Some history.

The Lunak (Sea Buzzard) was one of the early aerobatic sailplanes with inspiration coming from the DFS Habbicht. The designer, Vladimir Stros had two prototypes flying by 1948 which showed much promise as aerobatic sailplanes but they also had rather good soaring characteristics.

No design features were built in to reduce tip stalling or spinning as these were seen as favourable traits to be called upon by the pilots at will. The construction of the sailplane was very orthodox for the period and the canopy looks like it may have been designed to look and work as a jet fighter canopy due to the interest of the Czech Ministry of Defence wanting to use the Lunak as aerobatic trainers for jet pilots. Only 70 in total were built with only about 10 remaining today.

More information can be ready in Martin Simons' book ?Sailplanes 1945-65? or if you can translate from German there is a great site on the LF-107 - http://www.lunak.de/

We get introduced

I saw one of these flying in November at the Horsham Aerotow and thought they looked really neat. I called Mike at Model flight and said I was interested in one. Early the next morning my door bell rang and there was a huge box with a tiny Australian Air Express man standing behind it.

I always love this bit ? opening the box has a definite Christmas feel to it, and having a look in amongst all the packing, the model slowly appears.

Coming in one box, the kit is well packed and everything came in perfect condition

The first thing I noticed is how much presence this model has for a 4m glider. The fuselage is almost twice the girth of a modern sailplane of equivalent size and the wing roots are big! The second thing I you notice is the quality. Remembering that this is a $995 ARF, the quality is great. The yellow fuselage is well finished, although the seam is a little thick. The wings are finished in yellow Oracover, and already have all the surfaces tape hinged.

Going through the rest of the kit you find the crystal clear canopy and the tail surfaces ? all finished and ready to go. There is a bag of tricks, joiner bars for the wings and tail plane, and a set of decals for the scale finish.

The bag of tricks! You may need some time to work out what some of these things are without instructions.

Planning the attack.

Ok, everything is spread out on the table, and I look for the instructions? I found a manual (8 A5 pages in fact - 4 comprising the cover, a blank page, a page on the specifications and one on the list of parts) in Czech and a few basic drawings of the sailplane setup. These give you information on CofG and throws, but that is the extent of the instructions usefulness. I love that the Czechs are making our hobby more affordable, but I would really love it if they spent just a little time on instructions!

Spreading things out will need a bit of room... I can usually get six people on this table!

I know that in writing these reviews, one should build the kits as supplied, but given no instructions come with them, I believe it is better to finish the kits in a way that makes the most of its design... lets say we are giving these kits the opportunity to be "all they possibly can be!"

Starting at the end.

You have to start somewhere and the tails is as good a place as any. This is where we have done most of the modifications to the kit.

The first thing is the rudder. The supplied wedge shaped leading edge rudder with plastic hinges would be absolutely fine, but I always like a rudder that can be removed if need be, so I removed all of the covering and make a pin-hinged rudder. This also gives it a slightly more scale-like appearance.

Cutting off the leading edge wedge of the rudder, I glued a tube that fits a piece of piano wire down the centre line of the rudder at the hinge point. Then I pack this with Balsa and sand to shape.

The stripped rudder with new hinge and fin post

A bare rudder assembly in for a trial fitting.

The hinges are made from circuit board in the shape of half an icy pole stick and a hole drilled in the circular end to take the piano wire. The rudder control is via a closed loop system running up the fuselage to a bell crank that takes the closed loop load (rather than placing the load on the servo). Using the original control horns for the rudder made this whole process simple.

The closed loop system at the rudder end... yes, there is one on each side.

A view of the lower portion of the rudder. notice the circuit board hinges entering the leading edge of the rudder.

I replaced the balsa fin post with a 3mm light-ply post and cut the associated hinge holes for the rudder. We will fit it all later.

Focusing on the stabilisers? The Lunak has split stabilisers, thus requiring a dual connection system for the elevators. The supplied method is using one nyrod style push rod system with a ?Y? connector for the final push rods exiting the fuselage to the elevators. While flying at Camperdown one weekend, David Hobby, Anthony Peate (who was also in the process of building a Reichard Lunak) and myself, tossed around ideas on better ways to attach the stabilisers and elevators.

The first suggestion from David was to get rid of the steel joiner bars in the stabilisers and replace them with Carbon rods. This gave us a huge weight saving in the tail.

