The purpose of flight test is to find and correct construction errors and identify flight characteristics of the new airplane. Since no construction are perfect, problems better be found early then late. I will share with you the errors I made. Hopefully, it will help you to avoid making the same mistake.
Problems found during Phase I:
a. Right fuel tank wing root connection leak. After the first flight a small amount of fuel leaked out of the wing root connection. My first response was to tighten the nut on the fuel line. Unfortunately, the tighter I did the faster the leak. Finally, I pumped all the fuel out using the auxiliary fuel pump (disconnect the fuel line from gascolator to the mechanical fuel pump). I found that the segment of the line between fuel tank and fuselage is too short. When the nut was tightened, the flaring on the line fractured. Since it is too hard to replace the entire line (it goes through the landing gear mount), I decided to use a rubber hose to connect a short line from the fuel tank to the rest of the line.
b. Bad solder on headset jack. During the first flight the headset connection was lost. Fortunately I was at a non-controlled airfield with no traffic at the time.
c. Defective pullable circuit breaker for alternator field. Although the circuit breaker was pushed in, the circuit was not closed. I did not suspect the breaker problem and assumed it was the alternator. Only after swapping out the alternator and found the new one still not working, I tested the breaker. Earlier a similiar problem happened to my auxiliary fuel pump breaker switch. Only after less than 20 on/off motions, it failed to latch on the circuit when the switch arm is locked on the ON position. I thought that the pump failed. It took some testing to find out the real problem. The new breaker switch works fine.
d. Wing strobe lights failure due to poor wing root connections. The cable is not long enough that made checking the connection hard. If you have a finished RV-9, you will know how small a space there is between wing root and the fuselage. Leave enough cable at wing root so you can pull the connectors out for service.
e. Lost GPS signal during first flight. Although GPS is not important for the first flight, it is not good to have any failure during this high stress moment. It turns out the crimping on the antenna cable connector was bad. After the connector was replaced, it has not failed.
f. Carb Idle Mixture Adjustment. I forgot about this until today (3/27/2007) when I read on Van Airforce Forum about an RV-6A engine problem. I had the exact problem, however, it happened after I have landed. Following the instruction from the engine operators manual, I tried to adjust the idle mixture so at cut-off the engine would rise 50 rpm. I kept turning the screw loose until it fell off yet still don't see the 50 rpm rise. The local AP told me most of the engine he serves don't. I put the idle mixture adjustment screw back but not with too many turns. After I have done that I took the airplane up for just one circle in the pattern. While on the landing roll and the nose wheel was still in the air the engine run rough. Soon, the engine stopped. I immediately restart the engine and taxied the airplane off the runway. I noticed that at high rpm the engine runs fine (>1,000). I taxied back to my hangar and took off the cowl. I called National Aviation, Inc. (206- 762-7278) and had them FedExp me a new needle screw for $79 (needle $39.75, spring $3.99, packing $2.88, shipping $32.33). They did tell me that it happened to quite a few people. So, watch out, don't turn it too loose.
Well, finally I have my idle mixture adjusted to have the 50 rpm rise. Since it is done after 133 hr flight time, I will put the description on my maintenance page.
Other issues worth noting:
a. Lubricate all bearings. I forgot to do so. For a long time I always worry that something was wrong with the nose wheel pant. Whenever I turn the nose wheel on the ground it made some noise. After lubrication there is no noise anymore.
b. Check all engine block plugs. Due to my ignorance, I connected the oil pressure line to a location that already had a plug (of course I removed it then screw in a nipple). From the drawing in the engine manual it looks correct. What I did not realize was that there is another hole further below and it was not plugged. During my first engine start oil leaked out of that hole. The lessen is that you must check all around the engine and make sure all holes are plug. With all those hoses, wires, baffle, etc., it is hard to inspect all around. But, it is necessary to do so.
c. Carb temperature. I was puzzled that my carb temperature raised to 127 degrees and stays there soon after engine is started (in August). I called GRT and talked to Sandy several times. She even sent me a new carb temp probe. I checked and see that the probe is in the proper place and the head is expose in the midstream in the carb. At the same time my new engine could not turn the propeller (fixed pitch) beyond 2,400 rpm, full throttle in cruise. Without any experience, I thought the high intake temperature must be the reason. Now, in the winter, the outside temperature is much lower, carb temperature is no longer at 127 degrees. During run up, the temperature could be around 60 degrees. When I checked the carb heat, it does show one or two degrees raise. Now I am comfortable with the carb temperature sensor. By the way, in the Winter time, the carb temperature during taxi is often below 100.
d. Low engine rpm and cruise speed. I was getting much lower cruise speed than published. After posting my question on a forum, Cameron Kurth an RV-9 builder told me that he had the same problem before. He attributed it to the wheel pants and gear fairings. Since I was breaking in a new engine, I had no choice but to wait. After 25 hours, I put the wheel pants and gear fairings on, suddenly, the engine rpm picked up and the cruise speed increased. Landing gears induced so much drag, the engine did not have enough torque to turn the high pitch propeller faster. I would not have the problem if I had a constant speed propeller. Well, you get what you paid for.
e. Whisling sound. My airplane produces a whisling sound that is especially pronounced during low rpm operations (pattern and landing). When you watch my first flight video you will be able to hear it. I tried to identify the problem by taping over all the opennings on the airframe (except air inlets on cowl). The sound is always there. Cameron Kurth (see d. above) said that he heard other RV-9s have the whisling sound too. Some people speculate it is coming from the wind rushing over the wing. He said that A-10 whisle too, "just tell people it's the A-10 cannon you have mounting in the nose that's whistling."
7/2008. I got an email from 9 builder Alfrio of Canada concerning the whisling sound. He bet a beer that the sound comes from opening on elevator rod end (aslo rudder rod end). I taped all the openings and ask people on the ground to listen. None of them heard the whisling sound. Mistry solved by Alfrio!
Performance
Gear leg fairing and wheel pants are on. Airplane is dirty.
The curve flattens at the high end. It could be my data collection error (failed to wait for the airspeed to stabilize.)
1/3/2007 Airplane is polished
| 9,500 ft | 2510 | 155 |
1/10/2007
| 5,500 ft | 2590 | 164 |
1/17/2007: at 8,500 ft, OAT 25 degree F, Alt 30.77 inch
After airplane was polished (no paint, primer on fiberglass parts)
(Click the picture to enlarge)