Flexwing Controls

Flexwing aircraft are primarily controlled by means of shifting the weight of the pilot in relation to the weight of the aircraft. If the pilot wants to turn left, he will pull the left wing towards himself and therefore shift the weight to the left. As the right of the aircraft is now relatively lighter than the left, the left will lose height, and hence cause a turn and bank(or roll) to the left.

Pitching moments are achieved by moving the pilot to the back or front of the aircraft, and this is effected through the movement of the trapeze bar ie., by the pilot pushing or pulling the bar from his chest. If he pulls the bar, he will actually move his weight forward in relation to the aircraft, and thus the nose will become heavier and the aircraft will execute a forward pitching moment. If he pushes the bar forward, the tail will become heavier, resulting in exactly the opposite effect.

Aerofoils and Angle of Attack.

Certainly, one of the most important principles of flight pertains to why an aircraft actually flies. If you take a standard aerofoil (cross-section of a standard wing), it can be seen that the chamber at the top actually measures a longer distance from the leading edge to the trailing edge, had you to compare it to the distance below the wing.

AEROFOIL SKETCH.

THIN NORMAL AND THICK AERFOIL SKETCH

Bernoulli.s principal proves that the air which collides with the leading edge of an aerofoil as it moves through the air, has to split, causing some of the air to travel over the top of the wing and some across the bottom. The air over the top and the air across the bottom both have to reach the trailing edge at the same time. The air travelling along the chamber(the top) of the wing, has a much longer distance to travel than the air across the bottom, and therefore the air over the top has to accelerate to reach the trailing edge at the the same time as the air travelling across the bottom. As the air over the chamber then accelerates, it’s pressure has to decrease (because of its increased velocity) in comparison with the air travelling across the bottom, and hence a sucking force is provided in the direction of this area of decreased pressure. This sucking force is a force called lift, and the whole principle can be demonstrated by placing a teaspoon with it’s curvature side against a small stream of water running from a tap. It will be seen that the water sucks the spoon towards itself and does not push it away.

If the chamber is increased (ie., by using a thicker a i rfoil), the air travelling over the top of the wing has a longer distance to trael in comparison with the air travelling across the bottom of the wing. The result is then, more lift.

DOWNWASH AND DRAG SKETCH

As lift is created, a proportional amount of drag is created. Drag is therefore directly proportional to lift. The air washing down and behind the wing also produces a downwash force equal to the amount of lift created by the wing beacause, for every action there is an equal an opposite reaction. This re-action to the lift force can also be referred to indirectly as ground effect. The air washing down is trapped between the wing and the ground and forms a cushion at that height, which could improve the speed of the aircraft and its carrying capacity for that moment. It is iportant to note that ground effect can only be experienced more or less one half the length of an aircraft wing above the ground. So, if an aircraft flies any higher than that, the ground effect will not be in force any more.

Allthough every square centimeter of the wing’s chamber produces lift, the sum of all this lift is enacted through one single point, called the centre of lift. This could be demonstrated by suspending an object from a piece of string. On most standard wing shapes today, the most lift is produced at the highest point of the chamber, in other words, the thicker the chamber the more lift is produced.

CORDLINE SKETCH

A line drawn from the centre of the leading edge to the centre of the trailing is called the cord line. The air travelling in an opposite and parrallel path of the path of travel of the aircraft is known as Relative Air, and hence the airflow over the wing at any point is then reffered to as the relative airflow.

ANGLE OF ATTACK SKETCH

The angle derived by intersecting a line which depicts relative airflow with the cord line, is know as the angle of attack.

INCREASED ANGLE OF ATTACK SKETCH

As the angle of attack is increased, the distance between the leading edge and the trailing edge, via the route of the chamber, is also increased. This means the air flowing over the top has a longer distance to travel in relation to the air flowing across the bottom, and this in turn means the air over the top has to speed up more and hence, more lift is produced. It is now clear that lift can be increased by using a thicker chamber, or by increasing the angle of attack. It is also clear that when the angle of attack is increased, the lift force is increased, and as a direct result drag is also increased.