In my opinion, there is a big issue here with the design of the kit. Where the joining rods for the stabilisers go through the fuselage the kit only has the fibreglass as the barer. There are no compression struts stopping the fuselage from being squashed on a hard landing. To solve this, I added aluminium tubes that were glued in to the fin area to act as bearing for the carbon rod joiners as well as compression struts for the fin.

The original stabiliser

The second suggestion David had was to eliminate the elevator linkage from the front of the fuselage and replace it with mini servos in each stabiliser. Incorporating both of these changes still came in lighter than if we had used the originally supplied steel rods.

The servos were sunk into the root of the stabiliser by cutting out square opening in the root and then gluing two fixing points for the servos. A slot was cut so the servo arm can protrude through the lower skin of the stabilizer. Standard metal clevises and threaded rods are used to bridge the gap from the servo arm to the supplied control arm.

The hole created for the servo in the root of the stabiliser.

The control arm on the elevator made from circuit board.

The servo installed and the servo cable protruding out of the root from the front of the servo.

With that much area used in the tail plane for the servos, we came across another issue, using the elastic band system (one of the things you can make out from the instructions) of securing the stabiliser. Anthony came up with a neat solution to get around this problem using magnets. These were embedded in the root of the stabilisers and the fuselage to create a strong enough coupling for any flight forces, but also providing an easy and fast way to install the tail plane.

The fuselage side of the stabiliser root showing the installed magnet, aluminium tube compression struts and the holes for the servo connections.

The magnets used are Neodymium Iron Boron Magnets. Known as third generation of Rare Earth magnets, Neodymium Iron Boron (NdFeB) magnets are the most powerful and advanced commercialised permanent magnet today. Since they are made from Neodymium, one of the most plentiful rare earth elements, and inexpensive iron, NdFeB magnets offer the best value in cost and performance.

Check out the following link for more information: http://www.mmcmagnetics.com/

I have only placed the magnets at the front of the stabiliser, but Anthony suggest to put them on both sides of the servo for better coupling as he has done in his model. Dave's Aircraft World (http://www.aircraft-world.com/shopexd.asp?id=2095) is now selling 6x4x2mm magnets which would be ideal for the Lunak.

I left the elevators leading edge with the wedge shape that they originally had, as there would need to be too much reworking of both the stabilisers and elevators to create a shrouded hinge. I used the supplied plastic hinges and secured them using PVA. This gave me plenty of time to make sure they were all aligned properly. Note - some of the pre cut slots for the hinges did not match on mine.

The servos were connected via large extension cords. These were run through a special hole cut in the stabiliser root on the fuselage. The extensions running up the sides of the fuselage back to the receiver located in the front.

Putting the tail feathers together... and you get a sneak peak at the colours.

All up the changes to the kit design end up 1.6 grams lighter in the tail than the original components? and with what I believe to be a much more positive coupling on the rudder and elevators. Not only that, but a much easier way of fitting the stabilisers when preparing for flight.

Into the belly of the beast

Fitting the gear in the fuselage is relatively simple given the amount of room you have here. I decided to split the trays, rather than having one tray for all the gear using the tray provided, I decided to have one tray at the rear of the cockpit for the receiver, switch, rudder servo and rudder bell crank, and another in the nose for the battery tray and areotow release servo. This put as much of the weight as possible into the front of the fuselage rather than having to add more lead later.

The aerotow release servo and battery tray in the nose of the Lunak. You can just see the aerotow release wire coming back from the release mechanism.

The bell crank for the closed loop was made up from circuit board to the same width as the control horn on the rudder. This gives me a 1:1 control loop. The servo is then attached with a short threaded rod and clevises.

The rear tray showing the switch, rudder servo, rudder bell crank and wheel assembly underneath. Also visible is the joiner bar housing.

The Aerotow release is made up from aluminium rod and matching aluminium tube. The rod is cut at an angle to make the bevel that the line slides up. A hole is drilled through the rod slightly high of centre. A cross bar of spring steel is drilled through the tube. The reason for the bevel on the outer tube is due to the location of the tow release on the nose of the Lunak. The release is not in the centre, but down 30degrees on the centerline. A discussion of how to make these is available here, or alternatively you can purchase ready made Graupner tow releases from Model flight.

The tow release assembly. Once made, glue the outer over the inner and then glue the whole unit in place.

