DH87B Hornet Moth

[Danny Fenton]

Hi John, the Cub has been in two editions of RCM&E so hardly a secret

Cheers

Danny

[john stones 1 - Moderator]

Ooops that's me caught out then, i don't always get the mag

Superb Danny, and i can't see any trimming on the waggly bits

John

[Danny Fenton]

Tsk tsk John

I am quite pleased with the model, my first foray into indoor R/C design. It might be a free plan in a future RCM&E if my scratchings pass muster. I am however struggling with the C of G, these smaller models require a slightly further forward C of G than normal (20 - 25%) and it has cost me dearly..... 10g of nose weight

Interestingly the model turns quite well considering almost zero dihedral....

Cheers

Danny

[Martyn K]

Completed the tailplane:

Then scrapped it because it was too heavy (just over 3 grams)

Version 2 underway..

This one has the outline laminated from 1/32 soft balsa (3 laminations) and then are a mix of thin and thick 1/32 balsa ribs. (Thick ones where I need a stronger glue joint)

Need to add the trim tabs, sand to shape. I am then going to cover and water-shrink the bottom of the tailplane and a mist of thinned dope. Pin the tailplane back down again with the tissue side downwards then add the short riblets, gluing to the LE and tissue then add the upper covering. Hopefully that will give me the appearance of scale rib spacing without any excess weight.. It may also means scrapping it and starting again.. If it works, I'll separate the elevator using a single sided razor blade which is thinner than my Swann scalpel blade. Hinge using cotton like Danny did on his Cub

Just to add to the irritation, the hinge line for the elevator isn't quite perfectly straight

More to come

Martyn

[Danny Fenton]

Hi Martyn, good idea. The same thought sprung to mind as to attaching the riblets to the leading edge and tissue. The profile is no doubt flat on the bottom. Will be interesting to see if it works You could just leave the riblets off and give the rest of us a chance

You may want to pre shrink your tissue, and use banana oil to stop excessive shrinking

Cheers

Danny

[Martyn K]

The tailplane with the core used a NACA 0006 section. The new one will be mainly flat... The challenge will be not to let the glue seep through the tissue. I detect a Pritt Stick on the horizon..

Tissue on order - hopefully today. Got some Silver lightweight Esaki coming which will hopefully only need a smattering of paint to give it a nice sheen.

Martyn

[Danny Fenton]

Yep great choice on the silver esaki You might be okay to just add the riblets to the leading edge, and not actually glue to the tissue at all..... Let the dopebananaoil stick them

[Martyn K]

Don't trust myself. It will go pear-shaped when the tissue goes down and then its too late. They will be stuck to the tissue - just enough to hold them.

[Martyn K]

The new tailplane weighs 1.3 grams. 2 grams lighter than the version with the wooden core. Still a bit on the heavy side, the tail moment is quite long and the nose is quite short I don't want to add nose weight unless I have no choice.

I started sanding it down last night. Apart from being a bit porky I am quite pleased with it.

So, I bit the proverbial bullet and made a start on the wing last night. Wing construction uses 'hollow' wing ribs, basically a cap strip with no supporting wing rib - top and bottom.

One challenge is to get the LE located so it wont move while the wing is being assembled. The LE is diamond orientated so I made a jig from 2 strips of triangle stock - like this

The LE (and TE) is slightly curved so a sliver of clink film added into the Vee..

You can just see the LE in there somewhere

another view

TE and lower cap strips (1/32 x 1/16 wide) added

and then the lower spars added.

Left to dry overnight. I added one top cap rib this morning - 1/16 x 1/32 wide (precut to the upper curvature) this morning and left it to dry. This bit is going to take a while.. and I have 3 more wings to do..

More to come but now slowly

Martyn

[Martyn K]

Still plodding on with this. I cant pretend that the dialogue is riveting (sorry Danny). Its basically a case of add a few ribs, jig them in place using pins, let dry and then add a few more. However, my productivity has improved. It took 5 elapsed days to assemble the first wing, only 2.5 days to do the second..

