Basic Aerodynamics - Dartmouth Flying Club

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Basic Aerodynamics

Basic Aerodynamics

Dartmouth Flying Club

October 10, 2002

Andreas Bentz

Basic Aerodynamics

Lift

Bernoulli’s Principle

3

Energy


Definition
: Energy is the ability to do work.


Energy cannot be created or destroyed. We
can only change its form.


A fluid in motion has (mainly) two forms of
energy:


kinetic energy (velocity),


potential energy (pressure).

4

The Venturi Tube and Bernoulli’s Principle

kinetic energy

(velocity)

potential energy

(pressure)

velocity
increases

pressure
decreases

5

Lift: Wing Section


Air flows toward the low pressure area above the wing:
upwash and downwash.


Newton’s third law of motion: to every action there is
an equal and opposite reaction.


“The reaction to downwash is, in fact, that misunderstood
force called lift.” Schiff p. 8

upwash

downwash

relative low pressure

6

Angle of Attack


The angle of attack is the angle between the chord line
and the average relative wind.


Greater angle of attack creates more lift (up to a
point).

total
lift

7

Lift and Induced Drag


Lift acts through the center of pressure, and
perpendicular to the relative wind.


This creates induced drag.

total
lift

effective
lift

induced drag

8

Got Lift? Flaps


Flaps increase
the wing’s
camber.


Some also
increase the
wing area
(fowler flap).


Almost all jet
transports also
have leading
edge flaps.

9

Too Much Lift? Spoilers


Spoilers destroy lift:


to slow down in flight (flight spoilers);


for roll control in flight (flight spoilers);


to slow down on the ground (ground spoilers).

Basic Aerodynamics

Side Effects

There is no such things as a free lunch.

11

Drag: Total Drag (Power Required) Curve


50

100

150

200

Indicated Airspeed

(knots)

Drag
(lbs)

1,400

1,200

1,000

800

600

400

200



induced drag


parasite drag


resistance


total drag

max.
lift/drag

best glide

12

Wingtip Vortices and Wake Turbulence


Wingtip vortices create drag:


“ground effect”;


tip tanks, drooped wings, “winglets”.

relative low pressure

Basic Aerodynamics

Stability

Longitudinal: Static, Dynamic

Lateral

14

Longitudinal Stability


Static stability (tendency to return after control input)


up elevator increases downward lift, angle of attack increases;


lift increases, drag increases, aircraft slows;


less downward lift, angle of attack decreases (nose drops).

weight

down lift

lift

15

Aside: CG and Center of Pressure Location

weight

down lift

lift


Aft CG increases speed:


the tail creates less lift (less drag);


the tail creates less down force (wings need to create less lift).


This also decreases stall speed (lower angle of attack req’d).

16

Lateral Stability


If one wing is lowered (e.g. by turbulence), the
airplane sideslips.


The lower wing has a greater angle of attack (more
lift).


This raises the lower wing.

17

Directional Stability


As the airplane turns to the left (e.g. in
turbulence), the vertical stabilizer creates lift
toward the left.


The airplane turns to the right.

18

Speed Stability v. Reverse Command


Power curve:


Power is work
performed by the
engine. (Thrust is
force created by the
propeller.)


Suppose airspeed
decreases.


“Front Side”: Power is
greater than required:
aircraft accelerates.


“Back Side”: Power is
less than required:
aircraft decelerates.


50

100

150

200

Indicated Airspeed

(knots)

Drag (thrust required)

1,400

1,200

1,000

800

600

400

200


Percent horsepower

100%

50%

max.
endurance

ca. 75% of
max.
lift/drag

Basic Aerodynamics

Turning Flight

Differential Lift

20

Turning Flight


More lift on one wing than
on the other results in roll
around the longitudinal
axis (bank).


Lowering the aileron on one
wing results in greater lift
and raises that wing.

21

Turning Flight, cont’d


More lift on one wing than
on the other results in roll
around the longitudinal
axis (bank).


Lowering the aileron on one
wing results in greater lift
and raises that wing.


This tilts lift sideways.


The horizontal component
of lift makes the airplane
turn.


