DH71 by Chris Golds

[Geoff S]

Having been reminded about the DG71 by the recent build of the JB Aviation kit by Robert Parker I decided to see about building one myself and I ordered the drawings for Chris Golds version from Sarik. It arrived this morning. I regret delaying too long to buy an unstarted JB Aviation kit at a swap meet. When I went back it had sold.

It comes in 2 huge sheets on very stout paper and, in addition, is a printout of the original article which appeared in Q&EFI in December 2006. So lots of information and there's more , including instructons, on the drawings themselves.

It's 5th scale but still only 54" ws so it's a small model of a small full size. It's also electric - another plus for me. Chris used an Axi 2820-14. I'm using one in my Peter Miller Li'l Cub so I'll have an idea of its performance though I'll probably substiute a different motor - I have an unused Turnigy G46 which might serve. The energy is provided by a 10s 3AH NiCad pack which is roughly the equivalent of a 3S 3AH LiPo but a lot heavier. I may go for 4S 4AH LiPo.

There are some odd aspects of the design I've not yet sorted out but no doubt will become clear. This is now next but one on the bench after the Rans 9 Chaos. I'm looking forward to it. I have avery soft spot both for the era and DH aircraft in particular. The design calls for mostly soft balsa throughout which may well be a stumbling block as I don't think my stock is sufficient.

In 2006 the drawing was priced at £20 + £3 postage. This was £23 plus £3.50 postage, so really not too bad.

More in a few weeks

Geoff

[Bob Cotsford]

10S Ni, what's that, about 25oz going by the RS cells? A 3S 3000 LiPo is somewhere around 7 or 8oz so you could allow some latitude with the balsa weight as long as you keep the tail light-ish and still end up lighter than the original.

[FlyinBrian]

Bob, where do you buy such light weight LiPOs?

[Bob Cotsford]

A quick Google listed plenty of 3S 3000 Lipo packs, weights shown where 180 - 260gms. A Turnigy 4S 3000 was listed at 277gms which is only 10oz! Admittedly I haven't been out to the shed to weigh anything.

Edit - a handy 4S 3700 weighs 370gms so that's 33% more volts and 20% more capacity and it's only 13oz.

Edited By Bob Cotsford on 13/01/2021 15:38:19

[Geoff S]

That's what I was thinking, Bob. The elevator and rudder servos are near the tail, too, so I could lighten the back by moving them forward and use my normal 3mm carbon push rods which are quite light. I canalso install more watt.hours of energy for longer flight times and (hopefully) save weight at the same time. It's amazing how electric flight options have improved since even this was designed.

This is what's puzzling me on the wing construction:

The Xs in circles indicate where so-called building jacks go. I'm assuming I need to make those to support the spars at an appropriate height. The trailing edge near the root is raised 1/2" and near the tip (just before it starts to curve) is 1/4". As the mainspar is 0" and the LE jacks are both 1/4", doesn't that indicate wash-in?

There is non-scale dihedral (the full-size's wing is flat). The dihedral braces I've measured at 4 deg/side. There is, however that piece of 1/4" ply labelled as Building Jack which is at an angle of 2 deg along one long side and square everywhere else. One end is labelled 'Forward' so I guess it goes across the chord - perhaps that's something to do with dihedral and perhaps I need 2 of them (one at each end when I join the wing halves.

I've built a lot of wings but this is a new technique to me. Perhaps once I get the parts cut out and do a dry assemble it'll be obvious how it all goes together.

These are the wing building instructions printed on the drawing:

Geoff

[Bob Cotsford]

it does sound like simple packing pieces to support the LE and TE to give 1/4" of washin. I wonder if that's a misprint? I must admit the root TE packing looks like 1/8" to me but that's on a reduced size image. Is the bottom of the rib flat or cambered behind the spar? I'd be tempted to give it 1/4" of washout and call it done.

[Bob Cotsford]

As for the 'building jack', I would have thought it's just the usual packing to raise one or both tips when joining the wing so balsa should do. Which way it tapers will tell you whether it's supposed to be washin or washout.

[Geoff S]

Thanks, Bob. It is a bit of a puzzle but I like puzzles and it'll no doubt become clearer when I actually start. I just like studying drawings of a model I intend to build. At least the print quality is excellent.

I wonder if Chris Golds is still available? He's a very talented guy - Hunter pilot, artist and aero-modeller. I once wrote a joky letter to him about a humourous article he wrote and got a beautifully hand written reply.

Geoff