# Relationship between liftl drag and thrustt weight

### AP4ATCO - Lift/Drag Ratio, Forces Interaction and Use - SKYbrary Aviation Safety

Friction drag is not affected by the angle of attack change. We neglect the changed lift contribution of engine thrust when the angle of attack is. Relationship of forces acting on an aircraft. Rearward component of weight. Thrust. Drag. Flight path. Relative wind. Component of weight opposed to lift. Lift. CL. It is the properties of the air, changed by the shape of the wing, that generate the Lift, Weight, Drag, Thrust, relationship.

In the reverse situation, when during steady and level flight thrust is reduced, the aircraft will start to accelerate in the direction of drag the speed will start to decrease. The decrease in speed will lead to a decrease in drag. The shape of an airfoil and other lift producing devices i. Since the glide ratio is based only on the relationship of the aerodynamics forces acting on the aircraft, aircraft weight will not affect it.

The only effect weight has is to vary the time that the aircraft will glide for. The heavier the aircraft is, the higher the airspeed must be to obtain the same glide ratio. Both aircraft will cover the same distance but the lighter one will take a longer time to do so. From the practical point of view one should remember that although it is well known that winglets reduce drag and save fuel, their effect on speed control may not have been highlighted before.

Crews therefore need to be aware that reduced drag makes speed control on the approach more difficult. For example, if by design an airplane must be able to accelerate vertically upwards then the thrust must be greater than the weight and drag combined.

The engine creates thrust and moves the plane forward. Gravity provides the thrust for a glider. The engines push air back with the same force that the air moves the plane forward; this thrust force-pair is always equal and opposite according to Newton 's 3rd Law. When thrust is greater than drag, the plane accelerates according to Newton's 2nd Law. When the plane flies level at constant velocity, thrust equals drag. When the plane flies level at constant velocity, all opposite forces of flight are equal: How the 4 forces of flight interact Drag opposes thrust.

Imagine sticking your hand out the window of a moving car and flying your hand. Pressure Recovery Any object moving through the air will have a high-pressure region in front, but a properly streamlined object will have a high-pressure region in back as well, resulting in pressure recovery.

### 4 Lift, Thrust, Weight, and Drag

The flow pattern 2 near a non-streamlined object is not symmetric fore-and-aft because the stream lines separate from the object as they go around the sharp corners of the plate. Streamlining is never perfect; there is always at least some net pressure drag. Induced drag also contributes to the pressure drag whenever lift is being produced even for perfectly streamlined objects in the absence of separation.

The pressure drag of a non-streamlined object is much larger still. This is why on high-performance aircraft, people go to so much trouble to ensure that even the smallest things e.

## Class Discussion

An important exception involves the air that has to flow through the engine compartment to cool the engine. A lot of air has to flow through narrow channels. Unlike pressure drag, friction drag cannot possibly be canceled, even partially. Friction drag causes energy to be thrown overboard.

The energy gets carried away by the relative wind, and there is no practical way for the airplane to recover that energy. The way to minimize friction drag is to minimize the total wetted area i. The way to reduce form drag is to minimize separation, by making everything streamlined. However, there are other ways of looking at things. Similarly, there is a coefficient of lift: One nice thing about these equations is that the coefficient of lift and the coefficient of drag depend on the angle of attack and not much else.

If you could by magic hold the angle of attack constant, the coefficient of lift and the coefficient of drag would be remarkably independent of airspeed, densitytemperatureor whatever.

The coefficient of lift is a ratio 3 that basically measures how effectively the wing turns the available dynamic pressure into useful average suction over the wing.

A typical airfoil can achieve a coefficient of lift around 1. The left side of the figure corresponds to the highest airspeeds lowest angles of attack.

Note that the coefficient-of-lift curve has been scaled down by a factor of ten to make it fit on the same graph as the other curves. Airplanes are really good at making lots of lift with little drag. Coefficients versus Angle of Attack In flight, we are not free to make any amount of lift we want.

The lift is nearly always equal to the weight times the load factor. This leads us to rearrange the lift equation as follows: Weight can change, too, but usually only slowly.

Extending the flaps can change the area somewhat. Because of the factor of airspeed squared, the airplane must fly at a very high coefficient of lift in order to support its weight at low airspeeds. The lower coefficient of lift means the airspeed must be higher, in order to support the weight of the airplane. This explains why the the stalled regime of the power curve lies below the critical point. It plots the same four curves against airspeed rather than angle of attack.

Now the left side of the plot corresponds to the lowest airspeeds highest angles of attack. Coefficients versus Airspeed At higher angles of attack approaching or exceeding the critical angle of attack the basic-model approximations break down. The coefficient of parasite drag will rapidly become quite large, and the induced drag will probably be quite large also.

There will be no simple proportionality relationships.

• The Four Forces

We see that whereas the coefficient of parasite drag was more or less constant, the force of parasite drag increases with airspeed.