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RM Aerobat

Another electrified 1980s design

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Nigel R14/05/2018 09:37:16
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1202 forum posts
274 photos
Hello all
 
This summer's project is going to be this 1984 design by Dave Boddington. It is Part 2 of RM's Flight Training Course, as it was billed - of which part 1 I built over last winter (**LINK**).
 
The Aerobat was designed for glow (no surprise, given its vintage) of around 0.25. One of my favourite size of airframes. Big enough to fly nicely, they seem to make low stress models which need little notice to get ready for a trip to the field - which means I fly them a lot.
 
I'm planning on an electric install on my build.
 
This design has yet to make it to Outerzone - unusually! - so I found on Ebay an untouched copy of the plan (not even removed from the two issues of the mag) for the princely sum of £5.
Bob Cotsford14/05/2018 09:41:39
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7195 forum posts
406 photos

Nigel, it's a pity you didn't ask (or if you did, that I didn't see it) as I have both of the RM plans, almost certainly with the accompanying articles. At least you have the rest of the mags to read now and that's probably worth the fiver.

Nigel R14/05/2018 09:48:04
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1202 forum posts
274 photos

rm_aerobat_a_s-l1600.jpg

rm_aerobat_b_s-l1600.jpg

Nigel R14/05/2018 09:49:06
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1202 forum posts
274 photos
The airframe in numbers:
 
Span 48in
Chord 9in
Area 432sqin
Length 38in
 
I think a target of 3lb (or close to) including lipo is realistic.
 
This will give good manners (I hope!)
Nigel R14/05/2018 10:20:09
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1202 forum posts
274 photos

Bob - no worries, I figured it was worth a fiver for a couple of issues.

Nigel R14/05/2018 10:21:09
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1202 forum posts
274 photos
I have my favourite ways of doing stuff, so, I figure I would list my intended changes to the plan upfront.
 
First, tailgroup.
 
All sheet tailplanes are just dandy if you have a nice reliable supply of light quartergrain wood in a local shop from which you can pick and choose good sheets... which I don't. So instead of sheet, I will build a framework of 1/8" x 1/4" strips, and sheet it with 1/16" both sides. Another nice thing about this size airframe is the tail can just about be got from a single sheet of 1/16".
 
Fin will be the same. Controls will probably be solid wood - too small/narrow to make it worthwhile doing a sheeted framework.
Nigel R14/05/2018 10:23:43
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1202 forum posts
274 photos
Wing:
 
This is a perfectly reasonable design, if you like those egg box spar things, which I don't. The RM Trainer design (which I suppose was meant to be easier to build compared to the Aerobat) used a perfectly good I-beam made from spars and webs. You can guess which I will use.
 
I also like a fully sheeted wing. I find it no different in speed to fiddling about with cap strips and such like, plus it means covering is easier.
 
Wing retention will be dowel and bolts. A 1/4" dowel will sink into a slot in the root ribs, if I make them from liteply.
 
Ailerons will get a micro servo each. Torque rods would make the fuselage radio install a bit of a headache - there is not much room in there. Besides, servos are comparatively cheap these days anyway, it's not like one servo costs a weeks wages.
 
Lastly, I don't see much point building this one with dihedral and a semi symmetrical section - I just did that with the Trainer build. I am going with a dead flat wing and a symmetrical section. I've picked one from the library of airfoils in the Profili/DevWing software - RAF30 - which is a 12.5% section developed way back in the 1930s (and with a couple of % camber forms the section used on a few British WW2 designs).
 
Ultimately, all very standard stuff, just a bit different to the plan.
Nigel R14/05/2018 10:26:35
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1202 forum posts
274 photos
Fuselage:
 
Generally, I will build this in the way shown on the plan. I am expecting to use slightly lighter grade wood here and there (in this day and age with light radio and electric motors etc etc), and the ply doublers will get some big holes for lightness.
 
The lipo will sit on a liteply tray; the tray to run from firewall back to rear of wing root. Servos also go in the tray; RX can sit on it too.
 