The battery is placed on a separate cradle with Velcro attached for simple installation and removal. Enough space was left to allow strips of lead to be placed in the nose between the battery and the aerotow release. All in all, there ended up being 1.5kg of lead in the nose of my Lunak.

The inside of the fuselage showing the front compression strut made from carbon fibre.

The wing joining mechanism is fairly straight forward, but again here I think a crucial element is missing. I added two compression struts at the leading edge and just in front of where the flap starts. Especially with the wings being bolted to the fuselage, without the struts makes the fuselage more susceptible to being crushed on a hard landing. At least with compression struts the strut will take most of the force in this event.

The final item un the fuselage is the wheel. The system they have provided with the plastic cover seems overly under engineered. So I replaced it with a ply brace and aluminium arm. This also gives it a slightly more scale like appearance.

The installed wheel assembly

The wheel assembly with the ply brace and aluminium arms.

Its only a missile without wings

The wings have six control surfaces, even when you omit the spoilers (more on that in the flight test). In this instance, we have left the spoilers out.

Wires running to the other servos. A view of the servo well with servo included.

Now, installing the wing servos requires some preparation. When you make something, you need to remember that you need six, and each may vary slightly. Each of the wings has the servo bays pre cut and standoffs and covers already pre-made. All you have to do is secure the servo and make the linkage.

in order, the servo cover, the stand offs, the servo and servo tray and finally the servo well.

How I did this was to make a tray that the servo sat on. Each tray was a different thickness to make sure that the servo was at exactly the same level when compared to the bottom wing skin. Remember to take into account the servo bay covers when doing this. Once these are made, they were glued in. Then the control arms were made and finally the servo glued in. The stand offs were then glued in place and finally the covers held down with covering.

All the items above, but shown as a sandwich.

 A couple of items of note at this point:

- When choosing the servos for your wings, remember these surfaces are larger than an equivalent 4 meter glider. This is also a semi aerobatic sailplane. So the servos we suggest you use in the wings should be at least fitted with metal gears, and not be below 3.5kg capacity especially for the Flaps.

- When gluing the servos in, some people prefer to cover the servo in shrink wrap before gluing it in (others have suggested wrapping in masking tape). This allows for easier release of the servo should you need to do maintenance. (Although, I did not do just to expedite things a little... and regretted it later!)

The control connections again were just clevises and threaded rod. One thing that helped with the flaps was that the hole for the clevis was moved back away from the hinge point by 5mm to allow the throw to be a little tamer. With the hole on the hinge point made the flap very sensitive to control input.

Bringing it all together

The fin post, gear trays, battery cradle and lead were all fitted using silicon. This allows for some movement in harder landings without giving way. In the case of the fin post, it also allows for easy removal for any maintenance required in the fin area.

looking down on the top of the fuselage at the rear of the canopy opening. You can see the hole for the pin that comes down from the canopy tray.

The canopy tray with the pin for the canopy latch. (looking at it upside-down)

Masking tape holding down the canopy while gluing. note the vinyl between the tray and fuselage to stop the glue from sticking to the fuselage.

The canopy was a little tricky. There is no way to use a standard canopy latch in the traditional way, so I created an pin made from aluminium rod and used this to hold down the canopy frame with the latch in the rear. In the front, two brass pins made the forward connection. The Canopy was cut out and once satisfied with the fit, glued down with Pacer Canopy Glue. This is great stuff for this particular job. Stronger bond than using silicon, but more flexible than using cyano.

Dressing it up.

The colour scheme for the Lunak supplied in the kit is very simple using only the decals and some black for the nose. (see image at the top of page)

I opted to make my life interesting and do a fairly unique colour scheme of an Hungarian HA-4083 Lunak with a wonderful star burst (which I will come to regret later) scheme on the wings.

Working out of the star burst pattern from the full size took a bit of head scratching...

All the colours were added using sign vinyl and many hours of patient work. If I can give any advice when applying vinyl, the one most import thing to remember is the water and the squeegee! Your will go mad without them! Oh, why regret the star burst? ?well the three sleepless nights it took to get it done!

... but I eventually got there!

I think it was worth it? ?right?

Prep for flight.

The Cof G is checked and we find that to set it at 110mm (the instructions show it at 130mm, but we found that to be a little too far back) from the leading edge (which is just forward of the joiner bar) we need 1.5kg of lead in the nose. This brought the Lunak to an all up flying weight of 6.1kg.

