Help Files



Generation of Slope Lift

Slope soaring has been used by birds for aeons and was used by glider pilots for just over a century. Anytime there is an obstruction to the wind, such as a hill, dune, or building, the wind has to get around it somehow. Immediately in front of the obstacle, the wind is forced upwards to get over the obstacle resulting in rising airstreams capable of sustaining flights. The slope lift most commonly explored by RC slope soarers is those generated in front of mountain slope; see the picture below. The wind coming from the ocean or a plain hits the slope and is forced upwards. Depending on the prevailing wind and the topography of the slope, there may be lift even hundred meters above the mountain top. Safe slope soarers will fly their planes in front of the slope in the area where lift is strong and constant. The situation on the other side of the mountain is totally different. After the high pressure air have passed the mountain top, they are slowed down and become stagnate in the back side of the hill. Here, dangerous rotors and turbulence are generated and may crush your planes if you dare to fly over this area. Some advanced soarers may take advantage of wind speed difference between the compression side and the decompression side of the slope to do so called dynamic soaring, then they need to fly over the decompression side of the slope in high speed under a controllable manner.

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Slope soaring bird

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Generation of slope lift


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

Sailplane models: Depending on the wing and the tail structures, there are many possible forms of sailplane models. Generally, they can be classified into three major categories: conventional model with tail, tailless model and canard model. Models with tail can also be further divided according to the types of the wing sweep and the tail shapes. Tailless models are also called delta wing models because their wing shapes resemble a Greek delta. For the canard models, the elevator is ahead of the wing in the front side of the plane.

Conventional models with tail: The wings of these models can be swept forward or backward as shown. Some models also have winglets attached to their wing tips.

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Backward Sweep Wing Model

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Non-Sweep Model

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Forward Sweep Wing Model

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Model with winglets attached to it wing tips

Conventional models with tail can also be classified according to the shapes of their tails. See the pictures below. V-tail models are very popular for slope racing.

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Low Tail

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Mid Tail

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T Tail

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V Tail

Delta wing (tailless) models:

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Delta Wing with Fuselage

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Delta Wing without Fuselage

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Full Sweep Delta Model

Canard models:

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Sailplane Manufacture and Material Used: 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!

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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:


Mode I

Mode II

Channel 1



Channel 2



Channel 3



Channel 4



The major difference of flying using these two modes is that you can control a 2-channel sailplane with only one stick if you use Mode II while you need two sticks in Mode I.  In Hong Kong, most RC pilots fly their aircraft using Mode I. Some advanced programmable transmitters allow you to set and remember the parameters for a particular model, for examples, aileron/elevator mixing or flap/elevator mixing. Also there are factory preset parameters for models such as helicopter, aircraft and/or glider.

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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.

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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.

Coding Schemes: PPM is sufficient for most RC flying activities. Many expensive transmitters have dual PPM and PCM mode in which you can select either PPM or PCM coding at will. Many people say that PCM system is safer because it has error locking capability. Well, it depends, sometimes it is better not to lock the aircraft in case of transmission errors because if the plane is locked and continues to fly on its current course, it may finally crash in a remote area, for example, the sea, then you will have a total loss. Nevertheless, if your plane crash nearby in case of transmission errors, at least, you may be able to recover something! Meanwhile, in case of transmission errors, you will immediately notice the problem for PPM system, while for PCM system, the receiver will hide the errors until it is too late for you to react.


Prior to the First Flight

New comers to RC flying usually find it difficult to start because the time spent on repairing is much more than the time actually getting the planes flying. Quite often, after a few unsuccessfully tries and repairs, they may get frustrate and then start loosing interest. Ultimately, they may never come back to RC flying again! Well, learning to fly is just like learning to walk in the early childhood. You must be patient and always ready to accept failure. Before introducing the basic techniques, we would like to warn the newcomers that a sailplane is not a toy, it is a fast flying machine and is potentially danger if it is not handled properly. Over the years, we have been seeing innocent people got hit by fast moving sailplanes, some accidents were quite serious and the people got hurt had to be sent to hospital for operations. It is recommended that you should choose a flying site that has relatively few onlookers or vehicles nearby to start with. As an example, the Clearwater Bay Peak is one of the suitable sites in Hong Kong where you can find experience pilots nearby to help you and it is also far away from crowd of spectators. Prior to your first flight, you should ask an experience pilot to inspect your plane for the CG balance and proper settings of flight control surfaces. In this aspect, many experience pilots in the field are willing to help, don't be shied! Moreover, it is better to ask an experience pilot to give you a quick introduction to R/C soaring, the behavior of the plane that you will encounter during flight, and the responses needed to be applied. If you have an experience pilot to help you, it would be better to hook your transmitter to his transmitter via a trainer cord as shown in the picture below. During flight the power of the trainer's transmitter must be switched on while the learner's transmitter is switched off. There is a push button switch in the trainer's transmitter for handing out the control to the learner. When this switch is pressed, all the control inputs are from the learner's transmitter. The learner can fly the plane using the learner's transmitter as usual. But in case of emergency, the trainer can release the push button in his transmitter, then the control will be passed pack to the trainer so that he can act accordingly to pull the plane out of danger.

