Mystic 64

[Martyn K]

A couple of years or so ago, I built a 40 sized Mystic which I still rate as one of the best flying models I have ever had the pleasure to fly. I was asked by a few people at the time whether I was going to do a larger version

At the time I planned to do a 68" span version for a piped 60 and drew up a plan based on the 40 design using the same basic construction and dimensional relationships. However, time and other projects took their toll and the Mystic 68 was side lined. However, recently, I have been asked again whether I would do a larger version and my CAD systems had the dust shaken off them

Hanno Prettner built several versions of the Mystic, larger ones for 120FS power but eventually opted for a 60 sized 2 stroke of about 68" span.

The 68" span version needs a powerful motor. Hanno was very fortunate as he had access to the most powerful 2 stroke engines available, made to his specification by OS. These motors do appear on the second hand market as collectors items but the price tends to be astronomical.. A cheaper alternative was required.

I have been quite impressed with the OS55AX at UKCAA events. They seem to have adequate power and will happily fly a Curare with oomph to spare, providing the airframe is kept light. A heavy airframe, a long vertical manoeuvre and an under powered airframe is not pleasant to watch,

So I redrew the plans (actually shrank then by about 3 or 4%) to drop the wing span down to 64" span with the idea of using OS55AX with standard silencer but with an option of using tuned pipe if others wanted to put either a bigger motor in or simply add a pipe to the OS55.

After quite a lot of work this materialised.

I then made the mistake of publishing this on the UKCAA group page on Facebook. This seemed to generate a great deal of interest - especially from South America where I have had many requests for copies of the plan and the DXF files. Now, I have no intention of making any money from this design, its Hanno's design and it would be wrong to profit from it. However, if the model is a success then I'll make all the drawing freely available via the UKCAA website.

So, 2 sets of parts were ordered from SLEC, one for me and one for one of the UKCAA members who also wants to build one to prove the plans. I am going to keep one component ahead of the other Martin who will work from my annotated and corrected noted/plans.

SLEC have agreed to keep the parts on file (which are the 3mm lite ply parts only), there are some additional 6mm and thin ply parts needed but these are not too difficult for the typical builder.

I am hoping this will be a quick build - it has to fit in with the Fury build which is currently waiting for paint.

[Martyn K]

Making a start yesterday...

Day 1 of the Mystic build. The sides are from 3mm balsa with ply doublers front and rear with strength from spruce longerons and rigidity from the formers.

I start most of my models around the firewall and the fuel tank bay. Building this model I have made one difference. The fuselage sides were joined from 2 lengths of spliced 3mm balsa but not cut too shape. One edge is trued and the upper longeron and ply doublers added. The position of the formers is marked, they are dry fitted and this defines the location of the lower longeron. Once the glue has dried the excess 3mm balsa is then removed. Much easier than trying to pin prick through from a plan. Have to admit it was made easier because the wing cut out and straight rear fuselage lines are very easily defined.

The front formers added (the parallel section of the fus) making sure its all vertical or horizontal as appropriate.

Dry fit of the rear formers. This set the position of the lower longeron and the sheet then trimmed back to size. All the formers need their slots opening slightly but this ensures a really tight fit if doe with care.

Other side clamped into position and left to dry overnight. Front former is offset to build in some right thrust (3 degrees)

Tuesday, I add the rear formers and the top decking which is planked.

[Martyn K]

Day 2

Cracking on with the fuselage. SLEC fuselage jig in use, one of the best purchases I have ever made. No bananas here. This went together very quickly as the formers had all been dry fitted yesterday. All clamped up and the top longeron fitted and left to dry for a couple of hours.

The fuel tank bay has two dummy formers delimiting the hatch. This will be boxed in with 6mm balsa as it needs to be strong enough for handling

Next job was to start work on the planking. I did consider a foam and veneer deck but have never been too happy with my foam creations in the past. Coming along but it's fairly slow as expected. May get the decking finished tomorrow evening, if I do I'll park this and start on the tailplane and fin which I will need fairly soon.

Fus is still in the jig as it is possible to induce a twist when planking. I speak from a sad experience.

The planking around the front of the fus has been completed and the cockpit floor added. It will be trimmed back and sanded to shape tomorrow. Hopefully, I will lose about 1/32 balsa in the sanding process.

More to come. Probably not much will happen tomorrow though as its grand daughter minding day which is usually pretty exhausting..