STALL SKETCH

As the angle of attack is increased, so the centre of lift will move towards the leading edge. If the angle of attack is increased to much, the chamber’s curvature will be to steep

for air to flow around it, and the airflow will be disturbed, thus causing what we know as a stall. Most wings will stall at an angle o f f between 15 and 18 degrees.

A STALL IS CONDITION OF FLIGHT WHEREBY THE ANGLE BTWEEN THE CORD LINE AND RELATIVE AIRFLOW EXCEEDS THE CRITICL ANGLE OF ATTACK WHICH LIES BETWEEN 15 AND 18 DEGREES ON MOST WING SHAPES.

SPIN SKETCH

A SPIN IS A CONDITION OF STALLED FLIGHT WHEREBY AN AIRCRAFT WILL, YAW, PITCH AND ROLL UPON ITS 3-AXIS’ SIMULTANEOUSLY.

The Principle of Height and Speed.

FORCES SKETCH

There are four main forces enacting upon an aircraft at any point and time. Lift and it’s opposite, weight or gravity and Thrust and it’s opposite, drag. An aircraft flying at a constant altitude of 5000 feet at 100 mph, will have all these forces in equilibrium. In other words, lift = weight and thrust will be equal to drag. If the same aircraft accelerates from its starting position on the runway, thrust will be greater than drag and lift will be equal to weight.

An aircraft standing on the runway, cannot get airborne. As thrust is produced and the aircraft starts moving and generates airflow over the wing, the awing will start producing lift. Therefore it can safely be said that height is controlled by thrust.

As an aircraft flies along and the angle of attack is increased, drag is also increased, which means the aircraft will now fly slower. It can therefore also safely be said that speed is controlled by the attitude of the aircraft.

Height and speed can therefore be controlled by applying a simple formula.

CLIMB SKETCH

Climb: To initiate a climb, we first have to apply power, which will produce more lift. We then change the attitude of our aircraft. Once we obtain the correct speed, we can trim the aircraft to continue the climb

The pnuemonic used here is PAST
......................................................P - POWER
......................................................A - ATTITUDE
......................................................S - SPEED
......................................................T - TRIM


SETTLE SKETCH

Settle: To settle is to terminate the climb and fly straight and level again. To settle we first have to gain speed to compensate for the intented loss in power, which in turn will terminate the climb. So, to gain speed we change the attitude of our aircraft. Once we obtain the correct speed, we can reduce power and trim the aircraft to continue the straight and level.

The pnuemonic used here is ASPT
.......................................................A - ATTITUDE
.......................................................S - SPEED
.......................................................P - POWER
.......................................................T - TRIM


DESCEND SKETCH

Descend: To descend we first have to gain speed to compensate for the intented loss in power, which in turn will initiate a descend. So, to gain speed we change the attitude of our aircraft. Once we obtain the correct speed, we can reduce power and trim the aircraft to initiate the descend..

The pnuemonic used here is ASPT
.......................................................A - ATTITUDE
.......................................................S - SPEED
.......................................................P - POWER
.......................................................T - TRIM


LEVEL OUT SKETCH

Level out: To Level out, we first have to apply power, which will produce more lift. We then change the attitude of our aircraft. Once we obtain the correct speed, we can trim the aircraft to continue the straight and level

The pnuemonic used here is PAST
......................................................P - POWER
......................................................A - ATTITUDE
......................................................S - SPEED
......................................................T - TRIM


The Principle of the Horizon.

When flying an aircraft it is important to find a a way right from the start to keep the aircraft straight and level all the time. Whether flying a fixed wing or a flexwing, the rpinciple remains the same.

In a fixed wing aircraft it is easiest to judge the distance between the dashboard and the horizon. If the distance between the dash and the horizon increases, you are in a descend, and if the distance decreases, you are in an ascend. Once you have managed this, keep the distance the same, and the aircraft will stay level.

In a flex wing, it is easiest to take a mark on your profile tube allmost in line with your eyesight. If the mark on the profile tube goes below the horizon, you are descending, and if the mark on your profile tube goes above the horizon, you are in an ascending process.

The Circuit Layout.

