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DH84 Dragon

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Chris Reid09/02/2019 12:00:06
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216 forum posts
95 photos

Well, we finally got there. After just over a year in production, the Dragon is 99% done. I just have to calculate the CG position, then finalise the battery location, and do some weight and power measurements.

The final finishing was uneventful but took a long time. It serves me right for going for the vanity "Chrisair" colour scheme. However, now that it's done I'm pleased I went to the effort as it is an eye catching finish. The Hobbyking film went on well as always, and the minor wrinkles were easily ironed away.

The assembly of the wings into two permanent box structures proved straightforward . The wood tipped streamlined metal struts were plugged into their PVA glue filled slots in the wings and the whole, very wobbly, structure plugged into position on the wing centre sections to set. One the glue had set, the wings could be slid off the centre section in pairs as planned, and their replacement takes only moments. They are retained by what looks like a large staple that fits across the wing joints at the main spar position. The ailerons have a clevis and carbon rod connector bottom to top from the servos in the lower wings, and look to work well. At the top and bottom of each strut there's a small hole which I threaded up with cotton rigging. I'm not too happy with the result. I had found some bright silver cotton to simulate the stainless steel wire of the full size. However, it proved to have no strength at all, and was replaced with a stronger black thread. It sort of looks the part, but it serves no real purpose, will add drag, and you won't be able to see it in the air. I may remove it again.

My Dragon is not super scale as I have made some minor alterations here and there to ease the build, and also to slightly enlarge the main planes chord as I felt that the scale chord looked very narrow. I don't think any of the mods are obvious, and I'm very happy that my Dragon represents the full size aircraft pretty well. I just hope she flies as well as she looks. Test flight report pending subject to better weather and the availability of my Club Chairman test pilot.

final 5 rs.jpgfinal 4 rs.jpgfinal 3 rs.jpgfinal 2 rs.jpgfinal 1 rs.jpgfull size.jpg

GrahamWh09/02/2019 13:08:09
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356 forum posts
53 photos

Lovely build Chris and a clever way of dealing with the wings. Is there any chance you could show a close up of the staple method you describes that holds the wing sections together?

Chris Reid09/02/2019 13:59:03
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216 forum posts
95 photos

Hi Graham. No sooner said than done.wing joiner 1 rs.jpgwing joiner 2 rs.jpg

ken anderson.09/02/2019 14:04:30
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8454 forum posts
773 photos

hello Chris,very nice model- well done...

ken anderson...ne..1..rapide dept.

GrahamWh09/02/2019 21:07:35
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356 forum posts
53 photos

Thank you very much Chris - I have been puzzling about some sort of connection to use for some time.

Percy Verance10/02/2019 08:47:38
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8108 forum posts
155 photos

Very nice indeed Chris.

A most attractive colour scheme too. The Dragon seems an altogether less twitchy prospect than the Rapide. The Rapide's narrow chord wingtips just look scary before you even get any further......

I like the method you've used to keep the outer wing sections together sections together as a whole. Cuts down on the pre flight fiddling.........

michael harper 217/02/2019 15:27:28
14 forum posts
5 photos

Very impressive. Hope it flies well.

Chris Bott - Moderator17/02/2019 17:08:42
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Moderator
6670 forum posts
1382 photos
1 articles

Looks superb Chris. An excellent result. I'm really looking forward to a flight report.

Chris Reid17/02/2019 17:56:35
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216 forum posts
95 photos

Thanks for the compliments guys.

I've done the weighing and power checks now. The latter proved a bit of a puzzle as I'm only pulling 210W on 7x6" props from both motors from 2x2200 3S lipos in parallel; my standard for larger models. The Hobbyking spec for the Turnigy 2863/8 1100Kv motors says they pull 336W each at 18A and 15v, which I should have spotted is rubbish, as others have recorded on forums various. The model weighs 4lb 12oz ready to fly which around 40W/lb, rather less than some of the tables suggest for good performance. However, as fellow clubmates and I have found, brushless motor watts go further than old brushed motors watts, on which the tables are based. The static thrust is 2lb 2oz which ought to be enough. It's a vintage biplane not a pattern ship!

I've done the CG calculations which are not wholly straigtforward for a swept winged biplane. I hope I've got it right. I just need a fine day and a friendly test pilot now. I'm a safe flyer but I'm not up to first flight out of trim surprises.

Trevor17/02/2019 18:27:44
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389 forum posts
55 photos

7x6 props are a bit on the small side for 1100kv motors on 3s batteries. I’d be tempted to fit 8x6 or even 9x4 for the first flights.

Trevor

Chris Reid17/02/2019 18:33:10
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216 forum posts
95 photos

Alas, scale being what it is, 7" props are as big as I can go! No room for bigger.

Chris Reid17/02/2019 18:43:48
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216 forum posts
95 photos

Good thinking Trevor. I bought unsuitable motors having been fooled by the promise of 336W. I've just been out to measure the gap between the engines and fuselage. 8" props might just fit, they certainly will if I trim the tips a tiny bit. I'll buy some 8x6" props and give it a go.

