I had a great time with this project. It was rather easy to put together and a whole lot of fun to test. I look forward to playing with it more and may update the video with more antics in the future.
For more pics and videos check out crazybuilders.com
Step 1: Building the Skateboard, Mounting the Engine
This page contains information on how I built the propeller powered skateboard. It took approximately four hours of building time to complete the build spread over four days as I gathered parts and finalized the design between steps. The build was straightforward and mostly consisted of strapping an overpowered model airplane engine onto a plank and attaching skateboard wheels.
Bill of Materials
|What||Details||What For||Where From|
|Wood||1x10||Skateboard and Motor Mount||Home Depot|
|Bolts||#8 x 1.5||Mounting Trucks to Deck||Home Depot|
|Fan Cage||Prevent Major Injury|
|Skateboard Trucks||Cruiser with Risers||Smooth Ridin||Skate Shop|
|Grip Tape||Self-Adhesive||Looks/Traction||Skate Shop|
|Transmitter||HiTec Laser 6 Ch. FM||Transmits Signal||Tower Hobbies|
|Receiver||Receives Signal, Powers Servos||Bundled w/ Transmitter|
|Servo||Engine Throttle||Bundled w/ Transmitter|
|Battery||4-Cell 4.8V 600mAh NiCd||Power Receiver/Servos||Tower Hobbies|
Model Airplane Gear
|Engine||OS 1.60 FX||Powerplant||Tower Hobbies|
|Gas Tank||950cc||Holds Fuel||Tower Hobbies|
|Fuel||Fuel For Engine||Local Hobby Store|
|Glow Starter||Standard w/ Meter||Starting Engine||Tower Hobbies|
|Starter Motor||TorqMaster 180 Heavy Duty 12 V||Starting Engine||Tower Hobbies|
|Propeller||16x10 3-Blade Pusher||Transmit Force||Tower Hobbies|
|Flints||Tail Devil||Making Sparks||Amazon|
I had a hard time deciding whether to put the engine on the front or back of the board. Most, if not all, model airplane engines can only rotate in one direction. If you were to point the typical airplane engine setup backwards, it would pull the board backwards not forwards. If you want the engine to pull the other direction it is not possible to simply turn the propeller around to change the direction of pull, but instead you must get a "Pusher" prop - the opposite being a "Tractor" prop. Most model airplanes have the engine on the front of the plane, which means that there is a much wider selection of tractor props. In the end I decided it was worthwhile to me to put the engine in the rear, facing toward the rear, to keep the exhaust out of the way and to get the propeller as far away as possible. And I think it looks better that way too.
I had to make sure that I could still tilt the skateboard back to hit the flints on the ground and make sparks with the fan cage attached, while at the same time I wanted the motor as close to the ground as possible. This, along with the decision to have the engine in the rear of the board, solidified the engine's position.
Construction Steps1.) Sketch Board Design
The inspiration for the board design came from some old skool cartoons with rocketships. I thought this would be a good design for a board, and am surprised that I haven't seen any with this shape before as it works pretty well as a longboard. If you make skateboards and think we could sell a special edition board, hit me up! :-)
Then, I sketched the bolt pattern for mounting the skateboard trucks. I did this quickly by using my original skateboard deck as a template and marking each hole with a sharpie.
I took a jigsaw and cut out the rocketship pattern, then drilled out the skateboard truck mounting holes.3.) Grip Tape
I got some grip tape from a local skateboard shop, Beacon Hill Skate Shop. The grip tape I got was self-adhesive, pretty much like a big sticker, so it was rather easy to apply but took time to get the design to match on the interface between the yellow and black grip tape.4.) Engine Mount
It took a good deal of thought to get to the final design, which consists of two 1" vertical planks, screwed into the board from the bottom through the skateboard deck by a lot of screws. I added an electronics compartment between the uprights which kept the electronics out of the way and helped reinforce the engine mount. I spaced the uprights just far enough apart to allow the engine to sit perfectly in between. I decided to add grip tape to the uprights as well, for extra style points.5.) The Rest
From there, it was mostly screwing or bolting everything onto the board, including the skateboard trucks, fan cage, engine, electronics (including throttle servo), gas tank (which I attached with a hose clamp), and tail devil sparking flints.
