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Modern aerofoils for vintage scale sailplanes

How to improve performance of a scale model of a vintage sailplane.

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Royal Evo 7

Royal Evo 7

Simon Cocker unravels the Multiplex marvel

Peter Musselwhite 122/12/2019 18:30:54
4 forum posts

I want to follow up a subject Simon Cocker introduced in the June 2019 edition of RCM&E - namely building vintage wooden scale sailplanes with modern high-efficiency aerofoils.

I have an (unbuilt) short-kit for the 1937 German "Rheinland" (the 1/5th scale version by Mick Moore) which I'd like to update with a modern aerofoil.

This is new territory for me. Can anyone offer some advice, including choice of aerofoil(s), best software for printing them out, suggestions about computerised production etc?

Frank Skilbeck22/12/2019 19:36:01
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4748 forum posts
104 photos

Chris Williams uses the HQ35/14 at the center going to HQ35/12 at the tips with no washout. I've built/acquired several of his designs and they all fly well. Though you may want to also ask the question on scalesoaring.co.uk.

I've used Dev Wing to print out wing ribs and it works quite well (Dev Fus isn't so great though IMHO). If you draw up your wing ribs I know Cliff Evans at lasercut sailplanes will laser cut them for you from the cad files if you wish.

Simon Chaddock22/12/2019 21:49:14
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5727 forum posts
3034 photos

Peter

It does depend on the size of the model glider but unless it is really big are you sure there will be a significant difference between true scale and 'modern' sections. The 'efficiency' of any section falls away significantly at the low Reynolds numbers typical in model.

Modern full size glider wing sections allow higher top speeds, although this is also due to stiffer materials but the minimum sink rates are not significantly altered although that point does occur at a higher speed. They also use flaps to reduce the flying speed, but not the sink rate, to be able to thermal at a smaller radius.

It rather comes down to how scale you want the glider to be.

I have found that just 'thinning' a scale section, i.e. reducing the T/C ratio by a couple of %, works well for me.

Peter Musselwhite 123/12/2019 00:14:45
4 forum posts

Thank you, Frank. I’ll follow your scalesoaring suggestion, and will also look at Dev Wing.

Peter Musselwhite 123/12/2019 00:18:56
4 forum posts

What you say is interesting, Simon. What benefits do I really want? The reference in RCM&E wasn’t very specific about the benefits. Rather, it talked about ‘transforming’ the ‘effectiveness’ of a vintage section wing. I guess that an improved speed range would be useful, but perhaps thinning the section, as you suggest, might achieve that. Food for thought. I will try and contact Simon Cocker to explore what he sees as the potential benefits.

Nigel R23/12/2019 06:53:02
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3966 forum posts
714 photos

Agreed about devwing. Generally quite easy to use. Comes with s decent library of sections.

Piers Bowlan23/12/2019 09:02:20
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2163 forum posts
53 photos

As far as I am concerned, I think the benefit of using modern wing sections in model gliders is about improving the glide ratio and penetration due to a higher flying speed and reduced drag. Of course that might not be a great help if the design is intrinsically draggy (form drag, parasite drag) e.g. Grunau Baby, Sedberg etc.

Some years ago I built a little glider (Auriga 'Silent Flight' Aug 95) with an RG15 section. With it's built up balsa structure and my limited building 'skills' it was probably a distant cousin of an RG15 but nevertheless it was semi-symmetrical and thin. I remember being amazed by it's fast, flat, glide compared to my previous glider builds with their flat bottomed Clark Y-ish sections. The advantage was that it would slope soar very nicely in moderate winds despite it's low wing loading, try doing that with a 'Gentle Lady'.

Do you remember James Mays's 'Flight Club', where he set out to fly a 1/5th scale Slingsby T45 Swallow across the Channel? He found the old wing section in the Triplet plan only had a glide ratio of 1:14 so Brunel University were commissioned to design a new wing of the same span and planform, to improve that to 1:20 (they should have asked Chris Williams!). The English Channel turned into the Bristol Channel and he only made it to Lundy Island, but great entertainment non the less.

 

Edited By Piers Bowlan on 23/12/2019 09:12:13

Simon Chaddock23/12/2019 10:02:23
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5727 forum posts
3034 photos

That James May episode does rather show the effect of model size on aerodynamic efficiency.

The full size Swallow had a claimed glide ratio of 26:1 despite using a thick 18% NACA section at the root reducing to 12% at the tip.

Having flown a full size one quite a bit that claimed glide ratio is maybe a tad generous but it is nevertheless still significantly above even the Brunel improved glide ratio on the model.

Size does matter. wink 2

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