Curare 25

[Bob Cotsford]

a good question - but I wanted them to build the other wing and tail! In actual fact if the rods are stiff enough to keep everything aligned during construction then they will be heavier than you would want for spars. The wooden structure is plenty strong enough with hard spars, full webbing and 3/32" sheet surface acting as a stressed skin. I've never had a wooden wing break mid-span unless there are serious stress raisers (see any Flair Scout wing where the centre setion sheet transitions abruptly to quite a flimsy outer wing structure). The usual point of failure is at the centre - either the panel joint or at the fuselage. This will not happen with this win g as there's a 30mm ali tube joiner across 4 or 5 bays per wing. At 80" span this is an old-style aerobatic model, quite graceful in flight, some might say stately and reminiscent of a galleon under full sail. Not like the Curare (or my Tornado) which would be flown in a more ballistic high-G style.

All pretty irrelevant to the Curare 25 - sorry

[Magnus Hansen]

Hi,

I still fly my MK Curare20 from 1980. Agreed, I made a 20 odd years break.

At the time I had an OS40 (light as an OS25fsr...). Now I changed to an outrunner (RCtimer3542 with an APC9x6 and 3 cell 5000mA lipo) and added electric retracts. It weighs about 1.7kg. Flies well from a grass field but a tad heavy. Whissling... Adrenaline is lacking with the outrunner, however.

The information I'd like to give is that the MK curare is a bit simplified compared to a straigh down scaled .60 (that I still fly as well...). The stab is simply flat for instance. Just keep it light. I had colleagues building MK Curare20 with retracts and tuned pipe covered with fabric and dope of about 1250 grams. Needless to say, I was not even close...

Best wishes!

/Magnus

Edited By Magnus Hansen on 19/10/2012 15:05:38

Edited By Magnus Hansen on 19/10/2012 15:06:25

Edited By Magnus Hansen on 19/10/2012 15:17:12

[Martyn K]

Hi all

I agree that we need to keep the weight down. My AstroHog 25 was 3.5lbs (fully fuelled) with a slighly larger wing (with semi symetrical wing section). That definitely was not underpowered using an SC25. It eventually lost steam on vertical maneouvres though. I can live with that. I would like to keep this below 3.25lbs ready to fly.

Regarding the tapering of the spar and LE/TE doublers. Yes - this is to provide a linear join for the ply wing joiners. It also provides a nice taper from a high stress area so there are no sudden changes where a fracture may occur. I have to admit, I don't like aluminium wing joiners. In fact I hate using them, they are too stiff, they dont glue particularly well - even with epoxy.

I did consider carbon spars - I can see the benefit of them having looked at your photo, but I am trying to build a replica of an original using traditional (for the time) construction techniques. If you stray too much then the originality claim starts to look a bit hollow.

Liteply. I consider this to be very hard balsa - it has its uses. If the part warrants it then I would go for a 3mm liteply rib rather than a 1/16 ply birch rib. It is easier to cut and has a larger contact area for gluing. But that is a personal choice. Everyone to their own

Still working on prepping the ribs for laser cutting, it will take a couple of weeks to get that bit completed. I have decided to make allowances for the doublers in the laser cutting templates, but will be edging on the side of safety. . I never envisaged a partial kit when I started on this.

Martyn

[Bob Cotsford]

No need for carbon spars Martyn, I only used them as a jig in the Altair. Years ago anyone jigging a wing would have used 1/4" steel rods, I used carbon as it's easier for me to buy, is more rigid than steel over that length, and I can re-use it more easily than steel. For a 24" wing panel the rib tabs will be fine to build a straight wing.

I never expected that you would take on the task of preparing the drawing for laser cutting, but good on you for giving it a go.

I now have an ASP 28 as it was cheaper than a 25 (?), but as I also have 4 kits waiting to be built and winters nearly here, take your time!

[Andy Palmer]

Rather late I know but I have just found a photo of my MK Curare 25 from many years ago. It was powered by an OS25FSR and covered in orange nylon, it was a great flyer. Forget what happened to it unfortunately. Thought it may be of interest.

Andy

[Martyn K]

Thanks Andy

That gives me something to aim for. Can you remember what wingspan and weight it was please?

