Ground effect and downwash
By Nirmoy Ghosh, Yash, Rahul Kar
Downwash
can be explained as the deflection in the air or the change in the direction of
the air due to action of the wing or the airfoil. It’s basically a process in
which Lift is produced and depends on the velocity of the downwash or we can
say the velocity of the air deflected. Another definition could be as the by-product of lift is called as downwash.
The tornado type dusty formation of air is created around the helicopter due of
downwash. In aircraft, it is equal to a component of force produced by airflow
over the wings. The amount of air deflected is proportional to the lift of the
wing.
Here’s
a better explanation to the concept of downwash:
The
white fog generally seen at the wingtip vortices is actually circular motion of
air which produces upwash and downwash in aircraft. The air behind the wing is
going almost straight down when seen from the ground, but actually it’s not.
The pattern created by the airflow behind the wings are not a cylinder or tight
column, it is cone shaped which is created naturally. The air behind the wings
in the airflow on both the sides comes to equilibrium and they do not
contribute to the thrust of the system.
The
parameters which generally influence downwash are :
Angle
of attack, Lift, drag. The decrease in the velocity of the air diverted ,will
increase the angle of attack of the wing which results in increase of vertical
lift. As the speed increases the angle of attack decreases and the vertical
lift reduces. Since Lift is the main factor in both the case therefore it can
be concluded as the lift is proportional to the angle of attack of the wing. The effective angle of attack of the
wing is thus decreased by the flow that
is induced by the downwash.
The
downwash can be reduced to zero by keeping the chord line at a certain angle
below the horizontal but it also changes the lift force, noticeably and it is not
suitable for any type of aircraft.
Coming
to ground effect which basically occurs due to compressed vortices of air. The
increase in life created by ground effect is attributed to reduction in amount
of induced drag which as a result improves the lift/drag ratio.
Ground
effect is the reduced drag that is imposed in the wing when the height is
reduced and the plane approaches a fixed surface
The
earth helps in flying the aircraft but actually it doesn’t , as the air flow
below the aircraft is restricted when the aircraft is flying at a height equal
to half of its wing span which reduces the induced drag as the airflow over the
wingtips is very less and thus producing ground effect or cushion effect.
When
the aircraft is experiencing ground effect:
I.
It has more Vertical
lift, opposing weight.
II.
It has less rearward
lift, reducing drag.
III.
Smaller the vortices and hence
less downwash, reducing drag
During
the ground effect more power is available which allows lifting heavier weights.
So basically flying at lower altitudes reduces induced drag which reduces
ground effect. During take off ground effect facilitates by increasing lift
whereas during landing it produces a bouncy effect which is controlled by
reducing the velocity and gradually as ground is approached
These
are the concepts of downwash and ground effect and their effect on each other.
