How an RC Model Jet Turbine Works

A Jet turbine is such an awesome engineering feat, not to mention how cool they look, sound and perform! Being able to have the opportunity to use these engines in our Radio Control models is totally awesome.

How an RC Model Jet Turbine Works
How an RC Model Jet Turbine Works

A typical jet turbine used for RC is capable of spinning over 100,000RPM in order to produce the thrust force they generate. RPM ranges between about 117,000RPM for a large model turbine and 245,000RPM for a relatively small turbine. Thrust ranges between about 20N (4.5lbs) at the small end to over 220N (49.5lbs) at the larger end of the range. The 220N turbine would weigh less than 2kg (4.5lbs). This kind of performance is incredible!

Fuel Source for a Model Turbine

Turbines in general can run on just about anything. Model jet turbines use either Jet A fuel that you can purchase at an airport, diesel fuel that you can buy at the local gas station or Kerosene that you can buy at several locations locally. There are advantages and disadvantages of each fuel. Primarily, my preference is to use Kerosene because of one factor. Odour! Kerosene is very clean and the odour that it gives off is quite minimal. Since the fuel is used to heat homes/cottages and such, it needs to be not too overbearing in terms odour! The disadvantage is that it costs a lot. About $3 canadian per litre as opposed to deisel and Jet A that is much cheaper. About half the cost or more.

Fuel must be mixed with turbine fuel in order to properly operate the turbine. Mixture is usually between 2.5% and 5%. More on this below under the lubrication heading.

Deep Dive in to the Jet Turbine Inner Workings

The big question is How does one of these model jet engines work? We dive deep in to the model of a turbine to get a better understanding as to how one of these turbines is able to operate. Check out the video for full details:

Turbine Starting Sequence

It all starts from the transmitter. The starting sequence is initiated by performing a sequence of control input specific to the manufacture of the turbine. Typically it involved moving the throttle and throttle trim in a certain manner. From here the ECU (Engine Control Unit) begins the starting sequence.

Firstly, to get the model turbine started, the igniter must be pre-heated. At this point there is a delay to allow for the igniter to warm up. Once the delay is over, the starter motor which is an electric motor on the front of the turbine is used to spin up the turbine to a pre-determined speed. At this point fuel begins to flow in to the turbine usually through a starting fuel line. (Can be seen externally on some model turbines) The fuel for starting ignites and when a certain temperature is reached, the EGR (exhaust gas temperature) sensor reads this and initiates the next part of the starting sequence.

Transfer from Starting Fuel to Main Burner

Once the engine is at the next temperature stage, fuel is then brought in through the main burner. During this part of the sequence, the starter motor is slowly ramping up to the idle RPM as the fuel flow rate is increased. At some point during this ramp up stage, the starting valve is closed and fuel only enters the combustion chamber through the main burner fuel line. When the model turbine ramps up to idle RPM, the automated process is complete and all control is now transferred back to the radio. During the entire starting sequence, the ECU handles everything and the radio does not have control.

At this point you have an idling turbine ready for operation.

Mechanical Workings of an RC Model Jet Turbine

Mechanically, the amount of precision required in these engines is very high. Balance of the compressor wheel, shaft and turbine wheel is very important as the engine spins a significant amount of RPM. Much of the mechanical workings of the engine is discussed in the video with visual of a turbine model.

Lubrication of the Bearings

Lubricating the bearings of a model turbine is a very important function. The fuel that is burned in the turbine contains the lubricating oil for the bearings. unfortunately, all oil used in the fuel is 100% consumed. Oil making its way within the inside of a model turbine is required to find the bearings otherwise there is no lubrication. It is only a small percentage of oil that comes in contact with the bearings, otherwise the rest of the oil simply exits the nozzle of the turbine.

The most important part of the lubricating oil is not the lubrication part. It is actually most important for the lubricating oil to control the temperature of the bearings. Maintaining the proper temperature for the bearings is how we are able to get maximum life out of them before the time to rebuild expires. If the bearings were not correctly cooled during operation of the turbine, lifespan would be drastically reduced.

