Twin Viscount (oz1256)


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About this Plan

Twin Viscount. Radio control twin sports model, for (2x) .19 -.35 motors.

Quote: "Yes, there is a reason for it! Even if it is only the desire for more than one engine. For example, one of my flying buddies who had not been in R/C too long, announced that he would build his first model. Naturally, it was to be a twin-engined affair with a complete control system, full scale, fully proportional, of course. In addition, this dream boat would have a retractable landing gear. At the time, it was explained to him that all this was just a dream, even for the most advanced fliers of that day. Fortunately, he was persuaded to take on something just a bit less involved and he started with a 10-channel Bipe as this was as far down the ladder as he wished to go! Yet today, just a short time later, as you must be aware if you have been reading the magazine reports, his so-called dream boat has become a solid reality; as a matter of fact, it is most practical.

My reasons for trying to build a twin-engined plane also included some very sound aerodynamic ideas. First, I had recently read that Lockheed had spread the engines of the Electra all along the wing so that the slipstream from the props practically covered the whole wingspan. The reason behind the idea was that the slipstream would be present at whatever speed the aircraft was flying: hence there would be more available lift and control action at low speeds than there would be if the engines were placed in the conventional manner. Since our models have a problem when flying at low speeds, as in landing approaches for example, it appeared that Lockheed's idea could be applied to our models. For this you need multi-engines, therefore the Twin.

In practice, what I found did not prove to be completely correct, but it did indicate the idea would work. First of ll the model demonstrated increased lift at all times, even at high speeds where the slipstream would not be considered to be effective. To evaluate these ideas, I had chosen my single engine Viscount (oz5659) design as a planform, as I am extremely familiar with this model and its single engine form and thus could easily note any difference in performance. Unfortunately, for this slipstream effect at low speeds, this was not the best design from which to form any conclusion, as even at low speeds the single engine Viscount is exceptionally stable and controllable. I did note that the landing approach had to be longer than usual, apparently as the sinking speed was lower due to the added lift, and the landing speed faster because of the slight additional weight. Speaking of weight, it is interesting to note that the addition of the second engine added less than a half-pound to the weight of the model. Total weight ready for flight was only 6 lbs.

Second effect of the slipstream was much more pronounced. I found the aileron action to be much more responsive and had to tone it down to keep from overcontrolling. This added action seems most useful during landings where immediate reaction is sometimes desired.

Another idea leading to the use of a Twin was gleaned from Charles Grant's good book, Theory of Flight. If you are familiar with Grant's bible, you will note that he devotes an entire chapter to propeller design and backs it up with what appear to be good hard facts. The main substance of the chapter is that the propeller should be designed to 'fit the airplane' and not the engine, as we generally do. This makes sense because the propeller is intended to fly the airplane, not just the engine. Theoretically, we should design the propeller to fit the model and then choose an engine which would turn this propeller at the required rpm. Then we would be getting the most performance from our model.

I have reviewed the formulas several times toward developing a propeller to fit one of my models. It all seems to work out well except that for the average model, the propeller size comes to about 16 inch in diameter. This shocks me each time and I have never been able to bring myself to use a .60 engine in a model that should normally use a .35, just so that I could swing a big prop at the correct rpm! A twin airplane can be the answer to this propeller efficiency problem..."

Update 07/09/2012: replaced this with a rescaled version, at full-size.

Update 02/12/2016: added article, thanks to RFJ.

Update 30/12/2018: Replaced this plan with a clearer copy, scanned at 400 dpi from fullsize, thanks to dfritzke.

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