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3 Answers

Fixed pitch Cruise vs Climb prop

Asked by: 10015 views , , , ,
Aerodynamics, Private Pilot

How is a cruise prop more effective than a climb prop during cruise? How does taking a "bigger bite" out of the air create more thrust? How does it allow you to fly faster when it is also creating more drag than a climb prop?

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3 Answers



  1. KDS on Oct 08, 2017

    You probably had a multi-speed bike at one time. Remember starting in a lower gear and shifting to a higher one to get more speed once you got going. The pitch of the prop is analogous to the shifting of gears. A climb prop is like the gear you used to start on your way or for going up steep hills. The cruise prop is like the higher gears.

    A constant speed prop is intended to make the airplane perform at its best in various conditions. Of course, the downside is the expense and complexity.

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  2. Brian on Sep 22, 2018

    Your propeller, like the wing of an airplane, has a most efficient angle of attack. For the plane this equates to best glide, or L/D max, which equates to the most efficient angle of attack for that airfoil.

    Notice the final word there being airfoil. I do this in an attempt to show that the wing, the tail, the propeller, your hand out a window, are all airfoils. Each airfoil has a most efficient angle of attack. Got it?

    Good, because this is where it gets tricky with a propeller. As you likely know, the definition of angle of attack is the angle between the airfoil (chordline) and where it’s going (relative wind). The tricky part with a propeller is that where it is going, and as such the relative wind, changes as the aircrafts speed changes!

    To help us analyze this we need to make a few simplifications. First, we will assume the propeller disk strikes the wing dead on and as such will ignore the aircrafts changing angle of attack. We will also only discuss the motion of the down going blade for further simplicity. Note: The upgoing blade is effected in the same way. Finally, we need to remind ourselves that the relative wind is a force that is parallel to and opposite to where the airfoil is going.

    Let’s begin with an aircraft parked on the runway awaiting takeoff. In this case the blade goes straight down towards the ground, its path of travel can be considered vertically downward. As such the relative wind at this point is vertically upwards. When we release the brakes the aircraft moves forward. If we were to take a second by second snap shot of each downward path the propeller takes we would notice that the path taken is no longer straight down. Instead the propeller moves down at an angle because as it has moved down the aircraft has moved it some distance forward.

    The propellers airfoil has remained the same, but the path of travel is now downward and forward. To imagine this stand facing forward, point your finger vertically at the ground and then point it slightly forward in front of you. Recall the relative wind is parallel to and opposite. If we were to draw this on paper as a side view of the propeller there would be a chord line angled some degree forward of vertical and a relative wind that, when standing still would point vertically down. And, when the aircraft is moving forward, would tilt forward and decrease the angle of attack of the propeller.

    What does all this mean? Quite simply, as you pick up speed the propellers angle of attack decrease. With a fixed pitch climb prop the pitch will be less because the goal is to have the most efficient angle of attack (L/D max) occur roughly at best climb speed. A fixed pitch cruise prop will have a steeper pitch with the goal being to have the most efficient angle of attack reached near cruising speed.

    If you have been able to follow along with this discussion then you may have realized that both the climb and cruise props will be flying at nearly the same angle of attack when they are most efficient. The only difference is that this angle of attack will occur at a different speed.

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  3. Brian on Sep 22, 2018

    If you are still reading this, I’m curious as to where you got the idea that a cruise prop creates more drag than a climb prop?

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