Author Topic: stock vs. cutdown props (Read 2214 times)

Steven Kientz · « **on:** August 23, 2012, 05:08:03 PM »

Hopefully this is the right section for this question.
Is there a significant difference between a stock prop vs. a cutdown prop? What comes to mind is a 11x4 vs. a 12x4 that is cut to 11". I tend to believe that the cut prop would be wider at the last station, possibly loading the engine more. Any thoughts?

Steve

RandySmith · « **Reply #1 on:** August 23, 2012, 05:37:38 PM »

Quote from: Steven Kientz on August 23, 2012, 05:08:03 PM

Hopefully this is the right section for this question.
Is there a significant difference between a stock prop vs. a cutdown prop? What comes to mind is a 11x4 vs. a 12x4 that is cut to 11". I tend to believe that the cut prop would be wider at the last station, possibly loading the engine more. Any thoughts?

Steve

Steve there is no answer to that question unless you give many specifics, like what exact prop you are asking about, Many cut props ARE stock props, for example on many CF hand laid props there can be 3 sizes that come from 1 mold
If you are asking about props like APC s with pointy tips, then yes, they will be wider at the tips when cut down

Randy

proparc · « **Reply #2 on:** August 23, 2012, 06:24:35 PM »

I've cut down a gazillion props. I can tell you that a stock APC 12x6 will out-pull an APC 13x6 cut down to 12x6 on a Saito 72.

Too many combinations to list. Don't really think about doing it unless, you have a really good prop balancer like a Dubro.

But, you just have to try it. It is a lot of fun.

Chris Wilson · « **Reply #3 on:** August 23, 2012, 07:40:45 PM »

Had this discussion here before and the only real issue seems to be washout - if you trim the washout away then you drastically change the props overall pitch.

Jim Kraft · « **Reply #4 on:** August 24, 2012, 07:44:14 AM »

I was running 11-6 APC props on my McCoy 40 Red Head, and the engine ran OK, but fuel usage was not consistant, and the engine was not really happy. I tried a cut down 12-6 to 11" APC, and left the tips square. That prop runs really great, and the fuel usage is now consistant. The engine only breaks to a 2 stroke a few places in the pattern, and pulls good everywhere. I have been running that prop for 6 or 7 years now as I can't find anything else that works as well. There are so many variables that the only way I know to find a good prop is trial and error. That is starting with something that should be close.

FLOYD CARTER · « **Reply #5 on:** August 24, 2012, 11:13:17 AM »

Sometimes, as little as 1/4" in diameter will make a big difference. I was having trouble using a 12-5 fiberglass prop on a Stalker 51. It would struggle after a couple corners of the square 8's. Took off just 1/4", using a jig and disc sander and balancing with the "magnetic" balancer. Problem went away, and the engine is now happy!

Floyd

Brett Buck · « **Reply #6 on:** August 24, 2012, 11:23:50 AM »

Quote from: Chris Wilson on August 23, 2012, 07:40:45 PM

Had this discussion here before and the only real issue seems to be washout - if you trim the washout away then you drastically change the props overall pitch.

   I don't think this is the *only* issue - the distribution of area is typically different. A 13-6 cut down to 12" usually has more chord at the tip than a prop manufactured as a 12-6, for instance. It runs differently even if you match the pitch.

   Note that the washout is mostly a function of the way we measure it than the actual aerodynamic characteristic of the prop. The difference between the bottom of the prop and the zero-lift line has much greater effect in terms of pitch as you get to the tips. The "washout" is probably there to minimize this effect, and in every case I measured it and did the correction, the pitch actually goes up at the tips even of the measurement makes it look like it goes down. That's also why you can get such dramatic differences in RPM and airspeed with props that measure the same pitch, but with different airfoils.

   Randy had it right, there's no generic answer to the question without more details. I have been experimenting and considering the problem for ~40 years to various degrees, and all I can say is try it and see what happens.

   Brett

Steven Kientz · « **Reply #7 on:** August 24, 2012, 08:24:00 PM »

I am not looknig to cut down props, just one of those nagging questions that gets stuck in your head. The only way to get rid of it is to ask or experiment.

Steve

RandySmith · « **Reply #8 on:** August 24, 2012, 09:16:21 PM »

Quote from: Steven Kientz on August 24, 2012, 08:24:00 PM

I am not looknig to cut down props, just one of those nagging questions that gets stuck in your head. The only way to get rid of it is to ask or experiment.

Steve

Steve don't discount trimming, it is a great way to help add a little more load to a motor, many 12 x 5 cut to 11 inches have been used instead of a 11 x 5 of the same brand

Randy

Steven Kientz · « **Reply #9 on:** August 25, 2012, 05:39:16 PM »

I'm not discounting trimming props, just curious as to what the difference between stock and trimmed props might be. Sounds like another tuning tip to add to my bag of tricks.

