Turboprop vs Turbofan: How to compare Specific Fuel Consumption

Question

I understand that Turboprop engines have a lower Specific Fuel Consumption (SFC) than Turbofan engines, on average. I want to see this for myself by analysis, ideally by comparing values of SFC for typical turboprop engines vs turbofan. As turboprop SFC is usually given as the Fuel Consumption Rate / Power, and turbofan SFC is given as Fuel Consumption Rate / Thrust, it is tricky to directly compare them.

For a given airspeed and altitude, I want to show that a turboprop engine will require a lower rate of fuel consumption to produce a given amount of thrust. Is there a way I can show this without going into too much detail on things like blade pitch, thrust coefficient etc.

Ideally, I want to compare SFC of turboprop engines with that of turbofans, and show that the SFC for turboprop engines at a given thrust is lower.

Thanks

Answer 1

Well, propellers deliver thrust and turbofan engines deliver power to the fan axis, it is just a matter of choice.

Finding the thrust that a propeller generates at a specific airspeed and altitude, can be derived from the drag of the whole airframe in those conditions. Statistically, this sort of data can be found in reference books on pre-design of aeroplanes, such as Synthesis of Subsonic Airplane Design by E. Torenbeek. If we take a generalised approach:

$$D = C_D \cdot ½ \rho V^2 \cdot S \tag{1}$$ $$C_D = C_{D0} + \frac{{C_L}^2}{\pi A e} \tag{2}$$ $$C_L = \frac{2W}{\rho V^2 S} \tag{3}$$

So total drag follows from:

• design factors weight W, wing area S, wing aspect ratio A, Oswald factor e.
• environmental factors airspeed V and air density $\rho$

For the generalised $C_{D0}$ of aeroplanes, from Torenbeek page 149:

So you can use all of this to compute propeller aeroplane specific thrust. But the exercise will be futile, because:

• The engine of a turbofan looks remarkably like that of a turboprop. In fact, they are both turbine engines, powered by the Brayton cycle. Similar size and design engines deliver very similar gas generator power to the [fan or prop] axis.
• Turbofan engines tend to larger than turboprop ones, generating extra efficiency due to less boundary layer effect and more uniform internal flow.
• Speed is of the essence: at turboprop speeds, turbofans are not very efficient. At turbofan speeds, turboprop cannot deliver enough thrust due to propeller tip drag divergence.
• Turbofans utilise more of the hot exhaust flow for thrust generation due to the higher required flight speed. Lower efficiency, higher thrust.