30-Jun-22 - 10:30 AM

Thermal Circle Text - Thermal Circle 01

Written by Hayden Daley - 03 December, 2007.
CAWelcome to RCMN new soaring column (title of column). In this edition, as a follow up on Chris Carpenters excellent report on the Lameroo Aerotow event we will have detailed look at aerotowing as well as a competition report from the recent RCGA competition.

To many R/C pilots the idea of towing there sailplane behind a power model causes concern, they believe it is difficult and risky for their model. The facts are that aerotowing is an easy, safe way of getting your sailplane to height. Any glider with ailerons can be towed, unfortunately rudder/ elevator gliders tend to dutch-roll and yaw and are not really suitable to Aero tow. The advantages of this method of launching your sailplane is that you choose the height and where you release, the only restriction being visibility and legal height limitations.

Any reasonably powerful engine powered model aircraft can be fitted with a release and used as a tow plane. Some of the more succesful combinations I have seen are, 1/4 scale Cub fitted with an os 160 two stroke, an ultra stick powered by a zenoah 45 and the extremely powerful HOTS style models used in Adeilade fitted with Zenoah 80 twin. Although one of the models listed is a methanol burner, the os 160,but due to the price of glow fuel the majority of tow-planes are powered by petrol motors. Combined with these powerful motors are large diameter low pitch props which gives the tug a "low gear" that allows for steep climbs to height at a speed that is not to quick for the sailplane. The tow plane must be reliable, easy to service and be able to tow for a long time before needing refuelling and charging. Because of these requirements tow planes use big battery packs and battery backing systems that use two packs as well as larger than usual fuel tanks. Powerful servos are required, preferably metal geared for the rapid descent from height after a succesful tow. The more powerful servos draw larger current so the need to charge at the field is also a neccesity and vital for safety.

A second tow release is always fitted to the tug in case of the glider not being able release at height. The ideal place for the release being on the center of gravity, although just behind the trailing edge is fine. Model tow planes do not tow from the rear as in the full size because this positon cause problems if the glider is not perfectly in position. The tow release on the center of gravity is more forgiving for model use. Large diameter wheels and strong undercarriages are needed as they are more resilient in the large amount of takeoffs and landings that occur while towing.

The tow-plane pilot must be able to maintain model speed in relation to the sailplane and have good visibility to see the models at the higher than normal heights that are reached. The tow-plane pilot also must have good landing skills and better than average takeoffs as most problems occur when the tow pilot initiates the takeoff. If the tow-pilot does not apply power quickly the result is slow glider take off speed on the ground without enough aileron effectiveness and the subsequent dragging of the wing tip. This causes the glider to slew to the side requiring a quick release from the line by the sailplane pilot. These are not big problems but must be taken into consideration when aero-towing.

The tow lines that I have seen in use are generally dial cord or builders string with a weak link on either end. The weak link is 10-15lbs breaking strain fishing tackle attached in loops at either end. The need for a breakable weak link is required in case of a dual release failure. If this condition occurs the tow plane pilot will be required to break the weaklink, the best way to do this is with a large barrel roll or snap roll and the subsequent jerk will break the weaklink. Longer tow lines are more forgiving for beginners and the usual length is between10 and 15 metres.

Aerotowing by its very nature requires a long take off area and a place to hook up sailplanes off the strip. The reason for this is the high number of take offs and landings that occur and the safety of being able to land straight ahead in case of problems during take off. The field also must have a reasonable surface, scale gliders with retracts have a bad habit of damaging gear doors on rough strips and they also punish retracts on takeoff. The area to hook up sailplanes is required to keep the strip clear for takeoff and landing. It is aerotowing etiquette to keep the takeoff and landing areas as clear as possible,and to hook up the glider to the towline off the strip. Gliders also have priority when landing.

The most common models that are aerotowed are scale gliders. I have seen a southern sailplanes Eclipse fitted with a wheel and this combination towed successfully. Scale sailplanes can be broken down into two categories, modern and vintage. The modern sailplanes are models of the high quality full size that were built from about 1970 onwards these include the open class Nimbus 4d, Discus and Duo Discus sailplanes, there are too many to mention. The Vintage sailplanes are modelled off the earlier wooden sailplanes and I have seen models of the famous German Minimoa, Graunau Baby and others. There are a lot of choices in the vintage category and most of them are wood and fabric making them more friendly for the scratch builder. There are also a number of vintage kits available, the Flair company from England does an excellent kit of the K8. The manufacturers of the modern scale model sailplane cater to pretty much to everyone with many choices to fit the budget of most aeromodellers.

