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Old Feb 11, 2013, 05:12 PM
Crashing into the sky!
jackosmeister's Avatar
Auckland NZ
Joined Aug 2007
7,143 Posts
Thats not the best advice ever sorry


Pitchspeed is part rpm, part pitch.

If you have a massive prop, with very low pitch, turning very slowly, your plane aint gonna fly so flash.

Most airframes/props wont exceed ~90% of pitch speed, so if your powerplant is making a pitch speed of 30mph, the prop essentially becomes an airbrake at around 27mph.

Static thrust is a fairly useless number as well, when Im flying my plane is moving

As an example - a 10x8 prop might make a lot more thrust at 40mph than 11x5 does, However a 11x5 might make much better thrust at 10mph.

Think of pitch as akin to gears in your car, low pitch is first, high pitch is 5th (or 6th if your flash)

Some experience comes into play with what sort of pitch speed you should be aiming for, but educated guesses are very possible, ignoring pitchspeed is silly. You wouldnt try drive your car down the highway in 1st.

Excessive pitchspeed is useless as well, if your not making enough thrust to over come drag, you'll never approach that ~90% of pitchspeed figure.

Its about finding the right balance for the model, sometimes that involves trial and error with various props, but you can normally get ball park figures to start with.


My thermal glider runs a 13x11, at low rpm through a gearbox, and it hauls itself vertically easily. An 18x21 can still accelerate from a standstill pretty well as well...
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Old Feb 11, 2013, 05:18 PM
rip
ripacheco's Avatar
United States, FL, Niceville
Joined Jan 2013
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Quote:
Originally Posted by jackosmeister View Post
Thats not the best advice ever sorry


Pitchspeed is part rpm, part pitch.

If you have a massive prop, with very low pitch, turning very slowly, your plane aint gonna fly so flash.

Most airframes/props wont exceed ~90% of pitch speed, so if your powerplant is making a pitch speed of 30mph, the prop essentially becomes an airbrake at around 27mph.

Static thrust is a fairly useless number as well, when Im flying my plane is moving

As an example - a 10x8 prop might make a lot more thrust at 40mph than 11x5 does, However a 11x5 might make much better thrust at 10mph.

Think of pitch as akin to gears in your car, low pitch is first, high pitch is (5th, or 6th if your flash)

Some experience comes into play with what sort of pitch speed you should be aiming for, but educated guesses are very possible, ignoring pitchspeed is silly. You wouldnt try drive your car down the highway in 1st.

Excessive pitchspeed is useless as well, if your not making enough thrust to over come drag, you'll never approach that ~90% of pitchspeed figure.

Its about finding the right balance for the model, sometimes that involves trial and error with various props, but you can normally get ball park figures to start with.


An 18x21 can still accelerate from a standstill pretty well...
I see what you see...
Attempts to oversimplify the matter will fall short one way or another...
I need to "hit the books" and try to undrestand how this all relates to each other.

In the mean time I have been adviced that for the size motor and the glider I have the two props I picked are "reasonable".

Its me that wants to undrestand the "why" of everything that gets confused
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Old Feb 11, 2013, 05:23 PM
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Joined Apr 2012
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That's pretty much my theory, rip. It get's a little more complicated when make of prop and blade area come into play but, basically, static thrust is king with most sport planes. Case in point.

A club member and I had identical planes with identical engines. He used the 10X6 prop recommended by everybody and I went with an 11X4 that experience taught me worked better. As expected, I had better acceleration and vertical performance but the surprise was that my level flight speed was substantially greater than his. I had less pitch and a lower RPM but, due to greater blade area and efficiency, my plane was faster.

Of course, this becomes less true when dealing with much bigger props and low kv motors but that's another story.
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Old Feb 11, 2013, 06:09 PM
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That's all true, jackosmeister. However...

At the speeds attainable by an average 2 meter sport glider low speed thrust or static thrust is king. A higher pitch speed doesn't do one much good if the plane can't climb at it's best efficiency. After all; all that we are really interested in for sport gliders is climbing speed. "Better" thrust at speed doesn't do a whole lot of good if one has to fly level for a few seconds to get there and then the speed drops off once maximum climb rate is reached. Case in point.

I have a Specter 1800 with a @250 watt motor. With a 9.5X8 prop I have to fly level for a couple seconds before I pull back and then I have to keep the climb angle at around 70 degrees or the rate of climb suffers. My best time to height is around 12-15 seconds. With a 10X6 prop I can give it a good heave at around 70 degrees and immediately pull into the vertical. Climb to height is about 10-12 seconds and it's still accelerating when I push over at the top. With an 11X6 prop I could simply release it pointing straight up and it will take off like a 3D plane but the motor gets hot and it's a bit slower at the top than with the 10X6 because I can't get those last few RPM that I achieve with the 10X6. Climb to height is identical to the 10X6 but I don't have to throw it as hard to "pull it out of the mud".

To put it very simply: blade area and RPM are more important than pitch speed with a 2 meter sport glider and a direct drive motor with a moderate kv rating.

A good place to start is with the biggest blade one's motor can spin efficiently with a 6 inch pitch. Then one begins experimenting as one gets some results from an individual plane. If I am disappointed in the thrust I increase diameter, even if I have to drop in pitch, rather than pitch. If the plane reaches it's maximum vertical speed before it gets as high as I want then I increase pitch. If I have to hold the nose down until I pick up some speed then I decrease pitch.

