0:08
a propeller rotating through the air
0:10
creates an area of low pressure in front
0:12
of the blade this low pressure area
0:15
combined with an area of high pressure
0:17
behind the blade enables a propeller to
0:19
produce thrust the amount of thrust
0:21
produced depends on several factors
0:23
including the propeller Blade's angle of
0:25
attack speed and air foil shape the
0:28
angle of attack of a propeller blade is
0:30
the angle formed by the cord line of the
0:32
blade and the relative wind the
0:34
direction of the relative wind is
0:36
determined by the speed that aircraft
0:38
moves through the air and the rotational
0:40
motion of the propeller for example when
0:42
a propeller rotates on a stationary
0:44
aircraft the direction of the relative
0:46
wind is exactly opposite to the
0:48
rotational movement of the propeller in
0:51
this case the angle of attack is the
0:53
same as the propeller blade angle when
0:56
the aircraft begins moving forward the
0:58
relative wind direction shifts because
1:00
in addition to rotating the propeller
1:02
now has Forward Motion the result is
1:05
that the relative wind is much closer to
1:07
the angle of attack in this case the
1:10
angle of attack will always be less than
1:12
the blade angle based on the effect that
1:15
forward motion has on the relative wind
1:17
of a propeller blade the faster an
1:19
aircraft moves through the air the
1:21
smaller the angle of attack on the
1:22
propeller blade however if propeller
1:25
speed increases the trailing edge of the
1:27
propeller blade travels a greater
1:29
distance for the same amount of forward
1:31
movement as propeller speed increases
1:34
the relative wind strikes the propeller
1:36
blade at a greater angle and the angle
1:39
increases the most effective angle of
1:41
attack for a propeller blade is between
1:43
2 and 4° any angle of attack exceeding
1:47
15° is ineffective because a propeller
1:50
blade might stall typically propellers
1:52
with a fixed blade angle are designed to
1:54
produce an angle of attack between 2 and
1:56
4° at either a climb or Cruise air speed
2:00
with a specific speed
2:01
setting unlike a wing which moves
2:04
through the air at a uniform rate the
2:06
propeller sections near the tip rotate
2:08
at a greater velocity than those near
2:10
the Hub the difference in rotational
2:12
velocity along a propeller blade segment
2:14
can be found by first calculating the
2:17
circumference of the Ark traveled by a
2:19
point on that segment the circumference
2:21
of a circle is calculated with the
2:23
formula the circumference is then
2:24
multiplied by engine speed and RPM to
2:27
find rotational velocity for example to
2:30
determine blade velocity at A8 in from
2:33
the Hub that is rotating at 1,800 RPM at
2:37
a 18 in from the Hub the blade travels
2:42
23,5 75 in per minute to convert this to
2:51
63,360 and multiply the product by 60
2:55
the number of minutes in 1 hour the
2:57
speed of the propeller at station 18 is
3:00
192 7 mph you can now compare this to
3:04
the speed of the propeller at station 48
3:07
by using the same formulas you can
3:09
determine that at station 48 the
3:11
propeller is moving at 514
3:15
mph to compensate for the difference in
3:18
velocity along a propeller blade the
3:20
blade angle changes along its length the
3:22
gradual decrease in Blade angle from The
3:25
Hub to the tip is called pitch
3:26
distribution or twist blade twist en a a
3:30
propeller to provide a fairly constant
3:32
angle of attack along most of the length
3:34
of the blade in addition to Blade twist
3:37
most propellers have a thicker lowspeed
3:39
air foil near the blade Hub and a
3:41
thinner highspeed air foil near the tip
3:44
this along with blade twist enables the
3:46
propeller to produce a relatively
3:48
constant amount of thrust along the
3:50
entire length of a propeller
3:52
blade a rotating propeller is acted upon
3:55
by centrifugal twisting aerodynamic
3:58
twisting torque bending and thrust
4:00
bending forces centrifugal force is a
4:03
physical force that tends to throw the
4:05
rotating propeller blades away from the
4:07
Hub this is the most dominant force on
4:09
the propeller torque bending force in
4:12
the form of air resistance tends to bend
4:14
the propeller blades in the direction
4:16
opposite that of rotation thrust bending
4:19
force is the thrust load that tends to
4:21
bend propeller blades forward as the
4:23
aircraft is pulled through the air
4:25
aerodynamic twisting force tends to turn
4:27
the blades to a high blade angle
4:29
centrifugal twisting force being greater
4:32
than the aerodynamic twisting force
4:34
tends to force the blades toward a low
4:38
angle at least two of these forces
4:41
acting on the propeller's blades are
4:42
used to move the blades on a
4:44
controllable pitch propeller centrifugal
4:46
twisting force is sometimes used to move
4:48
the blades to the low pitch position
4:51
while aerodynamic twisting force is used
4:53
to move the blades into high pitch these
4:55
forces can be the primary or secondary
4:58
forces that move the blades to the new
5:01
position pitch is not the same as blade
5:04
angle but because pitch is largely
5:06
determined by blade angle the two terms
5:08
are often used interchangeably an
5:10
increase or decrease in one is usually
5:13
associated with an increase or decrease
5:15
in the other propeller slip is the
5:17
difference between the geometric pitch
5:19
of the propeller and its effective pitch
5:22
geometric pitch is the distance a
5:23
propeller should advance in one
5:25
revolution with no slippage effective
5:28
pitch is the distance it actually
5:29
advances thus geometric or theoretical
5:33
pitch is based on no slippage actual or
5:36
effective pitch recognizes propeller
5:38
slippage in the air geometric pitch is
5:41
usually expressed in Pitch inches and
5:43
calculated by using the
5:46
formula although blade angle and
5:48
propeller pitch are closely related
5:50
blade angle is the angle between the
5:51
face or cord of a blade section and the
5:54
plane in which the propeller
5:56
rotates thanks for watching
