Once more into the Breach…..
by Steve Davis
How to Choose Your Weapon
As a result of a Sunday afternoon outing to Lasham for the SSS Scale Competition 2000, I was smitten with Colin Bond’s 3rd Scale Wilga tug and decided ;I should get one of those Typically Maisey got in there first and ordered the 1/4 scale from Alex Frisch, so I sat back and waited to see what turned up. Six months later when he began to fly it I thought ;I should definitely get one of those!
But which size – or 1/3 scale? The merits of the 1/4 scale being its handy size and smaller cost. The merits of the 1/3 scale are presence and visibility at altitude. Having chewed it over for a month or two more I came to the conclusion that the 1/3 scale version was just too big and what was needed was something in between. A revisit to Alexs website proved very timely as he had just announced the arrival of a 1:3.5 scale model. So fate having taken a hand so nicely, I placed my order.
The Power Struggle
Choice of engine was the next important decision. Alex recommended without hesitation, King Engines. 80cc being adequate and 100cc being plenty. As with most things in life I believe you can never have enough power and a close inspection of the King range revealed a 140cc engine producing 14 horsepower and weighing only 1/2lb more than the 100cc engine. Easy decision then! A phone call to Alex Frisch to confirm my choice solicited the comment ;Oh my god!. So with the kit on order and the engine choice made I felt confident that I had the right combination to tow anything the brothers Brigg dared throw at me!
Six Weeks Later.
A call from the freight forwarder confirmed the existence of a rather large box awaiting collection. ;No problem I said ;Ill be there in a tick. An hour later saw me on my way home with the back doors of the van struggling to contain a portacabin sized box! Upon opening the box I discovered a kit of superb quality and 12 black bags full of polystyrene chips (it was three days before we found the children again!).
I tried to read the instructions but not knowing any German this proved to be a fruitless exercise. I was, however, able to confirm from the diagrams that all parts were present and correct. For an experienced builder the diagrams proved sufficient since I had my own construction techniques and preferences that I intended to employ. Closer inspection of the parts revealed that they were very accurately made. All the positioning marks moulded on the fuselage to indicate where the undercarriage and wing joiners etc could be trusted and the finish of the moulding was excellent. There were no pinholes and the seams were clean. I was concerned initially about the flexibility of the floor of the fuselage but this has since proved groundless.
The wings are white Styrofoam with 1.5mm veneer skins bonded with epoxy resin and again the consistency of the panels was the best I had seen.
The hardware supplied was adequate with lots of captivated nuts and plated bolts, which I replaced with stainless steel (a personal preference). The tank was from Tony Clark and of good quality as with the fuel system accessories. I included a Dubro fuel filler valve here to help with the refills.
All the window apertures have to be cut out by hand, but before doing this some internal woodwork is necessary to stabilise the structure. The apertures can then be cut with a hacksaw blade, the radius corners finished with a flap wheel and the straight lines finished with a fine file. I strengthened the area of the fuselage, which would see the majority of the undercarriage loads with carbon fibre cloth. This was also used around the tow release area.
Over 200 holes were drilled to accommodate the window screws; the positions for these again were marked on the fuselage moulding by Alex.
I discarded the mid fuselage bulkhead supplied by Alex in favour of my own which would also accommodate the rudder and tail wheel servos, this was fabricated from 1/4 inch plywood and epoxyed into position.
The engine mount supplied was for the smaller King 100cc engine and to fit the longer 140cc engine the front was shortened by 1 inch and reconstructed with carbon fibre and 1/4 inch plywood. I also strengthened the 1/4 inch balsa wood fin post supplied by laminating a layer of 1/64 inch plywood to either side and some blue foam ribs were made to help strengthen the fin moulding.
The undercarriage was built as per instructions with the exception of the pivot point of the trailing links. I machined a proper phosphor bronze pivot to replace the cap screw pivot supplied.
Wings -Flap hinges were used as supplied, but I designed my own aileron hinges which consist of an O ring used as a bush and a machined aluminium pivot in the aileron. The O ring gives a degree of flexibility to the overall hinge which helps absorb the vibration in the surface whilst maintaining sufficient rigidity for accurate control. This was used for the fin and the elevator too.
Radio -when reflecting upon my choice of radio I had not only too consider the obvious, such as what size of servo should be used to power the surface in question, but too look more deeply at the safety of an aircraft this size and hence the kind of systems I would need to give me some redundancy in the control system. After some deliberations I came up with the following shopping list:
2 receivers, one to control the left hand side of the plane and one for the right (Ive had crystal failure twice in the past)
Optical isolation of all channels
Separate servo and receiver power supplies
Twin servo batteries, 6AH total capacity
Heavy duty switches
SMC 20 DS Graupner dual conversion receivers x 2
SM Services Optoisolators x 2
SM Services battery backer x 1
3AH 5cell NIMH cell packs x 2 for servos
0.5AH 4cell NICAD x 2 for receivers
Voltspy x 2
Upon completion the whole model was finished with lightweight glass cloth and epoxy resin flatted back and painted with 2-pack car paint (instructions from Chris Williams here.)
