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Old Feb 12, 2013, 03:20 PM
rip
ripacheco's Avatar
United States, FL, Niceville
Joined Jan 2013
1,655 Posts
Interesting that Scorpion Calc considers the Grauper 8x4.5 the "optimal" prop for the SII 3008-1090 motor...

I think most people use this or that prop because of his personal experience or because others use that "kind of prop on that kind of airplane". Others go further and "try" several props and find the ones that "work best".

That is all great. We all learn from that. Yet there is science behind this. And the science escapes most of us (especially me). Or most likely "the science doesn't matter if I got it to work"

That is why i made the comment "like a college course" earlier in this thread.

All that said. I'm learning a lot from all this.

My wife just shakes her head when she sees me "plugging numbers" into the program.
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Old Feb 12, 2013, 03:37 PM
Ascended Master
Sparky Paul's Avatar
Palmdale, CA
Joined Oct 2000
13,472 Posts
Quote:
Originally Posted by jackosmeister View Post
I give up
.
Practice trumps numbers with this stuff.
Letting the smoke out is not a good thing.
Drawing too much current will release the smoke hidden in the battery, or the receiver, or the ESC, or the motor.
Any of these can be expensive.
Without any ammeter to let you know what the current draw is, smoke will follow, regardless of how often one waves the calculations at the complete system.
Calculations are merely a guideline, and cannot include the physical variables for any particular setup that can render the calculations moot, as the smoke comes out.
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Old Feb 15, 2013, 03:18 PM
rip
ripacheco's Avatar
United States, FL, Niceville
Joined Jan 2013
1,655 Posts
I talked with a scale fellow today (another one of my interests)

Fairchild PT-19
20lbs
18x8 Prop at 7,800 RPM
Generates 16lbs of thurst that is 80% of model's weight.

Since it rotates at 7800 RPM it generates enough thrust and pitch speed to fly the scale model without difficulty in the maneouvers expected of this model.

He specifies model weight, propeller size and RPM with his plans. That makes life so much simpler to pick a proper power system.

oh yea... don't forget the AMPs the beast is sucking out of your poor LIPOs...
Granted no 3D is expected or desired on this model.

Gliders need to climb fast in short time. What did we mentioned before in the thread? 70 degree climb for a competition glider? In that sense gliders behavig a bit more like a 3D model than the Scale bird mentioned above.

In either case static thrust is an important measure... without forgeting the other variables.
Dynamic thrust is much more difficult to estimate with the tools we have on hand...

Now if we put an airspeed sensor, and RPM sensor on the plane... hummm...

Of course... don't forget the AMPs that the power system is sucking of the poor LIPOS...
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Last edited by ripacheco; Feb 15, 2013 at 03:24 PM.
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Old Feb 15, 2013, 09:36 PM
Crashing into the sky!
jackosmeister's Avatar
Auckland NZ
Joined Aug 2007
7,327 Posts
Speedfire F5B + Jeti Phasor 2026/3600 (2 min 14 sec)


My 2m glider with 18x21 prop. Sorry for the swearing, was setting up the throttle ramp on this flight.
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Old Feb 16, 2013, 06:14 AM
rip
ripacheco's Avatar
United States, FL, Niceville
Joined Jan 2013
1,655 Posts
Quote:
Originally Posted by jackosmeister View Post
http://www.youtube.com/watch?v=OLPh0-h6gP8

My 2m glider with 18x21 prop. Sorry for the swearing, was setting up the throttle ramp on this flight.
Hotliner! What power setup u have on it? (besides da prop)
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Old Feb 16, 2013, 12:40 PM
Crashing into the sky!
jackosmeister's Avatar
Auckland NZ
Joined Aug 2007
7,327 Posts
Jeti phasor on 6S, 6.6kW.

Point is, you dont need to be stuck in 1st gear to accelerate.
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Old Feb 18, 2013, 10:02 AM
Grow old disgracefully!
dickw's Avatar
Luton, UK
Joined May 2006
1,188 Posts
Quote:
Originally Posted by ripacheco View Post
...........
Gliders need to climb fast in short time. What did we mentioned before in the thread? 70 degree climb for a competition glider? .......
It depends on the competition.
F5B gliders climb vertically accelerating all the way up - as seen in Jackosmeister's excellent video.
F5B is probably at the extreme end of electric gliders and the models (like his Speedfire) and his props (probably a GM) are built to withstand the forces generated.
The motor runs are so short the battery and ESC don't really have time to overheat - I have used 25/50C batteries at 100C peaks, and they lasted several seasons.

Data log diagram from an F5B competition glider at launch attached as an illustration of what is possible if you really want it. A motor run of less than 3 seconds puts the 2m Avionik glider at an altitude of 100+m doing close to150mph. (Jeti Phasor 2035/2100, GM 18x23 prop, 10s 1800 Lipo, YGE ESC).

Not many people need this sort of performance (it is great fun though ), but note how the power drops from 5800 watts to about 3500 watts as the plane accelerates. This is an extreme example of what also happens at lower powers, but is most noticeable with higher pitch props.

Quote:
Originally Posted by jackosmeister View Post
Jeti phasor on 6S, 6.6kW.
Point is, you donít need to be stuck in 1st gear to accelerate.


Dick
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