A few years back I was at an aerotow meet in Bordertown when one of the guys showed me a fuselage and wings for a 1/3rd scale cirrus. When I was told that the original purchaser had fallen through, I quickly snapped it up.

First some specifications:

1:3 scale

Wingspan 5m

Fuselage length 2.15m

Weight 12.7kg

The fuselage was of traditional glass layup, but with a lot of defects. There was no reinforcement in the fuselage in any of the high-stress areas. The wings were foam covered with 1.0mm ply.

The Standard Cirrus was first designed in the Schemmp Hirth factory during 1968 and first flew in 1969. It was designed for the 15 meter class and designed at the same time as the original Nimbus (open Class). It is well known for its all moving tailplane that also was renowned for not coupling properly to the elevator linkage. Later versions were tested with a fixed tailplane, but they did not have the same “look” as the original plan form.

As simple a design this sailplane is, it had a few interesting design elements to it. The all moving tailplane proved an interesting structural challenge given its location on the fin.

This required a sub framework to be made that carried the torque rods for the tailplane assembly. A separate linkage and shroud needed to be made to actuate the elevator from the front. In actual fact – I have used the same ratio of drive on the tailplane as was used on the full size.

To simplify the rigging, I make the tailplane in two halves that slid together on carbon rods and were held together using magnets. This proved to be a very effective way of quickly rigging the tailplane given the relatively unusual control linkage.

The other fairly unique element on the sailplane is the side hinged rudder. I ended up making a piano style hinge using nylon tube for the baring and kevlar and carbon to hold it all together. A scale wiper was made for the opening side and the servo mounted inside the fin to supply a short linkage internally.

With the fuselage lacking any reinforcement and the fact the tailplane was sitting atop the fin structure. Over and above the sub frame for the tailplane, I added reinforcements of carbon the crucial areas of the fin, lower fuselage where the retract was going to be mounted and near the wing seats to distribute the load. I also added a timber brace member that ran inside the fuselage from the nose to the wing seat area. This similar to one in the full size.

Mounting the retract using two gas struts provided shock absorption and the front also needed to pivot. Both of these locations were mounted to Carbon/Ply/Carbon formers. In front of the retract former was the mount plate for the Smart-Fly power-expander pro power distribution unit. I now will not build a large scale sailplane… or any large scale plane without using one of these units. The safeguards they provide are well documented elsewhere, but think that anyone building something this big without these kinds of safety measures is putting others in jeopardy… let alone for your own peace of mind!

Right up front went the two 4000mah packs and the tow release. Don’t worry, you can go bigger on the batteries – I ended up adding nearly 900grams of lead in the nose.

The wings were fairly simple with only ailerons and airbrakes. In hindsight I should have put in bigger brakes – the scale size does very little to slow it down… but it does break the airflow enough to bring it in a slight downward attitude.

The wings were mounted with a bent round joiner bar (no my choice – but what was already there). The wings are secured to the fuselage using two systems – nylon bolts in the wing root (to stop the wing sliding away from the fuselage) and small unbreako bolts screwed into the wing tube and bar at the end of the joiner from the bottom of the wing to stop the wing joiner rotating in flight.

The servos were mounted on plates for easy removal and the ailerons were hinged using a silicon hinging method. I have added tape to the hinge as I just was not quite happy to trust just the silicon itself.

The canopy and frame came together with the canopy being supplied, but I did have to create the frame. The Cirrus has a fairly simple canopy frame so I made it from layers of think ply/carbon/ply/glass/ply. This was all pinned in place on the fuselage and allowed to dry. Once set, the frame was trimmed, hinged and the canopy glued to the frame. I have yet to look at the cockpit detail, but at the moment I am in no hurry to compete that.

Prep for paint was a pain as the fuselage had thousands of tiny air bubbles in the glass. The only way to deal with this was with primer putty, rubbing, stop putty, rubbing, primer and see what you get. I think I went through this process 3 times before the fuselage was clear of holes and ready for the two pack white coat.

The wings were first covered in 3/4oz cloth and attached using Cabbott’s water based floor sealer. This not only acts like an epoxy for the cloth – it also seals the timber without the weight which is associated with epoxy. Then the primer, sand and paint and the wings came up a treat.

A few decals, some red paint and she looks almost like the real thing.

Flying

Initially, I thought she was going to be too heavy, but after a few flights and a little less nose weight, she fly’s as gently as any sailplane half its size. The great thing is that she can also keep up her momentum, so covering big sky is no problem.

The only thing is in landing, the small breaks need to be taken into account – or you go whoosing by…

I can see many long flights just cruising round the sky. She may not be a moulded ship, but you would be hard pressed to see the difference in the air.

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