Wings

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Wing basics

*Wing span is the distance from one wingtip to the other.

*Leading edge is the very front edge of a wing.

*Trailing edge is the back edge of a wing.

*Chord length is the length from the front of a wing to the rear, if we drew a line straight thru the wing from side on. This line would be known as the chord line. It would go from the leading edge to the trailing edge.

*The depth of awing at its thickest part is known as the thickness.

*An important ratio for wing performance is the thickness to chord ratio.  Wings with a thicker thickness to chord ratio produce more lift than thin wings. The ratio is generally 10-15%

*The ratio of a wings span to its chord is known as its aspect ratio. Wings with a high aspect ration eg gliders, that have long wings and short chord length produce more lift than wings with a low aspect ratio like a jet fighter.

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Wing aspect ratio

*Most light aircraft have a different amount of curve on the top and bottom surface. This is known as camber. The greater the camber the more the lift. Flaps are used to give a wing even more camber for low landing speeds.

With flaps in the down position, the air travelling over the top of the wing has to travel even further, creating a greater pressure difference between the upper and lower wing. The upper surface has fast moving air under low pressure. The bottom surface has slower moving air under high pressure.  More difference in pressure creates more lift, but it also creates more drag. This is why flaps are ideal for landing.

*When we look at a light aircraft from front on, most aircraft have the wing tips higher then the middle part (the root) of the wing. *This is called dihedral. Dihedral helps an aircraft fly straight and level by hanging the wight of the aircraft between the wing tips. The opposite of dihedral is called anhedral.

If we look at an aircraft wing from on top some wings have their tips much farther back towards the rear of the aircraft compared to the root of the wings. *This is called sweepback.

If you stand with your eyes looking directly along a wing from a few meters away from its wing tip, you may see that compared to the root of the wing, appears to be twisted as it goes outwards, so that the  wing tip has a  lower angle of attack.  This is called washout or twist. This allows the wing root to stall before the wingtip and is very important in helping an aircraft stall a little more safely than without it.

**Angle of Attack

**The angle between the chord line (line thru middle of wing from front to back) and the relative airflow is called the angle of attack. The angle of attack changes the amount of lift and changes with your throttle setting. At lower throttle settings you need a higher angle of attack (nose up) to maintain lift for level flight. As you increase throttle you can lower the angle of attack and maintain level flight.

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A of A changes as speed changes

**You must know that angle of attack is related to your flight path and relative airflow, not the horizon or your longitudinal axis.

In level flight  A of A = Angle relative to horizon

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A of A in level flight

In descending flight A of A = Angle relative to horizon + descent angle

Level flight Angle Of Attack

A of A in descending flight

Your stalling angle will look different in descending flight, when compared to your stalling angle in ground effect or level flight. Discuss this with your instructor, it is very important. Your stalling angle may also look different with each stage of flap. Each aircraft may have a different looking stall angle. Discuss this with your instructor.

Important Safety Note: Some aircraft “may” have very subtle stalling characteristics. This can result in significant rates of descent and can catch even experienced pilots out, eg Air France 447 which had multiple stall warnings, one for over 54 seconds that the pilots failed to notice and descent at over 10,000 feet per minute. This “error” killed 228 people in 2009. Always monitor airspeed.

See: http://www.flyingmag.com/technique/accidents/air-france-447-crash-final-report-points-pilot-error-confusion

 

For more info about wing design, you may optionally watch this video:

Some great wind tunnel observations on the airflow over various wing configurations. This helps with understanding what happens as the wing begins to stall.

A fantastic video showing the disturbed airflow as the wing begins to stall. Note around the one minute mark when the wing begins to stall, the disturbed airflow begins near the root of the wing and the edges of the wing are still relatively normal. This is a design feature to allow aileron control near the stall speed. Also note the A of A as the wing stalls and recovers.