DeHavilland DH90 Dragonfly

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Steve Rickett 2333
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DeHavilland DH90 Dragonfly

Postby Steve Rickett 2333 » December 11th, 2014, 1:05 pm

This project has been in the planning stage for almost a year now...but the recent spell of bad weather has put paid to any further spraying of the Boston, so I've had no choice but to start cutting wood on this one!

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This was the last of the production Dehavilland Multiengined Biplanes (there was a DH92 Dolphin...but that was scraped after 3 months due to being too heavy!)

Whereas the Rapide was built for the airlines of the day, the Dragonfly was the biz jet of its time. The 1930's equivalent to the Learjet! Only 5 seat and powered by two Gypsy Majors, it was smaller than the Rapide and featured a moulded ply sheeted fuselage which was pioneered on the Dh88 Comet Racer and went on to be used on the fabulous Dh98 Mosquito.

Ive chosen 30% as the scale so it fits in my van with minimal de-rigging...and also it matches the scale of the Libellula!

This gives the model a span of 157" and I'm hoping will weigh around 60-65lbs. I've been testing a Kolm EZ50 in my Topflite Thunderbolt all season and I'm really pleased with the reliability, ease of starting and the sound! So....I've ordered a second engine for this project.......

Here are a few photos of the progress so far.....

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Home made oleo legs...

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Last edited by Steve Rickett 2333 on December 11th, 2014, 1:24 pm, edited 1 time in total.

Steve Rickett 2333
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Re: DeHavilland DH90 Dragonfly

Postby Steve Rickett 2333 » December 11th, 2014, 1:10 pm

Wings are very conventional...Cyparis spars, balsa ribs and then sheeted in 3/32" balsa. 3/32" rib caps and 1/8" balsa webbing create a rock solid structure.....

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Collection of parts so far......

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A new engine mounted to its nacelle, ready for the wheel pants to be made.

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Steve Rickett 2333
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Re: DeHavilland DH90 Dragonfly

Postby Steve Rickett 2333 » December 11th, 2014, 1:14 pm

The wheel pants will be moulded from fiberglass...so first I need to make a plug...Blue foam clad around a simple ply frame and lots of elbow grease to sand it down.

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The best tool to remove Blue foam? Try a Wire brush!

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Left over Plaster board filler from the bathroom makes light work of the gaps.

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Steve Rickett 2333
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Re: DeHavilland DH90 Dragonfly

Postby Steve Rickett 2333 » December 11th, 2014, 1:18 pm

And to bring the thread up to date.....

The plug has just had a coat of Resin and 135g glass cloth...

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I'll keep working on this plug to get the finish smooth and even before tackling a little bit of detail and then the mould making.....

Bob Thompson1894
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Re: DeHavilland DH90 Dragonfly

Postby Bob Thompson1894 » December 11th, 2014, 2:50 pm

Looking forward to this one!

stuart knowles 1611
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Re: DeHavilland DH90 Dragonfly

Postby stuart knowles 1611 » December 11th, 2014, 10:00 pm

You do make it look easy. - I know its not but I do watch and learn. Thanks for posting
stu k

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paul needham
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Re: DeHavilland DH90 Dragonfly

Postby paul needham » December 11th, 2014, 10:30 pm

A very nice subject to model Steve, can I ask what glue you use on the blue foam blocks for the plug ?

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Rob Buckley
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Re: DeHavilland DH90 Dragonfly

Postby Rob Buckley » December 12th, 2014, 12:53 pm

That's some speedy looking building there Steve, you've not been using the table as a building board have you??
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LMA Secretary - I've got a reasonable idea where you live!

Steve Rickett 2333
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Re: DeHavilland DH90 Dragonfly

Postby Steve Rickett 2333 » December 13th, 2014, 1:49 pm

No Rob! I couldn't get the glue and scalpel past security....it's quite handy for resting an IPad on while check your favourite websites though!

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Rob Buckley
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Re: DeHavilland DH90 Dragonfly

Postby Rob Buckley » December 15th, 2014, 7:36 am

Fantastic stuff!!

Can you say where you were at the time?
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Steve Rickett 2333
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Re: DeHavilland DH90 Dragonfly

Postby Steve Rickett 2333 » December 15th, 2014, 10:52 am

Errrr....probably China!

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Re: DeHavilland DH90 Dragonfly

Postby Steve Rickett 2333 » December 21st, 2014, 10:16 am

After a good rub down, the plug has received one brushed coat of primer and one sprayed coat.

I'm not putting much detail on the plug as I don't trust my fiberglassing skills to pick out the fine detail! I'll add the detail after the moulding process!

However, I do need to add the little overhang that is very characteristic of Gypsy cowls, around the exhaust.

I've simply extended the cowling down with an offcut of pro-skin, tacked on with cyano. The resulting gap was then filled with auto-body filler and sanded flush.

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Two coats of Gloss Black Klass Kote brings a nice shine that I will be able to polish to give myself the best chance of a release when it comes time to lay-up the moulds.

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I'm going to put this plug to one side now for a few weeks, to allow the paint/glue etc to fully harden. I've learnt that gasses can still be given off by paints etc long after you would think they had dried. These gasses can play havoc with the moulding process and I think is one of the reasons I (like others) suffer from wildly different results every time I make moulds!

Time will tell!

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Re: DeHavilland DH90 Dragonfly

Postby Steve Rickett 2333 » December 21st, 2014, 1:21 pm

I've started to make some of the first parts for the fuselage.

I'm approaching this fus quite tentatively as it has a few very awkward curves and angles plus the fact that the rear end has be very light indeed. Jigs are going to be the most valuable part of building this fus, as well as building the thing in stages.

