intro to jet aerodynamics and handling-akg

8/10/2019 Intro to Jet Aerodynamics and Handling-Akg

http://slidepdf.com/reader/full/intro-to-jet-aerodynamics-and-handling-akg 1/51



2

Purpose

• Demonstrate aerodynamic differences

between piston and jet aircraft



3

Objectives

• State 4 types of jet airplane buffet

• Identify aerodynamic properties of a

swept wing

• Define the basic types of stability

• Define critical Mach

• Know the airplane reaction in regardsto speed stability, dutch roll, and

Mach tuck.



4

Compressibility and Mach

Number• At 18,000 ft, 185 KTAS:

TAT = -21C

PT = 7.7 lb/in2

TAT = -25C

PT = 5.2 lb/in2

OAT = -21C

P AMB = 7.3 lb/in2

• At FL350, Mach .78:

OAT = -54C

P AMB = 3.5 lb/in2



5

Mach Number and Speed of

Sound

Mach No. =True Airspeed

Speed of Sound Altitude

Temperature (oC)

Sea level:

15 oC

36,089 ft and above:-56.5 oC

15-56.5

FL360

0

Altitude

Speed of sound (KTAS)

Sea level:

661 KTAS

36,089 ft and above:

574 KTAS

661574

FL360

0



6

VMO and MMO

Crossover altitude

True Airspeed, V

Pressure

Altitude

Climb speedschedule

VMO is a typically a constant IASMMO is a constant Mach



7

High Speed Buffet

• At some even higher speed,

complete flow separation can occur.

This will result in buffeting in level

flight – High Speed Buffet will befelt.

Flight Speed = Mach .798



8

Critical Mach and Drag Rise

• Drag is adversely impacted by shock

wave formation

Drag

Coefficient

(CD)

Mach

Critical

Mach



9

Initial Buffet Envelope

Mach

A l t i t u d e ( 1 0 0 0 f t )

.60 .68 .76 .84

20

25

30

45

40

35

.52

Increasing weight

or load factor



10

Initial Buffet Envelope

Mach

A l t i t u d e ( 1 0 0 0 f t )

.60 .68 .76 .84

20

25

30

45

40

35

.52



11

“Coffin” corner

Mach

A l t i t u d e ( 1 0 0 0 f t )

.60 .68 .76 .84

20

25

30

45

40

35

.52

Coffin

corner



12

Stick-Shaker

• Many commercial airplane designs also

include a “stick shaker ” to artificially

augment natural pre-stall buffet in regions

where it is not naturally prominent… • The stick shaker is programmed to trigger

at a pre-defined margin above stall.

At low altitudes and for flaps down operation, the loweramber band shows stick shaker speed for a 1.3G

load factor, and the lower “barber pole” shows the

stick-shaker speed for current load factor .



13

Jet Transport Handling

Characteristics

• Static and dynamic stability

• Wing sweep, “dihedral effect” and

“Dutch roll”

• Speed stability and speed trim

system

• Mach “tuck” and mach trim system



14

Basic Stability Definitions

Airplane stability and control is critical to

safety of flight, as well as passenger

comfort.

• Stability – a tendency of the airplane toretain the same attitude in relation to it’s

flight path or it’s relative motion to the air;

the tendency to return to a state of

equilibrium.

• Control – the ability to adjust the

airplane’s attitude in relationship to it’s

flight path or it’s relative motion to the air.



16

Positive Static Stability

“Static” stability – indicates the airplane’s

initial tendency to return to a trim condition if

disturbed:

In these examples, the initial tendency is

to return to the equilibrium condition –

this is called positive static stability



17


“Static” stability – indicates the

airplane’s initial tendency to return to a

trim condition if disturbed:






18










19










20

Negative Static Stability


airplane’s initial tendency to return to

a trim condition if disturbed:

In these examples, the initial tendency is to

diverge further from the equilibrium condition –

this is called negative static stability



21

Neutral Static Stability

“Static” stability – indicates the airplane’s

initial tendency to return to a trim

condition if disturbed:

In these examples, there is no initial tendency to

either return or to diverge further from the

equilibrium condition – this is called neutral static

stability



22

Airplane Stability

• Airplane stability and control needs tobe evaluated in all axes:

