CUMUL

Fancy trying your hand at a Discus Launch Glider? John Stennard builds a well thought out DLG kit from Kavan words & photos

» John Stennard

There are two things to look at straight away: one is Kavan and the other is Cumul. Franz Kavan started his kit and accessories firm in Germany in 1965 and the Kavan brand (www.kavanrc.com) is now handled by the Pelikan Daniel firm (www.pelikandaniel.com) based in the Czech Republic. In addition to his high-quality products, Franz Kavan is perhaps best remembered as the designer and manufacturer of one of the first successful R/C helicopters, which was introduced in 1972. Kavan products are available from some retailers in the UK, and I purchased my model from Model Shop Leeds.

Now, as to the name Cumul, all I’ve been able to come up with here is that it’s an abbreviation of Cumulus which, to me, sounds more attractive.

Dlg Here We Come

I’ve always had an interest in DLG gliders but they are lower down my enthusiasm list than aerobatic electrics and EDF, although I’ve owned and flown several DLGs. Mine have been the type of model that I load in the car along with others, but not necessarily assuming I will fly it. Some of this reasoning comes from the fact that achieving successful flights with a DLG can be very dependent on weather conditions. So, the model will either get flown or stay in the car. Having said that, I have always really enjoyed the challenges inherent in a DLG, both when launching one and in the quest for lift.

Kit comes in an attractive box. CNC cut balsa and ply parts are all excellent quality.

Starting Point

One thing I’ve never done is build my own DLG. My previous models have all been of the minimal assembly type with ready covered wings etc. Finding myself without a DLG, and possibly some good flying weather ahead in 2023, I did a bit of hunting on the net and spotted the Cumul. This model is a bit different as it is a modern kit, with all mod cons. However, I had to be patient and wait until there was a delivery of this glider from the Czech Republic. Model Shop Leeds were on the ball and emailed me as soon as they arrived, and a surprisingly small parcel was quickly and safely delivered.

The size of the box is dictated not by the carbon fibre (CF) tube boom but by the beautiful, full size rolled plan. All the balsa parts are equally impressive being of excellent quality and accurately CNC cut. The many parts are separated and stowed in six separate bags. A curved and slotted length of balsa has you guessing, until you consult the plan and find that it’s a rib jig that can used in turn for assembling the two wing halves. Many of the parts interlock to ensure a speedy and accurate build. The ribs, for example, have jig tabs allowing for both for the undercambered build and to include washout. Non balsa parts include a CF tube main wing spar and beech dowel leading edges, plus the CF tube boom. A lightweight heatshrink covering is recommended. Although primarily intended as a DLG, a tow hook is also provided.

A 27-page A5 manual is included, which does make for a small font - time to put on the reading glasses! It also includes several pages with colour photos to help with construction. Kavan mention that this model is ‘maybe not an ideal first model for a newcomer without any previous experience’ . The model’s design certainly has a high performance feel about it.

So, with an excellent plan, comprehensive instructions and set of CNC cut parts all I really need do here is to highlight some points in the construction and finishing, and then report on flying the Cumul.

Enjoy The Build

Having read the instruction booklet and perused the plan I decided it made sense to build the model in the suggested stages. I confess I don’t always do that and sometimes I build a model in the order I fancy. Regarding adhesives, a selection of PVA, cyano (CA) and epoxy are required.

The instructions start with the tail parts, which is a nice introduction to the assembly process and the kit parts. There’s nothing to report here but it gave me the opportunity to dry fit the tailplane and fin in place and get an idea of what the tail end of the model will look like.

The instructions move on to the fuselage pod. Your first thought might be, ‘A pod, that’s simple to build.’ But this pod involves 21 pre-cut parts! Using the precision cut parts the pod assembled easily and I could now see the available space for the receiver (Rx) and flight battery. The pod includes a hatch for access to the servos. The Rx can be positioned under the wing, where there is adequate room for a small or micro receiver and a quick check showed that a Spektrum AR 410 would fit easily. But I soon realised that I did not have an Rx battery pack that would fit in the pod in front of the servos. As I fly mainly electric powered models I rely on the BEC for power. However, I found a very small Rx battery pack on the Component Shop website. It’s a 4.8v 150mAh NiMH of 1/3 AAA flat configuration. It only measures 10 x 14 x 43mm and weighs just 16.2g. When the servos were fitted I found that this pack would slide in past the servos so it can be removed for charging.