The maiden voyage took place in Caen during May 2002, upon returning home I visited the local club field for a “tuning” session, coming in for a landing I elected to go round again for another practice approach At that exact moment the engine cut. So with the flaps down, no airspeed, only 20 ft of altitude, no room to turn and too high to force a landing before running out of field I elected to stretch the glide into the next field. You know whats coming and so did I, it clipped the top of the field boundary (a rubbish heap) and rolled end over end!!! Bollocks!!! All that electronic safety gear couldnt prevent the dopey pilot running out of fuel!!!
Inspection of the wreckage revealed that the fuselage was badly crushed but incredibly all the foam parts were almost untouched.
Back in the workshop nothing much happened until the sick feeling in the pit of my stomach had worn off (about two weeks). Enquiries to Frisch for a new fuselage resulted in a quote of £450, so I repaired the original and two months I later was flying again.
Although very intimidating initially, It has proved to be a delight to fly and has real presence in the air.
All the controls are nicely harmonised, the engine produces some 14hp so roll out is very short, vertical climbs are unlimited.
The position of the main wheels in relation to the cg make landing a constant challenge, half flap on approach with full flap for the flair at the end just prior to touch down on the two main wheels is the current method. Then a stab of down to hold the tail up before gently lowering the tail down. Get it wrong and the tail slaps down and a kangaroo session begins.
1/4 scale gliders require only half throttle, 1/3 scale stuff two thirds, and anything Dave Briggs brings needs nitro!
Aerotowing is such a fascinating aspect of RC modelling and neatly combines my love of sailplanes with my interest in large power planes. Being a tug pilot at a meeting such as middle wallop, is both a pleasure and a privilege. It tests your spatial awareness, landing in the midst of a busy field time after time. As I said earlier the Wilga demands total concentration for each landing, and it is easy to become mentally tired after a couple of hours without a break at which point the likely hood of a mistake rockets skywards.
It is a privilege to launch the gliders. The builders who have invested a great deal of time and money in their models deserve the best launch possible. That means a reliable tug. A focused tug pilot, a good steady climb that the glider pilot is happy with, and the communication of any changes in course or climb rate prior to them occurring.
The coming season will see a set of ball raced main wheels – the existing wheels sport a plain metal bush on a stainless steel axle, over the summer the bush has worn a little so at some throttle settings the wheels rattle on the axle slightly causing the occasional momentary fail safe; a fully articulating tail wheel complete with shock absorber to take the impact when I frequently get the landings wrong. A bulletproof tow release (Re Bills accident at MW) based on a full size version I have acquired. Also on the list is a dashboard and pilot
Frisch 1/4 Scale Wilga Q & A
As a result of questions on the SSS forum, here are some words of wisdom from Bill Maisey
“I am currently putting together a Frisch 1/4 Scale Wilga. I will be grateful if someone could tell me what is the best place to position the servos for elevator, rudder and tail wheel control. The length from the servo positions on the undercarriage mount to the control surfaces is over a metre and I don’t particularly want to use very long Bowden cables. I therefore need to know if it is feasible to position the servos in the back end of the plane close to the elevator / rudder and, if so, where is the best place to put them.
It is also not clear in the instructions (which are minimal English ones (with diagrams supplied but with the text in German) how the servo connection is made to the flaps”
I’m pleased to hear of another 1/4 scale Wilga being put together. It’s a classic aeroplane for sure.
Although I haven’t personally built a Frisch Wilga, based on other giant scale models I have built I would recommend that as the flaps are bottom hinged I would be driving the flaps from a horn in the leading edge just beneath the top skin with all servo linkages internally and obviously one servo per surface.
As to your question about the elevators, here are my thoughts you could use carbon arrow shafts to drive each elevator with a support half way down the tail, you could also mount the servos on their sides inside each tail plane and drive each elevator separately with closed loop to the rudder. You could also cut a hatch in the bottom of the fuse and mount the servos in a tray and run short pushrods or you could just contact Collin Bond who I believe has already built one, I know other members in here have also built Wilgas such as yours, also have a look in tech tips as there are building notes from two different modelers. I also have a kit review from the states that I can E-mail you if you like.
It is a good idea to cut the elevator in half and use two servos. Also, use separate servos for the tail wheel and rudder if you can.
I would make up a ply mount for all four servos and position it just inside the neck of the tail boom behind the rear window.
Use thin wall carbon tube with 6mm id for the elevator push rods and closed loop wire for the rudder and wheel.
This will give you very positive controls without having the weight of the servos way back in the fuz.
I previously used DS3328 servos mounted in the tailplane. However, these were operating at the limit of their capability and I wouldn’t recommend you do that.
As for the flaps, I fitted a horn below the top skin of the flap. The servo box positions are marked on the underside of the wings and the wings also have hardwood mountings for the flap hinges that are supplied in the kit.
“I had planned to use 2 servos for the elevator and one each for the rudder and tailwheel. I will be using Hitec Digital Servos (10Kg/Cm) which should have more than enough power. I intend to use 2 Futaba PCM Receivers with Opto-isolators, two 6volt batteries for the servos and two 4.8v batteries for the receivers. I am currently looking at the Lithium-ion packs which, although expensive, are much lighter than Nicads”
Standard ( 4kg ) servos will do fine so anything stronger is a bonus.
