Airfoil choice

[Mike Francies]

Hi,

I have joined this site as I am about to embark on a scale project that may take some time to complete. I have a technical question that has probably been discussed many times before but I found no relevant threads on this forum.

The intended scale will be either 1:4 or 1:3.5 which will make either a 6.25 or 7.14 meter span model. I would like to make this as scale as possible and I am at the stage of drawing up the CAD model before machining the fuselage plugs. The problem is the airfoil section; at these scales can the full size section be used, with the understanding that performance may well not be as good as it could be or is it more advisable to swallow a little scale pride and use well know modelling sections?

Is the departure from the full size section acceptable for top flight competition?

Thanks,

Mike

[Mike Francies]

Hi Cliff,

I had to sign a declaration of non disclosure before I could get any drawings so I am a bit reluctant to say anything at the moment. However, I can say that it uses an HQ section. After a little bit of research I realised that HQ is not the Helmut Quabeck that all modellers know but HQ stands for Horstmann and Quast and the section on my subject is very similar to the one shown on this home page http://airfoiltools.com/plotter/index?airfoil=hq17-il

I would like to use it but how much would performance be compromised due to Mr Reynolds? Would the difference between 1:4 scale and 1:3.5 be significant as I appreciate the closer we get to full size, the better the full size section will operate.

Cheers.

[chris williams]

Choice of wing section is largely dependent on the type of model proposed, and the the type of flying it will be expected to perform. So, for instance, an aerobatic Swift or Fox would need a very different wing section to a Slingsby T21. It's not too difficult to guess that the type of glider you are proposing is to be of the all glass composite variety (unless someone's building a new type of wooden glider), so it might be worthwhile to look at a similar commercial model and find out what wing section is being used and copy it. For mild aerobatics and good low-speed behaviour, variations in thickness of the HQ35/12 (Quabeck, that is!) have proved to be excellent. Thus you can thicken the root section up to match the thickness of the full-size, and taper through the wing back down to 12% at the tip. At the same time, construction is simplified by the lack of the need for any washout.
Good luck with your project...

[Mike Francies]

Thanks Chris,

Yes it will be an all composite model with an eye on entering scale competitions and to add further to the intrigue it is a self launching glider. I was really interested in seeing if people were using the full size sections on scale gliders. I can see that the modern, all glass machines would benefit more from a change of section than perhaps the older, wooden gliders because of the wide range of speeds required.

Interesting what you say about thinner sections at the tip. I have just read and re-read Martin Simons 'Model Aircraft Aerodynamics' and he advocates using increased camber at the tips along with washout so that the whole wing reaches aerodynamic zero at the same angle and with a coefficient of lift nearly constant across the span. With a moulded wing this is easy to achieve though I have to admit that all my gliders have always used the same section throughout without any washout - just as you advocate.

I am currently looking at the HQ-3015-DS series of sections and they are not too far away from the full size.

Mike