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Carving down props

Old 05-03-2002, 09:22 PM
  #1  
Johnny_Rebel
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Default Carving down props

Hey guys,
Last night I messed around a bit with two 14x4 zinger wood props. I cut one down to a little over 10 inches and the other ended up at 11 1/2 " - What I'm trying to do is make an ideal funfly 3-d performance prop for my ZNline Madness. I haven't had a chance to flight test these props yet but it should be interesting. What I did do was start the engine up and saw at what power setting the plane would hover at. The shorter prop seemed to have more thrust, and was also much more responsive than the longer prop and the apc 12.25x3.75. At full power and held vertically it had a pretty good tug skyward. This all isn't saying much as flight testing always reveals the unexpected. Whats neat is how fat the blade is. My thinking is that since its a wood prop being so light the excess load of the fat blades will be compensated for -- but who knows. The plane is the Znline Madness and the engine is the os 46fx.The plane wieghs 4 lbs 10 oz. I have a webra 50 on the way which is supposedly more powerfull and lighter, which will allow more messing about with different props. What I'd like to find is the ideal combo for the least amount of torque, most thrust, and most downline brake effect. The apc prop works fine, I just have a feeling that better could be found with a cut down wood prop. What have been your findings with cutting down wood props?
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Old 05-03-2002, 10:58 PM
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Aerosplat
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Default cutting props

In my experience I have found that cutting a prop to a smaller diameter results in a prop with an effective higher pitch. On a given engine it will turn more rpm, have less pull (static thrust) but more top speed.
Hold a prop on a flat surface by pressing on the hub. Now, sight down the blade from the tip toward the hub. You will notice that the pitch at the tip is lower than at the hub. If you have say a 16x8 prop, the 8 pitch is in the outer half of the blade where the blade is moving the fastest. The inner half of the prop will have greater than an 8 pitch. Cut an inch off each tip to make this a 14 inch prop. You have reduced the area of the 8 pitch in proportion with the higher pitch inner half. This effectively gives the prop a higher pitch rating.

We all know that lower pitch results in more thrust. So, cut off the flat tips and increase the pitch, but reduce the thrust.
I have tested this on my bench setup several times. If you want more top speed, cut down a prop. If you want more pull, go to a larger diameter prop and/or lower pitch.

Don't take the 50/50 relationship to be an absolute. It varies with each prop design. But the theory holds up the same.

Just my $.02.
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Old 05-03-2002, 11:43 PM
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bugsiegel
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Default prop trimming

on my 32 funfly engines I like an 11/4 apc snipped about 1 inch.
I can just go with a 10/4 prop but when i look at the two next to each other, my altered prop has far more meat than the factory 10/4 or even a factory 10/5.
Off the line I don't reach the max rpm's as fast as a factory 10/4 but on instant throttle I feel I produce a little more prop wash which is good for keeping the sub 4lb plane in an angle of attack.
just my two cents.
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Old 05-04-2002, 12:34 AM
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xsuperfastx
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Default Carving down props

VERY INTERESTING!!!!!!!!!!!!!!
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Old 05-04-2002, 12:34 AM
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ceandra
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Default Carving down props

The wood prop will give you quicker throttle response than the APC, due to lower mass. Once up to speed, both (if same design) should have same power and RPM. Thats why the stick-and-boomer use wood, quick throttle.

If a prop was perfectly pitched, lets say 4" pitch, then the tip is half the angle of the midpoint. This is because in one revolution, the tip travels twice as far as the midpoint. If the pitch continued to the exact center of the prop (cant, there is a hub), it would be at 90 degrees pitch. If you look at a pitch guage, you will find that the angle varies with diameter for a given pitch. Therefore, unless you have a prop with some sort of progressive pitch, the 11" cut down to 10" should be the same tip angle as the 10" stock prop. Rev-Ups were known to have a flatter pitch at the tip. Some racing props, I beleive, actually have higher pitch at the tip.

That said, I kinda liked a 11x4 cut to 10x4. It gave a wider blade, which gave me good thrust. I thinned the airfoil (Top Flite) by about 30% to keep the RPM's up. I find it critical to thin the tip, less critical over the rest of the prop. In fact, if you taper the last 1/2" of a top flight, front only, to a knife edge at the tip, you get a tremendous improvement in RPM and thrust.
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Old 05-04-2002, 02:06 AM
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Default Carving down props

Thanks guys- this is really interesting - i think the 14x4 cut will prove to be too fat at the tip unless i do some serious grinding- I think i'll try cutting a 12x4 and an 11x4 like you guys said- I never thought about the change in pitch along the length of the prop issue - Right now those blades I cut are probably more in the 6 - 8" pitch range now - that might not work. I'll test fly them next week sometime and see what happens. Thanks alot for the input and observations.
-Jon
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Old 05-04-2002, 03:52 AM
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Default Carving down props

