The goal of this chapter
This chapter discusses the aerodynamics of flight—how design, weight, load factors,and gravity affect an aircraft during flight maneuvers.
Why this chapter essential？
Understanding how these forces work and knowing how to control them with the use of power and flight controls are essential to flight.
By using the aerodynamic forces of thrust, drag, lift, and weight, pilots can fly a controlled, safe flight.
Forces Acting on the Aircraft
Definition of Thrust
the forward force produced by the powerplant/propeller or rotor.
Definition of Drag
a rearward, retarding force caused by disruption of airflow by the wing, rotor, fuselage, and other protruding objects.
Definition of Lift
opposes the downward force of weight, is produced by the dynamic effect of the air acting on the airfoil, and acts perpendicular to the flightpath through the center of lift.
Definition of Weight
The combined load of the aircraft itself, the crew, the fuel, and the cargo or baggage.
Detail about aerodynamic forces – Thrust
In order to maintain a constant airspeed, thrust and drag must remain equal.
For an aircraft to move, thrust must be exerted and be greater than drag.
Angle of Attack （AOA）
The pilot coordinates angle of attack （AOA） and thrust in all speed regimes if the aircraft is to be held in level flight.
Definition of AOA:the acute angle between the chord line of the airfoil and the direction of the relative windthese regimes can be grouped in three categories:
- low-speed flight,
- cruising flight,
- and high-speed flight.
When the airspeed is low, the AOA must be relatively high if the balance between lift and weight is to be maintained.
During straight-and-level flight when thrust is increased and the airspeed increases, the AOA must be decreased.
AOA were not coordinated
If the AOA were not coordinated （decreased） with an increase of thrust, the aircraft would climb.
slightly negative AOA：high speed
Level flight at even slightly negative AOA is possible at very high speed.
Detail about aerodynamic forces – Drag
What is Drag?
Drag is the force that resists movement of an aircraft through the air.
There are two types of Drag:
- Parasite drag
- Induced drag
Parasite drag is comprised of all the forces that work to slow an aircraft’s movement.
There are three types of parasite drag:
- form drag
- interference drag
- skin friction
（1）Form drag – easiest to reduce
Form drag is the portion of parasite drag generated by the aircraft due to its shape and airflow around it.
Form drag is the easiest to reduce when designing an aircraft.The solution is to streamline as many of the parts as possible.
Interference drag comes from the intersection of airstreams that creates eddy currents, turbulence, or restricts smooth airflow.
For example, the intersection of the wing and the fuselage at the wing root has significant interference drag.
（3）Skin Friction Drag
Skin friction drag is the aerodynamic resistance due to the contact of moving air with the surface of an aircraft.
In order to reduce the effect of skin friction drag, aircraft designers utilize flush mount rivets and remove any irregularities which may protrude above the wing surface.
Induced drag is a result of anairfoil developing lift.It is an established physical fact that no system that does work in the mechanical sense can be 100 percent efficient.The more efficient the system, the smaller this loss.
Induced drag is inherent whenever an airfoil is producing lift and, in fact, this type of drag is inseparable from the production of lift.
the lower the airspeed the greater the AOA required to produce lift equal to the aircraft’s weight and, therefore, the greater induced drag. The amount of induced drag varies inversely with the square of the airspeed.
The spirals of air that trail off the tips of an airplane’s wings also contribute to drag. These wing tip vortices steal energy from the motion of the airplane, creating vortex drag.
The air tends to flow from the high pressure area below the tip upward to the low pressure area on the upper surface.
In the vicinity of the tips, there is a tendency for these pressures to equalize, resulting in a lateral flow outward from the underside to the upper surface.
This lateral flow imparts a rotational velocity to the air at the tips, creating vortices, which trail behind the airfoil.
Drag versus speed
As airspeed decreases to near the stalling speed, the total drag becomes greater, due mainly to the sharp rise in induced drag.
As the airspeed reaches the terminal velocity of the aircraft, the total drag again increases rapidly, due to the sharp increase of parasite drag.
At some given airspeed, total drag is at its minimum amount.
Drag is the price paid to obtain lift.
The lift to drag ratio （L/D） is the amount of lift generated by a wing or airfoil compared to its drag.A ratio of L/D indicates airfoil efficiency.
Aircraft with higher L/D ratios are more efficient than those with lower L/D ratios.
Note that the maximum lift/drag ratio （L/DMAX） occurs at one specific CL and AOA. If the aircraft is operated in steady flight at L/DMAX, the total drag is at a minimum.
Any AOA lower or higher than that for L/DMAX reduces the L/D and consequently increases the total drag for a given aircraft’s lift.
The CG may be considered as a point at which all the weight of the aircraft is concentrated. If the aircraft were supported at its exact CG, it would balance in any attitude.
It will be noted that CG is of major importance in an aircraft, for its position has a great bearing upon stability.