Online Flight Training
Lesson 2: Level Flight and Coordinated Turns
At the end of this lesson you should be able to:
- List the fundamental forces of flight
- Describe how the four forces are related to each other while the plane is in level, unaccelerated flight
- Describe the aerodynamics of how and ultralight turns
- Understand adverse yaw
- Know the three axes of flight and what control surfaces control movement about them
- Describe how lift is produced and its mathematical relationship to airspeed and induced drag
- Describe three ways a pilot can increase lift
In addition you will show in your practical lessons that you can fly basic maneuvers with the the plane.
The fundamental forces of flight
Four fundamental forces act on a plane when in unaccelerated flight: Lift, thrust, drag and gravity. Only two or these four forces are adjustable as you manipulate the trainer's controls: thrust and lift. The other two forces, drag and weight (gravity) change also, but are not controllable.
Lift is the force produced mostly by the movement of air over the wing of an aeroplane. As air strikes the leading edge of the wing, part of the airflow moves over the top of the Wing and another flow moves under the wing
Because of the curvature or camber of the Wing, the airflow directed over the top of the wing has a greater distance to travel. Therefore, it is travelling faster relative to the airflow travelling underneath the Wing.
In the 18th century, a Swiss mathematician named Daniel Bernoulli discovered that as the speed of a fluid (air can be considered a fluid) increases, its pressure decreases. This is known as the Venturi effect.
This principle, when applied to the airflow over a wing, means that an area of lower pressure exists on top of the wing, in contrast to the relatively higher pressure on the area underneath. The higher pressure under the Wing creates the lift an airplane needs to be able to overcome earth's gravity. In a sense, the wing wants to move from the area of higher pressure to the area of lower pressure.
Actually, about 70-75% of the lift an airplane's wing produces is due to this Venturi effect (pressure differential); the remainder is due to Newton's third law of motion which states: "for every action, there is an equal but opposite reaction". As large quantities of air are deflected downward from the Wing, the net result is that a reaction in the opposite direction results, adding to the total lift. This force accounts for nearly 25% of the total lift a wing produces
The amount of lift a wing produces is partly due to its speed through the air and partly due to its design. Generally, a wing moving through the air at a given angle will produce four times the amount of lift if the speed of the wing is doubled. (We will be discussing this angle, called the angle of attack in the next lesson).
Con versely, the total lift will be reduced by four times if the airplane's speed is reduced by one half. Wings also have different shapes and designs which affect the amount of lift they can produce. Every wing has an upper limit to the total lift it is capable of producing, and there is also a price to pay for an increase in lift. It is called drag.
The force that opposes lift, and the one that kept man on the ground for so long, is called gravity or weight.
When an airplane is in straight and level flight and is not speeding up or slowing down, the forces of lift and gravity are balanced (equal).
The force of gravity (or weight) acting on your ultralight trainer can change as it turns or otherwise maneuvers. Naturally, it also changes as you add or remove weight to your ultralight. This is an important concept to remember for later lessons. For example, during steeper turns additional forces called loads are placed on the trainer's wings. These loads are measured in units called 'G's, in which one G is equal to the force of gravity as if you were standing on the earth's surface, or flying straight and level.
In straight and level and unaccelerated flight the weight that the wing must support is 1 G. This represents the combined weight of the ultralight and its contents.
Loads imposed on an airplane of any kind also increase the speed at which the airplane's wing will stop producing lift at a given angle. This is called a stall. We will study about the phenomenon of stalls and stall speed in more detail in lessons 3 and 5.
A propeller produces lift and drag because it is essentially an air/ai/like the wing, only the resultant lift is produced horizontally. We call this force thrust.
Apropeller has a leading edge, a trailing edge, and camber, just like a wing.
With either a pusher-type (propeller in the rear), or a "tractor" (propeller in the front) ultralight design, the principle is the same: thrust is produced in a forward direction, opposite drag.
How a propeller works
The speed at which a propeller rotates varies along its length from the hub to the tip. The propeller tips are moving much faster than the part near the hub because they have a greater distance to travel in the same amount of time. To keep the amount of thrust the same along its length, propellers are designed with a "twist" along the length, that is, the blade angle changes along the propeller blade relative to its plane of its rotation.
This twist ensures that each part of the propeller is creating relatively the same amount of thrust. Without this shape, each part of the propeller would be creating a different amount of thrust, which is not good.
The angle at which the blade meets the air near the propeller's tip is reduced, along with the blade's thickness. This is done to reduce the higher stress placed upon the tip, due to its higher speed. Propeller speeds can also be reduced through the use of a gear or bell reduction drive unit, and these units are common on ultralight aircraft. The term pitch of a propeller is used to describe its efficiency.
The pitch of a propeller is a numeric value representing the distance in inches it will move forward through the air as it completes one revolution.
Most ultralighl trainers have a fixed pitch propeller; that is, the pitch cannot be changed. Some propellers are designed with a small pitch and are best suited for climbing. Others with a higher pitch number are better suiled for cruise flight. Ultralight aircraft generally have a propeller with a pitch that gives them a good compromise between climb and cruise. The pitch of the propeller is usually stamped into the wood along with its length. For example, a 66/ 34 propeller is one whose length is 66 inches, and its pitch is 34 inches (the propeller moves forward 34 inches in one rotation). Higher performance propellers allow the pilot to adjust the pitch of the propeller either on the ground or in-flight. This allows the propeller to be adjusted to give maximum performance. These propellers are called adjustable pitch propellers.
One disadvantage of propellers is an effect known as P-factor. This effect causes the nose of me trainer to drift to one side when it is at a steep angle (when climbing, for example).
P-factor is a turning tendency created by the descending blade of the propeller meeting the air at higher speed than the ascending blade.
P-factor is the result of the descending propeller blades moving faster through the air and therefore producing more lift than the other blade. This is the case when the plane flies with a high angle of attack and the propeller experiences air hitting it not only from the fron but also from below.
The graphic below shows a level flying plane which meets the airflow directly head on. The P-factor is 0.
In the next graph below the plane ploughs through the air with a high angle of attack at relatively low speeds. The result is that, from the plane's perspective the airflow does not come from straight ahead. Instead it also blows upwards through the propeller. That's why the descending blade moves faster through the air.
Your instructor will point out the need to hold a little pressure on one of the rudder pedals in order to keep the nose pointing straight ahead during these situations. In straight and level flight, P-factor has no effect because each blade of the propeller is meeting the air at the same angle. The direction of rotation of the propeller on your trainer will determine which rudder pedal you'll need to apply pressure to. Expect to find P-factor's effects most pronounced when flying with lots of power, particularly during steep climbs.
Another aerodynamic force affecting your ultralight flight is called torque effect, or propeller torque.
Newton's third law, applied to a rotating propeller, says that as the propeller turns in one direction, there is an equal but opposite force applied in the other direction. The torque from the propeller wants to rotate the trainer along its longitudinal axis, opposite the direction of the propeller's rotation. It is predictably most noticeable at high power settings.
There are other turning forces acting on your ultralight trainer, but the two most relevant to your night training are P-factor and torque effect. Consult the aerodynamics section of any textbook for additional information on this subject. For now, it is important enough to say that these two forces will cause the nose of the trainer to drift, especially during climbs. You will counter these forces by holding a little opposite rudder pressure.
Always start and end your pre-flight inspection at the same point and check the same things in the same order