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Forces Acting in an Airplane
Aerodynamic’s & Aircraft Devices
By: Edwin Pitty S.
Forces Acting in an AirplaneThrust, drag, lift, and weight are four forces that act upon all aircraft in flight.
Understanding how these forces work and knowing how to control them with the use of power and flight controls are essential to flight.
Forces Acting in an AirplaneThe four forces acting on an aircraft in straight-and-level, un-accelerated flight are thrust, drag, lift, and weight. They are defined as follows:
• Thrust—the forward force produced by the power plant/ propeller or rotor. It opposes or overcomes the force of drag. As a general rule, it acts parallel to the longitudinal axis. However, this is not always the case, I will explained in other slides.
• Drag—a rearward force, retarding force caused by disruption of airflow by the wing, rotor, fuselage, and other protruding objects. Drag opposes thrust, and acts rearward parallel to the relative wind.
Forces Acting in an Airplane
• Weight—the combined load of the aircraft itself, the crew, the fuel, and the cargo or baggage. Weight pulls the aircraft downward force because of the force of gravity. It opposes lift, and acts vertically downward through the aircraft’s center of gravity (CG).
Forces Acting in an Airplane
• Lift—upward force 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.
Forces Acting in an Airplane
Forces Acting in an Airplane Picture
WeightGravity is the pulling force that tends to draw all bodies to the center of the earth. 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.
LiftThe pilot can control the lift. Any time the control yoke or stick is moved fore or aft, the AOA is changed. As the AOA increases, lift increases (all other factors being equal).
Streamline flow:The flow of a fluid is said to be streamline (also known as steady flow or laminar flow), if every particle of the fluid follows exactly the path of its preceding particle and has the same velocity as that of its preceding particle when crossing a fixed point of reference.
Lift
Turbulent flow:The flow of a fluid is said to be turbulent or disorderly, if its velocity is greater than its critical velocity. Critical velocity of a fluid is that velocity up to which the fluid flow is streamlined and above which its flow becomes turbulent. When the velocity of a fluid exceeds the critical velocity, the paths and velocities of the fluid particles begin to change continuously and haphazardly. The flow loses all its orderliness and is called turbulent flow.
Lift
LiftBernoulli Principle (Venturi effect) The basic concept of subsonic airflow and the resulting pressure differentials was discovered by Daniel Bernoulli, a Swiss physicist. Bernoulli’s principle, as we refer to it today, states that “as the velocity of a fluid increases, the static pressure of that fluid will decrease, provided there is no energy added or energy taken away.” A direct application of Bernoulli’s principle is the study of air as it flows through either a converging or a diverging passage, and to relate the findings to some aviation concepts.
Bernoulli Principle (Venturi effect)
Lift
Dynamic & Static Pressure
• Static pressure is the pressure you have if the fluid isn't moving or if you are moving with the fluid.
• Total pressure is what acts on you as you face into the wind and the air collides with your body.
• Dynamic pressure is the pressure of a fluid that results from its motion. It is the difference between the total pressure and static pressure.
Lift
Dynamic & Static Pressure
Lift
AirfoilThe Airfoil is the shape of the cross section of the wing. The front of the airfoil is the leading edge and is usually a rounded section. The back of the airfoil is the trailing edge and usually tapers to nearly a point. The distance between the two is the wing chord. The top surface of the airfoil is usually always curved to allow smooth airflow and produce lift.
Lift
Airfoil Shapes• Flat Bottom — A Flat Bottom Wing is when
the lower surface of the wing is primarily flat between the leading and trailing edges. This type of wing has high lift and is common on trainer type aircraft.
Lift
Airfoil Shapes• Symmetrical — A Symmetrical Wing airfoil is
curved on the bottom to the same degree as it is on the top. If a line was drawn from the center of the leading edge to the center of the trailing edge the upper and lower halves of the airfoil would be symmetrical. This is ideal for aerobatic aircraft and most lift is created by the angle of incidence of the wing to the flight path.
Lift
Airfoil Shapes• Semi-symmetrical — A Semi-symmetrical Wing
airfoil has a curved bottom section but to a lesser degree than a symmetrical section. It is a compromise between the flat bottom and the symmetrical wing. This is a very popular airfoil on sport type aircraft.
Lift
Airfoil Shapes• Under-camber — An Under-camber airfoil has
the lower surface of the wing curved inwardly almost parallel to the upper surface. This type of airfoil produces a great deal of lift but is not common in R/C models.
Lift
Aerodynamic Force Is exerted on a body by the air (or some other gas) in which the body is immersed, and is due to the relative motion between the body and the gas.
