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Build Log for Krick Grunau Baby IIb (1/4 scale)

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Tim Mackey24/08/2011 18:41:23
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I'm beginning to wonder if this is the same model you know - the vertical stab and rudder construction look the same, as does the fuselage generally?
Did your front end small round windows come ready cut - mine has none.
Wings are 66" each panel, so seems like the same span?
The air-brakes are driven off a rather messy arrangement involving plastic snakes with ball ends, that are pushed onto servo ball joints, in the wing pod housing. The servo arm has to be removed each time - I may see if I can fit small servos in the wings instead.
All Ive done so far is give the fus a really good cleaning, and covered one or two small rips in the solartex with some reg numbers and non scale stickers
Ive removed the tailplane ( screwed rods and nuts, and struts ) for examination, and discovered that one of the screw holding plates is loose - so have just glued some 1/4 sq balsa strips to re-secure it. Otherwise, all looks good so far - the model appears to be well made, with strong, accurate and neat woodworking, hinges are secure, pushrods and metal rod ends etc seem strong and well bound.
Elevator has pushrod, rudder is closed loop via idler crank.
Ive changed the slightly rusty clevis'.
S148s waggle rudder elevator, and air-brake, but the elevator servo top housing/bushing is worn, so that will be replaced.
1.7 lbs of lead was rattling around the nose - so may try to reduce this dead weight by using A123 cells for power, and a nose release system needs fitting too, as I plan on aero-towing her. More on this later.
Found an unused WW1 pilot in the bits box, so may try to adapt him to fit /look right.
Scrubbed up pretty well me thinks

 

 
 1.6 lbs of finest church roof
 

 airbrake servo


 
 
elevator clevis access
 
first look inside
 
 
rear end


Edited By Tim Mackey on 24/08/2011 19:05:36

Tim Mackey24/08/2011 19:01:54
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Here's the tailplane seating area under repair.
And also a couple of pictures - as best I could - of what appears to be part of a tow release mechanism right up in the nose.


 
A plastic pivot tube runs across the nose block, with a rod through it, to which is attached a plastic and brass thingy ?
Being new to aero-towing, I cant quite see how it operates, but the chap I bought it off is sending me some more bits, so maybe then it will make sense.
 
Tim Mackey24/08/2011 19:07:36
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Few more piccies...

 


Martyn Johnston25/08/2011 15:44:05
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Looks like a 'Grunau Baby' to me, - but it's not the same 'Krick' kit.
Mine will be 1:4 scale; 3.4m (134") in total.
The wing seat is definitely different. My two wings will mate together, and then this assembly will sit 'on top' of the the fuselage.
Tail and fuselage look very similar though.
I like the foam blocks you have; I'll copy that idea if you don't mind.
The round windows were supplied, but it would have been easier not to cut the holes out. I think all the real Grunau Babies had holes though.
Your pilot looks rather small; what scale is he? I've been looking for a 1:4 scale bloke, and I'm only going to be able to fit just a head in !
Can't make out your tow-release though; I've bought a very simple looking Multiplex one (link here); I've glued it in but won't get round to setting it up for a while yet.
My plan suggests something similar for the ailerons and the airbrakes; but I'll be using seperate small servos in the wings instead.
I like the look of your closed-loop rudder idler crank thing. I've done something similar before but it ended up being very oversized and heavy.
Martyn Johnston25/08/2011 15:49:21
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Good info on the early versions of the Grunau Baby here.
Not all versions had the little round windows.
Tim Mackey25/08/2011 23:31:57
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Ok, it probably isn't the same ( although the span is identical I think, with 66" panels, and 2 " fuselage width ) - in fact, I am thinking that this may well be a scratch build from plan, as the more I dig - the more "home made looking" pieces I find.
All the horns, pushrods, struts and mountings, cross bracing where the struts attach etc, all seem home brewed - but very well done I add.
I have decided to strip out all the gear as the servos are all showing a bit of slop in the top "bearing" and the geometry of the elevator system means the resultant torque is marginal at best, from a simple old S148. The plastic servo tray is also flexing a fair bit under load, so I'm redesigning this and making a plywood radio tray that will house 3 servos ( elevator, rudder, and tow release ) along with Rx and switch and charge jack.
Im going to re-use the best of the existing servos for tow release where a little play wont matter too much, and fit higher torque, new, HiTec servos on rudder + elevator. Mini servos will be fitted in the wings somehow for airbrakes - when I finally get around to the wing work.
All wiring will be concealed, and battery pack made to fit right in the nose.
I thought about the foam "stand" as its a nightmare to try and get it sitting on the bench due the skid.
Now, on that subject, as I say...Im new to aerotow, and Im wondering how the thing is going to fare when being draggged along, balanced on a half inch wide strip of hardwood.
Whats stopping the wingtips from catching the grass, and maybe even digging in on take off or landing?
I've heard talk of using a dolly, but would really like to avoid that if possible.