A couple of photos - both upper wings are now just about complete.

Right Upper wing with riblets fitted.

Laminated wing tip for the Left upper wing. Riblets trimmed/sliced back to the spar.

its handy to be able to weigh the ribs down when the glue dries. the Swann Morton no 4 scalpel also gives a good sense of scale. I normally use a no 3 handle and razor blade on this sized model

 

Amazingly, I have only broken one wing rib so far.

The right upper laminated wingtip was fitted this morning.

I now need to think about how I am going to actually fix the upper wing halves s onto the fuselage top and then worry incessantly on how I am going to do the ailerons and the swivel panels for the wing fold mechanism.

Its now getting tricky.. 3 weeks into the project and 6 weeks left to get it completed. This is going to be tight. A Kwik Fli is an absolute doddle compare with this..

More to come.

Martyn

Edited By Martyn K on 24/01/2017 09:50:42

[Danny Fenton]

You will do it Martyn, I have every faith. Personally I would dump the wing fold idea unless it is part of crash resiliency wing attachment.

I know I have gone against the grain by fixing my Cub wings with no give. But my reasoning is don't crash!

Cheers

Danny

[Martyn K]

LOL,

I am not actually going to fold the wings but I want to delimit the Wing fold panels. I'll only kick myself if I don't do it now.

I have a plan.

I am still scratching my head on the ailerons. I am going to use torque rods - probably thin ali tube as I can easily crimp bend and glue these to a central servo in the wing root. Lower wing will be in one piece and will be removable for access. Probably held in place with a pair of 10BA bolts and central pegs. As the lower wings need to come off then so will the uppers. These are simply plug ins using thin short stubby 20g piano wire joiners. The idea is that the lower wing carries all the flight loads and the upper wing is supported using struts and wires.

You can see why I am worried...

Martyn

[Danny Fenton]

Sounds good to me Martyn, if you haven't a visible cockpit ceiling then piano wire and tubes is a good springy solution, put a slight kink in the piano wire and you will get a nice friction fit.

If I was doing ailerons in my Cub I would use fishing line and spring load the aileron down, then one line to each aileron to pull and lift it. Getting the spring rate right might be interesting, rubber band perhaps? it would need to be very soft.

Will be good to see how you crack it, torsion rods will work but the mechanics may be tough to keep light?

Cheers

Danny

[Martyn K]

its the weight that is bothering me. Fortunately, the ailerons are quite long so the rods will be short but at least I can bend them up to make the horns etc.

The upper wing joiners are a bit tricky as the full size has a perspex roof. My idea is to use a square 'U shaped joiner linked to the front and rear spar on each wing and then simply bind these to the upper longeron. It will be just over half the weight of 2 separate joiners because the fus is so wide

The upper wing will mate to a small faired in to the fuselage top rail that extends about 6mm in front of the cross member, It should give me a nice junction if my building skills are up to it. Also handy as there is a colour change here

Do I need a pilot?

Martyn

[Paul H]

Hang in there Martin, I suspect you might share the feelings I have occasionally of desperation and a strong desire to chuck it in the bin but once I push through confidence starts to build again. My learning curve on this project is very steep not least because time is against me. Definitely a case of if I was going there I wouldn't start from here but you live and learn and the challenge is (sort of) enjoyable

[Martyn K]

Cheers Paul

I have no intention of binning this unless the cat sits on it. However its now a balance of what I can reasonably achieve in the time I have available. I'll start work on the lower wings tonight. I thin I know what needs to be done. I will be happy when experience proves me wrong

[Colin Leighfield]

I'm looking at the details of my (much less ambitious!) Wicko project while I'm away. The wing is also complicated by a considerable glazed area in the roof, I'm pretty certain now that I will make the wing in two halves, they and the struts will be attached together and to the fuselage by tiny neodymium magnets. The model will therefore comprise seven parts:- fuselage with tail surfaces in one piece, two wing panels and four struts. That makes everything completely accessible and those magnets are strong so there's no way that it will come apart in the air. I've got quite a few in different sizes I bought through EBay. I attached the very large under cowling to the Super Marauder using only these things and it takes some shifting. No sign of it moving on the first two flights last week.

