A prop has number 8X6, what does it mean?

Others will explain it better for you, just give em time to see your post. The 8 is the diameter of the prop, in other words the prop is 8 inches long. The 6 is the pitch of the prop. This is the part that I'm still trying to grasp myself. In order for the plane to perform the way you want it to, you have to get the right thrust to weight ratio. The right prop size & pitch is a big part of acheiving the thrust to weight ratio you want or need, and will also determine the speed of your plane. I think this gets much more detailed when you get into 3d stuff. Guys feel quite free to jump in and correct me on this because it still confuses me too, and if I'm giving him bad info please clean it up for me ok? I still just go with the plane manufacturers suggested prop size for each model.
Ken or Bill can explain all that to you much better than I, but the root answer to your question is 8=in. 6=pitch. Good Luck, Mark T.

Pitch is the distance in inches the prop will move forward (duh) in one revolution of said prop. Of course that doesn't happen since there is always slippage.

Been working on this myself. You can build a simple spreadsheet to calculate prop area multipled by pitch to get a ROUGH idea of prop performance. Build a simple table in Excel and you'll be amazed at some of the figures. The old +1/-1 rule doesn't always work. 10x6 can be beaten slightly with an 11x5 but if you go 12x4 you end up with less than you started with.

Of course, you must take into account actual RPM. This can only be done when you have tested the prop and got some RPM readings. Going for a "bigger" prop can be a step backwards if it pulls the RPM down significant from the baseline prop.

Rememberalso to just keep in the back of your mind in case you change the prop pitch and or size. look for the following-

If pitch is too small, the spinning prop could actually slow down a vehicle.
If too large, you will be splitting air into vortices fore and aft or each blade

Prop Length, Too small, the prop spins fast, wastes energy churning air and does less than its best. Usually you can hear the over winding, it gets loud and just isnt going fast.
Too large, the prop's air drag is high, and you might not succeed in using up most of your allotted power, which you would prefer to do, to go as fast as possible.


Rob

Mark and JD explained it just fine.

There are allot of factors involved in determining the correct prop for a given airplane/enginr combo

A really good example is my YS110.

This engine loves to be in the 9000RPM range. AN APC 16X6 prop fits the bill perfectly and has quickly become the prop of choice for this engine. Now for the problem. I have that engine installed into a Funtana90. The 16 inch diameter is fine but the 6 pitch is too much for the Funtana. The problem is it generates too much speed for the plane and this plane is known to flutter at speed and blow itself apart.

So you ask, how do we correct that problem and still maintain the RPM's that the engine loves.

The fix is this, you go up one inch in diameter and go down 1 inch in pitch. But wait, we have another problem, No one makes a 17X5, (insert expletive here), so what we do is this. APC makes a wide blade which will add more load to the engine so what we do now is install a 17X4W, and now we use a tachometer to check RPM,s and we gained only 100RPM so now we are even more into the engines power band. BTW..a wide blade prop will generate more thrust that a pattern prop (pointed tips) or the square tipped props (Zinger)

The end result is this. We now have a prop that creates more thrust, keeps the speed down and the engine in the middle of it's power band.

Here are several good rules of thumb when deciding on a prop.

More diameter less pitch equals more thrust. This give pulling power which you want for 3D and extreme aerobatics and will slow down nicely for landing
Less diameter more pitch equals higher airspeed. Great for when you want the plane to fly faster, but on certain airplanes can make landings difficult.

So in the case of the YS110 example above, I could have stayed with the 16X6 and just practiced throttle management, but that prop made extreme 3D flight difficult because it created speed and not the thrust needed for 3D flight.

We have a guy at our field that flies pattern racers and to give you an example, he flies a Patriot using a SuperTiger 50ish turning a custom cut 11X8 on a tuned pipe. I was not there, but I understand it was clocked at 128mph. What you have here is more pitch, turning higher RPM's. The drawback is it takes a couple of passes for this thing to get up too speed but once it's done, it's quick. The landing is pretty fast and he uses all of our 300 foot runway

There are mainly two ways to decide on a prop for a given plane/engine combo.

Ask here on RCU, this will save you some money as you will not be buying tons of props trying to figure out which is best for you. Sooon you will see just how to figure it out and pretty quickly will be able to chose the correct prop

Start with he manufactures recommended props and change the pitch/diameter to what you need. This takes experimentation and can get expensive once you get into the bigger props

Oh yea, and just to really confuse you. Not all props are the same. APC props generate more thrust than most other props out there short of the high dollar props and are generally quieter, but weigh a bit more. Master Airscrew props are nice and light but flex and are quite noisy. You can see these flex where you blip the throttle to full while you are on the ground, the flex is causing a loss of thrust so imagine what happens in the air.

Wood props are pretty stiff and the Pro series Zingers are not bad, but the Menz, Bolly, and 3W wood props are great but are generally only available in larger sizes

A good tool to calculate the differences that different props will have is a program called Thrust HP. It will let you play with the different numbers each prop will have without having to go through the hassles of trying them all on your airplane.

You can download the program here:

[link=http://freespace.virgin.net/barry.hobson/software/thrusthpv20d.zip]Thrust HP[/link]

Hope this helps

Ken

Bubbagates' short treatise is excellent, and it cleared up some of the fog around a couple of things confusing me.

I use a comparison with boat props when comparing pitch; a low pitch prop (low second number) will get you to its max speed quicker, it's allowing the engine to rev more quickly because it's loaded more lightly, taking smaller"bites" of the air with each revolution. It's max speed (airspeed), however, is less than a prop of higher pitch (higher second number) of the same diameter at the same rpm.

A higher pitch prop loads the engine more, because it is taking bigger "bites" with each rev, so it may accelerate to it's max airspeed more slowly, but it will be a greater airspeed than the lower pitch prop of the same diameter at the same rpm. If you've got the horsepower to spare, the high pitch prop may give you both the acceleration and speed. In boats, the prop diameter is pretty much the same (outboards and stern drives), but you have the latitude in airplanes to start figuring diameter into the equation to get the best combination of both. You want to "prop" your engine so that it can reach the upper part of its power band without allowing it to over-rev. Kind of like "gearing" a racecar, you can fly in low gear, wind up quickly but not too fast, or start off in high gear, takes a while to wind up, but hauls butt when it does.

This is oversimplification, Bubbagates gave more of the variables in choosing the right prop. Sometimes you have to experiment and see just what works better for you.

Also, if you buy and try 8 x 6 props of different brands, you will find that your airplane does not perform the same on different props of the same diameter and pitch.

8-6 prop sounds like you are either using a 20-25 size engine or electric.if the plane is a trainer using a glow engine and weighs in the 3 1/2 to 4 1/2 pound range witha span of 44-55 inches a better prop may be a 9-5