kergan

I have a Sig Four Star .40 and am running it with a TT .46 pro. I am currently at college in Prescott, AZ where the elevation is 5000 feet. Having only ever flown at sea level, and being rather in experienced with prop sizes I was wondering if anyone could help me with my decision. I'm not necessarily asking for an exact prop to use, simply a few pointers to help me in my quest. I have so far tried an APC 11x6 with fairly good results, a MA 11X7 with slightly worse but still reasonable results, and am about to try an APC 11x7 and an APC 12.25x3.75 . I am interested in learning more about props, eg. Such as the effect a larger diameter but lower pitch prop will have or how RPM is affected by pitch/diameter. Having never flown at such high altitude, I am also curious as to how this will affect engine performance, prop size, ect. Thanks for your help

alan0899

G'day Mate,
At that altitude the engine will be down on power, & it won't make a difference, changing props, use the 11x6, & fly, just be aware that it ill not perform as it did at sea level.

sscherin

My Feedback: (1)

I fly at 6,200ft here in Colorado..

My .46 FX and .45 FSR run great with an APC 11x6 or a 10x7 If I want to get the RPM's up and go for more speed than acceleration.

A few of the guys here run the APC 12.5x3.75 with a MAC's pipe and like to combo.

Campy

My Feedback: (1)

To get more power (equivelant to sea level power ) increase your nitro content by at least 10%. You will probably need to readjust your low end and idle for best performance also.

If you haven't noticed already, your plane is going to need a longer takeoff run and will fly faster because the air isn't as dense (that is also why you need more nitro - less oxogen )

Your APC 11x6 or 11x7 should do fine. If you feel you want a bit more performance, TRY a 12x6, BUT KEEP A CLOSE WATCH ON THE ENGINE TEMPERATURE.

Clint H

My Feedback: (1)

I'm at 5,000 feet, and I like the 11x6 APC best for my .46.
If you’re interested, here are a couple of free thrust calculators:

www.lcrcc.net/thrust_calc.htm
www.gylesaero.com/_frames/f_propcalc.shtml

It might be fun to run a few numbers and see if you can narrow down your possibilities.
Just keep in mind these are a good guess of propeller performance, but, because of the large number of variables, it's still just a guess.

-Clint

da Rock

When you are at higher altitudes, it is worth trying props that seem to have more pitch than less. And it's for a basic reason. In classical aerodynamics they've come to the conclusion that the "squarer" a prop is, the more efficient it is. And with thinner air, efficiency is worth trying for. A square prop is one that has a pitch number that equals the diameter. If the only thing that mattered for efficiency was pitch/diameter, then a 12x12 prop would be the most efficient. As it is, the closer the pitch is to the diameter, the more probable it is that the prop is going to be more efficient. So..........

If you had a choice between two props, the higher pitched should be more efficient and might be the better choice to try when the going got tougher for the prop, like happens when the site altitude increases.

Truth is, if I had to choose between any two props based on just one detail, I'd say screw it to the theory, and try both props.

Clint H

My Feedback: (1)

You have to be careful with too high of pitch, however. If you go to high, it might be hard to slow the plane down on landing, and you can bog the engine down on vertical uplines. Let's say you try a 9x8 on your .46 instead of a 11x6. This means you lost 32 sq. in. of prop disc area. At idle, the prop disk acts like a speed brake, so you'll have much less drag on final.

Also, props are kind of like gears. Small diameter/large pitch tend to be speed props because for every revolution the 9x8 propeller is moving forward 8 inches instead of 6 as in the 11x6. So a 9x8 can be thought of as 5th gear in a car where an 11x6 is more like 3rd. Which one would like to climb a long, steep hill with?

Bottom line is what da Rock said, make an educated guess on two or three and go try them.

-Clint

js3

My Feedback: (3)

I live at 5400 feet alt. For a .46 sized motor I'd start with a 10 X 7 then experiment from there.