The force created by a propeller or a jet engine is called thrust and it is also an aerodynamic force. The aerodynamic force on a powered airplane is commonly resolved into three components: thrust, lift and drag.
Lift
Pressure Distribution and CP MovementWith changes in angle of attack there are pressure distribution changes.
We have already seen how the center of pressure moves forward with low angle of attack, and aft when we have high angle of attack.
Lift
Pressure Distribution and CP Movement
Lift
CamberIs the curvature which is present on top and bottom surfaces. The camber on the top is much more pronounced, unless the wing is a symmetrical airfoil, which has the same camber top and bottom. The bottom of the wing, more often than not, is relatively flat. The increased camber on top is what causes the velocity of the air to increase and the static pressure to decrease. The bottom of the wing has less velocity and more static pressure, which is why the wing generates lift.
Lift
WingspanIs the length of a wing, measured from wingtip to wingtip. It always refers to the entire wing, not just the wing on one side of the fuselage.
Lift
Chord LineAn airfoil is an infinitely long, straight line which passes through its leading and trailing edges.
Chord Is a measure of the width of an airfoil. It is measured along the chordline and is the distance from the leading edge to the trailing edge. Chord will typically vary from the wingtip to the wing root.
Lift
Aspect ratio (AR)
Is the ratio of the wingspan to the average chord.
Lift
Wing loading (WL) Is the ratio of an airplane’s weight to the surface area of its wings. There tends to be an inverse relationship between aspect ratio and wing loading. Gliders have high aspect ratios and low wing loading. Fighters with low aspect ratios maneuver at high g-loads and are desig ned with high wing loading.
Lift
Relative WindWhatever direction the airplane is flying, the relative wind is in the opposite direction.
Lift
Parasite Drag
Is composed of form drag, skin drag and interference drag. It is all drag that is not associated with the production of lift.
Drag
Parasite Drag • Form Drag, also known as pressure drag
or profile drag, is caused by airflow separation from a surface and the low pressure wake that is created by that separation. It is primarily dependent upon the shape of the object.
Drag
Parasite Drag• Skin drag, is created in the boundary layer.
Turbulent flow creates more friction drag than laminar flow. Skin drag is usually small per unit area, but since the boundary layer covers the entire surface of the airplane, skin drag can become significant in larger airplanes.
Drag
Parasite Drag• Interference drag, is generated by the
mixing of streamlines between components. An example is the air flowing around the fuselage mixing with air flowing around an external fuel tank.
Drag
Induced Drag
Is that portion of total drag associated with the production of lift. We can add the airflow at the leading edge and the airflow at the trailing edge of the wing in order to determine the average relative wind in the immediate vicinity of the wing.
Drag
Angle of Incidence
The angle between the chord line and the longitudinal axis of the airplane is known as the angle of incidence.
Angle of Attack (AOA)
The angle between the chord line and the relative wind is the angle of attack. As the angle of attack increases, the lift on the wing increases.
Angle of Attack (AOA)
Critical AngleIs the angle of attack which produces maximum lift coefficient. This is also called the "stall angle of attack". Below the critical angle of attack, as the angle of attack increases, the coefficient of lift (CL) increases. At the same time, above the critical angle of attack, as angle of attack increases, the air begins to flow less smoothly over the upper surface of the airfoil and begins to separate from the upper surface.
• Laminar Flow, the air moves smoothly along in streamlines. A laminar boundary layer produces very little friction, but is easily separated from the surface.
• Turbulent Flow, the streamlines break up and the flow is disorganized and irregular.
Critical Angle
Wing DesignWing design is constantly evolving. The number of lifting surfaces, shape, size and materials used all contribute to an aircraft’s performance.
Monoplane - one wing plane. The wing may be mounted at various positions relative to the fuselage:
– Low wing - mounted near or below the bottom of the fuselage.
– Mid wing - mounted approximately half way up the fuselage.
– High wing - mounted on the upper fuselage.
Wing Design
Planform: The shape of the wing when viewed directly from above.
