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Yesterday the Critical Mass flew for the first time. The winds were calm all day so there was no reason to put off that first flight any longer. I got to the field two hours before sunset. The winds were barely stirring as the Critical Mass was prepared for flight. Since many of the systems had been pretested, the start was easy, and take off normal. Line tension was very heavy, the airplane a little fast and not grooving particularly well. Inside rounds, squares and triangles were flown. Winds began to pick up quickly after takeoff. About a minute into the flight the winds were up to about 10 with gust much stronger. Engine operation was completely normal except for a moderate vibration in the last couple of laps before quitting. I think this was probably an artifact of fuel exhaustion. I will check the engine mounting carefully, and try a richer mixture on the next flight. When the engine quit, the glide was nice in spite of the wind and the landing was gentle on its shock gear. I decided to check out the engine mounting for vibration and wait for calmer weather before flying again.
There is not much to report from just one flight. The 16-5 prop seemed to be gyroscopically inhibiting the turn. Richard Oliver experienced much the same characteristic with the same prop on his Ro-Jett 90. The airplane is heavy but seems to have an OK corner. There are some trim changes I'll try to lessen line tension, but, for now, I'm going to fly a few more flights with the 16" prop because I know it runs OK on my Saito 91. I'm thinking that a somewhat overweight airplane with an OK corner and an obscenely large engine and prop might be a killer combination in the wind. I'm looking forward to more flying.
It looks really great in flight.
At home the cowl came off to check anything which might have contributed to the vibration at the end of the first flight. Everything was tight and secure. The filter was checked. It was clean.
To reduce the excessive line tension somewhat, tip weight was reduced. The Critical Mass has 1 1/2 oz built in and the first flight was made with an additional 1 1/2 oz. The removable tip weight was reduced to 3/4 oz. Also the rudder offset of two degrees was reset to zero.
The first engine run was unusually long for the fuel burned. Four stroke power is relatively insensitive to mixture control (needle valve setting). Duration is quite variable with needle setting. Lean mixtures float the valves and run long. Excessively rich settings give unusually short runs. In general, I adjust the needle to peak and then richen about a quarter turn for flight. The needle for the first flight was preset for test stand operation but turned out a bit lean for flight. This time I would use a richer setting for a more normal duration.
Takeoff was normal and this time I had my stop watch to time lap times. The Critical Mass was a bit fast at 4.9 sec/lap. Line tension was down a bit, but still very heavy, at least partly, due to the excess speed. I pulled up high and leveled off inverted with enough altitude to recover if the engine quit. The engine runs were all great, upright and inverted confirming the tank height adjustment. I did a couple of lazy eights which went fine, then flew all of the pattern maneuvers, leaving out most of the level laps, with a fair amount of confidence. The Critical Mass was grooving better today as the controls break in and the turns seemed less inhibited by the gyroscopic factors.
When all of the pattern maneuvers had been tried, I did another square loop and started an outside square. As the Critical Mass leveled inverted on the bottom, the engine quit. Rather than accept a highly destructive inverted landing, I pulled hard on down and started backing up pulling on the lines. The Critical Mass came over but turned well into the circle and started to roll. I could see the entire top of the airplane as it started its final dive. I was sure that the Critical Mass would be destroyed on this, its second flight. I thought I had fuel. I was only 5:40 into the flight and the engine was showing no signs of fuel exhaustion. Lesson one; this engine/tank combination runs steady to the last drop.
After the initial shock, I ran, and was surprised to hit the end of the lines and get control. The glide was nice, and the landing was flawless.
I changed props to the 15" carbon, filled the tank, confirming fuel exhaustion on the previous flight, and hit the starter. The engine started running backward. It sometimes happens with four strokes, but rarely. I think this was the third time in more than five hundred four stroke starts. No problem. With my fuel plumbing it could only run out the prime. After another start and normal take off, I timed these laps at 4.6. This was way too fast. Except for trying a couple of squares and triangles to check precession with the smaller prop, I flew the rest of the flight with two hands. The engine runs beautifully except that it gives very little warning when running out of fuel.
I had enough excitement for one day, and rolled up. Both props fly too fast. I'll work on both props before flying again. The Critical Mass needs no other routine maintenance. It hangs on the wall awaiting its chance to fly again with repitched props.
For now I'm satisfied with the trimming progress and the airplane seems to have all the competitive promise I had hoped for. On the next flights, if the speed is in the 5.2-5.3 area, that will reduce line tension to a more livable pull. Then I can try to put some tip weight back and maybe try some more noseweight.
The trimming process has begun. With a 91 four stroke powered semi-scale airplane, the process is lengthy. Each flying day ends with a maintenance session where I try to improve any undesirable characteristics and make sure the airplane is ready to go again.
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I tried the Saito 91 partly on Windy's recommendation. He was of the opinion that the Saito 91 was even a friendlier stunt engine than the saito 72 which I like very much. So far, I have to agree with Windy. The engine runs are as steady as can be and obviously powerful. It turns the Zinger 16-5 with ease and that is a big prop, not just the diameter, it has a lot of blade area. Anyway, I'm glad the engine run is all anyone could ask for. The saito 91 installation in the Critical Mass was a retrofit. It required some fairly heavy (expensive) custom machined parts to make it work. I have another airplane built expressly for the Saito 91. The BBFB Bearcat was designed for the Saito 91 making the installation much lighter. That airplane is currently on track to weight the same as my Saito 72 powered BBQB. Aerodynamically, they are the same.
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Here are a couple of the props I have run. The 16" wood is obviously much larger. The next prop is the 15-5" Eather and a 14" Bolly for comparison. The Saito turns the 16" wood so easily, I suspect that may be the best prop or lead to modifications which may be even better. Prop load is obviously not a problem.
Now if I could just find someone to take inflight pictures.
Al
