Aerodynamic forces
Essentially there are 4 aerodynamic forces that act on an airplane in flight; these are lift, drag, thrust and gravity (or weight).
In simple terms, drag is the resistance of air (the backward force), thrust is the power of the airplane’s engine (the forward force), lift is the upward force and gravity is the downward force. So for airplanes to fly and stay airborne, the thrust must be greater than the drag and the lift must be greater than the gravity (so as you can see, drag opposes thrust and lift opposes gravity).
This is certainly the case when an airplane takes off or climbs. However, when it is in straight and level flight the opposing forces of lift and gravity are balanced. During a descent, gravity exceeds lift and to slow an airplane drag has to overcome thrust.
The picture below shows how these 4 forces act on an airplane in flight
Thrust is generated by the airplane’s engine (propeller or jet), gravity is a natural force acting upon the airplane and drag comes from friction as the plane moves through air molecules. Drag is also a reaction to lift, and this lift must be generated by the airplane in flight. This is done by the wings of the airplane…
The generation of lift is a widely discussed and sometimes disputed theory, but there are some key factors that nobody argues. A cross section of a typical airplane wing will show the top surface to be more curved than the bottom surface. This shaped profile is called an ‘airfoil’ (or ‘aerofoil’) and the shape exists because it’s long been proven (since the dawn of flight) that an airfoil generates significantly more lift than opposing drag.
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During flight air naturally flows over and beneath the wing and is deflected upwards over the top surface and downwards beneath the lower surface. Any difference in deflection causes a difference in pressure (‘pressure gradient’) and because of the airfoil shape the pressure of the deflected air is lower above the airfoil than below it, hence the wing is ‘pushed’ upwards by the higher pressure beneath.
One of the argued theories of lift generation is related to Newton’s 3rd Law of Action & Reaction, whereby the air being deflected downwards off the lower surface of the wing creates an opposite reaction, effectively pushing the wing upwards. This may well be the case but it’s the pressure difference between both surfaces that is the primary factor of lift generation.
Above: the general movement of air over an airfoil
If you want to generate some lift yourself, try holding a sheet of paper in front of your face and blowing hard over its top surface. The air molecules above the sheet get deflected differently to those below as you blow, so a pressure gradient appears and the higher pressure below the paper pushes it up. Of course, the physics behind proper lift generation over an airplane wing are somewhat more complex, but the end result is the same.
Above: have a go at generating some lift yourself!
The faster a wing moves through the air, so the actions are exaggerated and more lift is generated. Conversely, a slower moving wing is less efficient at lift generation. It’s important to note, though, that different wing designs (airfoil and shape) generate lift more (and less) efficiently than other designs at different speeds, depending on what the plane has been designed for.
A direct reaction to lift is drag and this too increases with airspeed. So airfoils need to be designed in a way that maximizes lift but minimizes drag, in order to be as efficient as possible.
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Airflow over an airfoil Flight is one of the most important achievements of mankind. We owe this achievement to the invention of the airfoil and understanding the physics that allow it to lift enormous weights into the sky. All flight is the result of forces acting upon the wings of an airplane that allow it to counteract gravity. Contrary to popular belief, the Bernoulli principle is not ...
A crucial factor of lift generation is the Angle of Attack – this is the pitch angle at which the wing sits in relation to the horizontal airflow over it. As the Angle of Attack increases so more lift is generated, but only up to a point until the smooth airflow over the wing is broken up and so the generation of lift cannot be sustained. When this happens the sudden loss of lift results in the wing stalling and the weight of the airplane cannot be supported any longer. When a stall occurs a sudden loss of altitude is inevitable unless the pilot rectifies the situation immediately by increasing the airplane’s airspeed.
Airplane control surfaces
For an airplane to be controllable, control surfaces are necessary. The 4 main surfaces are ailerons, elevator, rudder and flaps as shown below:
To understand how each works upon the airplane, imagine 3 lines (axis – the blue dashed lines in the picture above) running through the plane. One runs through the center of the fuselage from nose to tail (longitudinal axis), one runs from side to side (lateral axis) and the other runs vertically (vertical axis).
All 3 axis pass through the Center of Gravity (CG), the airplane’s crucial point of balance.
When the airplane is in forward flight it will rotate around each axis when movement to any control surface is made by the pilot. The table below shows the appropriate actions…
Action:Axis:Controlled by:
RollLongitudinalAilerons
PitchLateralElevators
YawVerticalRudder
The following sections explain how each control surface effects the airplane…
Ailerons
Located on the trailing edge (rear) of the wing, the ailerons control the airplane’s roll about its longitudinal axis. Each aileron moves at the same time but in opposite directions i.e. when the left aileron moves up, the right aileron moves down and vice versa.
This movement causes a slight decrease in lift on the wingtip with the upward moving aileron, while the opposite wingtip experiences a slight increase in lift. Because of these subtle changes in lift the airplane is forced to roll in the appropriate direction i.e. when the pilot moves the stick left, the left aileron will rise and the airplane will roll left in response to the change in lift on each wing.
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The ailerons are controlled by a left/right movement of the control stick, or ‘yoke’.
Rudder
The rudder is located on the back edge of the vertical stabilizer, or fin, and is controlled by 2 pedals at the pilot’s feet. When the pilot pushes the left pedal the rudder moves to the left. The air flowing over the fin now pushes harder against the left side of the rudder, forcing the nose of the airplane to yaw round to the left.
Elevators
The elevators are located on the rear half of the tailplane, or horizontal stabilizer. Like the ailerons they cause a subtle change in lift when movement is applied which raises or lowers the tail surface accordingly. In addition, air hitting deflected elevators does so in the same way as it hits the rudder i.e. with an exaggerated effect that forces the airplane to pitch upwards or downwards.
Moving the elevator up (pulling back on the yoke) will cause the airplane to pitch its nose up and climb, while moving them down (pushing forward on the yoke) will cause the airplane to pitch the nose down and dive. Elevators are linked directly to each other, so work in unison unlike ailerons.
Flaps
Flaps are located on the trailing edge of each wing, usually between the fuselage and the ailerons, and extend downward (and often outward) from the wing when put into use. The purpose of the flaps is to generate more lift at slower airspeed, which enables the airplane to fly at a greatly reduced speed with a lower risk of stalling. When extended further flaps also generate more drag which slows the airplane down much faster than just reducing throttle.
Although the risk of stalling is always present, generally speaking an airplane has to be flying very slowly to stall when flaps are in use at, for example, 10 degrees deflection. Obviously though stall speeds and safe airspeeds vary from airplane to airplane.
So all these factors are why and how airplanes fly. Radio control model airplanes can of course be more simple – for example, just have rudder and elevator control or perhaps just rudder and motor control. But the same fundamental principles always apply to all airplanes, regardless of size, shape and design.
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