Large modern scale gliders when aerotowed have the advantage of being able to be seen at high altitudes. These large models come into a element of there own after a high tow. Long flight times are often recorded by these models and there size and efficiency allow for long distances to be covered looking for thermals, exactly like there full size counterparts. These modern model sailplanes like the full scale aircraft sometimes have working water ballast fitted which can be dumped before landing, providing ballast in windy conditions and more scale realism to the model. Sometimes the construction of the full size is mirrored in the way the models are built with full moulded construction using the modern composites like carbon fibre and kevlar that are used in the construction of the full size. Wing attachments are sometimes recreated in these modern gliders meaning the have the same scale wing joiner system of the full size and use a similar interlocking pin system to attach the wings.These modern gliders are pretty much all fitted with working retracts that are used during takeoff and have high aspect ratio long wings. Many kits are available of modern sailplanes and if this style of model appeals to you there are many foam- fibreglass semi kits all the way through to expensive moulded models fitted with full scale cockpits and with the features listed above.

Vintage sailplanes while not having the efficiency and aerodynamics of the more modern sailplane, definitely have more character. The gull wings, skids and struts as well as different wing planforms of these older designs look magnificent in model form.These vintage aircraft sported great color schemes and recreated in miniature definitely stand out from the crowd. No one can call these models another "white glider" which is sometimes levelled at the more modern aircraft. There are a massive number of choices in the vintage category some dating way back to the early 1900s. These vintage aircraft have been modelled to many different scales the largest I have seen being a half-scale Minimoa built by a German modeller ,with construction photos and pictures of it flying displayed on the internet.

As mentioned earlier pretty much any sailplane with ailerons can be fitted with a release and towed. Until you become more confident in the method and ways of towing , practicing with a sport model may be the best way to go. Do not feel that the only models that are aerotowed are Scale gliders, this is not true. If you wish to experiment with towing I have seen a 46 trainer tow a combat foam wing, both were fitted with releases and were exciting to watch sometimes staying on tow and doing circuits as well as being released at high altitude and trying to beat the tow plane to the ground.

Hopefully I have made aerotowing gliders seem exciting and a good way to enjoy flying model aircraft. Practically, I will discuss what will occur during your flight from hook up to release at height and finally, concluding the flight with the landing.

Ok, you have waited in line and are ready to hook up to the tug. With the sailplane off the strip and the tug in front you need to hook up your glider. If this is the first flight it is advisable to test your tow release. This is done by pulling the line tight and activating the release, this should release straight away under load. If this does not occur don't fly, release problems are dangerous and you will be risking your model if you cannot release straight away. If the release was succesful hook up the glider and prepare for a clear strip, ie. no one taking off or landing. Call "on the strip" and the tow pilot will taxi out and you will have to carry your glider onto the field and line it up behind the tow-plane. The tow-pilot will take up the slack while walking back so both of you are behind the aircraft and can talk to each other during the tow. Hopefully a fellow pilot will now be holding your fin or wingtip which allows the wings to be levelled and the final slack in the line is taken up. When you both are ready the pilot will apply power and your job is to keep the wings level behind the tug during the take off run. Your glider will be the first aircraft of the ground, its your job to keep station behind the towplane until it to is off the ground and you establish the climb. You will have to keep the wings level during this initial climbout and to stay in the "high-tow" position. High tow means above the tug to the rear not below it. We are now climbing out in a straight line and we need to turn. A good tow pilot will do nice, level, broad turns and its your job to turn slightly outside his turns, this means dont cut inside his turns this causes tow line slackness and must be avoided and puts the model under undue stress as the slack line is jerked. Its at this point I would like to discuss one of the main issues of aerotowing, You can release at any time. If you Mucked up the takeoff or turned inside the tug or have any other problem flick the switch and release, you can always tow again and this is always better than pushing a bad situation, you do not have risk your model. After a couple of turns we are now reaching launch height and are ready for release. Now flick the release and when you confirm visually that the model has released you can tell the tow-plane piot you are off the line and he can descend. Do not tow to high on your first tow as visibility can be a problem, as Chris Carpenter mentioned in his Lameroo report, it can result in long searches and lost models. This were the fun starts, as you are way higher then any winch or high start can take you and you can look forward to a long flight on the way down! Upon reaching the height were you have decided its time to land you must call "glider landing" and this will give you priority in landing, all aircraft must give way to landing gliders at all times. Having completed the landing it time to clear the strip, rejoin the que and do it all over again!

