Having completed two models for the build-off, the Zephyr and the yet-to-be-flown Mamba 2X, I felt like a complete change of pace and so I have decided on an electrified R/C version of the Witch II 36" rubber model, aimed at "park" (or rather in my case "garden") flying. The aim is to produce a model for as close to the weight of the rubber powered original as possible.



The power system shown below is a little 40 watt outrunner, 10 amp Robotbirds ESC and a 300 2s Lightstorm lipo, total weight 1.5 ounces (42 grams). I have a pair of 3.6 gram servos from an old indoor model and an 8 gram receiver, so the total equipment weight should be around 2 ounces/56 grams. Target ready to fly weight is 5 ounces/140 grams which would give a wing loading of around 4 ounces/square foot, coupled with the under cambered wing section that should give a nice slow flying speed and produce a good "summer evening" model - that's the aim anyway.

Unusually for me I'm going to stick as close as possible to the original structure - resisting the temptation to substitute laminated wingtips and add riblets - the only changes will be those needed to produce rudder and elevator control surfaces and the substitution of hardwood for the wing spar. Covering will be mylar/tissue; in my search for a 3 ounce airframe I am encouraged by the fact that the total weight of the finish on the Mamba (mylar plus, effectively, two layers of tissue and four coats of dope) was only 1 ounce, so the mylar plus single tissue plus less dope on the smaller witch should hardly be more than half an ounce maximum, probably less.

I'm hoping that Scott will allow this as an entry in the main 2012 build off as well, I have two models planned for that (an Avro Avian and a Black Magic), but it would be nice to get a head start with this which should be a "quickie" build, unlike the other two!.

Subscribed!

Jon

George,
A nice choice there..
Watching with interest
Mark

Originally Posted by Sundancer

The aim is to produce a model for as close to the weight of the rubber powered original as possible.

George, I think this may well be doable. A big FF model would have quite a weight of rubber in it anyway!

Jon

Originally Posted by Yak 52

George, I think this may well be doable. A big FF model would have quite a weight of rubber in it anyway!

Jon

Yes Jon, that's my thinking too. The recommended motor for the Witch is 8 x 25 inch strands of 1/4" x 1/30" rubber, for an all up ready to fly weight of 4 ounces. Although I can't remember for the life of me what 1/4" x 1/30" rubber weighs, I am guessing that for snappy sports performance the airframe/rubber ratio would be something like 2.1/2 oz of airframe to 1.1/2 oz rubber, so the rubber weight should be roughly equivalent to the weight of my electric drive train, motor, ESC and lipo. I'm being a little less ambitious by aiming for a target weight of 5 ounces, which would mean I have to build the airframe for 3 ounces, and I am hopeful of doing this or better since I am choosing my own balsa, and Frog kits did tend to have rather heavy balsa as a rule. We'll see, but whatever it should be an interesting exercise and a different challenge from the last two Frog models.

A sweet project...

Cem

Right, basking in the success of the Mamba's maiden flight today, it is time to get this build thread on the road.

As is often the case I am starting with the tail surfaces to encourage that "1/3rd finished" feeling. However, in this case there is another and more practical reason for doing it this way. At present I have absolutely no real idea about how to distribute the radio and electric hardware in order to get the CG to come out in the right place. This means that, except for the motor mounting, I am unable to decide on where extra structure will be required for the electronics. So the plan is to complete the flying surfaces, covered and finished, and the fuselage as a bare box structure of longerons and spacers, and then sort out where the gear has to go to get the CG right, build in the necessary mounting structure and finally cover and finish the fuselage.

First job then was to make the rib templates and cut out the wing and tailplane ribs, then trace and cut out the wing and tailplane tips. After drawing in the necessary modifications for the rudder and elevator control surfaces, I built the fin/rudder and tailplane. The rudder and elevator are built integral with the fin and tailplane and will be cut free before covering, then hinged afterwards. Some slight changes, such as double tailplane centre ribs to slot the fin into were necessary, but I resisted the temptation to change the wing and tail tips from assemblies of 3/32" sheet pieces to laminated outlines.

