Prop Unloading

What Is It?

We hear about prop unloading here and there, but not much attention is given to it. It is an important concept to understand when considering how our quadcopters (or any propeller driven device) perform. My goal here is to explain what it is and why it is important.

Contents:

  1. Unloading
  2. Pitch Speed and Slip
  3. Thrust is Dynamic
  4. Why Do We Care?

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Unloading

Unloading is exactly what it means: taking the load off of the prop. What type of load is this? It is the force created when the prop spins in the air. As we know, the prop spins by the power of our motors, or more specifically, the torque generated by the motors. In other words, when we talk about prop unloading, we are actually talking about motor unloading. Before going further, here is a breakdown of the motor/prop/air interaction:

  • For every action, there is an equal and opposite reaction.
  • At any given throttle level, the motor generates torque which directly spins the propeller.
  • As the propeller spins, pressure differences cause the air to move from the front of the propeller to the back of the propeller.
  • We measure the torque created by a motor my measuring thrust.

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Pitch Speed

When we say pitch speed, we mean the speed at  which the quad would be moving at a certain RPM and certain prop pitch (explanation of prop pitch here) if there were no prop slip.

What is prop slip? Think of a prop on a test bench. As the prop spins, it is going nowhere: the prop slip is at 100%. Since air is a fluid medium, the prop is able to push and “slip” through the air. Now think of the prop as you would a screw going into wood. As the prop spins, it moves forward and goes into the wood. Prop slip is at 0%. Since wood is (more or less) a solid, the prop can’t push aside the wood; the prop must move as it rotates. As we will see, we will always need some prop slip.

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Thrust is Dynamic

Now back to the bench prop. We have our motor and prop spinning on the bench at 100% throttle, and therefore, max thrust. Note that it is no coincidence that at 100% prop slip, there is 100% prop slip.

Now lets come up with a few easy to work with numbers:

  • Pitch speed is 150 mph. **A good way to imagine pitch speed is to think of it as being the speed of the air directly behind the propeller**
  • Max thrust is 1500g

Next, let’s think about what happens if we have our test motor/prop in an air tunnel. With the wind tunnel, we can change the speed of the incoming air to the prop. What will this do? Since thrust is basically created by the difference in pressure of the incoming air ( directly in front of the prop) and the pressure of the outgoing air (directly behind the prop), the thrust will change accordingly:

  • If the air tunnel is at 0 mph, then the difference of the incoming/outgoing air  is 150 mph. Since 150 mph is 100% pitch speed, then the thrust is 1500g and the prop is 0% unloaded.
  • If the air tunnel is at 50 mph, then the difference of the incoming/outgoing air is 100 mph. Since 100 mph is 67% pitch speed, then the thrust is 1000g and the prop is 33% unloaded.
  • If the air tunnel is at 140 mph, then the difference of the incoming/outgoing air is 10 mph. Since 10 mph is 6.7% pitch speed, then the thrust is 100g and the prop is 93.3% unloaded.
  • If the air tunnel is at 150 mph, then the difference of the incoming/outgoing air is 0 mph. Since 0 mph is 0% pitch speed, then the thrust is 0g and the prop is 100% unloaded.
chart1
As our quad gains speed, the props produce less thrust.

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Why Do We Care?

Efficiency. Although we will never see a 100% unloaded prop in the real world, we definitely want to be unloaded as much as possible and as often as possible. Why? Notice how the more unloaded the prop is, the less thrust is produced. Now remember about the equal and opposite reaction thing? That means less load is put on the motors which equals efficiency. How to do this?

Reduce air drag: There are many ways to do this, but to understand the relationship between unloading, air drag, and efficiency, consider the illustration below:

Prop unload
When thrust = air drag, our quad is at top speed and we no longer accelerate.

The horizontal component of thrust is used to offset air drag. Let’s say you are at top speed and travelling at 100 mph. Using the graph above, at that speed, each motor is producing 500g of thrust. This means that the total air drag is 2000g. What if we reduce the drag down to 1600g? Now each motor only needs to produce 400g of thrust each to offset drag, so if we look at our chart again, we should now see a top speed of around 110 mph.

Kind of makes you think a little… I’m going faster, but I’m producing less thrust!?!? A more in depth look in to drone physics is here.

Reduce weight: This is a very simple one. The lighter our quad is, the quicker it accelerates which unloads the props quicker.

Note that quicker acceleration is also a byproduct of reducing air drag.

Prop pitch: We all want to go fast so we like to have an aggressive prop on our quads. But in those instances where you know you won’t be needing a high top speed, use a milder prop. A milder prop usually means that the pitch speed is slower. A slower pitch speed means that for any given speed, the prop is unloaded more than if you were using a higher pitch prop.

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