Game Development Reference
acting on the aircraft, and it may be neglected. 5 An exception may be when the foil has
Flaps are control devices used to alter the shape of the foil so as to change its lift char‐
acteristics. Figure 15-6 also shows typical lift, drag, and moment coefficients for an
airfoil fitted with a plain flap deflected downward at 15°. 6 Notice the significant increase
in lift, drag, and pitch moment when the flap is deflected. Theory of Wing Sections also
provides data for flapped airfoils for flap angles between −15° and 60°.
The most notable force that we've yet to discuss is thrust—the propulsion force. Thrust
provides forward motion; without it, the aircraft's wings can't generate lift and the air‐
craft won't fly. Thrust, whether generated by a propeller or a jet engine, is usually ex‐
pressed in pounds, and a common ratio used to compare the relative merits of aircraft
powering is the thrust-to-weight ratio . This ratio is the maximum thrust deliverable by
the propulsion plant divided by the aircraft's total weight. When the thrust-to-weight
ratio is greater than one, the aircraft is capable of overcoming gravity in a vertical climb.
This is more like a rocket than a traditional airplane. Most normal planes are not capable
of this, but many military planes do have thrust-to-weight ratios of greater than one.
However, airplane engines rely on oxygen in the atmosphere to combust their fuel with
and to produce the force that propels them forward. As the plane climbs higher, the
engines will have less oxygen and produce less thrust. The thrust-to-weight ratio will
fall, and eventually the plane will again need lift from the wings to maintain its altitude.
Even when the plane is climbing vertically like a rocket, the wings still generate lift, and
in this case try to pull the airplane away from a vertical trajectory.
Besides gravity, thrust, wing lift, and wing drag, there are other forces that act on an
aircraft in flight. These are drag forces (and lift in some cases) on the various components
of the aircraft besides the wings. For example, the fuselage contributes to the overall
drag acting on the aircraft. Additionally, anything sticking out of the fuselage will con‐
tribute to the overall drag. If it's not a wing, anything sticking out of the fuselage is
typically called an appendage . Some examples of appendages are the aircraft landing
gear, canopy, bombs, missiles, fuel pods, and air intakes.
Typically, drag data for fuselages and appendages is expressed in terms of a drag coef‐
ficient similar to that discussed in Chapter 6 , where experimentally determined drag
forces are nondimensionalized by projected frontal area (S), density (ρ), and velocity
5. Aircraft designers must always consider this pitching moment when designing the aircraft's structure, as this
moment tends to want to twist the wings off the fuselage.
6. There's a large variety of flap designs besides the plain trailing-edge flap discussed here. Flaps are typically
referred to in the literature as high lift devices , and the references we'll provide in this chapter give rough
descriptions of the most common designs.