Viwanda light aeroplane, kenya.

Wing Basics

 

 

 

 

 

 

 

 

 

 

 

when air moves smoothly around a wing it provides lift to the aircraft. This is derived out the fact that the air moving on the upper curved part travells faster than that below and hence has lower pressure.( in compressible fluid dynamics, this pressure loss is related to as dynamic pressure)That pressure difference creates an upward force otherwise called lift.

Any discussion about wings start with its AIRFOIL shape. This can be easily understood as the cross section of the wing viewed from the side.

Every airfoil can be plotted by its data file. This file is an x y coordinates.

 

Every airfoil has unique performance characteristics

Performance data for an airfoil are . Critical angle . Drag characteristics, Pitch  moment characteristics, among others.

About four graphs can give the designer a quick understanding of the characteristics of the airfoil.

Different airfoils will be suited for different uses of the aircraft. Common uses are for efficient cruise, heavy lift, STOL etc.

Online utilities such as airfoiltools.com are very helpful for airfoil choice.

WING TYPES.

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THE WIND TUNNEL

This extracts the complete airfoil data. It consists of airflow blower, air speed meter. Angle of attack protractor. lift sensor, drag sensor etc.

I fabricated a homemade wind tunnel that can measure lift against angle of attack to test miniature airofoils.

 

 

THE BOUNDARY LAYER

Any fluid( air or liquid) flowing over a surface and bordering it sticks with the surface and acts like part of that surface. Fluid a little further outward away is the one that flows over. This boundary layer increses the dimension of that surface.

 

CAMBER

This is the curvarture of the wing and can be viewed by center line called the camber line midway the airofoil.

Maxmam camber against the chord gives the camber percentage. maximam thickness against the chord gives the % thickness. Rarely do aerofoils exceed 15% thickness. common figures are between 7-12 %.

 

UNDER CAMBER .

Under camber as oposed to flat bottomed makes the wing have hig lift at low angle of attack. The wing also has higher pitch moments. These can work for slow flights so long as elevator authority is ensured by either the size of elevator or increased moment of elevator to the wing quater chord.

 

STALL

 

Every wing stalls at a certain critical angle. When the air flow above the wing no longer flows smoothly, becomes turbulent nd separates.

Any control surface also can stall. This includes rudder, aileron even propeller blades since they are essntially airofoils. As such stall should be avoided at all times since lift suddenly falls and controll is adversely affected. There are many types of stalls, such as deep stalls etc .

 

Most wings stall at about 15 degrees angle of attack. High lift devices such as slats can extend this angle to 25 -30 degrees.

Coefficient of lift ranges up to 1.2 for most wings. Special high lift airfoils especially under-cambered can have max cl of 2.0 .Again a combination of flaps and slats can raise this to 2.7.

A good wing aerofoils should stall less easily have less drag and have low pitch down moments. Pitch moments is the natural tendancy for wings to flip or the nose down effect when in an aircraft. this counteracted by elevator.

 

WING PLACEMENT

biplane,mid wing, low wing, tandem, canard style.

 

CONTROL SURFACES ON THE WING

These are mainly flaps and ailerons. while some simplified designs can do away with flaps. ailerons are needed for roll controll.

flaperons are a combination of flaps and ailerons in one surface and are used in some light aircrafts.

 

SELECTING WING AREA

 

Most light aircrafts cruise at a coeficient of lift of 0.4. Knowing the Gross weight and desired cruise speed  of the aircraft that is enough to get the wing area from the Lift equation. Lift =Area x coeficient of lift x Air density x speed squared /2

Derived from this Wing area becomes Since at cruise Lift = gross weight.

 

Wing area A = 2 xGross weight / [ cl x v2 x density of air] keep units in metric and get area in meters squared. 1 meter squared =11.111 square feet.

 

WING GEOMETRY FOR STRAIGHT WING.

 

Aspect ratio  ( wing span/chord)

aspect ratio of 5- 7 works. 

 Chord x ( chord x aspect ratio) = wing area determined above.

Calculate chord then and you got the wing geometry.

 

CONSTRUCTION

The Spars: These are span wise structure . can be one or two depending on chord and strength required

Ribs: These are chord wise structure that takes the airfoil shape spaced along the span

Covering: can be aluminum or fabric.

Brackets: these are needed in order to attach struts

Materials: ribs and spars can wooden or aluminum.

 

 

 

 

 

 

                                                                                                                                    Typical wing internal structure for a production aircraft.

 

 

My aluminum tube welded ribs.

 

REYNOLD NUMBERS AND SCALING

 

Two different wings even if having same airfoil but operating on different speeds or having different dimensions will behave differently.

This is because the ratio of inertial forces to viscous forces are different for both. That ratio is called the reynold numbers.

For example a radio control wing measuring only 1 square feet and operating below 5 Mph will stall at a less angle that a 120 square feet wing operating at 80 mph. The small slower wing sees air as less dense than it is. lets say a clark Y wing in piper j3 will stall at 13 degrees. A clark y in a RC model will stall at 6- 8 degrees. The airfoil graphs are then plotted for a range of R-numbers to help the designer.