(To maintain altitude, more
total lift needs to be created:
higher angle of attack req’d)

Centrifugal
Force

22

Adverse Yaw and Frise Aileron


However, more lift on one
wing creates more
induced drag on that
wing: adverse yaw.


Adverse yaw is corrected
by rudder application.


Frise ailerons counter
adverse yaw:


They create parasite drag
on the up aileron.

Basic Aerodynamics

Stalls

Too Much of a Good Thing

24

Stalls


A wing section stalls when its critical angle of
attack is exceeded.


Indicated stall
speed

depends on how much lift the
wing needs to create (weight, G loading).

25

Stalls, cont’d


The disturbed airflow over the wing hits the tail and the
horizontal stabilizer. This is the “buffet”.


Eventually, there will not be enough airflow over the
horizontal stabilizer, and it loses its downward lift. The
nose drops: the stall “breaks”.

weight

lift

26

Stalls, cont’d


The whole wing
never stalls at the
same time.


Power
-
on stalls in
most light singles
allow the wing to
stall more fully.
Why?


Where do you
want the wing to
stall last?


Ailerons

27

Stalls, cont’d (Stalls with one Engine Inop.)


Stalls in a
twin with
one engine
inoperative
lead to roll
or spin
entry:


Propeller
slipstream
delays
stall.

28

Stalls, cont’d


Stall strips make the wing stall sooner.

29

Stalls, cont’d


Definition
: The
angle of incidence

is the acute angle
between the longitudinal axis of the airplane and the
chord line of the wing.


Twist in the wing makes the wing root stall first:


The angle of incidence decreases away from the wing root.

30

Preventing Stalls


Slats direct airflow over the wing to avoid
boundary layer separation.


Slots are similar but fixed, near the wingtips.


Delays stall near the wingtip (aileron effectiveness).

31

Stalls and Turns


Greater angles of bank require greater lift so
that:


the vertical component of lift equals weight (to
maintain altitude),


the horizontal component of lift equals centrifugal
force (constant radius, coordinated, turn)

32

Stalls and Turns, cont’d


Load factor
(multiple of
aircraft gross
weight the
wings
support)
increases
with bank
angle.

acrobatic 6G

Normal 3.8G


Stall speed
increases
accordingly.

limit load
factor:

33

Turns


As bank increases, load factor increases.


But: as airspeed increases, rate of turn
decreases.


In order to make a 3 degree per second turn, at 500
Kts the airplane would have to bank more than 50
degrees.


Uncomfortable (unsafe?) load factor.


This is why for jet
-
powered airplanes, a
standard rate turn is 1.5 degrees per second.

Basic Aerodynamics

High and Fast

In the Flight Levels

35

High and Fast


Mach is the ratio of the true airspeed to the
speed of sound.


Speed of sound decreases with temperature.


Temperature decreases with altitude.


At higher altitudes, the same indicated airspeed
leads to higher Mach numbers.


Conversely: at higher altitudes, a certain Mach
number can be achieved at a lower indicated
airspeed.


The indicated stall speed increases with
altitude (compressibility).

36

High and Fast, cont’d


At high subsonic speeds, portions of the wing can
induce supersonic airflow (critical Mach number Mcrit).


Where the airflow slows to subsonic speeds, a
shockwave forms.


The shockwave causes boundary layer separation.


High
-
speed buffet, “aileron snatch”, “Mach tuck”.

velocity
increases

velocity decreases,
shockwave forms

boundary layer
separates

37

High and Fast, cont’d


Vortex generators delay boundary layer
separation.

38

High and Fast, cont’d


With
altitude:


indicated
stall speed
(low speed
buffet)
increases;


indicated
airspeed
that results
in critical
Mcrit
decreases.


coffin corner

39

References


De Remer D (1992)
Aircraft Systems for Pilots

Casper: IAP


FAA (1997)
Pilot’s Handbook of Aeronautical
Knowledge

AC61
-
23C

Newcastle: ASA


Lowery J (2001)
Professional Pilot

Ames: Iowa
State Univ. Press


Schiff B (1985)
The Proficient Pilot

vol. 1

New
York: Macmillan


U.S. Navy (1965)
Aerodynamics for Naval
Aviators

Newcastle: ASA