The model sailplanes: Sailplanes are powerless gliders. They float in air by ridding on air currents generated by natural forces. Model sailplanes can be classified into two major categories. Slope gliders in many aspects differ from thermal sailplanes. They float on the upstream air in front of a mountain slope. This upstream lift is generated as a result of wind blowing towards the slope and pushing air up.  Slope gliders are typically faster than their thermal counter parts. Generally, they are also smaller in size. Their wings are made of balsa sheets with foam cores. Some high performance gliders are made by molding with carbon fiber. Many combat gliders are made of EPP material which are flexible and strong.

How it works: Under construction

RC equipment: Radio control equipment consists of a transmitter, a receiver, and a number of servos. The servos are plugged into the receiver with one servo for each controlling channel. The number of channels used depends on the model itself, for example, a two-channel sailplane will employ one channel for controlling the rudder and the other for the elevator. The receiver together with all the servos and the battery are installed on the model aircraft while the transmitter is carried by the pilot during flight. The pilot controls the aircraft by moving the sticks or flipping the switches in the transmitter.

Working Principle: Most modern RC equipment are based on proportional control where the amount of movement applied on the stick in the transmitter is proportional to the amount of servo's movement in the receiver. All RC equipment are operating on a principle as similar to broadcast radio. In broadcast radio the audio signal is modulated on a carrier signal. The frequency of the carrier signal is much higher than the frequency of the audio baseband signal so that it is easier for the modulated signal to leave the transmitter's antenna. There are two popular modulation schemes; frequency modulation (FM) and amplitude modulation (AM). Generally, FM has an advantage over AM in terms of noise immunity. There is a general misconception that AM and FM transmitters are mutually interference free. No! this is not the case. Provide that their carrier frequencies are the same, they will interfere each other. So please check the carrier frequencies of other RC pilots nearby before switching on your transmitter! Most RC systems allow you to change the carrier frequency by replacing the x'tals on both the transmitter and receiver. You can buy an extra pair of x'tals in case someone is using the same carrier frequency as yours in the flying site. Note that  this matched pair of x'tals consists of a x'tal for transmitter (typically marked as Tx) and a x'tal for receiver (marked as Rx). The system does not work if you insert a Rx x'tal in the transmitter or vice versa if you insert a Tx x'tal in the receiver. Also note that, some receivers utilize dual-conversion technology, the Rx x'tal used in a dual-conversion receiver is different with the Rx x'tal used in a single-conversion receiver even their carrier frequency markings are the same.

In radio control systems the baseband signal is a collection of all control signals each represents a movement of the stick on the transmitter panel. Basically, there are two popular schemes to represent and to combine those control signals; they are pulse position modulation (PPM) and pulse code modulation (PCM). Generally, PCM is only available for FM transmitter while PPM is available for both AM and FM transmitters. In PPM scheme, the baseband signal is constructed as a sequence of frames. There are about 50 frames per second (so it is no use if you move faster than 20ms!). In each frame, there are a number of pulses each representing one control channel. The pulse width varies from 1 ms to 2 ms according to the stick position. The pulse width is 1.5 ms if the control stick is in its neutral position. Therefore, RC equipment can maximally have 8 channels (note that some PCM systems may have 9 channels due to different coding structure). In PCM scheme, the range of stick movement is divided into a large number of small quantized steps. Typically, there are 1024 steps and hence each step can be represented by a 10-bits code. Totally, all channels of 10-bits codes together with an error check sum are inserted into the 20 ms frame for transmission. Since PCM scheme has error detection capability built-in, it has inherent advantage over PPM scheme when there are transmission errors either being interfered by other RC transmitters or due to lost of transmitter power. Typically, the PCM receiver will set all servos to default or preset positions when transmission errors are detected. Again, there is also a general misconception that PPM and PCM systems are mutually interference free. No! this is not the case either. The cause of interference is entirely dependent on the carrier frequencies used; check your x'tal and make sure that there are no more than one transmitter working on the same carrier frequency!

You may change the carrier frequency by replacing a pair of x'tals both in the transmitter and receiver

Transmitter: Most RC transmitters have two sticks, one on the left and one on the right. The stick longitudinal and latitudinal movements are related to the controlling channels as shown in the photo below. With respect to aircraft control, there are two popular modes of operation that relate the transmitter stick movements with the aircraft controlling surfaces. They are:

RC transmitter and its channel/stick relationship

Receiver: The size of the receiver must be small  in order to fit into the model. The size mainly depends on the number of channels available. Also, PCM receivers are typically bigger than FM or AM receivers. All receivers should have a long cable feed antenna. Mind you that when you install the receiver on your plane, you have to fully extend the antenna feed cable in order to achieve maximum transmission range. A receiver has a number of sockets for the servos to plug-in. There is also a socket for the battery power input. We normally use NiCad rechargeable batteries for long life and re-usability.

