lml@lancaironline.net Mailing List Archive

<<<<<<<<<<<<<<<<--->>>>>>>>>>>>>>>> << Lancair Builders' Mail List >> <<<<<<<<<<<<<<<<--->>>>>>>>>>>>>>>> << With a 77 inch prop at 2800 RPM and a TAS of 210 knots, your tip speed at 5000 ft (std day) will be 0.917 mach, and at 10000 ft (std day) it will be 0.935. My experience says: be very cautious with predicted performance curves when they suggest better than 86% efficiency anywhere, and beware of any numbers at tip speeds above 0.85. Calculate your "J" ("advance ratio") and power loading numbers for the anticipated performance envelope, and manage your expectations with real data. 'nuff said. >> Not quite. I have to wonder where this idea of "percentage" came into use regarding the propeller efficiency factor "np". "np" is a dimensionless number represented by the product of the advance ratio (J) multiplied by the ratio of thrust vs. power coefficients. It is NOT a percentage of anything. Thrust from a propeller is also inversely proportional to velocity. This has not been mentioned anywhere in these comments on tip speed, so it might be useful to state the equation for thrust: T = 550 * np * HP / V Where: T = thrust in pounds; HP = brake horsepower imparted to the propeller, SAE net for all you gear heads; V = velocity in feet per second; 550: necessary to convert HP (ft.*lb./min) to comply with V in fps; and finally.... np = the efficiency factor (not percentage) of the prop at that particular flight condition (this changes with velocity, advance ratio, tip speed mach limit and many other factors). Useable tip Mach and "np" varies considerably with prop type and technology. Thick wood props with (archaic, but still used) Clark Y airfoils have very low useable tip Mach, some as low as .75. Some (not all) composite and aluminum props with thin, swept, supercritical sections can achieve greater than .85 np at tip Mach of .91 to .94. It should be noted that certain exotic and multi prop disc combinations have achieved np of greater than 1. It is not a percentage. More important, thrust from a prop for a given HP goes down as we go faster. That makes our low performance at low speeds look even less impressive <LOL>. "Propulsion efficiency" is the more comprehensive factor that represents the actual net thrust minus all the losses (aerodynamic, cooling, etc.) that don't get counted in the basic HP vs. T equation. Yes, thrust causes drag; not just because we are going faster, but also because the greater thrust impinges on the airframe more and the greater HP creates more heat that costs us more drag to get rid of. All of these things are recorded by the aerodynamicist as "negative thrust" and constitute the difference between "uninstalled" (books, dynos, prop charts) and "installed" thrust. Increasing Peff often has a better return on investment than beating on the engine or prop for more efficiency. Time to toot my own horn here, we do this for a living. Eric Ahlstrom Star Aerospace LLC ps - if anyone actually is part of a company that has a vested interest in anything discussed on the boards, please have the courtesy to sign your name AND your company. LML website: http://members.olsusa.com/mkaye/maillist.html LML Builders' Bookstore: http://www.buildersbooks.com/lancair Please remember that purchases from the Builders' Bookstore assist with the management of the LML. Please send your photos and drawings to marvkaye@olsusa.com.