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.