
When calculating that thrust you would use the fan area which is usually very large compared to the core area.
If you close the throttle on any engine you can get high drag which exceeds the idle thrust, if you shut down the engine you will get even higher drag and no fuel consumption. But the drag comes from the fan, not from the engine intake.
You have to apply different equations with bypass engines you can not apply the simple.
However FS treats all engines as simple non-bypass as far as thrust and fuel are concerned.
Roy
 
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46
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spain
Thanks Roy. I see the difference and how FS treats the engine as non-bypass.

My question was actually more geared to non-FS (real life) situations. Googling, I find a lot of literature and figures relating TSFC to Mach, Altitude and/ or Thrust for turbofan engines (BPRs between 5 and 12). But all these figures have positive TSFCs, mostly characterizing take-off, climb and cruise phases; but none characterize descent profiles.

Just curious and also intrigued on how descent is being modeled in FS. Going over the N1 vs Thrust table, all thrusts are positive at different Mach. Does FS interpolate/ extrapolate to the negative side of the curve when descending?
 

Roy Holmes

Resource contributor
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1,721
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us-virginia
When cruising at constant altitude your energy comes from fuel being burnt and turned into thrust, so TSFC means something. When you climb you have an additional call on the fuel burn to generate potential energy. When you descend your fuel consumption is reduced because you use stored potential energy. That is why it is not just thrust and drag if you are climbing or descending. If you apply TSFC calculations designed for level cruise during the climb or descent you will get silly answers.
Roy
 
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