Lift & Pressure :
Airplane wings are created with a special design called an airfoil. The airfoil design bulges out more on the top than on the bottom, as shown in the figure. This causes the air that hits the wing to go off into two different streams, one that goes over the top and one that goes under, and they both meet up in the back.
moving over the top of the wing is caused to go faster than the slower moving
air on the bottom. Faster moving air has less pressure, so this causes the
pressure on the bottom of the wing to be greater and the plane is lifted. This
effect is known as the Bernoulli Principle. When a plane creates
sufficient lift it overcomes the force of gravity that is pulling the plane
Air pressure plays a big part in flight also. Air pressure is a force pushing on every square inch of an airplane. When a plane is parked the air pressure is distributed evenly around the plane's surface. When a plane is in flight the pressure on top of the wings pushes down less and the pressure on the bottom of the wings pushes more. This is what causes the plane to feel a lift.
The Lift Diagram shows some of the basic terms relating to a wing section. These terms are common to R/C flight.
|Airfoil||-||The cross section of the wing|
|Angle of Attack||-||The angle between the chord line and the relative direction of flight|
|Chord Line||-||The line between the leading edge and the trailing edge of the airfoil|
|Direction of Flight||-||The relative direction of the wing in relation to still air|
|Leading Edge||-||The most forward edge of the wing|
|Trailing Edge||-||The most rearward edge of the wing|
Another force that has a great part in flight is drag. Drag is the force pulling the plane backwards. Drag is the resistance created by the air molecules struck by the aircraft, being spread apart and flowing around the plane as it flies through them. Drag is created when the air collides with the airplanes wings and creates friction. This friction causes the plane to slow down and feel a drag. When wings are produced the designers make the wings in such a manner to create lift but also minimize friction with the air. Drag increases in proportion to the square of the velocity. So if the aircraft flies three times as fast, then drag is nine times.Total drag produced by an aircraft is the sum of the profile drag, induced drag, and parasitedrag. Total drag is primarily a function of airspeed. The airspeed that produces the lowest total drag normally determines the aircraft best-rate-of-climb speed, minimum rate-of-descent speed for autorotation, and maximum endurance speed. The following picture illustrates the different forms of drag versus airspeed
Curve "A" shows that parasite drag is very low at slow airspeeds and increases with higher airspeeds. Parasite drag goes up at an increasing rate at airspeeds above the midrange.
Curve "B" shows how induced drag decreases as aircraft airspeed increases. At a hover, or at lower airspeeds, induced drag is highest. It decreases as airspeed increases and the helicopter moves into undisturbed air.
Curve "C" shows the profile drag curve. Profile drag remains relatively constant throughout the speed range with some increase at the higher airspeeds.
Curve "D" shows total drag and represents the sum of the other three curves. It identifies the airspeed range, line "E", at which total drag is lowest. That airspeed is the best airspeed for maximum endurance, best rate of climb, and minimum rate of descent in autorotation.
Weight is the force of gravity trying to pull the plane back to earth. The important thing about this force is that it acts as though all the weight of the aircraft is centered at one point. That point is called the Center of Gravity or CG. When loading passengers and their baggage, always keep in mind that the CG must be located within specified limits for that particular aircraft.
Thrust is the force that causes a plane to move forward and is created by the plane's propeller or jet engines. Thrust is created by a propeller by using the same concept as lift. The propeller is specially shaped like an airfoil but it uses the lift to pull the plane forward instead of pushing the plane up.
Axes in a Flight:
An aircraft pivots about three (3) axes; the yaw or vertical axis controlled by the rudder, the pitch or lateral axis controlled by the elevator, and the roll or longitudinal axis controlled by the ailerons. It can pivot about any one of these individually or in combination based on the control surfaces that are moved and the direction of the movement.
When the rudder is moved to the right, the aircraft will rotate to the right about the yaw axis and vice versa. When the elevator is moved up, the aircraft will pitch the nose upwards. The ailerons move in opposite directions. When the left aileron is moved up and right one down, the aircraft will rotate to the left and vice versa.
Aircraft during Flight:
The aircraft moves forward because
of the thrust produced by propellers rotation or by Jet efflux in case of
jet engines. When Thrust produced is greater than ( Drag and Rolling
resistance ), the aircraft moves forward. As the aircraft moves forward,
air flows over the wings such that there is a Low pressure above the wing
and High pressure below the wing. Thus producing lift. Lift and
Drag increases proportionately with forward speed. Lift also increases
with angle of attack. When lift is greater than Weight of aircraft,
the plane flies and landing gear is retracted to reduce drag.
Lift, drag and moment ( Resultant force X arm of the aerofoil ) are the forces in a aircraft during flight. These values can be determined experimentally in a wind tunnel.
Major Components of Aircraft:
Major Aircraft Systems :Wings and Empennage are attached with Fuselage.
Ailerons and flaps are attached to wings.
Elevators are attached to Tail plane.
Rudder is attached to Fin.
Landing Gear / Power plant is attached to Wings.