And, as I've said, when one steps up in prop size and runs with lower kv ratings then more pitch becomes more desirable.
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Old Feb 11, 2013, 06:33 PM
Registered User
Chico, California USA
Joined Mar 2003
462 Posts
"Measure with a micrometer, mark with chalk and cut with an ax"

I have followed this thread closely and have learned a number of new ideas. However it seems that a number of points are at the 'micrometer' level when the best that can be expected is an 'ax' result. For example let me propose/ask the following: What is the effective prop diameter for a Graupner CAM folding 11-8 prop when performing these calculations?

Consider that:
-Pin to tip on a single blade is 128mm
-From both pins (one shaft) to both tips is 256mm
-11" converted to metric is 279mm
-tip to tip on a 29MM spinner is 287mm
-tip to tip on a 50mm spinner is 313mm

With this amount of variation the best that most calculations can do is get you in the ballpark. I agree that experimentation is the best way to get maximum prop performance.

Wayne
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Old Feb 11, 2013, 06:50 PM
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The best answer I can give you, Salmon-Run, is that blade area is generally the most important variable. The length of the prop yoke does make a "micrometer level" difference because there's a change in the tip speed of the prop but, if I were that good, I would be getting thousand dollar planes from my sponsors rather than spending hundreds of dollars myself. The change in overall length with different spinners/yokes just isn't a big deal for a sport flier.
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Old Feb 12, 2013, 10:26 AM
rip
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United States, FL, Niceville
Joined Jan 2013
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Wingspan: 78.5 in (2000 mm)
Wing Area: 676 inē (44 dmē)
Weight (*): 30 oz (850 g)
Wing Loading (*): 6.5 oz/ftē (20 g/dmē)
Fuselage Length: 39.25 in (1000 mm)
Motor: Scorpion SII 3008-1090
ESC: Scorpion 15V 35A
Batt: ThunderPower Pro Lite G6 TP2700 3S
Prop 1: Aeronaut FOLD 10 x 6
Prop 2: Aeronaut FOLD 11 x 7

(*) doesn't include power system: motor/esc/batt/prop...

Thinking about ordering some props in between for some "scienfic testing"

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Old Feb 12, 2013, 11:05 AM
rip
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United States, FL, Niceville
Joined Jan 2013
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I used Scorpion Calc to crunch some numbers.

I kept the motor and ESC constant and then went through all the Aeronaut propellers in the Scorpion Calc library starting with the "most efficient" prop and moving up (in size and pitch) to the limits of the motor.

I find it intersting how Scorpion calc deems the 8x5 prop the "most efficient" of the bunch.

I'm so tempted to order all those props and run tests to see how it all behaves...
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Old Feb 12, 2013, 11:28 AM
rip
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United States, FL, Niceville
Joined Jan 2013
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Quote:
Originally Posted by Salmon-Run View Post
"Measure with a micrometer, mark with chalk and cut with an ax"
I love that quote... I'm gonna use it...

We may be talking of a hobby here. But I can see that quote being applied to my line of work... (capturing information from under-the-pavement sensors)
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Old Feb 12, 2013, 11:51 AM
Ascended Master
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Palmdale, CA
Joined Oct 2000
13,341 Posts
I just put a Graupner Speed 600 direct drive with a Graupner 8x4.5 folder into the fuselage for my Spirit Elite, which I've put the wing from the ROC 2000 which now has upper and lower surface spoilers on. ISTR that prop and motor worked well way back when.
Weather permitting... flights today with that and the Easy Star with the flapper/spoilers and a GL with spoilers.
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Old Feb 12, 2013, 12:51 PM
rip
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United States, FL, Niceville
Joined Jan 2013
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Quote:
Originally Posted by Sparky Paul View Post
I just put a Graupner Speed 600 direct drive with a Graupner 8x4.5 folder into the fuselage for my Spirit Elite, which I've put the wing from the ROC 2000 which now has upper and lower surface spoilers on. ISTR that prop and motor worked well way back when.
Weather permitting... flights today with that and the Easy Star with the flapper/spoilers and a GL with spoilers.
Do you know the RPMs u were getting when you tested?
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Old Feb 12, 2013, 01:44 PM
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Palmdale, CA
Joined Oct 2000
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Don't know. It worked well. Draws 13 amps. Current draw is more important.
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Old Feb 12, 2013, 02:04 PM
rip
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United States, FL, Niceville
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Originally Posted by Sparky Paul View Post
Don't know. It worked well. Draws 13 amps. Current draw is more important.
Awesome pictures! Thanks for the input. I'm learning from you all.
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Old Feb 12, 2013, 02:39 PM
Crashing into the sky!
jackosmeister's Avatar
Auckland NZ
Joined Aug 2007
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Quote:
Originally Posted by ripacheco View Post
Do you know the RPMs u were getting when you tested?
Quote:
Originally Posted by Sparky Paul View Post
Don't know. It worked well. Draws 13 amps. Current draw is more important.
I give up
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Old Feb 12, 2013, 02:53 PM
The Sequel
mustflynow2's Avatar
Australia, VIC
Joined Sep 2011
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Originally Posted by jackosmeister View Post
I give up
Take heart, your post at the top of the page was very informative, thankyou.
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