– Pitch Longitudinal Stability

– Roll Lateral Stability – Yaw Directional Stability

• Secondary “coupled modes” also

evaluated…



23

Dutch Roll

The phenomenon of “Dutch” roll is an

interesting characteristic largely

associated with swept wing airplanes,

and is caused by “roll-yawcoupling”…



25

Dutch Roll

Due to primarily to wing sweep, the

advancing wing generates more lift,

causing a roll into the yaw…



26

Dutch Roll

The wing generating more lift also

generates more drag. This causes the

airplane to yaw away from the bank…



27

Dutch Roll

At the point where the airplane

develops significant yaw in the

opposite direction, the roll tendency

reverses also…



28

Dutch Roll

The roll due to yaw continues…



29

Dutch Roll

The advancing wing induces yaw in the

opposite direction…



30

Dutch Roll

The yaw

continues…



32

Dutch Roll

We are back to the initial yaw condition,

and one cycle of Dutch roll is

complete…

Depending on the dynamic stability of

the Dutch Roll mode, it may take

several cycles to damp this

oscillation…



34

Speed Stability

Thrust,

Drag

Mach

Min Drag

Thrust

limited



36

Speed Stability

Thrust,

Drag

Mach

At this Mach, the airplaneexhibits positive

speed stability



38

Speed Trim

• To compensate for this natural negative speedstability when hand-flying at slower speeds, a

speed trim system is added

• This system automatically applies stabilizer trim as

necessary to oppose the unintended speed change:

• Speed trim is most likely to activate on takeoffs,

climbouts, and go-arounds, at light weights, aft

center of gravity locations, and at high thrust

increasing speed nose up trim decreasing speed nose down trim



39

Mach “tuck”

Lift

Speed above Mcrit,reduces lift forward of CG, and

reduces downwash at the tailplane.

Will cause nose down movement.



41

Mach Trim

• To compensate for this effect in jet

transports that cruise routinely at speeds

above MCRIT, a mach trim system is

added• This system automatically applies

stabilizer trim as necessary to counteract

the movement in the center of pressure

• Inflight failure or MEL dispatch withinoperative mach trim, if allowed, can

impact speed envelope



42

Summary

• Flight characteristics of jets

• Different types of buffet

• Swept wing characteristics

• Airplane reaction to stability types,

dutch roll, and Mach tuck,



43

Review Questions

Mach number is the ratio of

a) the airplane’s true airspeed to the

speed of sound at sea level.

b) the airplane’s true airspeed to the

local speed of sound.

c) the airplane’s indicated airspeed to

it’s true airspeed.



44

Review Questions

The speed of sound ___ as

temperature decreases.

a) decreases

b) increases

c) remains constant



45

Review Questions

Critical mach is the

a) mach number that defines the sound

barrier.

b) lowest flight speed at which local

velocity on the wing first exceeds the

speed of sound.

c) the mach number at which a wing is

designed to cruise.

d) the maximum operating mach.



46

Review Questions

In cruise, flight at speeds above critical

mach

a) will induce a shock wave and

turbulent flow that increases drag.

b) can overstress the wing.

c) is not dangerous as long as high

speed buffet is not present.

d) all of the above

e) a and c only



47

Review Questions

In cruise, initial buffet might be

encountered when

a) speeding up.

b) slowing down.

c) climbing to a higher altitude.

d) rolling into a turn.e) all of the above



48

Review Questions

The stick-shaker is intended to

a) provide warning of impending stall at

conditions where the wing’s natural

buffet is not sufficient to act as adeterrent to further speed decay.

b) indicate that the airplane is

exceeding VMO.

c) automatically verify that controls are

free and correct.

d) wake the crew in time to eat dessert.



50

Review Questions

Dutch roll is

a) a coupled roll-yaw oscillation that is

common on swept-wing aircraft when the

yaw damper is off or inoperative.b) the rolling tendency that occurs when an

engine fails.

c) an inherently dangerous and unstable

maneuver that must be avoided.

d) an evasive maneuver pioneered by

Anthony Fokker.



Review Questions

True/False: A jet transport always

exhibits naturally positive speed

stability.

intro to jet aerodynamics and handling-akg

Documents