Bronze C Theory

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Bronze C Theory
The Principles of Flight
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Terms Wing Section Chord lineMean Camber
lineAirflow Relative Airflow Boundary
layerStagnation point Angle of Attack
(AoA)Typically 0 to 12 degreesAoA drawing
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Increase Lift by increasing speed or AoA The
forces are balanced unless accelerating.Main
couple.
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Lift increases with AoA until the critical angle
is reached when the airflow can no longer flow
smoothly over the wings upper surface. The
point on the wing where the air becomes turbulent
is known as the transition point
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If we exceed this angle lift reduces sharply.
The wing has now stalled. We have reached the
stalling angle.
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2/3 of the lift generated comes from the upper
surface and 1/3 from the lower surface. Lift
distribution diagram
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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.
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This CP moves forward and back as the AoA
increases or decreases in the normal range.
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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 thesame but at a higher
speed.Next - stall
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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.
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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.
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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.
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Drag is comprised of different elements. The
overall force is known as Total Drag. This is
comprised of Profile and Induced drag
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Profile drag is made up of Form Drag and Skin
FrictionForm 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.
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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
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When discussing the forces in flight the Lift
force is considered to be at 90 deg to the
Relative AirflowDrag is parallel to the Relative
AirflowWeight always acts vertically
downResolution of forces Diagram
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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.
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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.
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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.
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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.
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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.
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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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