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Beginner's Guide
to Aerodynamics
Altitude and Flight Forces
Answers
 Compare and contrast
the following forces on an airplane: weight, lift, thrust, and drag.
 Force
may be thought of as a push or pull in a specific direction.
 Weight
is a force that is always directed towards the center of the earth.
 Thrust
is the force provided by the engines and moves an airplane through
the air.
 Drag
is the resistance force provided by the air as the airplane moves
through the air.
 Lift
is a "remaining" aerodynamic force at a right angle to drag and
perpendicular to the flight direction. Several components work together
to produce aircraft lift, most of which is generated by the wings
of the plane.
 Netforce
is lift minus the weight of the plane.
 Alter the altitude
of the given airfoil. Enter values in the following table. As the altitude
changes, what happens to the density, pressure, temperature, and lift
quantities? (Note: Answers are written in the form shown in FoilSim.)
Altitude

4763

12864

17775

25500

30000

36000

44800

50000

Density

.00207

.00161

.00137

.00105

9.0
E 4

7.1
E 4

4.7
E 4

3.7
E 4

Pressure

12.34

9.03

7.42

5.34

4.37

3.28

2.15

1.68

Temperature

42
F

13
F

4
F

32
F

48
F

70
F

70
F

70
F

Lift

2.731
x 10000

2.123
x 10000

1.808
x 10000

1.387
x 10000

1.179
x 10000

9.356
x 1000

6.129
x 1000

4.773
x 1000

 Compare the information
obtained about the forces on an airplane with the table information.
What is the implication of the changes in altitude on the forces affecting
the flight of an airplane? Think this through carefully. As
the altitude increases, density, pressure, temperature, and the generated
lift decrease. With a decrease in density, the drag on the plane decreases.
In other words, in "thin air" there is no drag, essentially. However,
this in turn affects the lift that is needed to keep the plane in the
air. To compensate, the engines should produce more thrust to increase
the velocity of the plane. For this reason, an airplane has a flight
ceiling, the altitude above which that airplane cannot fly because there
will not be enough air mass to generate the lift required to keep the
plane and its weight airborne.
 As the altitude
changes, what happens to the graphs seen in the Plotter View Panel window?
The view on the Airfoil Plotter Panel retains
the basic shape formed by the yellow and the white graphs. When x >
0, the white curve seems to indicate a sharp decrease followed by a
"leveling out" while the yellow curve seems to show a steady increase
until it intersects the white. The y values shown change to reflect
the changes in pressure caused by the change in the altitude.
 What are the implications
or importance of the yellow and white graphs seen in the Plotter View
Panel window? Think carefully. In general,
the y values of the yellow curve are greater than the y values of the
white curve. The basic shape models the pressures acting above (white)
and below (yellow) the airfoil moving from the front to the rear. The
white curve remains below the yellowunless the pilot wants to fly
upside down!

Related Pages:
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Aerodynamics Index


