Transcript The Stall, Airfoil development, &Wing Lift and Span Effects

The Stall, Airfoil development,
&Wing Lift and Span Effects
Lecture 4
Chapter 2
The Stall
• What happens when we increase the angle
of attack?
• Can we increase our angle of attack too
much?
• A practical limit to the angle of attack is
the stalling point.
Factors that contribute to a stall
• Angle of attack increases the stagnation
point moves farther down on the forward
part of the airfoil-making a longer effective
upper surface.
– This creates friction that increases with travel
distance.
Factors that contribute to a stall
• Pressure gradient (pressure change)
– There is a decrease of pressure from the
leading edge back; that pressure decreases
with distance.
– This decreasing pressure tends to induce the
flow to move along the surface, promoting the
flow in the direction we want.
• We call this favorable pressure gradient
Factors that contribute to a stall
• Beyond the peak in the negative pressure
we find a reversal:
• An unfavorable pressure gradient
– As the angle of attack increases the center of
pressure moves forward and the unfavorable
pressure gradient becomes longer and steeper.
Factors that contribute to a stall
• Eventually, the combined effect of the
unfavorable pressure gradient and the
surface friction become greater than the
energy available in the airflow to overcome
them.
– At this point the flow will detach itself from
the surface.
Figure 2-25, p. 29
• With no flow over the top surface, there is
no mechanism to reduce the pressure over
the surface and lift decreases drastically.
• The upper surface separation causes a great
loss in lift production and stalls.
The Stall
• The lift does not go to zero because there is still
flow over the surface and at this angle of attack is
normally exerting positive pressure.
• The upper surface separation causes a great loss
of lift.
• The result on an aircraft in flight is a sudden loss
of lift; it will drop due to weight now being
greater than lift.
Reducing the abruptness of the
stall
• The roundness of the leading edge
– A very sharp leading edge can act as a barrier
to the flow at a high angle of attack.
• A stall Strip
– A stall strip causes the flow to separate at the
leading edge at an angle of attack somewhat
below the normal stall angle.
Stall Warning Devices
• Vane-type- which takes advantage of the
relation between the stall angle of attack
and stagnation point.
– There is a distinct stagnation point for each
angle of attack.
– The vane is positioned so that the stagnation
point is above it in normal flight.
Figure 2-27a p. 30
• The air stream hitting the vane is, then that
going over the lower surface, which holds
the vane down.
• The vane is connected to an electrical
switch-which is open when the vane is
down.
• As the angle of attack is increased the
stagnation point moves below the vane.
Airfoil Development and
Designation
• What is the typical airfoil?
• What is the simplest?
– The Flat plate
• It is not efficient because it creates quite a bit of
drag.
• The sharp leading edge also promotes stall at a
very small angle of attack; severely limits lift
producing ability.
• Figure 2-28 p.32
The National Advisory
Committee for Aeronautics
• NACA, the forerunner of NASA looked at
aerodynamic characteristics of airfoils in
wind tunnels
• They looked at the thickness form and
meanline form
• They then proceeded to identify these
characteristics in the numbering systems
for airfoils.
NACA 2412
twenty-four twelve
• The first number (2) is the max camber in
% of the chord length.
• The second number (4) is the location of
the max camber point in tenths of chord.
• The last two numbers (12) identify the
maximum thickness in % of the chord.
Four digit airfoil
• Four digit airfoils with no camber, or
symmetrical would have two zeros in the
first two digits.
– 0010, double-oh ten
The six series airfoil
• NACA 652-415
• The first digit is the series number (6)
• The second number is the location of the
minimum pressure in tenths of a chord (5)
• The subscript (2) indicates the range of lift
coefficients above & below the design lift
coefficient where low drag can be
maintained
NACA 652-415
• The next number (4) indicated the design
lift coefficient of .04
• The last two digits (15) represent the max
thickness in % of the chord.
– The 6-series airfoils were first used in the
wing of the P-51 Mustang for their low drag
qualities
Richard Whitcomb
• NASA research engineer
• Developed the supercritical airfoil
• The airfoil was intended to improve drag at
speeds near Mach 1, but the methodology
was also used to for low-speed airfoils.
– The general aviation {GA(W)} was
incorporated into Piper Tomahawk; p. 36.
Wing Span
• The profile shape has a great deal to do
with the aerodynamic characteristics of a
wing.
• The length of a wing or span, and the
planform of the wing also affect the
aerodynamic characteristics.
• Planform is the shape of the wing as
viewed from directly above or below.
Figure 2-34 p. 37
• 2-34A- Along the span of the wing the
pressure force exerted against the wing,
except at the wing tips
• 2-34B-Wing tip vortices, more commonly
called wake turbulence.
• 2-34C- Downwash results in a change of
direction of the incoming air stream in the
vicinity of the wing.
Quiz on Lecture 4
Chapter 2
Please take out a sheet of paper
Include today’s date and your name
Downwash effect
• Downwash- pushing downward on air
stream causing a rearward tilted lift vector.
• The downwash effect is greatest at the
wing tip, but is experienced across the
span.
• When the lift vector is tilted backward, not
all of the lift is acting perpendicular to the
incoming stream.
Downwash effect
• Because of the downwash a little more
angle of attack is needed to make up for
this loss of lift downwash creates.
• This additional angle of attack is called the
induced angle of attack.
– This angle is necessary because of the flow
induced by the downwash.
Aspect Ratio
• Aspect ratio is the span divided by the
average chord.
• Figure 2-37 p. 40 shows two wings of
different aspect ratios, but have the same
area.
Quiz on Lecture 4
Chapter 2
Please take out a sheet of paper
Include today’s date and your name
Quiz on Lecture 4
Chapter 2
• Explain favorable pressure gradient.
• List and explain two things that can affect
the abruptness of a stall.
• Explain NACA 2413.
• What is planform?