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The impulse generated by an engine is based on the total momentum sent out the back, which is proportional to the velocity and mass of whatever you're ejecting; whilst the energy consumed is proportional to the mass and the *square* of the velocity. Hence, an engine that puts out a large amount of mass
at low velocity consumes less energy than one that puts out a small amount of mass at high velocity, for the same impulse.
Airships have a large frontal surface area. I propose that the front of the airship should constitute a scoop, which would funnel large amounts of air through a central tube. This air would then be accelerated out the back, possibly by burning a small quantity of fuel to heat it (see: Air augmented rocket), providing substantially more thrust than a traditional propeller design could achieve.
Inner propeller airship
Sort of what I have in mind, but not for the same reasons. [Selky, Nov 27 2018]
Intriguing [8th of 7, Nov 30 2018]
||// by burning a small quantity of fuel to heat it
||I would recommend helium for the lifting gas.
||The overall propulsor efficiency, which relates the
energy required to overcome the aircraft drag to the
rate of supply of fuel energy, is the product of the
thermal efficiency and the propulsive efficiency. Your
arrangement will have an excellent propulsive
efficiency but a thermal efficiency of approximately
||A turboprop by contrast has an excellent thermal
efficiency (due to all the internal compression of the
working fluid) and achieves a modest propulsive
efficiency by using a propeller to accelerate a large
mass of air by a fairly small amount.
||Right. See also Genesis 5:21-24.
||Never was a fan of Genesis, just prog-rockers.
||Something like a Dyson "air multiplier" using multiple high-pressure jets around the inner circumference of an inflated duct, combined with an airfoil shape, might have potential. And don't neglect the possibilities of the Coanda effect to generate some extra lift.