Transcript Welcome to the - Kent Gliding Club

The Principles
of Flight
Bronze C Theory
Terms
Wing Section
Chord line
Mean Camber line
Airflow
Relative Airflow
Boundary layer
Stagnation point
Angle of Attack (AoA)
Typically 0 to 12 degrees
AoA drawing
Increase Lift by increasing speed or
AoA
The forces are balanced unless
accelerating.
Main couple.
Lift increases with AoA until the
critical angle is reached when the
airflow can no longer flow smoothly
over the wing’s upper surface.
The point on the wing where the air
becomes turbulent is known as the
transition point
If we exceed this angle lift reduces
sharply. The wing has now stalled.
We have reached the stalling angle.
2/3 of the lift generated comes from
the upper surface and 1/3 from the
lower surface. Lift distribution
diagram
To keep it simple think of the lift
force acting through one point which
is known as the centre of pressure.
This force acts perpendicular to the
chord line.
This CP moves forward and back as
the AoA increases or decreases in
the normal range.
Increase Lift by increasing speed or
AoA. This was slide three. Here is
an interesting point to consider. If
we fill the wing with water we must
fly faster to generate enough lift.
The glide performance remains the
same but at a higher speed.
Next - stall
In the classic stall the CP moves rapidly
from the leading edge at the critical angle
and attaches to the trailing edge producing the marked nose down pitch.
We now know the glider produces lift
when it goes through the air. In a
steady state this forward motion is
opposed by Drag.
If the glider continued to accelerate
the force would increase beyond the
strength of the airframe and so a limit
is imposed. This is called VNE.
Velocity Never Exceed.
While we are on the subject of
Speed, here is a picture of a typical
Air Speed Indicator.
We need to know what the coloured
arcs mean.
Drag is comprised of different
elements. The overall force is known
as Total Drag. This is comprised of
Profile and Induced drag
Profile drag is made up of Form Drag
and Skin Friction
Form Drag is caused by the physical
shape of the glider.
Form drag includes Interference drag
which is caused by airflow meeting at
sharp junctions, wings and fuselage
etc.
As the name suggests Skin Friction is
caused by the relative smoothness of
the wings surface (laminar flow)
Induced drag is lift related and it
reduces as speed increases. Induced
Drag is also load related, as load
increases so does the lift related drag.
Vortices. Winglets
When discussing the forces in flight the
Lift force is considered to be at 90 deg to
the Relative Airflow
Drag is parallel to the Relative Airflow
Weight always acts vertically down
Resolution of forces Diagram
Earlier we discussed the Stall. If we now
consider the wings as two separate items,
we can see that if one wing is going
slower than the other, it is possible for
one wing to stall before the other. At slow
speed Yaw can cause this and precipitate
a spin.
In a spin the inner wing is going slower
and is in a deeper stall, which produces
more drag than the outer wing. This extra
drag causes the glider to continue to
rotate or “Autorotate”.
So now we have to considered that
the ailerons can produce different
amounts of drag from each other.
Alieron Drag/Adverse yaw.
Differential Ailerons. The down
Aileron moves much less than the
up Aileron.
We must now consider how we
control the glider in Pitch.
We do this by increasing or
decreasing the lift generated by the
tailplane to achieve the attitude we
want.
The stability of the glider can be
affected by the cockpit load. If we
reduce the cockpit load the C of G
will move aft and the longitudinal
stability will reduce. On the other
hand if we increase the cockpit load
the C of G will move forward and we
could run out of elevator authority
and be unable to raise the nose.
One final point I want to cover is the
question of Compass errors. In the
northern hemisphere a normal panel
type ball compass will show a turn
to the North when you accelerate
and South when you decelerate.
A basic compass also has a lag
during part of a turn and a lead
during the opposite part. For this
reason to come out of a turn on a set
heading, “undershoot to the North”
& “overshoot to the South”. If you
want to come out on North exit the
turn 30 deg early. If you want to
leave the turn on South leave 30 deg
late. No adjustment East/West
.
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