Roebers Twin Astir (1980s) - RC Groups
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Aug 10, 2017, 02:13 PM
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Phaedra's Avatar
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Roebers Twin Astir (1980s)


Almost by coincidence I found this vintage model on a German second-hand webpage. These models are very rare, but finding one that was not even built yet, is even more rare.
Since I flew the real one in the eighties, I quickly considered effectively buying it.
These are the only pics I have of the one I flew:



When the price appeared to be only slightly above 200 Euro, I sent an email to the owner to seal the deal.
It took a bit of time to ship it to my place, but about a week later I had this big box in my living room:



After unpacking, I was amazed by the size of the fuselage


The canopy shows it age; there are some clearly visible spots that will need some treatment:



A fixed wheel was already installed:


At first sight, it doesn't seem to be the correct scale size. I put a retractable wheel next to it for comparison

I will have to do some research to find out which is the correct size, but I'm really considering a retractable wheel, whatever the size.

For the elevator, some sort of mechanism has already been built in; it looks a bit medieval if you ask me:

I think I will replace this by a servo mounted in the vertical stab....

The unpainted fuselage looks a bit beat-up...makes me wonder if all these kits were like that, back then.


The wings are still in a very good shape, no warping, not even on the ailerons:



They have mechanical spoilers already built in:

I will be replacing them for several reasons:
- they extend only about 1 cm
- they are operated by steel push-wires, and these will have to be connected to the servo(s) in the fuselage before each flight. I really dislike the idea of that.
Modern-day electrical spoilers weigh less, require no extra servos, and they extend to about 3cm.

On the bottom of the win, a cutout is provided to install the mechanical aileron control rods system.

for obvious reasons, I will replace that by servos installed in the wing. I will be moving their location a bit closer to the center of the aileron.

A nice included extra is the original "manual":


And then there were the horizontal stab and rudder:



I couldn't resist assembling the parts, to get an idea of the looks, the weight and the balance:


I was pleasantly surprised by the weight of the different parts; total weight as in the picture is 2,5 kg. When I add the weight of the other required electronics, motor and battery, I should be able to keep the weight around 4 kg.
Last edited by Phaedra; Aug 10, 2017 at 03:02 PM.
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Aug 10, 2017, 02:46 PM
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First thing I did was to remove the mechanical spoilers, to get exact measurements to order a new set.
That proved to be harder than I thought...they were glued in pretty good.



I don't think they came out unscathed....


Next point of attention were the wingtips. From what I read about this model, I suspect it came with two options: the short wings and the extended wings.
With short wings, the span is 3,5m, and that is what I got, with the tips glued on:



If my suspicions were correct, there should be some small tubes glued into the wing, to allow for wing extensions to be connected. The only way to find out, was to due some minor surgery and cut off the wing tips:



I was wrong....

But that shouldn't stop me considering adding some wing extensions myself.
I intend to use this glider purely for thermaling, and did some calculations of wing loading. I measured up the wings and calculated the wing area as they are now.
For a weight of 4 kg, I would have a wing loading of 59 g/dm2.
If I extend the wings to 4m, it would come down to 54,5 g/dm2. That is not spectacular, but for thermaling, it might help.
Only downside would be that the ailerons would be a little less effective. Since I have no intentions at all to perform any aerobatics, that wouldn't really be a problem.
I still have time to think about this one....

Next step is motorisation...
Last edited by Phaedra; Nov 13, 2017 at 11:05 AM.
Aug 11, 2017, 09:46 PM
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pete914's Avatar
Nice!
I love the classic lines of the Grob 102. What a workhorse with solid XC performance in the full scale.
I hate to see the nose get cut for a motor, but it's the simplest way to go.
Sub'd
Aug 12, 2017, 01:44 AM
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I flew the G-103 type, built in 1980, and from what I remember, its response was pretty sluggish. A very heavy glider, requiring the full length of our grass runway to get towed up...we even had to push it before the runway threshold, to get that bit of extra runway. Later we bought a Piper Pawnee and that solved the towing problem for then.
But compared to the Ka7 I learned to fly in, it was some sort of Rolls Royce in terms of comfort And the first plane with a retractable wheel I ever flew.
I remember one flight in France, where we forgot to deploy the landing gear when we reached end of downwind leg. The French guy on the radio kept shouting "le train, le train!" (French for "landing gear", but also for "train"), and my instructor's first response was "what's he shouting about, I see no trains here?". Luckily, we still had base leg to come to our senses