Wing Design
Rectangular
> Inefficient from a structural, weight and drag standpoint
> Good slow flight/stall characteristics:
1.Stalls first at the wing root
2.Displays adequate aileron effectiveness at stall
3.Usually quite stable
Wing Design
Elliptical>Most efficient in terms of weight and
>Expensive and more difficult to construct
>Provides Minimum induced drag for any given aspect ratio
>Provides poor stall characteristics:
1. Little advanced warning of a stall
2. Difficult lateral control because of poor aileron effectiveness
Wing Design
Tapered>Can be tapered in planform or thickness or both
>Relatively efficient with reasonable weight and drag
1.Tapering causes a decrease in drag (mostly effective at high speed)
2.Tapering causes an increase in lift
>Tapering uses less material - meaning savings in weight
>Reasonable construction costs as well as good slow flight capability
Wing Design
Sweptback>Used for high speed aircraft, Jet airliners
>Poor stall characteristics
>Low aspect ratio- more dragper lift produced
>High aspect ratio- less drag per lift produced
1.Stalls from the wingtips inward – reducing aileron effectiveness
Wing Design
Delta>Supersonic Wings
>Shape is aerodynamically cleaner
>Offer greater strength
>High wing loading and high speeds
Wing Design
Amount of Lift GeneratedThe amount of lift generated by the wing depends upon several factors:
• Speed of the wing through the air• Angle of Attack• Planform of the wing• Wing area• The density of the air
Lift-to-Drag Ratio and Best GlideThe lift-to-drag ratio, or L/D ratio, is the amount of lift generated by a wing, divided by the drag it creates by moving through the air.
A higher or more favorable L/D ratio is typically one of the major goals in aircraft design; since a particular aircraft's required lift is set by its weight, delivering that lift with lower drag leads directly to better fuel economy, climb performance, and glide ratio.
Best glide or glide ratio is a constant speed in still air a glider moves forwards a certain distance for a certain distance downwards.
Lift-to-Drag Ratio and Best Glide
FlapsFlaps are devices used to improve the lift characteristics of a wing and are mounted on the trailing edges of the wings of a fixed-wing aircraft to reduce the speed at which the aircraft can be safely flown and to increase the angle of descent for landing.
They shorten takeoff and landing distances. Flaps do this by lowering the stall speed and increasing the drag.
Plain Flap Is a simple hinged portion of the trailing edge that is forced down into the airstream to increase the camber of the airfoil.
Flaps
Split Flap Is a plate deflected from the lower surface of the airfoil. This type of flap creates a lot of drag because of the turbulent air between the wing and deflected surface.
Flaps
Slotted Flap Is similar to the plain flap, but moves away from the wing to open a narrow slot between the flap and wing for boundary layer control. A slotted flap may cause a slight increase in wing area, but the increase is insignificant.
Flaps
Fowler FlapIs used extensively on larger airplanes. When extended, it moves down, increasing the camber, and aft, causing a significant increase in wing area as well as opening one or more slots for boundary layer control.
Flaps
Leading edge devices such as nose flaps, Kruger flaps, and slats reduce the pressure peak near the nose by changing the nose camber.
Slots and slats permit a new boundary layer to start on the main wing portion, eliminating the detrimental effect of the initial adverse gradient.
Leading Edge Devices
Leading Edge Devices
Are unique in that they may also be fully deployed on both wings to act as speed brakes. The reduced lift and increased drag can quickly reduce the speed of the aircraft in flight.
Dedicated speed brake panels similar to flight spoilers in construction can also be found on the upper surface of the wings of heavy and high-performance aircraft.
They are designed specifically to increase drag and reduce the speed of the aircraft when deployed.
Spoilers
Basic Propeller PrinciplesThe aircraft propeller consists of two or more blades and a central hub to which the blades are attached. Each blade of an aircraft propeller is essentially a rotating wing.
As a result of their construction, the propeller blades are like airfoils and produce forces that create the thrust to pull, or push, the aircraft through the air.
Blade angle, usually measured in degrees, is the angle between the chord of the blade and the plane of rotation and is measured at a specific point along the length of the blade.
Basic Propeller Principles
The reason a propeller is “twisted” is that the outer parts of the propeller blades, like all things that turn about a central point, travel faster than the portions near the hub.
Basic Propeller Principles
Left-Turning TendenciesTo the pilot, “torque” (the left turning tendency of the airplane) is made up of four elements which cause or produce a twisting or rotating motion around at least one of the airplane’s three axes. These four elements are:
1. Torque reaction from engine and propeller,
2. Corkscrewing effect of the slipstream,
3. Gyroscopic action of the propeller,
4. Asymmetric loading of the propeller (P-factor).
Torque Reaction Involves Newton’s Third Law of Physics—for every action, there is an equal and opposite reaction. As applied to the aircraft, this means that as the internal engine parts and propeller are revolving in one direction, an equal force is trying to rotate the aircraft in the opposite direction.
