Conditions of level flight

Conditions of level flight

When a body keeps travelling at a steady height at uniform velocity in a fixed direction, this state of steady flight is known as equilibrium. In order to do this the forces acting on it must be balanced – the lift must be equal to the weight (this condition will keep the aeroplane at a constant height); and, the thrust must be equal to the drag (this condition will keep the aeroplane moving at the same steady velocity).

For an aeroplane, when there is no load on the tail plane the conditions of balance
for un-accelerated flight are these:
a) Lift = Weight, i.e. L = W.
b) Thrust = Drag, i.e. T = D.
c) The “nose-down” pitching moment of L and W must balance the “tail-down”
pitching moment of T and D.
The two forces, L and W, are two equal and opposite parallel forces, i.e. a couple;
their moment is measured by “one of the forces multiplied by the perpendicular
distance between them.” So, if the distance between L and W is x metres, the
moment is Lx or Wx Newton-metres.
Similarly T and D form a couple and, if the distance between them is y metres, their
moment is Ty or Dy Newton-metres.
Therefore the third condition is: Lx (or Wx) = Ty (or Dy)

Difficulties in balancing:

In practice, there are lot of difficulties in achieving
balancing for maintaining level flight. For normal flight modes, changes in AOA will
causes changes in the CP, thus the lift component which acts through the CP will
change as the AOA changes.
The weight which acts through the CG depends on every individual part of the
aircraft and will vary depending on the distribution of passengers, crew, and freight
and fuel consumption.
The line of action of the thrust is set in the basic design and is totally dependent on
the position of the propeller shaft or the center line of the exhaust jet.
The drag may be found by calculating its component parts separately or by
experiment with models in a wind tunnel.
Any change in the angle of attack means a movement of the lift, and usually in the
unstable direction; if the angle of attack is increased the pitching moment about the
centre of gravity will become more nose-up, and tend to increase the angle even
There is a possibility of movement of the centre of gravity during flight caused, for
instance, by consumption of fuel, dropping of bombs or movement of passengers.
The line of thrust is settled by the position of the engine or engines.

The four forces do not, therefore, necessarily act at the same point so that equilibrium can only be maintained providing that the moments produced by the forces are in balance. In practice, the lift and weight forces may be so designed as to provide a nose-down couple (Figure 7.2(a)), so that in the event of engine failure a nose-down gliding attitude is produced. For straight and level flight the thrust and drag must provide an equal and opposite nose-up couple.


However, the design of an aircraft will not always allow a high drag and low thrust line, so that some other method of balancing the flight forces must be found. This involves the use of the tail plane or horizontal stabilizer. One reason for fitting a tail plane is to counter the out-of-balance pitching moments that arise as a result of inequalities with the two main couples. The tail plane is altogether a lot smaller than the wings, however because it is positioned some distance behind the CG, it can exert considerable leverage from the moment produced (Figure 7.2(b)).

At high speed the AOA of the main plane will be small. This causes the CP to move rearwards creating a nose-down pitching moment. To counteract, this tail plane will have downward force acting on it to re-balance the aircraft. Quite clearly, following the same argument, for high AOA at slow speeds, the CP moves forward creating a nose-up pitching moment. Thus, tail planes may need to be designed to carry loads in either direction. A suitable design for this purpose is the symmetrical cambered tail plane, which at zero AOA will allow the chord line of the section to be the neutral line.
Most tail planes have been designed to act at a specified AOA for normal flight modes. However, due to variables (such as speed) changing AOA with changing load distribution and other external factors, there are times when the tail plane will need to act with a different AOI, to allow for this some tail planes are moveable in flight and are known as the all-moving tail plane.

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