History

Last year, I tripped over a particularly large banana skin.

I had enjoyed two seasons of successful aerotowing with my Frisch Wilga and had developed a high level of confidence in my equipment and method of operation.
I had recently posted my opinions (on the SSS forum) on the merits of various tow release mechanisms to the point of condemning some designs in favour of my preferred “pin in a hole” method. I was also convinced a standard (5kg/cm) servo was adequate for this since I had tested it by pulling the line firmly by hand.

Of course, you can guess what?s coming next. Yes, it?s one of those "banana skin" moments where you just wish you could turn back the clock.

At Middle Wallop International Aerotow, the gliders wing caught in the grass and flipped over onto its back just as the Wilga lifted off. This little scenario induced a far greater strain than my standard servo could overcome, and my Wilga finally submitted to the law of gravity. All blessings to Sir Isaac Newton..!!

Conclusions

My mishap produced a stream of comment in the UK on the subject of tow releases and towlines.
I have concluded that :-

• A weak link is only useful when there is a sudden snatch on the line. For example, if the towline gets caught on a fence or in a field crop as happened to me this year whilst on landing approach at Caen, France. I survived that one..!!
• The tow-plane release mechanism should be able to operate with the engine at full power (i.e. maximum static thrust) with standard servo power and should only release the towline when the pilot commands it to.
• I have now made a new mechanism that is a scaled down replica of the Tost system used in full-size gliders and tow-planes.
  

This new device :

• is very secure when closed for towing.
• only requires a very small effort from the servo to initiate release, regardless of line tension.
• once a release has been initiated, uses line tension to help pull the mechanism open.
• will be very easy to re-connect manually after say, a refuelling stop.

I spent some time considering what to do next. I had recently been lusting for a large scale sailplane and had just concluded a deal with Ekkehard Hermann of Rosenthal Flugmodelle. Then I realised, I just hadn’t got the Wilga out of my system and would have to put the glider plans to one side and get on with Wilga number two. Sorry Ekkehard..!!

I made a call to Alex Frisch in Germany and ordered another scale kit, minus the undercarriage and hardware pack. Six weeks later, it’s arrived…

The Build – Part 1

4thDec2002
So, now that’s the new tow release sorted, all I have to do is build the tug to go with it ..!
To begin, the two window struts have to be fitted before the windows are cut out or the fuselage shape and wing alignment would be lost. The throttle and choke servos will be located at the rear of the ply undercarriage mount with carbon pushrods passing through the ply supports underneath. This ensures all of the radio gear stays well away from the engine ignition system (highly recommended). I spend a lot of time here taking various measurements to ensure good alignment of undercarriage to fuselage
By the way, I have this recurring thought, Alex makes such a lovely job of his Wilga kits I always feel I am committing vandalism each time I cut, drill or apply glue to it. Strange eh..?? So, I try to give a lot of thought to each stage of the build process before cutting or gluing.
To reinforce the ply firewall, I have made up a doubler plate from 6mm MDF material. At this stage, I am wondering if it may be a good idea to include a layer of carbon cloth in the sandwich here to further help reduce the chance of ignition emissions reaching the radio gear. I am cautious because this might blank out the control signals when flying directly towards the transmitter…
17thDec2002
Part of my motivation for this build is generated by the opportunity to make improvements over the first Wilga. I plan to make the undercarriage struts stronger and more ‘true to scale’. This has been a slow process involving making new brackets and struts, and ensuring everything clears the exhaust system and engine cowl. With a project like the Wilga it is often the small details that absorb so much building time, but It’s very rewarding when things work out well. Anyway, I am very pleased with new undercarriage and feel I have made some real progress now that I have got it sorted.
I am cutting out windows today. I bought a small diamond slitting wheel from Dremel that is proving to be ideal for this. As I fit the glazing, I am already giving some thought to the next ‘improvement’, the tailplane and elevators. I will mount the elevator servos in the fuselage this time. The DS3328 ‘wing servos’ I used in the first build were mounted inside the tailplane and were a bit marginal so I will now use two standard servos. I have to consider how the linkages to elevators, rudder and tail wheel will be arranged before I install the fin post. At times like this I see why ARTF models are popular..!!
2ndJan2003
I find that because I am still undecided about the tail area, I keep finding other things to do up front. In fact I’ll probably complete the cabin area before tackling the rear of the fuselage. I tend to work like this. For example, I started to develop new fairings for the undercarriage legs. I got them half done and then I hit a snag so I put them to one side and started something else. I know I’ll go back to them later with a fresh attitude and it will all come together eventually.
I have installed a strong ply mount in the cabin roof for the new tow release mechanism and reinforced it with carbon cloth. I couldn’t resist setting up the servo at this point because it involved powering up my new radio setup. I am going to be using an Emcotec DPSI. With two receivers and two cell packs, this should provide radio redundancy and long cable filtering all in one package. The Emcotec unit also uses no mechanical switches (a common cause of failure) but has a single electronic switch that “latches on” when the peg is inserted into the ‘on’ position. The DPSI will only switch off if the peg is returned to the ‘off’ position. Take a look here http://www.hufitech.de/modellflug/modellflug.html for info. By the way… the PDF downloads are in English….

To be Continued