In flying model aeroplanes the performance of a machine will often vary unaccountably from day to day. With the same motor and winding, the model will fly much higher and more freely at different seasons of the year. It is well to bear in mind that in the summer months the heat causes a low density. The pressure exerted by the atmosphere is therefore correspondingly small; the model, or for that matter a large machine, travels much faster. A dry day also tends to cause low density. This will account for the excellent flights on warm, dry days and the crankiness of the machine when the weather is damp.
Cold weather on the other hand tends to increase the density of the air. The speed of an aeroplane is materially reduced but on the other hand the air will be found more buoyant. Such weather is better for heavy models, although they fly much more slowly. It will be seen that at high altitudes, where the density is least, models should fly both faster and higher than they will nearer the sea level.
One of the curiosities of the air is the effect of eddies created by the passage of aeroplanes. Let an aeroplane move fast enough through the air and it will create high density above its course, and low density below its line of flight. This condition may also be found in the wake of model aeroplanes. It will often be noticed when two models are flying near together that the disturbance caused by one will seriously interfere with the second machine. The contrast between the air above and below the models in flight in especially noticeable when the planes are flexed.
[Illustration: Two of the earlier Peoli models]
A great deal has been learned of the action of the air upon aeroplanes by photographing the air currents. The smallest eddies of the air have been made visible by taking instantaneous photographs of thin smoke as it passes obstacles of various size and form. It has been found that a square object causes a great deal of disturbance when in an air current. The air is compressed in front of it and eddies for some distance in its wake before finally coming to rest. An elliptical object causes less disturbance, but the air continues to splash in its wake for some distance. Even a perfectly spherical object offers a surprising amount of resistance to the air. In the case of a long narrow ellipse the disturbance is considerably reduced. A curved surface however, such as is used for the planes of an aeroplane, cuts the air with practically no resistance, and the air flows smoothly about and joins behind it with very little wake or splashing. A form which suits the air in this way or a "stream line" body as it is called is obviously just the right design for the wings of an aeroplane.