Step 2: Configure the RC Gear
Radio Control is the basis for a lot of fun projects. Here I briefly overview a system, show how a servo works, and give a demo.
The transmitter is often the most expensive part of an RC system. A transmitter, as with most RC gear, will survive through many projects, so it is wise to shop for a transmitter with future projects in mind.
The pistol grip is primarily used for controlling RC cars and trucks as the wheel simulates a steering wheel, the trigger being used as the throttle.
The stick interface is the most common, and is used for practically everything including RC cars, helicopters, planes, boats, and general use. The sticks are spring loaded to return to the center position with the exception of the left stick which has no spring for the vertical direction. The ability to hold the vertical position is commonly used to hold a throttle position making the left hand at least momentarily available for switching other controls. The exception to this is in special "helicopter" radios in which all sticks return to center.
Radio Transmission Technique
There are many types of transmission techniques available on the market today. Each newer generation improves on the last, so when selecting which one you want it is really a trade-off between the new features and price. I'll start with the oldest and describe the improvements through the generations.
Amplitude Modulation (AM) systems would transmit the data as changes in amplitude on a constant transmission frequency wave. The simplicity of the electronics made this method inexpensive. Noise directly effects the output signal causing a noisy output. AM is cheap, but the costs of FM transmitters have come down so much AM is not longer in the game.
Just like car radio transitioned from AM to FM, so did hobby RC gear. FM (Frequency Modulation) changes the frequency of the waveform as opposed to the amplitude. Most random radio noise is in pops in amplitude as opposed to consistent frequencies. Since FM is insensitive to pops in amplitude while that's all AM has got, FM can handle radio noise much better than AM can.
PPM is a way to encode the data before sending it over the air, (Pulse Position Modulation). This is how the hobby radio implementation works. The transmitter sends a series of highs and lows in a specific sequence which the receiver then decodes. The output is binary, high or low. The sequence starts with a long high followed by a low, which signals the start of the sequence. The position of channel 1 on the transmitter is indicated by the length of the next high, which is again followed by a low. The length of the following high corresponds to the position of channel 2 on the transmitter. And so on, and so on, followed by a long high which is the start of a new transmission. This sequence is repeated regularly, typically at 50Hz.
The receiver's job is to split the sequence up into isolated individual pulses on different channels. Each channel receives one of the pulses. The length of each pulse ranges from about 1ms to 2ms. The receiver knows when to change outputs when it sees a low. If the low was caused by noise, the rest of the channels would receive bad data due to an off by one error. This type of error is avoided in the next technology...
Referred to as PCM (Pulse Code Modulation), this technique is still uses FM at it's core but sheds the FM name. Whereas PPM created a pulse of a certain length specified by the position of the controls, PCM creates a digital representation of the position of the controls (8-bit, or 16-bit, etc) and sends the position data as a stream of 1s and 0s. For example, as opposed to a neutral stick being represented by a 1.5ms high pulse, it may be represented in 8-bit as a high pulse followed by seven low pulses (1,0,0,0,0,0,0,0 [binary]), the pulse length being determined by a clock.
One advantage of this is the ability to do quick error checking with a CRC check. The error checking allows for noise to be detected and for bad data to be halted before going out of the receiver.
The noise screen practically eliminates interference, yet we find a new problem: when the model gets out of range, the servos hold the last good transmission, possibly until a spectacular crash. To reduce this problem, some receivers have the ability to keep "fail-safe" outputs in memory. When the receiver has not received a noise free transmission for a certain amount of time it starts outputting the "fail-safe" values. These values may, for example, turn the throttle down or put a glider into level flight which could help avoid the spectacular crash.
