Planes with airframes inducing least amount of drag

[Laurelix]

These are planes that suffer the least from air resistance.

for example a plane like P-47N at Sea Level has 2800 horsepower but only does 590km/h at Sea Level

P-51D on the other hand does 604km/h at Sea Level with only 1630 horsepower because it is very aerodynamic.

 

[Laurelix]

So I'm going to go first:
Yak-3U - 655km/h with 1850 horsepower at Sea Level

 

[AMCKen]

Almost 300 mph from a (supposedly 100hp) 200 ci engine.
Sharp Nemesis - Wikipedia

 

[pbehn]

Almost 300 mph from a (supposedly 100hp) 200 ci engine.
Sharp Nemesis - Wikipedia

Why supposedly? Most migrating ducks and geese can fly at around 50MPH in level flight, how much horsepower do they develop? A 125cc racing motorcycle does well over 130MPH with all the rolling resistance, the rider out in the open etc?

 

[swampyankee]

Almost 300 mph from a (supposedly 100hp) 200 ci engine.
Sharp Nemesis - Wikipedia

Formula 1 engines are all O-200s, however, during races they are routinely run far beyond their normal operating rpm. Another pretty quick O-200 powered aircraft is Ed Lesher's Teal (Lesher Teal - Wikipedia), which still holds a number of records in its class.

 

[swampyankee]

Normal practice in the aero community is to measure airframe efficiency by zero-lift drag coefficient and lift/drag ratio. Zero-lift drag coefficient uses wing area as the reference area.

For sheer airframe efficiency, look to sailplanes. Some of these will get lift/drag ratios over 40, that a 10000 lb aircraft would have 250 lb drag -- it could maintain altitude with a 250 lbf thrust jet engine.