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RM Aerobat

Another electrified 1980s design

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Piers Bowlan12/01/2019 15:43:03
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'Redesign her', David? With a new wing structure, tailplane position, leccy conversion plus the latest bulkhead modifications, the model looks redesigned wink 2. All Nigel needs to do now is fly her, when the weather Gods allow!

Former Member12/01/2019 16:38:48

[This posting has been removed]

Nigel R13/01/2019 12:02:06
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2722 forum posts
445 photos
Ha yes I'm with Piers on that one! I simply don't have the time to build a second fuselage.

Servos are now set up, lead is glued in place - all that remains is to offer a sacrificial chicken.

Roast tonight

Edited By Nigel R on 13/01/2019 12:02:50

Nigel R15/01/2019 13:52:18
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2722 forum posts
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Battery hatch left on the bench at home.
 
Cowl not attached - didn't get time last night to trim the ply ballast cover.
 
Spinner not fitted
 
Despite all that, aviation was committed.
 
First thing observed, the prop was horribly out of balance. As yet, I have not put RTV on the wing saddle or canopy and the latter vibrated loudly in sympathy. With the missing hatch, a resonant effect at any speed made the noise even worse. During the second flight, the balsa chunk at the rear of the lipo came unglued.
 
Let's ignore the noise for now. As for flight itself, she goes very nicely. Take off was a piece of cake, plenty of go, and up she went. Very well behaved at low speed, as a plank wing should be. A nice moderate approach speed, with no real tendancy to drop a wing unless provoked. Smooth, surprisingly so, and very neutral. A few goes at the dive test show that a couple of degrees down thrust is about right, ditto for stall turns, showing no particular leaning to go one way over the other.
 
Could use a little more aileron, and a little more rudder. Knife edge is nearly there, but needs the extra few degrees movement that I didn't have. A slight pull to canopy was observed which I think may mean the CG could have gone a bit further back (!). The extra rudder will help initiate a snap cleanly. As will a few degrees more of aileron.
 
All that said, she is good, very good. I believe this will be a go-to model. Everything fits in the boot - no messing around putting seats down - and just two plugs and two bolts to assemble. Great aerobatic performance is icing on the cake.
 
A few snags to correct now, but, happy with that. Looking forward to a proper session with everything sorted out.
Nigel R15/01/2019 14:02:19
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2722 forum posts
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Couple of quick stats from the shop log, if anyone is interested

Total build time; 120 hours.

Fuselage took approx 40 hours, the wing 35, tail end 8, covering and rigging about 25, the remainder was sorting the CG and some other small jobs like soldering the electrics.

Overall cost, 150 pounds. I already had RX & servos, everything else was bought for this project.

Former Member15/01/2019 16:58:40

[This posting has been removed]

Nigel R15/01/2019 17:19:45
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Many thanks Dave

Engineering habits die hard!

The actual shop log is 143 entries in a spreadsheet, each being a different small (sub 2 hour) job. To make it even worse I also list them out ahead of time and estimate total required time for the project, as things go along the spreadsheet can be used to predict roughly when I'll finish the job - I knew almost for certain when I started back in May that this one was unlikely to fly before Christmas!

It sounds like way more bookkeeping effort than it actually is. A couple of minutes to update after every session in the shop. An hour or three up front to figure out the job list, and at the same time I'll run up a bill of materials and keep tabs on spending. Any notes on the design, structure, or whatnot get another tab in the spreadsheet.

Hmm, now, should I have admitted that all in public, or not!?

Former Member15/01/2019 18:52:16

[This posting has been removed]

Nigel R17/01/2019 09:12:19
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445 photos

There is one question that I had in mind during this build - "is this a suitable second RC model?"

And after having now finished the build, I would have to say, fairly emphatically, no. This design was a swing and a miss by Boddington.

Why?

Simple - it's too fiddly to build.

Someone progressing from a high wing box trainer is (most likely) still refining their build skills (I'm still refining my build skills!)

The wing, as drawn, was easy enough for a novice builder. That's ok, I have no particular problem with the wing at all. Great.

The fuselage, however, has way too many opportunities for mistakes. And a couple of very weird design choices that make no sense.

It could easily have been simplified:

Low wing layout - no awkward separate cabin area to build, and, more crucially for a novice - much more space for the critical radio install.

Simpler nose area - flat 1/4 sheet for the front deck/hatch, round the corners off, job done. Why use 3 bits of thick block? That makes awkward angles for a novice builder to cut. Especially when the plan article then seems to show the surfaces left flat. As for the cowl, that could remain as is or be substituted with some cheek pieces, like the archetypal cheap and easy ARTF front end.

Ditch the hardwood bearers and use a regular plastic motor mount. The firewall was already 1/4 ply. The structure really doesn't need the hardwood bearers for strength.

The plan was unclear about what to do with the tank hatch, or if there was a hatch at all.

The tail area was a mess. The tailplane was underslung, yet, still had to be built with an elevator joiner, because the fin, for some inexplicable reason extended down into the elevator. If you must have the tailplane on the bottom, make the elevator one single piece, and move the fin up a half inch. Madness!

