Page 199 - Understanding Flight

CH07_Anderson 7/25/01 9:00 AM Page 186

186 CHAPTER SEVEN

enough to achieve supersonic flight, so in this case the high service
ceiling is due to the high L/D rather than a high power-to-weight ratio.
The fact that this airplane was slow led to the incident with Gary
Powers, when he was shot down over the Soviet Union in 1960. This
political catastrophe helped push the development of a replacement
reconnaissance aircraft, the SR-71.
Since the U-2 could not fly fast, it was susceptible to antiaircraft
missiles. The SR-71 (shown in Figure 3.6) was developed to prevent
such possibilities. The SR-71 had to fly at high altitudes and at very
high speed. Its published service ceiling is 80,000 ft (24,000 m) and its
maximum speed is Mach 3.2 or 2300 mi/h (3700 km/h). In
order to be fast, the SR-71 has a high thrust-to-weight ratio.
The air pressure at 63,000 ft
The compromise for high-speed flight is that the L/D is low.
(19,000 m) is so low that water
So the SR-71 and the U-2, designed for the same mission,
will boil at body temperature.
made two drastically different choices in their designs.

Fuel Consumption

When we think of fuel consumption for a car, we think in terms of
miles per gallon (or liters per 100 km). These are natural units, since
cars have odometers and we measure the amount of fuel when we fill
up. On the other hand, these units are not appropriate for an airplane.
The airplane is flying in a moving fluid. A small airplane in a strong
head wind at a low power setting can actually fly backward with
respect to the ground, while measuring a substantial positive airspeed.
Pilots are more concerned with how much fuel is on board and how
long they can remain airborne. Recall that in Chapter 2 you learned
that induced power was proportional to load squared. The pilot of a
commercial airplane wants to fill up with as little fuel as necessary. By
the end of the flight, the fuel tanks should contain only the FAA
required reserves, which should be enough to reach an alternate
destination, if necessary. The important parameters in determining
the necessary fuel are the anticipated ground speed, which gives the
time in the air, and the rate that fuel is consumed. The rate of fuel
consumption is measured in units of gal/h (l/h) for small airplanes
and lb/h (kg/h) for large airplanes. Unlike a car, the rate of fuel