Regards

Martyn

[Andy Palmer]

Sorry Martyn, I honestly don't have a clue

Andy

[Martyn K]

I have actually started building this model. Work has started on the tailplane, photos and some words to follow shortly.

However, I have been having a think about the wing section. As you may be aware, the original full size model used a thickish symetrical section and in my variant, I originally went for a 14% symetrical NACA at the root moving to a thinner symetrical 10% NACA at the tip.

I have been having second thoughts about this and I am starting to think that a slightly semi symetrical section is a better way forward for this model. Consequently, I have produced a hybrid NACA, 18% upper surface and a 10% lower surface.

Here is the latest A3 layout.

I think that this will provide a more forgiving model, offering a lower landing speed, providing the weight doesn't increase and almost no difference in the model's aerobatic capability.

The A3 PDF sheet can be downloaded from here

As usual, your considered opinion will be welcomed.

Martyn

 

Edited By Martyn K on 31/10/2012 23:02:15

Edited By Martyn K on 31/10/2012 23:02:42

[Tony Bennett]

thats the same section i have on my own design and that flies like a trainer on low rates and a monster on full throws.

lands realy gently as well.

[Martyn K]

Thanks Tony

That sounds just perfect

Martyn

[Martyn K]

Hi all

I have just completed the DXF templates for the tailplane and about to start on the wing. As I don't want to do this twice (it is very time consuming) , I am planning to prep the semi symetrical section as laid out above unless someone (who will want a laser cut set) shouts very quickly 'don't do it' and then we can have a quick discussion. My prototype is going to have the semi symetrical section..

Regards

Martyn

[Bob Cotsford]

Martyn, is the wing thickness still the same with semi? I've got to say that for myself I'd stick with fully symetrical but semi may give a more forgiving model if your not intending to emulate the originals character. It all depends how you intend to fly it I suppose. My Scallywag (trainer version) has a semi section and while it's still fully aerobatic, the section needs to be taken into consideration when trimming it. For example, it needs more down than up elevator to get similar sized inside and outside loops, it needs more down holding in when inverted than I'd like and of course it needs downthrust to limit the rate of climb with the throttle open.

By way of contrast the MiniPanic has a fully symerical section yet is a doddle to fly at low power settings while needing little or no down for inverted, depending on how it's balanced. When it finally comes the stall is a little sharper than the Scally, but not much.

[Bob Cotsford]

another example closer to the Mini Carare- the Formosa II, symetrical but a real pussycat that can be floated around and hovered in a breeze.

[Reno Racer]

Just noticed a kit version available here; useful for reference.

[Bob Cotsford]

As it happens I'm putting the Wega-Sunshine Blue Angel together now, I only started it on Friday. Their models are designed using a glass fuselage and balsa covered (dense) foam wings/tails so I think their quoted 2.3kg for the Curare 40 is only representative for the 56" model if you go for a very basic installation and minimal paint. No Prettner drag flaps or retracts.

[Martyn K]

I mentioned that I had started work on the tailplane. It is a very conventional build, there are no tricky bits..

Laying out the tailplane.The LE and TE are levelled by sliding them up and down the pins accordingly. The tabs on the ribs keep the symetrical ribs in the correct relationship to each other.

LE and TE anhederal braces fitted. Yep - this is upside down. A problem with the plan, the ply braces as shown on v0.3 are at double angle. My error, I'll get that corrected and upload a revised plan

This shot show the spar doubling at the root and LE to provide a flat surface for the 1/32" ply braces. You can also see the tapered spars, simply cut from1/4" balse sheet. Very easy to do but you need to take care. Cut slightly oversize and "adjust" with a plane.

Wing tip blocks carved from 1/2" soft sheet balsa

Finally, very light 1/16" balsa infill at the root. I haven't quite finished this yet, I am only fitting one piece per evening to allow the glue (PVA) to dry so as to risk not disturbing each piece after it has been fitted.

Finally, I started cutting fuselage pieces tonight. Still thinking about the wing section..

More to come.

