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RM Aerobat

Another electrified 1980s design

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Nigel R18/06/2018 09:03:26
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1560 forum posts
336 photos

Did a couple more jobs these past few days.

First, I ran the motor. However, I'm a bit concerned. It seems a little spicey for its rating. On that Graupner 9x6 it shows 540W (steady, for about 15 seconds) on the 4S2200 pack! I was expecting about 100W less. I wondering if I've actually been given the right variety of the motor. I have a Hobbyking orange 3536 1250kv so will try that and compare. I think 540W will be faintly ridiculous in this model, perhaps I need to double check my result, or try it without spinner or with a different 9x6.

Second, back to the woodwork, I got the webs in place:

20180615_223807.jpg

and then buttoned up the wing with the top sheeting:

20180617_231025.jpg

Rather rubbish photo, but hopefully it shows the complexity (or lack of) in my wing building environment - some lines on the board (ribs, spars, jig strip position) and the two jig strips themselves.

As usual, a job for contact adhesive. The sheeting is aligned with a couple of toothpicks drilled into the spar where the spar overhangs. The reverse of the sheeting is marked for rib positions and then I use a 5ml syringe to get some contact adhesive on both sides. Leave a few minutes to dry (at this time of year, almost no time at all). Then make sure the wing is lined up over its jig strips, get the sheeting carefully aligned with the toothpicks, and press down gently in the jig, starting with the spar and working forwards and backwards, out from the centre of the panel. Presto. Providing the board is flat and the jigs are located correctly, that is one nice flat warp free panel and with sheeting all over it, (1) it ain't changing shape any time soon and (2) it is very, very strong.

The both panels together are weighing in at approx 6oz at this point. Happy with that.

Time wise, I'm guessing that I'm about half way done with the wing build.

kc18/06/2018 18:06:30
5574 forum posts
163 photos

Going to a motor with 1250kv on 4S will mean it revs to about 20,000RPM. It would be almost 17,000RPM with the 1050kv anyway. Needs lower kv for 4S or change to 3S I would reckon.

Nigel R19/06/2018 10:56:39
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1560 forum posts
336 photos

kc

I've not seen quite those revs. By the time the lipo is loaded up the voltage is down and with a decent prop load we're at around 80% of the "1050kv" figure - measured at about 13k. I might drop to a 9x5 with this motor to get the power to the right level.

Alternately, I have also measured the 900kv motor on a 10x6 at 470W. That might make a better fit here, I think.

edit - I did also check the 1250kv motor I have, that showed over 650W / 45A on the 9x6, so clearly the 1050kv I have is marked "correctly".

Edited By Nigel R on 19/06/2018 11:11:40

PatMc19/06/2018 16:47:30
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3802 forum posts
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Posted by Nigel R on 19/06/2018 10:56:39:

edit - I did also check the 1250kv motor I have, that showed over 650W / 45A on the 9x6, so clearly the 1050kv I have is marked "correctly".

If you're in doubt about the true kv figures why not measure the no load rpm & input voltage then do the maths ?

Nigel R20/06/2018 10:23:49
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1560 forum posts
336 photos

good shout Pat

I guess I could use a couple of strips of dark tape on the motor casing, for the tacho to read?

PatMc20/06/2018 11:51:40
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3802 forum posts
479 photos

Yep, I use white but any contrasting colour will do. I use a 2s 2200 lipo then a 9.6v 2000 nihm to keep the rpm within reasonable limits and by applying different voltages be reasonably confident of the result. It's also important that the batteries used have the capacity to hold a steady voltage whilst the readings are recorded.

BTW I think I'd use the 900kv motor as the greater dia prop will give better climb performance & smaller changes in airspeed during manouvers for smoother aerobatics.

Nigel R20/06/2018 14:39:31
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1560 forum posts
336 photos

I'm inclined to agree about the 900kv motor. Trouble is, it's already in use. The D3536 are only £17.50 or so from HK European warehouse (bizarrely, they're more expensive from the UK warehouse).

I'll see what the 1050kv tacho results are first, and go from there.

Paul Blakeborough 121/06/2018 13:57:58
7 forum posts
7 photos

A blast from the past... this is the RM Aerobat I built in 1987 (With my better half as then) the model was powered by an OS 25 FP and took me miles forward in flying experience. I built it so I could take the U/C off and fly from longer grass if I had to. Eventually it was moved on to Roger Bellingham (RIP) who had watched me using it from time to time and always liked the colour scheme. Another great design by D Boddington if I remember rightly.424813_109464849183752_2005747938_n.jpg

Piers Bowlan21/06/2018 17:06:19
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1387 forum posts
37 photos

Yes, a nice scheme indeed Paul, are you now tempted to build another for electric or 4 stroke? Also, would you build in the dihedral or leave it out if you built another?

