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.

Differential Fluid Performance in RC vehicles

Differential Oil

Different Differential oils provide different performance characteristics. These characteristics can be controlled by the different weights of oil placed in to the differential. The weight of a differential fluid is determined by the viscosity of the fluid. A higher viscosity represents a higher weight oil. Weightings can range anywhere from the low thousands to several hundred thousands. The diff fluid represented below has a weighting of 10,000.

Differential Oil

Differential Oil

Differential Operation

The differential operates differently with different weighted oils. An open differential or one that has no viscous fluid in it will allow all motor or engine power to be routed to the tire with the least resistance. This means all the power potential you have will travel to the wrong wheel. In most RC applications, this is not desired.

There is however a way to provide more bias towards the slipping wheel so that the non-slipping wheel will be provided with more power. The way to do this is tune the proper weight oil for your application. There are either 2 or 3 differentials in the typical RC vehicle. Each one must be tuned with the proper weighted oils. It is recommended to use the weights that the manufacture suggests. A baseline is needed to begin the tuning. A factory configuration would be best for this. First it will be necessary to understand what increasing or decreasing the weight will do in each differential for an RC car.

A very good starting point for most buggies and truggies out there is to use 5000 weight oil in the front diff, 8000 in the center and 3000 in the rear.

Differential Fluid added to an RC Car differential

Differential Fluid added to an RC Car differential

Differential Performance – Front Differential

Increasing the weight of oil will provide you with more on power steering. This will provide an advantage to those who drive your buggy hard out of a corner. On the flip side under off power steering, your buggy would be more prone to understeer through the turns increasing the difficulty for off power steering.

Decreasing the weight of differential oil will increase off power steering into corners. This is excellent if you prefer to drive harder into a corner. On the downside under power, your buggy is more prone to understeer through corners while under power.

Differential Performance – Center Differential

A heavier oil will increase acceleration potential but will decrease off power steering capability. Under heavy acceleration there is potential for the rear end to swing out. A lighter oil will provide more on/off power steering, but decrease acceleration potential. Decreasing acceleration potential will make it easier to get out of the hole with no throttle control as the front axle performs majority of the acceleration.

Differential Performance – Rear Differential

Lighter oil will decrease rear end traction. This may provide more predictable steering at a loss of straighline acceleration. Heavier oils will increase traction, but may decrease a positive feel for steering.

Best Setup

In order to achieve the best setup for your application, it will take some trial and error in order to match your cars performance with your driving preferences. If you already have an idea for this, it will be easier to achieve the best setup more quickly. In general all choices will be a trade off. You will select a specific weight of oil in order to gain a performance characteristic, however you are always going to be sacrificing something.
Also, it is important to consider that higher weighted oils will generally be more difficult to handle. It may provide excellent acceleration performance, but with a higher power setup and tight turns, experience and skills will be needed to keep it all under control with throttle, brake and steering management.

Cutting Brushless Motor Wires

Often times the question of motor wires is asked. The exact wire in question is the three motor wires that exit the can of the motor. They can be up to several inches in length down to only an inch or so from the can of the motor. Many would like to know if it is safe to reduce the length of the wires coming out from the motor. Most importantly, if this is a common question there must be some sort of significance behind it.

So, is it safe to trim the motor wires?

There is actually no exact answer for this. The best possible answer is it depends on the motor itself to determine whether or not it is safe to reduce the length of the wires. But here we will explain this in more detail so you can determine for your self if it is safe.

Why does it matter from motor to motor?

In every brushless motor there are several windings of copper wires that make up the stator. This is inside the motor itself. These windings are the wires you may be able to see through cooling vents in the motor itself. The windings must then be brought to the Electronic Speed Control. (ESC) There may be several strands of windings inside the motor and it is up to the wires termination points to take all these wires and make only three ends from them. Remember, there are only three wires coming from your ESC.

This motor wire termination is the important factor in whether you can cut your motor wires or not. In some motors the wire termination is actually done inside the motor can. In other motors, the wire termination point may actually be placed outside the motor. This means the wire coming outside the motor may contain the termination point.

This is important as if you cut the motor wires with a termination point inside the motor you will be fine. However, if you cut motor wires where the termination point is outside the can of the motor you may actually damage the termination of the motor. This may lead to shorts in the motor ultimately damaging it.

When is Cutting Brushless Motor wires Safe?

In general, if you don’t know for certain that your motors wire termination is inside the can, it is highly recommended to just leave them as is.

Motors that have long leads in excess of 150mm or 6″ usually have internal wire termination. This is motors similar to the Castle 15xx series. Motors that have leads only 25-75mm 1-3 inches in length are known to have wire termination outside the motor can.

RC Battery Connectors and Wire Gauge

RC Battery Connectors

In the application of electric motors and batteries, connectors play a large role in transfering the battery power to the motor and through the ESC. Not only does it transfer power but it also must be able to break apart quickly for battery or motor replacements. Choosing the right kind of connector for your application is a must.

Factors determining your selection for RC Battery Connectors

There are several connectors to choose from and this is due to the fact that each one serves an entirely different purpose. Amount of current needed to pass through a connector is the most important factor in selecting a connector. Then comes size, weight, ease of use ect. In general a connector should be as large as the maximum amount of average current it will see. This logic will provide the lightest and smallest connector as possible.