“In that case I may use some Multiplex Digital Servos I have which pull 6Kg/Cm at 6Volts. I will use the others on a large Funtana I have just ordered.
The engine I have to put in the Wilga is a new ZDZ 80 – I presume that this will be fine?
Some more questions if I may:
1. Do I need to reinforce the firewall?
2. Do I need to reinforce the fin which seems rather flexible?
3. What are the best hinges I should use for the control surfaces (other than the flaps)? I am currently planning to use Robart Super Hinges.
4. What is the best way to mount the wing servos?
5. How do I connect the bottom of the cowl?”
Either of the ZDZ80s will give you a very powerful Wilga capable of towing 6 Meter (13kg) sailplanes with ease and much larger models with a little care. All petrol engines, and particularly single cylinder versions, produce considerable vibration that can be very destructive. The best approach to managing this is to mount the engine very firmly so that the vibration is absorbed into the mass of the airframe. You can see evidence of this if you run the engine with the wings off as the fuz just goes berserk without the extra mass of the wings to absorb energy. ( A second piece of advice was – To help absorb the vibration and stop cracking in a glass cowl fit round servo grommets trough the cowl with the mounting screw through the middle. The screw head will pull up on the grommet, not the cowl and there fore eliminate excessive vibration.
As for the mounts use the yellow inner from golden rods or similar to act as a bush inside the screw holes to bolt your cowl with as they are vibration proof and if you strip the thread simply drill it out and super glue in a new one, also works a treat for servo mounts.)
The Frisch Wilga comes with a 6mm ply firewall already fitted. You should double this with ply or MDF. Also, make the engine mount holes a close fit on the bolts or that ZDZ will start skidding around and work loose. There is no need to glass inside the fuz as this looks messy and just adds weight.
The fin sides are quite floppy but as soon as the fin post is fitted the structure becomes rigid and easily handles all loads. If the floppy sides are a concern, make up a single foam rib to go half way up the fin.
The larger sized Robart hinges are fine. I believe it’s worth the effort making all hinges fully serviceable. If you just glue them in, it’s a hellish job to get them out again.
Regarding servos on the ? scale Wilga, I believe it’s preferable to use standard (20mm wide) servos throughout rather than anything larger in order to keep the weight of the model as low as possible. All surfaces bar the flaps can be safely controlled with standard 4 to 5kg servos providing the elevator is cut in two. For the flaps, I used the JR DS8231 which produce 6.5kg @ 4.8v. Line the wing boxes with hard balsa or 2mm ply and then make hardwood mounts or use commercial servo boxes. Remember to arrange the flap linkage so that the push rod is in line with the servo out put shaft when the flaps are fully extended.
To fit the cowl you have to make up a pair of aluminium brackets for the lower side. I came up with a neat trick here. I used 3mm Dural for the brackets and also fitted three small plates inside the fuz for the top side of the cowl. I fixed the cowl with 3mm button head screws and “captive” nylock nuts. To make the nuts captive, drill a hole in the brackets just a bit larger than the AF size of the nuts. Don’t break right through with the drill but leave a small internal flange. Then use a 3mm screw to pull the nylock nuts into the hole up to the flange. This method provides a vibration free mounting for any cowl that can also withstand repeated removals.
“Can I purchase a reliable tow release to fit the Wilga or will I have to make one myself.”
There is no commercially available, reliable release mechanism for a tow plane that I know of. Most tow pilots make up their own using a “pin through hole” system which will work very well providing it is driven by a strong servo (11kg minimum).
I have made a release that is based on the principal used in the full size Tost mechanism, and requires a just very light touch to trigger it. See my building review in the articles/tips page of this site. Martin Tigg will be using it in his 1/4 scale Wilga, which should be flying in 2004.
For the Tow Hook contact Langnickel in Germany, they supply a hook for tugs based on the pin through a mounting block and it looks neat. You can find it on their web page it is called a Schleppkupplung ref 0511002791 and they have an account in England for direct cash transfer.
“I just received my engine today and have mounted it on fuselage of the wilga found that the carb is every tight up against the bulkhead. I take it I will have to drill a hole in the bulkhead big enough to fit the trumpet. Please advise if this is the right coarse of action”
There are a couple of things to consider here…
The first is that it’s essential to ensure the small hole on the side of the Walbro carburettor is always at the same air pressure as the mouth of the trumpet.
If the trumpet mouth is inside the fuz and the body of the carb is in the cowl, the difference in pressure will cause erratic running. This is great news for dead-stick landings but bad news for your ticker ..!!
The solution is to remove the diaphragm cover and solder a short length of brass tube over the hole then, run a length of fuel tube through the firewall, ending close to the mouth of the trumpet. 3W now supply a plastic trumpet with a nipple in the side for this purpose….
Also, make sure the engine gets plenty of air. The opening at the rear of the fuselage has a negative value relative to the ambient air pressure and will actually suck air from the fuselage causing a loss of power. For maximum power, make a larger clearance hole around the trumpet or drill some extra holes in the firewall to take high pressure air from the cowl into the fuselage.