lml@lancaironline.net Mailing List Archive

<<<<<<<<<<<<<<<<--->>>>>>>>>>>>>>>> << Lancair Builders' Mail List >> <<<<<<<<<<<<<<<<--->>>>>>>>>>>>>>>> Mark Lally writes: <450 lbs of torque (at 2700 RPM) will move a plane of lancair size off the runway quite smartly I think.> Horsepower is the product of torque, in foot pounds, times RPM divided by 5252, or in this case 450 x 2700 /5252 = 231 horsepower. Not nearly enough. Actually you need about 700 foot pounds of torque at 2700 RPM to match the TSIO-550. Torque is proportional to the average cylinder pressure (aka Brake Mean Effective Pressure or BMEP) and the piston area. As the saying goes "If you want to make horsepower, the only thing better than cubic inches......is cubic feet." What about turbocharging to increase the BMEP? you ask. As it turns out a single stage turbo is limited to how much it can increase the deck pressure (the pressure just before the throttle butterfly) over the ambient pressure. This pressure ratio is limited to about 3:1 for a high performance turbocharger. If you want to make full horsepower at FL250 where the ambient pressure is about 11 in.Hg then you need to make full horsepower at 33 inches of manifold pressure. So, all you can hope for is about 10% over sea level naturally aspirated performance. If you want to fly high and fast then a turbo is a must BUT you don't get high, fast AND a lot more horsepower (over sea level performance). What about turning at higher RPM and using a PSRU? you also ask. PSRUs weigh a lot and they don't produce any horsepower so as you reduce an engines weight (by reducing it's displacement) you are also adding it back as PSRU. It works out that there is a "no man's land" between 2700 RPM and around 4000 RPM where decreasing displacement and adding a PSRU does not decrease the installed weight of the power plant. You need to be turing the engine around 5000-6000 RPM before a PSRU makes any real sense weight wise. There aint no such thing as a free lunch. TANSTAAFL. Regards Brent Regan PS. I know that I have made some broad statements and taken some liberties with definitions. I do know that BMEP differs from the mean cylinder pressure by the friction horsepower and other factors, for example. My goal is to provide the broadest view in the simplest terms. I am not ignorant of the minutia, I just don't think it is relevant, or interesting, at all levels of discussion. BR LML website: http://www.olsusa.com/Users/Mkaye/maillist.html Builders' Bookstore: http://www.buildersbooks.com/lancair Please send your photos and drawings to marvkaye@olsusa.com.