Steve

Chris Wilson · « **Reply #10 on:** August 26, 2012, 05:56:56 PM »

Quote from: Brett Buck on August 24, 2012, 11:23:50 AM

I don't think this is the *only* issue - the distribution of area is typically different.

Brett

Hi Brett,
as far as the topic of 'cut down' props go I still think that most else can be solved by 'cutting' except tip washout.

You simply have no material at the tips that enables cutting and restoring the washout.

Cheers.

Brett Buck · « **Reply #11 on:** August 26, 2012, 08:28:17 PM »

Quote from: Chris Wilson on August 26, 2012, 05:56:56 PM

Hi Brett,
as far as the topic of 'cut down' props go I still think that most else can be solved by 'cutting' except tip washout.

You simply have no material at the tips that enables cutting and restoring the washout.

?? I don't think I understand. Why does trimming it make it harder to change the pitch?

Brett

Chris Wilson · « **Reply #12 on:** August 26, 2012, 09:14:25 PM »

Quote from: Brett Buck on August 26, 2012, 08:28:17 PM

?? I don't think I understand. Why does trimming it make it harder to change the pitch?

Brett

I never said that it did.

Its probably just my dumb wording, but anyway the OP question was "Is there a significant difference between a stock prop vs. a cut down prop? "

My answer concerned washout, "There is a significant difference between a stock 'washed out' prop and its cut down partner" that goes beyond chord and thickness.

If you want to cut down a washed out prop you erode the washout area and the percentage of that vs the standard pitched area, and you can't really get it back by cutting, trimming or sanding as there is very little material left to do so at the tips.

So if I am wrong, how does one restore the same proportion of wash out with all cut down props?
(I know that some composite props will take heat and twisting quite well but with the others it is an issue and an added hassle.)

Some fliers obviously rely on the extra thickness revealed to re-pitch the tips but many props are so thin and narrow that they won't support material removal at all.

Steve Helmick · « **Reply #13 on:** August 29, 2012, 07:10:51 PM »

Even wood props will respond to heat for repitching the tips, tho most wood props we see over here are plenty thick enough to carve/sand to correct the pitch. But then you're needing a pitch gauge. Even experts will read the results differently, even using the same gauge and same prop with no heat alterations. I'm not that good with a pitch gauge, and I know it.

I have trimmed and reshaped APC's to a fair replication of the stock tip, with good results. I also once clipped a Thunder Tiger 11 x 4.5 to 10.75", leaving the tips square, and found that it would not rev as high as it did before. My conclusion is that the tip really needs to be thin and sharp, which makes sense. Still, I could see that a squared off tip might work better in the air, even if the tach says it's slower...maybe that's just what the engine/airplane needs.

Steve

Bill Little · « **Reply #14 on:** August 30, 2012, 06:37:26 PM »

I have an abundance of Rev Up 12-5 "government" props. I have often taken one and using a Top Flite paddle blade as a template made some 10 1/2-5 and 10-5 which worked very well. Even 10-6 by doing a bit of shaving There is plenty of wood on a prop that size to replicate a smaller prop. I often add Phillips entry for no apparent reason.........

BIG Bear
RNMM/AMM

Brett Buck · « **Reply #15 on:** August 30, 2012, 08:07:23 PM »

Quote from: Chris Wilson on August 26, 2012, 09:14:25 PM

I never said that it did.

Its probably just my dumb wording, but anyway the OP question was "Is there a significant difference between a stock prop vs. a cut down prop? "

My answer concerned washout, "There is a significant difference between a stock 'washed out' prop and its cut down partner" that goes beyond chord and thickness.

If you want to cut down a washed out prop you erode the washout area and the percentage of that vs the standard pitched area, and you can't really get it back by cutting, trimming or sanding as there is very little material left to do so at the tips.

Sorry, I think I misunderstood. I have never run into a case where I ran out of meat to repitch the tips with any of the typical wood props. Once you cut the tip off, it's really thick in most cases, and it doesn't take much shaving to change the pitch by 1/2" (which is about what we used.). Then you spend two hours shaving the airfoil down to look like a HLG wing or a *Bull* Y&O.

Brett

Author Topic: stock vs. cutdown props (Read 2214 times)

Steven Kientz

stock vs. cutdown props

RandySmith

Re: stock vs. cutdown props

proparc

Re: stock vs. cutdown props

Chris Wilson

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Jim Kraft

Re: stock vs. cutdown props

FLOYD CARTER

Re: stock vs. cutdown props

Brett Buck

Re: stock vs. cutdown props

Steven Kientz

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RandySmith

Re: stock vs. cutdown props

Steven Kientz

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Chris Wilson

Re: stock vs. cutdown props

Brett Buck

Re: stock vs. cutdown props

Chris Wilson

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Steve Helmick

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Bill Little

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Brett Buck

Re: stock vs. cutdown props