I haven't decided yet how to get at the lipo. A hatch somewhere - no way am I removing the wing every flight - but whether that is top or bottom has yet to be decided. I have a feeling an underside hatch will be easier to build.
 
 
One big outline change - the tailplane will be moved to the top of the fuselage sides. First, I think it will look better there, second, it is out of the way of the ground - this is a taildragger, after all.
Nigel R14/05/2018 10:28:32
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1202 forum posts
274 photos
Motor stuff.
 
This is the same drivetrain (nearly) as my Trainer build;
 
4S 2200 lipos.
3536 motor, 1050kv.
40A ESC
9x6 prop.
 
This pulls approx 400W (flat) and 480W (freshly charged) on the bench, which I think makes it equivalent (roughly) to a 0.28 glow.
 
The motor will mount on M4 standoffs; this means the ply firewall stays in the location shown on the plan for IC.
 
That's all for now. I'll check back in when there is something useful to report from the workshop!
Nigel R18/05/2018 23:34:27
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1202 forum posts
274 photos

A little progress.

20180518_225244.jpg

Kiting up for the wing. Skins joined. Spars, leading and training edges stripped. Ribs cut and ready. Mixture of liteply, 1/8 and 1/16. Thick ones by servo bay and tip, liteply at root to take the dowel.

Piers Bowlan19/05/2018 06:08:38
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1238 forum posts
36 photos

With a different wing section, wing and tailplane structure plus leccy conversion, some might ask; Is it still an RM Aerobat? devil wink 2

Always enjoy following your builds Nigel, I am sure it will be better than the original. yes

Nigel R19/05/2018 22:00:14
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1202 forum posts
274 photos
Piers you are quite right. How different can an airframe be before it is something else? This, perhaps it is at least a close blood relative of the aerobat. Plans are always just a suggestion of how to build and Structural changes you could argue away but I guess the aerodynamic changes are what truly starts am airframe down its own path.

And thank you! I will endeavour to keep the log as interesting add I can.
David Mellor20/05/2018 09:00:44
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662 forum posts
230 photos

Following with interest, Nigel.

I too have often wondered how different an airframe can be before it becomes something else.

One approach to answering the question is to use the 2-D planform as the "test". If it is the same planform, then the Neutral Points and CG ranges will be identical (these being independent of airfoil section, weight, power and construction details), so you could say the plane is "true" to design. So yours would be bang on by that test!

Dave

Nigel R23/05/2018 08:28:11
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1202 forum posts
274 photos

Hi Dave

Good to have you along smiley

Going on the 2D planform, mine will as you say be identical. So perhaps we will call it an Aerobat after all!

An Aerobat Redux, perhaps?

Anyway. Sadly no more photogenic progress, but I have now put bevels on the strip wood where needed, and drawn out the spar & rib locations on my board (it's just a plasterboard sheet on top of my bench, and this model will likely be the last before replacing with a fresh piece). I should have some structure by the next update...

Nigel R23/05/2018 22:34:05
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1202 forum posts
274 photos

Did a dry run on basic wing structure.

20180523_220434.jpg

Which showed that I had slightly over sized spars. Our undersized slots perhaps. So now I have slightly smaller spars which fit their slots. Servo cable tubes have to go in at this stage too. Also cut a jig strip for the te. All small stuff but needed to be done before moving on.

Mark Pidduck26/05/2018 09:53:05
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29 forum posts
7 photos

Looking good, Nigel, I can't see you for balsa dust! I have a little time this week, hopefully I'll get a little more done.

Nigel R04/06/2018 09:14:05
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1202 forum posts
274 photos

Hi Mark

You see you say that, and then I don't get anything done for a week!

Anyway. Had a couple of hours to make a bit more progress last night. Thought I'd take a shot of wing structure half way through framing up. Standard procedure is to pin down a couple of jig strips, and then I just use some metal blocks (retrieved from the bin at work) to hold things in place while I whip round with the cyano. I try to avoid pinning down the structure if possible. Another point is, I try and keep the spar slots as an ever so slightly slack fit, this means the LE and TE can be aligned and glued up, then the spars get a whip round with the cyano in their final position afterward.

