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What was the most efficient aircraft design (based on factors such as lift, drag, and fuel consumption) but not developed or scrapped due to issues with long term viability and maintenance, operations and repair?

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    $\begingroup$ but not developed - We can't truly know that, because companies generally don't publish their designs until they are being developed. I'm sure someone will come up with some things that reached the prototype stage, but this is inherently an unknowable thing. $\endgroup$ Mar 20 at 15:19
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    $\begingroup$ And not developed means we do not have data. From design to a certified aircraft there are many changes. Simulations cannot give you accurate figures. And you forget one important part of equation: engines (which are usually designed or modified for a specific aircraft, so to calculate drag and efficiency is impossible) $\endgroup$ Mar 20 at 16:01

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The most efficient designs, based on lift, drag and fuel consumption, for heavier than air craft, were the ones that could fly slowly enough and still have sufficient Reynolds numbers to generate lift efficiently.

Some, like the Handley Page HP 42 could lift 15 pounds per horsepower and fly at 100 mph. The contemporary Douglas DC-3 could lift 10 pounds per horsepower but fly it twice as fast.

The C-130J Hercules can lift 8 pounds per horsepower but fly it at 400 mph.

Aircraft gain advantage in True Air Speed by flying higher, thereby faster, to maintain the same Indicated airspeed in thinner air.

However, at altitude, a (turbine) compressor is needed to provide enough air to maintain enough thrust for the same IAS.

So the most efficient design may be ... a jet powered aircraft with an excellent, glider-like lift to drag ratio, flown at altitude.

Modern airliners are designed in precisely this way$^1$.

$^1$ ton-miles per gallon fuel consumed is one way of looking at it. The faster plane can then become more efficient.

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