If you're running an electric R/C and the receiver loses the signal sent from the transmitter, most of the time it will just sputter out and coast to a halt because the speed control also fails to receive a signal to "go." But if a nitro vehicle's radio loses a signal, the throttle servo will not receive a signal to return to idle and will stay just where it is. It's all too often that the engine is at full throttle when this happens. And off it goes, until it hits something or blows the engine. Following Murphy's Law, that "something" is usually an immovable object that demolishes quite a bit of the vehicle - or worse yet, cause property damage or physical injury.
Radio System
Just as described in electric powered RC vehicles, the receiver accepts the signals from the transmitter and sends steering instructions to the steering servo, but instead of the speed control it sends throttle instructions to the throttle servo. The battery slot, normally left unused in an electric, is connected to a receiver pack as described below. The radio systems used in nitro powered R/C's are often identical to systems used in electrics, except there are a few more precautions a nitro owner must take.
A nitro owner must be mindful of the following, without fail, every time, before they put the glow start on an engine:
- Verify the charge state of the receiver pack. Be 100% sure.
- Verify the pack is firmly fastened in the vehicle and cannot be knocked out on a hard hit, especially when racing.
- Verify that all wiring harnesses and connector ends are in top shape and firmly connected so that a hard hit will not cause anything to come loose.
- Use a zip-tie to firmly hold the receiver connectors in place, and also insure that the receiver itself is anchored to the vehicle and cannot be knocked loose.
- Some receivers come with a rubber grommet to slip around the receiver crystal, insuring a firm seating into the receiver. If one is not present, fashion one or zip-tie the crystal in but not so hard it cracks the crystal inside. Radio signal can die just as easily from a faulty or loose crystal.
- Insure the receiver antenna is in good shape, showing no signs of nicks in the insulation (except at the very end,) and never ever shorten your antenna wire or allow it to come into contact with metal parts. This can cause glitching, a form of radio interference characterized by jittery servos and loss of control.
- Install and use a throttle return spring as described in Throttle Servo above.
- Verify the functioning, strength, and reliability of the throttle servo.
- Are your electronics, both the transmitter and the vehicle electronics, turned on?
- Are you SURE all electronics are on and functioning?
- Range-check the vehicle; that is, turn the electronics on while someone remains by the vehicle, walk off in the distance, and using hand signals indicate which signals you are sending and verify the vehicle is reacting perfectly without glitching (showing signs of interference, such as jittery servos or other abnormalities.)
- Install and use a working failsafe device.
A failsafe is a device connected between the throttle servo and receiver. If the throttle fails to receive a signal, as with transmitter failure, interference, or severe radio glitching, the failsafe uses the power supplied to the radio to close the carburetor to idle.
While failsafes are not a 100% panacea to prevent runaways, they will prevent problems caused by signal loss or glitching. if the battery connector falls loose or the receiver loses battery power in any way, the failsafe will also lose power and cannot help you. A runaway could still result. If you do all of the above and still get a runaway, this is another area where ample portions of patience are required to endure the hobby.
The Receiver Pack
Electric powered RC's generally power the servos and receiver from the main 6-cell battery pack that drives the motor. Since this pack is not present in a nitro, they must carry a receiver pack to power on-vehicle electronics.
The receiver pack is more important to the servos and their correct functioning that anything. Most nitro kits and RTR's are designed to run with 4 standard AA cells mounted in a battery case for this purpose, supplying a nominal 4.8 volts to the servos. But as described in electric powered R/C vehicles, the more voltage supplied to the servos, the faster and stronger they will be. This is particularly important in a nitro for all of the reasons already mentioned - you must insure that power to your receiver and servo is reliable and consistent to avert nitro disasters.
Most nitro owners opt for a 5-cell 1100 mah pack in a shrinkwrap-sealed pack, which provides a solid 6 volts to the electrical system. The connections from cell to cell in these packs are soldered or spot-welded, providing one less thing to worry about over the standard 4 cells in a battery case. The connections in the case can fail or the batteries themselves can be jarred loose or fall out, resulting in a runaway.
Per the checklist above, always verify the charge state of your receiver pack and it is a very good idea to have a spare pack or two handy for those long afternoons at the track.
Throttle Servo
The throttle control on a nitro R/C vehicle is driven by one "horn" (actuating arm) on a servo connected to linkage which opens and closes the carburetor. When the throttle is released and the carburetor reaches idle, the idle stop screw in the carburetor stops the carb from closing any further, allowing just enough air to enter the carb to allow the engine to idle. The throttle linkage, however, is allowed to push past the idle position by means of a sliding idle pivot on the throttle linkage. This allows a different horn on the servo to continue moving in the "throttle down" direction and pull the brake rod, applying the brakes. In the idle-through-throttle positions, the brake linkage slides away from the brakes in a similar manner, doing pretty much nothing. This is how brakes and throttle are both operated by the single throttle servo.
The throttle servos used for nitro vehicles are the same servos described in electric powered RC vehicles, but require a few additional considerations.
The only link between you and your expensive new nitro engine is the connection from your throttle trigger to the throttle servo. Should the transmitter, receiver, servo, servo linkage, or battery malfunction, the results could be disastrous, ending your day with a blown engine at the very least. For this reason, do not skimp on any device in this vital chain. Use only equipment that you can verify is stable and reliable. If the throttle linkage is wearing or questionable, replace it.
Another weak link in this chain is the actual ball end attached to the carburetor. Even with an enclosed linkage end, the screw can work loose. Employ a throttle return spring, one strong enough to pull against your servo when it is powered down, that will pull the carburetor to the closed position should the linkage fall off or the servo should fail.
There is another important difference between electric and nitro throttle control. In an electric powered vehicle, changes in throttle are immediate. In a nitro vehicle, the signal you send is immediately received and transferred to the throttle servo, but it takes time for the servo to move from one position to the next.
If you are moving along at full speed and hit the brakes, an average servo* may allow the car to continue rolling up to 20 feet before the brake is actually applied to the vehicle. The faster the servo, the faster the reaction time you are going to have between your throttle finger and the brake. When selecting a throttle servo, try to get the fastest servo available.
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