Andrew Moore 7 Posted July 10, 2018 Share Posted July 10, 2018 Just acquired an unflown CMP Extra 33L 50" version with an OS 46 engine at a good price. Having read alot of forums about this particular model its very clear there are major C of G issues with the 50" version which I have when using a `small` OS46 engine as it is/will be way too tail-heavy. Just checked my C of G dry with a small battery pack and its 125mm and its supposed to be 90-96mm from front of leading edge of wing at fuselage. Dont want to go down route of adding loads of noseweight (would need alot), moving engine forward or big battery for receiver behind firewall. I would rather put a bigger (heavier) OS engine in so will help towards the C of G as well as power it much better as its apparently low powered on OS46 - can weight 3-3.5Kg depending on engine used. Ideally, I guess I'm on lookout for an OS 65-95 and even then may need to add noseweight. I'm told the larger version of this good looking plane does not suffer with same issue. I want to make it work but would appreciate your thoughts if you have one. Quote Link to comment Share on other sites More sharing options...
Nigel R Posted July 10, 2018 Share Posted July 10, 2018 Gone off the idea of electrifying it? A big lipo positioned a long way forwards might work rather nicely in your favour if there are CG problems. Quote Link to comment Share on other sites More sharing options...
Andrew Moore 7 Posted July 10, 2018 Author Share Posted July 10, 2018 Definitely gone off the idea, after reading all the C of G issues. Easier solution is to put a bigger and more powerful engine in. Think converting to electric in this case would be a nightmare 😀 Quote Link to comment Share on other sites More sharing options...
Nigel R Posted July 10, 2018 Share Posted July 10, 2018 Hmm, I just googled this airframe. Seems like it has the nightmare combination of small wing and rearward CG. Just thinking aloud - What can be moved forward? Where are the servos installed - tail end? Can they go forward and have a closed loop cable run to the controls? Can the RX battery go in front of the firewall? Extras have a pretty big cowl, if I remember right. Can the engine be shuffled forward on its mount? What sort of structure is there at the back end, and is there any scope to lighten it (without making it weak obviously)? Heavy tailwheel? Replace it with a light skid? Quote Link to comment Share on other sites More sharing options...
Andrew Moore 7 Posted July 10, 2018 Author Share Posted July 10, 2018 many have covered all of the above and I've considered each - best way is still bigger/heavier engine as apparently the OS46 will be under-powered anyway. When I was testing the C of G (dry) this morning, it was 125mm (supposed to be 90-96mm). Even when I say a 3300 3S battery just in front of Firewall (engine is an OS46) the C of G came down to 100mm so would need alot of lead if keeping same engine. Quote Link to comment Share on other sites More sharing options...
Denis Watkins Posted July 10, 2018 Share Posted July 10, 2018 Andrew, mine has the whole .46 bearing casting poking from the cowl and is not flush, the motor mounted virtually at the end of the engine mount The servo try is well forward already and have joined to horns using carbon tubes. A .90 motor throttle range could not be maintained high for long, so the motor would probably not fully warm up Or be in its useful torque range. Even a .52 is unnecessary, but it is your plane Bolt the lead or steel bar ballast to the engine mount The model suffers from a short nose, but loads of room for weight Edited By Denis Watkins on 10/07/2018 16:42:58 Edited By Denis Watkins on 10/07/2018 17:03:03 Quote Link to comment Share on other sites More sharing options...
Andrew Moore 7 Posted July 10, 2018 Author Share Posted July 10, 2018 Denis, are you suggesting I simply keep the OS 46, move it as far forward as I can on the mounts then add ballast to the engine mount ? No point say sticking a 75 or 90 in for the extra weight/power ? All advise gratefully accepted Quote Link to comment Share on other sites More sharing options...
Denis Watkins Posted July 10, 2018 Share Posted July 10, 2018 My view is that in old money, this is a 5 foot span, 6lb model, designed for .46. It is twitchy enough on the .46 without a .90 With 6 - 8 foot span models, 8 - 10lb, we have some leaway for .90 and above, and can play with power Just this one Andrew, I do believe is in the good .46 mould, mine being the red Irvine, and about 1.5mm reflex up on the ailerons. In the end it is your model, your choice Quote Link to comment Share on other sites More sharing options...
Denis Watkins Posted July 10, 2018 Share Posted July 10, 2018 Double post Edited By Denis Watkins on 10/07/2018 17:11:34 Quote Link to comment Share on other sites More sharing options...
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