I won't be cutting the nose, though. I intend to insert a longer shat in the motor, supported by a ball bearing in the nose.
Aug 12, 2017, 08:47 AM
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According to the drawing of a Twin Acro that I found, the wheel diameter should be around 1/20th of the fuselage length (nose to forward side of rudder).
Measuring up the model, my suspicion is confirmed: the installed fixed wheel is too small: it is 55mm diameter, while it should be around 72mm.
The retractable wheel I have is 70mm
Aug 12, 2017, 09:02 AM
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Motorisation has been another challenge so far.
I want to avoid any excess weight, and so the motor plus battery in the nose should be able to balance the airplane. If the battery has to be placed closer to the wheel, that means that it is too heavy.
A 4S setup is perfect on weight for balance, but a bit too marginal on thrust if I want to start this model from a dolly on a grass runway.
A 6S setup would give it the thrust of a rocket, but when I try to balance the model with the 6S setup that I have lying around, I have to put the battery over the wheel.
While the 4S setup weighs 730g, which appears to be the perfect balancing weight, the 6S setup weighs in at 1150g, more than 400g heavier. If I want to keep weight around 4kg, I can't afford to put in a 6S setup. (which would be overkill anyway).
Whatever 4S setup I calculated, I can not get the margin on thrust that I think I need. Some motors even get listed with expected thrust of 4,9 kg on 4S, which seems really unrealistic to me. Especially when eCalc gives me a number below 4kg, which in real life might come out even lower taking into account propeller and battery quality. It's not that I need a 1:1 thrust to weight ratio, but on the grass runway I like to have a safe margin on thrust.
So I started looking for a 5S setup, and started simulating with eCalc.
With a Turnigy 4250-500kV motor with a 16x8 prop, I would get 4,6 kg thrust calculated. Plus the total weight of this setup is very close to the ideal weight needed to balance, for 1100W of power.
I think I will be doing some research now on some real-life experiences with this combo, see if I can find any....
EDIT: I quickly found that these last numbers can not be trusted and are way off. There is a reason why Turnigy motors are a lot cheaper than the more established brands....
Last edited by Phaedra; Aug 12, 2017 at 10:20 AM.
Aug 13, 2017, 02:53 PM
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So Turnigy is out of the competition for me, after the bad reviews I have been reading. I found not even a single story online were anyone reported power readings that came even close to the specifications. Even worse, some even reported motors failing after only a couple of flights. Nope.

So I went back to my first choice: Topmodel XPower motors. I have very good experiences with these, as I have been using the 35mm version in my Ecotop/Topmodel Ka8: rock solid and consistent performance and reliability.
But they also list unrealistic performance for their 42mm version: 4,9 kg thrust, 900W of power, 63A with a 15x8 prop on 4S.

When I run this through eCalc, I get 3,9 kg of thrust and also 900W, which seems a bit more realistic. I have seen video of people hand-launching a 5 kg glider equipped with this setup, so I should be more than OK at 4 kg.



But the very neat thing about these motors, is that they come with an extended shaft, which makes it unnecessary to cut the nose off the glider. My Ka8 uses the same system in 35 mm, and it works like a charm.

Regarding the main wheel, I was able to find a profile picture of the G-103 version that I flew.



That picture clearly show that the fixed wheel in the model is too much forward, and so I already removed it to make room for the retract.
The existing hole in the fuselage for the fixed wheel means some extra work to close it, maybe only partly. In the latter case, I will need to make my gear doors from other material.

Next step: servos.
Last edited by Phaedra; Nov 13, 2017 at 11:06 AM.
Aug 13, 2017, 08:40 PM
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Big Nuts 181's Avatar
So what the reason for removing the air brakes ?
Aug 14, 2017, 01:31 AM
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Quote:
Originally Posted by Phaedra
I will be replacing them for several reasons:
- they extend only about 1 cm
- they are operated by steel push-wires, and these will have to be connected to the servo(s) in the fuselage before each flight. I really dislike the idea of that.
Modern-day electrical spoilers weigh less, require no extra servos, and they extend to about 3cm.
several reasons
Aug 14, 2017, 06:47 PM
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Big Nuts 181's Avatar
Nothing wrong with mechanical air breaks. In mine I've always put re servo next the air break or in the wing root and unlike electrics they are fully serviceable. Even if the servo syrups you can still open the break and disassemble it, you can't do that with electrics. As to the height they would have been enough. Anyhow good luck with the build.
Aug 20, 2017, 02:50 PM
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Meanwhile, I ordered the necessary motorisation components and the electrical airbrakes.
I also passed by the LHS for some materials for the wing extensions.
It bugged me a bit that I might deviate too much from the scale dimensions when I extend the wings, and so I looked for the exact dimensions of the G103 Twin Astir I type, and I found a drawing with everything I need.



