User Guide

Flight Instruction
237
Flight Instruction
236
JETS 101B:
High-Speed Aerodynamics
Until now, you’ve been flying straight-wing airplanes in a
relatively low speed range, one in which airspeed was the primary
factor. In the jet, however, you’ll be flying at a significant fraction
of the speed of sound, called Mach 1 (after Austrian physicist Ernst
Mach, who did much of the early research into high-speed fluid
and gas flow). As you approach Mach 1, the behavior of the air
changes: it becomes more like water, an incompressible fluid,
than a gas. Since air can’t readily move faster than the speed at
which sound propagates through it, in a sense it “can’t get out of
its own way” fast enough. Instead of flowing smoothly over a
wing, it “piles up” to form shock waves.
The speed at which this occurs, for a given airfoil, is called its
critical Mach number, and it applies to the speed at which the air
moves chordwise, straight from the leading to the trailing edge. If
the wing is swept, so the air moves over it obliquely, the speed of
the chordwise component is reduced, so the airplane as a whole
can fly faster without encountering Mach number difficulties.
This brings with it, unfortunately, some other problems. One
of the most common is something colloquially called “Dutch
Roll.” Alas, this isn’t something like a Danish pastry; rather, it’s a
coupling, or relationship, between roll and yaw that can make the
airplane difficult to control.
What was needed was something that could move air faster
than a propeller, but slower than a pure turbojet, and that’s the tur-
bofan. At the core of every turbofan is a straight turbojet - but
attached to the front is a big fan (or, if you prefer, a small, shroud-
ed propeller with a whole lot of blades). This fan is driven, like the
propeller of a turboprop, by energy extracted from the jet exhaust,
using its own set of turbine blades (and, in some engines, includ-
ing those on the Hawker, a set of reduction gears very similar to
those on turboprops).
Only the physical arrangement is different, since the fan is
an integral part of the engine. Once air has been sucked in by the
fan, however, most of it doesn’t go through the core turbojet
engine; instead, it bypasses it via a circular duct that surrounds the
core. The engines in the Hawker have a bypass ratio of almost 4
to 1 - i.e., some four times more air goes around the core engine
than through it.
The cool air passing through the bypass duct doesn’t get
accelerated nearly so much as the hot gases coming out of the tur-
bine, so it propels the airplane much more efficiently. In addition,
it provides a “sheath” around the hot jet exhaust, allowing it to
mix gradually with outside air, so turbofans are inherently much
quieter than pure jets. In fact, old “straight pipe” jets like early
Lears are now banned from many airports, especially at night.