ELECTRIC RTP (Round The Pole)

“That’s not flying – it’s just an electric motor whizzing round on the end of a wire”. Well yes it is, but it can be so much more.

I started investigating electric RTP over 40 years ago because I wanted to fly at my choosing rather than when the fickle UK weather allowed and as I didn’t live in a baronial hall I needed models that could be flown in a normal living room which meant lines of 6’ or less! Flying on such short lines was going to require small very light models and my initial attempts with the (then) smallest 3V motors were at best marginal. Motor life was short and getting the C of G right with a relatively heavy and low power motor was critical. Too far forward and the necessary flying speed could not be achieved, too far back and the tail did not even lift up. I reasoned that as you had to have two wires for the motor why not make the models control line as well. With some elevator authority I hoped a marginal model might be coaxed into the air. Obviously control line RTP makes the centre pylon mechanism a bit more complicated but on the plus side judicious use of elevator control does reduce‘unwanted contact with the ground’ so the models can last longer. In fact from a design point of view electric RTP control line models can be flown so gently that the airframe really only needs to handle flight loads, anything stronger is just excess weight. In small models this is actually quite difficult to achieve and requires much ingenuity to find light construction methods that are sufficiently rigid. The final element is motor gearing. Small (1 ounce?) electric motors tend to have a high ‘maximum power’ speed so gearing is necessary on even a small 6” propeller. The ideal model needed to be of small size with plenty of wing area and a big propeller. So after many trials what did I end up with? The Fokker Triplane Anthony Fokker’s triplane was just about the ultimate design for a compact aeroplane built around a relatively big & heavy engine. It has a generous wing area for its span, proper cantilever wings (no rigging!) and a huge propeller! Just about perfect for a short line model. This 16” span, 2¼-ounce model is true scale, both inside and out with 1mm round balsa for the fuselage steel tubing and elastic cotton for the bungee-sprung undercarriage. The control surfaces are moveable and are correctly cabled (with cotton) to the stick and rudder pedals in cockpit. The control line bell crank is just simply linked to the joystick; the control cables do the rest. Its looks great and has never given a moments trouble. The motor is geared 2.5:1 to a scale (in both diameter and pitch) carved balsa propeller and as a final touch, a nine cylinder Oberursel goes round with it inside the spun aluminium cowling. Just to complete the picture it actually flies quite well, with an impressive climb off the ground. The thick wing section and the slot effect mean it can ‘hang on its prop’ just like the original, and yes, control line RTP is indeed gentle. This most delicate of models was built in 1969, flown at that year’s Model Engineer Exhibition in London (on 12’ fixed lines) and its still going 35 years on. (the pictures were actually taken last month)   The Martin Baker MB5 Not so many people are even aware that Martin Baker (of ejection seat fame) built series of aircraft. Their final design was an impressive looking fighter with short broad wings and a Rolls Royce Griffon turning a contra-rotating propeller. Despite a good performance and many novel design features only the prototype was built. Clearly my model could not be of scale construction but to look right it needed to be all sheet covered (except the elevator and rudder, which were fabric covered on the original). To keep weight to a minimum the fuselage is 1/32” balsa planking on built up lightweight frames. The wing is similar with 1/32 sheet ribs. The result is a true monocoque where the skin takes all the loads. Double curve planking in such thin sheet is not for the faint hearted but the final result looks great and is remarkably strong and light. The cockpit shows all the instruments and the harness straps are neatly folded on the seat! With a separate motor driving each of the 8” diameter propellers, this 24” span model still only weighs 4½ ounces all up and the motors themselves account for 2 ounces of that. Compared to the Triplane it flies quite a bit faster, actually it is only 12 mph, but on 6’ lines it certainly feels fast. It is also remarkable noisy. The hollow sheet structure amplifies the gear whine so it sounds more like a turbo prop. It certainly turns a few heads. Of course landing requires great care just to keep all those prop blades clear of the ground. I have recently rebuilt the undercarriage with proper springing to ease the landing shocks but never the less the MB5 is over 20 years old!

