Rubik R-22 Futár

[StephenB]

I came across a plan for this Hungarian glider whilst searching the Sarik website, scant information other than at a scale of 1:3.7 the model has a wingspan of 165".

An enquiry to Sarik revealed it was designed by Tony Slocombe and John Elliot, wing section is G549 which replicates the full-size. Anyway, I fancy having a crack at this, any thoughts on the suitability or otherwise of the wing section, how easy to modify it if needs be and has anyone ever seen the model in flight?

[RobbieB]

Morning Stephen - some interesting questions there.

I have actually flown John's prototype and it flies very well indeed - but there is a caveat. Just like my Weihe on which I also used the scale 549 section (didn't know any better in those days) which is a great section for soaring and scale-like flight, especially if the wing loading is a bit on the chunky side - you will have good lifting properties at low speed. That's because it has quite high camber and with most, if not all high cambered aerofoils they are one speed sections. Speed them up and they take on the glide angle of a dead parrot - not good. In a good blow you will find it difficult to go anywhere apart from downwind.

For me anyway, a modern model section such as the Quabecks, Seligs etc with both moderate thickness and camber is the way to go if you want a more versatile flying machine; but if you prefer scale-like flight in ideal conditions then the old 549 is fine.

[Barry_Cole]

Quabeck every time...



BC

[StephenB]

Thanks for the replies chaps, interesting that you have flown it Robbie and report good flying characteristics. The full size is still flying and only a couple of hours up the road from me.

Would that be 2.5/9 Quabeck? How would I go about changing the aerofoil?

[Peter Balcombe]

Stephen,
HQ2.5/9 is a 2.5% camber 9% thick section.
I would have thought it more likely to go for a HQ3.5 section as used by Chris Williams on many of his designs. The thickness is more likely to be 13% or so, particularly at the root end to more closely match your 549 root section.

How you go about changing the wing design depends on what facilities you have available, or who you know
You obviously need to change each wing rib profile & spar etc. notch position to match the new external profile along the wing.
If you have a constant wing section, then that could be done mandraulicaly by scaling an airfoil section template to the chord length at each rib position to get a template for each rib. Then by transferring the spar positions etc.to each rib, you can identify the required notches.
Note that the HQ section will be different In thickness profile, so you will probably find less depth for airbrakes/servos etc. than on the original.

By far the best way to change the airfoil section is to use a CAD type wing design package such as Compufoil/Profilli/DevWing or similar as these (usually) allow you to progressively change airfoil and/or thickness profile along the wing and locate spars etc. to derive cutting templates for each rib.
That is the process that I used to convert the original Mike Trew Fauvette wing design to an HQ section alternative.

I’m sure others will chip in if there are other options or comments (e.g. making a new rib set using the ‘sandwich’ method).
Peter

I’m sure others will chip in

[StephenB]

Many thanks Peter for the detailed reply. I thought the 2.5/9 looked worryingly thin!

I'm hesitating over ordering the plan at the moment, at thirty quid plus postage it's a lot of expensive paper to waste if I can't work with it. I'm not totally averse to going with the original design and accepting it will work in a smaller performance envelope, back in the day we used to go to the slope with two or three models to cover all possibilities!

I'll chew it over for a few days and see if other suggestions come up.

[RobbieB]

Stephen,

If you really want to do this model, PM or email me to chat about some options.

[StephenB]

Well, the plan has arrived. Beautifully drawn and presented on four sheets which will be a nice fit for the workbench.

It mentions "all ply, unless stated otherwise, is packing ply". I'm assuming that was early 90's terminology for liteply??