David Young
Technical Subject Areas
1. Subject – Forces Acting on the Airplane.
2. Introduction – During all phases of flight, there are always four basic forces acting on
the aircraft. These include: thrust, drag, lift, and gravity. All four greatly affect the
behavior of the aircraft, and a thorough understanding of them will build a foundation for
understanding the aerodynamics of maneuvers.
3. Outline –
a. Draw a diagram of the four forces in straight-and-level flight.
i. Explain that the opposite forces are equal at this point.
1. They are only unequal when accelerating.
b. Explain what would happen if one vector is increased/decreased (for all four).
c. Explain lift.
i. Explain Bernoulli’s principle.
ii. Explain how it is affected by air density, wing surface area, and airspeed.
iii. Explain angle of attack’s effect on lift.
d. Explain weight.
e. Explain thrust.
f. Explain drag.
i. Define and explain induced drag.
ii. Define and explain parasite drag.
g. Draw a diagram of the four forces during a turn.
i. Explain the components of lift.
ii. Explain adverse yaw and how to compensate for it.
4. Contents –
a. There are four forces that always act on the aircraft:
i. Thrust – forward force produced by the engine and the propeller. It
opposes drag, and it points along the longitudinal axis, or the axis drawn
from tail to nose, of the aircraft.
ii. Drag – rearward acting force that is caused by the disruption of airflow by
the aircraft (i.e. wings, landing gear, etc…). It opposes thrust, and it acts
parallel to the relative wind, or wind that is created by motion (hand out
the car window).
iii. Lift – upward acting force that is created by the air flowing over the wings.
It opposes the force of gravity, and it acts perpendicular to the flightpath,
or the direction the aircraft is moving.
iv. Gravity – downward acting force that is created by the mass of the earth
pulling on the aircraft. It opposes lift, and it always points towards the
center of the earth.
b. When in straight-and-level flight, the opposing forces are equal.
Young 2, Forces
i. If a force is greater than its opposing force, then the aircraft will
accelerate, or change velocity, in that direction.
1. For example, if thrust is greater than drag, then the aircraft will
accelerate forwards.
c. Gravity pulls the aircraft down towards the ground.
i. This is why all objects have the tendency to fall to the ground.
d. The center of gravity is the point on the aircraft at which the aircraft could be
balanced.
i. It is easier to think of the force of gravity only acting on the CG of the
aircraft. This way, when drawing a force diagram, you can treat the entire
aircraft as only a single point in the system of forces.
e. Lift is the aerodynamic force that opposes gravity.
f. It is caused by the pressure differential both above and below the wing when air
hits it (refer to Figure A).
i. The air that travels over the upper wing must travel a further distance
because of the curvature of the wing.
ii. This causes the air to accelerate, which drops the pressure of the air.
iii. The air below the wing has a relatively higher pressure than the top of the
wing. This higher pressure pushes the wing upward and causes lift.
g. Lift is also created by the acceleration of the air downwards once it leaves the
trailing edge of the wing.
i. This plays off of Newton’s Third Law, which states that for every action
there is an equal and opposite reaction.
ii. In this case, the action is the air moving downwards, and the reaction is
the wing moving upwards.
h. If you reduce the angle of attack, the amount of lift is decreased, and if you
increase the angle of attack, the amount of lift is increased.
i. This is because in the equation to calculate lift, the coefficient of lift
increases with angle of attack until the critical angle of attack.
𝜌
1. Lift = CL 2V2A, where CL is the coefficient of lift, ρ is the air
density, V is the velocity, and A is the surface area of the liftgenerating surface.
i. If you are flying at 10,000 ft, the lift will not be as great as flying at 2,000 ft
because the air is less dense at higher altitudes.
i. A reduction in air density is essentially a reduction in airflow.
ii. Flying in warmer temperatures as opposed to colder temperatures has the
same effect because warm air is less dense than cold air
j. As airspeed decreases, a higher angle of attack is necessary to maintain altitude.