The VARMS club in Melbourne holds regular Aero-tow days at the Briggs field in Glen Waverly on the last Saturday of the month. Scale Aero towing is organised by Scale Soaring Association of Australia and they hold regular weekends throughout the year.The details can be found at scalesoaringaustralia.com.

DG 600 Review Hayden Daley

The H-model, DG-600 Carbonfibre, is a replica scale sailplane of the full size DG-600 racing class sailplane, it is modelled in 1-3.5 scale, with a dual wingspan incorporating two types of plug in tips , 4.8 metres with the larger tips and 4.5 metres with the smaller tips. It is distributed in Australia by the Model Flight company, whose home is in Keswick, South Australia. The DG 600 is one of a number of H-Model scale products, whose line up of models also include a 5.3 metre Nimbus 4D, a smaller span 4.3 metre Nimbus 4D as well as others.. These models are built to exacting tolerances and are accurate, fully molded replicas of their full size counterparts. There are options available for these models which include scale canopy detail, water ballast and if you inquire when ordering, you may find that their are many other options available! The model being reviewed had the options of a scale canopy; and working water ballast.

When the model arrived I was far from disappointed, I was actually dramatically impressed. The model was well packed for transport and arrived in perfect condition from the supplier. The model had all markings and decals already attached and they were excellent quality, well cut , cad designed, scale decals. “Backup” decals were also supplied The markings on my model were Swiss with the red band and white cross on the tail making the model really stand out from the crowd. All hardware supplied was of excellent quality and the instructions although sparse gave enough detail to prevent any building problems. The wings were very impressive with a scale wing joiner, which included a pin to pass through the two joiner boxes, exactly as in the full sized aircraft. The wings are full carbon fibre; and with the model utilizing the smaller tips can be flown very fast and is an excellent slope model; and when full of water has excellent energy retention. With the larger tips the model is a supurb aerotow – thermal model. The key to the two tip system is that while also being scale, makes the model an excellent all rounder, equally at home on the slope as well as aerotowing. The model that I built used 12 channels, however 3 can be omitted if not using the water ballast system and an aerotow release.

Construction Building this model does not require a great amount of work, so I will give a general overview of the models construction. I began with the fuselage and cut the appropriate number of plywood trays for radio gear, battery and filter leads. These trays were then epoxied into the fuselage. I used a combination of rubber bands and cable ties to attach the components. The aerotow servo was also attached to a tray and I chose to put the servo in front of the battery as I felt this gave the best acctuation and would help with the fit of the cockpit frame. A z-bend was used on the attachment to the aerotow release servo arm.. The receiver used was a JR/Graupner 12 channel FM unit. The switch used was JR gold switch, these switches provide reliable operation and a charge lead. To operate the switch I used a Dubro push-pull switch mount, and when attached to canopy frame looks like part of an aircraft control, difficult to pick as a switch. These dubro mounts make it difficult for foreign objects to turn off the switch in flight.

I have always used JR filter leads in my 12 channel scale models, as the JR filter lead also has a built in amplifier and excellent rejection of spurios signals. These amplifiers are a great help with the long leads and carbonfibre construction that are used in this model. The battery used is a 5 cell 3300mah nickel metal hydride pack which provides for a safe operation time of approximately 1 hour and 45 minutes. Some people may see this as sounding short, however with digital servos and 5 cells this is a safe assumption. The retract and rudder servo were mounted on the retract, and I used 90 degree aluminium that is available from most metal suppliers, with servo bays cut on a metal bandsaw. This 90 degree aluminium was then attached to either side of the retract with three small screws for each aluminium unit and cyano. The cyano adhesive is a backup, in case the screws shear under load which is doubtful; as the rudder and retract servo do not carry a great load. JR DS 811 Servos were then bolted in place for each control. The retract control used a short piece of wire. The retract and mounting system on this model are strong and have caused no problems over time, however keep in mind that if the retract mount fails you will lose rudder control when using this system. In regards to the rudder; I used a Dubro pull-pull kit commonly found in most hobby stores, attached to one piece printed circuit board horn epoxied into the molded rudder. A small amont of qcells mixed with the epoxy helps to hide the join as the horn passes through each side of the white rudder.