Stuff that is put on the plane

Selection of RC Equipment: There are a number of R/C equipment manufacturers. The big names are Futaba, JR, Hitec. There are a number of important factors influencing your choice of RC equipment. They are:

Cost: Of course, this should be as low as possible, typically, the cost of RC equipment depends on, the number of available channels, AM or FM transmitter, PCM or PPM scheme and the functionality of the system. The cheapest systems are those 2-channel AM systems while the most expensive systems are those 9-channel PCM systems with programmability.

Number of Channels: It very much depends on the models you are going to control. Most gliders require 2 channels, more advanced gliders may require 6 channels. Generally, a 4-channels system, such as Futaba's Skysport 4, is sufficient for most purposes. However, if you are flying delta wing gliders, you need some kinds of mixing mechanism, then you might need a more advanced RC system with electronic mixing in the transmitter, or you might add a mixer (either electronic or mechanical) on board the aircraft.

Modulation Schemes: AM systems typically operate at 27MHz frequency band and this band is widely used by RC cars and RC boats. The cost of AM systems is low but AM receiver is most susceptible to noise interference. The FM systems you can buy in Hong Kong are those that operate in 72MHz, 40/41MHz, 35/36MHz bands. So far 40MHz band is the most populous band in Hong Kong. Some expensive transmitters were designed to be flexible by using plug-in RF module, for example, Futaba FF7. By changing the RF module, you can  switch to use another frequency band, say, from 40MHz band to 72MHz band. Of course, the frequency of Tx x'tal on the RF module must match to the frequency of Rx x'tal in the new band in order for the system to function correctly. It is worth while pointing out that, in order to operate RC equipment in Hong Kong, you need to use the legal frequencies assigned by the Office of Telecommunications Authority (OFTA). The 27MHz band, from 26.960MHz to 27.280MHz is opened to public use without the need of a license,  however, it is for CB and other public wireless applications, so it is generally not suitable for remote control of model aircraft.  From April 2005 onward, OFTA has released a total of 18 new channels for r/c models; 6 channels in the 72MHz band, 8 channels in the 35MHz band and 4 channels are in the 40MHz band. There is no need to apply for a license to operate on these frequencies. For details, please see the information in here.

Prior to the First Flight

The second skill you need to learn is how to make a turn without loosing attitude. On a two-channels model with elevator and ruder controls, a turn is manipulated by ruder movement. On an aileron model, a turn is manipulated by aileron movements simultaneously on both the left and right wings. The behavior of a turn performed using ruder or ailerons are basically similar. There are two issues you need to concern about the turn. First, when you perform a turn by moving the right-hand stick on the transmitter, the aircraft will bank towards the desired direction and will loose attitude on its way (this is because the lift its wing generates is less as it banks), you need to apply a corrective movement of up elevator to bring the aircraft back to the same attitude. That means, if you using Mode I for operating your r/c model, you need to coordinate with your right hand (aileron or ruder) and left hand (elevator) for the level turn. It is a bit difficult in the beginning but this coordination will become automatic once you've acquired it. The second issue you need to concern about is that, after you have already performed the turn and returned the control stick to its neutral position, the plane will still continue on its course of turn and never get back to a straight path (unless your model have a large dihedral wing). This behavior is different to the way that we've learn to turn a car, we all know that the driving wheel of a car will automatically be adjusted to its neutral position after the turn and the car will go straight again. But this behavior is simply not available on aircraft. You need to apply a reverse movement of stick in order to bring the aircraft to go straight after the turn, note that just simply return the stick to its neutral is not enough!

Upwind Landing Approach

If there is no flat land suitable for landing at the mountain top, most likely you will have to land your plane using a downwind approach. For downwind landing, you need to initially make a dive away from the slope to loose attitude. You should gradually turn your plane on its way during the dive. Once your plane is heading towards the slope again, then give a bit up elevator to pull the plane up so that it climbs along the slope contour. The plane will loose its energy and slow down on its way up. Perfectly, your plane should be slowed down significantly to the speed just higher than the stall speed. Finally, you should make a tight turn to let the plane heading towards the wind again just before touchdown. That will save you from the risk of a flip-over and damaging your plane's wings. Mind you that as the speed of the plane for downwind approach is really high, it is rather difficult to land your plane on the right landing spot; too low in the approaching speed, you might end up landing your plane far down to the slope and make the recovery more difficult; too high in the approaching speed, you might overshoot the landing spot hitting people or other obstacles nearby. The approaching speed chosen is very much depending on the topography of the slope and the current wind speed. So be patient! You should dive a few times to work out the best path and speed for landing.   Please see a movie clip for downwind landing.

Downwind Landing Approach

Another possible landing approach is the crosswind approach. The use of this approach depends on the topography of the flying site, the wind speed and its direction. Typically, if the flying site has a long and straight ridge line and the wind is not too strong and also the wind direction is slightly tiled, then it is better to execute a crosswind landing because the ground speed is slower. The idea is to initially fly the sailplane downwind towards the slope, keep the plane at an attitude about the same as where you stand. Turn you plane gradually until it is pointing towards you and is also parallel and close to the ridge line. Then, slowly bring the plane to a touchdown on the ridge just in front of you. Please see a movie clip for crosswind landing.