The wings are going onto the board now, it is pleasant to be building a very light and entirely conventional structure, it's a long time since I last built a rubber model!

Even with crutches you're a fast runner!

Looks good George. That airfoil looks just right for a slow, floaty, modestly powered model.

For assessing the CG position can I recommend Dan P's CG Calculator. It allows you to put in the point masses of various objects and tells you where the CG will be. Pretty clever stuff, but easy enough to work your way through the spreadsheet.


Jon

Thanks for the link Jon, looks interesting. I will try this, but also continue with my original intention, so we should produce some comparative results, calculated versus practical, which might be interesting.

The wing structure is finished now; spring has come to the Haute Vienne I am glad to say, so I could do my sanding on the terrace, which is much more convenient and less messy - and a lot more pleasant.

I am enjoying this build as it really takes me back to my early days of building rubber powered models. I am trying to minimise any changes; the tail surfaces obviously had to have provision for the rudder and elevator, but the wing is pretty much as per plan except for a change of spar material to lime wood and the addition of a 1 mm ply dihedral brace, a few extra 1/8" square spars in the centre section to support the wing bands and a couple of extra gussets. Oh yes, and some 1/32" ply to protect the TE from the wing bands.

Way back in my youth, one of my modelling mentors, someone who had been building models since the days of "A" frame pushers, told me that sandpaper was one of the most important modelling tools if I was going to aspire to a decent standard of building. Well, I still believe he was right, this is nicely illustrated by the fact that even this little wing, with minimal structure, weighed 20 grams as first built, and 16 grams as now finished after sanding. That's a 20% saving and a good contribution to my hopes of holding the weight of the airframe down to 70 grams.

Now to pin down the first fuselage side, then I can cover the wing and tail in mylar whilst the glue is drying.

Sounds like the Witch will be the complete antithesis of the Mamba but then variety is the spice of life Following with interest.

Ray

I managed to get both fuselage sides built and covered the wing with 5 micron mylar in between, so a good day's work.

Tomorrow I'll join the sides and then cut the rudder and elevator free before putting the mylar on the tail plane and fin. If the weather co-operates like it did today I can tissue and dope the flying surfaces outside to avoid smelling the house out, and will then be ready to sort out the balance issues.

Another great looking build. After seeing your wing center section I wish I had done that on the wing I just built instead of sheeting to support the rubber bands. Try as I might I always find myself departing from plan to add something that wasn't there.

If it works when your taking pictures can you post one with just the mylar on part of the structure before adding tissue? I found a source for the Eske (sp?) tissue in the U.S. but not the mylar yet...probably not a problem but ran out of time in my search.

Regards,

Larry

Hi Larry

I'll try to get some photos with just the mylar on - doesn't show up too well. If you look at http://www.rcgroups.com/forums/showt...1515121&page=2. post #25 shows bits of the Zephyr with just the mylar on.

I shall be using the 5 micron on the Witch rather than the 10 micron. The 5 stuff is much harder to handle, but I want to absolutely minimise weight on this one.

If you still have trouble finding mylar, Mike Woodhouse at www.freeflightsupplies.co.uk will ship world-wide, shouldn't cost much for mylar as it is light enough. He also does all the grades of Esaki tissue.

Thank you, I think I'm good on the photo now. I have looked at most of these if not all threads before. I just reacquainted myself on the Mylar covering at the link you provided. Also, I did visit the free flight supplies a couple of days ago when I began my search. If I can't find it in the U.S. I will go there.

You do such a great job with the Mylar and tissue and the look is, well, so vintage. I will have to make a test project to try it on before risking something that has taken me several weeks to build. How large a model have you done using these materials? Is the a point silkspan would be more appropriate?

Larry