In summary, wing, great, fuselage, poor.

Actually I find this fairly typical of Boddington designs, the aerodynamics are generally ok, particularly his conventional sport types, if you can ignore his "size 10 shoe" type wing sections, but, the structures are often fussy and far from optimal. They feel a bit "prototype" to me, as if they needed a "proof reader" to do a red pen job.

Nigel R17/01/2019 14:27:53
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2722 forum posts
445 photos

Few shots at the field today:

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Nigel R17/01/2019 14:44:41
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2722 forum posts
445 photos

Today was cold. The Met office website suggested 4 or 5 degrees and wind of 10 gusting 27, I believe the 27, I'm not sure I believe the 10!

Anyway.

This visit, I had the thing completely assembled - with hatch and cowl. The balanced prop has stopped the drumming I found on the maiden - good. Fitting the cowl and hatch has much reduced the odd resonant noise, although not stopped entirely - I suspect I should tidy up the air passages through cowl and ESC area, there are a few sharp lips.

I enjoyed the flights immensely today. Ignoring my comments about whether or not this is a suitable build for a neophyte, it is certainly a nice performer. Very neutral and capable. The conditions were a bit borderline, hardly smooth air, but the Aerobat handled it well.

I had wound up the travel adjust on the rudder. This gave sufficient throw - near 45 deg - to obtain a nice knife edge. Only a fraction of roll correction (it rolls with rudder) and pitch (a small pull to canopy) are needed for a horizon to horizon knife edge.

Balance is just dandy at 1/3 of wing chord. Stall is noticeable, but perfectly manageable, and not sharp or unpleasant. and no tendency to drop one way or other. As one should expect with a rectangular wing planform. The increased rudder now makes snap rolls a cinch and gives a nice spin. It recovers instantly. Snaps are better to left than right, to the left I can release aileron and hold rud/ele for a true snap, to the right it needs a little helping hand from some aileron movement. Fairly typical for a rectangular wing, in my experience.

Lateral balance is good, and loops are absolutely true - always nice. Rolls are a decent speed in both directions. Inverted needs the merest breath of forward stick at mid throttle - perfect.

Landing is also a piece of cake. She slows up quite easily and can be flared very nicely for a 3 pointer without fear.

I will need to shim the motor for a touch more right thrust I think. I don't think much other trimming will be needed.

Overall, it's good, very good, this will be a regular visitor to the field laugh

alex nicol17/01/2019 16:01:19
243 forum posts
6 photos

Nice looking plane

Peter Christy17/01/2019 16:17:50
1375 forum posts

I built an stock Aerobat for my son many years ago, as his first "full-house" model. I found the build quite painless. The only slightly fiddly bit, IIRC, was that due to space issues, the free plan in the mag had to be cut and joined, as otherwise it wasn't big enough to accommodate the fuselage.

Ours was powered by a Super-Tigre 21, with home made radio gear. It flew well, and despite only being 11 years old at the time, my son was soon doing all the fuelling, checking and starting himself! He learned a lot from that model, which I thought had delightful handling.

It finally met its end due to the dreaded "black wire corrosion".... blush

I thought it was a cracking little model, with excellent ground handling characteristics, as well as being an extremely good flyer. Landings and take-offs were very easy, and an ideal follow on from the rudder-and-elevator trainers he had flown up to that point.

His next model after that was an Acro-Wot, another fine flyer, for which the Aerobat proved to be perfect preparation.

So I would have to respectfully disagree with Nigel's question as to whether its a suitable step on from a basic trainer. Ours fulfilled that role admirably, and was remarkably robust too! The fields we flew from back then were not very forgiving....!

Must see if I can find a picture of it, somewhere....

--

Pete

kc17/01/2019 16:32:23
5859 forum posts
168 photos

Well somebody has to say something to pacify the lynch mob of Boddo's fans looking for Nigel!

The Aerobat was part of the flight training course which was designed to get more difficult to build and fly with each design. I found no difficulty building the Aerobat as a second RC model after completing the RM Trainer. I think it succeded in teaching various building techniques.

But it could have been simpler as Nigel says. Frankly it was a little too small and out of date. At the time -1984- trainers were mostly .40 size like Hiboys etc so a .40 Trainer and Aerobat would have been better. Hardwood motor bearers and wing bands were also old fashioned apart from use on trainers. It's the dowels for the bands that really get in the way. The articles failed to inform beginners that Veco 19 used by Boddo and his pals were much more powerful than a OS .25 Max that the typical 1984 beginner was sold. And as a beginner I built my first few models far too heavy which made it worse.

Like most Boddington designs they were scale ups or down from his previous designs and that is both a delight and a curse. They mostly use the same building methods which can be handy but also restricting in that the construction is not optimum for every size.

Credit for the wing section for many Boddo designs should really go to Vic Smeed because he used them first.

Nigel R17/01/2019 17:08:29
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I thought that would go down well!

