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Black Magic by Pete B


Pete B
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Now the CG Cub is flying, the building board is clear and Xmas is over, it's time to step up to the plate and make a start on the Black Magic, an elegant 60" high-wing monoplane designed by Fred Hempsall way back in 1946, just a few years before I was born.......
 
Free-flight initially, it was updated in1956 to take advantage of early R/C flying and several changes can be seen on those early plans. I'm going to try to keep faithful to the 1946 outline but with electric power (no asterisks here....) and rudder/elevator control.
 
I managed to pick up a plan-less kit on Ebay for a song and, thanks to Bert (), a copy plan and instructions were forthcoming after an appeal on the forum.
 
I've been following 35mhz's thread with interest and have found a couple of threads over on RCG that have provided some useful tips, which I shall shamelessly use, mentioning the variations as I go.
 
Plenty of bits in the box. The balsa looks to be of good quality but much of it is rather dense and I'll be making some substitutions from my stock:
 
 
 
The bits that will make it go. I'm using a Turnigy-type C3542 1000Kv, 40A ESC with 3S 2200mAh battery, and with just 3 channels needed, an AR500 Rx for guidance. With a specified AUW of 'about 4lb', which I'm sure I can undercut, I reckon about 250 watts will be ample, say about 70W/lb and I know this is easily achievable with a suitable prop:
 


I've decided to start with the wings. Many of these pictures and processes I describe may be rather basic for the more experienced forumites, but I'm always learning on the building side and it may just encourage someone to get a kit and start chopping balsa - go on, you know you want to! .
 
As an aside, I am getting far more pleasure from flying my kit-built models than from any ARTF I've assembled - so it's really worth it if anyone watching is thinking of taking that 'big step'!
 
Anyway, back to the build. The wingtips are built up from a number of shaped parts cut from 3/16 x 1/2 strip and I wanted to get them roughly cut to shape before they are glued - makes final shaping and sanding much easier. I copied the relevant parts of the plan, cut out the shapes and Pritt-sticked them onto the strip:
 

 
After cutting them out, I had two sets ready to start the wing:

 
Looks good for a start, if I might say:
 
 
That's the first wing well under way. I've cheated a bit by using some trailing edge stock rather than enduring the tedium of planing and sanding down strip. The leading edge, 1/4" square, has been replaced with 3/16" square. This means the leading edge sheeting will fit nicely flush over the 3/16" rather than butting up to the 1/4". Makes for easier sanding and saves bit of weight:


I'm using aliphatic glue for most joints (with a bit of CA here and there), so I'll leave it all overnight to set nicely..........
 
Pete
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Great start, and thanks for starting the wings first! As I have yet to start mine and i can borrow all your tips.




I have near finished the fuse on mine, I will post updates and pics in the next few days when I can get some time to sort the pics from those of xmas!
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I built one of these last August, powered by an SC 25 engine.
It is quite nippy for an old timer, but flies well.
Just make sure you make the stab support absolutaly solid as it breaks so easily. 
 

Darryl

Edited By Djay on 04/01/2012 12:35:45

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Thanks All, your comments are always appreciated (apart from Terry's - cheeky blighter!)

That looks really nice, Djay - I'm planning to have the fin permanently fixed, which will save a lot of faffing about with disconnecting linkages, I think. I like the idea of the enlarged versions, Barrie - getting a bit keen on the larger model now.........

Right, progress today. First job was to fix the upper spar, kerfing the underside to the left of R10 which helped the downward curve to the wingtip. The trailing edge has been raised 1/4" at R10 to introduce washout and the remainder of the trailing edge is supported to keep it nice and straight. Shear webs were then fitted to the fronts of the spars between each rib and the ply dihedral braces glued either side of the inboard end of the spars:

 
Once the shear webs were dry, I wetted the upper surface of the sheeting and came close to emptying my pin box to hold everything in place. It was a bit difficult at the wingtip with its compound curve and there is a slight hollow in the finished product. Even with nice soft sheet, balsa bends better across rather than along the grain! This was given plenty of time to dry thoroughly with the all-important washout being locked in with the sheeting and the diagonal inter-rib spars (not yet fitted in this photo):
 
 
Next stage was to release the wing and plane off the sheet excess. Using 3/16" square for the leading edge made it a lot easier to get a good bond with the sheeting. The lower sheeting, again wetted, is fixed to the wing 'freehand' and, much to my surprise and thanks to the rigidity imparted by the inter-rib bracing and 'D' section sheeting, the washout remained intact!:

It is surprising just how rigid the wing has become now everything is in place, which is quite reassuring. A rough shaping with the plane and the wing is finished, requiring just some overall sanding and a bit of filling towards the tip on the leading edge, shown on the bench with the next wingtip in place, ready to do it all again:
 

Now on to my biggest challenge so far - building the second wing so that it is a fair imitation of the first !