Trevor18/02/2019 09:03:33
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389 forum posts
55 photos

In case it helps, the stats for my Rapide are here: **LINK**

It has plenty of power at 55w/lb so I'm sure you could fly at around 30 - 40w/lb but it wouldn't leave much in reserve while coping with any trim issues on a maiden flight.

Temporarily fitting 3-blade props or going to 4s batteries would be other ways of getting a power boost if prop diameter is a constraint.

Good luck!

Trevor

Chris Reid18/02/2019 11:30:15
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216 forum posts
95 photos

Hi Trevor,

My Dragon is nice but your Rapide is very nice! Lots of interesting stuff on your link Did your weight of 5lb include the 3 batteries? If so your build was much lighter than mine.

Chris.

Trevor18/02/2019 11:39:28
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389 forum posts
55 photos

The weight includes one 3s x 3300mah LiPo. The Rapide was kept light to reduce the tip stalling risk. However, it is arguably too light! In anything other than flat calm it looks very untidy being bumped around by slight turbulence. I added an Eagle Tree Guardian 3-axis stabiliser a couple of years ago and that has made flying in a breeze a lot more pleasant.

At the risk of going off topic, if anyone is fancying building one of these DH biplanes from an Ivan Pettigrew plan, I should mention that the Rapide is one of Ivan's earlier plans and he's refined his design and drawing methods a lot since then, so the Dragonfly and Express are much more straightforward to build.

Manish Chandrayan18/02/2019 12:13:33
599 forum posts
70 photos

Trevor, I have seen the youtube video of your Rapide multiple times. Did not know about your blog till you earlier post. Beautiful models and nicely flownyes

MattyB18/02/2019 17:30:55
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1960 forum posts
30 photos
Posted by Chris Reid on 17/02/2019 17:56:35:

Thanks for the compliments guys.

I've done the weighing and power checks now. The latter proved a bit of a puzzle as I'm only pulling 210W on 7x6" props from both motors from 2x2200 3S lipos in parallel; my standard for larger models. The Hobbyking spec for the Turnigy 2863/8 1100Kv motors says they pull 336W each at 18A and 15v, which I should have spotted is rubbish, as others have recorded on forums various. The model weighs 4lb 12oz ready to fly which around 40W/lb, rather less than some of the tables suggest for good performance. However, as fellow clubmates and I have found, brushless motor watts go further than old brushed motors watts, on which the tables are based. The static thrust is 2lb 2oz which ought to be enough. It's a vintage biplane not a pattern ship!

I've done the CG calculations which are not wholly straigtforward for a swept winged biplane. I hope I've got it right. I just need a fine day and a friendly test pilot now. I'm a safe flyer but I'm not up to first flight out of trim surprises.

I have a lightweight foamie parkflyer Spad from HK that was originally designed for 2S, but because of their strange design decisions ended up on 3S. I put it on an diet to be able to fly it on 2S in small spaces, and it has ~45W/lb.

It is perfectly flyable (from a hand launch, ROG is impossible due to it's small size), but was very hairy on the first flight. Given how much time and effort you have put into building this beauty I would encourage you to think of a way to get at least 60W/lb for your first flights, whether that is by going up to 4S or fitting new motors. The additional cost will be forgotton when you see it soaring confidently overhead, but it would be horrible to lose it because of a powertrain miscalculation.

Edited By MattyB on 18/02/2019 17:32:43

Chris Reid19/03/2019 21:46:32
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216 forum posts
95 photos

I've done some more work on the power front. I cut down a couple of 8"x6" props a bit to give me the largest prop that would fit in the nacelle/fuselage gap. A power check gave me 260W and almost 8oz more thrust which was the improvement I was looking for. Foolishly I then tried a 4S battery pack. The fresh batteries gave a surprising 500W dropping to 460W after 10 secods or so. More than enough, and not necessary, so I decided that 3S batteries would do for the test flying. I reinstalled the 3S batteries only to find that one of the 25A ESCs had given up - too many amps for too long! The ESC should have survived but it has taught me not to overpower cheap Hobbyking ESCs. A 40A one will be substituted. Even that's not going to be straight forward as the original ESC signal plug and socket join is buried in the wing. I will have to cut the signal lead off the busted ESC and join it to the new one. I have some Futaba plug and socket kits to make up a new socket so I wil probably do that.

This has delayed the test flying, but the weather has reverted to normal March so I haven't really missed any flying opportunities. I hope to get to it soon. If Dragon flies well the big decision will be whether I can face drawing up a decent plan for RCM&E. I don't have the energy I once had.

michael harper 205/07/2019 09:20:38
14 forum posts
5 photos

Any news on test flight ?

Chris Reid05/07/2019 14:25:48
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216 forum posts
95 photos

Hi Michael,

I've been a bit tardy in getting the Dragon airborne, although she is ready to fly. On one recent trip to the field my favourite test pilots were elsewhere. Brilliant weather this week, but family issues and a large garden have taken priority.

It will be soon.

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