So what, exactly, do the numbers associated with props stand for? Example, a 20X10 ... I know that the diameter is 20", so what is the 10 all about? Does it have to do with the pitch of the blades? please enlighten me. Thanks in advance for any responses.

Correct. 10 would be the pitch. In an ideal situation, the prop will move forward 10 inches with one revolution.

You are correct. The first number is the diameter, usually in inches. The second number is the "pitch", also usually in inches. For those of you in the rest of the world, read mm or cm for inches. Pitch is the distance the prop will move through a SOLID in one revolution. Since air is NOT a solid, but compressible, there is slippage. Due to this, the pitch number is theoretical.

Dr.1

Is it true that low speed props have a higher diameter and finer pitch while a high speed prop has a corse pitch and small diameter?

Exactly right

Think of it this way.

Large diameter/low pitch equals more pull/thrust less speed

Small diameter/large pitch equals less pull/thrust more speed

To chose the correct prop for a given situation, it's sort of an art and science. For example, a Four Star 40 using an OS46AX flies very well with nice speed on an APC 11X6. An APC 11X7 will really make the plane scream but you just increased your takeoff roll and the time it takes to get to that speed not to mention the plane will be difficult to slow for a landing.

Same plane/engine combo now turning a 5 pitch prop will take off in a shorter distance, not fly as fast, but when you go full throttle and pull vertical it will climb like there is no tomorrow.

The other thing to remember is chosing a prop that keeps the engine in it's power band and works well with the plane.

A YS110 loves to be running right at 9000 rpm and an APC 16X6 prop puts it right there, put this same engine/prop in a Funtana90 and that prop delivers to much speed for the plane as it's very lightly built and will blow itself apart quickly (flutter). So I use a 17X4 prop on it. It now flies nice, not to much speed and when I go full throttle and pull vertical it just climbs until I tell it to stop and the engine is only getting 100+/- more RPM's.

If you want to see the effects of a low diameter, high pitch prop, here is a video of a Patriot using a ST 50. The prop is a custom cut 10X8.5 and you'll see that it needs lots of runway to take off and land and until he makes the first dive at the ground it does not have a lot of airspeed.

http://www.hiddenhangarrc.com/video/...ll_Patriot.wmv

This nextr video is me flying the Funtana90 with a low pitch prop. You'll see right at takeoff how I can pull vertical with little airspeed and go straight up

http://www.hiddenhangarrc.com/video/..._Funtana90.wmv

Thanks for the great post Bill. I'm downloading the videos now

GIVEN THE SAME RPM (very important):

1. Larger diameter/less pitch will produce more thrust, less top speed.
2. Smaller diameter/more pitch will produce more top speed, less thrust.

Case #1 is like low gear in a car. It accelerates quickly, but doesn't go very fast.
Case #2 is like high gear in a car. Lots of speed, but it takes a while to get up there.

Dr.1

I don't know about you guys but I think a 20x10 is pretty impressive for a beginner.

Yea Bruce I saw that but thought better of picking on him although it would have been in good fun

I wonder if we could bolt the 20X10 to a 46AX and slap it on a trainer...Ya think the engine will develop any rpms above idle??? or even idle at all

Sorta like the other thread a day or so ago where someone wanted to bolt in an OS46Ax into a 35% Edge to learn to taxi it.

Can you say "troll post" and it no longer exists

But you bring up a good point,. Bill. Most engines have a RPM limit, with both undersized props AND oversized props. Past a certain diameter, the engine just won't turn any more RPM, despite what the throttle setting is.

Dr.1

Dr. 1,

Yep, good point. Between all of the posts in here, I think we got our point accross. I just bookmarked this thread to help me find it when needed to answer this question once it comes up again

I don't really suspect anything underhanded. He probably just pulled the numbers out of the air. I'll accept that for now.

He also started the thread about the counter rotating props and said he was a college kid. You remember those days? [sm=confused.gif][sm=bananahead.gif]

There is an additional prop number you see sometimes. This is primarily on the Zinger wood props. I think Top Flite also has some like this. They will have numbers like 18x6-10. This is a multi-pitch prop with a 6 pitch near the prop hub abd a 10 pitch at the tip.

The idea was to lower the pitch near the hub to reduce the drag, but keep the higher pitch at the tip. The lower drag allows the prop to turn a little faster, more than overcoming the loss of thrust by the lower pitch at the hub.

Had not seen those. BTW - put in my RC Reports subscription yesterday, now that RCM seems to be gone I need some more reading material. Looking forward to your column Ed.

I feel like i ought to clarify things a little bit. I am an aeronautical and astronautical engineer at my school, and a group of us have gotten together to compete in a nationwide competition among colleges. We dont have any experience in building RC planes, so this is a huge learning experience and we dont expect to do everything right the first time. This competition involves heavy lifting and we are limited to electric motors, hence the large prop. i am not going to go into much detail about our design (just in case any other teams happen to monitor this board) but i can say that we will be doing analysis on everything that goes into it, both mathematically and through wind tunnel testing. Thanks for all the help so far, and you will probably be hearing from me again.