Page 75 - Understanding Flight

CH03_Anderson 7/25/01 8:56 AM Page 62

62 CHAPTER THREE

The smallest radio-controlled wing as viewed from above. How big should the wing be?
1 Should it have a high or a low aspect ratio? Should the wing
airplane weights 10 grams ( 3
ounce). be swept? Should it be tapered? In this section we examine
how to choose the basic planform of the wing.

Wing Loading

The first design parameter to determine is the wing loading. This is
2
the ratio of wing area to weight of the airplane, measured in lb/ft or
2
kg/m . Many basic performance parameters are determined as a
function of wing loading, which is discussed in Chapter 7, “Airplane
Performance.” Wing loading will determine cruise performance, take-
off and landing distances, and power requirements. Typical wing
loadings for various aircraft are shown in Table 3.1. Note that light
trainers have low wing loading while commercial transports and mil-
itary aircraft have a wing loading as much as 10 times greater.
There are many tradeoffs to be considered with wing loading.
One consideration is that the higher the wing loading the higher the
stall speed. That is why trainers have lower wing loading. But low
wing loading also limits top cruising speed. A higher wing loading
is desired for faster airplanes. Also, airplanes with higher wing
loading are less susceptible to clear air turbulence. The inertia of
the airplane against the gusts of turbulence makes the airplane
harder to blow around. Details of these tradeoffs are discussed in
Chapter 7.

Aspect Ratio
The aspect ratio of the wing is the span (measured from tip to tip)
divided by the mean chord. In Chapter 2 the advantage of high aspect
ratio (long narrow wings) was discussed for low-speed air-
Insects have a wing loading of
craft. High aspect ratio wings are more efficient, because of
2
0.01 to 0.5 lb/ft (1 to 10
the reduced loading due to upwash. A high aspect ratio wing
2
N/m ). Birds have a wing loading
can use a smaller engine and needs less takeoff and landing
2
of 0.1 to 1.0 lb/ft (10 to 100
distance than a low aspect ratio wing. However, there are
2
N/m ). Airplanes have a wing
many low-speed aircraft with low aspect ratio wings. Why
loading from 10 to 200 lb/ft 2
would a designer select a low aspect ratio when it is known
2
(1000 to 20,000 N/m ).
that the wing will suffer induced power and drag penalties?