Whilst learning to take off and land, you will be required to fly a certain pattern to facilitate, taking off, turning, flying straight and level, lining up for the runway, performing a landing and so on. This procedure or pattern is called a circuit. All circuits are flown in a rectangular format.and there are very specific and important rules pertaing to this format.

CIRCUIT SKETCH

All runways on the surface of the earth are marked on a magnetic headwind. A compass rose would show north as 0 degrees, south as 180 degrees, east as 90 degrees and west as 270 degrees. If a runway lies direct east west, the western point of the runway would be number as 9 and the eastern end would be marked as 27. In aviation, the last digit is ommited for ease of use. Also in aviation, one always points towards a direction and wind always comes from a direction. Therefore, if the aircraft is parked, ready for take off on the eastern end, it would point in the direction of 270 degrees, and hence it would be standing on runway 27.In the same token, if it was parked on the western end and pointed towards heading 90 degrees, it would be ready for take-off on runway 9. This take-off position is also called the holding point.

All circuits are allways performed to the left off the runway used. If the aircraft therefore took off on runway 27, it would have to turn 90 degrees to the left to position itself on the crosswind leg. The turn onto crosswind and downwind may never exceed a 15 degree angle of bank and it may also not be performed below a height of 300 foot above ground. The aircraft would again have to turn 90 degrees to the left (now 180 degrees from it’s original direction) to position itself on the downwind leg. This leg is called the downwind leg, because one will always take-off into wind. A further 90 degree turn to the left will position the aircraft onto a left-base for runway 27. The turn for left-base may only be made once the aircraft lies at 45 degrees to the holding point and the angle of bank may

now exceed 15 degrees. The last turn is also 90 degrees to the left again, and now the aircraft is positioned on a final for runway 27.

Landing.

AIRCRAFT LANDING SKETCH

TRIKE LANDING SKETCH

It is important to be able to fly the aircraft straight and level by using the methods as explained earlier, for once you have mastered that part, the same principle can be applied to landing the aircraft.

The same way you line up the dash with the horison or your marker on the profile with the horizon, the same you will line up your aircraft with the first 100 meters of your runway. If the dash or the marker on your profile tube is higher than the mark on your runway, you are to high and so on.

Sedcondly it is also most important to approach the field using your descending principles as explained earlier.

Turns

LIFT IN TURN SKETCH

A wing flying straight and level, would produce x amount of lift. The lift produced, would be measured 90 degrees to the surface upon which it acts. When the wing is banked, the same amount of lift is still produced. Allthough gravity still acts vertically upon the wing, the lift vector is 90 degrees to the flying surface, which now lies at an angle, and therfore, it can be seen that less lift is produced in relation to the amount of gravity enacting upon the wing.

It is evident that when the aircraft is banked to execute a turn, lift will be lost, which in turn will cause the nose of the aircraft to drop. To counteract this, more lift must be generated in a turn. If you apply the principle of the horizon, you should have no pro b lem in executing turns.

It is also most important to remember lookout at this stage. No turn should ever be executed without looking out to either side of the aircraft. If a turn to the left is anticiptated, first look to the right and sweep the vision across the horizon to the let and behind the aircraft. If a turn to the right is anticipated, lookout to the left first, sweep vision across the horizon to the right and behind the aircarft, and then execute the turn if safe to do so.

Clinbing turns should never exceed a 15 degree angle of bank and medium turns should never exceed a 30 degree angle of bank. In a fixed wing, with the wing above the pilot, a 15 degree angle of bank would be indicated by the lower wing touching the horizon. A 30 degree angle of bank would be indicated by the horizon intersecting with the leading edge approximately halfway between the wing tip and the wing root (where wing is attached to fuselage) of the lower wing. In a fixed wing aircraft with a low wing, a 15 degree angle of bank would be indicated by the higher of the two wing tips touching the horizon. A 30 degree angle of bank would be indicated by the horizon intersecting with the leading edge approximately halfway between the wingtip and the root of the higher wing

In a flex wing, the same principle as that of a fixed wing with the wing above the pilot, applies. Most flex wing aircraft, by nature of their design are not allowed to do more than 30 degree angle of bank in turns, and therfore we do not discuss those in this section.