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Old Oct 04, 2010, 09:24 AM
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Trying to find a motor to get 1350 grams trust with a 7" prop (space restriction), and no success.
Anyone have a hint ? Three blades welcomed
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Old Oct 04, 2010, 09:42 AM
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Originally Posted by RENATOA View Post
Trying to find a motor to get 1350 grams trust with a 7" prop (space restriction), and no success.
Anyone have a hint ? Three blades welcomed
How about the specs on the plane?
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Old Oct 04, 2010, 09:48 AM
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Should it matter ?
Is the thrust required for a glider different than thrust required for a 3D ?
I tried with webocalc, not able to solve my equation.
Is a big 2800mm glider, 1600 grams AUW, 3s power, and 40A max ESC.
The 7" prop is definitelly the bottleneck, because is a kind of pylon plane, like EZ*.
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Old Oct 04, 2010, 09:58 AM
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Originally Posted by RENATOA View Post
Should it matter ?
Is the thrust required for a glider different than thrust required for a 3D ?
I tried with webocalc, not able to solve my equation.
Is a big 2800mm glider, 1600 grams AUW, 3s power, and 40A max ESC.
The 7" prop is definitelly the bottleneck, because is a kind of pylon plane, like EZ*.
Wing area? Or average width of the wing?
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Old Oct 04, 2010, 11:03 AM
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44dmp
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Old Oct 04, 2010, 11:37 AM
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Here ya go...
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Old Oct 04, 2010, 11:41 AM
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Here ya go...
Wow, is it safe to spin that prop at that required RPM?
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Old Oct 04, 2010, 11:56 AM
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Wow, is it safe to spin that prop at that required RPM?
Hi richie,

WebOCalc won't let you pick a prop that is unsafe. It's one of the nice features it has...

Chuck
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Old Oct 04, 2010, 11:58 AM
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Thank you, I never would have believed it but there is a first time for every occasion.
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Old Oct 04, 2010, 12:03 PM
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Th only thinkg that is different is the top speed, for me was 68hm/hr, as suggested by webocalc, because my chosen model was gentle scale, not unlimited aerobatics.
Even if I switch to fast unlimited aerobatics, the suggested top speed is 87hm/hr.
Why so big top speed in your case? And, isn't it unrealistic for a glider ?

Rev, the RPM is not so big, more than 20k RPM can be found even for EZ* migrated to BL, with 6" prop.

What is the equivalent 3 blades prop for a two blades ? What about the folding props, are they 100% equivalent to a fixed blade prop ?
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Old Oct 04, 2010, 12:06 PM
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Originally Posted by RENATOA View Post
Rev, the RPM is not so big, more than 20k RPM can be found even for EZ* migrated to BL, with 6" prop.
This is all new to me, Thank you.

I knew EDFs turned some very fast RPMs, but nothing like this.
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Old Oct 04, 2010, 12:18 PM
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This is all new to me, Thank you.

I knew EDFs turned some very fast RPMs, but nothing like this.
Richie,

APC Thin Electric, or "E" props have a safe operating rpm of 190,000/diameter.

Let's calculate the safe rpm for an APC 7" E prop then....

190,000/7 = 27,142 rpm

For a 10" APC "E" prop it would be 190,000/10 = 19,000 rpm

See how it works?

Chuck
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Old Oct 04, 2010, 12:23 PM
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Why so big top speed in your case? And, isn't it unrealistic for a glider ?
It is VERY unrealistic for a glider! But the problem is we are working within the 7" prop you said was the physical limit. In order to generate your required 1300+ grams of thrust, we have to spin that 7" prop like crazy. And spinning it like crazy means the plane is going to have a wicked top speed. Very inefficient!

In fact, it's so inefficient, that WebOCalc doesn't even want to compute the combo, and we must rely on manipulating numbers just to get it to work. WebOCalc is a model of efficiency, and when it sees that we want to use a 7" prop on a 110 inch wingspan, it thinks we are absolutely nuts for trying to do so.

Chuck
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Old Oct 04, 2010, 12:33 PM
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Richie,

APC Thin Electric, or "E" props have a safe operating rpm of 190,000/diameter.

Let's calculate the safe rpm for an APC 7" E prop then....

190,000/7 = 27,142 rpm

For a 10" APC "E" prop it would be 190,000/10 = 19,000 rpm

See how it works?

Chuck
Yeap, makes good sense. Thanks
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Old Oct 04, 2010, 01:16 PM
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Yeap, makes good sense. Thanks
For APC Slow Flyer props, the magic number is 90,000/diam instead of 190,000/diam

For GWS RS props (the ones commonly referred to as slo-flyer), the magic number is somewhere between 40,000/diam and 50,000/diam

A 10" GWS RS prop would have an absolute maximum of around 4000-5000 rpm

Chuck
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