Class 19Forces Acting On An

Aircraft

Engineering Concepts and Methods

Introduction to Aeronautical Engineering

In normal flight a light aeroplane derives its forward motion from the thrust provided by the engine-driven propeller.

If the aircraft is maintaining a constant height, direction and speed then the thrust force will balance the air's resistance (drag) to the aircraft's motion through it. The forward motion creates an airflow over the wings and the dynamic pressure changes within this airflow create an upward acting force or lift, which will balance the force due to gravity – weight – acting downward.

Thrust = Drag

Lift = Weight

Thus in normal unaccelerated flight the four basic forces acting on the aircraft are approximately in equilibrium. The pilot is able to change the direction and magnitude of these forces and thereby control the speed, flight path and performance of the aeroplane.

The mass of a body is a measure of its inertia – i.e. its resistance to being accelerated or decelerated by an applied force increases with mass. The unit of mass we will be using is the kilogram [kg].

The air also has mass and thus inertia and will resist being pushed aside by the passage of an aeroplane. That resistance will be felt as pressure changes on the aircraft surfaces.

An aircraft in flight is 'airborne‘ (flying in the air) and its velocity is relative to the surrounding air, not the Earth's surface.

When the aircraft encounters a sudden change in the ambient air velocity - inertia comes into play and momentarily maintains the aircraft velocity relative to the Earth or – more correctly – relative to space. This momentarily changes airspeed and imparts other forces to the aircraft.

Although we said that lift acts vertically upward with thrust and drag acting horizontally, this is only true when an aircraft is in straight and level flight. In fact, lift acts perpendicular to both the flight path and the lateral axis of the aircraft, drag acts parallel to the flight path and thrust usually acts parallel to the axis of the aircraft.

Aerodynamicists have found it convenient to resolve that resultant force into just two components, that part acting backward along the flight path is the wing drag and that acting perpendicular to the flight path is the lift. The amount of lift, and drag, generated by the wings is chiefly dependent on: AOA, AEROFOIL, . FLIGHT AIRSPEED,

Is there a way to calculate the lift and drag?

The amount of lift, and drag, generated by the wings is chiefly dependent on:

•the angle at which the wings meet the airflow or flight path, •the shape of the wings particularly in cross section – the aerofoil, •the density (i.e. mass per unit volume) of the air, •the speed of the free stream airflow i.e. flight airspeed, •and the wing plan-form surface area.

AvCLift L2

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AvCLift L2

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The values in the expression are: • (the Greek letter rho) is the density of the air, in kg/m³ •v² is the flight speed in meters per second •A is the wing area in square meters •CL is a dimensionless quantity – the lift coefficient. Mostly depends on the ANGLE of attack and the SHAPE of the wing.

The diagram shows a typical CL vs. angle of attack curve for a light aeroplane not equipped with flaps or high-lift devices. From it you can read the CL value for each “aoa”, e.g. at 10° the ratio for conversion of dynamic pressure to lift is 0.9

Calculate CL for the an 408.2 kg aircraft cruising at 6500 feet at 97 knots ( 1 knot – 0.5148 m/s). The wing area is very close to 8 m²:

   • lift = weight    • = 1.0 kg/m³ (the approximate density of air at 6500 feet altitude)

AvCLift L2

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AvCDrag D2

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The drag equation is similar to the lift equation with the exception that we have a DRAG COEFFICIENTrather than a LIFT COEFFICIENT. As CL

depending on “aoa”, the CD depends on the SQUARE of the “aoa”. We can make this assumption based on graphical data.

So the result of decreasing airspeed, while maintaining straight and level flight, is an increase in the lift coefficient; and that has two contributors – the shape of the wing and the angle of attack

As the pilot can't change the wing shape (unless she/he extends flaps) the angle of attack must have changed. How? By the pilot adjusting control pressure to apply an aerodynamic force to the aircraft's tailplane ( or some other control surface) which has the

effect of rotating the aircraft a degree or so about its lateral axis.

Without the needed thrust, weight has more influence than lift and pulls the airplane toward the ground. Helping the force of weight is drag. Drag is present at all times and can be defined as the force which opposes thrust, or, better yet, it is the force which opposes all motion through the atmosphere and is parallel to the direction of the relative wind

Induced drag is the unavoidable by-product of lift and increases as the angle of attack increases

Newtonian or DYNAMICDRAG is caused by the INERTIA of AIR.

