Caro 1 MG Flight Test Blog
Caro 1 has successfully made a second “first” flight, this time with short wings. The design allows it to fly as an airplane with the outboard wings removed and short wing tips installed to cap off the ends. In this configuration, the wing span is only 30.6 ft. Obviously, it flies different this way. Although pitch control feels about the same, yaw and roll are more sensitive. The climb performance is nearly the same, but moved to 10 mph higher speed. The top speed is also increased. By going from a nearly elliptical wing planform to a nearly rectangular wing planform, the stalls are aft stick limited at all cg’s tested, with a slight pitching and increased descent rate, wings level. Roll rate is much faster with the short span and the aileron forces are quite light. With the spoilers, which are part of the inboard wings, steeper descents for landing are possible. Because of the reduced wing area, the wing loading is higher, which increased the takeoff and landing distances by about 1/3.
This is the sporty version. After the removal of the outboard wings, it takes up a lot less room.
Flight with the short wing span.
My flight test equipment
Now that I have installed an acoustic vario, we finally we had great soaring weather, which I was able to make use of with Caro 1. With 3.5 h without the engine, this was the first real soaring flight with this aircraft. It did well, although that fixed pitch prop is a considerable speed brake.
These are the sandbags I loaded onto the back seat, with some more in the baggage compartment, to get the plane to gross weight. The takeoff took quite a bit longer and the climb performance was not exactly overwhelming with that load, but otherwise it flew well loaded up like this. Now it is time for a configuration/loading change and some maintenance.
All of a sudden it is summer, at least as far as temperature goes. A modification had necessitated another break from flying, but Caro 1 is up and running again. Some intake tubes and couplers were replaced to eliminate small leaks that had affected the engine operation. I moved the CG close to the aft limit, which makes it fly nicer. I increased the temporary Vne to 140 mph. This seems to be fast enough for now since the rate of descent at that speed is quite high. Engine shutdown and restart in flight work very well. Gathering performance data has been quite limited, because smooth, calm air is so rare here, but some very preliminary numbers are 600 fpm rate of climb at 70 kts, at medium weight and ISA, at 6000 ft. Top speed is in the vicinity of 110 kts true at 8500 ft, stall speed is about 40 kts CAS at medium weight with full flaps. In slow flight, the air coming through the vent into the cockpit is insufficient in warm weather, I need to come up with a small mod or the pilot will overheat. Total flight time so far is 19 hours.
The weather in the last month did not allow much flying, but I got a few things done. The spoilers are working well now after a modification to the actuation. The wheel pants are on, and it gained 5 kts in speed with them. I modified the airfilter installation on the airplane while almost freezing my fingers off, to get rid of a few small problems. I flew this configuration with forward CG, light weight, for another longitudinal stability and landing test. With the CG at 20.5% MAC, the power off stalls with the positive flap settings are now aft stick limited, but the full flap landing was still acceptable.
It had been stormy for a while, and when things finally calmed down, I went flying to gather more flight test data, with one flight lasting over two hours. I have reached 11,000 ft, 120 mph and found the first lift, both wave and some thermals. I also remembered to pull the camera out for some in-flight pictures. Overall, Caro 1 is very well behaved. The only complaint I have at the moment are the high spoiler forces, but I think I have a way to reduce them.
Today I flew Caro 1 for a little over one hour. The engine was running smooth, the richer mixture helped but there may be a few more iterations needed. I climbed to 9000 ft in slightly turbulent air. The tests on this flight included steep turns, roll rate, stalls at all flap settings, transponder operation, engine operation while switching tanks and some sideslips. The flap-aileron interconnect was in the lowest hole in order to check roll rate and stalls in the most benign configuration. Since I do not yet have an airspeed calibration, the stall speed numbers may not mean much, but the stall characteristics are very benign. The CG was fairly far forward. At all flap settings, the motorglider showed very little roll or yaw (+-5º), both easily controllable with ailerons and rudder, while the tufts on the wings showed full span flow separation on the upper surface.
Finally the weather was suitable again for another test flight. I had made a change to the carburetor installation, hoping this would solve the engine operation issue. Again I did a short hop on the runway, which worked well, then I took the Caro 1 up for a proper flight. The engine ran steady, no power fluctuations, but from the high EGT’s it was obvious that the mixture in flight was still too lean. I stayed in the pattern and waited with pulling the throttle fully aft until I was on final. I sort of expected it might quit again, and it did, and I logged another glider landing. I could have kept it running if I had kept a little throttle in, but I decided not to and restarted it normally once I was on the ground.
Now it hopefully is just a matter of adjusting the mixture towards rich until the EGT’s are where they are supposed to be.
I am looking forward to being able to focus on flying the plane and not having deal with engine issues.