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Two transmitters can be connected by a cord; one for the trainer and one for the learner


Learn to fly

Wind conditions and the flying area: Sailplanes are powerless gliders. Their ability to float on air is ultimately determined by the lift generated as the prevailing wind hits the slope. On windy days with wind speed greater than 40 km per hour, almost all kinds of sailplanes are flyable. But on a clam day with wind speed less than 5 km per hour, only those lighter sailplanes with streamline design (less drag) can fly. Don't throw your plane out if the wind just isn't there! Meanwhile you must be cautious to note the direction of wind. If the wind is coming normal to the slope, the lift is maximum, otherwise the lift is proportionally reduced.  The best area for slope soaring is along the ridge on the compression side of the slope. On windy days when the lift is strong, you can fly your plane far always from the slope where you will have more airspace to practice your skill and also in case of bad maneuver you will have more time to react. However, when lift is weak, you will need to fly closer to the slope. If you consider your flying skill is not yet good enough to fly close to the slope then, ground your plane until the wind prevails and the lift is stable; that will save you lots of repair time and money.
Launching the sailplane: For launching the sailplane on a slope, point  the plane slightly downward and release it gently towards the heading wind (see a photo on the left for a good launching) The plane may loose altitude initially but as the speed gradually picks up you may apply a bit up elevator to level the plane. Don't point the plane on an upward angle for launching! Since the wind may blow the plane to a stall or a flip over and render the plane to an uncontrollable state (see a photo on the right for a bad launching). Please see a movie for a good launching.

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Point the plane slightly downward for launching

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Don't point the plane upward for launching!

Basic skill and controlsMost first time R/C slope soarers would have the problems with the sensitivity of the control sticks. Unlike the control sticks in arcade game machines, to control a model sailplane, you really need to be gentle to apply force to the sticks, since even a small stick movement in the transmitter may result in a drastic reaction of the aircraft on flight. There are some basic knowledge you need to know prior to the first flight. First, your plane cannot float if it doesn't move; unless the wind is really strong and the wind speed alone is high enough to generate sufficient lift for the craft. So the first skill you need to master in soaring is the level flight. In level flight, you try to keep the plane to fly in a straight course with constant attitude. The basic control to keep the plane in level flight is the elevator. You need to monitor and adjust the speed of your craft, if the speed is too low, the lift generated is insufficient to float the plane and it will gradually loose attitude. Note that it is no use to apply up elevator to pull the plane up if the speed is too low because the plane will be further slowed down until it comes to a complete stall. If the stall is happen close to the ground and the plane does not have sufficient time to recover from a stall, then sorry go home and repair it! In a straight course with enough speed, you should not apply too much of  up elevator because the plane will shoot up as a result of applying up elevator and it will loose speed on its way and finally may come to a stall rendering the plane uncontrollable. Therefore, you only need to do some minor elevator movements in order to keep the plane in level flight under a steady cursing speed.

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!


Best  flying pattern for practicing: You should fly your plane in the compression side of the slope unless you are really good and have mastered the art of dynamic soaring. If the lift is plenty, fly your plane away from the slope where you will have more airspace, that will reduce the chance of collision with other fast moving sailplanes near the slope. The best and safer flying pattern for learning basic flying skill is a figure-8 pattern as shown below. First, you level your plane in a course parallel to the slope face, then at the edge of the flying compound, turn your plane away from the slope and head towards the prevailing wind. The advantage of making the turn towards the heading wind is that the ground speed of the plane is lower than the speed you'll have if you turn your plane downwind towards the slope. Remember the ground speed is equal to the craft air speed minus the wind speed in case of upwind turn. After the turn, you should level your plane and fly towards the other side of the compound again in a course parallel to the slope face. At the edge, also make your turn towards the heading wind and repeat the flying pattern again. By practicing to fly in this pattern you will be able to learn how to make turns and how to keep the plane in level flight under a safer condition.

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The best flying pattern (figure 8) for beginners

Landing: To land a sailplane is much more difficult than to launch a sailplane. The choice of  landing approaches is very much depending on the topography of the flying site. A perfect landing area is the one that is on the mountain top with a flat grassland and no obstruction nearby. At there, an upwind landing approach can be executed by flying your sailplane firstly on a downwind path toward the slope and then turn your plane slowly on its way passing the crest of the mountain top while at the same time gradually decent your plane to an altitude suitable for touchdown. You should continue to turn until your plane is leveled and pointing toward the prevailing wind just before touchdown. As the aircraft's speed is the lowest for upwind landing, this will give you more time to react in case of poor landing approach. Mind you that there are always turbulence near the ground and this may give you problems especially when your plane's speed is slow, you should anticipate that! See a movie clip for upwind landing.

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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.

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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.

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Crosswind Landing Approach


Advanced flying skill and dynamic soaring

Under construction. Please see an article on dynamic soaring

construction.gif (2098 bytes)    This page is constantly under development. Last modified on 02 Feb 2006.


Copyright 2000, Stanley Chan
All Rights Reserved