Tim Mackey25/08/2011 23:33:47
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Posted by Martyn Johnston on 25/08/2011 15:49:21:
Good info on the early versions of the Grunau Baby here.
Not all versions had the little round windows.
 
Yup, found that link a few days back - I think mine maybe a 2a.
That looks a nice simple ( simple is best on something like this ) tow release
Not sure what scale the pilot is, certainly nowhere near 1/4 scale, but oddly enough, his head and shoulders dont look too wrong in the cockpit. 

Edited By Tim Mackey on 25/08/2011 23:37:31

Fubar27/08/2011 22:20:54
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Gosh Tim she does look clean.
That plastic servo tray needed to be removed
Tim Mackey27/08/2011 22:55:21
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Yes, scrubbed up really well Chris - wanna buy her back ?
Tim Mackey30/09/2011 13:01:36
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OK, she's pretty well finished ( hows yours coming along Martyn ?).
First off I made a large 1" diameter, 7" long aluminium tow release that fits right up tight against the inside of the nose, but the hooking mechanism is visible inside the cockpit area. I prefer this as it allows me to actually see the towline is properly "hooked". I forgot to take pictures of its construction, but its a solid 1" round bar, conical at the front, bored out right through with a slot at the rear, through which a hooking lever is swivel pinned and servo operated.The hook/lever was simply filed down from a piece of 1/8" flat bar, and the pivot is a cut down old drill shank, hammered home and peened over slightly.
This is the business end just inside the cockpit area and the operating servo is under the new rear ply false floor.

The new radio bay.

I did away with the idler crank for rudder, as it was binding a little and the rudder would not always centre correctly. New longer closed loop lines now run directly to the new rudder servo. Elevator is pushrod driven using original rod and fittings. The front tray also houses the on/off switch - radio power is from 2 x A123 2300 mahr cells in the nose - along with much reduced lead ballast.
The charge/balance socket for the battery can just be seen behind the elevator servo.
 
Rear false floor.

This hides all the wiring and tow release servo, and just visible is the wing wiring harness running up along the former, and the spektrum Ar7000 master receiver, with the satellite on the floor to the rear. Much tidier install now - and no flexing under load
 
I made a "padded headrest" from an old mousemat for the soon to be installed 1/5 scale pilot

 
He's slightly smaller than true scale, but at 77mm height and with real woolen clothes and fetching nylon hair, not bad for 14 Euros. Pity the postage cost 15 !!
....."Max" is on his way from France, having now become too old for leading roles in those dodgy 80s "movies"

I didnt fancy fiddling with little 3mm nuts and bolts for the wing struts, so made up some locking pins on the lathe - the struts will remain bolted to the panels, but these new fuselage fittings are retained by a small "R" clip, itself retained by a safety line to avoid losing them in the grass.
Heres the pins and clips in more detail.



