Beginner's Guide to Aerodynamics
Altitude and Flight Forces
Answers

1. 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.

    

2. 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

3. 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.
   
   
4. 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.
   
   
5. 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 yellow--unless the pilot wants to fly upside down!