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1/4 Scale ASK21

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Peter Balcombe
Posts: 1256
Joined: 18 Mar 2015, 10:13
Location: Clevedon, North Somerset, U.K.

Re: 1/4 Scale ASK21

Post by Peter Balcombe »

After a bit of a delay (to remake the elevator in order to avoid added filler weight), the tailplane top sheeting has been added and the elevator dry hinged to check servo operation.
Everything looks good, so the lower building tabs have now been removed and lower spars fitted ready for application of the lower sheeting.
Note that tip blocks have still to be added to give the final shape.

I will investigate fitting of a servo hatch to allow access, plus fitting the tailplane to the top of the fin.
Rudder construction has also started, so I hope to be able to check the overall fit of the tail feathers soon.
Attachments
Tailplane & Elevator underside
Tailplane & Elevator underside
Tailplane & Elevator top
Tailplane & Elevator top

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Peter Balcombe
Posts: 1256
Joined: 18 Mar 2015, 10:13
Location: Clevedon, North Somerset, U.K.

Re: 1/4 Scale ASK21

Post by Peter Balcombe »

With rudder built up and hinged onto a Liteply/balsa finpost, I finally found time between other builds to fit the tailplane fixing Tee nut at the top of the fin and also cut out the tailwheel slot in order to check that the rudder & elevator were clear with the tailplane in place.
Happily it all fits, so I can now tidy everything up before moving on to the wings.
The tailplane overhangs the front of the fin a little more than true scale, but that is because my fin is a little less deep than it should be (3-view scaling error :o ) - but hopefully no-one will bother to much when it is all done.

Note that the bit projecting above the tailplane in the photos (looks like a flag) is actually a piece of masking tape attached to the rudder hinge pin wire to hold the top of the rudder in place as the fin-post is only dry fitted. The wire will be replaced with a removeable carbon rod in due course.
Attachments
Tail group check.jpg
Tail group check2.jpg

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Peter Balcombe
Posts: 1256
Joined: 18 Mar 2015, 10:13
Location: Clevedon, North Somerset, U.K.

Re: 1/4 Scale ASK21

Post by Peter Balcombe »

A little more progress today after printing off a wing root rib, complete with datum line in order to transfer the datum line to the fuselage root ribs, together with the main joiner & rear incidence tube centres.

With the wing datum defined, I strapped a length of substantial hardwood strip against the wing root, with one side along the datum line. I also made an 1/8” balsa negative tailplane root section template so that with this sat on the top of the tailplane, I had a flat surface parallel to the tailplane datum.

Thus equipped, I was able to check the longitudinal dihedral & was pleased to find that the wing sits at +2 degrees, or just over, wrt the tailplane 🙂

So, out with the P38 filler to make a smooth tailplane bed at the top of the fin (essentially just filling the 1/8” gap at the very rear of the tailplane. Once that had set & been tidied up around the sides, short lengths of 16swg brass tube have been let into the top of the fin & tailplane for an anti-twist peg.

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Peter Balcombe
Posts: 1256
Joined: 18 Mar 2015, 10:13
Location: Clevedon, North Somerset, U.K.

Re: 1/4 Scale ASK21

Post by Peter Balcombe »

After more than a years delay, I at last have the bench clear to get back on this build.
So..... with a wing parts set kindly cut by Cliff at LCS, let the build recommence.

The wings use top & bottom main spars, bonded full length to a central Ply web to form an 'I' beam.
So after making up the main spars, the lower one was laid over the plan, supported on blocks to achieve the correct height. The outer ply web was then glued to the top of the spar, using a few ribs to correctly position it, before adding the remaining outer section ribs, plus Balsa Dummy LE, Dummy TE, Aileron hinge spar & Aileron LE to produce the complete structure apart from LE & stock Balsa TE.
K21 Outer panel start_small.jpg
The process was then copied for the Inboard section, including the rear spar & web to tie everything together and give a pretty complete structure apart from the upper main spar & lower rear spar.
The brake box will be defined by 1/8" Spruce spars yet to be added. The photos also show the carbon joiner tube which slots through the root ribs/spar web. The main joiner will be a 16mm diameter high carbon steel (machine slide) rod, with 5mm rear incidence rod.
Attachments
K21 Wing structure1_small.jpg
K21 Wing build2_small.jpg
K21 Wing build3_small.jpg

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Peter Balcombe
Posts: 1256
Joined: 18 Mar 2015, 10:13
Location: Clevedon, North Somerset, U.K.

