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If plane or car has Cd of 0.35 and 5L/100km, how much will be fuel consumption if Cd is reduced to 0.20 and everything else stay the same(weigth, frontal area,power etc)?

Is it possible to calculate this directly if we only know reduction of Cd?

user707264
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1 Answers1

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For Airplanes

Yes, use the Breguet range equation. Follow the link and use your numbers: First run it with the L/D that you get with the high drag coefficient, then repeat with the lower drag coefficient. You should look for the increase in landing mass which the drag reduction allows while range stays the same. This increase can be interpreted as the additional payload you can take along. Alternatively, you can look for the reduction in take-off mass to see how the reduced fuel mass due to the drag reduction allows to fly with a lighter plane.

There is more to vary, if you want: Fly both versions of your plane at the same lift coefficient, or go with a lower lift coefficient when drag is lower so you fly both at their optimal L/D. The Breguet range equation shows that range is proportional to L/D.

As an aside: May I assume that you have the zero-lift drag coefficients in mind?

If not, the drag coefficient grows with the square of the lift coefficient and a drag coefficient without an accompanying lift coefficient will be meaningless.

If yes, your values look too high by an order of magnitude.

For Cars

Strictly taken, this question would be off topic here.

Apart from that, the first question is how much of total drag is aerodynamic. Heavy and slow vehicles have non-negligible rolling drag. On the other hand, the drag of a passenger car at Autobahn speed is mostly of aerodynamic origin.

Next question: How much of the energy in the fuel is used to keep the engine and its accessories running? With the AC running at full power, reducing aerodynamic drag will have less effect.

I advise to be more specific for a more precise answer.

Peter Kämpf
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  • Since the question starts with "If plane or car", perhaps it's worth mentioning that the Breguet equation and this answer are only valid for planes, not for cars. +1 – Robe Sep 15 '22 at 14:44
  • @Robe Oops, overlooked that. Thank you for spotting this. – Peter Kämpf Sep 15 '22 at 16:02
  • @PeterKämpf 0.35/0.2=1.75 Can I simply divide consumption by 1.75 in car case? – user707264 Sep 16 '22 at 16:51
  • @JurgenM No, only if 100% of the chemical energy in the fuel goes into overcoming aerodynamic drag, which is clearly wrong. Haven't you read my answer? – Peter Kämpf Sep 17 '22 at 06:38
  • @PeterKämpf Yes. It cant be proportinaly because resistance forces add up in formula. – user707264 Sep 17 '22 at 11:04