Our program with the simple F2A trainer model has come to an end, and it's time to summarize our experience. In this article, I will describe the engine modifications, tuning and some operational aspects.
After the implementation of changes of the airframe mentioned in the previous article, the junior pilot was perfectly happy with the airplane performance. Using the Brodak .15 motor, we made over 50 flights with the main goal to practice takeoffs. This really paid off later because our motors do not produce a lot of power until the airplane is airborne. Thus, it is crucial that the pilot is able to whip and pull the airplane on takeoff in order to maintain proper line tension. The two-wheel landing gear also proved to be a good choice: it saved the model from serious damage in a couple of touch-and-go's.
Switching to the ЦСТКАМ 2.5КР required some effort (motor is often referred to as UCTKAM. In fact, the first character is the Cyrillic 'Ts' and stands for 'Central') . All motors of this type I saw were not good for any use the way they come out of the box. However, after a little bit of work they can be quite adequate and the one we used certainly served it's purpose. Below I cover what can be considered an absolute necessity.
- Take the engine apart, clean it and check the liner for sharp port edges and pieces of chrome protruding into the cylinder. If found, touch them carefully with an emery wheel (don't forget to clean the liner afterwards).
- Check the bearings. The front bearing that came with our engine had metal screens on both sides. They should be removed in order to allow better lubrication. Tighten the prop and check that shaft rotates freely and that there is some axial play.
- Replace the connecting rod with something better than the factory unit. The original conrod is made of very soft metal and will break. There were many replicas of Rossi 15, all of which had ok conrods.
- I don't like the long needle in the needle valve and suggest that it's cut and converted to screw driver adjustment. It's a good practice to think before turning the needle.
- The most important aspect is the compression ratio. The engine came with a head that is integral with the clamp ring and drilled and tapped for standard glow plugs. It's ok to use standard glow plugs although the performance with Aluminium drop-ins could be slightly better. However, the combustion chamber volume was totally unacceptable (0.22 cubic centimeter). One has two options: replace the head with something that works or modify the original. We chose the latter approach. First, use a depth micrometer to measure the stroke, exhaust port height (top edge to BDC) and the deck height (TDC to the flange of the liner). For our motor these number were: stroke 14.02 mm, exhaust port height 6.17 mm, deck 2.72 mm. These correspond to the exhaust duration of 182 degrees. Some motors come with 170 deg. in which case it's advisable to shim the liner up to bring the exhaust phase to 180. Now put the head in the lathe and machine its flange down to decrease the volume (checked with an insuline syringe with 0.01-0.02 cc graduation). My head measures 0.18 cc. Turn the outside of the head to make the depth 0.05 mm shaller than the deck height. Shim the head to have the clearance of 0.23 mm. This makes the trapped compression ratio of about 7.3.
It is important to understand that proper functioning of an F2A model is achieved by a combination of the engine, tuned pipe, propeller and fuel tank setup. Each of the components must be properly tuned, or the entire system will not work.
- Tuned pipe. We operated the engine with the stock ЦСТКАМ pipe and with a Rossi pipe. The latter produced faster speed and allowed the engine to pipe-up easier but broke on one of the flights.
- Propeller. The best speed was achieved with a Master Airscrew 7x5 S-2 G/F Nylon Gray Propeller cut to 140 mm (5.5 inches) diameter. With this prop the engine was turning about 30,000 rpm in the air. I suggest starting with 6 inch diameter and cutting the prop until the engine pipes up. Too small propeller will turn a lot of rpm with no thrust. We also tried more pitch (up to 6.5 inches) but this did not result in faster speed.
- Fuel tank. The suction type tank must be properly positioned to provide adequate fuel flow on the ground, at take off and in the air. Our model did not like when the tank was inboard of the needle valve: the engine was either too lean on the ground and would quit upon release, or too rich in the air. The optimal tank position was when the feeding tube was 1 mm outboard of the needle valve.
With these modifications and settings the system performed quite well. The fastest time achieved in a practice session was just over 200 kph, which is excellent considering the exposed lines, landing gear and no engine cowling. The airplane routinely did around 160-180 kph with a Rossi pipe and slowed down to 150 kph on the stock pipe (on FAI 80/20 fuel). The total of about 20 flights were made, all with one glow plug. Finally, we removed the landing gear and switched to taking off from a dolly. Surprisingly, this was easier than with the landing gear, probably because the dolly has a better grip on the asphalt, and also does not allow the airplane to jump on the bumps.
I would like to thank Bill Hughes for help in acquiring parts of the system and for useful advice. Thanks to Chris Montagino for letting us use his dolly.
July 25, 2011