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

Reducing Prop Dia.

Reply

Old 11-30-2006, 10:29 AM
  #1  
BCarter
Member
Thread Starter
 
Join Date: May 2006
Location: Richmond Hill, ON, CANADA
Posts: 52
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default Reducing Prop Dia.

Can I get some ideas on the best way to reduce the prop dia. of a APC prop. I have a 12x10 and would like to reduce down to approx. 9 or 10" dia. What shap should I try for? leading edge does and don'ts, best method (I am thinking of using a table belt sander). Thanks in advance.

Brian Carter
BCarter is offline  
Reply With Quote
Old 11-30-2006, 10:41 AM
  #2  
mmattockx
 
Join Date: Apr 2005
Location: Calgary, AB, CANADA
Posts: 2,427
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default RE: Reducing Prop Dia.

Brian,

For the sizes you are talking about, just go buy the right prop. My LHS lists APC sport props in 9x8, x9, 10x8, x9, x10 all for about $3 each. Then there are the pylon props in various sizes for around $5 each. Unless you really just want to try modding a prop, it isn't even worth it to go out the shop for that kind of $$$. $20 will buy you a wide selection of props to try out and the shape and balance will be really good right out of the bag.

My $0.02...


Mark
mmattockx is offline  
Reply With Quote
Old 11-30-2006, 10:25 PM
  #3  
buddude8
My Feedback: (10)
 
buddude8's Avatar
 
Join Date: Dec 2002
Location: Placentia, CA
Posts: 385
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default RE: Reducing Prop Dia.

just install the prop on the engine installed on your plane. Put a line at the tip where you want to cut it down too. start the engine and run it to full throtle, now nose it over and grind it down on the pavement.......................................... ...[sm=spinnyeyes.gif]. JUST JOKING, but i did do that one time at the field with a 11x8 when a 1/4" got cut off.


im gathering you want to do this because you want a stronger hub than a stock apc 10x10 would give you. The easiest way to get the prop even is to clip it off striaght and then balance it. I think the idea of the sander would work if you wanted to keep the same shape, but i would still clip it down close enough before you start sanding it down.

ken
buddude8 is offline  
Reply With Quote
Old 11-30-2006, 11:30 PM
  #4  
Mike Connor
Senior Member
My Feedback: (5)
 
Mike Connor's Avatar
 
Join Date: Jan 2002
Location: Tulsa, OK
Posts: 2,025
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default RE: Reducing Prop Dia.


ORIGINAL: buddude8

start the engine and run it to full throtle, now nose it over and grind it down on the pavement.......................................... ...[sm=spinnyeyes.gif]. JUST JOKING,
ken
That only works with MA props and not APC.
Mike Connor is offline  
Reply With Quote
Old 11-30-2006, 11:53 PM
  #5  
BCarter
Member
Thread Starter
 
Join Date: May 2006
Location: Richmond Hill, ON, CANADA
Posts: 52
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default RE: Reducing Prop Dia.

thanks all for the replies. The reason I was thinking of doing this is because I have only been able to find a 12x12 prop (it's the 12 pitch I'm interested in). I know that the dia is too big so I figured I could cut it down. The price of these props up here are 16 bucks ! The smaller props (I'm talking APC only) are quite a bit cheaper and I agree that if I could find a 9 x 12 I would just buy it. I was talking to the LHS and they said that cutting down the prop is probably not a good idea. He said it would probably sound good but due to changing the airfoil shape I would probably do more harm to the efficiency then good.

Thanks again all.
BCarter is offline  
Reply With Quote
Old 12-01-2006, 09:35 AM
  #6  
MJD
My Feedback: (1)
 
MJD's Avatar
 
Join Date: May 2003
Location: Orangeville, ON, CANADA
Posts: 8,629
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default RE: Reducing Prop Dia.

