Engine-out cruise SFC/Range?
 

After reading the A380 engine shutdown thread, I started wondering - how does engine-out SFC and TAS at LRC compare to the "All Engine Operative" case, both for a twin and for a quad?

I am not interested in exact numbers (as they are type- and mass- dependent), but rather physics behind single engine cruise

For a twin it'll depend whether the engines are wing or tail mounted.
Twins with tail mounted engines will occasionally have better specific range on one engine only, while with wing mounted engines there will normally be a loss of specific range, in a dimension of 30% approx.

Thanks Safelife!

So, am I right thinking that that the remaining engines operate at higher efficiency (even on a lower cruising FL) and the decrease in performance is due to windmilling and asymmetry drag?

Re SFC: Classical turbojets reached their best SFC at max thrust (TO), and at part throttle, SFC is degraded.

But modern turbofans are tuned to best SFC over a range of normal cruise settings. OEI ops may require pushing the throttle beyond this optimum range.

But every engine is a bit different; your mileage may vary. :ooh:

Several things come into play: The remaining engine is typically operating well above it's optimal efficiency range and hence is less efficient than during a normal cruise. Cruise altitude is much lower, meaning more drag. Windmilling drag and lots of rudder to compensate for the thrust asymmetry adds more drag.

A B-777 @ 10.000' uses a fraction more fuel with one engine at MCT than with 2 engine at a lower power setting to maintain the same speed, hence fuel to an ETOPS alternate is based on one engine at MCT. It might be caused by trim drag.

In practical terms, the max altitude with 1 engine inop is significantly lower at high gross weights than the optimum normal cruise altitude. That is the more significant factor in a long-range flight. Comparing SFC at a constant altitude is meaningless for most/all of the flight.