After getting back from Arizona I returned to work and kicked off a build of the X-Pack. While I was away I missed the launch of SP1 into beta and I missed the new P-51 engine tech being checked in. So I grabbed the files and built the product, then proceeded to try it out. Due to a minor bug, my engine exploded after about 20 seconds with a dead-stick landing at Reno/Stead. I’ve seen it in real-life and it happened to me too. But as I said, it was due to a bug, so once that was fixed and I had done a little tuning on the numbers, I was flying the course at speed and not blowing up my engine :). I measured my average lap speed using a manifold pressure (MP) of 120 at 3400 RPM (supplied by one of our partners who flies at Reno) and I was turning laps within 10 mph of his time. I’m sure he is a much better Mustang pilot than I am, but I was impressed that the sim was that close this early in the process.
So with the initial tuning out of the way, I got Brandon to fly the course with me. The P-51 engine management stuff was new for him, so we came down the starting chute and by the time we were done with one lap I was happily screaming around the pylons and Brandon was cursing me and his aircraft as it detonated and blew holes in his pistons. Okay, let’s try it again… Hmm, same problem… So thinking there might be something wrong with his build, we switch computers and I do fine and he blows his engine. That ruled out a bug and this was definately user error. This time I watched him fly, and coached him along the way. He was activating his anti-detonation injection (ADI) late, not backing off on the power early enough when the engine would get too hot, and in-short allowed the engine to get damaged. Essentially the system was working perfectly and there was just a little learning curve to address. It was good for me to see so I can address it in a tutorial or video. The next race he kicked my butt, so I had to have a rematch and he kicked it again! We both agreed that adding the engine management element to what is a very basic oval course really made it a lot more interesting and tended to even out the race more.
So why do racers need ADI? Good question… When you increase the pressure in the intake plenum by using a supercharger, compressing the air and fuel increases the temperature. Not only does this increase in temperature decrease power by having less-dense air, but it also increases the likelihood of detonation in the cylinders and increases cylinder head temperatures. Detonation is when the fuel air mix prematurely explodes before the piston gets to the top of it’s stroke which can damage the piston, pushrods, crank, and valves. In a car it’s called “pinging” and us usually the result of not using high enough octane in cars that require higher octane fuel (a higher octane fuel will ignite properly under greater pressure and has some other properties as well). Anyway as you may have guessed, ADI is designed to counter this behavior by injecting a water/alcohol mix into the intake where it cools the intake charge and reduces the likelihood of having detonation in the cylinders. Last year at Reno, a highly modified Grumman Bearcat name Rare Bear ran a lap without ADI on and that resulted in a huge hole in one of the pistons. Fortunately he landed safely, but it was the end of their race week. A stock Merlin engine (the engine in a P-51) is designed to operate up to 60 inches of manifold pressure and can go as high as 65 when needed. Racing Mustangs have pushed the Merlin to run as high as 150 MP (I’ve even heard of as much as 160) by using the -9 blower (which pumps more air than some of the earlier models) and allowing the engine to run at higher than normal RPM’s. Redline for a Merlin is 3000 RPM, but some racers run as high as 3700 RPM. Considering the supercharger (blower) is directly linked to the engine RPM, the faster it turns the more boost is generated and the higher the MP goes. Of course running an engine well beyond twice it’s normal limits is dangerous at best, and most pilots will not run their engines much beyond 130 MP and 3400 RPM. Beyond that and the engine starts resembling a grenade more than it does an engine… ADI is definately a requirement when running at such high manifold pressure.
So there you go, a little tidbit about some of what we’ve been working on. Superchargers and ADI. This tech will come in handy should we want to support WWII era aircraft in the future as most of them had superchargers. Even my little radial on the P-12 has a supercharger…