Back to the board.........

Pete
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Hi Pete.
i made my fin fixed but the fuse area below the sub fin is very flexible so will need beefing up. The tailplane is very thin and weak so the subfin/fuse area needs to be strong.
Do not worry about adding excess weight as the whole aircraft is so light and requires little power to fly it anyway.
Another thing to do is make a block on the top of the fuse above the front screen where the wing sits for the wing L/E to butt up against, as I find that mine tends to slide forward or go askew when the bands are on. Also the wing has such a deep diehedral that it does not sit correctly on the fuse sides, so trial runs before covering everything, as you may need to add or modify slightly the wing seating area. I had to fit two 1/4 inch blocks under the wing T/E (glued to the fuse) to get the plane to fly level, so be prepared to mess around with the wing incidence once you have it flying.
 
Fuse area behind wing T/E is also very weak as it is only 1/8 square balsa, so maybe fit 1/16 balsa sheeting for aprox 2 inches long and across the whole width of the upper fuse underneath the 1/8 formers and butting up against the bulkhead, just to add some strength.
All a lot easier than appears in writing.
 
Enjoy
 
Darryl
 
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Thanks very much for those tips, Djay, all recommendations welcome! I'm sure 35mhz is watching, too.
 
Right now I'm just concentrating on making a usable pair of wings and TBH, haven't looked closely at the fuse issues, which I'll deal with as and when I encounter them. It helps having the benefit of previous builders, that's for sure.
 
Pete
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Grim weather today so the building went on apace. I had to move into the kitchen as the workbench/kitchen table () is longer and the second wing is attached to the first at a fairly early stage, before the sheeting and diagonal braces are added:


Bottom sheet was added after the top had dried out and after planing and a bit of rough sanding, the wings are now just about complete. They do build quickly without ailerons! No mention of it on the plan but I'll epoxy some bandage around the root section. I had a nasty episode when the wings folded on my WW Fournier earlier this year, having relied only on the dihedral brace and glue!:
 

With the wings out of the way, I've started the tailplane. Very straighforward 1/8" frame, sheeted both sides but I've increased the midline front to rear spar from 1/8" sq to 1/8" x 3/8" to beef it up a little, taking note of Djay's comments:
 

More tomorrow, all being well!
 
Pete
 
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I've been pushing on over the weekend but still doing too much ruminating and sucking of air through teeth about the front end, so let's see where we've got to.
 
Continuing with the tailplane, the plan shows a single 1/8" square strip down the centre-line which is marginal to say the least, so I've replaced it with 1/8" x 3/8", with an additional 1/8th x 1/4" outboard of that, for luck! A couple of extra diagonals under the sheeting, too:
 

Fin, rudder and sub-fin assembled:

 

The plan shows that the tailplane is mounted on a diamond-shaped ply plate fixed to the top of the sub-fin, with the tailplane assembly banded onto this. I'm looking to permanently mount the tailplane so will probably omit the ply. Djay mentions the fragility of the tail and, rather than sand thesub-fim to a fin profile as per plan, I'll leave it as the full 3/8" width for the greatest contact area with fuse and tailplane. I'm thinking of running some dowelling or toothpicks through the assembly for additional support:
 

On to the fuselage, which is a 1/4" square built-up structure. The strip supplied in the kit, to be honest, would have been suitable for cricket stumps, and I'm wishing now I'd replaced more than just the lower longerons from my stock. The upper longeron is straight but will have a pronounced inward curve at the nose and I'm thinking I will need to kerf it to achieve the bend necessary - not a problem. Building the second side on top of the first ensures a matching pair:
 

The two sides were left to dry and then the built-up formers were glued to one side then the frame was assembled and clamped, being left overnight to dry:


Which is where I'd better break off for a moment.........
 
Pete
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Very difficult to concentrate when Mrs Browns' Boys is on the box......
 
