Medusa 4000 DD for Cox Micro Warbirds
For my second Cox Spitfire I wanted to try a Medusa 4000 DD.
I surgically altered an IPS gearbox frame by removing the lower half - leaving the motor "rings" and the 8x5 stick mount "box". With a 1.5mm shaft prop saver and a 4.5x3 GWS DD prop, the Medusa 4100 with GWS heatsink, in this partial frame, weighs in at 23g. The stock 130 motor and Cox prop and spinner is 27g. The LensRC 17T with 3mm shaft prop saver and Graupner 5x4 with a Jerry Blake mount, weighs 31g.
Now we have a light motor - but which prop to use?
The choices are pretty limited, and it would seem that even with the smallest (3x2 GWS DD) 3s is out of the picture (probably 7-8A, and the motor is rated at ~5A/44W max).
So let's try 2s. I briefly tested several props with a 2s TP 480 Gen II and at the 4A-6A draws I saw, the voltage under load was generally about 6.5v.
So, I put the little beast on the test stand and used 6.5v as my input voltage. [I also tried ~6v but this was not a good move - with the Phoenix 10 the LVC (set @ 6v) was cutting in and out, and the motor speed fluctuated too much for readings] - but 6.5v was fine.
The 3x2, 4x2.5 and 4x4 GWS props have square-tipped, essentially rectangular blades and I've called those DD's.
The 3x3 and 4.5x3 GWS props have round-tipped blades shaped, like the larger GWS HD props, so I've called those HD's.
The 3x2 and 3x3, having 1.5mm bored hubs, were mounted using a GWADP 001 prop adapter, the remainder, with 3mm bored hubs, were attached using a prop saver for 1.5mm shaft.
3x2 GWS DD:
6.5v, 3.45A, 22W, 21480rpm (40.7mph), 108g (4.91g/W)
3x3 GWS HD:
6.5v, 4.70A, 31W, 19410rpm (55.1mph), 106g (3.42g/W)
4x2.5 GWS DD:
6.5v, 5.25A, 34W, 17970rpm (42.5mph), 150g (4.41g/W)
4x4 GWS DD:
6.5v, 7.00A, 45W, 14610rpm (55.3mph), 127g (2.82g/W)
4.5x3 GWS HD:
6.5v, 6.50A, 42W, 15570rpm (44.2mph), 171g (4.07g/W)
I think I favor the 4.5x3 since it gets the best thrust, at decent pitch speed, and though it takes the motor well over the rated 4-5A max, it doesn't exceed the rated 44W max. [and it looks like a real prop - those square paddle like props look ugly!].
My first Spitfire came in at 142g with the LensRC 17T etc - the next one, if I go the Medusa route, should come in a fraction lighter - so better than 1:1 thrust:weight @ ~40mph seems okay to me.
Remember the stock Cox 130 motor and 4x2.5 prop gets you only about 65g thrust @ ~33mph. The LensRC 17T with a Graupner 5x4 is going to get you ~140g @ `33mph on 2s and ~300g @ 46mph on 3s!
For aircraft with larger noses than the Spit - like the Corsair and P-47 - I really think that a geared Medusa might be the way to go - I imagine that the Medusa 5300 on S2 (3.5:1) and 3s could really spin perhaps a 6x3 very well!
Here are a few pictures:
Last edited by Dr Kiwi; Oct 01, 2006 at 11:09 PM.
Fitting the Medusa in
Not recommended practice - but here I am starting a second version of the Cox Spitfire before I've test flown the first! It was raining today, so flying was out - and the fingers got itchy.
I figured my fuselage former and stick mount worked out well for the LensRC 17T, so here is the Medusa variant. The former was, this time, made of 1/8" Liteply and the 5x8 stick (to fit the butchered IPS gearbox frame) was from hard balsa. The 5x8 stick was glued through a slot in the ply former with a little downthrust (less than stock) and a little right thrust. Some carving of foam was necessary to fit the stick, the gearbox frame and especially the heat sink into the narrow nose of the Spitfire.
A potentially serious complication reared its ugly head when I realized that I needed to be able to remove the motor if necessary - it cannot simply be slid forward off the 5x8 stick, 'cos the nose gets in the way! I am going to use "the big starboard hatch" on this model too, so that I have access to the motor/battery/ESC from the right side. I cut out a roughly circular hole in the ply former, directly behind the motor, so that, with the prop removed, the motor can be slid out of the IPS frame, backward into the battery bay. QED!
So - here are the first pictures:
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