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RM Aerobat

Another electrified 1980s design

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David Mellor02/01/2019 15:44:25
1188 forum posts
599 photos

Nigel, I think Bob is right.

I have a variation on his theme..... if you have a smaller, lighter battery that would fly the plane for a shorter time, but still give you sufficient current, then use that for the maiden with even more lead as far forward as you can possibly fit it. The reasoning is that lead is far denser than battery and the further forward the less you need. So you get an initial, trial balance and still keep the weight down, even though you've added lead (because you have added less weight in the battery).

That would give you a lighter plane to maiden, but for a shorter flight. It gets over the first hurdle - the CG that suits you.

Then - as Bob says - you can look at your surgery option armed with new information....


Nigel R02/01/2019 16:27:35
2108 forum posts
381 photos

I think through you two the voice of sanity hath spoken clearly. You're both right. Minimal effort, nail on the lead, get it flying right, then look at the harder options. It's sitting at 3-1/4lb right now (2oz over my initial guesswork). I reckon I'm looking at about 3oz - 4oz to move the CG to 33%. At 3-1/2lb it will still fly nicely enough to maiden.

This is what I love about this place, chuck some ideas around and one of you chaps will come up trumps with something good to throw back.

Nigel R07/01/2019 09:09:48
2108 forum posts
381 photos

Madness bit.

I don't think I'll need this bit any more...


Nigel R07/01/2019 09:11:05
2108 forum posts
381 photos

It was removed with a bit of chain drilling, and tidy up by dremel.

Half way through tidyup:


Nigel R07/01/2019 09:11:52
2108 forum posts
381 photos

And with a box glued in place. Epoxy, for once, as my dremel skills do not include sanding a perfect flat edge at 90deg:


Nigel R07/01/2019 09:23:27
2108 forum posts
381 photos

As it stands, it needs a little more 1/4" tri reinforcement around the glue joints with the firewall.


The lipo will snug right up against the motor now. That's as much as I can easily do to fix the CG without lead.

The ESC will be relocated above the front of the lipo, inside the motor box.

Theory is:

- Some sort of lipo tray extension will go inside the box.

- I'll add a couple of baffles to make sure air goes over the ESC to manage the heat. Or, I'll box in the front of the lipo, which would have the same effect. Not sure yet, I'm kind of making this up as I go along.

Of course at this point I realise I've made the box too shallow, but only by the thickness of the firewall (5mm). Never mind - perhaps I will need the offcut of firewall after all - I can make a few ply washers to go between motor box and motor itself.

Mental note for the future:

- this fuselage had a reasonable amount of wood at the back, in the deck and the crossgrain sheeting

- servos were located close to the rear of the wing saddle

I could and probably should have put the servos forward in the wing opening. Live and learn. Maybe the thing to do would be to provide a couple of possible locations in the servo tray, and move things to suit when it starts getting close to time to balance the thing.

It wouldn't be impossible to move the servos now, but it's more surgery (knocking out the back end of the lipo tray, putting in a new tray for servos, sorting new control runs). I've seen snakes "extended" with an inch or two of outer stuck in place, so never say never, but I'll see where things lie with the new lipo location first.

Bob Cotsford07/01/2019 09:39:00
7636 forum posts
427 photos

Here you go, extended snakes using 2mm pushrods threaded into the snake inner and a Z bend at the servo.

servos moved forward.jpg

Nigel R07/01/2019 09:55:23
2108 forum posts
381 photos

Now there's a thought. Thanks Bob.

Mike T07/01/2019 11:30:27
365 forum posts
28 photos

FWIW I do the same sort of thing at the other end:


...keeps the wobbly bits inside their supporting tubes...

kc07/01/2019 12:53:12
5698 forum posts
164 photos

Using closed loop on the rudder instead of snakes might save some weight at the tail end, especially if you put the adjustable clevis at the front and just loop through the rudder horn. Saves moving the servo perhaps.

For major surgery on the ply firewall ( next time! ) consider using one of those vibrating saws from Bosch etc instead of chain drilling and hacking. Handy for specialist jobs but not worth buying specially but the sort of thing best borrowed when you need it for just 5 minutes...........

Nigel R07/01/2019 13:01:37
2108 forum posts
381 photos

Chain drilling was definitely not the best answer, more like the only one possible with my dremel type tool. One of those fancy reciprocating saws must go on my Christmas list next year.

I remember Dave M did a few weight comparisons with closed loop and snakes and these red sullivan snakes came out quite well.

Nigel R07/01/2019 22:58:41
2108 forum posts
381 photos


Nigel R07/01/2019 23:02:55
2108 forum posts
381 photos

Surgery nearly done.