Paul Blakeborough 121/06/2018 18:14:24
7 forum posts
7 photos

No, not tempted now. It had a perfect position in my progression. A good flying machine and yes I would leave the dihedral in.... it adds a little stability in the correct attitude, unless you want it to fly and behave like a Cassutt racing machine.

Nigel R02/07/2018 09:02:53
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1560 forum posts
336 photos

Nice Aerobat Paul!

Not much photogenic progress to report from my bench, I'm afraid. Weather has been too good to spend much time building smiley

Despite not looking like much, sometimes these small jobs can take time. The overhanging wing sheeting was knocked back in line with the sub-LE and sub-TE strips, using a long sanding block, ready to attach the LE and TE proper. These were stuck on and bevelled at an appropriate angle (using masking tape to protect the wing sheeting). After that, aileron blanks are cut, and then hinged:

20180701_224614.jpg

Next job is to tack glue the blanks in place, and then start sanding away for a couple of hours, to get everything to a nice shape.

Nigel R10/07/2018 09:18:59
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1560 forum posts
336 photos

Making lots of dust this week.

From:

20180701_224626.jpg

to:

20180709_224624.jpg

LE, TE and ailerons are all shaped. I have to be very careful not to ding the panels now!

Excuse the fairly poor picture, it is only to show the 1/2" wide scrap strips of 1/32" balsa that I use to tack glue the aileron in place (to get a good hinge gap) before attacking it with the sanding blocks:

20180709_224641.jpg

It doesn't look like much, but taking a blank down to a nice TE shape can soak up a good few hours of careful sanding. All interspersed with lots of checks against straight edges and a TE template, to make sure I'm approaching the right shape and not removing material in the wrong place, or worse yet introducing a warp. Not the quickest of techniques to get an aileron, but it does giver a nice end result. Quick therapeutic as long as you enjoy the woodwork. I already hinged the aileron, so they will have a perfect fit after being cut free and beveled.

Thinking about it, it's probably quicker to sheet right back to the TE and cut the aileron from the wing. That wouldn't work with strip ailerons on something this size, but a barn door shape, that would be ok on this size of panel I think. Food for thought on the next build.

Next jobs, tidying up the root and tip, and then joining the panels.smiley

 

Edited By Nigel R on 10/07/2018 09:22:05

Nigel R13/07/2018 12:39:12
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1560 forum posts
336 photos
A little more dust-making, just knocking the overhanging sheeting down to length and making sure the root is nice a square ready for joining.
 
The LE needs a bit more shaping. It's close but just not quite the right shape yet:
 
20180712_230032.jpg
 
Panels end to end. I've checked the root rib with an engineers square from a few directions, and checked the LE along a straight edge when the panels are pushed together. So far so good:
 
20180712_230015.jpg
 
Weight at this point:
 
20180712_225859.jpg
 
Happy with that, it's 6.5oz in old money.
Piers Bowlan13/07/2018 14:11:00
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1387 forum posts
37 photos

A very tidy build Nigel, light too. Fuselage shouldn't take too long to put together. Have you decided on the final motor option yet?

Nigel R13/07/2018 22:44:45
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1560 forum posts
336 photos
Thanks Piers.

I'm not sure on the motor front. I might run with the one I have and just trim the prop down 1/2" to reduce the watts a bit.

The fuselage is pretty standard stuff on this one so hopefully it'll be quite quick.
Nigel R20/07/2018 12:13:08
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1560 forum posts
336 photos

Well, another week, another small chunk of progress. Building goes slowly in this sunny weather!

First up, I decided to get another 900kv motor and run a 10x6 prop. I know this combination has the power I am aiming for.

On the woodwork front, I cut the ailerons free of their temporary attachment. They were then marked up for the fixed root and tip pieces, then it was out with the razor saw to cut them off. I made a few strips of 1/32", and then permanently attached the fixed portions. Lastly, I fettled the root rib and joined the wings with regular wood glue.

Fixed bits and a nice small hinge gap:

20180717_220107.jpg

20180717_220121.jpg

Time to add the centre bandage next, and then put the wing aside while I get on with the fuselage.

Nigel R26/07/2018 23:06:12
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1560 forum posts
336 photos

20180722_222238.jpg

Nigel R26/07/2018 23:06:39
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1560 forum posts
336 photos

20180726_223221.jpg

Nigel R26/07/2018 23:14:05
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1560 forum posts
336 photos

Made some tail feathers, the fixed bits of tailplane and fin. Simple enough, 1/8 x 1/4 strips to form the basic shape and then sheeted with 1/16.

Made a daft mistake on the tailplane and made it half inch too wide, front to back. If course i only noticed after sheeting the framework. Easily fixed by slicing off some of the leading edge.

I also got the wing bandage done. Two different width layers of glass tissue stuck with plain pva. Then a couple of further coats to fill the weave a bit.

Nigel R06/08/2018 10:15:20
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1560 forum posts
336 photos

20180802_225928.jpg

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