|
Wayne Jones Builds a Cmelak |
|
|
|
|
Written by Wayne Jones
|
|
You either love them or hate them, but you have to agree that the Wilga's and Cmelack's do a fine job of towing sailplanes into the wild blue yonder!
Wayne Jones is almost there and has sent in the floowing article. Many thanks Wayne, we all love to read about projects like this and a scratch build no less.
So without further adieu here is Wayne's Story so far:
Building a Cmelak on a shoestring.I started having strange thoughts about a year ago “ maybe I should build a tow plane”; but what to build? I needed something a bit different and had to tow well, preferably with a low wing, and then I saw it on the Airworld web site.  A Cmelak ! The Zlin Z-37A. So ugly it looks beautiful ( well I think so).
From the Czech Republic it is a purpose built agricultural aircraft, about 677 were built.
Also used as a glider tug, it holds the world record for towing nine gliders. I found lots of great photos on airliners. net. I searched the internet for a three view and then worked out how big I would build it. I had in mind a tug big and powerful enough to tow any third scale glider. At first I was thinking 50cc, but that changed to a DLE 111cc twin as my plans developed and the size actually dawned on me. Just under quarter scale brings it to 2.86 meters, big enough, when you work out that the wing chord is 536 mm and the tail plane is just over a meter. (See Pic 2). I decided to use NACA2414 for the wing section as it has quite good stall characteristics. Over the years I have looked at the different tugs that have been around and I think the best ones have been the ones with a lifting section. Tow planes with semetrical sections always look like they are using a lot of energy just trying to fly.You may remember John Gottschalk’s tug “ The Barge” one of the best tugs I have ever seen, I think it had a Clark Y section. I started by enlarging some of the three view to same size and from that I was able to take measurements. The only problem I had was that the lines were about eight millimeters thick after I enlarged them to same size. I have stayed as close as possible to a scale outline but it will be a practical scale tow plane.The front half is made from 6mm ply, and the back is made using Obeche for the formers with hardwood stringers. I intend to cover the fuz with Sig Coverall.  The fuselage went together fairly easily but the 6mm ply was quite a job to cut, with all the cutouts done by hand. (See Pics 3a and b). I sheeted the fin and rudder with 1.5mm balsa and is thick enough to put a standard size servo in the base. (See Pics 4 and 5) The tail plane is sheeted using 1.2 Obeche. I have split the elevator just to be on the safe side. The elevators are pretty big. The two servos fit nicely inside the underside of the tail plane. (See Pic 6) I think I will need to cut some holes in the elevators just to get the weight down a bit. At the moment I am working on the wings. The center section is built up. This part of the wing has to be incredibly strong as it has to support the undercarriage legs as well as the outer wing panels. I have used a steel tube to support the wheel struts which will be at the outside of the wing center panel. (See Pic 7) Andy Andy Smith was kind enough to cut the ribs for me using his laser cutter. That saved me a lot of work. Have a look at his web site, www.rcplans.com.au I sandwiched 0.5 mm ply and 2 mm balsa to make the inner section skins. I did this by pressing them in the foam cores using watered down PVA glue to bond the ply to the balsa. This produced quite a strong wing skin with the correct shape ready to go on the ribs. (See Pic 8 ) The outer panels will be just Obeche on foam. Even without the outer wing panels it’s quite an imposing model (See Pics 9a and b).The foam for the wings was cut by me using my Step-Four CNC foam cutting machine. You may like to have a look at my new web site www.foam-wings.comThe flaps present a few challenges as will the undercarriage struts, but that’s what it’s all about isn’t it? To Be Continued
Wayne Jones
|
|
Last Updated ( Tuesday, 16 March 2010 )
|