With this many surfaces on the wings, I would suggest that you make sure you have a radio that is able to deal with 6 servo wings (possibly 8 if you add the air brakes). Using my Graupner MC24 (A more cost effective alternative may be the Graupner MC22) and the new Rom upgrade that has programming specifically for this kind of wing, it made the process fairly easy. Now, remember these words? and read on.

The receiver I used is the new JR synthesised 10 channel RS10DS receiver. Pretty darn good value for a 10 Channel receiver... and no crystals! The wings were programmed in such a way that the inner surfaces worked as flaps only. The centre surfaces could be flaps and/or ailerons, and the outer surfaces were ailerons only. The two inboard panels also acted as the crow breaks while leaving the outboard panel as ailerons.

The elevators were run on separate channels and mixed with the transmitter, you could ?Y? lead them as did Anthony. The one thing he found was that you had to electronically reverse one of the servos which is very delicate stuff, or you can use a commercial servo reverser.

Time for a maiden? ...anyone?

Well I have waited for three days at Camperdown for the right conditions and we have finally got great wind but slightly in the wrong direction. I have asked Colin Collyer to maiden the Lunak for me. His years of experience will count as well as giving me the opportunity to take some photos.

All dressed up and we actually have somewhere to go!

Before moving it out to the slope, we go over it with a fine tooth comb. All throws, all mixes, and all connections are checked again? then Colin spots it? asking me to check the aileron direction? What? ...DOH! Left was right and visa versa. That?s right; I got the aileron throws reversed. Probably when setting it up on the bench, I was looking at the nose and set it up the wrong way.

Moral of this part of the story ? always check everything ? no matter how confident you are! Even better ? get someone else to check it as well!

So having averted a near disaster, we head out on the slope, throw? and? it?s flying!

Actually, it?s flying well, no that?s really well!

The view from the point at Camperdown. Tearing around at speed posed no problems.

Colin trims a little, but within a minute, with my back turned as I was picking up my camera, I asked Colin how it was going? He responded saying ?great! ?it?s upside down! Hope you don?t mind!? having trimmed the model out, he proceeded with a full set of aerobatics? and a cheeky grin. I suspect he was having fun.

Well it did everything that it was supposed to, and flew like a dream. Having set different rates on the wings and using different mix combinations we were able to get the aileron response to range from docile to very responsive. Cruising around the sky, the Lunak kept its momentum easily and looked great when flown in a scale like fashion.

No problems seeing this one in the sky!

At one stage the lift did drop a little, and keeping the speed up and the wing efficient, there did not seem any adverse drop in performance. We did not have an opportunity to see the thermal performance on the slope, but will get a much better idea when we take it to Jerilderie in a few weeks time. Oh, and yes, I did have a fly!

Landing? Well I did say that the wind direction was a little difficult and required that the landing be a cross wind landing? but in true Collyer style, Colin nailed it second time round. One thing here, if you are going to build one of these for the slope, you may want to think about adding the air brakes as well. The crow braking did slow it down a little, but was not overly effective given the momentum this model has.

Conclusion

Here is a kit for $995 that has the presence of a much larger model in only a 4meter footprint. The finish is very neat for the price and the performance is no less impressive.

This is probably one of the better value for money kits actually available here in Australia.

With a lack of documentation on how things should be done, there are many alternatives as to how the parts could fit together. What I have explained here is just one set of possible alternatives. This kit does allow a bit of freedom to make your own decisions, and with an extra bit of work could be fully fitted out with all the scale elements including the cockpit.

This is not a ?beginners? model as some experience is required to handle the sailplane in the sky and in landing, and you definitely wont be able to launch this one on your own (unless you are aerotowing).

The structural quality seems to have worked well with a combination of weight saving and strengthening materials, this combination has produced a good aerobatic performer (we have been told that she can thermal well too - we shall see). The wings stood up well to all of Colin?s tests and as we said before, the quality of the fuselage is great except for the slightly unsightly seam.

All in all ? great value for money! No wonder it won the model of the year in 2002.

More images of this model at the Maiden flight are available in the gallery

The Facts

Price	$995 plus radio gear
Value for money	8.5/10
Performance	7.0/10
Experience required building	Moderate
Experience required flying	Moderate +
Satisfaction factor	8/10
Approximate building time	40 hours (with the changes we made, not including the colour scheme)
In Australia available from:	Model Flight
Manufacturer:	Reichard (click here for the specifications on the kit)