THE EFFECT OF WIND

; Days with no wind at all are rare. Strictly, the effect of wind on flight is not a problem in :» aerodynamics but one of human perception, psychology and understanding. It nevertheless f seems necessary to make a brief statement since despite innumerable attempts to correct > falsehoods, the errors come up repeatedly in conversations at club level and in otherwise f reputable model magazines. Even some of those very experienced persons who set out to f teach beginners how to fly are seriously confused about this topic and perpetuate the №• misunderstandings. The true facts have been known for more than a century and have been j> amply demonstrated in practice.

: It is wise to take off and land into the wind because this reduces the speed over the ground

-i at the moment of leaving it or arriving on it. Landing or taking off downwind or across the wind produces a much longer ground run, with more chance of the aircraft swerving to one side, running out of room or striking a bump and tipping over. Once airborne this effect disappears and airspeed, not groundspeed, is what matters.

The air low down is slowed by contact and friction with the ground, producing the so – called wind gradient. Coming down through the wind gradient to land has the model passing from a fast moving airstream into one that is nearly stagnant. This can precipitate a premature stall and heavy arrival, so a little extra airspeed is advisable during die final approach, to allow for this. Climbing out after take off, the model passes from the slow moving air at ground level into the brisker flow a few metres above. This causes a surge in airspeed which may require some slight trimming action from the pilot. Above a certain level, and maintaining a more or less constant height, the wind gradient does not affect the model.

On a windy day, the air low down tends to be more turbulent so it is necessary to maintain slighdy higher airspeed when near the ground, to ensure that control is retained. This is true whether the model is flying into the wind direction, across it, or downwind when the gust strikes. At higher levels, the air is usually relatively smooth. An occasional gust can still occur but there is enough height to recover without danger.

The effect of wind on a glider attempting to make headway against it, is dealt with in Fig.

4.2 and associated text.

Inside the hangar the air is moving with the ship. An indoor flying competition is being held. The models are flying in a mass of air which is moving at 20 knots. The models behave just as they would in a steady wind of this speed. There are no strange inertia or momentum effects caused by turning this way or that. The model fliers are moving with the air so they feel no wind.

The more serious muddle concerns flight when the model is well above ground.

Despite innumerable authoritative published corrections and clarifications, there are many modellers who still believe that they should trim and control their aircraft differently when flying upwind or downwind and making turns. They do not understand that airspeed and groundspeed are two quite different things, but judge the speed of the model by its apparent motion relative to their own position. Certain types of model, and certain wing profiles, are. said to be sensitive to wind direction, models are said to surge upwards when faced into wind, and sag when flying downwind, and so on. It is even claimed sometimes that model engines run faster when the model is going against the wind and lose revolutions or overheat when they are facing the other way. This is all nonsense. The corrective actions which are sometimes recommended actually cause accidents rather than preventing them.

A wind is the movement of a huge body of air as a whole. When a model is in flight it is totally in the air and all forces and reactions on it, including inertia, kinetic energy and momentum, result from its passage through the air with no influence at all from the ground below other than gravity. The aircraft does not feel the wind passing over the ground — it is in the air and the flow over it is generated by its own airspeed. This has nothing whatever to do with the motion of the air mass itself as a whole over the ground.

Two analogies may be helpful: A balloon floats in the air and if there is any wind, moves with it. Passengers in the basket feel no wind blowing them along. If they put out a flag it hangs straight down even if flags on masts below are fluttering briskly. Tbe balloonists see the ground moving by at the speed of the wind. If one of them could launch a model aeroplane from the basket and control it from this position, it could be made to fly round and round the balloon in circles with no reference whatever to the ground. There would be none of the supposed surges and trim alterations because the pilot would be moving with the air in which the model would be flying. Flying upwind, downwind or turning in any direction, would be all the same.

Imagine flying a model aeroplane inside a large enclosed cabin, such as an empty furniture van moving on the road, or inside the enclosed hangar on an aircraft carrier at sea, or in the cabin of a huge airliner flying at 600 knots. The package of air inside the enclosed space is moving rapidly relative to the ground. The model may fly in any direction at all inside the moving air package, with no effects whatever coming from the motion of the air itself relative to the ground or sea.

The model pilot, however, is on the ground and feels the wind as a flow of air in a certain direction. From this fixed position it is easy to forget that the model is not influenced by the sensations felt on the ground. To control a model safely the pilot needs to think of the model as a thing in the air, and fly it accordingly. Pilots of full sized aircraft do this automatically for the most part and there is no reputable text book or flight instructor in full scale aviation, which confuses airspeed with groundspeed in the way modellers commonly do.