FORCES IN FLIGHT
The 4 forces in flight
Gravity, Lift, Thrust and Drag.
Gravity is a force that is always directed toward the centre
of the earth. The magnitude of the force depends on the mass of all
the aircraft parts. The gravity is also called weight and is
distributed throughout the aircraft. But we can think of it as
collected and acting through a single point called the centre of
gravity. In flight, the aircraft rotates about its centre of
gravity, but the direction of the weight force always remains toward
the centre of the earth.
Lift is the force generated in
order to overcome the weight, which makes the aircraft fly. This
force is obtained by the motion of the aircraft through the air.
Factors that Affect Lift
Object : Shape & Size
Motion : Velocity & Inclination
Air : Mass, Viscosity & Compressibility
Lift force = 0.5 *
V2 * wing's lift coefficient * wing area
The wing's lift coefficient is a dimensionless number that depends on the
airfoil type, the wings aspect ratio (AR), and is proportional to the angle of
attack (AoA) before reaching the stall angle.
is the force generated by some kind of propulsion system. The
magnitude of the thrust depends on many factors associated with the propulsion
- type of engine
- number of engines
The direction of the force depends on how the
engines are attached to the aircraft.
The glider, however, has no engine to generate thrust. It uses
the potential energy difference from a higher altitude to a lower altitude to
produce kinetic energy, which means velocity. Gliders are always
descending relative to the air in which they are flying.
Drag is the aerodynamic force that opposes an aircraft's motion
through the air. Drag is generated by every part of the aircraft (even the
There are several sources of drag.
One of them is the skin friction between the molecules of the air and
the surface of the aircraft. The skin friction causes the air near the
wing's surface to slow down. This slowed down layer of air is called the
boundary layer. The boundary layer builds up thicker when moving from
the front of the airfoil toward the wing trailing edge. Another factor is
called the Reynolds effect, which means that the slower we fly, the thicker the
boundary layer becomes.
drag is another source of drag. This one depends on the shape of
the aircraft. As the air flows around the
surfaces, the local
velocity and pressure changes. The component of the aerodynamic force
on the wings that is
opposed to the motion is the wing's drag, while
the component perpendicular to the motion is the wing's
the lift and drag force act through the centre of pressure of the wing.
With cambered airfoils, the Centre of Pressure position varies
with changes in Angle of Attack, whereas with
its position is fixed and coincident with the airfoil's Aerodynamic
Centre (AC). For rectangular
wings at subsonic speed, the Aerodynamic
Centre lies at 25% of the chord length measured from the leading
Induced drag is a sort of drag caused by the wing's
generation of lift. One cause of this drag is the flow near the
tips being distorted as a result of the pressure difference between
the top and the bottom of the wing, which
in turn results in swirling
vortices being formed at the wing tips. The induced drag is an
indication of the amount of
energy lost to the tip vortices. The
swirling vortices cause downwash near the wing tips, which reduces the overall
lift coefficient of the wing.
The picture below shows the downwash caused by an
The Cessna Citation has just flown through a cloud. The downwash
from the wing has pushed a trough into the cloud deck. The
swirling flow from the tip vortices is also evident.
The wing geometry (aspect ratio AR) also affects the amount of induced
drag :Long wing with a small chord (high AR) has low induced drag,
whereas a short wing with a large chord (low AR) has high-induced drag.
For the same chord, the wing with a high AR has higher lift coefficient,
but stalls at lower angle of attack (AoA) than the wing with a low AR.
Also, aircraft with high AR wings are more sensitive to elevator
The induced drag increases with increasing of the wing's actual lift
coefficient being generated and it's proportional to the square of the
angle of attack. And since a slower airspeed requires a higher
angle of attack (AoA) to produce the same lift, the slower the airspeed
is, the greater the induced drag will be. So, the induced drag is
also inversely proportional to the square of the velocity.
order to minimize tip vortices some designers design a special shape for
the wing tips. With drooped or raised wing tips, the vortex is
forced further out.
However, this method will cause an increase in weight
since they need to be added to the wing tip.
An easier and lighter method is by cutting the wing tip at 45-degrees.
With a small radius at the bottom and a relatively sharp top corner, the
air from the secondary flow travels around the rounded bottom but can't
go around the sharp top corner and is pushed outward.
There's also the Interference drag, which is generated by the mixing
of streamlines between one or more components, it accounts for 5 to 10%
of the drag on an airplane. It can be reduced by proper fairing
and filleting which allows the streamlines to meet gradually rather than
All drag that is not associated with the production of
lift is defined as
The graph below shows the induced and the
parasitic drag versus airspeed. Total drag is the induced
drag plus the parasitic drag.
Since during constant speed and level flight the thrust is equal to the
total drag the graph also shows how much thrust is needed at different level
At take-off (just above the stall speed), a high AoA is
needed to get enough lift which increases the total drag and also the thrust
needed. As the speed increases, the AoA needed to get the same lift
decreases and so does the total drag until the minimum drag speed is reached,
above which the total drag starts increasing exponentially (and so does the
thrust needed). The plane's max level speed will be limited by the prop's
pitch speed or by the max thrust available, which altogether means by the max