20180601_220127.jpg

The open wings then got their lower skins attached. Contact adhesive is the tool of choice here. As ever, I use a pair of cocktail sticks drilled into the overhanging spars to keep the skin lined up while I'm getting the glue on both surfaces and whatnot. The framework is just dropped in place over the jig strips with just two pins to hold the spar in proper location, and I press lightly down to get the contact adhesive stuck everywhere, work over the structure rib by rib to make sure its proper stuck. Quick and easy.

20180603_214127.jpg

Last job, I cut some webs from the wasted wing skin material. I've cut these at the right height to avoid having to sand them off after fixing them in. Once these chaps are in the spars won't bend at all, so the wing will be weighted flat on its jig strips while they are stuck in.

20180603_220621.jpg

After getting the webs in place, the wing will be ready to have the servo gubbins sorted out. I'm expecting to mount the servo flat in the wing, so this will probably be a 1/16 ply plate with some liteply/balsa mounts for the servo screws.

Nigel R05/06/2018 09:37:28
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1202 forum posts
274 photos

"After getting the webs in place, the wing will be ready to have the servo gubbins sorted out."

Minor change of heart, thought I'd get the servo stuff sorted first.

20180604_223229.jpg

I cut curved supports (to go at the side of the bay, following the rib profile) out of some 1/8", about 1/4" deep. These were used to position the length of 1/8" at the back of the servo bay (supports not glued in at this stage). Then the sheeting is carefully cut away and sanded back. To complete this job, I need a hatch, then I can figure out what depth to glue the side supports at.

 

Edited By Nigel R on 05/06/2018 09:37:59

Nigel R13/06/2018 09:05:16
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1202 forum posts
274 photos
The trouble with this nice summer weather is that flying and repairs get in the way of building...
 
The last few hours on the bench, in between that other RC stuff, have been sunk into the somewhat time consuming job that is wing servo hatches.
 
First off, 1/32" ply cut and fitted in to the openings in the wing panels:
 
20180607_222506.jpg
 
Next, front and side support rails were positioned in the bays at the right depth to make the hatch just a nose beyond flush (to allow for covering):
 
20180612_223250.jpg
 
From the inside:
 
20180612_223301.jpg
 
After a bit of careful measuring, the "rails" the servos screw on to are stuck to the hatch. These are about 1/2 x 3/4, cut from some scrap 1/8 ply, doubled up behind with some 1/8 balsa to give a bit more support. At the glue joint with the hatch, there's an extra bit of scrap balsa for moral support:
 
20180612_223330.jpg
 
I cut the slot using a dremel sanding drum, again careful measuring before cutting:
 
20180612_223410.jpg
 
Looks ok. And all together now:
 
20180612_223525.jpg
 
Nearly there. I have a few squares of ply to add to the wing (to receive the screws going through the hatches). And servos have to be screwed to the hatches.
 
Quite a fiddly job, all told. If you want a quick build, don't do a hatch with a flat mounted servo, mount the servos upright in the wing! However, doing it this way does look satisfyingly tidy when it is complete.
Nigel R14/06/2018 10:01:05
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1202 forum posts
274 photos

Finished up the hatches, pictures tell all:

20180613_230532.jpg

20180613_230608.jpg

20180613_230827.jpg

However, with about 6 hours sunk into these hatches (!), I'm wondering if I might take the shortcut of using SLEC's moulded servo mounts next time...

I also spent some time fiddling about finding those prop shaft reduction thingies and reaming a prop to get this lot sorted out:

20180613_230913.jpg

One nice shiny spinner and prop. Why the old IC prop? I have a drawer full of them, all perfectly useful even if they are a little beefier than the electric only props.

So, of course, the spinner and prop are bored suitable for 1/4UNF (as was the spinner nut), and the motor's shaft is 6mm metric. Fortunately the largest size on my reamer (3/8) seems to nicely fit those reducer widgets that come with electric props, so by enlarging the holes I could fit the appropriate spacer.

I also cut some 50mm (ish) lengths of M4 studding, ready for installing the motor.

It was jumping ahead of things a bit, but I fancied a short break from the woodwork on the hatches.

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