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Old Oct 09, 2012, 03:50 PM
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Prop Comps?

So pardon the "newbie" question here, but I was kind of stuck when it came to appropriately defining the search terms I needed to find my answer...

Without getting into all kinds of technical scientific specifics, would it be safe to say that the product of a prop's ratio is a reasonable-ish number to use when making a prop-to-prop comparison?

In other words, would an 9x5 prop (=45) provide "about" 40% more propulsion than 8x4 prop (=32)? Whereas a 10x4.7 (=47)is pretty much just the same thing as a 9x5 (=45)

I've read going up a size and down a pitch is comparable, (8x4= 32 ~ 7x5=35) Is a 10% difference "relatively close" ?


I'm just trying to find an easy way to experiment with propellers without have to buy 6 different (and potentially unusable) items...
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Old Oct 09, 2012, 04:06 PM
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Trip;
Without getting into technical scientific specifics, it would not be safe to say that the product of a prop's ratio is a reasonable-ish number to use when making a prop-to prop comparison.
Regards
Rick
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Old Oct 09, 2012, 04:21 PM
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Perfect answer! Thank you

Now to do some math-heavy research
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Old Oct 09, 2012, 04:28 PM
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Hi trip,
Just like wings thrust (or lift) is also dependent on speed through the air, angle of attack and blade (airfoil) thickness.

If you are assuming the same(ish) revs, in other words a straight substitution of props of the sizes you mention and they are of the same type or manufacture, you probably won't be far different over-all, BUT remember that the current draw of a prop of higher pitch will draw more current on the bench until it 'off-loads' in flight. The benefit will be of a higher potential airspeed once flying.

Also don't forget that propellers, like wings, can also stall...
and thin plastic props are more suited to high revving motors than wooden ones (which are more efficient at lower revs)... all of which should make clear that you will probably build up quite a collection of props in your modelling career. Most of us do!

If playing around in this area it is well worth getting yourself equipped with a watt meter. It doesn't have to be an expensive item, but so long as it is reasonably accurate it will save you a lot of money in motors and speed controllers that you DON'T burn out!

Best wishes,
Robin
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Old Oct 09, 2012, 04:59 PM
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I personally just plug the numbers into a motor calculator like eCalc and let it do the maths, but there was a recentish thread on prop equivalency rules of thumb.

The whole thread is here: http://www.rcgroups.com/forums/showthread.php?t=1698408, but the rule of thrumb is at post#10 from jrb:

"Old RCM RoT -- for same "load": 2D+P >> 8x6 = 22 = 9x4! "

I was pretty impressed by how close it worked out to both practical experience and the calculator results.
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Old Oct 09, 2012, 05:02 PM
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Quote:
Originally Posted by eye4wings View Post
Hi trip,
Just like wings thrust (or lift) is also dependent on speed through the air, angle of attack and blade (airfoil) thickness.

If you are assuming the same(ish) revs, in other words a straight substitution of props of the sizes you mention and they are of the same type or manufacture, you probably won't be far different over-all, BUT remember that the current draw of a prop of higher pitch will draw more current on the bench until it 'off-loads' in flight. The benefit will be of a higher potential airspeed once flying.

Also don't forget that propellers, like wings, can also stall...
and thin plastic props are more suited to high revving motors than wooden ones (which are more efficient at lower revs)... all of which should make clear that you will probably build up quite a collection of props in your modelling career. Most of us do!

If playing around in this area it is well worth getting yourself equipped with a watt meter. It doesn't have to be an expensive item, but so long as it is reasonably accurate it will save you a lot of money in motors and speed controllers that you DON'T burn out!

Best wishes,
Robin
Could you recommend a lower priced watt meter ? I have just stayed with stock props,as i keep reading about needing a watt meter to test when changing prop sizes on specific motors.
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Old Oct 09, 2012, 06:45 PM
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Could you recommend a lower priced watt meter ? I have just stayed with stock props,as i keep reading about needing a watt meter to test when changing prop sizes on specific motors.