The elevator servo is in the tail of the aircraft and requires a long lead to reach the servo. The elevator is controlled by a piece of wire that runs up the rudder to the elevator; and is acutated by sliding the wire into a small piece of brass epoxied into the elevator itself. Some thought too mechanics must be used during this part of construction and must again recommend using filter leads on any control with a long lead. The elevator servo being mounted in the tail of the aircraft is supurb; with very little slop and accurate acctuation of the control surface. A fairing is incorporated into the lay up of the fuselage for the elevator servo, making accurate installation easy.

The wings have the following controls, spoilers, flap, aileron and water ballast taking the wing’s channel count to eight. Standard servos will fit into the wings, I used DS811 servos on all surfaces making the servo purchase cost comparatively low. The ailerons and flaps require printed circuit board horns installed with epoxy and these horns are supplied with the model. Servo hatch covers are all supplied and are of excellent quality. I attached the servo covers to the wings with small hex head screws, making them easily removed in case of any dramas at the field. Water ballast control was easily acquired with only a short run of wire to the water dumping valves.The ability to have or make long length leads is required in the construction of this model. Please contact Model Flight for these leads.

The model was completed by doing the center of gravity. The CG location was provided in the construction drawings and a small semi circle was cut from cardboard providing an accurate template. 2 millimetre plywood was cut and epoxied in place and the appropriate amount of lead shot was mixed with slow cure epoxy, poured into the front of the fuselage and left to dry. This completed construction.

Radio Setup

With twelve channels; and flight modes being required this really pushed the MC24 to its limits. I ran 2 flight phases on the throttle, and also mixed the flaps to the spoilers. This mix gives very little pitch movement when the controls are operated. I used full span aileron control, with 70 percent differential. The second phase actuates at one-third throttle control giving reverse differential on the flaps to still provide full span aileron while landing. The flaps were set to give 30 degrees downward movement at full acctuation. Rudder to Aileron mix was set at 40 percent and this worked well with no adverse yaw whilst in flight.

The water ballast controls were mixed together and are activated by switch with a servo slow function, this dumps the water in a more scale like fashion. The elevator was set up to give 40 degree movement up and down. These models have a small, scale elevator so more movement is better on this control. Rudder was set at maximum available movement, about 45 degrees, and a bigger horn was substituted on the servo to provide this movement. Retract, aerotow release and all other controls were set up as per usual. I then checked the batteries by cycling them 5 times and with no other problems this completed the radio programming, and the model was prepared for flight.

Flying

The model was flown by aerotow for the first fights and used the long length tips on these fights. This model has excellent control authority, and is easy to fly. Because of the broad root chord it is also easy to see at the higher than normal heights achieved by aerotow. The CG location was moved rearward 5 millimetres after approximately 8 flights and provided better elevator control. This is one of the best flying scale models I have ever flown. It is accurate, covers a lot of ground when hunting thermals and brings a smile with every flight. It is easy to land, and the flaps and spoilers really slow the model when landing. On the slope with the small tips and water ballast this model is magnificent. The wings on this model are strong, being carbon fibre and the model will easily achieve 150kph plus speeds without a problem. Aerobatics are fine with the small tips but I do not recommend aerobatics with the long length wing tips (pushing fate I guess!). An excellent, strong, supurb flying model.

Conclusion

Overall this model acheives the goal of being an all round scale model. If you have the chance to purchase one of these models they come highly recomended. All the H-Model products are of exceptionally quality. They are well made, strong molded models of good quality and excellent flying charecteristics, that only modern scale models of modern aircraft can provide. The DG600’s carbon fibre wings are strong and stiff and hard to bend even in extreme manouveres with the small tips. This is one of the favourite models in my collection. Overall , and in conclusion , it is an excellent scale reproduction of the racing class DG600 Carbonfibre.

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