08 Nov 2019, 18:03

Fuel fractions.

The F-22 in an AA mission is around 64,000lb TOW with 18,000 in fuel for a 0.28 fuel fraction.

The F-35A in an AA mission is around 49,600lb TOW with 18,500 in fuel for a 0.37 fuel fraction.

Even with a fuel fraction of 0.37, a 30,000lb fuel load would be a TOW of 81,000lb. That is a HUGE plane.

Remember that range is a function of cruise speed, weight, cruise L/D, cruise TSFC, and fuel fraction. If you keep speed, L/D, TSFC, and fuel fraction the same, and just increase the weight, you will not see an improvement.

If we say 20% of the internal fuel is used on takeoff and climb to cruise and 15% is reserved for descent,landing, and reserves, then we can get the aircraft weight at the beginning and ending of the cruise phase. Combining this with cruise L/D and we get drag, and that combined with TSFC we get fuel flow, for the cruise phase for the begging and ending of the cruise phase. Combine that with cruise speed and we get range over the cruise phase.

So we will initially hold cruise speed constant at 0.95M, TSFC at 1, and L/D at 12 for all aircraft. Fuel fraction is the only variable.

F-22 goes 697nm on 11,648lb of cruise fuel on an average of 4,549pph dropping from 60,416lb to 48,768lb

F-35 goes 975nm on 11,929lb of cruise fuel on an average of 3,330pph dropping from 45,930lb to 34,001lb

PCA goes 975nm on 19,500lb of cruise fuel on an average of 4,549pph dropping from 75,081lb to 55,581lb

So in that first run we can see that having more fuel weight does nothing if everything else is equal. Now, let's say that the F-22 has an L/D of 15 as its size allows optimizations for wave drag, and that the PCA also gets this advantage. This would give the PCA the F-22s design wave drag optimizations and the F-35s fuel fraction.

F-22 goes 871nm on 11,648lb of cruise fuel on an average of 3,639pph dropping from 60,416lb to 48,768lb

F-35 goes 975nm on 11,929lb of cruise fuel on an average of 3,330pph dropping from 45,930lb to 34,001lb

PCA goes 1219nm on 19,500lb of cruise fuel on an average of 4,355pph dropping from 75,081lb to 55,581lb

Now, what is important is the PCA fuel FRACTION not the fuel LOAD. The design question becomes how small of an aircraft can we build that has the drag optimizations we need, the power and payload we need, and can still hit a 0.37 fuel fraction.

This is why a G650 has nearly the same max range as a 747-8.

G650ER

cruise speed 0.85M

fuel fraction 0.465 (48,200 / 103,600)

L/D (Wing Loading 80.7 - Aspect Ratio 7.73 - Sweep Angle 36deg LE)

TSFC (Engine BPR 4.1 - OPR 43)

Range 7,500nm

747-8I

cruise speed 0.86M

fuel fraction 0.43 (426,109 / 987,000)

L/D (Wing Loading 165.6 - Aspect Ratio 8.43 - Sweep Angle 37.5deg LE)

TSFC (Engine BPR 8.0 - OPR 52)

Range 8,000nm

So the Boeing has a slight edge in cruise speed and a huge edge in TSFC, but the Gulfstream has the advantage of fuel fraction. L/D overall likely goes in Gulfstreams favor as the wing sweeps are very similar and while the Boeing has a somewhat better aspect ratio the Gulfstream crushes it in Wing Loading which impacts Cruise CL which squares for the Drag.

So here we have two aircraft with very similar flight profiles and shockingly close ranges given that one has nearly 10x the fuel of the other.

"Spurts"

-Pilot

-Aerospace Engineer

-Army Medic

-FMS Systems Engineer