Ron Jensen wrote:
> 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
Thanks for the analysis Ron!
I'll take a look at the JSBSim config and try tuning the prop thrust.
-Stuart
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