I started by making a simple jig to replicate the curve from the rear of the wing to the nose. These are simply 1/4" ply rectangles that raise the stringers off the work board. The stringers themselves are then laminated from 2 x 1/4" sq Cyparis lengths using Aliphatic glue.

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Once the glue on the stringers had set I cut out a 1/16" ply sheet that will form the main strength of the nose section. This ply sheet will sit on the inside of the stringers.

This sheet and the pre-laminated stringers were re-installed in the jig while the glue set.

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Vertical posts added between the stringers at the former locations help to support the outer sheeting. This outer sheeting is going to be 1/32" ply infront of the CG and 1/64" ply behind the CG. I've done a few experiments with the weight of 3/32" balsa versus 1/64" ply and they end up basically the same. The benefit to 1/64" ply is that I can get it in 4ft sq sheets and it won't need so much resin when I come to fiberglass the whole lot. Though not the cheapest wood in the world, Stuart at Solutions did me a very good deal (along with most the other wood for the project.) Thanks Stuart.

I've had wood from all the UK suppliers (both balsa and ply) and I have to say that Stuarts' really is the best you can get. I've had some 'high quality' Birch ply from other suppliers which is quite frankly not up to the job.

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Re: DeHavilland DH90 Dragonfly

Postby Steve Rickett 2333 » January 1st, 2015, 10:51 am

Happy New Year everyone,

Not too much been happening over the past few weeks but I did get the new engine installed in the Thunderbolt so it can be run in. I've had 50 flights with my first Kolm 50, which is about 8 hrs of running. I'll do the same with the new engine to get them as matched as possible.

Had a great day at Hucknall on the 28th and another good day at Winterton on the 30th...cold, but crisp. Perfect running-in weather!

Thanks to Neil Hutchinson for the photos!
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Peter Siggins
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Re: DeHavilland DH90 Dragonfly

Postby Peter Siggins » January 1st, 2015, 2:56 pm

Lovely subject Steve,some years ago Maurice Thompson from Doncaster built one about the same size and test flew it at Goosedale .unfortunately it was a bit underpowered - two 22cc Zenoahs and the resulting tips tall damaged it severely.
It looked fabulous in a light blue finish.
All the best with the build
Pete

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Re: DeHavilland DH90 Dragonfly

Postby Steve Rickett 2333 » January 12th, 2015, 9:13 pm

Thank you Peter.

Thats very interesting....I wonder what happened to Maurices' model?

I've built a jig for the fuselage to be built in over the past couple of weeks.

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Laser line used to ensure a truly accurate datum.

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Then the plan view is pinned to the jig.
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Steve Rickett 2333
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Re: DeHavilland DH90 Dragonfly

Postby Steve Rickett 2333 » January 12th, 2015, 9:15 pm

The centre section has been mounted and strapped down to the correct incidence. It is sat on blue foam supports and the chip board goes across the top of the wing...a snug fit so it doesn't move but doesn't damage the wing either.

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Cheap kitchen unit blocks and three screws hold everything together tight.

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Steve Rickett 2333
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Re: DeHavilland DH90 Dragonfly

Postby Steve Rickett 2333 » January 12th, 2015, 9:16 pm

Supports for the rear formers made out of chip board...

Those horizontals mark the very bottom of the fuselage and have been lined up as accurate as I can using my trusty meter rule.

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The foremost nose former is also perched on a jig and clamped into place....

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Steve Rickett 2333
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Re: DeHavilland DH90 Dragonfly

Postby Steve Rickett 2333 » January 15th, 2015, 12:13 pm

All that weight up the front is very good...but I am 100% certain that it won't be enough to get the balance point in the right location. With such a short nose, and a loooong sheeted tail I can admit defeat right now and accept the inevitable 1/2 a church roof of lead. With this in mind, I set out a very rough sketch of the airplane and the weights of key assemblies and used my rather no existant Maths skills to produce some moments. (head scratching moments mainly).

I won't bore you with the gory details but suffice to say that I estimate a whopping 5lbs of lead up the front to get the correct CG. Clearly this is only a guesstimate, but it gives me a rough idea and means I'm not so shocked later on! Securing such a large chunk of 'hardware' to a sharp and pointy nose is never going to be easy so I've elected to build some of the lead permanently into the structure. This way, it'll not take up any space, can't break loose and will give them impression that I've built this thing with only a tiny bit of ballast required (shhhh, don't tell anyone.)

So, that's two ounces shy of 3lbs epoxied into the void between the fuselage skins. It won't be enough...but it's a good start.

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While fettling the fit of the formers I had to pre-assemble the two formers that make up the fuselage join. These are made from hard 1/4"balsa with 45 degree joins so that the grain is always traveling across the 'span'. 1/8" ply load spreaders were cut out by tack gluing four bits of ply together and then cutting them all out at once......

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M4 blind nuts installed to the rear of the pair of formers and then installed in the fuselage. I've added some 1/16" packing between the formers to allow for some ply facing and also the thickness of the fiberglass and paint. It also gives me a bit of clearance for a hand saw when it comes time to chop the tail section off.....but I won't do that until the sheeting is in place.

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Peter Siggins
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Re: DeHavilland DH90 Dragonfly

Postby Peter Siggins » January 15th, 2015, 4:42 pm

Hello Steve,
I don't know what happened to Mairice's Dragonfly - never saw it again.I know what you mean about lead - I built a 27 1/2 % Cessna 421 c once and was totally shocked at having to put 61/2 lbs lead in the nose even with its long nose and engines way in front of the CG.Just a thought I secured the extra lead in the nose using an aerosol of expanding foam - pushed in blocks whilst it was wet - never moved.
A question on your mold for the cowl/pants - do you slit the glass off the mould or split the mold in any way to get it released,I'm jumping the gun a bit as I know you'll get to it..
Cheers Pete


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