DeHavilland DH90 Dragonfly

[GrahamKennedy]

My favourite DH aircraft Steve - I'll need to make the effort to see this one.

Now that I've "downsized", I'm tempted to have a go at one for indoor RC - I'm not sure on it's suitability for free flight....

[Steve Rickett 2333]

Thanks Phil,

Hi Graham, I saw one at Gaydon a year or so ago, looked to be about 25-30" span. Might have been Peter Iliffes friend. I think it was for Free flight rubber of all things!

[GrahamKennedy]

Not sure who that could be - I've missed the last couple of Indoor Nats cos of my role within CIAM/FAI.

I'll do some investigation.

PS: What's your projected weight?

[Steve Rickett 2333]

It wasn't finished when I saw it, and I'm not so sure it would have been an indoor model. It was very nice though and gave a little inspiration to this project. Im hopeful that it'll come out at Between 60 and 70lbs.

[Steve Rickett 2333]

Aileron servos have been fitted.... 1/8" ply hatch mounted onto Cyparis bearers, the servo is mounted directly to the hatch with 2mm Aluminium Angle. The servo arm just pokes through the bottom to allow for the linkage.

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I've also tidied up a few areas around the strut mounting bolts. I made some balsa reinforcing for the covering to stick to and then lined the hole with a suitable length of brass tubing.

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[Steve Rickett 2333]

The interplane struts are primarily Cyparis with steel brackets at either end. Onto these brackets I've brazed two 4mm nuts to accept the mounting bolt. The bolts are simply inserted from the opposite side of the wing and clamp the two wings tight.

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These struts will then get clad in balsa, shaped and glass clothed.

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Those blocks are not a new version fitted with skis but simply jacking the model up to the correct angle (Wheels yet to be purchased!)

[Steve Rickett 2333]

While assembled I discovered a bit of a faux pas.....

Its a little tricky to explain in writing but essentially, when the lower centre section twists (only slightly) during ground handling, the resulting loads pass up through that 'A' frame strut and transfer into forward and aft movements...only a slight twist on the centre section gets amplified considerably and is trying to move the top wing forwards and backwards. So, I thought it wise to stop the wing from moving too far by building in a steel strap near the trailing edge of the top wing....these can be bolted onto the very sturdy main fuselage former which should lock it all tight....

A little bit of surgery....

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The steel strap is glued and screwed to the rear spar with ply webbing either side the spar to lock it all together.

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This is where the strap ends up at the moment...I'll have to make a spreader plate to get the loads into that rear former. Shouldnt be too much work.

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I made up some mounting plates for the front wing tubes too. These lock into the former with a tongue and groove. There isn't really much stress on these points as it is all spread about by the two wings, struts and rigging....plus, it's never going to see any real 'g'.

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[Peter Siggins]

That's beautiful Steve - your progress is astonishing considering some of the problems .
Shame to cover it.

Pete

[Steve Rickett 2333]

Not had a lot of time for building these past few weeks, but in dribs and drabs I have clad the interplane struts in balsa and fiberglassed them. Then added lithoplate fairings.

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I ordered some rigging from Aeroscale in Switzerland which arrived today. Absolutely beautiful pieces of kit, and really add an incredible amount of strength to the wings. Even uncovered, these wings are now rock solid.

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[Steve Rickett 2333]

Once the rigging was installed, I had a 'Thank Goodness!' moment when the wing panels slide off the fuselage without binding. I had been worrying about that since I first started building! Now it means that the whole model can be disassembled to fit in the awning with just 4 bolts!

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[Steve Rickett 2333]

I have started glassing the bottom the fuselage...simply because I felt like doing some glass work! This is 48g cloth applied with West Systems epoxy. I bought a job lot of West systems about 6 months ago (maybe more) and noticed that all the tins had crystallized. 2 hours in the oven at 50 degrees C soon solved that. It's amazing seeing the difference when after the heat treatment.

[Steve Rickett 2333]

Next up, I felt like a spot of covering! It's not until I've done some of these build threads that I realised how much I 'dart about' from job to the next...I hope you lot can keep up!

