is it true that the prop size depends on the altitude? Ihad a discussion with some friends at the club last weekend about this, they say that because of our city's altitude (1890 m) Isuppose to use a 10 X 7 prop on my .46 engine instead of the 11 X 6 that the manual states... what do you guys think?</p>

Check this thread out:

http://www.rcuniverse.com/forum/fb.asp?m=8014215

I believe that discussion applies to glow fuel as well.

I might be missing something but I didn't find anything about prop size in that thre.

Sorry, my mistake.

I have edited my post with the correct link.

You can swap an 11 x 6 for a 10 x 7 anywhere. They are approximately equivalent in engine loading. The rule-of-thumb is that you can go up an inch in diameter if you go down an inch in pitch. The 10 x 7 will give you a bit more speed while the 11 x 6 more torque. At higher elevatons the air is less dense so you might want to try an 11 x 7 or a 10 x 8 for better "grip" in the thinner air.

so my friend wasn't wrong, but it is not exactly that I must change my prop right? </p>

Props are a personal thing in the long run

Besides altitude, it depends on your plane and flying style.

You should try both props and see which one you like

Charlie was on the money with his explainatioon on prop pitch being traded for prop lenth or visa versa.

         But alas a little more insight.  Your engine given it's size makes a rated HP (horse power) at sea level. at a given air temperature.
It's called density altitude....and as you go up higher in altitude or air temp, your engine makes less power and your prop makes less thrust.  This is because the air is thinner.  That is why someday we'll all have super chargers on our RC engines.  But for now only the big boys do. [X(]
      When someone tells you to choose a prop by "the way you fly"  it's because.......the manufacturer of that engine has set forth what size prop that engine is rated to turn.  We'll say you have 2 choices......10X7 or 11X6.   The both put roughly the same load on the engine....meaning how hard the engine has to work to turn the prop.  But the manufacturer has worked that out for you and has told you that, that is what's best for that engine.  If you want a bigger prop....get a bigger engine.    Now....How do I decide between the 2 choices.   In overly simplifed terms......the 11X6 is a tractor prop.
It will pull more weight (your airplane) but at a slower speed than the 10X7.  The 10X7 will take a little longer to get going, but should give you a higher top speed once it gets moving,  But it won't have the same thrust ooomph (go power) at slow speeds that the 11X6 will have.

          And That ladies and gentlemen.....is why they say.......buy how you fly.  Fast or slow???

For CHOCORROL......your club assumes that because you're higher up....the air is thinner and so you need to push harder on the air.  Some lojic to it.
                                     But as others have said....try both sizes and stick with the one that makes you the happiest....pull or speed???
       

As already stated, flying at higher altitudes REDUCES output power from a normally aspirated plane engine.

You can do nothing to increase the power output, but you can help the engine not overheat, by reducing the load on the engine accordingly.

That is you can install a smaller diameter prop to reduce the engine load....

Now that said, bear in mind that the air is thinner anyway, so there is no real benefit in reducing the prop size, as the engine will have less air to bite into so to speak.

If you are attempting to get more power at higher altitudes, you'll either have to

- Increase nitro content ( this only works to a point... )
- Use a supercharger
- Get a bigger engine.

That's about it though.

You can usually remove a head shim to raise the compression to compensate for the lower density.

well actually the 11 X 6 that I'm using on my .46 engine works great for me, I guess there is no need to try another one unless I want to try a different type of flying.

You may or may not want to experiment with different prop sizes, just to see how the engine and airplane are affected, but I wouldn't let altitude be the primary factor in the decision.

My brother took his Astro Hog from Greenville Texas (541 feet ASL) to Thayne Wyoming (5993 feet ASL) and was concerned about flying at altitude. He runs a .91 four stroke with a 14x6 prop. He said the plane flew the same in WY as it does in TX and the carburetor required very little adjustment.

Carrell