Roof Raiser (oz11300)
About this Plan
Roof Raiser. Radio control ultra-light model for Cox TD engine.
Note the Roof Raiser and Beam Bender (oz11301) both appeared in the same article in RCME, June 1995.
Quote: "Roof Raiser and Beam Bender, by David Boddington.
An indoor R/C aerobatic duo, equally at home outdoors in calm conditions. Roof Raiser is a 31-1/2 in wingspan low winger for .010 engines or, if the drawing is increased by 112%, it produces a model suitable for .020 engines. Beam Bender is an aerobatic biplane of 31 in span for .049 engines. Two or three function micro radio equipment is required.
Challenges are a vital part of any hobby or sport if the enthusiasm and interest is not to decline. Personally, I have never found any problem in finding new challenges in the field of aeromodelling. Just the opposite in fact, there has never been sufficient time to accomplish all the intended projects. However, a new and different concept will occasionally come along which really stirs the imagination and cannot be ignored. So it was with the prospect of flying, on a daily basis, indoor R/C models at the Model Engineer & Modelling Exhibition in the National Hall at Olympia. The meeting and the prospects for further indoor R/C models and flying are covered elsewhere in this issue of RCM&E, this article deals with one of my approaches towards the challenge of flying in this marvellous facility.
Although I had limited experience in flying the Micro Barnstormer (oz8142) and the Handley Page Sayer (oz8137) powered glider model indoors, there was no immediate precedent for an indoor aerobatic style model. I say 'style' because it was by no means certain that aerobatics were going to be possible within the limited confines of the hall with an I.C. powered design.
Electric power had been used for aerobatic aircraft, notably by Robin James with his biplane series, but this power source and the resulting propeller/rpm combination gives a different style of flying to that with high revving IC engines. It was a case of breaking some new ground and all the more exciting for that.
One of the biggest enemies when flying in confined areas is speed - an excess of it for the model and a lack of it as far as the pilots reactions are concerned. The faster the model is going, the sooner the obstructions are reached. Moderate flying speeds were, therefore, an important part of the specification for my models - and reactions!
To get out of trouble, when the model is rapidly approaching one of the liberally distributed cast iron structural elements, the model must have excellent control responses, these also being necessary for executing any aerobatic manoeuvres in the limited space. Over-control can also get the pilot into trouble, so there has to be a fairly fine balance between good control and 'twitchy' movement.
Aim light: To achieve a high degree of control at sensible speeds invariably leads to a lightly loaded airframe and with pretty well constant R/C equipment and engine weight (the latter will not vary enormously over the size range we are considering) it also points to a reasonably large model for indoors. Assuming that simply because the model is to fly in a small space that the model also has to be tiny is wrong. We could not cope with giants - the turning radius would be too large - but ultra small models will inevitably have a high wing loading, fly fast and, well, perhaps your reactions are better than mine!
For the model to be truly aerobatic a symmetrical section, or near symmetrical, would be sensible, with a farily thick section to slow things down. A 'slippery' and aerodynamically clean aeroplane might not be an advantage in this case. Lightness also equates to simplicity, so I had no intentions of using a throttleable engine, just two function control, elevator and ailerons. Two servos only limited the radio weight, allowing the use of 50 mAh batteries to give a reasonable operating time, and keeping the airframe and radio weight low reduced the engine power requirement.
No exotica: Being a realist (for some of the time), I knew that I would have limited time to design and build the model and there was no point involving sophisticated structures - plain and simple was the answer. After a few 'doodles', I set about drawing the 'Roof Raiser' monoplane with a wing span of just less than 36 ins and a chord, including ailerons, of 7 inches. 250 sq in of wing is not that small and I decided that a 0.5 cc Dart diesel would be adequate power (bearing in mind that Club 20 racers, with 300 sq in of wing area used engines of 3.2 cc capacity). A built-up wing, of course, for lightness, a sheet fuselage from 1/16 balsawood and sheet tail surfaces, with lightening holes, formed the basis of the structure.
It was unlikely that take-offs would feature in the flying programme, so a monowheel would be sufficient for landings, with the tailplane tip fins acting as the second and third sup-ports for the model on the ground.
With the model drawn up, all I had to do was to wait until after the Christmas festivities before making a start on the construction. It took just over two days of fairly concentrated building to complete the model, allowing for the occasional surfacing for the social niceties required over the festive period. The model looked quite promising - you get a gut feeling about the models when they become three dimensional.
Diminishing returns: After flying the HP Sayer models for the Handley Page Trophy contest in '94 - over which a veil will be drawn - it was time to check out the 'Roof Raiser'. The choice of the Dart diesel was partly made because diesels are much easier to regulate, for rpm, than glow engines. By reducing compression and adjusting the needle valve you can obtain reliable slow running. The idea of the first flight (no, I hadn't had time to flight test outdoors and, yes, I should have done) was to have just enough power to fly around and test the general han-dling and control responses. This was duly carried out, with the model even-tually coming to the floor after a tight turn. What was required, I decided, was a little more aileron movement, plus more power.
Although slow flying is an attribute, too slow and there is little airflow over the control surfaces and less slipstream effect from the pro-peller over the surfaces. Up with the compression, lean the fuel supply and with the Dart singing beautifully on a 6 x 4 ins propeller, away she went. Response from the controls was now very positive..."
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