Sir - In reply to the letter "The dangers of down-wind turns" (Flight International, 13-19 December, 1995), I believe that Mr Maskens is barking up the wrong tree. The dangers of low-level turns have nothing to do with "the sum of potential and kinetic energy", but everything to do with pilot perception and wind gradient.
On a day when there is a wind, for example, if Mr Maskens were to fly an aeroplane in a trimmed, steady, level circle at, say, 2,000ft (610m), would the aircraft descend when going downwind and, conversely, climb when going into wind? The answer is no, the aircraft will maintain its level circle. In my experience, this would also be the case were the exercise to be conducted at low level.
When turning at low level, the view out of the window is different; when going into wind, the ground speed is lower; and, when going downwind, the ground speed is higher. The increase in ground speed when turning from into wind to down wind is a powerful visual stimulus and the automatic, natural, response of any pilot not used to low-level flying would be to slow down the aircraft.
If the aircraft is climbing or descending, then wind gradient becomes a factor and, when going down wind, climbing through a wind gradient will lead to a loss of some airspeed. If flying near the stall, either of these problems could be deadly - the two together are doubly so. Hence the old sayings about flying low and slow.
D le ROUX
Horsham, West Sussex, UK
...Sir -The majority of engineers confuse aerodynamic effects, with mass dynamics effects, resulting in the belief that kinetic-energy effects are relative to the air mass, not that of earth.
An understanding of the effect may be aided by answering the question, "How do you measure [or detect] the accelerative force, required to increase the ground-speed, in a downwind turn?"
D C BENOY
Poole, Dorset, UK
Source: Flight International