The first thing I noticed on this drawing is that the ailerons do not extend up to the wingtips, and on my model's wings, they do.
Another thing is that the wingtips on the model don't have the "inverse winglet" shape that I remember from the real one, and so I want to change those anyway.
I then measured the length of the model fuselage, and calculated the scale wingspan from the dimensions on the drawing. That gave a calculated scale wingspan of 3,5m, exactly as it is now on the model.
But when I calculate the length ratio of the aileron to the wing length, I get 40%. On the model, with the wingtips as they are now, this ratio is 46%.
With the current aileron length, I calculated the corresponding wingspan for a 40% ratio to be 3m90 I found my excuse to extend the wings.....

Next step is to cut some ribs and design the new wingtips...
Aug 22, 2017, 04:43 AM
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Today, my package finally arrived. When the delivery guy handed me the box, I got a bit apprehensive, though....



Before I signed for delivery, I first wanted to check the survival rate of the contents. That soon proved to be all good, luckily...

The motorisation components all lined up:


Can't wait to bench-test the Xpower motor:


The electric spoilers work just fine, and they look like they can make for a decent descent rate:


The opening in the wings would have allowed for 35cm long spoilers, but it was either 30cm or 44cm...which is a bit overkill.
You would think a 30cm spoiler would make the model drop like a brick, but yesterday, a rather turbulent flight with my 3m Ka8B model with 25cm electric spoilers proved that the sink rate at full spoiler deployment wasn't as impressive as I would have wished for, and I really had to fly the model to the ground to get it down in time...
Sep 11, 2017, 03:08 PM
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Weather went really crappy lately, making me feel more like building instead of flying. And so I picked up the Twin Astir build again.
I have been thinking about the wing extensions lately, how to keep the weight down without compromising strength too much.
From my calculations (see above) I determined the length of the extensions to be 25 cm.
The profile can be determined by tracing the wingtip profile onto 2 mm plywood, and then making it smaller by 1,5 mm to take the sheeting into account.



To get the outer profile, I first had to determine the chord at the extended wingtip. I determined that by just putting a piece of paper under the wing and then extrapolate the leading and the trailing edge over a length of 25 cm.



I then extrapolated the height of the profile from the wing root and wing tip profile.

To get the strength, I intend to insert steel wires span-wise into the extensions and into the existing part of the wing.

Now, to construct the extensions, I figured I might try to cut some foam cores and sheet them with 1,5mm balsa.
So I built a quick and dirty hotwire bow with some scrap wood and some steel wire of around 8 Ohms per meter. A first quick test with my car battery charger proved sufficient for my needs.



I glued my plywood profiles to the ends of a 25 cm piece of insulation foam, and did some freehand hotwire cutting, to be amazed by the first result of my trials:



But when I also cut the bottom part, precision was off a bit:



I'm going to think about how to proceed with this, but it might just work....
Thinking about making a cut from the bottom up to insert the steel wires and then glue the sheeting to this core with some PU expanding glue to compensate for the rough core surface....
And I have to think about the leading edge too...glue on some balsa profile or something.
Sep 12, 2017, 01:38 AM
Registered User
Nice one. I used to go full size gliding in the 80s, the club's twin astir was nicknamed "the twin pig"!
In case you don't know the trick, the canopy can be made crystal clear by a wipe of Johnsons Klear over it, or airbrush a coat of clear fuel proofer etc.
Sep 12, 2017, 02:10 AM
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Phaedra's Avatar
Yeah...she could be a bit heavy, the club Twin Astir. Before we had our Pawnee tug, we had to push the Twin before the runway threshold, just to be sure she would get airborne in time.

Thanks for the tip on the canopy, I will have a look at that. I'm afraid though that it might be something inside the plastic, instead of superficial...not sure yet.


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