| Home | Privacy | Contact |

Airplane Flying Handbook
Transition to Jet Powered Airplanes
SPEED MARGINS

| First | Previous | Next | Last |

Airplane Flying Handbook

Preface

Table of Contents

Chapter 1,Introduction to Flight Training
Chapter 2,Ground Operations
Chapter 3,Basic Flight Maneuvers
Chapter 4, Slow Flight, Stalls, and Spins
Chapter 5, Takeoff and Departure Climbs
Chapter 6, Ground Reference Maneuvers
Chapter 7, Airport Traffic Patterns
Chapter 8, Approaches and Landings
Chapter 9, Performance Maneuvers
Chapter 10, Night Operations
Chapter 11,Transition to Complex Airplanes
Chapter 12, Transition to Multiengine Airplanes
Chapter 13,Transition to Tailwheel Airplanes
Chapter 14, Transition to Turbo-propeller Powered Airplanes
Chapter 15,Transition to Jet Powered Airplanes
Chapter 16,Emergency Procedures

Glossary

Index

To observe both limits Vmo and Mmo, the pilot of a jet
airplane needs both an airspeed indicator and a
Machmeter, each with appropriate red lines. In some
general aviation jet airplanes, these are combined into
a single instrument that contains a pair of concentric
indicators, one for the indicated airspeed and the other
for indicated Mach number. Each is provided with an
appropriate red line. [Figure 15-8]

Jet airspeed indicator.
Figure 15-8. Jet airspeed indicator.

A more sophisticated indicator is used on most
jetliners. It looks much like a conventional airspeed
indicator but has a "barber pole" that automatically
moves so as to display the applicable speed limit at all
times.

Because of the higher available thrust and very low
drag design, the jet airplane can very easily exceed its
speed margin even in cruising flight, and in fact in
some airplanes in a shallow climb. The handling
qualities in a jet can change drastically when the
maximum operating speeds are exceeded.

High speed airplanes designed for subsonic flight are
limited to some Mach number below the speed of
sound to avoid the formation of shock waves that begin
to develop as the airplane nears Mach 1.0. These shock
waves (and the adverse effects associated with them)
can occur when the airplane speed is substantially
below Mach 1.0. The Mach speed at which some
portion of the airflow over the wing first equals Mach
1.0 is termed the critical Mach number (MACHCRIT).
This is also the speed at which a shock wave first
appears on the airplane.

There is no particular problem associated with the
acceleration of the airflow up to the point where Mach
1.0 is encountered; however, a shock wave is formed at
the point where the airflow suddenly returns to
subsonic flow. This shock wave becomes more severe
and moves aft on the wing as speed of the wing is
increased, and eventually flow separation occurs
behind the well-developed shock wave. [Figure 15-9]

Transonic flow patterns.
Figure 15-9.Transonic flow patterns.

If allowed to progress well beyond the MMO for the
airplane, this separation of air behind the shock wave
can result in severe buffeting and possible loss of
control or "upset."

Because of the changing center of lift of the wing
resulting from the movement of the shock wave, the
pilot will experience pitch change tendencies as the
airplane moves through the transonic speeds up to and
exceeding Mmo. [Figure 15-10]

Example of Stick Forces vs. Mach Number in a typical jet airplane.
Figure 15-10. Example of Stick Forces vs. Mach Number in a typical jet airplane.

For example, as the graph in figure 15-10 illustrates,
initially as speed is increased up to Mach .72 the wing
develops an increasing amount of lift requiring a nosedown
force or trim to maintain level flight. With
increased speed and the aft movement of the shock
wave, the wing's center of pressure also moves aft
causing the start of a nosedown tendency or "tuck." By
Mach .83 the nosedown forces are well developed to a
point where a total of 70 pounds of back pressure are
required to hold the nose up. If allowed to progress
unchecked, Mach tuck may eventually occur.
Although Mach tuck develops gradually, if it is
allowed to progress significantly, the center of
pressure can move so far rearward that there is no
longer enough elevator authority available to
counteract it, and the airplane could enter a steep,
sometimes unrecoverable dive.

 

15-7