Plus. Radio control sport aerobatic model, for electric power. Wingspan 39 in, for 550 motor and seven cells.
Quote: "Plus and Minus. Build Paul Thompson's low cost, low power electric duo based on buggy motors.
'Plus' and 'Minus' are both relatively easy models to build and both have been designed around inexpensive 540 or 550 type buggy motors. However, their performances are quite different and 'Plus', especially when powered by a 550, is fast and manoeuvrable and not recommended as a beginner's model while 'Minus' is a super slow, docile flier just right for lazy (and, above all silent) relaxation. Basic construction of both is straightforward and described at the end of the article. However, let's start with some general notes about their design and my own approach to electric flight.
As stated earlier, to help keep the cost down I have used the bottom of the range 540 and 550 motors. Both motors are the same diameter and therefore interchangeable so if you are not happy with the power of the 540 motor then just change to the 550 motor. The 550 will, of course, run the battery out quicker, but when you consider the cost of a 550 motor compared to a hot 540 motor the shorter flight doesn't seem to matter as much!
Another important consideration however is engine life. I find that the bottom of the range engines are unfortunately only good for around 5 to 7 hours of use. After about 5 hours you'll notice that the aircraft will not climb as well as it first did and by 7 hours or so the climb performance is very bad. This is not a problem if your model is a motor glider, as the motor is only used for a short time on each flight.
There are a few things that can be done to increase the engine life. One is to use a propeller that allows air through the nose cone to help cooling. Also giving the motor time to cool down after each flight will help. Using only 6 cell batteries will reduce the current through the motor, which will help save the brushes and also keep the motor cool. Lastly, using a smaller or finer pitched propeller will help.
The batteries I use are the Astro 7 cell 900mAh (8.4 volt) in 'Plus' and the Tamiya 1700mAh or the Tamiya 1200mAh 6 cell (7.2 volt) battery in 'Minus'.
I would have to recommend the 900mAh 7 cell battery for the 'Plus' design, as it is lighter than the longer life 7 cell batteries and able to give very good power for aerobatics. With only 6 cells you have to be very careful flying this aeroplane; it is very important to keep the nose down and the speed up, or it will fall out of the sky!
'Minus' with the 540 motor, 7 x 3 propeller and 6 cell batteries runs a lot cooler. Since this model is so light, 6 cells is all that is really needed - using a 7 cell battery would really just wear the engine out quicker, plus the 6 cell fits in better. The battery I would recommend for this plane is the Tamiya 7.2 volt 1200mAh or equivalent. I have had flights of 5 to 15 minutes with this battery and a typical flight (while not simply gaining height and looking for lift) should last around 5 to 6 minutes.
Construction: The easiest way to make the engine mount is to place F1 on the front of the motor, then place the stringers on F1 while holding them onto the motor with rubber bands. Next slide F2 onto the stringers and align it to its correct position and allow the whole thing to set. Then slide the motor out the back and place the soft balsa blocks on the engine mount.
On 'Plus', the wing is built upside down on a flat surface, so that the dihedral is produced by the taper of the ribs. 1/16 balsa covering is used as shown, with soft balsa used for the wing tips.
The aileron servo is mounted on its side as shown (Hi Tech mini servos were used). The spars are spruce with 1/16 balsa used as a spacer. Plywood is used in the centre section, so there is enough strength for the dowel..."
Plus, Radio Modeller, September 1993.
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