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

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?

• Do you have a source about the powerplant and the maximum speed of the Eurofox? According to the official website, the Eurofox can be powered by a Rotax rated either 80, 100, 115 or even 141hp but the relevant max speed is not given. For the Ikarus those values are listed but without the correct couple powerplant/max speed of the Eurofox, any answer would merely be speculative... Commented Aug 8 at 21:23
• @sophit the Eurofox POH (available on their website) states speeds for each engine. I am talking about the 80HP 912UL (not the "s") - as that's what the C42 has Commented Aug 9 at 8:24
• Do you have a link? Commented Aug 9 at 11:28
• @sophit I googled "Eurofox POH" for you - bottom of this page: eurofoxaviation.co.uk/factory-built-microlight Commented Aug 9 at 11:41
• Perfect, thanks ðŸ™‚ Commented Aug 9 at 13:22

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.

• Thanks Rob. Is the wing size on line 2 a typo? Does the C42 have a smaller wingspan? Commented Aug 9 at 8:25
• @Cloud Line 2 of what? The wing areas I saw on Wikipedia were 135 for the Ikarus and 112.5 for the Eurofox. That is a ratio of 1.2. Bigger wing, lighter wing loading, slower airplane. Commented Aug 9 at 16:04
• So the larger the wing ratio, the slower the plane? Commented Aug 12 at 8:44
• All else being equal, the larger the wing, the slower the plane. In these equations, S is the wing area. W is the gross weight. W/S is the wing loading -- since S is on the bottom of wing loading, it behaves opposite. Light wing loading (small W and big S) is for slow airplanes. High wing loading (large W and small S) is for fast airplanes. Commented Aug 12 at 16:15