[Martyn K]

A bit more progress over the past couple of days

Got the upper decking planking completed. The planks are quite firm and are too narrow to curve transversely. However, the joints all seem good and I wanted to reduce thickness from 3mm to about 2mm.

Before with bindings in place

and after

with the planking done and trimmed back, I made a start on the fuel tank hatch.

This is quite tricky because the curve at the very front is very challenging. Very narrow planks will be needed. I may have to rethink this a bit...

The decking has sanded up quite nicely. Still needs a polish and a tiny bit of filler but really quite pleased with that.

This morning, I made a start on the tailplane. Its not flat plate, the ribs are 10mm thick and will be sanded to a symmetrical shape, about 7 or 8% thick.

Tomorrow, I am planning on adding the elevators and sanding the whole tailplane to shape in one piece.

Fin and Rudder to build then I'll start on the wing.

More to come

Martyn

[kevin b]

Coo. A real modeller's workshop !

[Braddock, VC]

That splice reminded me of the first wot4 I built back in the last century, a mk2 . Chris Foss had cut the components on echelon as it were, I've used the idea several times on models, most recent on a wot 4, ironically, that succumbed to switch failure. On the mk2 wottie the joint was abaft the wing seat..

[Martyn K]

Thanks.

The splice joint is reinforced with the spruce longerons. I intended it to be at the front behind the ply doubler. That was an errr oversight ..

[Nigel R]

Looking good Martyn.

Easily scaled back up to 68" ??

I have an OS 110a that would work nicely in that size I think... One day maybe when I've cleared the board of current projects...

[Andy Stephenson]

Martyn,

Many thanks for posting this here as I always like to see your classic pattern builds which I follow on the UKCAA FB pages but not having a FB subscription I find it difficult to see the expanded range of images because they are always greyed out when I look.

Andy.

[Ace]

Like AS I don't do FB so really appreciate you taking the time to post hear - fantastic project

[Martyn K]

Hi both

Thanks for the kind comments.

Happy to scale the plan and dxf files to a larger size, but you will get some odd wood sizes but nothing that couldn't be adjusted on the building board.

Caveat. This us a clone, Hanno never produced a commercial plan for this design, there were two varints kitted but his 60 sized version plan was as far as I know kept in the Prettner vaults.

So this is scaled up from the 30 then to my 40 version then to this size. It has a slightly different laminar wing and rail section and the fuselage layout is my own. Hopefully, it will be OK, the 40 sized version certainly was/is. Either way, it will look right and probably be far better than my piloting skills.

Martyn

[kc]

Interesting project Martyn. What wing section did you use and is it the same as Hanno used?

It's too big for me but the Mystic 30 at 51 inch is more my thing and the plan is on Outerzone but minus individual wing ribs..

Looking at the Mystic 30 the wheels seem a bit far back -probably for the retracts - but Martyn's seems (possibly it's not clear) further forward and more conventional.

Edited By kc on 10/12/2020 13:04:27

[Martyn K]

I don't think that Hanno ever disclosed which section he used. However, he was one of the first to use laminar wing sections with max thickness at about 40% chord. I can't remember exactly which I used, but I think it was a 5 series naca, 14% thick at 40% chord. I'll have to get the pc out and remind myself

[Martyn K]

Regarding the retracts. They have the legs bent forward at the retract unit so they swing forward. In hindsight I wished that I had used the same layout that I used on my Touché as they sit better in the wing.

[Rod Parsons 1]

Please keep the build thread running.

Very keen on classic pattern.

i don’t do FB either.

Rod

[Nigel R]

Posted by Martyn K on 10/12/2020 12:36:42:

Caveat. This us a clone, Hanno never produced a commercial plan for this design, there were two varints kitted but his 60 sized version plan was as far as I know kept in the Prettner vaults.

Detecting any detail differences is beyond my skills in the air...

[Martyn K]

Just remembered there were actually 3 versions. Mystic 90 and 120 were I think ARTFs, Mystic 30 was an EZ kit and the only version where a detailed plan was produced.

A day late with this update. I scrapped the first tailplane as it was a bit too flexible when I sanded it to shape. Upon investigation this was because I used 10mm 3/8" instead of 12.5mm balsa. That 3mm makes a big difference.

The new tailplane was sanded to shape, glued into place , checked it was at the right angle and triangulated so it was straight and left overnight (on Friday) to dry.

Elevators were again glued in and sanded in situ.

The fin was fairly straightforward using very light 3/8 sheet balsa.

