| At last a few minutes available – and I have made a decision, I am going electric.
This raises questions requiring answers, some of which are guesses. For instance, what i/c engine will I be replacing? Well, generally it would seem that a 6ft biplane weighing about 15lbs (7Kg) needs a 120 4-stroke so I had in mind an RCV120SP which is fully in-cowl and radial-mounted. Because the RCV SP–series are geared and run at about 5,500 rpm maximum they can deliver high torque and hence can use quite large diameter, and necessarily large pitch, props hence a 4-blade 15.5x12 seemed very suitable. Many SE5A aircraft were fitted with 4-blade on Wolsley Viper 200h.p. engines and to emulate that set-up seems reasonable. So what electric motor matches this? Well, searching around an AXI5345/14 outrunner with a battery of 10 LiPo cells in series seems to match up so that is my thought but there is much to consider. Firstly the RCV120 weighs about 1.3Kg but the AXI only about 0.895Kg so I would make the nose very light – not a good idea. But 10 LiPo 4000mAh cells weigh about 0.65Kg as opposed to a 250ml tank of i/c fuel weighing about 0.350 Kg. However such a tank-load would probably give a flight time of 12-15 minutes whilst a 10S 4000mAh battery would give about 4 minutes. To equal the i/c duration I would need 30 LiPo cells connected 10S3P which would permit about 12 minutes duration – but such a pack would weigh about 2Kg giving motor plus battery all-up of 2.85Kg whilst the i/c set-up is only 1.65Kg. An extra 1.2Kg, not good. So O.K., compromise on flight duration and settle for 10S2P battery giving 37.5 volts (well nearer 40 volts actually) and 8000mAh of life and hence about 8 minutes flight time. Category 10C LiPo’s will permit an 80amp/3.2KW burst of power for take-off, should be enough.
So there is plan A, AXI plus 10S2P LiPo weighing 2.15Kg total, perhaps about 0.5Kg more than the RCV120 set-up. O.K., give it a go – but now it is “location, location, location”, I cannot have a light motor in front of the aircraft CofG and heavy battery behind so a bit of maths is essential. Remember maths lessons? A small boy, say 70lbs (5 stone) sits at one end of a see-saw say 10 feet from the central pivot, the fulcrum, thus he exerts a “MOMENT of LEVERAGE” of 70x10 =700 ft.lbs. If now a large oversize boy weighing 98lbs (7 stone) sits on the other end then in order to be in perfect balance he must sit 7.14 feet from the fulcrum – his Moment would be 98x7.14 = 700 ft.lbs so the 2 kids are balanced.
It is the same with my model, with the design Centre of Gravity (CofG) being the fulcrum and the Moments needing to be equal. So what are these Moments? I must assume that the plans were drawn up with i/c engine and fuel similar to those of my “guess” so lets look at figures. To have the 1.3Kg RCV at the correct position, permitting the prop to sit correctly, the weight centre of the RCV would be 29cms from the aircraft CofG and hence have a Moment of 1.3x29= 37.7Kg.cm. Now the centre of the fuel tank is best positioned directly under the CofG of the aircraft because as the fuel is used up and the weight drops it does so equally each side of the aircraft CofG and hence balance remains correct (if it was correct in the first place!) But my plans show the centre for the tank actually 1.5cms ahead of the aircraft CofG so there is 0.350x.1.5 = 0.52Kg.cm of moment which makes the total 38.22Kg.cm. I have to assume that the design takes this into account so if I go electric my motor plus battery must also have a total Moment of 38.22Kg.cm.
Well, the AXI in the right place has its centre 32cms ahead of the CofG and so it exerts a Moment of 0.895x32=28.65Kg.cm. Now as I need a total of 38.2Kg/cms my battery assembly needs to exert 9.57Kg.cm and since it weighs 1.3Kg its centre must lie 7.36cms ahead of the CofG (1.3x7.36=9.57) No problem, plenty of room, so now I have an electric flyer balanced just as its i/c counterpart, weighing about 0.5Kg more and with only two-thirds of the flight duration. Perhaps not too much of a bargain, but there are advantages. No messy fire-wall, no messy exhaust, no start-up problems, no dodgy plugs, etc. so here goes.
But then, think on. My models generally end up tail-heavy – a not uncommon occurrence with other chaps too so I gather – so I end up putting lead weights in the nose which always aggravates me. But now I have another option. The battery will be about 15cms long and at the balancing point, as calculated above, the front of it will sit about 7cms behind the motor. So if I make the battery holder a “trough” 25cms long placed 2cms behind the motor I can slide the battery forward or backward by 5 cms in order to compensate for other weight problems in my construction. Now this 5cms can give me an extra nose-down Moment of 1.3x5 = 6.5Kg.cm if I slide the battery forward. That would be the equivalent of putting, say, 500gms of lead on the nose some 13cms ahead of the CofG without actually adding anything! And of course if by chance I ended up nose heavy I could slide the battery backwards with the same effect. A certain winner. So that is Plan A. If it all turns out a disaster I can still go to Plan B – back to i/c!
And since up to now I have only joined up the front of my fuselage with pins, screws and tape I can pull it apart, take out most of the fuselage inner and replace with a battery trough – very open-structure to permit air-flow. In the same way, a new AXI radial-mounting bulkhead can easily be installed and since I am making the upper cowl removable then access to the battery and motor will be easy. Access to the servos will be by removing the upper fuselage cockpit area and now my thoughts on making a removable upper wing centre section and cabane can be abandoned – no need.
So, next, covering experiments. Interesting, well to me anyway.
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