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Lessons Learned and Hints from 2006/2007:

Transmitter and Reciever:

These are usually a matched set sold together. The most common type of radio is a 72mhz radio. These come in two different types: PPM and PCM*. *PPM and PCM refer to the encoding of the signal being sent to the receiver. PPM can "glitch" due to interference (either from outside sources, or from EMI interference from within the airplane's system). Glitches show up as random servo and throttle activation. PCM is considered better in some cases as it checks for signal integrity, and once signal integrity is lost the receiver defaults to a failsafe condition. This prevents any glitching. The failsafe condition is user definable (AKA set throttle channel to zero along with full up elevator and full right rudder for a controlled spin). Once signal integrity is regained the receiver exits failsafe and resumes normal operation.

A new type of radio system is called Spektrum. Spektrum is on the 2.4 ghz band, and is considered to be glitch free. Spektrum uses a dual link receiver (actually two separate receivers that compare signals for optimal reception). Use caution when dealing with Spektrum systems however, as it has only recently become a "full range" system (the original Spektrum systems had limited range). As far as I know there is only one manufacturer of Spektrum radios and receivers, and only one "full range" system. Unfortunately, because only one system is made you are limited to one specific transmitter, which if I remember correctly has only 7 channels and limited programmability. The Spektrum system does not require choosing a "frequency" (see below for more…). The system automatically selects an open frequency and syncs the transmitter to the receiver… this is part of why the system is said to be immune to glitching: the receiver will listen only to YOUR transmitter. Transmitters may be programmed with several different models. Setups such as aileron to rudder mixing, canard to elevator mixing, and flaperons (ailerons that can be deployed as flaps as well) are all readily possible. With a high end radio (such as the Futaba 9C used last year) just about anything you want can be programmed. If going with the 72mhz radio setup, there are a wide variety of receivers available. Varying numbers of channels are available as well as size. Make sure that if you buy a separate receiver that it is compatible with your transmitter (PPM receivers will not work with PCM transmitters and vice versa). Finally, the 72mhz radios are set up on "frequencies" These are literally small segments of the 72mhz band. What frequency your system is using is usually determined by a small swappable crystal that goes in both transmitter and receiver. Higher end systems have "frequency modules" that can be set to any frequency (on both transmitter and receiver). The power for all servos is routed through the receiver. Power can be input to ANY of the channels on the receiver (even though some receivers specify which channel to apply power to, it is a common bus) With this being the case make sure you buy a quality high-channel count receiver. Using a cheap 4 channel receiver to ferry power to your high amp draw digital servos won't get you very far.


Servos come in two major categories: Analog and Digital. Digital servos provide faster transit times, more torque, and better accuracy. This comes at the expense of higher amp draw. As a general rule, if you have enough power available, go with digital servos. On a side note, digital servos "buzz" almost constantly, analog servos do not. Be absolutely sure that you know how much all of your servos draw at "stall torque" Larger digital servos can draw in excess of 1 amp EACH at stall… with a complicated and large aircraft the power requirements can grow rapidly. Servos are rated in terms of how fast they travel a certain number of degrees, and how much torque they are able to put out. Be sure to perform calculations for each surface as to the torque required throughout the flight envelope… an under-rated servo WILL fail in short order. Also, keep in mind that you can apply further mechanical advantage (and thereby increase or decrease the torque of the servo) through your linkage system to the control surface. Be aware that all servos are usually dual rated: one set of ratings for 4.8V, and one set of ratings for 6V. What you use depends on what you power your servos with


There are a couple of ways to power your system. If you are going with an electric propulsion system, the Electronic Speed Control (ESC) may have what is called a BEC (battery eliminator circuit) This is essentially a linear regulator that takes the voltage of your propulsion battery pack (which is usually much greater than 5 or 6 volts) and steps it down to 5 or 6 volts. However, you must use caution in this situation. All BEC's have an amp rating, usually around only 2 or 3 amps sustained. For larger aircraft this is simply not sufficient… you will be drawing well over 2 to 3 amps through your servos, and the BEC will fry… kiss that plane goodbye! Typically a separate battery is used for the systems of larger models (discussed later).
Similarly to the BEC that is embedded in some ESC's, there are separate switching regulators available that perform the exact same function at a more efficient rate (the UBEC comes to mind, made by KoolFlight, as well as ones made by Medusa Research…). Units can be found that have higher sustained amp draws as well as selectable voltage output. However, once again the sustained amp draw that these devices provide will most likely fall short of what will be required by a larger model.
The final option is a separate battery pack for powering your receiver and servos. This requires either a NiCD, NiMh, or small LiPo pack ranging from 4.8 to 6 volts nominal. Be aware that all rechargeable batteries charge to a higher voltage initially than what you will see during nominal usage (for example, a 5 cell NiMh pack rated at 6 volts nominal (1.3 volts per cell) will charge to over 7 or 8 volts) While this option is sometimes heavier (you are now carrying around two battery packs) it is the most reliable. You do not have to worry about a separate regulator failing, and you don't have to worry about not having enough power to drive your servos in case your main propulsion battery pack fails. In addition, it is much easier to change between 4.8 and 6V systems: simply choose between a 4 cell pack for 4.8V and a 5 cell pack for 6V operation.

Stability Augmentation:

There are a couple ways of achieving simple and effective stability augmentation systems. The basis of these systems are called "gyros" These are solid state devices that allow you to correct for angular heading variations. They plug in between your receiver and your servo, and sense changes in heading along a single axis. Any disturbances in this heading that are not commanded by you are corrected automatically. This can be applied to any channel you want, whether it be steering, roll, yaw, etc… Furthermore, higher end gyros are "heading hold" enabled. Not only will the gyro correct for heading disturbances, but it will bring the aircraft back to its original heading, no matter how large the disturbance.

There are systems available that are essentially full autopilots. These consist of multiple gyros that sense roll, yaw, and pitch. Once you give the command the system will restore the aircraft to right- side-up level flight (useful for those "oh crap" moments where you are inverted and heading for the ground…) The one system I have heard a fair amount about is the "Co-Pilot" made by FMA.

Data Logging:

Data logging is done with aftermarket systems that plug in between the servos and receiver. They sense the commanded signal and record it before passing the signal to the receiver. Furthermore, these systems can be outfitted with separate sensors for airspeed, GPS, altitude, alpha, beta, etc. Eagle Tree is one company that makes such systems (the same that has been used for the past two years)

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