Aircraft Maneuvers :
The Landing Gear:
On a modern single engine the landing gear (or undercarriage) consists of a nose wheel and a right and a left main wheel. Older aircraft with a tail wheel were said to have "conventional" landing gear. The modern type gear is called a "tricycle" landing gear. Although some tricycle gear are fixed in place, most are retractable into housing to reduce aircraft drag in flight. Multi-engine aircraft usually have a steer able nose wheel which is controlled by the rudder pedals. Many aircraft accidents are caused by pilots attempting to land their aircraft without lowering their landing gear. On a normal landing approach, the gear should be lowered when leaving or passing through airport pattern altitude which is usually about 1200 feet AGL. On take-off, the gear should not be raised until the pilot is certain he could not land on the runway if his engine should quit. On real aircraft it's a good idea to touch your brakes before you retract your landing gear to stop the wheels from spinning and save wear and tear on your tires.
The basic flight instruments :Magnetic compass -- Like the compass in a car or boat, it tells about the airplane's heading -- the direction it's flying. It requires no power source.
Angle of Attack :
The angle made by the chord of aerofoil to the relative air flow. Chord is obtained by line joining lead edge with trailing edge.
It is the condition where the air separates from wing surface and causes loss in lift and drag. C L and CD increase with increasing angle of attack. But at 16o there is a drop in CL and rapid rise inC D. At this point, ( Stall ) the aircraft cannot fly safely.
The ratio of Lift to Drag is important from the point of fuel economy. The optimum value of L/D is obtained at an angle of attack of 4o.
Designer should consider the ratio of Thickness to chord. For low speed aircraft the ratio is 12% - 15%. In case of supersonic aircrraft, it is about 4%. This leads to thin thickness which cannot support other structural members. But delta plan solves the problem. MIG has a Delta plan.
Sweep Angle :
is the angle made by Quarter chord line along the lateral axis. In Mig 27 and Tornado, the sweep angle can be varied during flight.
The curvature of mean line between top and bottom surfaces is called Camber. Increase in Camber increase Lift and Drag. Leading edge radius also affects lift and drag. A large radius is used in low speed aircraft.
Aspect Ratio :
It is defined as Span / Mean Chord or Span2 / Area.
If Lift / Drag is high, when aspect ratio is high. For passenger aircrafts Aspect Ratio is around 8 - 12. But in fighter aircrafts, long wings are not acceptable, hence 2 - 4 is preferred.
Control Surfaces in a Aircraft :
Flaps are attached to the trailing edge of the wings, they are used to change the camber of wings. Flaps augment lift at the time of take off, but during normal flight, it is retracted back to reduce drag. Slats are attached to the front portion of wings. Flaps are relatively large, movable, hinged panels located inboard of the ailerons on each wing. Lowering them into the airflow under the wing increases both lift and drag significantly. Their main purpose is to permit a slower airspeed during a landing approach. They are also useful in shorten the distance required to take-off from short runways or from airports at higher altitudes. It is recommended that one-third flaps be set routinely for any take-off. Hanging low to the ground they can be easily damaged by debris being kicked up by the propeller or the landing gear.
The Rudder :
The Vertical Stabilizer is the vertical fin at the rear of the aircraft. The rear portion of the vertical stabilizer is a hinged section called the rudder. The rudder is moved on its hinges by pedals on the floor of the cockpit. The truth of the matter is that the rudder is of little or no real value in controlling the aircraft in the air during normal flight. On the ground, in single engine aircraft, it is useful for taxiing and take-off when the other control surfaces are still ineffective due to the low speed of the aircraft. In multi-engine aircraft it can be necessary to counter the loss of an engine. It is useful, and sometimes necessary, during stall and spin recovery when other control surfaces have lost effectiveness, and it is helpful in maintaining co-ordinated flight during turns. In a turn the inside wing has lost some lift and without the use of the rudder the aircraft would "skid" through the turn, a very sloppy and uncomfortable way to fly. Once this option is set permanently, your aircraft will fly with simulated rudder movement and keep your turns coordinated automatically.
The Ailerons :
The ailerons are a set of hinged surfaces found at the rear edge of each wing. They are controlled by the side-to-side movement of the joystick or yoke (control wheel). By moving the aileron, on the side of the aircraft you wish to turn toward, up into the flow of air over the wing, some lift is lost and the wing drops. At the same time the aileron on the outside wing has been moved downward, producing additional lift on that side and that wing rises. The combined effect is to turn the plane in the direction you have moved the stick. Are ailerons the only way to turn an aircraft? The answer is no. A Piper Cub, for instance, can be turned by merely sticking your arm out of the open cockpit as if you were signaling a turn in that direction. The drag produced will turn the plane in the direction you signaled.
The Trim Tabs :
Holding the control surfaces in one position for long periods of time can be tiring. This is especially true of the elevator. So small tabs on the edge of the control surfaces can be set to hold the surface in a steady position. On most modern aircraft this allows the pilot to remove his hands from the controls and/or use only light pressures to maintain steady flight conditions.
The Brakes :
At the tip of each rudder pedal
is a foot brake. These brakes are not only used to help stop the aircraft,
but are necessary to steer the aircraft on the ground if the plane does not have
a steerable nose wheel. This is especially true at lower speeds when the
rudder is ineffective. Aircraft are also equipped with a parking
brake. Any time the aircraft is stopped with the engine running the
parking brake MUST be set for safety reasons. Brakes should be used
sparingly on landing to avoid blowing out the tires. Jet aircraft use
spoilers which are inserted into the air stream to slow down the plane and most
jet engines are capable of reversing thrust to slow the aircraft. This
feature is also helpful in backing out of parking spaces at the gate.
Different types of Loads on Aircraft :
Lastly updated on Thursday, December 18, 2003 , 07:10 PM