What is the lowest speed a full-size fixed-wing aircraft can fly without stalling?
• A competent answer would have been 40 kilometres per hour. There is a class of light aircraft known as microlights that stall at speeds slower than 43 kpm. Some specialist aircraft can fly slower, but at the expense of a poor top speed. This is because the main limit to minimum speed is the weight per unit area of wing surface; called the wing loading.
“Microlight aircraft will stall when flying at speeds slower than 43 kilometres per hour”
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Trailing edge flaps, leading edge slats and slots can assist a little, but the complexity and weight of such devices may degrade top speed while only reducing stall speed by around 10 per cent and increasing costs considerably.
An extreme example would be the aircraft that won the for crewed flight after being pedalled across the English Channel at a cruise speed of less than 30 kilometres per hour.
John MacDonald, London, UK
While we were thinking mainly of commercial or military aircraft, we take John MacDonald’s point that our question was – because of reasons of space – ambiguous.
As John says, on 12 June 1979, the Kremer prizewinning Gossamer Albatross completed the 35.8-kilometre crossing of the English Channel in 2 hours 49 minutes, achieving a top speed of 29 kpm and an average altitude of 1.5 metres. It was piloted by amateur cyclist Bryan Allen, whose leg power drove its large two-bladed propellor. Fantastic – Ed
• The lowest stalling speed I know for a powered light aircraft is 50 kilometres per hour for the , a German liaison aircraft from the second world war. The Storch has a low wing loading and is fitted with full-span leading-edge slats, Fowler flaps and ailerons that droop with the flaps.
These all combine to improve airflow over the aircraft’s wing and prevent airflow separation over the rear part of the wing. This contributes to its low stalling speed and allows the aircraft to take off from a runway only 45 metres long and land in just 18 metres.
Thanks to these high-lift devices, at full throttle a Storch can be flown at less than its normal stalling speed. Its nose is well up, so the engine thrust is helping to support the aircraft. I’ve read that it could stay airborne at 35 kph when flown in this fashion.
Martin Gregorie, Harlow, Essex, UK
• The lowest speed at which a plane can remain airborne is stationary, as in Harrier jump jets during vertical take-off. Here, the lift is provided by Newtonian reactive force, but this is achievable only when planes are not fully laden. In conventional flight, where a wing’s lower surface encounters the air ahead of it obliquely and the airflow leaves the wing’s rear obliquely, the thrust required to provide sufficient downforce far exceeds engine capabilities.
At the other extreme, the lowest reported airspeed a Boeing 747 can achieve without dropping as dead weight is 154 kph – if minimally laden on approach for landing. If approaching into a headwind, for every kph the headwind increases, the plan’s airspeed can drop by an equivalent amount and still stay airborne.
Len Winokur, Leeds, UK