Ceandra is right about the tip--90% of the work is done by the last inch at tips of the prop. I don't clip my props, but I do rework all my sport props, usually starting with an off-the-shelf Zinger. I shape the tip so it looks like an APC tip. Think of a propeller blade as a wing. Efficient tip shape is leading edge curving back in a more or less quarter circle arc to the trailing edge. Trailing edge along the full length of the blade should be a feather edge, nmt 1/64". Stock props are way thicker than they need to be, and this adds drag where you definitely don't want it. Tip should be small radius on the leading edge, and taper pretty close to knife edge at the extreme tip. Also, most props have thick, heavy, not very smooth varnish finish. I sand that away completely and refinish with 3M fine and superfine sanding sponge, then seal the wood with a few light coats of clear spray lacquer, quick drying. I use the lacquer to bring the prop to perfect balance on magnetic balancer. With a little sanding and spray lacquer you can make your own premium prop, just as good as the $20 jobs. Use a dremel sanding drum for the initial shaping--cuts pretty fast.
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Old 05-05-2002, 02:49 AM
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DKjens
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Default Carving down props

I've been looking for the best ff props for YS91ACs and YS120. For my YS91AC on my 60SuDoKhoi, I started with a Zinger16x6 and cut two new props. One had tips shaped like the APC, and it was a wast improvement. The other had very pointy tips, and I love that prop. I have attached a picture. The Zinger props are much to thick, but leaves a lot to work with. I bought a $60 belt and disc sander from Harbor Freight, made a 1 hour minimum job into a 10 minute job.
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Old 05-05-2002, 02:19 PM
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sherman-RCU
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Default Carving down props

APC 12.25 x 3.75. THE BEST fun-fly prop out there!! I run my MVVS 45 with em and WOW!
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Old 05-07-2002, 11:55 PM
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Johnny_Rebel
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Default Carving down props

Hey Guys,
I got to test fly those props today - Looks like I need to point the tips a bit. The thrust just wasn't there. I think next I'll try a a12 or 11 x 4 and carve that down and make sure to point the tips. Thanks for your help.
-Jon
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Old 05-08-2002, 12:25 AM
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airjeffro
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Default Carving down props

Is it important to reseal the props after removing the finish?
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Old 05-08-2002, 12:27 AM
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xsuperfastx
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Default Carving down props

why whats the difference
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Old 05-08-2002, 12:56 AM
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Default Sealing the props

Yes, it is important to reseal a wood prop after sanding or reshaping. Wood will react to atmospheric conditions - Soak up moisture, or dry out in hot dry conditions. This changes the physical properties of the prop and can cause it to become out of balance, or make it brittle so that it breaks under load.
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Old 05-12-2002, 02:33 AM
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Default Carving down props

Ceandra is right about the tip--90% of the work is done by the last inch at tips of the prop.

the prop guy say's if it is out side the spinner it fly's. in other word's if it is spinning it will either create lift, drag, or reverse thrust. work backwards. if you are spinning the long low diamiter props at a higher rpm the tip speed is reaching a high mach #. all the sanding and fooling around you are doing with the tip might be working against you.

not often will you see a prop with more pitch at the hub than the tip. most props are variable pitch and some are constant pitch. the variable pitch prop will work better for you on the f/f. as the lower pitch hub will let it spool up faster.

the constant pitch prop will require more hores power to get the same result. and will work better on the faster airframes. i.e. pattern style plane's.

the air foil on the apc props at the hub is semi semitrical, this allow's the hub to create less lift and takes less horse power to turn. allowing the rest of the prop to use more constant pitch and be more efficent.

the zinger props are more often than not variable pitch. the hub is often only 1/2 the pitch of the tip. the flat bottom of the prop is the reason for this. unlike the apc with the round hub the zinger prop fly's from hub to tip.

the thickness of the airfoil has alot to do with safety "on the wood" prop. if you thined down a zinger like an apc it would more than likely flutter and sling a blade.

if you want to re-work a zinger, the tricks are simple. de-pitch the hub. on the bottom only. use a single edge razor blade, dragging it like you are sanding. be sure to measure the pitch as you go. and the last 30% of the prop on the top, carefully sand the top to move the high point back to around 50% progressive to the tip. thinning as you go.

dont leave squared off tips. as this will cause flutter. radius it back like a butter knife.

i'll take some pics if any one is interested to show you. i've been re-working props for years and have had national record's in 3 different country's. in j.s.o. RR
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Old 05-12-2002, 03:34 PM
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funflysteve
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Default Carving down props

Team futaba,

I am very interested in some more pics of what you are talking about.....and for clarification would you describe what you mean when saying the "bottom" of the prop. I am in the unlimited funfly competition realm of the hobby and have messed with modifying some props. I have seen a great deal of rpm gain when messing with them but i am not sure i am gaining the greatest effiency of the prop.


