Section 1 EO M231.01 – IDENTIFY THE FOUR FORCES THAT ACT UPON AN AIRCRAFT

Every aircraft has weight, which influences the design and performance of the aircraft.

The weight of an aircraft is the force that acts vertically downward toward the centre of the Earth and is the result of gravity.

The gliders used in the Air Cadet gliding program are towed to their determined altitude by a tow-plane. There are other methods of getting altitude, such as using a winch to get up to speed on the ground.

An aircraft gains energy as it gains altitude. The energy that the glider gains as it is taken to its determined altitude can be spent quickly in a rapid descent to Earth or it can be spent slowly in a long descent.

Drag is the resistance that any object experiences as it moves through the air.

Cadets will have experienced the resistance of air on their bicycles or just walking on a windy day.

Effort is put into aircraft design to minimize drag.

Cadets avoid drag when they lower their head and shoulders on a bicycle to gain speed.

The design of an aircraft can minimize drag but cannot avoid it entirely. The faster an aircraft is designed to fly, the more sleek and streamlined its design is likely to be.

A parachute is designed to maximize drag by catching air and using it to slow descent.

An aircraft can use drag to control flight and manoeuvre by pushing on the passing air.

As demonstrated in TP3, a glider moves forward as it descends, rather than falling straight down. It accomplishes this by acting on the air in a manner similar to a cadet diving into water.

A glider is always gliding downwards through the air, but by locating atmospheric lift (rising air) to offset the downward motion of the aircraft due to gravity, the pilot can actually gain altitude and fly great distances without needing to use artificial lift again.

Thrust is a force that moves an aircraft forward. A glider spends the energy it has gained and moves forward by trading the speed of descent for forward motion. It gets this control by using its weight to push upon the air below. With its nose lowered, it slides forward over the air below.

A glider’s wings are designed to project out into the passing air. Glider’s wings are usually very large for the size of aircraft because a glider depends on its wings to develop lift without help from an engine or a propeller. As air moves over and under the wing, the air is used by the wing to generate lift.

The purpose of a glider’s wings is not to go fast to minimize descent. The object of soaring is to get as much forward distance as possible, while losing as little altitude as possible for each unit of energy that the glider loses in descent. The distance travelled forward compared to the altitude lost is referred to as glide ratio. This should be a very large number such as 30 metres forward for each metre of descent.

The glider’s wing is designed to develop lift because lift reduces the rate of descent while allowing forward motion. The lift of the aircraft’s wing will counteract the aircraft’s weight, to a degree, and this will improve the aircraft’s glide ratio. Generally, the larger the wing, the more lift can be developed.

A wing generates lift by acting upon the passing air in a highly sophisticated manner that will be explored in the next lesson.

A powered aircraft also experiences weight, drag and lift as does a glider. However, while a glider can gain forward motion only by trading the energy of its descent for thrust, a powered aircraft can generate thrust by running its engine. In this case, thrust is provided to the aircraft via a driven propeller or a high-speed jet exhaust.

Show the cadets a slide (OHP or PPT), or paper handouts, of the four forces that act upon an aircraft shown in Figure B-1.

On the other hand, the engine adds weight to the aircraft and both the propeller and engine body add to the drag that the aircraft experiences. A powered aircraft, therefore, will usually not have the high glide ratio of a glider.

A powered aircraft, though, can attain equilibrium, which is something a glider cannot do. Equilibrium is a condition where lift equals weight or thrust equals drag. Pilots often refer to this as flying straight and level.

• If lift is greater than weight, the aircraft will climb higher. • If weight is greater than lift, the aircraft will descend. • If thrust is greater than drag, the aircraft’s forward speed will increase. • If drag is greater than thrust, the aircraft’s speed will decrease.

A glider can fly even though it does not produce its own thrust. It can fly even though its weight is greater than its lift. However, in the Earth’s gravity, its flight is limited by atmospheric conditions and the pilot’s skill. On a day without wind, even the most skilful pilot will soon return to Earth after being released.

With a powered aircraft, descent can be delayed by turning the energy of burning fuel into thrust because thrust can then be turned into lift by the aircraft’s wings.