First job was to install some wiring for the Nav lights. I used servo wire because that's what I had, I could have unwound one wire, but I thought it would look neater like this! Anyways, I'll probably use a 3 pin ashlock at the wing joint, so it would look odd with just 2 wires! lol.

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These wires are then protected with some masking tape as they exit the wing.

I am using Diatex 1000 fabric, which is a fullsize covering material used on microlights and gliders etc. It's really nice, about the same weight as Solartex (maybe a shade more) but much cheaper and has a nice tight weave (as opposed to Sig Koverall which takes hundreds of coats of dope to seal! ugh!)

As this fabric doesn't have any adhesive (though it can be heat shrunk....more on that in a mo) I apply it with a fabric cement. Again, as used by the Fullsize bods. I have found that it needs thinning about 60% thinners to 40% cement. Otherwise it is so thick you can feel the glue underneath the fabric!

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[Steve Rickett 2333]

What's even better is that Diatex comes on 1.5m wide rolls, so there is less waste.

Before you start with the fabric, give the main surface areas a coat of the thinned fabric cement and leave the dry (only about 5 mins). I don't do the ribs, just the LE, TE and root and tip.

Then cut the fabric leaving about 2" all the way around.

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Apply a little cement to a corner of the fabric with a brush and it should 'grab' within seconds.

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Work all the way around the wing, but don't do the ribs.. I find it just isn't necessary as the final coats of dope will do this.

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The edges were trimmed down to about 1/2" and folded over. The fabric will tend to bow out from the tight corners, this isn't a problem as the heat shrinking stage will pull it all tight.

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Don't shrink those wrinkles yet...wait until the top has been done!

I had an awkward rigging attachment to cover around. I find it easier to cut a slit first off and then slide the covering over it. Saves awkward cuts later on.

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This wing panel wasn't so tight to begin with, but I guess it proves the amount of shrinkage in the Diatex.

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Covering iron set to 120 degrees, no more than 125 degrees as this is the fabrics limit. Going over this temperature could set you up for saggy covering in high temperatures!

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I shrank both top and bottom sides as evenly as I could to try to avoid any warps... I think it worked!

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[Peter Siggins]

Hi Steve,

I tried sig coverall and found it the weave really hard to fill.Using Diatex what thinners do you use for the adhesive?
Looks a really good finish,do Skyways sell small quantities?
So many question!Ah well

Cheers

Pete

[Steve Rickett 2333]

Yes, I had the same problem on the Libellula which was covered the same as I described above, but I used Sig Koverall instead of the Diatex. It must have taken 6 plus coats of Nitrate dope to fill the weave (and even then there was still to odd patch of pin holes!)

I'm hoping that the Diatex will only take 3-4 coats.

I thin the Fabirc cement with standard cellulose thinners, seems to be ok. The Libellula is now 6 years old and the covering is still in great condition and no sign of bubbling in the heat or cracked/crazed dope. That tin is the smallest that Skycraft do, and it is certainly way too big for our uses. I think it is 1 litre, it does seem to store fairly well though.

The final coats of Dope will be Nitrate Dope (as most paints will stick to it!) thinned about 50% with the same cellulose thinners. The first coat brushed to get it into the weave, but the subsequent coats can be sprayed...I'll see how I get on with that, I've always just brushed it in the past.

[Peter Siggins]

Thanks for info - really useful

Pete

[Stuart Solomon]

Siggy, do it the old fashioned way and use nitrate dope as the adhesive. Remember the old nylon or silk method ?

[Tony morris]

Hello if you need any interior pics or other detail ones of G-AEDU let me know .
cheers

[John Greenfield]

Steve, and others

I now use Balsaloc to attach the Diatex as I find it much easier than using the fabric cement. Apply the Balsaloc round the edges of the part to be covered with a piece of sponge and just iron the covering down. Reapply the Balsaloc where there is an overlap of the covering.
I then dope all round the edge of the covered piece including any overlaps and then a very light sanding with 400 grade will feather the edge of the overlap before shrinking and doping the whole structure.

John

[Steve Rickett 2333]

Hi Tony,

Thank you for the offer....I visited Henry before G-AEDU was sold and got loads of external photos and a few internals but I think I could do with some more!

I would love to take you up on your offer...I'll be in touch.

Thank you.