2.5 EMS Duo Discus by Andy Blackburn

I've always fancied a scale sailplane but have usually been rather put off by the size and the apparently large investment of time required. However, I happened to come across the European Sailplanes website a while ago and was instantly attracted to the ARTF scale sailplanes on offer. Some of them are truly enormous with price tags to match, but there are smaller models (I hesitate to use the words "kits") at a steep but nevertheless reasonable price considering that they really are "install the radio and fly". After a short chat with She Who Must Be Obeyed and a solemn promise to do the washing-up more often, a cheque was duly dispatched and a large cardboard box arrived a few days later. There were so few components that I initially thought they'd missed something out; a finished epoxy glass fuselage with the retract unit installed, a separate cockpit seat pan and rudder, covered tailplane and wings, and a wing joiner. The control snakes are pre-installed; all control surfaces are actuated by thin (approx 20 swg) wire running in what look like Graupner white snake inners. No instructions were supplied but Chris Barker of European Sailplanes has been very helpful in providing advice and technical support.

Equipment choices are basically single or dual aileron servos, but the connection arrangements took a bit of thought. The servos had to be removable if required, and the connection arrangements had to simple and reliable. Obviously, the best solution would be a single servo for each wing but although the wing cutouts are wide and long enough, they aren't quite deep enough. The front edge of the servo pocket is 10 mm deep but the rear edge is only 8 mm deep, which means that even the thinnest metal-geared servo will project about 2 mm below the lower wing surface. The aileron servos therefore had to be mounted in the fuselage, using the pre-installed snake linkages. The Winter 1997 issue of "Sailplane and Electric Modeler" reviewed the EMS Duo Discus and Nimbus 4D and showed four servos mounted on servo rails just behind the cockpit; unfortunately the retract unit takes up most of that space so it wasn't an option. The servos would have to be mounted on some sort of servo tray in the rear cockpit area.

Inside the canopy: rudder, elevator and retract servos held in place using a hot glue gun.

The undercarriage servo was installed first - a servo with a large extension horn is required as the travel is about 38 mm. A retract servo would of course be suitable, but a standard Hitec servo was used because it was available. A scrap 3/8" balsa base was sanded to fit the fuselage floor and glued with a hot glue gun - it's very important to allow the hot glue to cool and cure properly for 25-30 minutes before putting any load on it. The servo was covered in heat-shrink (as used for nicad packs) up to the lugs and the remainder protected with a small amount of tape, then hot-glued to the base. One of the formers needed filing away and the retract mechanism horn needed some excess plastic paring off, but it eventually worked faultlessly.

The elevator and rudder were actuated with short pieces of scrap snake outer epoxied directly onto the surface, and the snake inner was simply bent 90 degrees and inserted into the tube; the free play was reduced using a single layer of tape around the snake inner. The rudder was then hinged with Tesa tape and the stabilizer bolted on with a couple of nylon bolts.

Close-up of the rudder horn

A Z-bend made in the wire at the servo end and the elevator and rudder servos were attached to the inner cockpit sides. Final adjustments were made with sub-trim.

A switch was installed next to the elevator servo, and the receiver was attached to the cockpit floor with Velcro stuck to a balsa base, attached with the obligatory hot glue. By now it was obvious that the cockpit seat pan was redundant as the cockpit was full of radio gear - shame really as I'd ordered a full-length 1/8 scale pilot from Pete's Pilots, which will now have to be truncated when finally fitted.

The wing root. From front to rear - aileron snake & linkage, wing joiner hole, exit for aileron servo wires (not fitted - Ed), incidence peg.

The aileron servos had to be mounted last, as the aileron servo tray would have to be removed if the retract unit linkage needed any attention. The servo tray was made from a 3mm liteply plate the width of the fuselage with a lip at the left and right edges. The servo lugs fit into a couple of slots, and the servos (HS85’s) can just be wedged in side by side. A central fixing plug and a 1.5 mm ply clamp plate was held in place using a spare wing bolt, and the completed servo mount was epoxied into the fuselage at just the right angle for the servo connectors to attach with a minimum of stress (refer to the pictures). This seems to work pretty well as both servos are solidly held but can be removed in a matter of seconds.

The aileron horns were cut from paxolin, drilled with a 0.9 mm hole (using a drill and pin vice from Hannants) for a snug fit to the wire inner and epoxied into the ailerons - this works OK provided that the holes are both in the same position relative to the surface, but it's a tricky job as they should only project about 3.5mm above the aileron to ensure that sufficient movement is available - a simple jig helps a lot.

At the aileron end the wire is bent outwards 90 degrees and is retained in the horn by the (considerable) springiness of the inner snake. The exposed snake and horn are protected by a couple of very cheap but very suitable vac-formed covers from West London Models, simply taped to the wing.

The servo end was soldered into some shortened threaded adapters (space is a bit tight) and a ball-link screwed on. The wings are a very tight push-fit onto the fuselage, but will be retained using yet more tape.

Finally, the Rx antenna was taped to a strip of hard 1/16" square balsa and fed carefully down the fuselage. The 1200 mAh battery was wrapped in thin packing foam and tape, then epoxied into the nose and packed with some pipe lagging. About 34g of nose weight was required which was taped to the battery before installation, and 2g on the right wing was required for lateral balance.

The completed model weighs 51.5 oz which gives a wing loading of about 16.4 oz/sq ft, which is a tad higher than expected but probably still acceptable.

The main wheel in the "down" position. I don't think I will be using this at the Beacon - those doors would last about three seconds!

The model was completed in March and after a substantial wait caused by Foot and Mouth, it has at last flown. I must confess to being a trifle apprehensive about the rather hefty wing loading, but I needn't have worried; with the c.g. in the design position and the control movements as detailed here, it really is a very docile aircraft, and looks like a much bigger model in the air. It is, however, extremely fast when the nose is held down and is capable of accumulating more than enough energy to perform all sailplane-type aerobatics. It will fly in very little lift and thermals very well, so well in fact that the usual problem is getting it down before it goes out of sight; applying the spoilers usually does the trick.

On the down side, landings can be a bit fast for comfort if you get your switchology mixed up - in my own defence, I feel I ought to point out that the landing switch that activates the spoilers is right next to the aileron rate switch... :-). Those wing tips have a very narrow chord and if you use too much down aileron movement, it *will* tip stall. The rudder could do with more movement to be on the safe side, but to do this you'd have to increase the chamfer angle on the moulded rudder before attaching the control horn.

Control Throws

Rudder: +/- 10 mm
Elevator: +/- 7 mm
Ailerons: +8/-4 mm, 100% rudder mixed in
Spoilers: Ailerons + 11 mm, Elevator -2 mm

Wingspan:	2.5m (98.4 inches in real numbers, approx 1/8 scale)
Weight:	51.5 oz
Wing Loading:	16.4 oz/sq ft
Wing Section:	E205 / E207
Functions:	Ailerons, Elevator, Rudder, Spoilers, Retract
Price:	About £274

Article by Andy Blackburn from Ivinghoe Soaring Association
9th Sept 2001
Originally Published in June 2001 issue of "The Beacon"

This review has been republished with the kind permission of the Ivinghoe Soaring Association

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