Prop Pitch
 

Can any of you guys with Tesla coils in your basement (you know who you are) please explain to me why speed can't be measured with prop pitch-say a particular engine is turning an 8 degree prop at 20,000 rpm, is the speed it will fly able to be estimated?
I'm not a scientifically-inclined guy, but I am curious.
Thanks for any light on the subject--WW

Prop Pitch
 

I'll give it a try... Let's use that prop that has 8 inches of pitch. If everything was perfect the plane would move forward 8 inches for every revolution. What is the diameter of that prop you say? Well, if you had a prop that was 3 inches in diameter and another that was 10 inches in diameter (both with 8 inches of pitch) and you spin those suckers up to what 20,000 RPM while they are mounted on identical Senior Telemasters, which would be faster? The 3" prop would just be a noise maker, while the 10" prop would actually get the plane off of the ground.

It boils down to the old formula where a plane is in unaccelerated flight (straight and level at a constant speed) that lift equals weight and THRUST EQUALS DRAG.

How much air do you want to move and how fast do you want to move it measured against what you are dragging through the air determines your airspeed.

Hope this helps.

Prop pitch
 

I wish I knew how prop pitch was measured.

I put an APC 10X9 next to a MA 10X9, edgeways, and they look completely different.

The APC looks a lot higher pitch.

APC vs MA
 

I know what you mean-they do look completely different, but if they have similar methods of measuring pitch it MUST be an illusion. Either that, or the completely different prop SHAPE allows MA to spread the pitch over a larger area thereby reducing the angle of the pitch somewhat. Let's hear from you guys that have lots more math smarts than me 'cause I'm starting to sound stupid, even to myself--WW

P.S. See what I mean? meant to post a reply, but started a new thread instead! LOL--WW

Prop Pitch
 

They make pitch gauges so you can measure your pitch instead of guessing about it.

I have one. I know how much pitch my props have.

Even so, it makes absolutely no difference in guessing how fast your plane is going.

Prop Pitch
 

I think the APC's must use some kind of average pitch
formula, or something....I don't know.... :rolleyes:

They have Mega pitch near the hub....and near nothing
at the tips.... :confused:

Prop Pitch
 

There is more pitch toward the hub, but not that much more. The advertised pitch is at the tip.

Prop Pitch
 

windwalker,

There is a formula, to which you seem to be referring, that is used to estimate maximum airspeed using the prop pitch and ( expected ) RPM. That formula could be thought of as predicting the maximum airspeed at which the prop ( at the specified RPM ) could generate any thrust at all. In order to predict the actual airspeed, you would need to know the drag of the airframe, and also to "correct" for the idealized assumptions that are made in the formula. The simple answer is that a high-drag plane with a given prop pitch and RPM will not fly as fast as a low-drag plane with the same prop and RPM. You can't predict max. speed using the prop pitch and RPM, because it's just not enough data.

banktoturn

Prop Pitch
 

It is the Master Airscrews and the likes (constant pitch prop) that have average pitch...

Props like APC have TRUE pitch... that is alot of pitch at the root to compensate for the very low rotational speed, and almost zero pitch at the tip to compensate for the very high rotational speed... the object is to produce the same amount of lift along the entire prop blade... which is impossible, but they try...

Constant pitch props (like MA's) just have a sort of a fixed pitch, which produces drag at the root and a lot of lift near the prop tip.

Prop Pitch
 

Rudeboy is correct; APC's are true pitch. This means that at any point along the blade; The pitch is what is shown. Remember: The angle must be much greater at a lesser circumference to travel the stated prop pitch forward in one revolution.

All wood props are limited by their manufacturing techniques. To make a true pitch wood 12X12 prop with anything wider than a pylon toothpick blade would require over a 1" thick hub.

MA props have true pitch only one measurement station a little inboard of the tip. everything inboard of that station is less than stated pitch, outboard of that is greator than stated pitch.

From an engineering evaluation standpoint the True Pitch is better but since about 80% of the thrust comes frome the outboard 20% of the prop the inefficiencies of uneven pitch are barely noticeable.