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Pilot's Handbook of Aeronautical Knowledge
Aerodynamics of Flight
High Speed Flight

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Pilot's Handbook of Aeronautical Knowledge



Table of Contents

Chapter 1, Introduction To Flying
Chapter 2, Aircraft Structure
Chapter 3, Principles of Flight
Chapter 4, Aerodynamics of Flight
Chapter 5, Flight Controls
Chapter 6, Aircraft Systems
Chapter 7, Flight Instruments
Chapter 8, Flight Manuals and Other Documents
Chapter 9, Weight and Balance
Chapter 10, Aircraft Performance
Chapter 11, Weather Theory
Chapter 12, Aviation Weather Services
Chapter 13, Airport Operation
Chapter 14, Airspace
Chapter 15, Navigation
Chapter 16, Aeromedical Factors
Chapter 17, Aeronautical Decision Making




Mach Number Versus Airspeed
It is important to understand how airspeed varies with Mach
number. As an example, consider how the stall speed of a
jet transport aircraft varies with an increase in altitude. The
increase in altitude results in a corresponding drop in air
density and outside temperature. Suppose this jet transport
is in the clean configuration (gear and flaps up) and weighs
550,000 pounds. The aircraft might stall at approximately
152 KCAS at sea level. This is equal to (on a standard day)
a true velocity of 152 KTAS and a Mach number of 0.23.
At FL 380, the aircraft will still stall at approximately 152
KCAS but the true velocity is about 287 KTAS with a Mach
number of 0.50.

Although the stalling speed has remained the same for our
purposes, both the Mach number and TAS have increased.
With increasing altitude, the air density has decreased; this
requires a faster true airspeed in order to have the same
pressure sensed by the pitot tube for the same KCAS or KIAS
(for our purposes, KCAS and KIAS are relatively close to
each other). The dynamic pressure the wing experiences at
FL 380 at 287 KTAS is the same as at sea level at 152 KTAS.
However, it is flying at higher Mach number.

Another factor to consider is the speed of sound. A decrease
in temperature in a gas results in a decrease in the speed of
sound. Thus, as the aircraft climbs in altitude with outside
temperature dropping, the speed of sound is dropping. At
sea level, the speed of sound is approximately 661 KCAS,
while at FL 380 it is 574 KCAS. Thus, for our jet transport
aircraft, the stall speed (in KTAS) has gone from 152 at sea
level to 287 at FL 380. Simultaneously, the speed of sound
(in KCAS) has decreased from 661 to 574 and the Mach
number has increased from 0.23 (152 KTAS divided by 661
KTAS) to 0.50 (287 KTAS divided by 574 KTAS). All the
while the KCAS for stall has remained constant at 152. This
describes what happens when the aircraft is at a constant
KCAS with increasing altitude, but what happens when the
pilot keeps Mach constant during the climb? In normal jet
flight operations, the climb is at 250 KIAS (or higher (e.g.
heavy)) to 10,000 feet and then at a specified en route climb
airspeed (such as about 330 if a DC10) until reaching an
altitude in the "mid-twenties" where the pilot then climbs at
a constant Mach number to cruise altitude.

Assuming for illustration purposes that the pilot climbs at a
MMO of 0.82 from sea level up to FL 380. KCAS goes from
543 to 261. The KIAS at each altitude would follow the
same behavior and just differ by a few knots. Recall from
the earlier discussion that the speed of sound is decreasing
with the drop in temperature as the aircraft climbs. The Mach
number is simply the ratio of the true airspeed to the speed
of sound at flight conditions. The significance of this is that
at a constant Mach number climb, the KCAS (and KTAS or
KIAS as well) is falling off.

If the aircraft climbed high enough at this constant MMO
with decreasing KIAS, KCAS, and KTAS, it would begin to
approach its stall speed. At some point the stall speed of the
aircraft in Mach number could equal the MMO of the aircraft,
and the pilot could neither slow up (without stalling) nor
speed up (without exceeding the max operating speed of the
aircraft). This has been dubbed the "coffin corner."

Boundary Layer
The viscous nature of airflow reduces the local velocities on
a surface and is responsible for skin friction. As discussed
earlier in the chapter, the layer of air over the wing's surface
that is slowed down or stopped by viscosity, is the boundary
layer. There are two different types of boundary layer .ow:
laminar and turbulent.

Laminar Boundary Layer Flow
The laminar boundary layer is a very smooth flow, while
the turbulent boundary layer contains swirls or "eddies."
The laminar flow creates less skin friction drag than the
turbulent flow, but is less stable. Boundary layer flow over
a wing surface begins as a smooth laminar flow. As the .ow
continues back from the leading edge, the laminar boundary
layer increases in thickness.

Turbulent Boundary Layer Flow
At some distance back from the leading edge, the smooth
laminar flow breaks down and transitions to a turbulent flow.
From a drag standpoint, it is advisable to have the transition
from laminar to turbulent flow as far aft on the wing as
possible, or have a large amount of the wing surface within
the laminar portion of the boundary layer. The low energy
laminar flow, however, tends to break down more suddenly
than the turbulent layer.

Boundary Layer Separation
Another phenomenon associated with viscous flow is
separation. Separation occurs when the airflow breaks away
from an airfoil. The natural progression is from laminar
boundary layer to turbulent boundary layer and then to
airflow separation. Airflow separation produces high drag
and ultimately destroys lift. The boundary layer separation
point moves forward on the wing as the AOA is increased.
[Figure 4-58]