# Basics of Flight

Forces on a Plane :

When a plane flies there are four forces at work that keep the plane flying. These forces are lift, thrust, gravity, and drag.  For the aircraft to generate lift, it wings have a special airfoil section as shown in the figure.

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.

The air 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 down.

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

Drag:

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
• Profile drag is the drag incurred from frictional resistance of the blades passing through the air. It does not change significantly with angle of attack of the airfoil section, but increases moderately as airspeed increases.
• Induced drag is the drag incurred as a result of production of lift. Higher angles of attack which produce more lift also produce increased induced drag. In rotary-wing aircraft, induced drag decreases with increased aircraft airspeed. The induced drag is the portion of the total aerodynamic force which is oriented in the direction opposing the movement of the airfoil. Think of it as lift which is in the wrong direction.
• Parasite drag is the drag incurred from the non lifting portions of the aircraft. It includes the form drag and skin friction associated with the fuselage, cockpit, engine cowlings, rotor hub, landing gear, and tail boom to mention a few. Parasite drag increases with 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:

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:

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:

1. Wings
2. Fuselage ( Passengers and Pay load )
3. Empennage ( Tail plane and fin )
4. Control surface ( Ailerons, flaps, elevators and rudders ) and
5. Landing gear.
The wings and Empennage are attached to Fuselage.  Following are the different attachments.
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.
Major Aircraft Systems :
1. Brakes
2. Navigation
3. Communication
4. Fuel and power plant
5. Instrumentation
6. Blind landing aids.
7. Weapons and electronic counter measures.
8. Cabin furnishing
9. Cabin equipment
10. Cargo equipment
11. Air pressurization and
12. Air-conditioning & Oxygen.
The uses of different aircraft components are given below.
• Wings - Produce lift.
• Control surface - Control the aircraft movement in different direction
• Landing gear - Support the aircraft in land.
• Power plant - Provides Thrust.

Aircraft Maneuvers :

1. Take off
2. Climb
3. Cruise
4. Turn and bank
5. Descent and
6. Landing.
Fighter aircrafts do additional maneuvers which include,
1. Loop
2. Roll
3. Dive
4. Tight turn
5. Side slip
6. Inverted flight and
7. Spin.
Mach Number (M) = Speed of aircraft / Speed of sound.

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.

Airspeed indicator -- It shows the speed of the airplane through the air.  This instrument measures the speed of the aircraft through the air NOT OVER THE GROUND in knots.   The speed it shows is called True Airspeed (TAS) if it has been corrected for the density of the air outside the aircraft.   Without this correction it is called Indicated Airspeed (IAS).   The instrument operates by comparing the pressure of the air being pushed into a probe under the aircraft, called a Pitot Tube, with the static air pressure inside the aircraft.  Many pilot had crashed on take-off because they neglected to remove the cover from his Pitot Tube.

Attitude indicator -- This instrument is like the horizon we see looking out from the pilot's seat. It tells whether the nose of the airplane is pointed above or below the horizon and whether the airplane is turning (banking) to the left or right (left wing down or right wing down).  This instrument uses a gyroscopically controlled horizon, moving behind a fixed representation of an aircraft, to indicate your plane's movement about both the lateral axis (nose position) and the longitudinal axis (wing position) relative to the actual horizon.   When flying under Visual Flight Rules (VFR), the pilot should ignore this instrument and use the actual horizon instead.   In fact, the other instruments should be only checked periodically. in a quick scan pattern, as the pilot should be mostly looking outside the aircraft.

Altimeter -- This instrument shows the airplane's altitude in feet above sea level.  The altimeter uses a sealed tin called an aneroid barometer to measure altitude.   It compares the atmospheric pressure outside your aircraft to the pressure inside the tin.  As outside pressure changes, the tin expands or contracts from the pressure sealed inside.   Each tick mark on the instrument face represents one hundred feet of altitude.   Each rotation of the needle represents 1000 feet.

Vertical speed indicator -- This instrument tells you how quickly the aircraft is climbing or descending in feet per minute. When in level flight, it reads "0".  It operates by measuring the relative change in atmospheric pressure.   Since the change is relative, unlike the altimeter, it does not require calibration.  There is a lag in the measuring  process, however, and therefore it is important not to take any corrective action until you have given the instrument time to do it's thing.  A second or less is all it takes.

Heading indicator (directional gyro) -- This instrument is another compass. It shows the direction that the airplane is flying. It's usually bigger and easier to read than the magnetic compass, but requires some source of power to work.

Turn coordinator -- When turning the airplane, this instrument shows the rate and the direction of the turn. In this way we can adjust to a slower or faster rate of turn.  This instrument has a representation of an aircraft which banks to indicate the direction and rate of the aircraft turns.   It also has a liquid-damped ball which will be in the center of the glass tube when the aircraft is in a coordinated turn.   If the aircraft is skidding around the turn, the ball will be displaced to the side of the tube in the direction of the turn.   Application of rudder in the direction of the turn will bring the aircraft back to coordinated flight and the ball back to it's middle position in the tube.

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.

Stall :

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 :

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 :

• Gust load ( Vertical air )
• Control load
• Thermal load
• Crash load
• Landing load
• Power plant load and
• Pressurization load.
Loads in Consideration from Design Point of View :
• Limit Load :- The maximum load on any aircraft member.
• Proof Load :- Limit load X ( 1.1 or 1.125 ).  When loaded upto this limit, safety of aircraft should not be impaired.
• Design Load :- Limit load X 1.5.  When a load lower than this is applied there should not be any breakage but may deform permanently in any way.

Lastly updated on Thursday, December 18, 2003 , 07:10 PM