I have been trying to come up with my own formula (that differs from the various CAFE formulas to have a weighting that suits me better). I found another interesting comparative formula, the AEI.
AEI = (W0*U0)/hp
where W0 is the gross weight in lbs
where U0 is the free stream velocity in ft/s
where the hp is the horse power required
In other words for Diamond DA40 this is:
W0=1200 kg = 2640 lbs
U0= 214 ft/s (127 kts cruise at 10000 ft)
hp_cruise=90
=>
AEI(Diamond DA40) = (2640*214)/90
AEI(Diamond DA40) = 6277
Unlike the CAFE formula this has no weighting for these parameters.
If I would like to make my formula based on this, I might want to weight the hp a bit more.
That is because the lower the hp figure gets, the better is the fuel economy if everything else remains constant.
Why this index is good? Because it isolates aerodynamics from structural engineering and does not care how much useful load the craft has. It only considers the aerodynamically important point, how much power is required to move the mass forwards and at the same time keep it on air.
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