Why do two similar planes with the same engine have such different performance?

Question

Why do two similar planes with the same engine have such different performance? For example, the Ikarus C42 and the Eurofox both use 912UL engines, but the Eurofox cruises 15 - 20 knots faster.

What factors contribute to this?

Answer 1

Wing loading -- mostly wing area because these aircraft have essentially the same gross weight.

The Ikarus has almost a 20% larger wing.

For these aircraft, the drag polars will be substantially similar. Importantly, the lift coefficient for best L/D will be about the same. This means that the cruise lift coefficients will be about equal.

The definition of lift coefficient in level flight:

$C_L=\frac{W}{q\,S}$

Can be re-arranged

$V=\sqrt{\frac{2}{\rho}\frac{W}{S}\frac{1}{C_L}}$

Dividing this expression by itself (but applied to airplanes 1 and 2), then cancelling all the constant terms.

$\frac{V_2}{V_1}=\sqrt{\frac{S_1}{S_2}}$

Plugging in numbers...

$109\times\sqrt{\frac{135}{112.5}}=119$

Which is pretty close to the 120 claimed.

I would expect the Ikarus to have better stall speed, takeoff distance, and landing distance -- unless the Eurofox uses more sophisticated high lift devices.

Note, the Wikipedia article for the Eurofox has a metric to standard conversion error on the wing area. Correct value used above. 10.45 m^2 is 112.5 ft^2.