Will some one please clarify for me what the pitch does and what the length does?(which gives thrust or torque and which gives speed) I have been told one thing by one person and another tells me the opposite and would like to know.Thank you

Both diameter and pitch contributre to thrust. Generally for a speedier plane you use lower diameter and higher pitch (e.g. 9x7). But for slower planes you use higher diameter and lower pitch (e.g. 10x5).

Pitch is the amount the prop will travel foreward in inches in one revolution. So the higher the pitch, the higher the top speed IF the engine can pull it.

Larger diameter propellers pull more air volume which equals more thrust.

The best prop is usually a compromise between thrust and pitch.

Pitch is the THEORITICAL distance (in inches) a prop will travel through the air in one revolution. There is always slippage so this figure is never totally accurate.

GIVEN THE SAME RPM:
Lower pitch and larger diameter gives more thrust but less top speed.
Higher pitch and smaller diameter gives more speed but less thrust.

For example, a .40 engine on a pylon racer may use a 9-8 prop. It'll go like stink, but takes a full lap to get up to full speed. An old-timer with the same .40 may use a 11-6. It flys very slowly, but gains top speed quickly.

Generally, planes with more drag need a prop with more pull than speed.

Dr.1

Just to add to what DR1 posted:

10-7
10 inches in diameter, 7 inch pitch. The prop will travel 7 inches for each full 360 degree revolution (pitch).

15-10
15 inches in diameter, 10 inch pitch.

The prop "absorbs the engine's power". That's the way propeller text books say it. They say, "absorbs".

The diameter and pitch together load the engine, give the engine something to work with. You need a combination of diameter and pitch that suits the power of the engine. Once you're in the ballpark you can focus on the two details.

You choose pitch to control the speed and climb of your airplane. After you've found a prop that combines dia/pitch to suit the engine, if the airplane is too fast for your purpose, the prop pitch needs to be adjusted. We can't adjust almost any of our model props, so we just buy a differently branded prop to get the different pitch. If the prop with the different pitch now lets the engine run out of it's power range, we can adjust with more or less diameter. Since we can't adjust......... etc.

Keep in mind that pitch isn't a perfect nor an accurate specification. It's really just a suggestion of how that model prop should act. Pitch is a very complex detail. Ain't no sound bytes that really work worth spit about it.

And keep in mind that we're slapping these props onto a broad range of engines and an even broader range of airplanes. The airplanes are extremely variable in how they affect any prop's performance. So we often need to try different combinations of dia/pitch that are not the "best" suggested one for the engine. We've got to match the prop to both the engine and the airplane. And that often results in using a prop that wasn't the "sound byte best" for that engine.

Also, keep in mind the comparison of pitch to the transmission in your car. The low pitch numbers could relate to the lower gear in your transmission. You use a lower pitch to gain power (poor word, trying to think of a better word but you get the idea), but at a slower speed. So, pitch down for more climb, for instance just like shifting to a lower gear to climb a hill. The engine will rev but the car (plane) will go slower, but have more pulling power. You can pitch down for a shorter take off distance, but your overall speed in the air will be lower, but pull on the elevator and the climb rate improves.

The opposite is true. Increase the pitch, your plane will go faster, but it will take a longer time to get there.. then pull to vertical, and you will lose that air speed more quickly as the engine strains to maintain. Take offs are longer because the plane takes longer to be pulled the take off speed when lift exceeds weight.

I've experimented with several props on my Venus II. I found that for my needs, with the OS 1.20 AX and Venus II combination, this combination really likes a 15-10 prop or a 16-8 prop. Both give me exactly what I want. And I maintain an operating RPM of right ar 9800 RPM at full throttle.

Hope all this helps.

DS.

Hi all,

Just I've been never got the correct answer as this topic is of props is being discussed so I wanna ask one question. What is the difference between a thrust and a speed?

Mody

Thrust is a measurement of force and speed is a measurement of velocity.

They measure the force the prop exerts to pull the airplane forward. Often in pounds of thrust.
They measure how fast, the speed, the airplane flies. Often in mile per hour.

thrust is the amount of pull if you will - so you can get 14 pounds of "pull" starit up speed is jsut the maxium throreticle speed with a specific rpm and pitch, no thrust but alot of speed is usless because you cant get off the ground a plane that has tons of thrust but no speed is also useless if you cant get above stall speed, we never get somthign for nothing its a hand off.

I see you got a REALY answere to your question, we posted at the same time

Thanks Da Rock & Flying Freak for your replies,

It means both thrust and speed play a major role in plane, more thrust is useless without speed, and more speed is useless without thrust right? Both thrust and speed should be good enough to take the plane airborne and fly it good and it also depends upon the engine prop and the plane right?

Mody

What all this kind of boils down to is for you to make a choice on what kind of flying your going to do with a specific plane, engine for which you will need the appropriate prop.

One the average most 3d'r's use a prop of less pitch. Warbirds and general all around planes install whatever and those who want speed go with a higher pitch prop.

So when you come on here and ask "what kind of prop should I use with this engine?" before this question can be logically answered, we need to know what kind of plane and size and whatkind of flying you want to do with it.