Re: 1/4 Scale ASK21

Post by Peter Balcombe »

With all ribs in place, the joiner tube has now been epoxied in position and then fully boxed in with ply sheet & epoxy to make a solid lump tied to the spars.
The rear incidence tube has also been added & boxed in using hard balsa, faced with 1/16" ply.
Airbrake fitted & pushrod routed through to the servo bay.
Lastly, a 1/8" balsa doubler strip has been fitted around the wing taper break rib in preparation for skinning, which I have decided to do as separate Inboard & outboard sections.
The structure now just needs to be sanded over to get the False LE & TE to profile prior to applying the full width upper skins.
Attachments
Joiner tube_small.jpg
Brake installed_small.jpg
Joiner tube boxedin_small.jpg
R21 Rib doubler_small.jpg

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Peter Balcombe
Posts: 1256
Joined: 18 Mar 2015, 10:13
Location: Clevedon, North Somerset, U.K.

Re: 1/4 Scale ASK21

Post by Peter Balcombe »

Upper sheeting has been applied in 2 sections once rib doubler had been fitted at the taper break.
I now need to find the Brake blade outline in the upper sheeting - if I can find the piece of paper I wrote the measurements on ;)
Aileron servo cable threaded though & now awaits more cable to be able install the Brake servo cable.
Photos below show the current state, noting that a 1.5" TE strip will be fitted once the sheeting is complete.
(The front part of the ailerons is already included in the build so far, complete with angled aileron LE spar).
Attachments
Wing top sheet_small.jpg
Wing tops sheet below_small.jpg

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Peter Balcombe
Posts: 1256
Joined: 18 Mar 2015, 10:13
Location: Clevedon, North Somerset, U.K.

Re: 1/4 Scale ASK21

Post by Peter Balcombe »

Underside now sheeted & servo hatches opened up.
1.5” TE stock now being applied to the rear edge, starting with outer section - aligning upper surface to wing profile (excess lower surface to be removed later when profiled to wing section).
Attachments
29D2B662-3E9D-4A8B-A943-0CFF4BC2541E.jpeg
5D875D66-EAE4-4B58-B0DE-E34799D7E99B.jpeg

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Peter Balcombe
Posts: 1256
Joined: 18 Mar 2015, 10:13
Location: Clevedon, North Somerset, U.K.

Re: 1/4 Scale ASK21

Post by Peter Balcombe »

With bottom spar tapered as required this morning, the 2nd wing structure is well on its way - with not a drop of Cyano in sight!
The Ply web sitting on the top of the wing is for the rear spars yet to be fitted. (The web fits from the underside anyway).
Attachments
DE72EF6C-FD96-4953-92CD-9AA19FE5731A.jpeg

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Peter Balcombe
Posts: 1256
Joined: 18 Mar 2015, 10:13
Location: Clevedon, North Somerset, U.K.

Re: 1/4 Scale ASK21

Post by Peter Balcombe »

With the top skin now on the 2nd wing & the lower building tabs removed, I couldn’t resist putting the joiner tube in the fuselage in order to try an initial assembly.
Tailplane also sat in position to get an impression of the overall size.
Still a lot of work to be done, but slowly getting there :)
Attachments
3AFC2C4D-E574-457A-ABE7-13C8ED76DE21.jpeg

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Cliff Evans
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Joined: 29 Dec 2019, 15:13
Location: Bristol
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Re: 1/4 Scale ASK21

Post by Cliff Evans »

Looking good Peter.
Laser Cut sailplanes

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