Not all my models are as complex, here are just a few: A sort of “Liberator” bomber A lightweight tissue covered model with direct drive 3-volt motors. I had found that putting all 4 motors in series and applying 12 volts did not produce as much power per motor as with 3 volts applied to one, so it has a 6-volt series /parallel cross connected set up that works well. It is almost silent and looks very impressive in the air but has to be stored carefully to ease the load on the wing spar as each of its four motors weighs more than the whole airframe! The Flying Brick For a display at a local school I built this twin-engine transport to carry a full size house brick (or rather a very light replica) and to drop it in flight. A 3rd motor (and a third wire) works the rear door and the full span flaps. The rear view shows its cavernous interior. Fairy Gannet A model based on the old Kiel Kraft scale rubber design & built for carrier deck landing. By careful selection of the arrestor system it is possible within the space of a 8 foot ‘carrier deck’ marked out on the floor, to land, pick up the wire & stop, let the arrestor system pull the model back to the start, drop the wire and take off again. It’s very impressive!

Small size control line RTP may not be to every ones liking and today’s electric RC models can do nearly everything but being able to fly in my own house has enabled me over the years to have great fun designing, building and flying a whole range of models and all at a very modest cost. I have even managed to fly (but not yet hover) a twin rotor helicopter. Of course I have had my fair share of failures but I hope my successes might encourage others to give it a go. Unfortunately as all my models are ‘one offs’ I do not use detailed construction plans but if anyone wants any more information Email Simon Chaddock

Article by Simon Chaddock for Bumpy Green Model Aerodrome
April 2004

Comment by Peter McCarthy on 20040623063627. Rating 5

Simon, I was very interested to come across your article on RTP. I played around with the system in my back yard on and off for years and finally worked out how it could work using inexpensive bits and pieces mostly available at the local electronics store. Your beautiful models are way beyond my very basic efforts, though I did make an all sheet Triplane and a Neiuport for display at a fete at one stage (did you need lots of negative incidence on the Tripe tailplane as I did?). Eventually I developed an RTP approach for school kids using a quickie cardboard model (see: www.ctie.monash.edu.au/hargrave/mccarthy_rtp.html Unfortunately, insurance concerns have exploded in Australia and these days I probably would not dare run the system for school groups as I once did. Since retiring I have maintained an interest in the hobby, building and flying micro electric models (using modified RC car radio gear) at the local park - when the weather allows. Thanks for giving me a glimpse of what could have been possible with RTP. Peter McCarthy

Comment by Greg (London UK) on 20040707101922. Rating 5

A fascinating account of RTP flying, and some tremendously well built and detailed models, congratulations. I have been flying RTP on and off for about 27 years. I started with some basic kit from the now long-gone Harry Butler Models and flew as a young lad in my parents back garden, where ‘trimming’ the flowers was my speciality! In those days I generally used simply constructed models and slot car type 12v motors driving 3” x 2” props on direct drive. I maintained my interest and once I was older I was able to finance the hire of a sports hall, and along with some friends, we had some good flying evenings, using 15 foot lines, with a myriad of aircraft types including aerobatic biplanes and multi-engined types. I was always inspired by the RTP displays at the Model Engineering Exhibitions and at Sandown Park symposiums, but alas, RTP rarely seen demonstrated publicly these days. Although I also do R/C electric and an awful lot of C/L flying, I have still maintained my interest in RTP and am currently looking around for a suitable venue to hire locally (and cheaply!) to allow indoor flying on decent long lines (say 20 foot radius) during the winter months and am also investigating electronic motor control (as opposed to variable resistors) and will experiment with using the readily available R/C electric flight motors (like speed 280s and 400s) rather than the 12v ‘Slot-car’ type, as these are getting harder to obtain these days. Most of the models I now build for RTP are converted from small flying scale F/F rubber kits, the old Keil Kraft range were all beautifully suitable but are alas hard to come by nowadays, but there are still some from Guillows, Sterling, etc. etc. plus the wonderful range from SAMS with their super Aerographics models which have a really good and diverse range of subjects. The convenience and safety of the system allows otherwise quite delicate and fragile models to be flown in complete safety, which is part of the system’s appeal. Of interest to some of you is the only remaining UK seller of specifically designed RTP equipment, the lovely guys at Ballards models shop in Tunbridge Wells, Kent. Go to their web site at: http://www.ballardshop.com/ And click the link for ‘RTP’

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