“Why bother with endurance, when air-to-air refuelling can make this effectively unlimited? Well, because it gives me an excuse to feature two of my favourite aircraft, and to discuss the difference between endurance and range as aerodynamic drivers.

Looking at the endurance equation, endurance is increased by maximising aspect ratio (a large aspect ratio means long skinny wings), maximising fuel fraction (making sure the a high percentage of the aircraft’s loaded weight is from fuel), maximising propulsive efficiency and minimising profile drag (Ed: I’ll ask Jim to kindly explain what profile drag is in the comments section). 

The Rutan Voyager holds the current record for unrefuelled manned flight, and has a wing aspect ratio of 33.8, and features a twin-boom + fuselage layout, which uses the distributed loading principle to enable the high aspect ratio by reducing bending loads in the wing. The fuel fraction for the aircraft (ratio of max fuel mass to max take off mass) is an extraordinary 0.79 (compare that with the 0.31 of a Eurofighter Typhoon), with the fuel carried in the twin tail-booms. Flight for endurance is undertaken at the minimum power speed*, which for the Voyager varied between 70 and 130 kt, decreasing as fuel was consumed. Profile drag is reduced to the minimum consistent with the volume necessary to contain the engines, fuel and two crew. 

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The second aircraft is the Airbus Zephyr S, current holder of the world endurance record for unmanned aircraft, at 25 days, 23 hr and 57 min. This is the latest development in the Zephyr series of solar-powered unmanned aircraft, conceived and initially developed by my late close friend, Chris Kelleher. The Zephyr again has an extreme aspect ratio, and, although looking relatively conventional in layout, again features distributed loading as the solar panels are distributed across the entire upper surface of the wing. Profile drag is minimised by the minimal fuselage, and by flying at very high altitude.

Other aircraft having military long-endurance application, the Lockheed U-2 and the Northrop-Grumman Global Hawk, are perhaps the most prominent operational systems. But I would nominate the Zephyr here, as its endurance, although nominally set at three months, is essentially indefinite.”

Jim Smith had significant technical roles in the development of the UK’s leading military aviation programmes. From ASRAAM and Nimrod, to the JSF and Eurofighter Typhoon.

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*There are two types of drag: Parasitical, which increases as you go faster (stick your hand out the car window and feel the resistance increase as the car goes faster, that is parasitical drag) and Induced. Induced Drag results from the production of lift. This drag increases as you slow down as the wings have to work harder on the air to provide the same amount of lift. There is a sweet point where these two drags will be the lowest. This is where the engine can keep the aircraft airborne with the minimum amount of power (and so the minimum fuel consumption) which is why it is called Maximum Endurance.