Here is a list of Connectors by Amp ratings

Connector Continuous Amperage Rating
Servo/Battery Lead
< 800mA
JST
1-5 A
Mini T Plug
5-18 A
3.5mm Bullet Connector
10-35 A
4mm Bullet / Banana Connector
15-50 A / 0-10 A
Deans T Connector
20-60 A
5.5mm Bullet Connector
55-110 A
6.5mm Bullet Connector
65-150 A

Connector Placement

On the motor to ESC connection it is common and recommended to use the best bullet connector that fits your specific use. On the ESC side, this is where the female plugs go. On the motor side this is where the male plugs should go.

When using bullet connectors on the battery, it is best to follow and remember the phrase “Red Hot Female.” This refers to the battery as being the “hot” source having a “female” plug on the “red” or positive terminal. The negative side of the battery would then get the male connector for polarizing the connection.

The ESC of course would get the opposite connectors to properly mate up to the battery connectors. In order to prevent any shorts a piece of PVC tubing is recommeded to be placed over the male battery connector when the pack is not in use. This is very important to prevent any electrical shorts.

Wire Gauge Chart by Amperage Rating

This chart assumes a short less than 6-8 inch run of wire.

Wire Gauge Continuous Amperage Rating
18 AWG
10-18
16 AWG
18-25 A
14 AWG
25-40 A
12 AWG
40-75 A
10 AWG
75-120 A
Dual 12 AWG
80-150 A
Dual 10 AWG
150-240 A

Brushless Motor Poles – Radio Control Info

When one refers to How many poles an electric motor has, they are commonly referring to how many magnetic poles there are. Many often wonder the differences between motors having more or less poles. Here, we will focus on a brief discussion concerning Brushless motor magnetic poles. Main focus will be placed on In Runner Motors, where the rotor spins inside of the stator.

Common in runner motors have 2 or 4 magnetic poles. There are some motors containing 6 poles as well. Outrunner motors may have in excess of 6 magnetic poles. You may want to read more on outrunners vs inrunners and what the differences are.

4 Poll Brushless Motor

4 Poll Brushless Motor

Motor Poles vs Motor Characteristics

The largest characteristic that a motor with more poles displays when all else is equal is a drop in the KV value. The KV value as we all know is the amount of RPM per volt. When all else is equal between 2 brushless motors, the one containing a higher pole count will have a lower KV value. This is the number one important characteristic of differences in a motors pole count. All else that we will talk about is based off of this.

How Cogging is effected by Pole Count

If you haven’t already read the article on cogging, I’d recommend following the link to read on this further. It may not be what you thought it was.  Pole counts do influence cogging torque on slotted motors. Generally on inrunner motors, cogging torque increases as you increase in magnetic pole count.

A drop in KV, what does that do? – Brushless Motor Poles

When a motors KV value drops when we keep the motor the same physical size and the number of motor electrical winds (wind / turn) the same, we gain motor torque. The torque is gained as a result of lower KV. Depending on the exact application this may be an advantage or disadvantage. More torque may mean greater acceleration but a lower KV value will reduce maximum RPM achievable.

How Does a Manufacture maintain a usable KV? – Brushless Motor Poles

There is one route a Motor Manufacture may take in order to bring the KV back up to a usable amount. The best option is to decrease the amount of winds in the motor. A 2 Pole motor for example will typically have many more winds than a 4 Pole motor. When a 4 Pole motor has the amount of windings reduced, KV will increase back to the amount needed. When a winding from the motor is removed, the physical size of the motor remains the same resulting in a void. The best solution to this created void is to add more copper. More Copper is added to the winding. This is done in such a way similar to moving down in the Gauge type. In other words the wire used as a wind is now increased in diamater.

As we know from electrical theory, when the cross section of a wire is increased the resistance decreases. This decreased resistance would allow a greater current load to pass through the motor.

Advantages and Disadvantages – Brushless Motor Poles

When comparing a 4 Pole motor that has the same physical size as a 2 Pole motor and the same KV as a 2 Pole motor, the 4 Pole motor in theory would be able to handle greater current loads. The greater current loads will result in greater power. This is similar to saying a 4 pole motor has better overall efficiency when compared to a 2 pole motor.

Although it sounds like there are many advantages, there are also some down falls to this as well. It is very common for 4 pole motors to not have as many motor options as a similar 2 pole motor. Less motor options may make it difficult to get the exact KV value you need. This is even with 4 pole motors using both wind types. A wind type is the difference in how a wind is terminated before it gets to the 3 wires hanging out the brushless motor that you typically see. These wind types are commonly known as the Delta wind and the Wye wind. Using one wind type over another does provide different motor characteristics yet again. We will note this as a possible disadvantage as well.

Which Pole Count Works best for me – Brushless Motor Poles

Overall, it will come down to testing, testing, and testing. These differences explained above may be very subtle to the point where you don’t even note any differences. There may be many reasons to this including your driving habits. In order to determine which works best for you, it is recommended to give both types a try. You may prefer either one of them.