Just ordered HobbyKing one for $23

Hopefully it comes off of back order sometime soon... I never ordered from them before; anyone know if they typically get restocks? I've noticed a lot of RC sites never seem to get a replenishing

Thanks for the info!
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Old Oct 09, 2012, 09:57 PM
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Trip;
Hobbyking
"Stock updated frequently.
Goods will be avaliable within 30 days"
Then, expect about 2 weeks for ship time.
Be patient.
If you write or call, be respectful, thank whomever you speak with.
They will assist you as best they can and name their firstborn after you.
Do not go Rambo on them
Regards
Rick
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Old Oct 10, 2012, 07:46 AM
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Ah really Rick? I wonder if the '30 days' applies to the Turnigy 9X system I have had my eye on since last Christmas and is perpetually marked as on backorder. No doubt that's due to high demand, but I assumed they just couldn't get them any more.
Is it worth just ordering a set and waiting do you think?
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Old Oct 10, 2012, 12:51 PM
buyer of the farm
United States, FL, DeLand
Joined Mar 2009
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Math won't help you. Take six 7x4 props and measure their wide open RPM, thrust, watt draw and motor temp and you'll get six wildly different sets of numbers. Why? The University of Illinois tested over 130 hobby grade commercial props and found their quality varied wildly from a terrible efficiency cooefficient of .28 to about .65. That means that keeping the diameter and pitch the same you can have a 100% difference in performance.

Therefore, the quality of the prop makes even more difference than the pitch and diameter. Therefore all the math based on diameter and pitch goes out the window in favor of test results, which alone will tell you the correct information.

Web calculators or downloaded programs? If they don't work based on manufacturer and model number they're junk.
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Old Oct 10, 2012, 12:54 PM
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Quote:
Originally Posted by eye4wings View Post
Ah really Rick? I wonder if the '30 days' applies to the Turnigy 9X system I have had my eye on since last Christmas and is perpetually marked as on backorder. No doubt that's due to high demand, but I assumed they just couldn't get them any more.
Is it worth just ordering a set and waiting do you think?
it is in stock right now and lower price
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Old Oct 10, 2012, 01:50 PM
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JetPlaneFlyer's Avatar
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Quote:
Originally Posted by tripn4days View Post
Perfect answer! Thank you

Now to do some math-heavy research
No need for math heavy research to get the basic concepts.
  • The first number is diameter. In general terms diameter diameter is related to thrust. larger diameter gives more thrust, smaller diameter less thrust. if you don't mind betting slightly technical the relationship is squared, e.g. if you double diameter you quadruple thrust.
  • The second number is pitch. Pitch dictates how fast the plane can go before the prop stops making thrust, so as a general rule; more pitch = more speed.

So it's wrong to conclude that you can compensate for diameter by adding pitch, or vice-versa.
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Old Oct 10, 2012, 05:54 PM
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Quote:
Originally Posted by eye4wings View Post
Ah really Rick? I wonder if the '30 days' applies to the Turnigy 9X system I have had my eye on since last Christmas and is perpetually marked as on backorder. No doubt that's due to high demand, but I assumed they just couldn't get them any more.
Is it worth just ordering a set and waiting do you think?
I Think:
Giving out money for something that may not be available is unwise.
Raises one's stress level.

Establish a HobbyKing account (membership)
It allows me to put interesting things into my wish list (72 as-of today)
A checkbox tells HobbyKing (HK) if I want to be e-mailed when item is back in stock.

When I want something from HK, I "Buy Now" (add item to "Cart").
When I check out, double-verify the item is NOT BACKORDERED
Pay with PayPal.
If BACKORDERED, delete item from "Cart" check box in wishlist.

I generally get the parcel in 14 days. May take longer.
HK and I have no control over:
Hong Kong post office
Contracted shipper
US Customs

When I communicate with HK, I ask for their help.
I am polite with them, and thank them for helping me with my problem.
I find they are eager to please and like to be appreciated.

I have never had a bad experience while following the above.
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Old Oct 11, 2012, 02:48 AM
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eye4wings's Avatar
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Originally Posted by fastfwd View Post
it is in stock right now and lower price
Thanks fastfwd, I had a look this morning... should have mentioned that I fly mode2 !
Mode2 version still shown on backorder but I went ahead anyway (well. it's cheap enough!) and after I paid saw it marked as 'in stock'. Don't know what to believe now! (but at least there's a fair chance it'll be here before this Christmas!

Thanks to Rick also for cautious and wise advice. Been following that all year but it got me nowhere! Time for a shot in the dark!
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