| Home | Privacy | Contact |

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

| First | Previous | Next | Last |

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




Wing airflow.
Figure 4-56. Wing airflow.

Speed Ranges
The speed of sound varies with temperature. Under standard
temperature conditions of 15 °C, the speed of sound at sea
level is 661 knots. At 40,000 feet, where the temperature is
–55 °C, the speed of sound decreases to 574 knots. In highs peed
flight and/or high-altitude flight, the measurement of
speed is expressed in terms of a "Mach number"—the ratio
of the true airspeed of the aircraft to the speed of sound in
the same atmospheric conditions. An aircraft traveling at
the speed of sound is traveling at Mach 1.0. Aircraft speed
regimes are defined approximately as follows:
Subsonic—Mach numbers below 0.75
Transonic—Mach numbers from 0.75 to 1.20
Supersonic—Mach numbers from 1.20 to 5.00
Hypersonic—Mach numbers above 5.00

While flights in the transonic and supersonic ranges are
common occurrences for military aircraft, civilian jet aircraft
normally operate in a cruise speed range of Mach 0.7 to
Mach 0.90.

During flight, a wing produces lift by accelerating the
airflow over the upper surface. This accelerated air can, and does,
reach sonic speeds even though the aircraft itself may be
flying subsonic. At some extreme AOAs, in some aircraft,
the speed of the air over the top surface of the wing may be
double the aircraft's speed. It is therefore entirely possible
to have both supersonic and subsonic airflow on an aircraft
at the same time. When flow velocities reach sonic speeds at
some location on an aircraft (such as the area of maximum
camber on the wing), further acceleration results in the onset
of compressibility effects such as shock wave formation,
drag increase, buffeting, stability, and control difficulties.
Subsonic flow principles are invalid at all speeds above this
point. [Figure 4-56]

The speed of an aircraft in which airflow over any part of
the aircraft or structure under consideration first reaches
(but does not exceed) Mach 1.0 is termed "critical Mach
number" or "Mach Crit." Thus, critical Mach number is
the boundary between subsonic and transonic flight and is
largely dependent on the wing and airfoil design. Critical
Mach number is an important point in transonic flight When
shock waves form on the aircraft, airflow separation followed
by buffet and aircraft control difficulties can occur. Shock
waves, buffet, and airflow separation take place above critical
Mach number. A jet aircraft typically is most efficient when
cruising at or near its critical Mach number. At speeds 5–10
percent above the critical Mach number, compressibility
effects begin. Drag begins to rise sharply. Associated with
the "drag rise" are buffet, trim and stability changes, and a
decrease in control surface effectiveness. This is the point
of "drag divergence." [Figure 4-57]

Critical Mach.
Figure 4-57. Critical Mach.

VMO/MMO is defined as the maximum operating limit speed.
VMO is expressed in knots calibrated airspeed (KCAS), while
MMO is expressed in Mach number. The VMO limit is usually
associated with operations at lower altitudes and deals with
structural loads and .utter. The MMO limit is associated with
operations at higher altitudes and is usually more concerned
with compressibility effects and flutter. At lower altitudes,
structural loads and .utter are of concern; at higher altitudes,
compressibility effects and flutter are of concern.

Adherence to these speeds prevents structural problems due
to dynamic pressure or .utter, degradation in aircraft control
response due to compressibility effects (e.g., Mach Tuck,
aileron reversal, or buzz), and separated airflow due to shock
waves resulting in loss of lift or vibration and buffet. Any of
these phenomena could prevent the pilot from being able to
adequately control the aircraft.

For example, an early civilian jet aircraft had a VMO limit of
306 KCAS up to approximately FL 310 (on a standard day).
At this altitude (FL 310), an MMO of 0.82 was approximately
equal to 306 KCAS. Above this altitude, an MMO of 0.82
always equaled a KCAS less than 306 KCAS and, thus,
became the operating limit as you could not reach the VMO
limit without first reaching the MMO limit. For example, at
FL 380, an MMO of 0.82 is equal to 261 KCAS.