Im not happy with the wing seating, as can be seen below - theres a very noticeable gap at the front of the right hand panel. The wings slide onto two metal joiners, and are held together with "O" rings across two hooks, front and rear. Even smaller stronger rings are not pulling hard enough for a good fit - so I am still deliberating a method to exert more force. I have ordered some small "bottle screws" like turnbuckles, but mnay need to fix new hooks to the wing root to allow room to operate them - I could just use tie wraps, but would prefer something better.

 







 

Edited By Tim Mackey on 30/09/2011 1

Tim Mackey30/09/2011 13:07:13
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I tidied up the skid - but time will tell how successful it will be for aerotowing purposes.
The two small yellow lines denote the COG, and the black stripes on one panel should make it go faster ( Cheat lines to cover previous damage )
I also bought some white solartex to recover similar damage on the other wing panel underside
 
Cockpit floor fitted and painted and windscreen fitted - original instrument panel just visible

So apart from sorting the wing fitting and installing Max, she's done.
Looks rather well I think
With the Indian summer here at last, I may even get someone to give her a tug next week
 
Or, of course, I could just chuck it off a big rock

 
 
Martyn Johnston30/09/2011 15:34:47
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You're making me feel a little jealous now Tim.
I feel like I've got years to go before I'm at that stage.
 
In fact I'm slowing down a bit now as I feel I've spent too much time on it this summer instead of actually doing any flying of other models.
I think I'm going to finish the fuse, then pause and start the wings over the winter.
I keep thinking the fuse is finished but there always seems to be more to do.
 
I'm half tempted to start covering the fuse, but I really want to get a photo of it fully built, but uncovered; I like them like that.
 
Tim Mackey30/09/2011 16:50:11
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I know what you mean Martyn, a long build can sometimes get tiring, and if like me, you occasionally get fed up with the whole thing, it gets even longer!
Rather than feel jealous, hopefully the pictures will help to inspire you again - your plan sounds a good idea to me, get the fus finished, then have a short break, and start the wings later.
Tim Mackey05/10/2011 23:44:03
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The bottle screws didnt work well - there just wasnt room to "operate" them.
I decided to use a ply strap across the wing roots - with 4 cheese head 4mm screws that fix through into threaded insets which I mounted at the wing root, and then recovered. This also has the advantage of covering the wiring aperture on the fus top.
I couldnt go all the way to the very leading edge due to the curvature of the wing profile being too steep to allow the ply to flex and fit neatly.
Seems to be secure, and certainly holds the wings aligned better - and I think - looks fine.
 


 
I reckon Max looks pretty reasonable in his office too
All finished now, and ready and waiting for a nice day to throw it off a big cliff.
 
Martyn Johnston06/10/2011 09:18:13
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What's the blob on the top of the front of the fuse, just in front of the cockpit ?
Is that the Sat-Nav ?
Tim Mackey06/10/2011 12:25:55
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Funny you should mention that...its a rather ugly attempt to cover up a sliding spring loaded hatch retaining pin - I have just finished turning down a pitot tube assembly which will replace it, and I shall use magnets to retain the hatch.
I'll post some pictures when its done later today
Tim Mackey06/10/2011 14:56:06
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And here's the result.
 

I opened up the side windows too
 
NOW she's finished
Stefan Hafner06/10/2011 15:21:23
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Looks very nice Tim,
 
Am slightly jealous, I wish I had a lathe for that sort of stuff, filing to shape get a bit tedius
Tim Mackey06/10/2011 15:30:33
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Cheers Stefan - you know, there are plenty of pretty inexpensive small model makers lathes around these days, and S/H are of course even cheaper. I have lost count of the number of things I have made for use in this hobby - and apart from the usefulness, I get a great sense of pleasure from turning pieces on the lathe... very satisfying.
Off topic...sorry .
Fubar06/10/2011 18:35:00
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Well done Tim , looks great !.
Like the piece of window frame on the glazing, nice touch.
What is the shank size of your lathe tools Tim , I'll have a rumage at work, Have you a miller or vertical slide ?
Perhaps I missed something but why a 1/5 scale dummy in a 1/4 scale model ?
 

Edited By Fubar on 06/10/2011 18:37:45

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