A Propeller's Pitch
I haven’t seen very much written on pitch; but if you do not understanding it, you can make some bad decisions. Here are some effects.
- Increasing the prop’s pitch or length will cause a higher amp draw.
- Decreasing the prop’s pitch or length is the easiest way to solve a heat problem with the motor.
- Low pitch numbers tell you it’s a torque prop. With this type of prop, the motor generally does not work as hard so it pulls fewer amps.If you want to do a lot of stunts, a torque prop has a lot more acceleration, and the power system will rarely need any heat management.
- High pitch numbers tell you it’s a speed prop. This type of prop will push your plane to faster speeds. This is not your best choice for stunts. A speed prop has a lot less acceleration, and the motor will run hotter.
The measure of a prop:
Pitch is measured in inches for a specific reason. A 6x4 prop is a 6 inch long prop, and the prop is at such an angle as to (during one rotation) move forward 4 inches. In real life with air, there will be losses. But in this tutorial, to make things a little simpler, we will need to look at this from a "no loss" perspective.
The available props will have a usable balance between speed and torque. Let’s look at a 6x4 prop and a 12x8 prop. Both have a different pitch number, but in reality, the pitch angle on those props will be basically the same. Pitch wise, a 12x8 prop is an enlarged 6x4 prop. That is why you will never see a 12x4 prop, it's too slow to be practical.
Let’s look at a 6x4 prop and a 7x4 prop. Both have a 4 inch pitch, but the pitch angle on those props will not be the same. But it’s even more complex than that, the pitch angle changes as it moves away from the center of the prop. Let's see why this is true.
To better understand these explanations, you need to look at thrust as "the high pressure area behind the prop". There are ways of making this thrust work for you more efficiently. Let’s take a look at how the blades do their job. Circle Circumference = ? × diameter = ? × 2 × radius. SO a 6 inch prop x 3.14 = 18.84, This means the end of a 6 inch prop travels 18.84 inches in a circle to complete one rotation. Using this trig Triangle Angle Calculator
, for the end of a 6x4 prop to travel 18.84 inches to complete one rotation and to move 4 inches forward, the prop angle must be at 11.99 degrees. On that same prop, let's look at what happens half way down each blade: For the middle of a blade on a 6x4 prop to travel 9.42 inches to complete one rotation and to move 4 inches forward, the prop angle must be at 23.1 degrees. As you can see, for the prop to move forward 4 inches and push the air straight back, the prop angle must progressively decreases as it gets farther away from the center of the prop.
Let’s take this to the next step. What happens if the prop designer does not want the air to go straight back, what if he wants the prop to be designed with the blade shaped to focus the air flow toward the middle of each blade? To find out how he does that, let’s talk about one blade of a prop. With respect to what the prop angle in each particular spot should be; If we made the prop to where each of its blades had less thrust in its center part, then in both directions going away from the center of each blade, progressively increase the prop angle. This would organize the thrust toward the center of each blade. It would greatly increase the props efficiency because the air volosity would be faster 3 inches behind the prop then it would be at the prop. Organizing (focusing) the prop’s thrust is like having a nozzle behind a jet engine.
To some it all up:
The pitch shown on the blade is the average pitch. The prop’s pitch angle progressively decreases as it moves away from the center of the prop. Most blades are shaped to focus the air flow toward the middle of each blade. Focusing the thrust will increase its velocity and effectiveness. The propeller's length will give you power, and its pitch will give you speed.
Why do we care?
APC (Advanced Precision Composites):
- An EDF's (Electric Ducted Fan) fan does not progressively change the pitch angle the same way as a prop does. This is why an EDF needs a graduated tube behind it to organize the thrust. The EDF Thrust Tube behind the EDF needs a 15 to 25% diameter reduction at its exit, and it needs to be long enough. With it, the EDF has more than 20% more power. Not having it is like removing the nozzle from behind a jet engine.
- Never trim or shorten a prop: On each side of a propeller, the blade is computer engineered to cup and focus the air that it pushes. The blade’s tapered prop tips are computer engineered to be aerodynamic and quieter as to minimize the shockwave at the ends of the prop. Both things let the prop and the thrust it produces work more efficiently. If you cut the ends off of the prop or change the prop in any way, you drastically change the prop's characteristics, this destroys its design. Even if you are able to balance it correctly, it is now a poorly designed less efficient propeller. Many have tried and found that an untouched 6x4 easily out performs a trimmed 6x4 prop. If the prop is damaged, or you need a shorter prop, Buy a new prop.
- It’s nice to know how and where the thrust is coming off of the prop, this will help you design planes to better utilize the strengths of the thrust by not interfering with the air flow. It will also help you place motors and control surfaces.
- If you install the prop backwards, you spray the thrust. When the prop is correctly installed, the thrust is organized, the air velocity increases because you are funneling it through a smaller area. An improperly installed blade will have at lest 40% less power, its unorganized thrust moves slower because it’s the same amount of air moving through a much larger area.
This prop’s design is the one I’ve been mostly talking about. It is a harder more ridged prop. I feel that this, for us, is the best all-around prop because it has reliably good power all throughout the entire RPM range. RCPowers recommends this prop, and it is the one I see on our jets the most.
GWS (Grand Wing Servo):
This prop is made of a softer material and it is engineered simpler. The softer material lets the prop deform at high RPMs, so it is better suited for a Lower KV applications. It is designed to do a specific RPM range very well, it is strongest and most efficient at the top of is operating range.
The shape of a prop is far too technical for someone to fix one with a balancer, a cutter, and some sandpaper. The rough, or sharp edges on a prop are supposed to be there. I sanded the edges of a prop smooth, and it lost more than 20% of its performance. Balancing is the only maintenance you should ever do on a propeller. Never use an unbalanced prop; it will shorten the life of a motor or even destroy it.