Extreme Speed Prop Planes Discuss the need for speed with fast prop planes (Screamin Demon, Diamond Dust, Shrikes or any REAL sound breakin'''' plane)

Prop Pitch

Reply

Old 08-04-2003, 05:58 PM
  #1  
Windwalker
Senior Member
Thread Starter
My Feedback: (2)
 
Join Date: Jul 2002
Location: Sedna, ,
Posts: 302
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default 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
Windwalker is offline  
Reply With Quote
Old 08-05-2003, 12:53 AM
  #2  
Kaos1
Senior Member
 
Join Date: Mar 2003
Location: Charles Town, WV
Posts: 170
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default 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.
Kaos1 is offline  
Reply With Quote
Old 08-05-2003, 01:26 AM
  #3  
Dustflyer
My Feedback: (13)
 
Dustflyer's Avatar
 
Join Date: Dec 2001
Location: Abington, PA
Posts: 913
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default 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.
Dustflyer is offline  
Reply With Quote
Old 08-05-2003, 02:04 AM
  #4  
Windwalker
Senior Member
Thread Starter
My Feedback: (2)
 
Join Date: Jul 2002
Location: Sedna, ,
Posts: 302
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default 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
Windwalker is offline  
Reply With Quote
Old 08-05-2003, 03:01 AM
  #5  
ChuckAuger
Senior Member
My Feedback: (12)
 
Join Date: Dec 2001
Location: Pampa, TX
Posts: 5,133
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default 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.
ChuckAuger is offline  
Reply With Quote
Old 08-05-2003, 05:57 AM
  #6  
Flyboy Dave
My Feedback: (21)
 
Join Date: Mar 2002
Location: Pinon Hills, CA
Posts: 13,775
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default Prop Pitch

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

They have Mega pitch near the hub....and near nothing
at the tips....
Flyboy Dave is offline  
Reply With Quote
Old 08-05-2003, 12:53 PM
  #7  
ChuckAuger
Senior Member
My Feedback: (12)
 
Join Date: Dec 2001
Location: Pampa, TX
Posts: 5,133
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default Prop Pitch

There is more pitch toward the hub, but not that much more. The advertised pitch is at the tip.
ChuckAuger is offline  
Reply With Quote
Old 08-06-2003, 03:40 AM
  #8  
banktoturn
Senior Member
 
Join Date: Aug 2002
Location: Bloomington, MN,
Posts: 762
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default 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
banktoturn is offline  
Reply With Quote
Old 08-06-2003, 05:35 AM
  #9  
Rudeboy
 
Rudeboy's Avatar
 
Join Date: Dec 2002
Location: Kortessem, BELGIUM
Posts: 3,508
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default Prop Pitch

Originally posted by Flyboy Dave
I think the APC's must use some kind of average pitch
formula, or something....I don't know....

They have Mega pitch near the hub....and near nothing
at the tips....
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.
Rudeboy is offline  
Reply With Quote
Old 08-06-2003, 03:14 PM
  #10  
ewspears
Senior Member
My Feedback: (12)
 
Join Date: Sep 2002
Location: Ocala, FL
Posts: 242
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default 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.
ewspears is offline  
Reply With Quote

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off


Thread Tools
Search this Thread

Contact Us Archive Advertising Cookie Policy Privacy Statement Terms of Service