Can someone explain the effects of prop size and pitch on engine and plane performance. I was told that the longer the prop and smaller the pitch the more speed you will have. The shorter and bigger the pitch the better vertical. ( im not sure I understand the term vertical).

I will be putting a .91 fx in an 9.5 lb giles 300 and Im not sure what prop size to use. Im not looking for speed.

Dave

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

That is some what contrary to what I was told.

Assuming that having alot of vertical means needing torque, I would use a large diameter prop with a low pitch. Therefore the plane would hover well but not fly very fast. Is that a correct analogy?

Dave

Not necessarily. Most of our planes, especially the 3D-types, are drastically overpowered. When you have prop-turning power and RPM to burn, you can increase both diameter AND pitch, and get the best of both worlds. Many 3D-trypes are running props with 8-12 pitch, and hovering quite well.

Dr.1

In my case I have plenty of power to burn and I can spin a high pitch, large diameter prop pretty well. So your saying use the large dia and high pitch prop which will give me good torque (vertical) at low RPM and speed at full throttle.

Dave

That is correct -- however, prop selection often isn't that simple -- it also very much depends on the particular airframe, as well as the operational characteristics of the powerplant.

Despite the general rule that small diameter, steep pitch props will make a plane go faster, it is entirely possible that a small steep prop on a bulky airframe with a big wide cowling, will make the model go slower than it would with a larger diameter flat prop.

Similarly a big flat prop on a 4-stroke engine may give poorer climb performance than a prop with more pitch -- 4-stroke engines have a limited rpm range, & when they are operating near their peak rpm, they can't spool up any faster with a flat prop, so thrust actually falls.

Prop selection is part application of "general rules" and part black art -- which is learned by practical experience.

I have the following conditions
1) a large 2 stroke power plant
2) Large cowl
3) I want to keep speed down since I dont think the airframe can handle high speeds

So based on all the commentary so far and the above conditions I think the the big dia, low pitch prop is my best (safer) choice. This is opposite my original statement adn concurs with DR1drivers original analogy


dave

That is correct.

The individuals who originally advised you (as per your original post) have it backwards -- unless you interpreted their information incorrectly

In making your prop selection, you should ensure that the prop will load the engine correctly -- ideally, the full throttle RPM should fall somewhere between the engine's torque peak & its power peak (closer to the torque peak would be better).