From: CHRISTOPHER_ZAVATSON@udlp.com
Date: Fri, 15 Jan 1999 19:05:24 -0600
Message-ID: <001EB39E.C21254@udlp.com>
Subject: Props, CS vs fixed pitch
To: Lancair.list@olsusa.com
Mime-Version: 1.0

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Just for reference, with a wooden, fixed pitch prop, I could get no 
more than about 2350 RPM for a sealevel takeoff and takeoff was a bit 
on the anemic side, to say the least. With the CS unit, I always get 
2650-2700 RPM and the takeoffs are now quite sprightly affairs.
     
Do the numbers; that's better than a 10% increase in takeoff horsepower
(HP = T x RPM / K [K is a constant around 5200 for these regimes]).
Nearly all of it is "excess power" needed to climb, which relates to
better takeoff performance.
     
Cheers,
     
Dan Schaefer
<<<<
          
     An addition to Dan Schaefer's response to Sven (SE-XOP) regarding 
     props. 
     
     The constant mentioned in the HP = T x rpm / k is fallout from the unit 
     conversions required:
     hp = 550 ftlb/s
     T = ftlb
     rpm = (2*pi rad/s)/(60s/min)
     
     put it all together and you get:
     
     HP [550 ftlb/s] = T [ftlb] x rpm [(2*pi rad/s)/(60s/min)] 
     HP = T x rpm /(550 ftlb/s x (60s/min)/(2*pi rad/s))
     HP = T x rpm / 5252
     
     With a CS prop, in addition to the engine putting out more horse power 
     at the higher take off rpm, more of the prop blade is flying and 
     providing thrust.  A fixed pitch cruise prop can have a substantial 
     portion of the blade in a stalled condition until sufficient forward 
     velocity is obtained.  If the pitch is too steep, enough of the blade 
     can be stalled such that the aircraft can't accelerate at all.  In 
     this case all of the horse power being generated is going into 
     stirring a lot of air and providing no thrust.
     
     Dan, your % thrust increase was probably much higher than your % power 
     increase.
     
     CS all the way.
     
     Chris Zavatson
     N91CZ