Thanks,
Steve
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Old 05-12-2002, 07:40 PM
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Default no problem

will do tomorrow, props 101. RR
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Old 05-16-2002, 06:38 AM
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Airwurthy
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Default Carving down props

I have been anxiously awaiting Props 101, did I miss it? Where and when was it?
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Old 05-16-2002, 09:02 AM
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Rocket
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Default holy props batman

my prop shop is at work, ive been so busy i cant think straight. i'll have the total rework of a rev-up 10x4 f/f prop for you with pics. and a few tricks for the apc aswell. keep the faith. RR
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Old 05-16-2002, 09:55 AM
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Fastlif
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Default Carving down props

anxiously awaiting your workup on the rev-up prop team Futaba. Have started the shaving props bit but am kinda blind as if me and Steve are doing it correctly, or even close. We gain RPM for sure, but not sure if there is a better way??? Anyway, I will keep checking back, and thanks. Jim
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Old 05-17-2002, 07:52 AM
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Aerosplat
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Default More about Zinger's

Teamfutaba,
You said:

the zinger props are more often than not variable pitch. the hub is often only 1/2 the pitch of the tip. the flat bottom of the prop is the reason for this. unlike the apc with the round hub the zinger prop fly's from hub to tip.
Is this correctly stated? On the Zingers I use (standard, not Zinger Pro) in the 15x6 to 18x6 range it appears that closer to the hub the pitch is greater than at the tip. (higher numerically)For example, on a Zinger 16x6 there is noticeably more twist closer to the hub (the flat bottom is at a steeper angle relative to the prop shaft hole).
You also said:
if you want to re-work a zinger, the tricks are simple. de-pitch the hub. on the bottom only. use a single edge razor blade, dragging it like you are sanding. be sure to measure the pitch as you go. and the last 30% of the prop on the top, carefully sand the top to move the high point back to around 50% progressive to the tip. thinning as you go.
This seems to support my observation, but seems contradictory to your above statement.
Did I misunderstand something?

Also, could you include some pictures of a Zinger with the hub de-pitched. I am most interested in doing this.

One more question, what is a good tip speed, or what speed is considered too fast? If my math is correct, an 18 inch prop at 9,000 rpm the tips are making 481.8 MPH.

Thanks so much for sharing your Experience and expertise.
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Old 05-17-2002, 09:06 AM
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Default illusion's

if you look at the prop like you said it would seem that the pitch at the hub is more that at the tip. the distance travled is the difference. not forward, but rotation. how many inches did your prop travel per revloution around the arch. thats where the illusion comes in to play. what you see at the hub travles less distance around the arch than the tip.

the prop i started to rework was not the 10x4 revup it was 10x8 zinger. the first series of photos show the pitch gauge. the pitch on that prop was 5.5" of pitch at station # 2 and 8.0" of pitch at station 10. station 10 being the tip and 2 being the hub.

the de-pitch thing i spoke of has to do with usable horse power. when you de-pitch a prop at the hub you dont use as much power to turn it. now you can take advantage of more pitch at the tip where the prop is more efficent. the hub is still creating lift just not as much.

i did the first series of photo's on thursday of the rework and will try to finish them today. RR
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Old 05-17-2002, 09:59 AM
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Default Got IT

OK, Thanks for the explanation. I understand the theory and practical application of what you are saying. I was just applying a more traditional and stricter definition to "Pitch"

Looking froward to viewing the pictures.

How about the tip speed question? I am assuming efficient tip speed would be faster for a lower pitch prop ? Or putting it another way - I assume a 12 pitch would cavitate at a lower speed (rpm) than a 4 pitch ?

Thanks again
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Old 05-17-2002, 06:47 PM
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Default 101 in session

the first pic is of the hub. the tool i use is 4way file, top 1/2 round with fine and course. bottom 1/2 flat with fine and course.

the pitch at station #2 was 5.5" and the tip was 8.0". i highly recommend you get a pitch gauge if you are going to alter the pitch.

working from trailing edge to leading edge with the fine/flat side of the file, remove the wood making sure to maintain a flat filed area. do this working from the hub towards the tip aswell. always check the pitch as you go.

the pitch was changed to 3.0" progressive out to 7.0". you can use prop's such as 10x7 or 10x6 or 10x5 to start. what we are doing here is taking a heavy pitched prop at the tips and tricking it to work on f/f's
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Old 05-17-2002, 08:11 PM
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Default 101 continued

the next pic is the bottom of the prop after the de-pitching. you can use a razor blade to check the blade for true flat. hold the razor across the blade and look for light. this also works well to smooth out ruff spots, and flatten the blade by dragging it like a squeegee.
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Old 05-17-2002, 08:14 PM
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Rocket
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Default 101 continued

use a dremmel with the barrel to finish the hub.
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