That was then glued and trued and laser aligned. I use 4 pins - two on the fus and two on the fin. If the laser lights all 4 pins up then I know its straight. Its also triangulated against the tailplane as a sanity check.

Pins located

Lights off - laser on.

Side cheeks added this morning after the fin glue had set. carved from 1/2" light balsa.

Now being sanded to shape. Test fit of rudder and elevators, starting to look like a Mystic..

I started a dry fit of the wing this afternoon. Found a couple of drafting errors that caused a bit of a panic but they are both rather trivial.

One problem that I do have though is that if I install the retracts as per the plan then they will foul the fuselage. Need to have a rethink on that before I get too far with the wing build. Apart from that, its coming along quite nicely I think

More to come

Martyn

[Gunnar Borseth]

Great work

Can`t wait for the plan to become available. My OS 61 RF needs a home.

[Nigel R]

Tailplane structure looks very light for this sort of size aerobat, I guess I'd expect a sheeted structure... was there a reason behind the choice?

[Martyn K]

Hi Nigel

Simply to keep the weight down. Mystic nose is actually quite short so every gram back there counts. It has a spruce brace in there. It should be OK, the diagonals keep. It fairly stiff and hopefully anti warp.

Weight of fus, tail and fin is 470g at the moment.

[Martyn K]

Mystic 64 Day 7 - quite a productive day today one wing half well on the way.

Using the building tabs and lots of weight to make sure the wing stays absolutely flat. Basically, my build sequence is to start from the lower spar add the ribs, add the upper spar, add the false te, leave it weighed down until the glue dries then add the false le.

I almost forgot the doublers for the uc mounting beams. You can also see the ply servo tray fitted as well.

I will assemble and sheet as much of this side as possible before it gets removed from the board.

Flip it over, cut the slots and add the retract mount beams and pipe in the air retracts. Sheet the other side and add the cap strips.

Build the other wing half then invert, fit the braces and doublers at the centre, then add the true LE, Trailing edge and ailerons.

Seemples but that will take a few days and I need to build a nocal indoor scale model before next Wednesday.

Also managed to cut and fit the mount for the tail skid while the glue was drying. 16swg piano wire glued and stitched into a 3mm ply support. The little washers are there to provide some support and hopefully stop the skid from rocking

[Martyn K]

Mystic 64. Day 16

A bit late with this update, sadly I had a corruption on the sd card in my phone and lost 1500 photos, including the latest build sequence. Hey ho

So a brief summary on where I am up to.

Second wing half assembled

Wheel wells laminated from two and a bit laminations of 1.5mm balsa wrapped wet around a 3" id aerosol and slotted into a cut out in the wing rib.

Wings joined with thick balsa rib in the centre to carry the locating dowel

Wing centre section doublers are 1/2 *1/4 spruce tapered to slot into the swept bck main spars. The lower doubler is also tapered span wise to accommodate the implicit dihedral due to the tapered wing. 1.5mm balsa webs on both sides of the spar combination over the centre 4 bays.

Lower le sheeting added with the wing jigged and weighed down to hopefully lock it warp free.

Added the upper and lower centre section sheeting and started work on fitting the retracts.

Coming along..

Merry Christmas

[Martyn K]

Forgot to mention. Weight of the wing so far is just under 500gms

Looking at just less than 1kg for the complete airframe less covering, radio, retracts and engine

Martyn

[Martyn K]

Mystic 64 day 22.. Wing is just about completed, weight is 590g.

And

Wingtip is laminated from one inner layer of 0.8mm ply with 6.5mm and 12 mm balsa top and bottom each side with the top hollowed out to save weight

Started fitting out the fus. Engine is canted through 120degrees so the silencer sits almost centrally in the tunnel. The side thrust skews it a bit.

Throttle linkage was a bit more challenging but it looks like it will clear the top of the silencer. Using a lightweight snake for the linkage terminated in a ball link at the throttle arm.

Radio installation is pretty straightforward

Weight of fus with motor and radio gear is 1.4kg.

Still got the ailerons, lower fus and cowl to make. Hopefully may just make 2.5kg but the cg is looking dodgy. With the flight battery at the front of the fuel tank bay it currently balances without wing, 20mm in front of the desired cg. Need to keep the covering at the rear very light. I'll move the motor forward by about 20mm on the final version of the plan.

More to come

Martyn

[Rich too]

Great stuff Martyn 👍