I have added some more pictures
I had asked a few friends to come in as ground crew, and after getting the motorglider ready, I briefed Tom Phy, the president of the local EAA chapter, who would also take some video, on the test and emergency procedures. Because I had made another adjustment to the carburetor since the last test, and because I now had to use runway 34, which is downhill, for the first time, I decided to do another short runway flight first. This was ok and I got in line for takeoff again. There was a lot of traffic at the Bend airport, and I was number 3 for takeoff. I would have preferred to have the airport to myself for this occasion, but with much fewer days of good weather this late in the year, everyone is out and flying when the wind is not blowing with 30 kts.
Was I nervous? Not at this point anymore. It felt like any other test flight. The Caro 1 accelerated normally and instead of pulling the power back like before, I kept the throttle forward and waited to see what would happen. It left the ground smoothly, accelerated quickly to 80 mph and climbed at about 500 fpm. At some point I reduced power somewhat and leveled of at 6500 ft. The controls felt steady and the noise level was lower than what I was used to from my other aircraft. It was balanced in roll and yaw but I had to hold a small amount of back pressure in pitch, which was expected since I had not adjusted the elevator trim springs since adding a tab to the elevator. I liked the visibility with the large canopy. The ventilation worked well and there was no indication of CO in the cockpit.
I checked that I had good roll control, checked flight at all flap settings, the operation of the cowl flap and the instrument readings. Then I tested slow flight down to about 55 mph indicated and the spoiler operation. I was relieved that I was able to get about 500 fpm descent rate, which would be sufficient for a normal landing. This had been a big question mark before the flight with such a low drag aircraft. This was also the main reason for not installing wheel pants and keeping the cowling air inlet wide open.
The maximum speed reached was about 90 mph, and it would easily have gone faster if I had not set this as my temporary red line. The engine readings were looking good, but now the power started to fluctuate. It would run well at 2700 rpm and higher and steady below 2200 rpm, but fluctuate in between. Ignition tests and boost pump on/off did not change anything. I had stayed within a few miles of the airport and was now descending back towards pattern altitude. The engine temperatures had dropped steadily since I reduced power, and the somewhat unsteady engine operation continued. I was debating with myself if I should shut it down when the engine decided the question, it quit when I was on downwind. Since I was flying a glider and was ready to land anyway, I did not attempt to restart. I announced on the radio that my engine had quit and I was coming in as a glider. Some of the responses were rather funny. Someone behind me flying some heavy iron suggested that I should go to Redmond, (about 15 nm away), “where they had a fire truck and rescue equipment”. Ha, ha, not from this altitude. I pulled the spoilers and landed uneventfully. Since this was about my 7th landing in the Caro 1, there were no further surprises. I cleared the runway before getting out and a quick look at the engine did not reveal anything unusual. The flight time was 25 minutes.
After pushing the glider back to its parking spot, I started the engine up again and it ran normally. At this point, I don’t know why the power fluctuated in flight or why it quit. My best guess is that the mixture was fluctuating, which could have been caused by the carburetor, in which case it may be something easy to fix but difficult to find. I had not seen this during engine ground runs. This carburetor has no mixture control, but the overall mixture setting can be adjusted at the main jet when the engine is not running. Some more troubleshooting will be next.
After a lengthy delay to fix some engine oil leaks and another lengthy delay to get the aircraft inspected, I am finally able to start flight testing.
N1124S received its airworthiness certificate this week, despite the fact that the FAA lost my paperwork somewhere between Portland and Seattle. Because the Caro 1 is a new design, the FAA could not delegate the inspection to a DAR and I had to wait for an FAA guy to come down to Bend from Seattle.
My first tests were taxi tests, where I soon progressed from slow speed to check directional control with the tail on the ground (good) to faster with the tail raised. This allowed me to get to know the operation of the airplane and get used to the visibility from the cockpit, which is similar to the front seat of an Extra 300.
Now my left hand had to learn how to use the vernier throttle, something my right hand had mastered long ago.
S-turning during taxi to see what is in front of me is necessary, but not much fun with that wing span. After checking the initial control responses, a little tweaking was next, for example to the mixture at the carburetor, which was too lean, and to the directional trim.
Since this motorglider has 5 flap settings, I did several runs along the runway with different flap settings and cg’s to check pitch control and trim. I also lifted off and landed several times after flying at an altitude of a few feet along the runway. The Caro 1 is well controllable at slow speed in ground effect. I felt more comfortable doing this in this aircraft than I would have in the Pulsar.
The wind also provided a certain unintended buildup, as it increased from about 3 kts at runway heading to about 10 kts at a 45º right crosswind during these initial tests. Next I need to stick some tufts to the wings and wait for another day with little wind, which is rather rare here this time of the year.