Every since the first fragile ‘stick and string’ aircraft staggered into the sky during the first years of the 20th century, the pilot has needed to know just how fast he is going. Not only because he uses this value to help him determine his position by ‘dead reckoning’ (time x speed = distance run), but also when he is entering dangerous areas of the ‘flight envelope’ of his aircraft. Being close to the stalling speed of the machine is lethal, but also there is a ‘never exceed speed’, above which lies structural failure. There are various other speeds which it is important to know, such as the safe speed at which flaps may be deployed, or the speed at which the undercarriage must not be extended (if retracted, of course).
Modern aircraft often have a ‘glass cockpit’ display which gives readouts of airspeed from digital sensors, but even these have an old-style analogue dial, as a back-up. The usual two inputs needed to produce an ASI readout are the static and ram-air pressures, one being the ambient pressure (which changes with altitude and atmospheric conditions) and the other being the pressure due to the aircraft’s movement through the air. These are usually obtained from a Pitot tube (named after the French engineer, Henri Pitot), located a little distance from the fuselage, to avoid air disturbances, although large, modern aircraft can have several 'static ports' flush to the skin of the aircraft, in various locations, to give an average of the static pressure. The ASI became part of the ‘basic six’ instruments ,which were instantly recognizable by any Royal Air Force aircrew member from the 1930s onwards.
Up until the mid-1930s there was another method – crude but adequate – used in some low-performance machines. Here we see a photograph of the air speed indicator on a DH 85 Leopard Moth, G-AIYS, parked with its wing folded, at the Great Vintage Flying Weekend at Keevil Airfield. All it consists of is a flat plate, with a few holes drilled in it, secured to a length of spring wire. As the air pressure builds up due to movement of the aircraft, the plate is forced backwards, giving a readout of the estimated speed on the graduated scale. There is even a red section, indicating when you are approaching stalling speed! Needless to say, this device IS crude, but it works well enough. The Leopard Moth’s bigger brother, the De Havilland DH 84 Dragon airliner has a similar indicator, and I dare say that this was the last time a commecial airliner flew with such a device. By the way, the first time I entered the cockpit of a DH 84 Dragon, I noticed a flashlight clamped to the left hand side of the bulkhead. When I asked what it was, I was told that it was an essential piece of the night-flying equipment….at night, you shone it out to port, and illuminated the graduated scale between the wings to find out your airspeed!