like a wing, the prop has one angle of attack at which it produces the most thrust with the

least amount of drag. Manufacturers frequently design fixed-pitch props to be specialized for

a specific operation. Banner towing, for instance, requires optimal climb thrust and rarely

reaches any significant speed, so those types of operations normally use climb props, props

with a low pitch/blade angle to allow for high RPM. What we'd like is to produce this optimal

angle of attack at any airspeed. We can't get optimal performance throughout our range of

airspeeds, but we can greatly increase our efficiency with the use of a constant speed prop.

In order to maintain the optimal angle of attack, we must be able to change the pitch of the

prop blade as airspeed increases. For this reason, constant speed props are also called

variable pitch.

Let's assume that we've just leveled off at cruise altitude and we're now accelerating to

cruise airspeed. As we go faster, our effective angle of attack decreases and the prop tries

to spin faster. The prop governing mechanism then opens the pitch of the blade, increasing

its angle of attack and keeping its original RPM. Were we to slow down, the effective angle

of attack on the blade would be increased and the prop would try to slow down. The prop

governor would then reduce the pitch of the blade, reducing the angle of attack and

allowing the prop to maintain RPM. Think of this as the difference between walking and

running. As you're walking along, you're taking fairly small steps. If you want to walk faster,

you need to either take larger steps or take more of them in the same time period. It's

more efficient to take larger and larger strides the faster you go. Try taking a lot of small

steps to walk across the room and then running in strides and see which gets you there

faster and with less spent energy. On the other side of this, when we're walking slowly, we

want to take small steps instead of large strides. Taking big strides at a slow pace requires

much more energy then normal size steps. It is possible to open up the prop pitch too much

in flight and start to reduce performance. Think of the constant speed prop as an "automatic

transmission" instead of a one speed. Our constant speed propeller gives us the effect of

having several different fixed-pitch efficiency curves with one system. Now, let's just review

a couple of operating procedures for the constant speed prop system. You'll notice that I

didn't go into the actual construction of the prop governor and then prop hub system, which

actually moves the prop blade in flight. Let's talk a bit about the flight

controls. Instead of just having a throttle, we now have a throttle and a propeller lever. Our

throttle now controls the manifold pressure, or in the case of a turboprop, the torque. Our

prop lever is used to set the actual prop RPM. As we've said above, for takeoff, we like to

have the prop set to the high RPM/low pitch setting. As we increase our airspeed, we want

to increase pitch/lower RPM. This allows us better efficiency at lower RPM, which in turn

burns less fuel and makes less noise.

You can then see what the most efficient prop setting is for a given airspeed. Remember

that it is possible to increase pitch too much and reduce performance. Typical props can