How gliders fly

7.   Altimeter, (calibrated in feet or metres).

 

8.   Compass.

 

9.   Electric variometer control panel.

 

10.  GPS moving map display.

 

11.   VHF aeronautical radio.

 

12.   Joystick, (operates ailerons & elevator).

 

13.  Yellow, (launch) cable release.

 

14.  Black Undercarrige lever, (if fitted).

 

15.  Trim lever, (usually painted green).

 

16.  Wheel brake.

 

Basic cockpit instruments & controls

1.   Airbrake lever, (blue handle). 

 

2.   Canopy release lever.

 

3.   Turn & slip indicator.

 

4.   Variometers, (mechanical and electrical).

 

5.   Canopy emergency jettison.

 

6.   Airspeed Indicator, (calibrated in knots).

 

The images above illustrate the typical basic instrument layout of a modern glider.

The blue lever, (1) on the left is the air brake lever. The lever has an over-centre lock to lock the air brakes closed. When extended, the air brakes create drag and slow the glider down, the nose of the glider needs to be lowered to maintain speed while the brakes are open resulting in a steeper glide angle. The air brakes which extend as paddles from either the top of the wing, (or both top and bottom), allow the descent for landing to be controlled. The air brakes are also used to loose height quickly.

 

A glider canopy may have either one or two red canopy latches, (2) used to lock the canopy before flight. All canopies have an emergency jettison lever, (5) to be used in the very rare occurrence where the pilot may have to bail out.

 

The turn and slip indicator, (3) has two main components, a ball in an oil filled glass tube. When the ball is centred, the aircraft is being flown in co-ordinated flight, (being flown correctly without any 'slip or skid'. A glider may also have a simple piece of string taped to the outside of the canopy in front of the pilot above eye level. This string is called a 'yaw string' and helps the pilot to maintain a zero side-slip angle by keeping the yaw string centred on the canopy.

The turn indicator is a gyroscopically driven instrument which indicates the angle of bank, (turn) at which position the aircraft is being flown. A needle is used to indicate turn rate against a calibrated scale. The turn & slip indicator is the minimum required instrument for cloud flying.

 

The variometer, (4) is either a mechanical or digital instrument, (or combination of both). A simple mechanical variometer indicates the rate of descent or climb rate of the glider. Calibrated in knots or metres per second, the variometer will indicate a descent/climb in either knots per minute or metres per minute. If the needle on the instrument, (calibrated in knots) showed + 2 knots, this would indicate the glider is climbing at 200 feet per minute. Without any lift the glider will be descending and this descent rate indicated by the instrument. Just like a bicycle free-wheeling down hill, the glider maintains it speed by descending through the pull of gravity toward the Earth. If the glider flys into air that is going up faster than the glider is descending, it will climb and this is how gliders can maintain flights of many hours, sometimes over great distances.

 

An electric variometer will provide much more information to the pilot, average rate of climb and will tell the pilot the best speed to fly at for the best performance, generally speaking the stronger the lift the faster the pilot can afford to fly. Some digital variometers record and show the path of the glider while it is circling or flying in thermal lift, useful to determine the centre and strongest part of the thermal, (thermals are the warm updraughts of air caused by the Sun heating the ground, typically on a summer day). Electric variometers often have a loudspeaker which indicates the strength of lift or sink by an audible tone. The faster or higher the pitch of the tone, the faster the glider is climbing. A lower tone indicates sink. Such audible devices allow the pilot to maintain a good look out without having to refer to the instrument all the time which could be hazardous when thermalling/climbing in close proximity to other gliders. Often an electric variometer will have a separate control instrument, (9) to adjust its settings and volume.

 

The airspeed indicator, (6) is normally calibrated in knots, (sometimes MPH). Our airspeed indicator above is calibrated in knots and the scale calibrated with 10 knot increments. One knot = 1.15 MPH. Gliders often cruise in the range of 40-50 knots, depending on the design of glider and this would be its best glide speed in still air, (air which is neither going up nor down).