I only mention this because it simplifies the design and adds a lot of convenience. I just need to think about how I set up the ailerons doing it this way, I think I can do it.

[Martyn K]

Cheers Colin. The risk with something that needs a lot of force to separate it, is that there is an increased risk you'll break it in the process. I was going to use a magnet to secure the lower wing but have since decided against in..

I cant wait to see how you progress..

Martyn

[Colin Leighfield]

I did think about that Martyn, it's certainly a risk if you give it a direct pull! Sadly I can't do anything to test the idea until I get home on 11 February, but this is a pretty simple project so hopefully I can quickly resolve what works and what doesn't.

[Martyn K]

Just found this cracking YouTube video of a Hornet Moth. Lots of scale detail that I wasn't aware of

Watch it in HD - Full screen. Very worthwhile

[Colin Leighfield]

I enjoyed that. Something about these thirties and forties flying machines that doesn't seem to date when it comes to actually flying in them. It's an element of character and personality that can't be improved on. A timeless quality that can't be substituted. When I flew the Auster AOP6, in spite of it's tube and fabric construction, it had an air of solidity and smoothness in the air that I never felt in any Cessna. It might have been more fiddly to operate and less convenient, but once it was airborne I greatly preferred it. Hard to explain really, but our wish to model these things might be some reflection of that perception.

[Martyn K]

Still working on this. 24 days into the project, 30 days left to complete...

Wing 3 is now on the board and ready to lift and add the wingtip

The ailerons on the DH87B were lower wing only, and quite long.

The plan is to use aluminium torque rods which I can also bend and crimp and make into horns. I only need about 100mm per wing - a 300mm length weighs this much:

< 1.5 grams. about 1 gram for both halves plus glue

The ailerons look like this at the moment

and

the spar and the aileron LE will be carefully built up so they are flush with the upper and lower wing ribs.Mainly 1/32 sheet balsa. I can then separate them..

With that well underway and the glue drying, I decided to tackle the upper wing mounting.

My plan - it may be a bad one - is for the lower wing to carry all the flying loads. The upper wing will essentially be a cantilever supported by pins at the root and the struts and the landing and flying wires.

So using very short pins epoxied into the wing root and short ali tubes in the fuselage root the first mounted wing looks like this.

Gently jigged into place with the correct dihedral angle set (by tilting the fuselage.

You can also see where I have had to extend the upper stringers. Its amazing how everything starts to click when you see it as an object.

There is also a soft balsa upper fairing above the cockpit that has been added and sanded to shape. The root ribs are balsa on both sides - hope it will be rugged enough.

I have spotted one 'major' error in my drafting though. The side windows should have been inclined slightly (about 3mm) - mine are vertical.. (They looked vertical in the photos - except 1. It means that the curved fairing is totally inboard of the root ribs - it should have been partly outside the root as well. Hey-oh.

A close up showing the pins and tubes.I still have to make the wing fold panels but at least they will be in the right place..

The rear cross member will also need to be reinforced slightly to carry the additional load

It's coming along though. Wing no 4 will be started tonight and probably completed over the weekend then I can think about the aileron servo and the rest of the flight gear

More to come.

[Danny Fenton]

Looking good Martyn, you do seem to be enjoying the challenge

Cheers

Danny

[Martyn K]

Its very different. I am quite pleased with it, mainly with the engineering challenges I am facing. I could quite happily not build a 4th wing though,, A bit like pulling teeth. Still once done, then its done and I can concentrate on some of the more interesting bits, Wing fitting was fun last night.

Still dreading covering and painting though

Martyn

[Colin Leighfield]

The structure detail is getting close to full-size. Achieving strength by structure design rather than material mass. You really need incredible patience to do something like this.