WINGS AND THINGS …………………………… Mike White

(or why an aeroplane flies, not how it flies!)

I expect most members have seen, at some time, the diagram entitled “The forces acting on an aeroplane in flight” (Fig 1) which says that :-

thrust = drag and lift = weight

It then says that if any one of these forces changes, it will affect the others. As written Fig 1 is saying then, that the diagram only applies to an aircraft flying at a fixed altitude and at a constant airspeed. Please keep this in mind for the duration of this read.

There are two main opinions as to how lift is generated. One is the Newtonian Theory where his Third Law of Motion is used and the reaction is from the wing downwash. This is not the popular opinion, though. The more accepted one, using Bernoulli’s Theorum, says that if the speed of a gas/fluid is increased across a surface, the surface pressure decreases. It can be seen in wind tunnel demonstrations (Fig 2) how the airflow follows the wing skins and, as the top surface is more highly curved than the bottom, it must travel a little faster and, therefore, be at a lower pressure than that of the bottom surface. If the aerofoil shown is divided into a number of sections and manometers (pressure reading instruments) are placed at each division, top and bottom, the pressure difference may be measured and recorded and then plotted as in Fig 3.

The highest point shown is the Centre of Pressure or Centre of Lift. Although the aircraft is flying as stated in para 1, ie straight and level, there is a force known as the Moment Coefficient (Cmo) which is exerting a nose down rolling moment to the wing about the centre of gravity. (Fig 4). A counter force must now be exerted to prevent this and this is the job of the horizontal stabiliser which is set at a negative incidence** to the wing to provide the down force necessary. This down force changes with speed and, on aircraft with a fixed stabiliser, this change is countered by the elevator trim. The elevator does not make the aircraft climb and descend, it is only a trimming device which adjusts their rates! (Please read that again). On larger, faster aircraft, an all moving stabiliser is fitted. Next time you go “across” and enter the aircraft at the rear steps, look up at the stabiliser and you will see a bright plate on the fuselage just in front of the stabiliser leading edge on which are some numbers. This is its range of movement, the more negative settings being those for changing flap positions.

** Incidence. The angle measured between the fuselage datum and the surface chord line.

Regarding the previous statement about the elevator not making the aircraft climb and descend. Return now to para 1. ie. if any force changes it affects all the others. If you want to climb and put in and hold some up elevator so increasing the wings angle of attack, you will increase the lift which increases the induced drag (induced drag = drag due to lift). Speed now reduces resulting in a reduction in lift and the aircraft descends to its original height approximately. This is called an auto stabilised system.

How then is the aircraft made to climb and descend? To climb the thrust has to be increased, speed therefore increases, as does lift, and up you go. The elevator setting is now changed to give the required rate of climb. The reverse is, of course, the case for a descent.

Many of our models are grossly overpowered and flown too fast which gives the impression that the elevator makes it climb and descend. Thrust still is the climbing force, (even with models), the elevator only changes the trim to make the model climb at a higher or lower rate. Next time you fly, trim for straight and level flight, cross wind, at about three-quarters power. Now, without touching the elevator stick, put on full power. Your model will climb.

If high do not push the nose down because the airspeed will increase, lift will increase and the model will climb back again. Holding in some down elevator to get you to the spot will increase the airspeed, the model will touch down too fast and a heavy, bouncy landing will result. In this case all you can do is go around and try again If too low, then add a little burp of power which will get you back to a better height to make a greaser landing where you want it to be.

Click below to view illustrations:

Figs 1, 2, 3 and 4