 

The altimeter is either calibrated in feet or metres. Our altimeter indicates a height of 950 feet. The small hand on the dial indicates thousands of feet, the large hand indicates hundreds of feet. The altimeter has a rotating knob that allows the pilot to set the altimeter to zero on the airfield, (QFE). When flying locally most glider pilots use an airfield height setting, (QFE) which will need to be set to zero each day to compensate for changes in atmospheric pressure. In flight the pilot will know his/her height above the ground. Above sea level we refer to height as altitude, (QNH).

 

Aircraft will often set their altimeters to sea level height (QNH) especially when travelling distances. As the barometric pressure in the atmosphere is always changing, this allows all aircraft to have the same sea level pressure settings applied to their altimeters. This is very important otherwise aircraft will be flying with different pressure settings and thus flying at different altitudes, you don't want this situation when flying off on your holiday in an airliner! 

 

The compass, (8) is self-explanatory. In this case the compass ball is floating in a fluid filled chamber.

 

Many pilots now fly with GPS systems and moving map displays which can be used on cheap mobile devices. One such application is PocketFMS  which will show the position of the aircraft relative to controlled airspace and provide a useful navigation aid but is no substitute for a regular map/chart and looking out of the cockpit!  Click on the number ''10'' graphic in the cockpit layout schematic to learn more about a typical GPS application for an Android or Apple iOS device.

 

Many gliders carry either a portable VHF aeronautical radio or one that is permanently installed in the glider instrument panel. There are a few specific special glider frequencies that a glider pilot can use without having to own a pilot's radio licence issued by the Civil Aviation Authority. Such radios will still require a Home Office licence which only costs a few pounds for a three-year licence period.

 

The bank and pitch of the glider is controlled by the stick, (joystick). Push forward on the stick and the glider nose will go down and the glider will gain speed. Pull back on the stick and the glider will climb and loose speed, much like a bicycle free-wheeling uphill. The stick is moved left and right to bank/turn the glider, rudder pedals, (feet operated) are used along with the stick in the turn to balance the drag caused by the ailerons which are located, (control surfaces that move in opposite directions) near the tips of the wings. When the stick is moved either left or right to initiate a turn, the ailerons which control the rate of turn generate drag which has noticeable leverage over the gliders very long wings. We call this drag, aileron drag or adverse yaw, the effect of aileron drag is offset by using the rudder operated by the feet, in the direction of the turn, (right stick input with right rudder) to ensure that the glider fuselage is always pointing in the direction of the air flow. If we just used the stick to initiate the turn to the right without rudder, the glider will start to turn in our chosen direction to the right but then yaw away from the direction of the turn to the left while inducing a lot of undesirable drag from the fuselage. The rudder is used to correct for this undesirable effect of the ailerons. Powered aircraft have shorter wings and therefore less tendency toward the effect of aileron drag/adverse yaw.

In order to stop a turn and level the wings, we simply turn the glider in the opposite direction using both ailerons and rudder, centralising the controls when the wings become level.

We reduce the bank when the glider is at the required bank/turn angle, by taking off the bank by moving the controls in the opposite direction until the required bank/turn rate is achieved. The glider will then continue in the turn with the controls more or less centralised, except we need to maintain some backward pressure on the stick, (up elevator) to stop the nose lowering while in the turn. Corrections will need to be made on the ailerons, elevator and rudder to maintain a constant turn rate while thermalling, especially in gusty conditions.

 

All aircraft have a trim lever, (15) or wheel which is used to reduce the heavy feel of the airflow acting on the elevator at the back of the glider, the elevator control surface that controls the pitch, (up and down) movement of the aircraft. The pilot simply moves the trim lever forward or aft, (backwards) until the pressure needed to hold the stick is reduced for the given speed that the pilot decides to fly at. This feature reduces pilot fatigue and the aircraft can be trimmed to fly 'hands off' the stick without no tendency to pitch up or down unless disturbed by air movements.