Martyn

[Pat (rActive) Harbord]

Hi Martyn, have I missed the tail ribs on the plan? I'm itching to get started

your build looks great so far

[Martyn K]

rActive,

The wing ribs are here: The wing uses a NACA 0014 tapering to a NACA 0012. The tail is a NACA 0011.

Sheet 1 (Symmetrical Section)

Sheet 2 (Symmetrical Section)

Tailplane

All should(!) print full size on A3 paper...

I am still thinking about using a semi-symmetrical section for the wing although Bob has swayed me back to sticking with Symmetrical. If you go with the assymetrical section the PDF sheet can be downloaded from here (semi-symmetrical Section)

Now that I have got the pro version of Profili, I can incorporate them into the plan.

I'll try and get the plan updated later this week - I'll probably split it into 2 sheets. I am also starting work on the fuselage and I need to add some detail on the upper decking on the 25 version.. I also need to think about the cowl. I am not sure if the original had a removable cowl or the engine was simply built in. Whichever way we go, its a very tight fit. I have some more photos to upload shortly, but I have not overly committed myself yet.

Best wishes

Martyn

[bouncebounce crunch]

Wht the Blardy hearl Is Martyn K doing with that ring on his finger? it will interfear with my radio system again again and again I have crashed three models this month blast himm.

[Martyn K]

Crumbs - is that why I keep crashing...? I thought it was incontinence.

M

[bouncebounce crunch]

Back to your model Martyn, I was wondering why? or If? you thought of sheet with fake ribs rather than the fully built up girl there.

Thats it shot self in foot for not reading full text. whoops! there goes another shot.

 

Edited By bouncebouncecrunch on 07/11/2012 14:31:08

[Martyn K]

Hi BBC,

in a word - weight. The model has quite a long moment arm and a very short nose. Every 10g at the back needs about 60g at the front to compensate.. I have deliberately ommitted cap strips and webbing to keep the weight down and have used tapered spars from light (not cheesy) balsa. It all helps.

Not stated above so your remaining foot is safe.

Martyn

[Bob Cotsford]

Martyn, most models of this period had a fixed cowl with a big hole in the side large enough to wiggle the engine into place and secure it to the commonly used wooden beam or ply plate bearers.

I think the ARTF revivals tend to go with a f/g cowl and conventional radial mount.

Edited By Bob Cotsford on 07/11/2012 16:30:05

[Martyn K]

Cheers Bob

The SLEC long mount that I am using is exactly the right length from the firewall to the front former behind the prop driver. I will be screwing the former to the engine mount then sheeting to the firewall. I am not sure I want to get into moulding G/F however, I would like to get a removable cowl. I have cut the 1/32" ply doublers so that they protrude forward of the firewall and these could be used to restrain a removable cowl. I just need to weigh it all up when I start gluing - probably this weekend.

Martyn

[Martyn K]

I have cut the two fuselage sides(from 1/8" x 4 medium balsa) plus the 1/32" ply doublers. Standard 36" sheet needs extending by about 3" to get the fuselage to fit.

Alternatively, build the cowling separately.

The firewall/engine mount is 1/4" plywood, the remaining formers are cut from 1/8" liteply.

Additional 1/8" balsa doublers (not shown on plan yet) around the tailplane area.

The Firewall is drilled for the SLEC engine mount.

It is quite literally exactly the same width as the firewall.. I said there wasn't much spare space.. There is insufficient space to use the usual captive nuts so I placed M3 bolts from the back and epoxied them into place...Nyloc nuts from the front hold the mount into place..

Using the batteries that I use to keep things flat and square, this assembly was left to dry overnight while the glue hardened..

The upper 1/2" sheet triangle reinforcement has been added.

The steerable nosewheel mount is simply a suitable hole drilled through the thickest section of the engine mount. It is a bit further forward that I intended, but it looks like it will be OK.

The remaining 2 rear formers (F3 and F4) have been glued into place.1/4" triangle section helps add to the adhesive contact area.

Finally (for tonight), the assembly around the tank has been started. These pieces are made from 1/8" ply. Again, there is very little spare space. The idea is that the tank can be slid out from the area in front of F3. I am still not convinced that this is a good idea.I would rather have the extra strength rather than the convenience of a removable tank

More to come...

BW

Martyn