CONTRA CLUB

Are two props better than one? John Stennard attempts to find out as he tests a new contra rotating propeller system for small models words & photos

This is nothing to do with contrary club members (who has those anyway?) but is to do with flying models using contrarotating electric motors. In general, if you fly a model with contra-rotating props you are likely to be an F3A pilot or indoor F3P ultra-light wizard. If so you will be using high end units designed specifically for a demanding style of flying, often in a competitive environment. For the average modeller contra-rotating motors (CR from now on) can be a complication too far and it offers few advantages over a conventional power system.

» John Stennard

So why use this motor system? As far as models are concerned it eliminates the torque action and for competitive aerobatic flying this is a valued function. For indoor models larger and slower revving props can be used, enabling models to be flown very slowly. Of course, some very iconic full-size aircraft have used CA power and there is an attraction in replicating them in a scale model with a functioning CR unit.

But, as far as I was concerned, I just liked the idea and I saw it as an opportunity to try something a bit different. There are several CR units of different sizes available commercially and I have tried a variety of them.

These all flew the models well whilst making a unique sound.

But before launching into these it’s worth a quick look at the historical full-size scene.

Cr In Full Size

To balance out the technical and engineering difficulties of a CR system it should give comparable power and/or efficiency as a conventional power system, plus give the benefits of reduced torque effect. Research led to two design variations for CR systems: one engine driving both props or use an engine for each prop.

The superb Martin Baker MB5, which first flew in 1944, used the one engine system. Capt. Eric Brown said, ‘In my opinion this is an outstanding aircraft.’ Power came from a single Rolls-Royce Griffon 83 liquid-cooled V12 engine driving three-blade props.

The Spitfire was developed into the Spiteful, with a laminar flow wing, and this was further developed into the Seafang. This was a naval version with the Griffon CR engine and it first flew in 1946. Eric Brown was less happy with this aircraft due to its stall characteristics.

By this time jet fighter development had taken centre stage so further development was abandoned. However, the contra-rotating system still seemed to offer advantages for certain aircraft designs, particularly naval types and this led to three iconic aircraft: the Wyvern, the Shackleton and the Gannet.

The Westland Wyvern was designed for CR and was powered by a 3,500 hp (2,610 kW) Eagle engine driving large and distinctive CR propellers. The first flight was in 1946 and 127 were built. The next aircraft in the contra line up was the Avro Shackleton, first flying in 1949 and 185 were built. Initially Merlin engines were used but these were replaced with the larger, more powerful Rolls-Royce Griffons with 13ft (4m) diameter CR propellers. For the Gannet, Fairey selected an engine based on the Armstrong Siddeley Mamba turboprop, the Double Mamba (or Twin Mamba). This utilised two Mambas mounted side-by-side, coupled through a common gearbox to coaxial CR propellers. The Gannet first flew in 1957 and 303 were built. The Shackleton had a long and successful service life and was only retired in 1991. However, maybe this was due more to there being no replacement aircraft being immediately available, until the Nimrod entered service.

Cr In Miniature

I know that several ingenious and very capable modellers have built and flown successful CR powered aircraft using their ‘own design’ power systems. I am not a member of this league of gentlemen, but I have always been interested in different approaches to powering model aircraft.

The first CR unit I purchased was quite small. It used 3-blade 5-inch GWS props and it appealed to my interest in small models. The design made the unit quite difficult to mount and it required two ESCs to function correctly. Initially I installed it in a small foam Sea Fury and I was pleased with the results. The model certainly flew no better that it would have done with a standard single motor, but it had an interesting alternative propulsion system and sound. I later modified this model to look like a bit like a Gannet – well, from a distance, anyway! It flew fine and looked surprisingly good in the air.

The second contra-rotating unit was much bigger and equally difficult to mount. The PJS unit used 2-blade 8 x 6-inch props. I fitted this in a semi-scale Tucano, and it required two 20A ESCs and two 3S flight packs. The model flew nicely but again it was no better with the contra-rotating props. It was just a bit alternative and sounded good. This added a different dimension to the model and its flight performance, and it always created interest.

