Mark said: RPM: 2900 Exhaust temp: 1250 Oil PSI: 35 CHT: 280 Indicated MPH: 125
I don't believe Mark's numbers are that far off either. Given his prop, I made right at that speed, with RAF 48 wings, 54x48 prop (really 50 pitch) turning 2900 rpm on a VW and 125 to 130 mph. I think he has a clean plane, or as clean as mine was, and as our discussion agreed earlier, cruise is more of a function of lack of drag, then other factors (although they all play a part for sure). Speaking of drag, Mark may be up against a little bit of what the Spam cans are up against. He is heavier AND more stable with a more forward CG. This means at cruise he has a higher angle of attack than say Mark L, or maybe even mine, since mine is a KR2 plans built and header only tank (no comment for or against). How much more angle of attack I don't know, but it seems with these little planes, every little bit of something makes a difference. I know mine is definitely more "busy" at cruise with the weight of two on board, and my CG hits 1/2" from the agreed upon limit (not using the last 2 inches Ken originally published). This may account for similar speeds with lower hp and less pitch, but same disc area, and similar fuse (we are a tailwheel also), due to lower angle of attack/drag. I do think that Mark probably did not have carb icing, as Brian hinted at, especially if it cleared up nearly immediately. This almost never happens with carb icing. It usually seems to worsen with the application of the carb heat, because it further richens the mixture aggravating the condition temporarily. Gradually the rpms will recover to almost the point prior to the rough running. If when Mark applied carb heat, the engine almost immediately began running better, I would be more inclined to believe that the heat was now causing better atomization and mixing, restoring the lost power. The 20 minutes of flight prior would have been time enough for the engine heat to be lost after his warm-up and engine run up check, and the heads to be cooled to the point that even inside the cowling air was just not hot enough, especially with that same air circulating around those free standing intake tubes. Couple this with cooling from the fuel itself, and you would have a significant problem. At least for Winter use, I would plumb a tube from the exhaust, maybe with a valve for shut off, or with an inexpensive thermac vacuum sensor that responds to temp, directly blowing on the carb to warm it, or duct the intake air that way full time, just like a car has for cold startup cold running. Even today alot of fuel injected cars have it. Gotta be a reason, and this seems like a good reason to me... Colin Rainey brokerpi...@bellsouth.net
