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Martin

Cougar 2000 CoG

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I've been fettling the Cougar 2000 I bought off Emay some months ago.

 

This has a non-standard steerable tailwheel which Emay had started but not finished. I've managed to tighten up all the closed loop cables so the rudder and tailwheel now move appropriately, but it does seem to add significant tail weight, which is affecting the CoG.

 

I found a post on RCGroups http://www.rcgroups.com/forums/showthread.php?t=115294 which lists the manual-recommended CoG as 100-115mm back from leading edge. When I picked the plane up I also measured where Emay had the CoG on his other Cougar, which I have written down as "125mm, 40mm back from trailing edge of spar". (On my Cougar this doesn't fit: 125mm from LE is only 30mm from the TE of the spar, so I may have written this wrongly or the wing design may have changed slightly.)

 

Right now, with a 4S 2200mAh as far forward as it will go, the CoG is about 150mm back, which is a long way behind the manual's recommendation and still an inch beyond Emay's.

 

So, the first question is: I know there are quite a few Cougars in the club - where do you all have your CoG?

 

The second question is: assuming that it is indeed too far back, what's the best way to move it forward?

 

  • I could add lead, but given it's supposed to be a super-light funfly plane I am loath to do this, and with the motor sticking quite a way out and no cowl I can't add it very far forward. I could maybe try adding weight via a metal spinner or something to get it further forward, but again I'd rather not add dead weight if possible.

  • I could try a bigger battery, but there is very little spare space inside the fuz as it is and I'm not sure anything bigger will fit. I'm already concerned about the lack of airflow / cooling around the battery and ESC, and I don't want to make it worse.

Finally, I could try moving the motor forward. Currently it's mounted on four metal spacers which look to be about 10-12mm long. I have some longer spacers (about 30-40mm, I think) which came with the Seagull Boomerang electric conversion kit. These would move the motor quite a bit forward. If I do this, are there any things I should be aware of? One concern I have is that the bottom spacers seem to extend into the firewall (possibly to get the thrust angle correct) so I will have to make sure that I keep the same thrust angle if I do extend the motor mount.

 

 

Any suggestions welcomed!

 

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IMO the manual recommendation is ridiculously conservative, more based on Weston's insistance that a Cougar can be a good trainer.

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I used to fly mine at about 120mm back from LE. I would start at 100-110mm and work backwards if I was you.

I notice that some people mount the motor forwards on long standoffs or build a box on the front to get the CoG right without adding lead. I dont remember the Cougar being specially sensitive to thrust angle. Maybe you could add a couple of spacing washers at the motor end to change the angle and leave the standoffs flat on the bulkhead?

 

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Any suggestions welcomed!

 

On the belief this model has flown safely before, I'd fly it and see how you like it before going in for any mods. Obviously with a rear-ward CoG elevator sensitivity will be increased, you can counter that in the short term with plenty of expo.

 

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On the belief this model has flown safely before, I'd fly it and see how you like it before going in for any mods.

 

I don't think it has flown since the tailwheel mod was done, which appears to have added tail weight.

 

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Then "eyeball" the extra gear weight approx and temp add 2/3 of that as lead up front, which will err on the forward side.

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Thanks for all the advice!

 

I'll add 20-30g of nose weight temporarily and see how it goes.

 

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Returning to this after some time...

 

I did try flying the plane with the existing CoG and it was definitely very tail-heavy. (The advantage of something like a Cougar is that it can at least be landed in one piece in this situation - I took off directly away from me and others and then chose to land straight ahead in the rough rather than risk a turn with horrible CoG.)

 

So, I have been returning to how to get the CoG somewhere sensible. Given how far out it is and how this is a plane supposed to have a low wing loading, I didn't really want to add lots of lead. Instead, I tried moving the motor forwards as an experiment just to see how far it would need to go.

 

This picture shows the result:

[*]Try to find a commercial aluminium standoff motor mount long enough

[*]Try to construct a box of plywood or similar to move the motor forwards (but then it would cut down cooling airflow to battery and ESC inside fuz)

[*]Try to support the rods better - maybe a piece of hardwood on each side drilled for a tight fit and epoxied to the firewall, so that less of the rod is unsupported and it doesn't rely only on the tightness between blind nut and front nut to grip the firewall

 

 

I've already rejected the idea of trying to stiffen the rods by enclosing them in 3mm I.D. carbon tube, as is done for pushrods on bigger planes, as it doesn't solve the problem of the rods bending where they meet the firewall (or just slipping in the hole in the firewall) - it just concentrates the stress in that location.

 

Any ideas?

 

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Have you got this sorted yet Martin? I think I've got a solution I'm going to use on my cougar!

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