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A Simple Auto Engine Conversion
On Aug 27, 3:32 pm, "Peter Dohm" wrote:
"Jim Logajan" wrote in message .. . Ron Wanttaja wrote: On Tue, 26 Aug 2008 21:07:46 -0700 (PDT), " wrote: everyone says "ooh -- auto...dangerous" but no one can explain exactly why. 1. Ignition systems with insufficient redundancy. 2. PSRU failures. 3. Difficulty in implementing efficient liquid cooling systems. But doesn't the Rotax 912 have reduction gearing and liquid cooling? It is getting put into an awful lot of aircraft models - particularly LSAs. That's true, and the biggest annoyance (of which I am aware) is that they have increased the recommended "idle" speed to increase the service life of the PSRU--which is of the spur gear type. I don't know whether any of the belt or chain type PSRU installations have a similar requirement. As to cooling: there were a lot of liquid cooled aircraft engines in WWII, but the the aircraft they in which they were installed looked a lot different from their air cooled counterparts. Peter Even belted PSRUs have vibration nodes. The Glastar in which we put a PSRU'd Soob didn't like 1400 engine RPM; it semed to be an argument between the flywheel's inertia and the prop's. Running it at that RPM for long would have torn the teeth off the belt. I didn't notice if there were further nodes at 2800 and 5600. Adjusting belt tension didn't change anything. I've read about (and encountered) cases of cooling problems in auto conversions. Many builders underestimate the amount of heat that needs discarding, and also make mistakes in radiator installation and baffling. I've seen rads mounted out in the breeze where they not only slow the airplane but suffer from airflow problems created by the vortices generated around the rad. I've seen a couple of small rads mounted behine the front cowl openings, where they're supposed to get ram air, but without proper baffling to separate the incoming air from the air behind the rads the pressure differential is minimal, causing low flow, and air eddying around the rad further interferes with flow. In the Glastar I mounted the big, full-size rad (from the same car as the engine) behind the engine, at an angle so that the top edge was at the firewall and the bottom was forward about 8". Baffling around the rad made sure that ALL air leaving the cowl (except for a bit leaving around the hot exhaust pipes) had to go through the rad, so I had maximum flow. A lip on the cowl outlet to accelerate air away from the opening lowered the pressure further so that max differential was maintained between the front and rear of the rad. And even with all this the engine's coolant temp reached max in an extended full- power climb on a summer day. The P-51 had an underbelly scoop and a variable-geometry outlet behind it. The rad was in this housing. Inlet and outlet shape and size were critical, and I've heard that the designers were so clever that they even got a little thrust as the cooling air expanded and was accelerated a little when it left the outlet. OWT, maybe, but there's lots to learn from their design anyway. It's worth noting that the inlet was much smaller than the rad's area; Mr. Bernoulli tells us that pressure increases as airflow slows and decreases as it accelerates, so the divergent duct between the inlet and rad face slowed the air and increased its pressure. Same principle used in numerous places in a jet engine. Dan |
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