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Author Topic: Props and undercamber - how much UC?  (Read 939 times)

Online Dennis Toth

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Props and undercamber - how much UC?
« on: October 23, 2018, 09:02:36 AM »
Guys,
Having just gotten solid consistent engine runs (free air uniflow vent thing) that I can set anyplace I guess its time to see what we can mess up. I used the RSM prop which their website indicates has a 0.020" undercamber. This prop was really strong on my engine (FOX 35)/ship and pulled strong through the vertical 8's. I have used other undercambered props on other ships but never gave much thought as to how much UC is actually in the blades.

Since contest season is about over for this year it gives time to test some new stuff for next year and props are easy and quick to work with. Has anyone tested various amounts of UC and how much is beneficial and where does it get to a point of diminishing returns? I've done a search here on SH and the posts indicate that different amounts have been used but did not define how much and where on the blade it was applied. How far back from the leading edge should it start and does it work the same with narrow blades as with wide ones?

Best,   DennisT

« Last Edit: December 12, 2022, 09:53:16 AM by Dennis Toth »

Offline MikeyPratt

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Re: Props and undercamber - how much UC?
« Reply #1 on: October 23, 2018, 11:24:04 AM »
Hi Dennis,
Over the years I've tried various amounts of undercamber on props and always come back to the same thing, .020 to .030 st about right.  Less doesn't seem to do much and over, loads the prop (less RPM).  There are some really good undercamber prop out there and they work great hands down.

Another tip on props is to add pitch to the tip of the prop, about about 1/4 to 1/2 more pitch (really easy on C/F props) no so easy on wood props but still worth the effort.  I'm sure other will chime in on this so good luck.

Mikey

Online Dennis Toth

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Re: Props and undercamber - how much UC?
« Reply #2 on: October 23, 2018, 11:46:35 AM »
Mikey,
Good information on the undercamber, did you sand it in under the whole blade or somewhat back from the leading edge?

I understand about the increase pitch at the tips, this is suppose to give a little more line tension and re-acceleration from the corner. I would think that adding in some undercamber out there would do something similar.

Adding the pitch to the tip on a wood prop is very easy, they repitch just like doing an APC prop with a heat gun. The technic is to heat the area were you want to pitch (turning over the prop to heat both sides) let it heat soak a bit then reapply the heat (I use a monokote gun, does not blister the finish). Twist the blade just like it was the APC (they like the heat soak and a little extra twist) hold a couple seconds and check the pitch, adjust as needed. You will get a feel for how much twist and heat after a few tries. I have done this on several brands - RSM, Rev-Up, Top Flite, BBYO, of wood props and they hold the new pitch just like a CF prop, had them sitting in the sun in a hot car for hours never lost pitch setting.

Best,   DennisT
« Last Edit: December 12, 2022, 09:56:41 AM by Dennis Toth »

Offline MikeyPratt

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Re: Props and undercamber - how much UC?
« Reply #3 on: October 23, 2018, 01:19:58 PM »
Hi Dennis,
I sand the outer 1/3 of the prop blade for undercamber.  At the tip of the blade I found it easy to increase the pitch by sanding the back of each blade to increase the pitch.  I've never tried using heat to repitch blades, on wood props, but I'll try it and see how it works.

Thanks for the tip,
Mikey

Online Lauri Malila

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Re: Props and undercamber - how much UC?
« Reply #4 on: October 23, 2018, 01:49:56 PM »
Repitching wood props by heating & twisting works just fine and stays well when done correctly. You have to heat slowly so that center of blade also gets hot without burning the surface. Then twist a little over and return to desired pitch.
I only twist from root but tips are also doable. You just have to be carefull not to deform the blade airfoil. Hold something flat against the bottom of blade while twisting. L

Offline Vitalis Pilkionis

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Re: Props and undercamber - how much UC?
« Reply #5 on: October 23, 2018, 02:09:36 PM »
This is some really usefull information about UC by Brian Eather.

Vitalis

Online Brett Buck

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Re: Props and undercamber - how much UC?
« Reply #6 on: October 23, 2018, 04:06:58 PM »
This is some really usefull information about UC by Brian Eather.

Vitalis

  Brian wrote that? 

  I wonder how "performance" was defined, because it's not obvious to me whether L/D or Igor's "drag bucket" effect is more important or how it would be affected by the modifications shown.

   Brett

Offline Steve Helmick

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Re: Props and undercamber - how much UC?
« Reply #7 on: October 23, 2018, 07:02:56 PM »
I've been running Brian Eather 3-blade undercambered props on a PA .51 and a Randy Smith AAC OS .46VF for about 8 years or so. Nominally 4" pitch, I tweek the tips pitch up for the last 2" or so. The increased load on the engine is something to keep in mind, but otherwise, I don't see any downside to the undercamber, and the extra pitch on the tips does seem to improve line tension overhead. 

It took me awhile to figure out a pitching method that worked for me, but the way I came up with is pretty easy. I use a 16 oz "Pounder" (beer) glass and an induction heating water pitcher. It makes it easy to keep the water hot...every time I change blade station, I dump the water out of the glass back into the pitcher. With a full 12" dia., the blade hits the bottom of the glass. With a smaller diameter, the other two blades make an effective stop. I put a short piece of electrical tape on the beer glass to tell me how much boiling hot water to put in. It's not the same pitcher my wife drinks hot water from, OBTW.    H^^ Dyrbr

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