Henryk and I started our aero towing activities approximately two years ago when our long standing club tug pilot departed to pursue other modeling activities. Our current tug was developed as a result of experience and learning’s gained from first model which was based on a “Giant Stick”. Whilst the “Stick” performed admirably on a stock standard Zenoah G62, and provided an ideal platform for learning the art of aero towing, it was a little over weight at 12kg so a key aim for the new tug was a target weight of 8-9 kilos. We were also spurred into action following the demise of the Stick due to an unexplained power failure even though we were using dual batteries via a commercial battery backup unit!

With over 18 months of hectic aero tow events and hundreds of tows under our belts there were some additional learning’s that also feature in the new tug. We have towed up models from 2m semi scale ships up to third scale 12-plus kg molded beauties so we felt reasonably happy sticking with the Zenoah G62 before venturing into a bigger project (which is currently in the planning stage – more on this in a later article).

We chose to base our new tug on a development of the Hots airframe because it is a relatively straight forward build and there are a number of various sized Hots doing sterling tug duty in Australia. The standard Hots is a mid wing sports model with a stylish raised turtle deck and simple flat plate tail surfaces. The wings are rectangular planform and the undercarriage is a tail dragger setup – all good features for a tow plane. The final product was modified as described below and met all our expectations for a versatile workhorse tow plane.  It is now in regular use at our monthly VARMS club aero tows in addition to various events held at our sister clubs throughout Victoria.

The following issues were considerations in development of the new tug:

Why not a scale tug? – The aim of the project was to provide a workhorse tug that could cycle sailplanes into the air in the shortest possible time and keep up the action all day. There are also the questions of quick preparation at the field and ease of maintenance during the day. A scale tug by its very nature is likely to sacrifice some of these attributes for the sake of scale appearance thus the development of a purpose built machine.

Coming in at just over 9kg AUW our modified Hots has proved to be a very stable towing platform.  We have successfully towed gliders up to 14kg AUW whilst maintaining a safe performance margin although towing these weights to altitude takes considerably longer than the typical 5-6kg ¼ scale gliders commonly found in Australian clubs. The low wing loading and the large control surfaces have given the model the ability to fly slowly with the smaller models, but also the ability to keep the speed up to the more slippery ships.

The mid-wing arrangement of the Hots design provides a great compromise between good ground clearance for the easily damaged wooden props and safe ground handling in cross winds.  It does not suffer the typical “wingover” problems experienced by high wing tugs in strong cross winds.

We modified the standard Hots wing saddle and turtle deck to allow a large hatch to be installed from the firewall through to the bulkhead at the wing trailing edge. This hatch provides for easy access to verify fuel and battery status. Careful management of the fuel and battery states, facilitated by the easy access, has all but eliminated fuel related “dead sticks”. The large hatch also makes setup and maintenance much easier on the field and in the workshop.

The wing mounting was changed from the original bolt down system to a plug on side mounted system using a carbon tube wing joiner. This allowed for easy access to the batteries and fuel tank which is positioned directly over the CofG.

The tail plane was made removable using carbon joiner tubes and retaining magnets which we found to be an extremely convenient arrangement on our previous tug.  It takes less than a minute to install and remove the tail plane which makes a tremendous difference during transportation. For additional convenience the elevator control rods are connected using quick release ball couplings. 

We selected the E195 airfoil for the wings and incorporated 50:50 flap and aileron surfaces. This combination has provided excellent control response at all speeds with the ability to adequately control the decent angle and landing over a wide range of headwinds. Experience with excessive aileron and flap vibration on our previous tug led to the use of top driven control horns with bottom hinged flaps and over centre locking of the flaps in the up position. We also cantered the motor at a 45 degree angle which also reduced vertical vibration of the wing and tailplane surfaces. The servos are mounted directly to the removable ply covers for easy access. Wing construction uses the common balsa over foam cores with the addition of 50mm uni-directional carbon reinforcing strip above and below the joiner tube sleeves.

The undercarriage assembly is bolted to the fuz with four 4mm unbreako cap screws and T-nuts. This allows easy removal which provides additional flexibility with transport. The lighterweight undercarridges all seem to suffer similar problems with not being able to withstand the high number of rotations that a glider tug endures. The location of a stress spring and transfer arms limiting the energy transferred into the landing gear has provided us with longer livfe for the undercarridge and with the added benefit of taking out some of the spring from those not-so-neat landings.

To ensure positive ground handling whilst protecting the drive servo the tail wheel is mounted on a carbon trailing arm and is driven by a spring isolated pull-pull system off the rudder servo. Oversize (6”) wheels are used to ensure good ground handling on rough fields and minimum resistance during the critical take off run. We elected NOT to use pneumatic tyres as some of our Aussie strips are covered in very aggressive “bindies” (spiked seed pods) during the summer months!

We kept the tow point close to the trailing edge to minimize pitching loads. It is a little higher that we would have preferred, but we did not want to have to resort to a bridle system due to the shape of the fuz. The release design is very simple and uses a 2.5mm music wire retaining rod that passes through an 8mm hole in the aluminum upright.  We used a 6kg metal gear  servo that has never failed to release if a glider got into trouble. 

