ORIGINAL: sportrider_fz6
Carl Goldberg Piper Cub
OS .46LA
APC 10X6
idle RPM 2500 pitch speed 14.77 MPH static thrust 0.20lbs
MAX RPM 12100 pitch speed 68.75 MPH static thrust 4.39lbs
WING AREA 744 SQ IN
MODEL WEIGHT 7.5lbs
wing loading 23.21oz.
I read an article in sport aviator mag about selecting a prop to match the model. if I understood it correctly,
the wingloading of a model is equal to the stall speed.giving this plane a stall speed of 23 MPH. acording to the
article the cruise speed is equal to twice the stall speed giving this plane a cruise speed of 46 MPH. for a
plane of this type they recommend a 1/3 thrust to weight ratio (vs a sport model like an extra which is recommended
a 1/1 ratio or greater) if I understand this right the idle pitch speed being 1/2 of the stall speed will act as an
effective brake which should allow a nice slow landing, the max pitch speed is actually much greater then the required
cruise speed so maybe a 10X5 prop would be a better selection.
does this make since?
following this theory,using a 10x5 prop, if I was able to keep the same idle RPM the pitch speed would be 12.31 MPH
and should slow the plane more, even adding 1000 RPM to the max RPM the pitch speed would be 61.55 MPH still more then
necessary for cruise speed and the static thrust would slightly increase to 5.07lbs still more then enough at .7/1 ratio
what do you think?
Actually NO, it does not make sense. As others here have told you, only what happens in the field with a given prop and aircraft/power mode is all that really makes sense.
Using a specific diameter/pitch as an absolute in your equation/s right there devalues the answer. No two model airplane props are exactly the same, especially in the differences between manufacturers. Way back in the late '70s, early '80s when I did considerable pylon racing, I recarved many a prop. I could have two identical props, one would GO and the other might be an "also ran." There are no recognizable traits between an APC and a Zinger, of the same stated sizes, except those stated sizes. [:-]
You are on the right track using lower pitch for better landings, however by just reducing the pitch from a given standard prop for the given engine size will only increase the specific throttle position RPM, and may not be conducive to obtaining good in-flight characteristics.
Remember that a glow engine has no prop control other than the throttle and the installed propeller load. If you unload the engine with a prop that is smaller than a previously used prop then all you get is an increase in RPM for the fuel-mixture setting that the engine operates at with any given throttle position. That will also upset your formula.
If you wish to keep a given RPM, at a given throttle setting, then you must adjust the prop size to load for that RPM. As previously stated, an 11-5 may suit your LA 46, but a 10-5 will not do what you seek. A
rule-of-thumb is longer - lower pitch for bigger airplanes and/or maneuvering flight, and higher pitch for smaller airplanes and GO-FAST operations. A Cub definitely fits the low pitch requirements.
Although a commonly used term in the aviation world, there really is no such critter as a "Stall Speed" per se. A Stall can occur at any airspeed, or never happen, depending on how the machine is flown. Another discussion ??? [sm=confused.gif]
