On Thu, 2009-12-03 at 19:40 +0000, Ron Jensen wrote: > Hmm, climb too high and cruise too low... Someone installed a climb > propeller on our aircraft? :D > > For example: > http://forums.cessnaowner.org/read/1/7599 > "I have a 172H and this summer I had the pitch changed from cruise to > climb. I lost approx 2 knots but saw a noticeable increase in takeoff > and climb performance. " > > We're 10% off max cruise, and 25% up on climb performance.. > > Our propeller model really starts to fall off around an advance ratio of > 0.60. Advance ratio is > > Speed / ((propeller revolutions/time)*(propeller diameter)) > > Some advance ratio calculations for our c172: > 107 knot/ ((2500/min)*75 in) ~= .69 > 120 knot/ ((2500/min)*75 in) ~= .78 > 120 knot/ ((2700/min)*75 in) ~= .72 > 60 knot/ ((1000/min)*75 in) ~= .97 40 knot/ ((1000/min)*75 in) ~= .65
And converting our advance ratios (J) into thrust: Thrust = Ct*density*(rpm)^2*(prop diameter)^4 J Ct 0.65 ~= 0.054 0.69 ~= 0.05 0.72 ~= 0.045 0.78 ~= 0.04 0.97 ~= 0.019 Sea Level density ~= 0.00238 slugs/ft3 8000 ft density ~= 0.00187 slugs/ft3 107 Knots, 2500 RPM, 8000 ft: (0.00187 slug/ft3) * ( 2500/min)^2 * (74 in)^4 * 0.05 ~= 235 lbs thrust 120 knots, 2500 RPM, 8000 ft: (0.00187 slug/ft3) * ( 2500/min)^2 * (74 in)^4 * 0.045 ~= 188 lbs thrust 120 knots, 2700 RPM, 8000 ft: (0.00187 slug/ft3) * ( 2500/min)^2 * (74 in)^4 * 0.045 ~= 245 lbs thrust So, as you can see, not allowing the propeller to accelerate as the speed increases actually reduces thrust with the current propeller. Accelerating the propeller to 2700 (red-line for this engine?) yields only a modest thrust increase. At the other data point mentioned: 60 knots, 1000 RPM, sea level: (0.00238 slug/ft3) * ( 1000/min)^2 * (74 in)^4 * 0.019 ~= 20 lbs thrust 40 knots, 1000 RPM, sea level: (0.00238 slug/ft3) * ( 1000/min)^2 * (74 in)^4 * 0.054 ~= 50 lbs thrust As the airspeed drops from 60 knots to 40 knots, the advance ratio moves to a powerful region causing a 150% increase in thrust. It appears we may want a propeller thrust table that is flatter in the 0.6 - 0.8 J range. A corresponding flattening of the power table may slow the engine below 1000 rpm when idling at higher speeds thus causing a more appropriate drop in thrust. Ron ------------------------------------------------------------------------------ Join us December 9, 2009 for the Red Hat Virtual Experience, a free event focused on virtualization and cloud computing. Attend in-depth sessions from your desk. Your couch. Anywhere. http://p.sf.net/sfu/redhat-sfdev2dev _______________________________________________ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
