Transcript Basic Aerodynamics - Dartmouth Flying Club

Basic Aerodynamics
Basic Aerodynamics
Dartmouth Flying Club
October 10, 2002
Andreas Bentz
Basic Aerodynamics
Lift
Bernoulli’s Principle
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).

3
The Venturi Tube and Bernoulli’s Principle
kinetic energy
(velocity)
velocity
increases
potential energy
(pressure)
pressure
decreases
4
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
relative low pressure
upwash
downwash
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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
6
Lift and Induced Drag


Lift acts through the center of pressure, and
perpendicular to the relative wind.
This creates induced drag.
induced drag
effective
lift
total
lift
7
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.
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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).

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Basic Aerodynamics
Side Effects
There is no such things as a free lunch.
Drag: Total Drag (Power Required) Curve
1,400
1,200
max.
lift/drag
1,000
best glide
800

induced drag
parasite drag


resistance
total drag
400
Drag (lbs)

600
200
50
100
150
200
Indicated Airspeed (knots)
11
Wingtip Vortices and Wake Turbulence
relative low pressure

Wingtip vortices create drag:
“ground effect”;
 tip tanks, drooped wings, “winglets”.

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Basic Aerodynamics
Stability
Longitudinal: Static, Dynamic
Lateral

weight
down lift
lift
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).
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
weight
down lift
lift
Aside: CG and Center of Pressure Location
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).
15
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.

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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.
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Speed Stability v. Reverse Command


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.
1,400
100%
Percent horsepower
Power curve:
1,200
max.
endurance
1,000
ca. 75% of
max.
lift/drag
Drag (thrust required)

800
50%
600
400
200
50
100
150
200
Indicated Airspeed (knots)
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Basic Aerodynamics
Turning Flight
Differential Lift
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.
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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
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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.
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Basic Aerodynamics
Stalls
Too Much of a Good Thing
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).
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Stalls, cont’d
weight

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”.
lift

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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
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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.
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Stalls, cont’d

Stall strips make the wing stall sooner.
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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.
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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).
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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)

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Stalls and Turns, cont’d


Load factor
(multiple of
aircraft gross
weight the
wings
support)
increases
with bank
angle.
Stall speed
increases
accordingly.
limit load
factor:
acrobatic 6G
Normal 3.8G
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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.
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Basic Aerodynamics
High and Fast
In the Flight Levels
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).
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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
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High and Fast, cont’d

Vortex generators delay boundary layer
separation.
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High and Fast, cont’d

With
altitude:



indicated
stall speed
(low speed
buffet)
increases;
indicated
airspeed
that results
in critical
Mcrit
decreases.
coffin corner
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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
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