the science of prop selection?
 

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?

RE: the science of prop selection?
 

wouldnt a 10x5 be a small prop for that engine, i run 10x6 on .40 engines. although a 11x5 would be fine, might slow you down even more w/ the increased drag

RE: the science of prop selection?
 

If a cub is stalling at 23 mph, how on earth would you be able to slow it down enough to land without it first falling out of the sky? I'm not sure about all this analysis, but most cubs I've seen can slow down way under 23 mph and keep on trucking. I'd say prop an engine to get it within it's useable power band, and the style of flying you will be doing. Generalizing prop numbers for any given airframe is just guess work. There are far too many variables involved, like drag coefficients, Reynolds numbers, prop manufacturer, density altitude, etc.

Jesse

RE: the science of prop selection?
 

While I do appreciate your analytical approach to solving the problem and the care and thought you have spent determining the correct prop; I can offer no further advice then to give your theory a field test.

I would never have though of using a 10 X 5 prop myself. But when I spend a few minutes thinking about it, my gut instinct tells me that might just work, ( if the engine likes it).

RE: the science of prop selection?
 

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RE: the science of prop selection?
 

The real science to prop selection is to first go with what is recommended (if of course you have experience and can just select a prop size out of the clear blue..) then fly the plane. If it performs as you expected, stick with it. If not, change it. Repeat until you are happy then fly it.

This does two things. First, it gets you to the right prop for your style of flying and where you are satisfied with the performance, and second, it gets you a small collection of props for the next plane.

See? Very scientific.

CGr.

RE: the science of prop selection?
 

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] :)

RE: the science of prop selection?
 

That's way over thinking!!:eek: Read the instructions that came with your engine then start flight testing with those sizes until you find the prop that will fly the plane the way YOU, the pilot likes the plane to fly. You don't want to over rev or lug the engine down but there is a lot of testing space between the prop sizes that OS says you can use. Most of my planes I used the LA .46s on flew the way I liked with an 11X6 but the key word there was {I}!!!!!!!!!! There is no such thing as the perfect prop!!!!!:eek:

RE: the science of prop selection?
 

Sportrider:

The factory recommended propellers for your engine are 11x6 and 11x7.

http://www.osengines.com/engines/la.html

These load the engine and avoid over-revolutioning and operation at inefficient regime.

In flight, the 6” pitch not always produces the speeds you have calculated, and the generated thrust should be bigger than the static.

If 5” is your conclusion for best pitch, then you should keep the engine loaded increasing the diameter to 12, for which your model has enough ground clearance, I think.

The Cub manual recommends 10x6, 11x4 and 11x6 for two-stroke engines in the range of 0.40-0.51.

Here you can read four interesting articles about matching propeller, engine and frame:

http://findarticles.com/p/articles/m...g=content;col1

http://findarticles.com/p/articles/m...g=content;col1

http://findarticles.com/p/articles/m...g=content;col1

http://www.lcrcc.us/id9.html

The ultimate selection is made in the air, though.

RE: the science of prop selection?
 

Keep in mind that several props of the same size but from different manufacturers have different performances, so I am with Gray Beard when saying:

[Read the instructions that came with your engine then start flight testing with those sizes until you find the prop that will fly the plane the way YOU, the pilot likes the plane to fly.]

Some Master Airscrew props give more revs than APC
And some wood props give less revs than APC but with better efficiency.
This is a matter of try and find the right one.

RE: the science of prop selection?
 

Hi!
A 10x6 APC is way too small for a OS .46LA. The best prop for it ...in a CUB is a 11x6 or 12x4. 11x7 has too much pitch for a CUB.

RE: the science of prop selection?
 

thanks for the replys, I may over think things sometimes. but I was reading that article and found it interesting and thought it would be a good topic of discussion [link=http://www.masportaviator.com/ah.asp?ID=72&Index=0]Matching propeller to mission[/link]

RE: the science of prop selection?
 

Great discussion,
Here is a chart I stole, without regret from another RCU guy, so long ago I don't know who he was;)
It is a good tool to save.

RE: the science of prop selection?
 

I have run an APC 11 X 5 on a OS Max 46 ball bearing engine with good luck. ITs on a little aerobatic airplane, and the prop give good climb performance with low thrust at idle. But its still very powerful. Maybe a 11 X 4 would be worth trying.