On A Wing And A Prayer

The wing build proved to be quite challenging, mainly because it uses a completely different technique to the type of construction I am used to. Checking things out I realised that the wing ribs have no markings on them and there are 21 ribs/riblets in each wing half. Comparing the sheets of ribs with the plan I was able to identify and mark all the ribs prior to construction. The trailing edge and the leading edge jigs have rib slots in them which matched up but the slots did not match up with the wing drawn on the plan. I was puzzled by this and an investigation showed that the plan was incorrect.

Pod access hatch is secured with a ply finger and a magnet.

Datafile

Name: Cumul DLG

Manufactured by: Kavan

The wingspan is listed as 1096mm but in fact it’s 1010mm, according to the pre-cut balsa parts! This made the construction a slower process as the ribs/riblets, other than the tip rib, did not line up with those drawn on the plan.

As I mentioned the wing construction is unusual as one has to thread all the ribs and riblets onto the CF tube and carry on the construction from there using the slotted trailing edge and leading edge jig. It was not a quick build, but it has resulted in a very strong wing.

Late January and a pre-test flight photo shows the attractive lines of the Cumul.

The final addition before covering was to glue the discus launch CF tube in place. This is reinforced with glass matt.

My choice of heat shrink covering was dictated by what I had left over from my last model! I covered the wing in a transparent yellow film as I like to see the wing structure through the covering and I used transparent red on the tail surfaces and pod.

UK Stockist: Model Shop Leeds https://www. modelshopleeds.co.uk/ catalog/index.php

Part No: KAV02.8023

RRP: £88.99

Wingspan: 1096mm

Length: 880mm

All up weight: 180g

Wing section: YA0801

Functions: Rudder and elevator

Required to fly: Small Rx, two 6g servos & Rx battery

Fitting Out

Two 6g servos fitted the pre-cut balsa servo mount perfectly and ball type control rod connectors were fitted to the servo arms. An unusual feature is that in order to suit the position of the snakes and wire inners these fittings are positioned on the underside of the output arms. A brass threaded connector must be soldered to the servo end of the wire control runs. The control surface end uses a 90-degree bend or Z bend.

After the fin/rudder assembly and the tailplane/elevator had been covered and hinged using the covering film they were glued to the CF tail boom. The plastic snake outers can then be threaded through the boom and into the servo area of the pod. Connecting up the control runs is more fiddly rather than difficult but it’s important to make sure the servos are centred and the piano wire inners operate freely.

With the battery in place a C of G check showed that some nose weight was required. I had to fit 20g of weight ahead of the battery to achieve the recommended position, which increased all up weight (AUW) to 200g. The instructions show 180g AUW. Test glides will show if the nose weight is required.

LET’S FLY

The instructions include a section on test flying and a detailed explanation of the discus launch technique. Once a level glide has been obtained you are ready for a discus launch. The instructions recommend using a small change in rudder and elevator settings for the discus launch. This can be achieved using a ‘Flight Mode’ switch set up.

It’s 30th January, the sun is shining, it’s not too cold and there is a light wind. So it’s a promising day to try some test glides at my local farmer’s field, just five minutes away. Only test glides of course, none of that discus stuff! The test glides were almost perfect, with the model being just a tad nose heavy. Elevator seemed a touch sensitive, so I put in some expo to calm this down.

Cumul looked great in the air and a nagging voice in my head kept alternating between ‘Go discus, go!’ and ‘No discus, no!’ Having discus launched before I decided that it had to be done and started with a shallow launch. This was encouraging, with the model showing no unforeseen tendencies. I quickly built up to steeper launches and they proved to be perfect. This is possibly the easiest DLG model I have ever launched. The longish grass was not conducive to fast pirouettes so I was never going to reach any great height. However, I was absolutely delighted with the way the Cumul performed. She came off the top of the climb cleanly and flies so smoothly. I went home a very happy man!

Subsequent flights were equally satisfying and with faster pirouettes and stronger throws I was achieving my best ever discus launch heights and flights.

PILOT’S NOTES

What a delightful model this is. I think the kit is good value and the building process is interesting. The finished result is a very attractive built-up model with not a scrap of foam in sight. The Cumul is easy to launch well and is then responsive and smooth to pilot. I’ve already had more satisfaction from this model than any of my previous DLGs.