Once you've got the thing built, it's all good, I'm not denying it flies well or (as designed) would be right as a aileron trainer. Thing is Pete, you weren't a learner builder - I imagine you had a number of models under your belt, so to speak. I just reckon a second time around builder would find things unnecessarily tricky.

The primary job this model ought to do is to teach flying an aileron model, and one just a small bit faster and heavier loaded than the trainer.

To that end, the structure ought to be as simple as. Don't dally about the workshop, a thing to get the learner up in the air.

I think kc says it all really, this wants to be a low wing, sized for a cooking 40 - 500 to 550 sq in of wing, box fuselage with a bit of deck on top for looks (or maybe just a big old canopy), large enough to have lots of space for radio install, etc.

Ironically (or not) that was exactly what my low wing trainer was like. It was pretty good at its job. Next thing I had was a Flair Meteor, which was a different kettle of fish entirely...

Former Member17/01/2019 17:38:35

[This posting has been removed]

Peter Christy17/01/2019 19:57:28
1375 forum posts

I knew I had it somewhere.....

His previous model is the half-size Super 60, in the middle of the group. The other Sub-Mini is mine, complete with single-channel and rubber driven escapement! Both the Sub-Minis still exist and fly - and bear in mind that the little lad in the middle will be 39 this year!!!

To be honest, though, I will probably have to retire his Sub-Mini this year. The front bulkhead is coming loose again, and its now so oil soaked I don't think it can be salvaged!

So he went straight from the Sub-Mini to the Aerobat with no problems at all. Its flying characteristics are excellent! BTW, the trophy was from the great Boddo himself, at Old Warden!

As to the difficulty of the build, well, I wouldn't class myself as a great builder. I tend to build heavy - too many years of flying out of rough farm fields. But any trainer, primary or secondary, needs to be robust as well as handling well. On this score, again, the Aerobat excels.

Its around 30 years since I built that one, but I don't recall cursing and swearing anymore during the build than I usually do. Indeed my recollection is that it was all pretty straight forward. I particularly liked the way the undercarriage was mounted, making it both strong and easy to replace! Very important when flying from patches that resemble lunar landscapes......!

--

Pete

Nigel R17/01/2019 21:43:58
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2722 forum posts
445 photos

Now there's a happy fellow! Somewhere, there's a few similar pictures of me, of a similar age at a similar time - I'm 42 now...

For the record, I think Boddo did a huge amount for us lot, well, not to put too fine a point on it, his life was aeromodelling. He had an eye for drawing up good looking airplanes, and churned them out at a rapid rate. He wrote a ton of material to help learners, made some great kits, at some point edited pretty much all of the magazines around. He was a massive influence on the UK aeromodelling scene.

His RC Primer book - which I would still recommend as a 'how to' in building techniques - has the Excellence plan in reduced scale. It's almost identical in size to this. It's also a simple box fuselage low winger and fixes almost all the things I just moaned about!

Anyway. It's a smashing flyer. And whether we reckon (or not) it's an all round good second model is rather moot, 35 years after it was published...

Nigel R23/01/2019 13:56:08
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2722 forum posts
445 photos

Ok, as a sort of a postscript to the log, I have been flying this a few times now and got 15 flights on the airframe.

I have - physically - tweaked almost nothing really.

I am now certain a little more right thrust is needed. A pair of washers to shim the motor, should do it, nothing much needed.

Power level is good. At full throttle she will go vertical and keep going as far as needed; there is not that much vertical acceleration, but, it is enough to power through tall vertical maneuvers. It has around 130W/lb at the 3lb6oz take off weight.

I have played a little with several mixes to reduce the corrections needed for knife edge. So far I have added two mixes, one at 5% rudder -> opposite aileron, and the second, 5% rudder -> down elevator. It's not much, but, it has reduced the workload during a slow roll by a large amount.

Today was only the lightest of breeze at the field, perhaps 4 or 5mph. Despite not having a lot of draggy stuff hanging out in the breeze, she seems to slow down by herself easily enough. I guess that's the plank wing at work - very different to a slick pattern type model. I'm finding it makes a neater job of landing to take advantage of this behaviour - she seems best when coming in with throttle completely cut, and then flared out just prior to touchdown, vs slowing down using elevator and bringing her down using the throttle.

I'm enjoying flying it, and it'll be a tough gig to find a better sport model.

Nigel R29/01/2019 14:49:51
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2722 forum posts
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I have now shimmed the motor as eluded to above. This seems to work rather nicely and the model displays no particular pull to any direction when pulling into a vertical, and if balanced right it will prop hang (just) without tipping. Of course my cowl is not quite lined up now!

A smidge more rudder to elevator mix was also needed, to tidy up knife edge.

I changed the antique Kavan yellow prop to a Graupner Super Nylon prop, it is certainly more rigid and perhaps a touch more efficient.

Looking now at trying to dial in the aileron differential. I think it may need a tiny bit more up than down. Or perhaps my geometry of servo horn and aileron horn is producing an unintentional differential in the wrong way. Either way a couple of flying sessions should sort it.

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