Page 9 - PDF_Flip_Book

Chapter 2 - Aerodynamics: The Wing Is the Thing
2-65

Power, Climb, and Thrust
Your Propeller: Trust Your Thrust
It’s power that is transferred to the engine’s crankshaft and most folks understand that concept intuitively. For
instance, suppose you have two cars of the same size, shape and weight. One car has a 100 hp engine and the other
has a 200 hp engine. With all else being equal, the car with more horsepower will accelerate faster and go faster (all
else being equal). That’s intuitive. The same thing applies to airplanes. With all else being equal (i.e., same airplane,
same propeller, same everything), the airplane with more horsepower will accelerate faster, climb faster and go faster.
Power, however, is only one factor in determining what an airplane will do. Another
factor is thrust. Let’s face it, an airplane with a 100 hp engine and no propeller is
nothing more than an expensive noise generator. Add a propeller and you can
generate thrust (the force created by spinning the propeller). We typically
think of a force as something pushing or pulling on something. That’s what
propellers do. They generate their thrust by accelerating air from a lower
velocity in front of the propeller to a much higher velocity aft of the propeller
disc. In this way they either pull the airplane forward or push it forward,
depending on where the propeller is located on the airframe.
If there’s any part of an airplane deserving of your respect and admira-
tion, it’s the propeller. Two or more blades spin at several thousand RPMs
producing sufficient thrust to pull a small training airplane forward at over
100 knots. Impressive. So let’s take a closer look at how the propeller works.
It’s a Wing that Rotates

Similar to most wings, one side of the propeller is
curved or cambered, while the part facing you from the
cockpit (called the propeller’s “face”) is not as curved
and relatively flat. The chord line extends from the
leading edge to the trailing edge of the propeller.
Since the propeller rotates about the crank-
shaft, we can say that the propeller blade angle
is the angle between the chord line and plane
of rotation (Figure 115, position A). This isn’t,
however, the propeller’s angle of attack. The
angle of attack is made between the chord line and
the relative wind, which just happens to change
direction as the airplane moves forward. Let’s
assume Figure 115 represents a fixed-pitch pro-
peller. Speed up the airplane and, for a given pro-
peller RPM, the propeller’s angle of attack decreases
(position B). Slow the airplane down and, for a given Fig. 117
Fig. 115
RPM, the propeller’s angle of attack increases (position A).
The propeller’s pitch is not its blade
angle. Pitch has two definitions: effective
pitch and geometric pitch. The pro-
peller’s geometric pitch is the amount
it would move forward if the air were
a solid medium (Figure 116). Think
of a wood screw advancing in a block
of wood as the screw is turned
(Figure 117). The effective pitch is
the amount the propeller actually
advances in air, which is clearly less Fig. 116
solid than wood. Turning a wood- The propeller’s geometric
screw into something as soft as a cup- pitch is the amount the airplane
would be pulled through the air if there were no pro-
cake would limit the forward advance peller slippage. The effective pitch is the actual amount
of the screw. The difference between the airplane is pulled through the air. The difference
the geometric and effective pitch is known between the two is the amount the propeller slips.