Internal Combustion Stages in a Turbine

I hate to say it like I did when I was in high school but for the record, it makes me remember the sequence actually quite well. A turbine follows that same internal combustion engine logic that we all know. Suck, Squeeze, Bang, Blow. However, it just does all of this at the same time!

There is a compressor wheel that operates centrifugally to intake the air (suck). The air is then compressed by the wheel (squeeze) as it enters the turbine. From here, the compressed air enters the combustion chamber as it ignites and expands from the heat of the flame front. (Bang) This expanding air can not stay in the combustion chamber. It makes its way to the rear of the engine out of natural easiest route or path since air on the other side is compressed. As the hot gases exit the turbine, they pass over the turbine wheel spooling up the compressor wheel to start the process over again.

Importance of Reliable Operation – Flame Outs

Now just imagine that all of what we have just covered is happening at the exact same time. If the flame were to go out by having a tiny air bubble in the fuel line, the engine quits and needs to be restarted using the entire starting sequence again. Any type of issue that can result in the flame burning out will stop the continuous combustion process. This is known as a flame out and a big part of the reason we need to be able to operate a turbine reliably through its entire on time.

RC Jet Turbine Conclusion

The operation and precision required to operate a turbine is very high. I hope this has given you some insight as to how these marvels work. If you haven’t seen one of these in person, my recommendation is that you check it out. Simpy speaking, they are just impressive.

3D Printed Radio Box for 1/8 Scale Hobao VSE Buggy Free Download

Download this Printable File on Thingiverse: https://www.thingiverse.com/thing:4300018

3D Printed Radio Box for Hobao 1-8 scale VSE Buggy
3D Printed Radio Box for Hobao 1-8 scale VSE Buggy

The trigger for me to design and print this radio box came from a requirement that the physical internal size of the stock box was too small. I could not get the radio that I wanted to use in the stock box. What was required was a new box that maximized the available room inside. But the problem was the physical footprint where the radio box is installed on to the buggy was rather limiting. Also, this box must have an easier method of accessing the radio inside. Here we make use of a standard body clip to lock the cap in place. You can see this in the image above.

Just for reference RX refers to the receiver.

The stock radio box also includes the steering servo mount. The steering servo mount has been designed in such a way to maintain high strength as a number one priority.

There are 2 Printable Versions

One version has side flanges in side the box for a receiver. The internal distance between these flanges is 26.5mm. The other version of the box does not contain any flanges for installation of a more generic radio receiver. You may see these when you load the files in to your printing software.

3D Printed Hobao VSE Buggy Box with Flanges
3D Printed Hobao VSE Buggy Box with Flanges

The internal opening size of the box is 32.1mm long x 24.8mm wide x 40.8mm Deep. These dimension include the space that would be occupied in the cap.

Model of the 3D Printable Hobao Buggy Radio Box
Model of the 3D Printable Hobao Buggy Radio Box

3D Printer Settings

Printer brand: MakerBot
Printer: MakerBot Replicator (5th Generation)
Rafts: Yes
Supports: Yes
Resolution: 0.2mm
Infill: 35% (Recommended)
Filament Brand: MakerBot
Filament Color: White
Filament Material: PLA
Shells: Use a Minimum of 2 shells

Post Printing

The Radio Box must be Tapped for M3 Fasteners. Once the Radio Box is completed, your 3D printed may have shrunk the size of the holes required to be tapped. If this is true be certain to drill the hole back out to be suitable for a M3 tap. Once the hole is at the correct size tap the 4 holes for an M3 fastener.

Upon Installation of your RX box, there are two wire exits that you can use for the ESC. One is located at the bottom corner of the radio box. The other is located right under the cap. Choose which location you would prefer to use. As for the steering servo, use the wire entrance slot at the bottom of the box to pass the connector through.

Use a body clip to secure and lock the cap of the box in place. This allows quick and easy access.

Hiding Wire

The box has a convenient area where excess wire can be tucked away. This is located right under the lip where the cap locks in to place on the side where the steering servo wire exits the box. Use this area to hide excess wire.

Other Considerations:

The box is not waterproof and is never intended to be. Place your receiver in a balloon if you operate in wet condition.