Left-Turning Tendencies
Corkscrew Effect or Slipstream The high-speed rotation of an aircraft propeller gives a corkscrew or spiraling rotation to the slipstream. At high propeller speeds and low forward speed (as in the takeoffs and approaches to power-on stalls), this spiraling rotation is very compact and exerts a strong sideward force on the aircraft’s vertical tail surface.
Left-Turning Tendencies
Gyroscopic PrecessionPrecession is the resultant action, or deflection, of a spinning rotor when a deflecting force is applied to its rim. When a force is applied, the resulting force takes effect 90° ahead of and in the direction of rotation.
Left-Turning Tendencies
P-FactorWhen an aircraft is flying with a high AOA, the “bite” of the downward moving blade is greater than the “bite” of the upward moving blade. This moves the center of thrust to the right of the prop disc area, causing a yawing moment toward the left around the vertical axis.
Left-Turning Tendencies
Controllable Pitch Propeller A controllable pitch propeller (CPP) or variable pitch propeller is a type of propeller with blades that can be rotated around their long axis to change their pitch. If the pitch can be set to negative values, the reversible propeller can also create reverse thrust for braking or going backwards without the need of changing the direction of shaft revolutions.
Adverse YawAdverse yaw is the tendency of an airplane to yaw away from the direction of aileron roll input. When an airplane rolls, it has more lift on the up-going wing than on the down-going wing. This causes an increase in induced drag on the up-going wing that will retard that wing’s forward motion and cause the nose to yaw in the opposite direction of the roll.
Stability, Maneuverability,
ControllabilityStability Stability is the tendency of an object (airplane) to return to its state of equilibrium once disturbed from it. There are two kinds of stability: static and dynamic.• Static stability is the initial tendency of an object
to move toward or away from its original equilibrium position.
• Dynamic stability is the position with respect to time, or motion of an object after a disturbance.
Static StabilityPositive Static Stability
If an object has an initial tendency toward its original equilibrium position after a disturbance, it is said to possess positive static stability.
Negative Static Stability
Is the initial tendency to continue moving away from equilibrium following a disturbance.
Static Stability
Neutral Static Stability
Is the initial tendency to accept the displacement position as a new equilibrium.
Static Stability
Dynamic StabilityPositive Dynamic StabilityAfter it is released, it will roll back to the bottom and up the other side. It will roll back and forth, oscillating less and less about the equilibrium position until it finally came to rest at the bottom of the bowl. It possesses positive dynamic stability.
Neutral Dynamic Stability
If the ball oscillates about the equilibrium position and the oscillations never dampen out, it possesses neutral dynamic stability.
Dynamic Stability
Negative Dynamic StabilityIf, somehow, the ball did not slow down, but continued to climb to a higher and higher position with each oscillation, it would never return to its original equilibrium position, depicts negative dynamic stability.
Dynamic Stability
Maneuverability The quality of an airplane that permits it to be maneuvered easily and to withstand the stresses imposed by maneuvers. It is governed by the airplane’s weight, inertia, size and location of flight controls, structural strength, and powerplant. It too is an airplane design characteristic.
The capability of an airplane to respond to the pilot’s control, especially with regard to flightpath and attitude. It is the quality of the airplane’s response to the pilot’s control application when maneuvering the airplane, regardless of its stability characteristics.
Controllability
Flight Controls and Effectiveness
Dutch Roll Is the result of strong lateral stability and weak directional stability. The airplane responds to a disturbance with both roll and yaw motions that affect each other
Most airplanes are designed so that the outer tips of the wings are higher than the wing roots attached to the fuselage. The upward angle thus formed by the wings is called the dihedral, and is usually only a few degrees.
Dihedral Angle
The rolling action of an airplane caused by gusts is constantly being corrected by the dihedral of the wings. If one wing gets lower than the other when the airplane is flying straight, it will have a different attitude in relation to the oncoming air. The result is that the lowered wing has a greater angle of attack and thus more lift than the raised wing and consequently will rise.
Dihedral Angle
If this rising action causes the wing to go past the level attitude, the opposite wing will then have a greater angle of attack and more lift. A dynamically stable airplane will oscillate less and less and eventually will return to its original position as the oscillation dampens.
Dihedral Angle
SweepBack AngleIs the angle between the lateral axis and a line drawn 25% aft of the leading edge.
WeathervaneLike the feathered arrow, the most important factor producing directional stability is the weathervaning effect created by the fuselage and vertical fin of the airplane.
It keeps the airplane headed into the relative wind. If the airplane yaws, or skids, the sudden rush of air against the surface of the fuselage and fin quickly forces the airplane back to its original direction of flight.