One of the things that often poses a problem when towing sailplanes to great heights is visibility. We decided to continue our previous practice of alternate wing colors for orientation and high visibility colour for contrast. The fuz is bright orange and the wings are grey (left) and white (right) both having contrasting tips. This scheme works well in both low light and against bright blue skies.

The last flight of our old tug did do some damage to the motor which required a new crank shaft and cylinder barrel. The great thing about the Zenoah G62 is that  parts are readily available and easy to install. The rebuilt motor looked and performed like new and was soon thundering away through the 23x8 Xoar wooden prop. 

Whilst there is nothing wrong with the standard Zenoah magneto system we decided to install electronic ignition for the new tug. This provides a number of benefits including a reduction in all up motor weight (especially if you take off the spring start which is no longer necessary). The electronic ignition provides slightly smoother running in the air but most of all a much smoother and slower idle. The lower and more reliable idle speed are a big advantage given the amount of time a tug spends on the ground taxiing and waiting. The only real disadvantage with the electronic ignition is the management of an extra battery pack however we are operating all day on a single charge with a 2,200Mah 4 cell pack.

Large capacity single cylinder two strokes at not known for their ultra smooth operation which led to the following actions:   

After spending a fair bit of time (and money) testing various props on both tugs we found that although a carbon prop was stronger, it creates a situation when the forces from a prop strike are transferred to the motor rather than absorbed by the propeller breaking.  It is far more time & cost effective to replace a prop than a crank shaft. With that in mind, we have settled on Xoar wooden props which have a good balance between cost, performance and availability.

The downside of using wooden props is their vulnerability to tip damage from rough surfaces, grit and even grass.  It’s also likely that a number of light prop strikes may occur over the course of many landings in one day (we all mess up every now and then). Even relatively insignificant prop strikes can chip away at the tips and quickly unbalance a prop. We also found that the inevitable run-offs into long grass also play havoc with the tips and blade leading edges. Our solution to the problem was to laminate about 2 cm of the tips with light weight carbon cloth and epoxy resin. This is a relatively simple task using a layup of mylar, foam (to form to the prop shape), and ply clamped together using G claps. A little sanding and checking with a prop balancer and your tips are much more resistant to erosion with very little weight penalty. This technique has significantly increased the lifespan of the wooden props.

To safely operate a tow plane all day, the capacity and reliability of the power supply is a significant issue. Whilst we had used a basic dual battery system in our first tug, we were not entirely happy with its operation in conjunction with the 2.4Ghz radio equipment. To provide the maximum level of reliability we decided to install a sophisticated power distribution system in the new tug. Most of the commercially available power systems provide isolated feeds to each servos from dual batteries rather than drawing the servo power though the receiver power bus. After looking at the various options available we eventually settled on the Smart-Fly PowerExpander Sport Plus as it provides the features that we wanted for very reasonable price. The Sport Plus also provides dual filtered and regulated 5.0V power to receiver which is very important for 2.4Ghz installations in addition to RF filtering of all signals in and out of unit. Another excellent feature is the optical remote ignition cut-off which effectively isolates any ignition noise from the radio gear and has the added advantage of providing an additional failsafe ignition cut if the receiver power supply fails.

One of the great advantages of the Sport Plus unit is that is enables you to use twin Lipo batteries. The twin 2500Mah Lipo packs that we installed in the Hots provide ample capacity for a full days aerotowing.

The regulations covering large models (9- 25kg) in Australia requires the installation of an “on model” ignition kill switch. In addition to its safety functions this switch is also very convenient when priming the motor so we placed on the side of the fuz near the firewall.

Since the introduction of 2.4Ghz radio gear there have been many different opinions as to the reliability and advantages of the various competing systems however the ability to operate a tug without clashing with glider frequencies is a very great advantage. In our experience the JR RD921 9 channel receiver, in conjunction with the Smart-Fly PowerExpander unit and twin Lipo batteries, provides excellent reliability. At the time we put this article together the Hots has accumulated in excess of 60hrs of aero towing work and we have not encountered any observable radio related issues. 

Not withstanding this excellent operational record, we have not forgotten previously observed radio or power related failures therefore regular maintenance and management of batteries and other hardware is crucial to ensure that the on board systems can function as designed. No mount of expensive backup gear can replace good basic maintenance practice!

A final word of caution when using systems that provide a regulated receiver power feed: The regulated receiver power on some distribution systems may not be designed to power ANY servos. If you run out of free channels on the power distribution system you should consult the user manual or manufacturer before plugging servos directly into the receiver. When using JR/Spektrum gear it is also important to understand how the receiver binding system operates in conjunction with the power distribution system. If in doubt, or you experience some unexplained system behaviors, seek advice from the relevant manufacturers.

We would like to thank Mike O'Reilly and Model Fight for their support in both of these tug projects. We would also like to thank the support of Quest Engineering & Development for their assistance with the Smart-Fly PowerExpander Sport Plus.

More images of the build and other close ups are available in the gallery