Check Out more 3D Printed Projects

All About RC Car Gear Pitches – Why are there so many?

In brief, RC Cars utilize a transmission that contains a gear set. Different applications will require different “pitches”. A gear pitch given in imperial terms is related to the amount of teeth that would encircle a one inch diameter. This one inch diameter is known as the pitch diameter. The metric module is how pitches are defined in the metric system. The metric module is known as the ratio of the diameter of the pitch circle per tooth. Do not confuse the metric and imperial pitch sizes as they represent different sizes of pitch.

MOD1 and 32P Gears of Different Sizes
MOD1 and 32P Gears of Different Tooth Counts

What Gear Pitches Exist

In particular, there are both metric gear pitches and Imperial gear pitches used in RC cars. Below is a list of the common gear pitches used:

Imperial Gear Pitches:

  • 32 Pitch
  • 48 Pitch
  • 64 Pitch

Metric Gear Pitches:

  • 0.5 Module
  • 0.8 Module
  • 1.0 Module

If you are unsure of which pitch your specific RC car vehicle uses, look up the parts in your manual. In any event that you can’t find the pitch of your spur gear or pinion gear in the manual, try out the gear pitch calculator.

As the imperial gear pitch increases, the size of the teeth and gear actually decrease. They get smaller. Furthermore, as the Metric Module increases, the size of the teeth and gear increase. Hence, the gears and teeth get larger

Can you Use a Metric Pitch in Place of an Imperial Pitch?

A Metric Module of 1.0 converts to an imperial pitch of 25.4. Gears with a Metric pitch are not designed to match an imperial pitched gear.  I would recommend not using a metric pitch in place of an imperial pitch. In short, this does not mean that you will never find a combination that is close enough to substitute. One could argue and even use a 0.8M gear in place of a 32P gear and it may work well. Even so, I would not recommend using the pitches interchangeably unless you are OK with an increase in gear wear that could result.

Why Use Different Gear Pitches?

Not all applications are alike, this is why we must use different gear pitches in RC. Generally speaking there are two main reasons to use a different pitch. The first reason being physical size. If you are operating a smaller indoor 1/18 scale RC car, it is not ideal to use a large gear with a big pitch. The size of the gear would be far too big to place in to the RC car. It’s also worth it to point out that moving to smaller gears also allow for a decrease in friction allowing more power to make it to the wheel. This is also important for smaller RC cars in order to get the limited power they make, to the ground more efficiently.

LaTrax Prerunner Teton 3D Printed Drift Wheels Free Download

Download this Printable File on Thingiverse: https://www.thingiverse.com/thing:4235548

Here we have a 3D printed LaTrax Teton or Pre-runner (by Traxxas) Drift wheel. The idea was inspired by a friend talking about having an indoor RC car that can drift. I know how much I like the LaTrax line of RC cars and decided that adapting one that I already have should be very easy. That is with the help of 3D printing!

Latrax Prerunner 3D Printed Drift Wheels
Latrax Prerunner 3D Printed Drift Wheels

I jumped on the design software that I use and came up with a simple solution. Something that would be a good balance between print cost, print time, appearance and performance. The end result is a lot of fun.

These wheels work very well with the brushed (stock) setup. The 3D printed wheels have been optimized for use on carpet, and it is quite easy to hold a drift on short carpet. It is also quite easy to hold a drift when running on any hard surface.

The overall Diameter of the wheel is 50mm. (2.0in)
The overall Width of the wheel is 30mm (1.20in)

Latrax 3D Printed Drift Wheels
Latrax 3D Printed Drift Wheels

3D Printer Settings

Printer brand: MakerBot
Printer: MakerBot Replicator (5th Generation)
Rafts: Yes
Supports: Yes
Resolution: 0.2mm
Infill: 15%
Filament Brand: MakerBot
Filament Color: White
Filament Material: PLA
Shells: Use a Minimum of 3 shells

Latrax Teton 3D Printed Drift Wheels
Latrax Teton 3D Printed Drift Wheels

Post Printing Steps

Step One – Clean Up Part

After your wheel has been printed, remove any loose printed material from your wheel. The important point is to make certain the hex insert is free of any loose material. Loose material that has not been cleaned out of the hex area may allow the wheel to sit incorrectly.

