Pilots have to promptly know the speed at which they are moving in the mass of air that surrounds the aeroplane and the anemometer is responsible for measuring it.
The anemometer, as it is known today, was designed in 1926 by John Patterson.
Up until then, aeroplanes used anemometers similar to that designed by Leonardo Da Vinci, a flat metal piece that moves against a calibrated graph. When speed increases, the resistance against the sheet increases, making it move.
In order to determine speed, the anemometer uses two values: total pressure and static pressure. When the latter is deduced from the former, the dynamic pressure is obtained.
Total pressure – static pressure = dynamic pressure
Total and static pressure are obtained thanks to the pitot tube or probe, (see photograph), a device that is usually situated on the inside of the wing of a plane, outside the sheet layer.
The anemometer has a scale of colours to aid its comprehension:
Además, el anemómetro tiene una escala de colores para que su comprensión sea aún más fácil:
We can talk of different speed according to the type of aeroplane and altitude at which it flies. For example:
The Mach number is calculated by dividing our speed by the local speed of sound. To calculate the local speed sound, LSS, in English, we have to apply the following formula:
LSS = 38.95 √ Temperature expressed in Kelvin degrees
If you do not know how to convert Celsius to degree Kelvin, it’s really easy: just add 273. That is, -60 degrees Celsius are 213 K.
It is important that we remember the speed of sound only depends of the temperature of the mass of air in which we are.
If an aeroplane can fly at 60 knots and has facing wind also of 60 knots, the speed with respect to the ground is of 0 knots. The aeroplane is still even though the pilot will continue to read 60 knots on their anemometer.
On the contrary, if an aeroplane has wind on tail at 60 knots, the speed with respect to the ground will be 120 knots.
Do you now understand why it is so important to know about the wind in aviation to organise routes, manage fuel and of course, fly safely?
If you think this fact is curious, read on to find answers to the most frequent questions about aeroplane speed.
The speed at which an aeroplane takes off is probably the most repeated question and its answer will vary a great deal, depending on multiple factors.
The take-off speed of commercial flights is calculated before each flight, taking into account the weight of the plane, its altitude, the airport’s altitude, the temperature and many other factors.
Still, as a rule of thumb, we can say that a short and medium radium aeroplane such as an Airbus A320, takes off at 220km/h whereas an Airbus A380, the biggest passenger aeroplane in the world, will do so at 270 Km/h.
In our case, with our Diamond DA20 C1, we take off at approximately 95 km/h. As you can see, take off speed can really vary from one aeroplane to another.
If you want to know more about this topic, we recommend our post about the best angle of climb and the speed for best rate of climb – you’ll love it!
The same as in taking off, speed varies all the time when landing and is dependent on many factors. Even so, to give you an idea we can say that commercial aeroplanes land around 240km/h. In our case, general aviation aeroplanes touch ground at 85km/h.
In Addition, landing speed is very much affected by the flap configuration used. You know flaps are moving slats on wings which open up so aeroplanes can fly more slowly. They are used during take-off and landing.
You can learn more about flaps and other aircraft parts in our post about their similarity to birds ‘wings.
Aeroplanes have limited speed for various reasons. The first one is the maximum time engines can be in use at the maximum speed.
At low altitudes, the maximum speed will depend on structural limitations whilst at high altitude the limitation will be for aerodynamic reasons. In the anemometer, as we saw earlier, aeroplanes have a set VNE, (velocity not to exceed), which varies from model to model.
For example, Airbus 320 cannot exceed a cruising speed of M.79; and general aviation aeroplanes even have a more limiting maximum speed. Our Diamond DA42 have a VNE of 300km/h.
As explained above, commercial aeroplanes measure their cruising speed in relation to the Mach number. We can generally say that they fly around a M.77; this is equivalent to 860km/h or 14 km per minute. Incredible, don’t you think?
The Concorde, the fastest passenger aeroplane ever, would reach a cruising speed of up to 2.35 Mach. That’s 41 Km per minute!
But this is not all! The fastest aeroplane in history, the fighter jet Lockheed SR-71 “Blackbird” would reach an unbelievable 3,540 kilometres per hour, or to put it in other words, almost a kilometre per second.
Imagine travelling almost three times as fast as the speed of sound… From Malaga to New York in a couple of hours!
If you’d like more curiosities about the speed of aeroplanes, don’t miss the post about the effect of Coriolis in which we explain why an aeroplane flies faster towards the east than towards the west.
