Visual Flight Training (VFR)

Basics Of Flight

Relative Wind

Relative wind is the direction of the wind in relationship to the cord of the wing.  Relative wind always flows to the rearward of the plane.  In order to produce lift the chord of the wing angle in relationship to the relative wind is changed by the pilot thru the pitch of the wing which is called angle of attack.  Angle of attack is basically defined as the angle between the chord line and the direction of relative wind.

Camber

Camber is the curve of the airfoil.  The upper Camber and lower Camber of an airfoil are generally not proportional to each other.  The upper camber generally has more of an upward arc than the lower camber of the wing.  This has some interesting affects on how lift is produced.

Lift is produced by relative wind flowing over the wings surface.  The relative wind  hits the leading edge of the wing the wind is then split where the leading edge of the airfoil strikes it.  The molecules are separated from each other but wish to rejoin on the other side of the airfoil.  Since the camber of the upper and lower airfoil are of different shapes the a wind over the upper surface moves faster than the wind over the lower surface.  Exactly how lift is produced can be explained with Bernoulli's Principle of Pressure.  

Bernoulli's Principle states that the pressure of a fluid (liquid or gas) decreases at points where the speed of the fluid increases.  In the case of airflow, high speed flow is associated with low pressure and low speed flow with high pressure.  The airfoil of the aircraft is designed to increase the the flow of air over the upper camber of the wing and decrease the flow of air over the lower camber.  The combination of pressure increase on the upper and decrease on the lower produces lift.

Basic Aerodynamics

There are four forces that must be dealt with during flight they are Thrust, Drag, Weight and Lift.  The following will explain these forces in straight-and-level, unaccelerated flight.

Thrust - is produced by the power plant by use of a propeller.  It opposes the force of drag.

Drag - is the force to the rear, it is caused by the disruption of the airflow about the airfoil (wing, tailfin, etc...).  Drag acts rearward opposite of relative wind.

Weight - is the total weight of the aircraft.  This includes crew, fuel and cargo.  Weight is the downward force that is being applied to the aircraft by the earths gravity.

Lift - is the opposing force of weight.

In steady (straight and level, unaccelerated flight) all opposing forces are equal.  This is not to say that they are equal to each other.  This basically says that Thrust equals Drag and Lift equal Weight.

There are several factors the  affect both lift and drag on the airplane they are:

- Wing Area-  Lift and drag action on a wing are roughly proportional to the wing area.  As a pilot you can change wing area by using flaps.

- Shape of Air Foil -  As the upper camber of an airfoil is increased (to a certain point) the lift that is produces will increase.  Lowering of the aileron or a flap will accomplish this.  Ice and frost on the wing also changes the shape of the airfoil while also adding weight to the plane.

- Angle of Attack - As the angle of attack is increased, both lift and drag are increased, up to a certain point

- Velocity of air - An increase in velocity of air passing over the wing increases lift and drag.  Increase the speed of the plane you get more lift, decrease the speed you lose some lift.  In a head on gust the plane will generally receive a slight lift from the increase of airflow over the wing.

- Air Density - Lift and drag very directly with the density of the air.  As air density increases lift and drag increases.  This is why on warm humid days it takes longer to get the plane up and off the runway.  On cold dry days the air is less dense and the aircraft is easy to liftoff.

Torque Effect

Torque Effect involves Newton's Third Law.  For every action there is an equal and opposite reaction.  Applied to a prop engine aircraft, this means that as the propeller rotates to the right, from inside the cockpit there is an equal force trying to rotate the aircraft in the opposite direction.

- In Flight this opposite acting force tends to want to roll the aircraft to the left around the longitudinal axis of the aircraft.  Depending on the year and make of the aircraft some manufacturers rigged the wing so that the one being forced downward would produce more lift.  Also the engine can be offset to counter this force.

- On the ground the opposing force is pushing downward on the main left landing gear.  This in turn creates more friction between the left wheel and the ground causing the plane to want to turn to the left.

Torque Effect is only one thing that causes the plane to want to turn to the left.  As you may have notice that on takeoff in a single engine it requires right rudder to counter this effect.  The other items that that also work against you are the cork screwing effect of the propeller slip stream.  At low speeds and high RPM setting the slipstream of the propeller spirals rearward around the planes fuselage and hits the left side of the tail, this causes the aircraft to also want to turn to the left.  The last thing that gives you a further turning to the left is Propeller Precession.  A propeller is nothing more than an airfoil that is spinning.  During this spinning process it produces lift.  The lift you feel from the propeller is transferred into forward movement because of it's vertical positioning.  It is said that when a force is applied that the resulting action is felt at 90 degrees opposite from which is applied.  In the case of the propeller at slow speeds and high RPM settings or high RPM with a high angle of attack the initial force is being applied on the upward rotating blade on the left side.  The actual resulting force is being felt on the downward stoke on the right.  This in turn means that the propeller is getting a better bite on the relative wind on the right hand side producing more lift on the right causing the plane to want to turn to the left.

Stalls and Spins

An airplane stalls the the critical angle of attack has been exceeded.  The critical angle of attack is reached when the nose pitch is increased to approximately 18 to 20 degrees.  Lift is produced by the smooth flowing of air over the airfoil (wing)  once the airflow can no long flow smooth over the wing you start experiencing an airflow separation.  A wing stall starts at the outer edge of the wing.

As the stall becomes more aggravated the stalling area increases to the point of total wing stall and loss of lift.  In this next picture you see that stall and air separation is starting to happing in the pre-stall stage.  Notice the airflow is not exactly the shape of the wing as the air can no longer move and reshape fast enough to maintain the shape of the wing.

In this next picture we have tall wind separation and lift is loss.