A few months ago I started putting together the Getting Started pages. In Getting Started Part Two (Types of model suitable for learning) I recommend the Multiplex Twinstar Mk2 as a good first model for those looking for a electric trainer.
I still stand by these views, but a recent enquiry (May 08) from a prospective new member regarding getting a Twinstar Mk 2 in the air poised several questions, this has set the grey cells thinking and has persuaded me to do this write up.
Inspired I ventured into my attic and dusted off my old and much abused Mk I model. It wasn't until I went in search of battery packs that the problems arose. The model uses a pack of eight Nickel Cadmium (NiCd) or Nickel Metal Hydride (NiMh) batteries.
These were located, a label stated they had last been tested in June 2002! A couple of packs of 8 X Sanyo 2400 NiCds proved serviceable after a few cycles, but the two packs of Sanyo 3500 NiMhs didn't want to come back to life. Sanyo were masters of the NiCd, but never really got the hang of making NiMhs.
So using the serviceable packs the model flew as always giving 8 - 10 minute flights, which is fine except when Stan flies his Mk 2 he gets over twenty minutes.
Try E.bay the members shouted, wrong, no 8 cell packs, plenty of 6 cell packs and a few 7 cells good for car racing but no use in a Twinstar.
So why am I being so mean?
Well I have a huge investment in Lithium Polymer (LiPo) cells, the Hyperion Litestorm 3700 3S1P is very similar in potential energy to 8 X 4200 NiMhs while being nearly 200 grams lighter. The nominal voltage of the LiPo pack would be slightly higher at 11.1 volts against 9.6 for the NiMhs so should increase performance. Plus I already have these.
So just plug in the Lipo pack and away we go?
Oh, if only it were that simple.
It could have been if I had a Mk2 Twinstar. Now most electric models have the battery on or near the centre of gravity (C of G) allowing use of a wide variety of battery pack. Alas the Twinstar has it's battery in the nose thus any reduction in weight (200 grams in my case) will put the C of G back to far. Solution bore a 1/2 inch hole in the nose and fill with 2 oz/60 grams of lead shot and epoxy.
In the Mk 2 Twinstar the battery compartment is extended forward in to the nose which allows the battery to be moved to get the C of G correct, Stan's locates his 4200 pack as far back as it will go to get the C of G right.
So problem No 1 solved at least for my old Mk I, avoided if you have a Mk II.
A couple of pictures just to remind you of what we are talking about.
Twinstar Mk I on left Mk 2 on right, remember this as there will be a short test at the end!
|So just a quick warning before you fly
your Lipo equipped Twinstar.
Is the speed controller you are using Lipo compatible?
It is a given that it won't be unless you have changed it, the Twinstar Mk 1 & 2 uses a pair of small brushed 400 size motors. Most brushed speed controllers are old technology and are set up to work with NiCd/NiMh cells they are rarely Lipo compatible. In the Mk 2 RR (radio ready) package Multiplex states not to use LiPo batteries. (There maybe other reasons for this more on Part 2).
It doesn't mean LiPo's won't work, it's just the speed controller will be a bit confused.
The problem arises with the cut off voltage, the cut off voltage for NiCd/NiMh is usually set at 0.9 volts/cell, for an eight cell pack this would be as low as 7.2 volts discharging to this level will not damage NiCd/NiMh cells they will simply bounce back when charged.
LiPo's must not be discharged below 3.00 volts/cell as they can be permanently damaged. So your 3 cell pack needs a cut off at 9 volts. A setting of 7.2 would damage LiPo cells.
Despite some "literature" speed controllers cannot count, they calculate the number of cells and hence the cut off point from the initial voltage (auto mode), or will have been programmed to operate with a set number of cells.
I could have left it there and simply always started with fully charged cells and limited the flight time. A quick check with a watt meter and a bit of maths would have given me a safe flying time.
But then that would have been far to simple?
Another couple of pics to keep you interested.
Now I knew that I had up my sleeve, or at least stored
in a shoe box some brushed speed controllers that I had
purchased a few years back with conversion of this model
I won't mention the brand simply because I cannot find anywhere that still sells them! I had purchased 2 with 20 amp rating and one with 40 mp rating. The two smaller ones are in models but I've no idea of which. Anyway the 40 amp ESC was fitted with suitable plugs and connected up, it worked but needed setting up for a nine volt cut off.
I won't go into the details but an hour later, blood had been shed and the said ESC was heading for modification using two bricks. Programming is by moving the throttle stick in time with a flashing LED whilst counting and reading the instruction. I phoned Paul G. as I knew he had one working for advice, he said he had programmed his purely by luck, and agreed the best way forward would be with two bricks.
A quick browse around the web and a Jeti Brushed 030 contoller (£30.00) was ordered from Puffin Models, with next day delivery it was programmed (four options join pins with jumper provided) and the old Twinstar leapt into the air. The controller works fine, but you do have to land as soon as the power goes. On one flight I kept the model airborne by closing and opening the throttle to reset the controller, this took the battery down to 8.3 volts!
I managed to recover the battery, and it has suffered no ill effects.
Several members had fly, with Darren having a few sessions on the buddy box before soloing with his "Buzz". Yes, it's still a great fun model and a great trainer.
In part two I will do a full modernization to include "brushless" motors and maybe an even bigger battery packs.