ORIGINAL: BCarter

thanks all for the replies. The reason I was thinking of doing this is because I have only been able to find a 12x12 prop (it's the 12 pitch I'm interested in). I know that the dia is too big so I figured I could cut it down. The price of these props up here are 16 bucks ! The smaller props (I'm talking APC only) are quite a bit cheaper and I agree that if I could find a 9 x 12 I would just buy it. I was talking to the LHS and they said that cutting down the prop is probably not a good idea. He said it would probably sound good but due to changing the airfoil shape I would probably do more harm to the efficiency then good.

Thanks again all.
Then again, don't believe everything your LHS tells you.. since when are you changing the airfoil shape? You only said you were planning to clip the tips.

Square tips are actually pretty good drag wise. Rounding the tips (as in like a rounded wing tip, not the planform) often increases the size of the tip vortices over a square cut tip. If you try to artfully create a new scimitar tip in imitation of the unmodifed version, your chances of doing a beneficial job feehand are not great. Similar situation to building elliptical wings - unless you can nail the compound curves just right and faithfully reproduce the intended airfoil seciton right to the tips, drag wise you are probably better off with a simpler planform done well, such as a Schuemann wing or straight taper or whatever. The gains are tangible but small, and you don't realize them unless everything is chickety-boo.

Myself, if I was to use a belt sander, I would fashion a quickie jig out of scrap plywood, and pin the prop to it with a bolt or some such arrangmeent to get the tips as close to identical as possible. I have access to vertical mills at work, so I go a little berserk with the concept and fit my props to a bolt held in the jaws of an indexing head to mill the tips. But in truth that's as much for the accuracy as it is for my obsessive state of mind with regard to props.

MJD




MJD is offline  
Reply With Quote
Old 12-01-2006, 11:58 AM
  #7  
bob27s
My Feedback: (19)
 
bob27s's Avatar
 
Join Date: Apr 2002
Location: Cleveland, OH
Posts: 5,576
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default RE: Reducing Prop Dia.

I just cut the prop blades to length, and round the tips. Then CAREFULLY re-balance the prop. Do not do any other modificaitons.

No special consideration on the LE or TE..... just a transition to where you cut and rounded it. Make it look nice.

Start off with the blade a bit longer than you actually believe you will need to start with. Always easy to shorten it further after you run it on the engine.

What engine are you using this on?

Be careful.

(image is an example of what I mean about rounding off the blade tips)
Attached Images  
bob27s is offline  
Reply With Quote
Old 12-01-2006, 12:04 PM
  #8  
MJD
My Feedback: (1)
 
MJD's Avatar
 
Join Date: May 2003
Location: Orangeville, ON, CANADA
Posts: 8,629
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default RE: Reducing Prop Dia.

Bob, are you thinning the tips as well? Looks like some sanding there.

MJD
MJD is offline  
Reply With Quote
Old 12-01-2006, 12:10 PM
  #9  
bob27s
My Feedback: (19)
 
bob27s's Avatar
 
Join Date: Apr 2002
Location: Cleveland, OH
Posts: 5,576
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default RE: Reducing Prop Dia.

No thinning per say, it is just cleaned up and rounded over. That photo there is not one of my props, but I do exactly the same thing when trimming to size.

A couple of months ago I started playing around with a carved wood prop for the 90L engine But the weather now sucks. Started out with Zinger 12x10 props, applying some QM40 technique. Trying to get an 11.5x9 when I am done that spins about the same as the APC 11x7. Time will tell.
bob27s is offline  
Reply With Quote
Old 12-01-2006, 01:17 PM
  #10  
vicman
My Feedback: (10)
 
vicman's Avatar
 
Join Date: Dec 2002
Location: Valdese, NC
Posts: 9,910
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default RE: Reducing Prop Dia.

applying some QM40 technique
do tell my good man?
vicman is offline  
Reply With Quote
Old 12-01-2006, 01:33 PM
  #11  
Troy-RCU
Senior Member
My Feedback: (1)
 
Troy-RCU's Avatar
 
Join Date: Jan 2002
Location: Oceanside, CA
Posts: 525
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default RE: Reducing Prop Dia.