 

All gliders have a yellow cable release, (13) which is located on the left side of the cockpit or on the centre pedestle of the instrument panel. The yellow cable release handle is pulled to release the glider from a towing aircraft or release the winch cable at the top of a winch launch. If the glider pilot does not pull the cable release at the top of the winch launch, the release mechanism has a safety feature that will release the cable when backward force is applied to the release mechanism on the bottom of the glider fuselage.

 

Some gliders have retractable landing gear, (14) which allows the main wheel to be stowed away to reduce drag from the airflow.

 

One piece of equipment not mentioned in our list is the parachute. Parachutes are worn by most glider pilot's because gliders fly in close proximity to each other, especially while climbing in rising air currents. There is a small risk of collision especially in competition gliding when there can be a high concentration of gliders in one thermal all competing for the best lift. 

Parachutes provide a cushion for the back, many gliders have a recessed seat back to accommodate the parachute. The parachute is also a handy way of adding extra ballast, particularly if the pilot is very light. Our parachute has a slim design, manufactured for aircraft that fly slowly such as gliders. Weighing in at up to twenty pounds, once the ripcord is pulled the time to full deployment can be as little as three seconds!

When you fly with us, we will provide you with a parachute for your comfort in the cockpit.  Charles Forsyth.

How a wing generates lift

by Mike Harris

The theory of lift is a large subject and can not be fully covered here. However what follows is a brief introduction to how a wing generates lift and therefore how a glider flies.

This is a picture of Daniel Bernoulli a Swiss Mathematician born in 1700 who pioneered a lot of the early work on energy and in particular how energy applied in fluid flow. Incidentally he also qualified as a Doctor and Anatomist but that’s another story.

Back to the fluid flow which has a direct link to lift and how a wing works.

His mathematical work on fluid flow was governed by the principal of conservation of energy.

Simply put this states that;

Total Energy = Potential Energy + Kinetic Energy

Where Potential energy is (air in our case) the 'fluids static pressure',

and Kinetic Energy which is its energy due to movement or its 'Dynamic Energy'.

 

This statement means that if you increase the speed of a fluid (in our case Air), in order for the total energy to remain constant the pressure has to drop.

Above is a diagram demonstrating Bernoulli’s Theorem.

In it you can see that as fluid (air for us) passes through a restriction and the volume flowing remains the same on the left and right of the restriction, then the speed of flow must increase through the restriction and in order for the total energy to remain constant the pressure must drop.

 

Look at the shape of the restriction used here and compare it to the cross sectional shape of a glider wing shown below.This shape by the way is known as an Airfoil.

Can you see that the upper surface looks similar to the lower surface of the restriction?

This means that air flowing over the wing is forced to move over the upper surface of the wing faster and therefore MUST be causing a reduction in pressure which we see as LIFT.

This simply is the explanation of lift and is how a glider (or in fact any aircraft utilising an Airfoil, and this includes Helicopters and Drones) is able to fly.

 

Finally there is an experiment you can try at home that proves in a practical way all that has been said so far.

If you set your kitchen tap to provide a steady smooth stream of water and hold a spoon (note that the convex side of the spoon resembles an aerofoil shape) suspended between thumb and fore finger like a pendulum then offer the concave side of the spoons bowl to the flow of water you will see it repelled from the flow. Then offer the convex side into the flow and you will see that it is sucked into the stream of water, beautifully demonstrating that a streamline flow over the upper surface of an aerofoil (wing) creates lift.

 

Mike Harris is our club Chairman, airline captain and gliding instructor based at Essex Gliding Club.

Learn about the types of lift available to glider pilots

To learn how glider pilots exploit

 rising air, please CLICK the CLOUD BUTTON to go to Wikipedia where the types of lift used by glider pilots are explained. 

If you have read this far you might be planning to visit us to try gliding for the first time, it would be a good idea for you to bring along three items of kit show below, don't forget the fluids, especially if it is a hot day. Airfields can be very hot and exposed places during the summer!

Published on Sep 21, 2014

This video introduces the basic principles of flight.

Cambridge University Gliding Club.

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