Propellor Strength and Performance

Have you ever wondered if the type of material makes a difference in the performance of your RC Boat or RC Airplane? We will now take a look at how propeller strength and performance are effected. This does scale up to even full scale Boats and Airplanes. Many full scale boating enthusiasts have experienced negative characteristics with the strength of their propellor hanging off the back end of their boats.

For airplane props is Carbon Fiber really worth it over Wood?

How about for Boat props, is Stainless Steel worth the cost over a composite prop?

Some Background Information – Propellor Strength and Performance

Naturally the higher the performance we desire out of our propellers, the higher amount of stress they must endure. Stress in a part can be thought of as the amount of pressure placed on the material of that component. Stress in a material has a side effect, this side effect is strain. We will consider a simplification of strain to represent deflection as this better suits what we wish to talk about. Now of course, much like when our jobs becomes stressful we can snap due to the pressure of the stress. Same applies for material, when a material strength threshold is exceeded the material may fail. This threshold is known as the Yield strength. We should never see this point when operating with a safe and properly selected setup.

When we take a ruler and hold it at one end and use our finger to press on the other end we create stress in the material and thus create a deflection. A simplified example can be found below.

Stress/Deflection Example with Stress shown.

Stress/Deflection Example with Stress shown. The maximum stress in this example is 80.435 MPA and we can see that the Yield strength of 239.6 MPA is not exceeded and therefore our part will be safe from the load applied. The areas of stress is colour coded and and can be seen visually on the part where they occur.

Stress/Deflection Example with Deflection shown.

Stress/Deflection Example with Deflection shown. The maximum deflection in this case is right where the load in purple arrows are. In this case it works out to 3.2mm.

Generally when considering a propeller, the stress during running increases with the speed of the propeller. A higher RPM motor driving the propeller at very high revs will create a much higher stress value then driving that same propeller at slower speeds. Propeller strength and performance go hand and hand. When strength increases, performance increases. This is a parallel relationship.

 

However, if we consider a weak material we may not get the results out of the propellor that we are hoping for. This purely comes down to the amount of deflection in the propellor. If we have a significant amount of deflection in the propellor, this essentially means that the propellor is changing its shape. When we change the shape of the propeller the pitch is effected the most significantly. A significant change in pitch can greatly effect performance depending on the amount of change.

Effects of Propellor Strength and Performance

 

So now we have to answer how propellor strength and performance is effected. When pitch is changed the amount of thrust and the amount of speed the propellor can achieve is decreased. We now waste power to change the shape of the propellor. This decreases the overall efficiency of our setup, it can contribute to  the amount of noise the prop creates and can make our boats or planes handle differently and most importantly diminish the performance that we were expecting.

Below is a Boat Propellor example. We can see the difference in deflection from a 46mm prop loaded equally between the 2 materials Stainless Steel and Aluminum..

Displacement - Stainless Steel Prop

In this example we may note that the total deflection of the prop is 0.686mm. This is lower then the Aluminum Example.

In this example we may note that the total deflection of the prop is 2.31mm. This is significantly higher then the stainless steel example.

In this example we may note that the total deflection of the prop is 2.31mm. This is significantly higher then the stainless steel example.

In this simplified example we can see there is a significant difference in performance between these 2 props and the only difference is material selection. Top speed performance can be reduced by as much as 5%. Although this may not sound that significant, it may be worth noting that the handling characteristics may make an even more significant difference.

How to decide on a Propeller material.

This is where we have to decide on whether or not to go that extra mile in terms of cost to purchase the better performing material. The best answer here is to re-evaluate your performance goals. If you have a very expensive setup running tons of power, you should not consider the cheaper option. Along the same lines, if you are just out having fun and not trying to squeeze every ounce of performance out of your setup, the more inexpensive material should be selected.

High Performance – Stronger Material

Sport Flying/Boating – Weaker Material

If you still don’t know, try and borrow a propellor off a friend. Compare using metrics such as top speed or handling characteristics that you can note during the test. Trying a different combination of props is by far the easiest way to determine which one is the best.

 

Give it a shot.

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