Bronze C Theory

The Principlesof Flight

Terms Wing Section

Chord lineMean Camber line

Airflow Relative Airflow Boundary layer

Stagnation point Angle of Attack (AoA)

Typically 0 to 12 degreesAoA 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 thesame 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 Form DragDrag 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

AirflowDrag is parallel to the Relative

AirflowWeight 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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