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Author Topic: why cut down props  (Read 3407 times)

Offline Horby

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why cut down props
« on: February 10, 2015, 08:49:00 PM »
Hi All,
I have been scratching my head about this one for a while.
why is every one cutting down their props? What is the purpose?


Warren
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Offline Bob Hudak

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Re: why cut down props
« Reply #1 on: February 10, 2015, 09:15:29 PM »
Ground clearance is one reason,but another reason is to get a crisp corner in maneuvers.
.                 Bob
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Offline Chris Wilson

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Re: why cut down props
« Reply #2 on: February 10, 2015, 10:26:01 PM »
Quote from: Horby on February 10, 2015, 08:49:00 PM
Hi All,
I have been scratching my head about this one for a while.
why is every one cutting down their props? What is the purpose?


Warren
Define 'cut' - if its diameter then it could an rpm tuning aid incrementally raising the rpm in exchange for decreasing the area until satisfied but it could also be pitch as a thicker larger prop is far easier to sand a new pitch into.
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Offline Horby

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Re: why cut down props
« Reply #3 on: February 10, 2015, 10:43:15 PM »
OK , I get it, smaller prop gives less presstion,  so why not just buy a smaller prop? I am starting to compile differences I am finding in electric vs IC but not quite sure what is going on with a cut down prop. Could you not get APC to just make them that way?

Warren
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Offline Crist Rigotti

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Re: why cut down props
« Reply #4 on: February 10, 2015, 11:55:03 PM »
A larger diameter prop will usually have a wider blade than a smaller one.  You cut it down the longer prop and you have the diameter you want plus a wider blade.  Both good.
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Offline Steve Helmick

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Re: why cut down props
« Reply #5 on: February 11, 2015, 12:08:10 AM »
More prop load = more fuel/electrons burned. If your fuel load or battery size is marginal, a smaller prop (1/4" diameter change) can make a lot of difference.  Even APC doesn't make props in 1/4" diameter increments.

Restore the tip shape to as close as possible to the OEM as possible. For example, I buzzed an 11 x 4.5 TT Cyclone on my Magnum .36, and it wouldn't rev as high as usual during my normal warm-up drill (pinching the muffler pressure line while watching the tach). Not unexpected. So I trimmed it to 10.75" but left the tips square and balanced it. It still wouldn't rev as high as normal. The thin sharp blade tip apparently is important!  H^^ Steve
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In 1944 18-20 year old's stormed beaches, and parachuted behind enemy lines to almost certain death.  In 2015 18-20 year old's need safe zones so people don't hurt their feelings.

Offline Howard Rush

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Re: why cut down props
« Reply #6 on: February 11, 2015, 02:32:32 AM »
Quote from: Steve Helmick on February 11, 2015, 12:08:10 AM
More prop load = more fuel/electrons burned. If your fuel load or battery size is marginal, a smaller prop (1/4" diameter change) can make a lot of difference.  Even APC doesn't make props in 1/4" diameter increments.

Why?  I'd figure that the more efficient the prop, the less electricity the airplane would use, assuming the same speed, flight profile, and air density. 
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Offline Igor Burger

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Re: why cut down props
« Reply #7 on: February 11, 2015, 02:54:55 AM »
May be we have 2 different efficiencies - maximally achievable and actual. Smaller prop and larger prop will have probably similar maximal efficiencies, but one of them can work closer to its sweet point then the other, so one of them will burn more elctrons then the other ... it can also burn copper in worst case  VD~
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Offline Howard Rush

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Re: why cut down props
« Reply #8 on: February 11, 2015, 12:45:15 PM »
And making the prop smaller doesn't automatically raise efficiency.  It would probably lower it.  Cutting the tips of a prop that's designed correctly would more surely lower efficiency. 

I do think that the APC 13" props we were using a year or two ago are too big for our airplanes.  Buzzing 1/4" off the tips on takeoff didn't seem to hurt them.  Many of us have switched to 11"-diameter props.  They allow us to fly better stunt, but they use more electricity to do a pattern.  They are optimized 11" props, not cut-down larger props. 
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Offline Igor Burger

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Re: why cut down props
« Reply #9 on: February 11, 2015, 12:56:50 PM »
Cutting tips will make wider blades what will probably make maximal efficiency lower (I say probably because cutting will make tips wider, that means enlarge induced drag at tips, but it will make also better P/D what can efficiency improve). There is certainly some optimal dimater of similar props (the same blade design) when prop works at its best, the only question is if choosen prop is closer to its max or not. Cut tips will make the prop probably worse, but at some circumstances it can push it to better actual efficiency ... so SOMETIMES it can improve consumption and sometimes not.
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Offline Steve Helmick

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Re: why cut down props
« Reply #10 on: February 11, 2015, 04:28:13 PM »
I am waiting for Howard to demand that I show the math. I'm not quite done yet. The blackboard is too small. Burma-Shave. H^^ Steve
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"The United States has become a place where professional athletes and entertainers are mistaken for people of importance." - Robert Heinlein

In 1944 18-20 year old's stormed beaches, and parachuted behind enemy lines to almost certain death.  In 2015 18-20 year old's need safe zones so people don't hurt their feelings.

Offline Steve Helmick

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Re: why cut down props
« Reply #11 on: February 11, 2015, 04:40:58 PM »
The technique has been used for more than half a century. Experimenting with propellers is part of the package. Math will not solve it for you. The CF props are more likely to work better than an APC or other plastic prop when trimmed, because after all, the tip shape and thickness are already hand work and not fresh from a CNC milled mold (APC, etc.).

I recall reading recently that after WWII, somebody took the prop off a Mustang and put it on a P-39. The P-39 performed very well with the Mustang propeller. You'd think the Bell factory would have tried that sometime before they made thousands of the damned things? No, the math said the OEM prop would work swell!  H^^ Steve 
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"The United States has become a place where professional athletes and entertainers are mistaken for people of importance." - Robert Heinlein

In 1944 18-20 year old's stormed beaches, and parachuted behind enemy lines to almost certain death.  In 2015 18-20 year old's need safe zones so people don't hurt their feelings.

Offline Howard Rush

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Re: why cut down props
« Reply #12 on: February 11, 2015, 05:44:36 PM »
We could try cutting the tips off Igor's prop.  It might make a cool whistle.
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Offline Russell Bond

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Re: why cut down props
« Reply #13 on: February 11, 2015, 10:37:37 PM »
OK Howard, I'll do that if Igor comes over to our Worlds.  H^^
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Offline Igor Burger

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Re: why cut down props
« Reply #14 on: February 12, 2015, 01:26:17 AM »
piped electro ? ? ? mnmnmnmnmnm ...  n~

... I will rather install better belt ...
« Last Edit: February 12, 2015, 02:10:48 AM by Igor Burger »
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Offline Russell Bond

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Re: why cut down props
« Reply #15 on: February 12, 2015, 02:08:16 AM »
 ;D ;D ;D ;D ;D ;D ;D ;D
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Offline Alan Resinger

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Re: why cut down props
« Reply #16 on: February 12, 2015, 08:28:38 AM »
Igor,
My understanding is that Russell has just started a business making suspenders.  I think he has done some advanced marketing.
Alan
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Offline Igor Burger

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Re: why cut down props
« Reply #17 on: February 12, 2015, 08:33:18 AM »
or may be he just shows his experience with dogs catching pants  VD~
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