An idiot’s guide to aircraft design – Part 2: How to build an aircraft with great endurance
“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.
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.
Hush-Kit 
Some explanatory comments:
Profile drag
There’s a lot of confusing nomenclature out there around drag. From my perspective (and I recognise that different perspectives and usage exist) the three main drag components are
1. Profile Drag, which is the drag of the aircraft when it is producing no lift, alternatively called the zero-lift drag. The profile drag itself may have different components – I’ll come back to that, but for the moment profile drag is the drag when there is no lift.
2. Lift-dependent drag is the drag caused by the generation of lift. It is usually represented as being proportional to lift coefficient squared, and inversely proportional to Aspect Ratio. Aspect Ratio is defined as (span squared)/(wing area), and is a measure of , as Hush_Kit puts it, the skinniness of the wings. The higher the Aspect Ratio, the lower the lift dependent drag.
3. Wave drag, which occurs once locally supersonic flows appear on the aircraft. This starts when the aircraft is flying below the speed of sound because the shape of the aircraft, and the generation of lift by the wings, speeds up the local air flow, and reduces its pressure.
There are some other causes of drag – the ones above are the main three airframe-related ones. Other drag components include some propulsion ones, including
Intake momentum drag – caused by the slowing down of the air so that it is at an acceptable speed for the engine;
Spillage drag – if the airflow into the intake is too large, the excess may result in additional drag
Boat-tail drag – arises if flow separates over the rear nozzle or rear fuselage.
Other airframe related drag can arise due to:
Excrescence drag – where small components of the aircraft interfrere with the local flows and cause additional drag
Interference drag – generally, but not exclusively where components intersect with each other, the junction flows can cause undesirable drag. In addition close proximity between, for example, stores and drop tanks, can raise local Mach numbers and cause high speed interference drag
Drag due to flow separation – if the smooth orderly flow breaks down, for example at the stall, or because a strong vortex is being shed, this will generally also increase drag.
If you’ve got this far, you may be interested in the components of the three main drag sources mentioned above.
Profile drag is sometimes considered to have components due to the shape of the aircraft, or its form, thus sometimes being called form drag, and due to skin friction, or boundary layer drag.
Form drag arises as a result of the pressure distribution around the aircraft, and is generally reduced through the use of high fineness ratio ‘streamlined’ shapes, which maintain smooth flows and avoid rearward facing suction surfaces.
Boundary layer drag is caused by the loss of momentum of the flow as it decreases from the free stream value outside the boundary layer to zero at the surface of the aircraft. Streamline forms which avoid flow separations os disturbance of the boundary layer help here.
Lift-dependent drag is a consequence of the deflection of the air as the lifting surface generates lift. It is also seen in the momentum deficit in the trailing vortices behind the aircraft, and. indeed in other shed vortices, for example from flaps, strakes or slender bodies.
Wave drag is a result of locally supersonic flows in transonic flight, or wholly supersonic flows in supersonic flight. If local supersonic flows exceed Mach numbers of about 1.2, research (by myself in the 5m wind tunnel) and experimental observation, going back at least as far as the Sabre, shows that local shock waves are likely to appear. A shock wave is a sudden increase in pressure, and drag may be caused through loss of energy in the shock wave itself, and through the impact of the pressure rise which will thicken the boundary layer or cause it to separate, in either case resulting in increased drag. Because local increases in Mach number may be caused by the increased velocity over lifting surfaces, or by increased velocity around non-lifting bodies (under-wing stores, or tanks, or the aircraft fuselage for example, wave drag may have both lift-dependent and lift-independent components.