Young 3, Forces
i. This is because lift is lost with lower airspeeds.
ii. Increasing the angle of attack also gives the thrust force a vertical
component, which can add to the lifting force.
k. The only way to increase surface area is to use a certain type of leading or trailing
edge device, which will be discussed in a later lecture.
l. To most effectively control lift, the pilot can change the angle of attack as well as
the airspeed.
m. Thrust is a result of engine power.
n. To maintain airspeed, thrust and drag must cancel one another out.
i. If engine power is increased, then thrust will increase. Thrust will then
become greater than the force of drag, and the aircraft will accelerate
forwards.
o. During a climb, the thrust vector will actually point slightly upwards, and it will
contribute to vertical lift.
i. This splits the thrust force into a horizontal component and a vertical
component. The vertical component of thrust is what aids vertical lift.
ii. During slow flight, vertical lift is significantly reduced because of the
reduced airspeed. By increasing the angle of attack, the thrusting force
can contribute to the vertical lift and help the aircraft maintain altitude.
p. There are two types of drag: induced drag and parasite drag.
i. Induced Drag – drag that results in the production of lift. It decreases
with airspeed (refer to Figure D).
1. When the wings are producing lift, the high pressure air under the
wings tends to flow over the wingtip to the low pressure (as is the
tendency in the movement of air). This disrupts the airflow over
the wings by effectively increasing the wingtips’ angle of attack
(refer to Figure B).
a. Note that the wing does not bend upwards, but rather the
relative wind changes, which gives the wingtip a higher
angle of attack.
2. The disrupted airflow has the same effect as bending the lift vector
rearwards.
a. Part of this rearward lift vector points opposite thrust, and
therefore acts as drag.
b. Vortices, which are circulating masses of air, are created by
the disrupted airflow. They are potentially hazardous for
small aircraft because of the turbulence they produce.
Young 4, Forces
3. Induced drag decreases with airspeed because, as the aircraft
speeds up, the angle of attack needed to maintain altitude
decreases. In the case of induced drag, it is the angle of attack, and
not the airspeed, that determines the amount of drag.
a. Therefore, in slower flight, a higher angle of attack is
necessary to maintain altitude, and thus the increased
induced drag.
ii. Parasite Drag – drag that is created by the friction of air as it flows over
the aircraft’s surfaces. It increases with airspeed (refer to Figure D).
1. As objects are thrown into an airstream, they disrupt the airflow
and cause friction.
a. When you stick your hand out the window of a car, your
hand is blown backwards because of parasite drag.
2. This friction is manifested as drag.
3. There are three types of parasite drag: form drag, skin friction, and
interference drag.
a. Form Drag – drag that is created by a non-streamlined
body (a brick creates a lot of drag because it isn’t
streamlined).
b. Skin Friction – friction manifested as drag, which is the
result of the friction caused by the air flowing over a rough
surface (bolts sticking out of the wing creates friction).
c. Interference Drag – drag caused by parts of the aircraft
disrupting the air flowing over other parts of the aircraft
(wings disrupting the elevator’s airflow).
q. In a turn, the lift vector is split into a horizontal component and a vertical
component (refer to Figure C).
i. A new force comes into play during a turn – centrifugal force.
1. Centrifugal Force – an apparent force resulting from the effect of
inertia during a turn (Cajun Cliffhanger).
ii. The horizontal component of lift now acts opposite the centrifugal force.
1. If the horizontal component exceeds the centrifugal force, then the
ball in the inclinometer will swing to the inside of the turn. This is
called a slipping turn.
2. If the horizontal force exceeds the centrifugal force, then the ball in
the inclinometer will swing to the outside of the turn. This is
called a skidding turn
3. Keeping the ball centered means that the two forces are equal and
that the turn is coordinated.
Young 5, Forces
r. During a turn, one wing is raised while the other descends (called bank). Induced
drag is created on the ascending wing because more lift is being generated on that
wing than the descending wing.
i. This drag slows down the ascending wing, causing the airplane’s nose to
point towards the high wing.
ii. This is known as adverse yaw.
iii. To compensate for it, use rudder in the same direction you are turning.
1. For example, if you are turning to the right, simultaneously apply
right rudder while turning the yoke right.
s. More lift is needed in order to maintain altitude because the vertical lift is reduced
during a turn due to the existence of a horizontal component.
i. This extra lift can come from both an increase in power, which will
increase the thrust vector, or from more elevator back pressure, which will
compensate for the loss of vertical lift.
5. Evaluation –
a. The student can identify the four forces during flight.
b. The student can draw a force diagram for both straight-and-level flight and turns.
c. The student can explain what a slipping and a skidding turn is and why the occur.
d. The student can explain the need for elevator backpressure during turns.
e. The student can identify and briefly explain the two types of drag.
6. References –
a. AFH, Ch. 4
b. PHAK, Ch. 3
Young 6, Forces
Figure A
Figure B
Young 7, Forces
Figure C
Figure D