Even the Zen master of props debates with himself on prop tip shape. [link=http://www.supercoolprops.com/articles.php]SuperCool[/link]But, I've found his props work amazing and he often suggests a nice clean, square tip. The problem is, APC is more Simitar and lift distribution is laid out differently. I think the main point of SuperCool in all of his theories, is that the tip airfoil is super critical to reduce induced drag on a prop. Therefore, don't mess up the airfoil or make it a better, thinner foil.
Troy-RCU is offline  
Reply With Quote
Old 12-01-2006, 02:21 PM
  #12  
vicman
My Feedback: (10)
 
vicman's Avatar
 
Join Date: Dec 2002
Location: Valdese, NC
Posts: 9,910
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default RE: Reducing Prop Dia.

Troy, have you read all of that?[X(]

Here is how Troy trims his prop
Attached Thumbnails Click image for larger version

Name:	Rp43907.jpg
Views:	2
Size:	130.4 KB
ID:	569990  
vicman is offline  
Reply With Quote
Old 12-01-2006, 02:35 PM
  #13  
bob27s
My Feedback: (19)
 
bob27s's Avatar
 
Join Date: Apr 2002
Location: Cleveland, OH
Posts: 5,576
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default RE: Reducing Prop Dia.

Ideally you would want a prop shape that is optimal and designed for the application. In which the tip shape is important.

But for sport use (my example) we are looking to create a quick, easy speed prop from an existing piece. In some cases, this is done to best match the application and rpm requirements of the engine. The larger center hub of a larger prop is sometimes prefered. In which case, clipping an existing prop to reduce the load and allow the rpm to climb works fairly well.

The rounded tip shape is more a result of 'rounding over' or tapering the end of the blade due to the airfoil shape. Looks more like a P-40 wing tip.

As for the QM40 prop cutting.... I'm not sure there are any on-line tutorials or photos on this. Same as had been done for F-1 for years, just slightly smaller prop diameter. Darrol Cady's site has this info .... http://www.darrolcady.com/Propellers...eller_faq.html

Basically for QM40 it involved starting with a Rev Up 400 serise or Zinger 9x7.5 pylon series prop as the master. First the pitch is checked on a gauge. (if the pitch on the blades was different, usually the hub had to be squared up a bit). It gets trimed it to 7.5" diameter. You then locate a point about 3/16" aft of the LE of the blade. Draw a line. Sand a flat from the blade TE to that line. (this thins the blade). Then it is a bit of finess rounding out the airfoil from that high point, and creating a properly radiused LE. They end up looking something like these shown here. In this case, note the tip shape. This one is typical for many that carve QM40 props. Some prefer rounded, others square. I rarely re-worked the back of the blade other than to correct the blade pitch. Some work carefully with gauges and micrometers to check the blade thickness and shape. In the past, we were looking for 0.100" to about 0.110" about mid blade (station 5 on a pitch gauge). The blade thickness tapers evenly from the hub to the tip, passing through that thickness and location. After final finishing and sanding, the blade ends up appx 7-3/8 diameter

Although these days the APC QM40 props have relieved many of us of endless work. Making 6 props, nearly identical, only 2 or 3 would be usable - the others didnt pull or turn up. Total crap shoot sometimes. For those good at this proceedure, they worked right or very close the first time. For me, I ended up re-working most props to thin them slightly or more commonly took another 1/16" or 1/8" off the diameter until the rpm was where it needed to be.

I was playing around basically making a bigger "QM" prop out of the 12" blade. It will take several iterations to get the blade thickness figured out just right.
Attached Thumbnails Click image for larger version

Name:	Ca82182.jpg
Views:	2
Size:	31.3 KB
ID:	569998  
bob27s is offline  
Reply With Quote
Old 12-01-2006, 03:04 PM
  #14  
vicman
My Feedback: (10)
 
vicman's Avatar
 
Join Date: Dec 2002
Location: Valdese, NC
Posts: 9,910
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default RE: Reducing Prop Dia.

Excellent explanation. Thank you.
vicman 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