the STUKA ?
Well, there are several good reasons to build this airplane. The first reason is that I needed an airplane for the 2009 Vintage Stunt Championships in Tucson. There’s a large number of designs to choose from. It had to be a design which can be flown in Oldtime as well as in Classic. At VSC 98 I had entered a Classic airplane, but the Classic event was cancelled because of El Ninjo and I didn’t even make one single flight. An airplane which can be flown in two classes should minimize this risk.
|The second reason is that the Oldtime scene in Germany isn’t exactly very healthy. If I’m very lucky I can attend at ONE contest a year. Oldtime doesn’t have the big following as it does in England and the US. So I was looking for a design which - if need be - can be flown in FAI contests without making a fool out of me.|
|My first steps in serious stunt were made around 1960, and that was when I discovered the article about Don Still’s STUKA in the January 61 Aeromodeller magazine. I was impressed by what was written about Don’s performance. Quote: “ Don surprised everyone by pulling out of wingovers with his wheels on the ground, keeping them there for half the circuit and then repeating the performance over and over”. WOW ! I didn’t think of daring such a show then, not even later, and even less today ! While this performance says more about the pilot than about the airplane, you cannot deny that only a very good airplane will allow such spectacular manoeuvres. This was the impetus in 2008 to decide on building the STUKA.|
|All I had was this sketch in the Aeromodeller issue. I know where plans are available, but these wouldn’t help me very much. Since this would have to be a fully detachable model, some definite changes would have to be made in the construction.|
|. Firstly, a different engine might call for a slightly different fuselage width and maybe nose length. The removable wing called for a very solid fuselage centre section. The removable tailplane required a solid seat to be mounted to. At last a detachable undercarriage needed some suitable way to be attached securely, easily, and rigidly. So I didn’t try to buy a plan, but instead figured out an enlargement factor to draw my own plan. Enlarged copies of the Aeromodeller sketch helped to get the dimensions right.|
|To help decoration of the airplane I have quite a library of suitable books, and after all we have the Internet to come to our rescue. No, I didn’t want to take the effort to paint all those delicate insignias. As far as possible I relied on commercial decals (in fact the big “crosses” on the wing are self adhesive decals). For the rest I designed my own decals with Corel Draw. The decal base is not 100% opaque and the print ink can never be. However if colours are suitably chosen this way offers a great opportunity to produce our own decals, whatever kind we want.|
|The next problem was to decide on a suitable engine. According to Aeromodeller Don Still had used a Fox 25. I wasn’t sure whether an OS 25 FP or LA was able to pull around this airplane convincingly. Especially since a take apart model usually increases the weight by a few ounces. Looking at my collection I discovered an old OS 25 FSR laying around for too many years|
|Suspecting that this engine is much heavier than Don’s original Fox (in fact it weighs 200 Gramm), I knew that the fuselage nose had to be as close ( =short) to the original dimensions as possible. Indeed the engine is sitting in the correct place. But since it is about ½ inch longer, this length was added to the fuselage nose to “cover” the front part of the engine. Fuselage width had to be increased slightly to allow for the wider engine crankcase, and it is also increased at the elevator hinge line. This is necessary|
|to allow for a solid seat for the tailplane ( fuselage WIDTH is the only insurance to mount a tailplane solidly) plus enough room for detachable pushrod and adjustable elevator horn. According to this the fin construction had to be changed completely. As I had expected some lead was necessary at the fuselage rear end.|
|Since this engine had never been used before, I put it on a test bench to run it in and to find out fuel consumption. With my usual 100 ccm plastic tank the engine only ran for about 3 ½ minutes. I quickly made a new venturi with a smaller diameter. As I found out in flights, these ground tests are not very reliable. Now I’m using a 6,2 mm diameter venturi opening. Please note that my spraybar is an old Graupner item and is only about 3,3 mm thickness ( the Super Tigre spraybars are 4 mm ). This combination makes for an acceptable compromise between power, easy needle setting, and reliable engine run. I use a 10% Nitro fuel with 20% oil ( half and half castor/ synthetics ).|
|I’ve tried different propellers. So far a Brian Eather 3 blade carbon prop proved to be the best solution. The prop started as a 10 x 5 , but was depitched to about 4,5 . I don’t know how people do this “prop twisting” 100% exact, I tried to do it as precisely as I can; pitch is now slightly below 4,5 .|
|Ground revolutions is 10200 RPM and it doesn’t differ much in different flying conditions ( temperature ). Line tension is not overwhelming and I cannot get any slower than 5 seconds/lap. Above this the Horizontal Eight and the vertical manoeuvres tend to get “demanding”. This is on 18,20 meter = 60 feet line length ( Don Still used 57 feet ).|
|Tank volume is 132 ccm ( 4,6 ounces ). All the room of the tank compartment is needed. I couldn’t install a bigger tank if that was necessary - some few millimetres are left but are needed to “shoe in” the tank. Run time is about 6.30 minutes, and this is sufficient for either FAI or AMA or Oldtime patterns, whatever I’m asked for. Considering that this design is 57 years old now it's amazing what performance you can still get out of this (by modern standards) little airplane.|
|A click on that button will get you to an album with a few construction fotos.|