Now comes the latest generation of wireless transmitters, using a technology much similar to WiFi computer networks. Unfortunately, there are currently many implementations of transmitters/receivers using the 2.4GHz spectrum, making generalizing difficult. The previous technologies used a crystal to determine the transmission frequency, allowing for interference from someone using the same crystal (of which there are a limited number), and requiring additional crystals to change frequencies. This new generation of transmitters get around this problem in different ways. Some scan for clear frequencies then latch on to a clear one, others use spread spectrum frequency hopping, to continuously evade noise by continuously changing channels. Other advantages of this new generation include the reduction of size in the transmitter antenna, smaller receiver size, and better battery life. Of course, these new features will hit you in the pocketbook.
The receiver's job is to split the incoming signal into however many channels you require. Although transmitters and receivers of different transmission technologies can't talk to each other, the output of all current receivers is the same: a pulse of a certain length that corresponds to the position of one of the controls on the transmitter. It is then left up to the servo, motor controller or other accessory to decode that pulse into a position, speed, etc.
It is most common to buy a receiver in a set with a transmitter. That way you know they are compatible and get a better deal on the purchase.
Servos, Motor Controllers, and More - Oh My!
A servo is a motor that has position feedback, allowing the servo to rotate the motor to an angle, then hold that specified angle of rotation. Check out the video for more on how servos work.
Motor controllers control the output (speed or torque) of a motor connected to it given various types of input. Many motor controllers accept the servo signal output from an RC receiver, allowing drop-in functionality with an RC system. With electric motor powered airplanes becoming more popular due to advancing lightweight and powerful battery technology more motor controllers are hitting the market and they are coming down in price. Since electrical power loss goes as I^2*R, twice as much current produces twice as much heat. In order to prevent part loss due to high heat, motor controllers are usually rated in terms of current (here is a link to an example motor controller, rated for 2 channels outputting 25 Amps each). Many motor controllers have onboard heat sensors and will shut down the motor controller if too high of a temperature is reached.
High electrical loads and spikes that come with motors and motor controllers may disrupt the signal going into the receiver causing potentially disastrous interference. Error checking radios (PCM or later) may help a great deal in these situations.
One cool part I have used is a relay that is triggered by the servo signal called the: Battle Switch
(Wikipedia Article For More Info )
Step 3: Test It Out!
I started by doing some feasibility tests with a household fan, 12V battery, and an inverter on a skateboard. The results were disappointing, but not surprising. I needed more power!
Enter: The OS 1.60FX 3.7 HP model airplane engine. The most powerful engine in the world! Ummm.. not quite - but it packs quite a punch for an engine that weighs under 1kg. Much better than the heavy, low-power house-fan and battery setup! Even better than a 5 HP lawnmower engine for the weight. Additionally the engine is very high RPM (1,800-10,000), which allows us to use the small (model airplane) propellers. The main drawback is that the engine runs on Glow Fuel, which is made of methanol, nitromethane, and an oil, needs to be purchased at a hobby store and is much more expensive than gasoline. That said, I probably only used a couple bucks of fuel during the entire time I spent using the board and tuning the engine.
The first test ended shortly in propeller failure. The cage was just too close to the propeller, and with a slight vibration the propeller was caught in the mesh of the fan cage. Totally preventable problem, but I thought the tolerances were good when it really needed some more space. The next day, after reconstructing some of the engine mount which cracked in the propeller incident, it was time for test #2.
Success! The board cruised around the parking lot like an unstoppable freight train. Now this parking lot wasn't like most parking lots you might thing of. It was the parking lot to a long-neglected factory, and itself had been long-neglected. In other words: there were tons of rocks and grooves and cracks! So, what one might think of as quite mundane empty parking lot skating was actually quite exciting plowing over these rocks which would normally grind my skateboard to a halt when freeskating - in addition to the fact that there is no brakes and that the board wanted to take off even at low throttle.