Step 2 – Remove Existing Wheels

Remove the wheels on your LaTrax Teton or Prerunner by unfastening the single fastener in the center of the wheel. Use a 2.5mm Allen key.

Step 3 – Check center Wheel Hole Clearance

Use on of the fasteners that was removed from the existing wheels to check the size of the wheel fastener hole on the printed wheels. This may occur if there was shrinkage of the hole during printing. If so drill it out to 4mm (0.150in)

Step 4 – Installation

Use the fasteners to fasten the wheels to your RC Traxxas LaTrax Prerunner or Teton. Do not over tighten.How I Designed This

The hex drive has been oversized slightly to allow for a small amount of shrinkage during printing.

Download this Printable File on Thingiverse: https://www.thingiverse.com/thing:4235548

The RC Car Gear Ratio Explained and Calculated

For every mating pair of gears in an RC car, there exists a gear ratio. The gear ratio is simply the number of teeth of one gear divided by the number of teeth of a mating gear. Gear ratios are important as they allow us to better control speed and torque. We are able to sacrifice speed in order to benefit from increased torque.

This relationship allows us to achieve optimal output RPM by adjusting the gear ratio. Another point worth noting, it is much easier to extract power from a high RPM brushless motor vs a slower turning motor.  It is all made possible by utilizing the gear ratio.

How Gear Ratio is Calculated

The gear ratio is calculated using the number of teeth on a gear. The other option instead of using the number of teeth can be using the overall diameter of the gear. Either way will work, for simplicity and consistency, we will use the number of teeth as our preferred method. Watch the video for a more visual demonstration.

Determine the number of teeth on the two meshing gears that you are calculating the ratio between. Use the number of teeth on the spur gear  and divide by the number of teeth on the pinion gear.  The resulting value will more than likely be greater than 1.0. A good way to always remember this relationship is dividing the Output gear by the Input gear.

Multiple Gear Sets and Ratios

Gear ratios can be confusing when there are multiple sets being used. In the example below, the output shaft of gear set number one is connected to the input shaft of gear set number two. The gear ratio is determined in each set first. The above method is used to calculate the ratio in each set. Next, the calculated gear ratios for each set are then multiplied together. The resulting value is the gear ratio for the entire system.

2 Meshing Sets of Gears - Output of Set one Connected to Input of Set 2
2 Meshing Sets of Gears – Output of Set one Connected to Input of Set 2

You may also use the RCI gear ratio calculator.

Determine Torque Multiplication from a gear Ratio

Torque multiplication always occurs when there are two meshing gears of different number of teeth. The torque multiplication can be calculated directly based off of the gear ratio. As an example, we will use a gear ratio of 10:1. For every 10 turns of the input shaft, the output shaft turns once. We can use this exact number and multiply it by the amount of torque the motor produces. For example, if we say the brushless motor can deliver 0.05 ft-lbs of torque at the input shaft, we would expect 10 times this at the output shaft. The resulting value would be 0.50 ft-lbs of torque at the output shaft.

Putting it together – Application of Gear Ratio Sets in RC

In many RC vehicles, multiple sets of gears are used. Quite commonly, one set will almost always be found directly on the motor itself. The pinion gear on the motor is the first gear that is in a set. The next set of gears in a transmission is typically the differential on a shaft driven drive train. The output shaft going to either the front or rear differential contains a pinion gear. This pinion gear mates with the differentials ring gear, that ultimately drives the differential. In order to calculate the gear ratio, the Motor pinion gear mated to the spur gear must have the ratio calculate first. Next, calculate the gear ratio of the differential setup. Lastly, take the 2 resulting gear ratios and multiply them together.

The total value that you have as a result is what would be considered as your final drive ratio. For every one turn of the output shaft leading to a tire, the final drive ratio represents how many times the pinion gear on the motor shaft must turn. You could then use this value to determine the total amount of output torque produced as long as you know the total input torque.