Pressure Induced Dragoccurs when the “aoa”Is too large and the airFlow becomes turbulent.

There are also skin-friction drag and form drag, which are referred to as parasite drag. All drag other than induced drag is parasite drag.

Skin-friction drag is caused by the friction between outer surfaces of the aircraft and the air through which it moves. It will be found on all surfaces of the aircraft: wing, tail, engine, landing gear, and fuselage

The LIFT/DRAG ratio can be found by taking the lift coefficient and dividing by the drag coefficient.

D

L

C

CratioDL /

The L/D ratio is a measure of EFFICIENCY!!!

The tangent of the glide angle is equal to the vertical height (h) which the aircraft descends divided by the horizontal distance (d) which the aircraft flies across the ground.

What good is all this for aircraft design?

d

h

C

C

D

L

L

D

D

L

tan

tan

1tan

From the last equation we see that the higher the L/D, the lower the glide angle, and the greater the distance that a plane can travel across the ground for a given change in height

Glide :- Glide is that condition of flight in which the aircraft is losing height without power at a constant speed maintaining lateral level and direction.

Gliding Angle :- Gliding angle is the angle between earth’s horizon and the path of the aircraft.

The forces acting on an aircraft in a glide are

Weight

Lift

Drag

Weight :- Weight acting vertically down wards.

Lift :- Lift acting at right angles to the glide path.

Drag :- Drag acting backwards along the glide path.

Glide Angle, W cos = LW sin = D

D

W

L

Flight Path

Gliding Distance

Glide Angle,

Flight Path

Ground

Altitude h

Gliding Distance = h/tanh * L/D

D=W sinwhere is the equilibrium glide angle. L= W cosTan = D/LGlide distance = h/ tan = h ( L/D).

Because lift and drag are both aerodynamic forces, we can think of the L/D ratio as an aerodynamic efficiency factor for the aircraft. Designers of gliders and designers of cruising aircraft want a high L/D ratio to maximize the distance which an aircraft can fly. It is not enough to just design an aircraft to produce enough lift to overcome weight. The designer must also keep the L/D ratio high to maximize the range of the aircraft.

LIFT INCREASING DEVICES

The problem of increasing the maximum lift co-efficient to the speed of the lower speed range aircrafts has always been as much attention as the search for the high speeds.

The use of high speed aerofoil sections helps the designer to achieve higher speed by reducing the drag.

As most aircrafts using these aerofoils also have high wing loading, the stalling speed are proportionately higher. This requires longer landing run.

The following are the chief devices used to augment the lift co-efficient :(a) Slats(b) Flaps

(c) Boundary layer control

Slats are aerodynamic surfaces on the leading edge of the  wings  of fixed-wing aircraft which, when deployed, allow the wing to operate at a higher angle of attack.

Slats are the small auxiliary aerofoil surfaces of highly cambered section is fixed to the leading edge of the wing along the complete span and adjusted so that a suitable slot is formed between the two. The lift coefficient is increased by so much as 70% and more. At the same time the stalling angle is increased by 10°.

There are mainly three types of slats:

(a) Fixed Slats(b) Controlled Slats

(c) Automatic Slats

Fixed Slats :- In this case the slats are kept permanently opened with a slot and it was found that the extra drag at a high speed would be a greater disadvantage than the advantage gained by the extra lift at low speeds.

Controlled Slats :- In this the slats would be moved to open the slot and close the slot by control mechanism attached to a lever in the cockpit which gains the advantage at high and low speeds.

Fixed Slats

Controlled Slats

Automatic Slats :- In this case when the slat is not in use it lies flush against the edge of the wings. At high angles of attack the low pressure peak near the leading edge of the upper surface of the wing and the lift generated by the cambered slot itself lift the slat upward and forwards to the open position, thus forming the required slot.

Automatic Slats

Automatic Slats

Introduction :- The plain or cambered flap works on the same principle as on aileron or other control surface, it is truly a ‘variable camber’.

Flaps, like slats can also increase the drag. Lowering of flaps produce an increase in

the lift co-efficient at given speed but at the same time the greater camber also causes an increase in the total drag.

The best lift drag ratio is obtained with the flap at some angle between 15° and 35°.

There are various types of flaps in use and they are mainly :-

(a) Plain or Camber Flaps(b) Split Flaps(c) Slotted Flaps(d) Fowler Flaps(e) Jet Flaps(f) Zap Flaps(g) Variable camber Flaps