Thrust impact of SFC for Turbofan engines

[Hu gePanic]

I am currently working on a project where we want to compare engine selection options for a turbofan driven airplane.

I found that the existing turbofan engine network (modified from the B737 example) does not show any effect of thrust (or throttle) on the SFC.

So the SFC is identical, e.g.. for 50% or for 100% throttle setting.

As I would have expected, this seems not to be realistic.

Some discussion is done here: Bensel, Characteristics of the Specific Fuel Consumptionfor Jet Engines

My questions:

- Is this "as intended" for the SUAVE turbofan energy network?

- Are there any plans to change that?

As I want to use SUAVE also to model that effect:

I found there is a "SFC correction factor" available in the network. --> I assume this is the easiest way to put in any formula to adjust the SFC according to throttle (once I found a good way to model it).

Did I miss anything? 

Was something done before?

Thanks

Matthias

[Emilio]

Hi Matthias,

That's normal for the assumptions of this model. What happens is we run the engine at full throttle and scale it. We realize it's not physical. 

HOWEVER, it actually works great. The reason is that at low throttle settings (where SFC is higher) the thrust setting is lower. So a small error in SFC doesn't result in a large change in fuel burn.

If you need to model the engine in detail you can use PyCycle. There's both a network that can be used to import an engine (this takes a bit of work) alternatively you can prerun cases and use the propulsor surrogate. In PyCycle they also don't have throttle settings by default, so this will take some work.

I ran a very detailed comparison on the 737 and the SUAVE model vs throttle PyCycle model was insignificant in fuel burn for a conceptual. It's good enough for conceptual level analysis.

-Emilio