With the information that the area around the trailing edge of the wing seat is a bit fragile, I fixed a 1/8" sheet behind the former, which provided useful support for the fuselage stringers, the gaps between being infilled with offcuts of 1/8" square.


Rather than sand down the end of the fuselage to a fine taper, I've left it at 3/8" across, the same as the sub-fin. I'm hoping this will make it a little sturdier. It will be smoothed off before covering and fixing:
 
 
 
The rudder and elevators have had the CA-type hinges, rather than the supplied thick Mylar hinges, slotted in for now. Not an easy job with the elevator at only 1/8" thick but the slots are just a scalpel-thickness wide. I don't expect the flight loads on this model to test these hinges too much!:
 


This is progress so far, then:


Plenty more to do - and I've still got to decide how I shall arrange the front end!
 
Pete
 
 
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Hi Pete.
Infill the whole sub fin, cutting the slots for rods as required, and sheet the outside with thin balsa for maximum strength instead of just infilling the centre part.
I still fitted the diamond ply plate, but glued all the tailplane parts on so it is non removable.
Despit being very thin and flexible it does take a lot of knocks without breaking.
Mine get battered around in the workshop with other planes resting on it, and in the car, and the tailplane being flexible does not break.
It is plenty strong enough for flying.
The weakest point is the 1/8 stringers on top of the fuse at the wing T/E bulkhead, put a strip of 1/8 stringer glued to the underneath as reinforcement to stop them breaking.
 
looking good so far, now you will have to decide on a colour scheme and covering material.
 
Darryl
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Cheers, Gents! There's a theme developing here so your points are being noted and I'm off to the shed for some 4 x 2.......
 
Steve, It's called retirement.......

I picked up some black Solarfilm when I was last in UK, so that indicates my lack of imagination Just pondering on the contrasting colour and scheme at present.

Progress is being made on the front end and I'll post more when I have something worth reporting.

Pete
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A little more done.......
 
I couldn't see a simple way of fitting a 3S 2200mAh in through the cowling that I liked and, being mindful that the u/c fitting is non-negotiable, I've settled for a hatch into a compartment immediately behind the u/c mounting. This puts the battery forward (just) of the C of G and with the slightly heavier motor, shouldn't result in too much noseweight having to be added.
 
I made up a plate from lite-ply, tongued into the F2 former and cut around the fuselage spars. The slots are for the Velcro battery strap:
 
 
Located. As you'll see in later photos, this tray has additional support added as the build progresses:
 
 
The motor will be mounted on an X-mount bolted to a 5mm ply bulkhead. I've built in about 3 deg right-thrust and 2 deg down-thrust (using the TLAR system!) reasoning that, if it's too much, it's easier to reduce it by a degree than to increase it by two. This is set so that the backplate of the prop driver will be about 10mm forward of the nose:
 

 
Plenty of agonising and measuring before chopping into that pristine block of balsa! If I have one item of kit which flatters my abilities it is my Dremel-clone (from Aldi). So useful for whittling away large amounts of excess material.
 
I don't overlook the value of 'full-size' tools, either. Using my Bosch orbital sander, it was easy to reduce the depth of the lower front of the nose neatly, too, which will faithfully follow the profile of the prototype:
 
 
Inside face of the nosecone all done. The upper section, which will be attached using magnets, will form the forward end of a removable hatch, dowelled at the back into the lite-ply former:
 
 
Nosecone now glued to lower part of fuselage and ready for side-sheeting and hatch making before final sanding to shape. I'm very relieved to have reached this stage without making a hash of it:
 

I now hope I don't screw up the final shaping of the nose.........

That's it for tonight.........
 
Pete
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Pete I am well impressed! All looking very good and craftsman like - I like the way you did the cowl and the rear face is very neat. I can't see you having a problem with the front face.
 
Gonna look really good isn't it? The lite ply plate was also a work of art and looks the business. Where you lead others (me!) follow.
 
Terry
 
 

Edited By Terry Walters on 13/01/2012 07:27:51

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Hi Pete, really looking good, very neat installation.
Just a thought and not everyones cup of tea I know. But 4S A123 would probably have fitted in the fus, and could be charged in situ, with only access to the leads for charging. I have a wot4 and cells are stuffed in through the wings and sit where the tank was. No need for hatches at all as the cells live in the model.
Cheers
Danny
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