24-Jul-21 - 08:04 PM

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.

RCGA Competition Report

These report details courtesy of Rod Watkins of the RCGA.The RCGA or Radio Control Glider Association is the Victorian based special interest group representing the interests of competition glider pilots to the respective state and national aeromodelling bodies. Part of the RCGA’s role is to organise a state based thermal soaring league, which we have done for the past seven years. The league consists of ten events held throughout the state of which the best six scores are counted towards a competitor’s final yearly score. The league also includes the Vic State Champs and scores from the LSF Tournament at Jerilderie.

One such event in this year’s league was held in the Melbourne region on the 22 Feb. A total of 16 competitors arrived for the event to cool conditions for Feb with a moderate breeze blowing from the south-west and a forecast indicating strengthening winds as the afternoon sea breeze kicked in. At the pilots briefing, the CD announced his intentions to fly seven (or more if possible) rounds with no drop scores. The inclusion of a drop score has been a source of endless debate over many years but it seems to me that the best pilots always come out on top regardless of including a dropper or not. I just wish we could decide once an for all to have dropped rounds or not in our competitions.

This event was also fortunate to include the attendance of Carl Strautins from Sydney who, along with Gerry Carter and David Hobby, are all members of the current Australian F3J world championships team. Carl’s presence enabled the team to fly and work together in a competitive environment and gain some valuable team practice prior to their journey to Canada in August.

The conditions throughout the day where generally good with weak but regular thermals in the morning growing to strong by mid afternoon. As most pilots found lift and flew the required time, the heat results were largely decided by landing accuracy (sounds familiar). However, even a 1m landing was no guarantee for a heat win as several “perfect” scores where achieved throughout the day. By late afternoon with the sea breeze in full swing the conditions became tricky with several pilots experiencing line breaks and then missing the thermal that moved very quickly down wind.

Overall a very successful and competitive day with David Hobby, Max Kroger and Gerry Carter filling the first three places after seven rounds.

Position Name Round Round Round Round Round Round Round Total 1 2 3 4 5 6 7 Score 1 David Hobby 997 1,000 1,000 1,000 1,000 1,000 995 6,992 2 Max Kroger 1,000 1,000 1,000 989 985 989 998 6,961 3 Gerry Carter 984 1,000 984 994 984 1,000 1,000 6,946 4 Jim Houdalakis 991 948 988 992 975 995 997 6,886 5 Theo Arvantakis 998 984 1,000 1,000 1,000 872 1,000 6,854 6 Carl Strautins 1,000 1,000 997 965 997 994 857 6,810 7 Rod Watkins 997 1,000 997 1,000 1,000 1,000 762 6,756 8 Marcus Stent 992 896 997 828 992 1,000 1,000 6,705 9 Steve Keep 595 989 1,000 998 1,000 994 985 6,561 10 Gregg Voak 905 998 991 994 615 985 722 6,210 11 Daniel Haskell 544 990 994 991 989 981 518 6,007 12 Tom Dupuche 1,000 893 992 316 1,000 778 994 5,973 13 Norm Ripp 940 923 684 383 971 987 1,000 5,888 14 Rob Benton 1,000 411 972 499 934 985 992 5,793 15 Graham Norman 951 997 982 524 984 696 648 5,782 16 David Pratley 497 842 819 903 975 736 992 5,764

Position Name Total Model Score 1 David Hobby 6,992 Pike Superior X 2 Max Kroger 6,961 Tangent/Tragi 3 Gerry Carter 6,946 Estrella 4 Jim Houdalakis 6,886 Mongoose 5 Theo Arvantakis 6,854 Cobra 6 Carl Strautins 6,810 Pike Superior X 7 Rod Watkins 6,756 Ellipse 4/Estrella 8 Marcus Stent 6,705 Mongoose 9 Steve Keep 6,561 Mongoose 10 Gregg Voak 6,210 Elita 11 Daniel Haskell 6,007 Escape/Tragi 12 Tom Dupuche 5,973 Escape/Tragi 13 Norm Ripp 5,888 Starlite 14 Rob Benton 5,793 Tangent 15 Graham Norman 5,782 Pike Superior X 16 David Pratley 5,764 Tangent

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