CR REVIVAL

I had not actually planned a CR revival but when I saw the DZP30 unit my dormant interest was awakened. The DZP30 is a beautifully designed and made unit with two 1500kV motors driving conveniently sized GWS 3-blade 7035 props. It features an easy-to-use mounting system and I was delighted with the unit, which weighs around 35g. Using a 3S LiPo the quoted thrust is 240g per motor. Tests have shown that it will operate using a single 20A ESC providing the throttle is opened slowly. However, two separate ESCs do improve the handling and performance. Not wishing to build a scale/semi-scale model until I had tested the motor’s potential, I utilised a redundant model donated by a friend. It is larger than required but it gave me an idea of the performance of the motor unit before embarking on a more suitable design and build project. But it definitely won’t be a Shackleton!

Flight tests have shown that the motor functions really well in the 800mm (31.5”) wingspan X-PLANE test model, with an AUW of 433g (15.3oz) using a 3S 1000mAh LiPo. The unit ran very smoothly, and I could roll and loop the model with ease, so it is definitely a motor with potential.

Feeling really pleased with my first flight using a 3S 800mAh LiPo, I fitted a 3S 1000mAh for my next flight. This pack produced an even better performance and a more suitably sized model would, of course, enhance this further. I am still enjoying flying the X-PLANE so currently I have no plans to fit the DZP30 unit in another model.

New Gannet Unit

The small original small CR model and Gannet conversion date back to 2013 and eventually they became unfit for service. However, you can’t keep a good idea down and the small CR unit is available again as the DZP13 or YX-13DZ. It is the same basic unit but now has a really neat mounting and is available as a package, complete with two 7A ESCs and 3-blade GWS 5 x 3 props. Unable to resist, and with a new Gannet in mind, I ordered the unit via Aliexpress.com. It arrived very quickly and after fitting the motor mount (beware the microscopic screws!) and following a power connector swap, I was able to test it. Although the motor unit only weighs 13g, once connected to the ESCs and with the props fitted the AUW rises to around 34g. So, not exactly a heavyweight combo and the thrust is listed as 180g.

DZP30 motor unit gives plenty of power for steep climb outs

I soon had the motors running in the advised directions, the rear anti-clockwise and the front clockwise. Prop fitting is unusual as the rear prop must be a tight push fit on the shaft while the front prop uses a standard prop adaptor. The video of setting up the motors and props shows that the hub of the rear prop is carefully enlarged to 5mm so that it is a tight push fit on the shaft. In fact, the supplied props already had a hole slightly larger than 5mm. This means that the prop is not at all tight on the shaft. I was very reluctant to glue the prop on and was thinking along the lines of putting heat shrink tube on the shaft and making an enlarged hole in the prop when I remembered I had some props of this size. A quick check showed that these had a smaller hole and were a tighter fit on the rear shaft. The front prop uses a plastic insert to reduce the size of the hole to suit the front prop shaft. Different batches of these props must vary slightly, which would normally not be a problem. Tests showed that the CR unit ran well but I was unhappy with having so many connectors. So, I disposed of the Y-lead connector used to couple the ESCs to the receiver by soldering the ESC leads to a single Rx connector.

Twin XP-7A ESCs are supplied with the DZP30. However, it will operate using a single 20A ESC providing the throttle is opened slowly.

This motor is destined for a new purpose-built Gannet. This will have a 6mm foam / 3mm ply / 6mm foam sandwich fuselage and a 6mm foam wing of 500mm span. The tail parts will be from 3mm foam. There are quite a few challenges with the design, so it won’t be a fast build. Both the DZP30 and DZP13 are available online, as are several other larger units. Check them out at Aliexpress.com. It is always rewarding to try new motor and airframe opportunities and fitting these CR units into models and enjoying a different flying experience has been very satisfying.

X-Plane has proved to be a really nice model to pilot and with the CR unit it has a good turn of speed if required.