Just got to sort the ply washers and remount the motor.

It's tight in the cowl now. But it all just about fits and airflow still seems free enough past the motor and out the back of the fuselage.

Ron Gray08/01/2019 07:55:17
1223 forum posts
295 photos

Try a keyhole sawblade next time, like these **LINK**

kc08/01/2019 13:03:00
5698 forum posts
164 photos

Next time when modifying the engine bulkhead to change to electric etc consider cutting the bulkhead off by cutting the balsa just behind. Then make a new bulkhead to suit- of course the new structure needs to be 6mm or so longer but it's easier.

Nigel R08/01/2019 13:39:43
2108 forum posts
381 photos

kc, that's an idea worth throwing in the mix, in this case I had a bit too much time invested in making the hatches and the covering - plus the cowl would have need modifying and remounting - but if I had a glass cowl fixing on the outside of the fuselage it could have been the easy route forward.

Ron - nice one - mentally filed away.

Nigel R09/01/2019 10:09:29
2108 forum posts
381 photos

Ok, motor box surgery done, lipo secured in (new) place, esc in its new home, motor back on with its 5mm ply 'washers' etc etc. That little lot has saved about one third of the lead from the nose - previously I needed nearly 7oz (!) to balance and now I need 4oz of lead. Not only has he lipo moved forward, but the available location to glue the lead has also moved forward - result!

So. I'm thinking about servos. I've worked out (using some "weights x distance" type sums) several further options related to the servos -

- without any lead at all, prior to any surgery, with NES507 standard servos in the fuselage, the airframe weighed 52oz.

- just sticking in lead, 6.75oz, total weight 58.75oz.

- surgery on motor box, 4oz lead, total weight 56oz.

- swapping the NES507 servos for HS225s will save about a half oz lead, and the servo swap saves a further oz - this gives a total airframe weight of 54.5oz, now only 2.5oz more than my first "pre lead" figure.

- putting HS225s in at the front of the wing saddle will save another 1.5oz lead - total airframe weight would be 53oz, only 1oz more than the first 'pre surgery' figure.

The straight swap from 507 to 225 is an easy win. I'm good with that.

Moving the servos to the front of the wing saddle will need a lot more work. I would need to remove the back of the existing lipo tray (no longer needed now the lipo is moved forward into the motor box area) and replace it with a new tray for the servos. Then I have to extend the snakes (might be quite easy, if I use Bob's idea of regular 2mm threaded pushrods rods into the snake inner).

I think I'm ok with the airframe weight being 2.5oz more than my 'pre surgery' figure. So my plan at this point is to go with 225s, in the same place as the 507s are now, and use about 3.5oz of lead in the nose.

I'm also considering a motor swap, a 3542 would fit and is about 1oz more than the 3536. Still means lead, just less of it.

If anyone else is thinking of building one of these for electric, I'd suggest making the nose about 1" longer. And thinking about the structure at the rear - I have slimmed things down compared to the plan, but perhaps I could have taken the rear deck sheeting down to 1/16" and the crossgrain underside sheet to 1/16" as well.

I also get the feeling that the HobbyKing covering film is a bit heavier than the old Solarfilm, that will not have helped, as the weight of the covering is centered (approximately) on the wing's TE.


Live and learn!

Edited By Nigel R on 09/01/2019 10:17:48

Bob Cotsford09/01/2019 11:15:47
7636 forum posts
427 photos

I'd agree that the HK film is heavier than original Solarfilm, it's more like Solarfilm Polyester, Oracover or Toughlon in my experience.

To be honest I don't think a couple of ounces here or there is going to be a problem though when I moved the servos on my J60 it did make a notable difference to how much lead was needed (that was probably a more extreme repositioning). On the Magnatilla in my pic above moving the servos forward 3" or so was quite easy and did save me from using any lead at all. It was undergoing radical surgery anyway to convert it from IC to electric.

Me, I don't like flying in cold weather so I'd get the saw out and move the servos during my off-season downtime. As it isn't my model I'd better get back to my winter task redecorating the bedrooms ready for the new carpet due tomorrowsad

Edited By Bob Cotsford on 09/01/2019 11:17:14

Nigel R11/01/2019 09:01:51
2108 forum posts
381 photos

Well, for now I have done a straight swap, and am down to 3oz lead required. Overall, it is now only 2oz more than it was prior to the last minute surgery:


Now I just need to actually fit the lead, then go and maiden the thing!

David Mellor12/01/2019 09:36:03
1188 forum posts
599 photos

Nice work, Nigel. Good job.

If you like how she flies, you are in an incredibly good position to redesign her to get the CG bang on with a lighter structure. If you could bear to, that is......

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