Control Line: Aerobatics
Frank McMillan 12106 Gunter Grove, San Antonio TX 78231
The story of, and solutions to, things that went wrong during the past competitive season continues. This part of the tale needs a preface to demonstrate why it took me so long to find out what my problem was. The root of this situation, and many that you run into, is assumptions. Making assumptions will devastate your program, and you won't know it.
Many fliers do it, but the more aware you are of the traps, the quicker and more accurately you can solve your problem.
Real World Example
Many years ago, when I was working with full-scale airplanes and helping to develop prototype electronic systems, install them, and execute test programs, I ran into a major assumption that created a serious problem.
I was working with a major aircraft firm to install and integrate a series of black box systems. The components were mounted in large equipment racks that were shock-mounted; the individual units were not. After the airplane flew, there were equipment malfunctions related to loose components and broken wires — a vibration problem. When the situation was analyzed, it turned out to be the system of mounting the units and an assumption made by one of the design engineers.
He reasoned that by shock-mounting the massive racks, they would not vibrate. However, the result was that the vibration period shifted from a very low cycle to a much higher one, because of the increased mass. That vibration period was very close to component resonance and the internal components in the electronics started failing. After the individual boxes were retrofitted to the racks, the problems disappeared.
Stunt fliers are subject to the same condition: trying to troubleshoot problems but not examining the whole picture from the beginning. Everything must be subject to scrutiny.
My Pipe/Cowling Problem
I'm going to relate assumptions to a situation that plagued me for several years without my knowing it. Although I'll talk about the care and feeding of pipes in this scenario — about heat and cooling — the same principles apply to conventional setups. I pay particular attention to cooling and interior airflow, and typically balance inlet and outlet areas. That assumption would come back to haunt me.
When I was acquiring knowledge about how to make pipe setups work, I looked at everybody who had been successful and mentally noted what the installation looked like. I merged those ideas into the configuration used on my first successful all-piped model, an SV-12.
At first there seemed to be no real problems. However, I noticed I could not fly the airplane with the cowling off: there was no directed airflow over the pipe aft of the coupler. The engine did not act predictably — power was down slightly and the "locked-in" feel was gone. Put the cowling back on and everything was sweet. It was a good lesson.
Because this showed what happens when a heating problem exists in the system, I began to look at how pipes have to deal with the entire package. A less-than-complete muffler installation, unless it's enclosed, may not show the problem until later in the sequence. I was getting good engine runs with different engines and pipes and wasn't aware anything was significantly wrong. I was aware, though, of having to patch holes in pipes while others had fewer problems.
At this stage I began questioning everything except what I was doing — making subtle changes in areas still having persistent leaks after fewer than 100 flights. Flying on the order of 300 flights a year, I'd change pipes, fix and go. After the engine ran excellently I would be satisfied. I also began using the Super Caudron series of designs, which had a very deep drop to the cowling and resultant internal airflow changes.
What finally brought the problem to the surface was an event like a ballistic missile sub-launch at the past Nationals team trials. After the contest, when I checked the pipes, I found major leaks I hadn't picked up earlier. I didn't pick up on it quickly because the P.A. 61 is so broad on power and tolerant of many factors. I wanted to do some flying while waiting for the pipes to be repaired, so I used an older design pipe I had coated with JB Weld epoxy — a good high-temperature gray epoxy.
After eight runs on that older, slightly smaller pipe (designed for the .51), the front of the pipe had severely discolored from the coupler back about six inches. Around the same time, Bill Werrauge told me that the interior of my pipes had the resin burned away. It hit me hard that there was a major heat problem I had been wanting to ignore.
Diagnosing Airflow — the Hair-Dryer Test
I needed a way to check the airflow. If in doubt, stop and think. I set a hair dryer on cold and fixed it to the air inlet of the cowling. It was a reasonable fit; you don't want to bleed air around the coupling. Here's what I did and found:
- Turn the hair dryer (cold) on and observe where the air exits the cowling.
- I found the air literally dumping out the bottom through the hole over the manifold coupling — no flow to the back.
- I blocked that hole and tried again. Then the small cutoff at the aft of the major opening was blasting out the air — still no flow to the back.
- I blocked that small hole and tried again. This time the 1/4-inch cosmetic holes I'd put in the side of the cowling were releasing air.
- After blocking all of these holes, I finally got strong airflow to the back.
Looking at the cowling shape, I suspected the severe droop caused the airflow to follow the bottom curve and exit the openings instead of flowing aft over the pipes. After flying with the revised setup (holes blocked), I couldn't detect any signs of discoloration and the runs were excellent. I believe the problem is solved and reliability will be significantly increased.
There's no magic in the hair-dryer procedure to direct air in the cowling, but it worked well for me and I recommend it for everyone. On a gross level, look for airflow where you think it should go. On a more refined level, you may be able to make adjustments to improve the situation.
Reading the airflow is dependent on touch. I tried a little heat and did raise some bubbles, but next I'm going to look at something such as smoke that I could get into the airflow to give a better visual indication. Any ideas out there? Not being a smoker, I couldn't come up with something convenient right away.
New Items
As a complement to the availability of the Brodak Nobler kit, Windy Urtnowski is working on a construction video that will be available as you read this column. There is nothing better than seeing a fine craftsman showing you how to do the tasks required to complete your model.
Windy is a many-time concours winner, which speaks volumes for the quality of construction and finishing. Also available is a fiberglass cowling to fit the Nobler. Contact Windy at:
Windy Urtnowski 93 Elliot Place, Rutherford NJ 07070 Tel.: (201) 896-8740
Transcribed from original scans by AI. Minor OCR errors may remain.
