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From: StarAerospace@aol.com
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Date: Sun, 6 Jan 2002 02:20:50 EST
Subject: Thrust and np
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          <<<<<<<<<<<<<<<<--->>>>>>>>>>>>>>>>
          <<  Lancair Builders' Mail List  >>
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>>
<< 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.
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