Cmelda (oz13626)

About this Plan

Cmelda (Bee). Radio control electric-powered sailplane. Uses a variable-incidence wing mount.

Quote: "I first came across the plan for this model in the offices of the Czech magazine Modelar, it was obviously a plan that would be a popular subject in the UK so I enquired about the designer and flyer. Later in '92 I had the pleasure of meeting the designer, Josef Janis and the pilot and co-designer, his wife Hana Janisova, at the F3E world champs where Hana was managing the Czech team. Incidentally at the time Josef was flying a Sukhoi (I think) in the Czech Air Force.

The model was designed for the Czech F3E/7 class this is a duration and landing contest very similar to our Electroslot competitions, so the model should be perfectly capable of handling competition work and general sport flying. The original model used variable wing incidence for reasons explained in the text. We have also put a standard wing fixing on the plan for those of you who are not quite so adventurous.

The quality of building in the Czech Republic is very high and they have a few techniques that will be familiar to free flight builders but may not be so familiar to the everyday balsa butcher. Where materials that may not be generally available are specified we have included alternative suggestions. For instance, it is pretty well standard practice to make all your own control horns and pivots, western modellers are spoilt in this respect. The general level of equipment used is also at a much lower level than we are used to. This works in our favour because the models have to be simple, so that a model like this can be operated with basic radio gear and servos.

The model is well thought out and not difficult to build for someone with a moderate experience of balsa models. Of particular interest is the blown turbulation on the wing tips. This is a very efficient method because it is speed sensitive, it works at low speed but is ineffective and therefore not contributing drag at higher airspeeds.

Design Considerations: The Cmelda (Bee) has been designed to get the maximum climb efficiency. The motor used was a Keller 35/5 which turned an Aeronaut 320 x 165 prop at 6700 rpm whilst burning 41 amps during ground tests. The model should fly easily on a Speed 600 motor and can handle hotter motors easily. The best climbing regime for models equipped with E2I4 is at about a coefficient of lift (CI) of 0.3. The best minimum sink
performance occurs at a CI of 1.2. The difference in angle of attack between climbing and gliding is about 8 degrees. We are able to manage the change between the two regimes by the following methods:-

1. Down elevator compensation - difficult on R/C sets without mixing, in addition the fuselage drag coefficient increases.
2. Down thrust - about 3 to 4 degrees are required to make the difference between the thrust line and the angle of attack 8 degrees. In the climb we are losing 1 % in the climbing direction. But the power component working upright to climb direction (sic) is P x sin 8 degrees, this is about 14% of the thrust P.
3. Change the wing's angle of attack.
4. Combine all the above methods.

We chose option 3, the wing angle mechanics seem to be difficult but it has worked reliably.

The Cmelda has achieved 180 flights during the season during which Hana took part in 8 competitions and won 5 of them.

Wing Construction: The wing uses a standard E214 at the root, changing to 9% at the polyhedral break and 8% at the tip. We would recommend that you build the wings on auxiliary foam beds (a common Czech method, DJ) cut to follow the bottom contour of the wing. The tip section beds will need to be cut with the washout incorporated. The alternative is to make up some packing pieces.

The model uses carbon fibre sandwiched between the balsa at the trailing edge, together with some rovings run from the trailing edge up the side of the ribs to stiffen the rear of the wing. A carbon capped spruce spar is used, although for sport use a simple spruce spar would suffice. The shear web is 4 mm wide at the root and 2 mm wide in the tip panel, the grain is vertical and only epoxy is used as an adhesive. The mainspar is wound with Kevlar thread, 10 winds between the ribs..."

Cmelda, Silent Flight, October 1993.

Direct submission to Outerzone.

Supplementary file notes

Article pages, thanks to RFJ.

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