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Old May 19, 2007, 10:55 PM
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jmendell's Avatar
Boston, MA
Joined May 2007
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Flying Weight / Lift / Motor Correlation

Does anyone have any formulas for calculating the motor(s) required for specific flying weights? Obviously wing lift is a big piece of it, but is there a general consensus for determining which motors and batteries are best for specific flying weights?

Reason I ask is I have been flying gas planes and recently converted to electric and FPV flying (have yet to take off or land, but getting there) and I ended up getting this plane. It's got brushless motors but i think they are on the "low-end" and need something stronger. I flew it today with lead fishing weights imitating weight and placement of my FPV gear and it *barely got off the ground with a decent head wind, and I just about lost it coming back in to land 60 seconds later.

Wondering if anyone has any suggestions on motor(s) to weight ratios.
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Old May 20, 2007, 03:45 AM
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What kind of FPV gear are you using?
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Old May 20, 2007, 09:24 AM
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Elmhurst, NY (Queens in NYC)
Joined Apr 2004
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Quote:
Originally Posted by jmendell
Does anyone have any formulas for calculating the motor(s) required for specific flying weights? Obviously wing lift is a big piece of it, but is there a general consensus for determining which motors and batteries are best for specific flying weights?

Reason I ask is I have been flying gas planes and recently converted to electric and FPV flying (have yet to take off or land, but getting there) and I ended up getting this plane. It's got brushless motors but i think they are on the "low-end" and need something stronger. I flew it today with lead fishing weights imitating weight and placement of my FPV gear and it *barely got off the ground with a decent head wind, and I just about lost it coming back in to land 60 seconds later.

Wondering if anyone has any suggestions on motor(s) to weight ratios.
I prefer LOTS of power. As long as you can stop the motor at low throttle, and the motors don't tear out of the mount at full power you are in a usable situation. The good part of that is that you can get to altitude very rapidly.

I've run half the capacity out of my pack just getting to an altitude where I could look around, so now I go for lots of power.

Don't worry about how long the batteries will last. As long as you can feed the motors at full power you will get normal run time since you will be throttled way back for normal cruise.

Of course you could get into a situation where you have enough power to rip the wings off or cause flutter if you go wide open in level flight or a dive, so you have to think about what you are doing.

Basically I am saying that you almost can't have too much power, and with normal throttle management you will still get lots of flight time.

Pete
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Old May 20, 2007, 02:38 PM
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Crawley, West Sussex, UK
Joined Jun 2004
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Try this web page:
http://www.eflightwiki.com/eflightwi..._Power_Systems
Here's the key bit, but the rest is relevant.

How much power do we need?

The simplest approach to figuring power systems in electrics is input watts per pound of "all up" airplane weight. The following guidelines were developed before brushless motors were common but it seems to hold pretty well so we will use it regardless of what kind of motor is being used.

* 25 W/lb = minimum for level flight, with a reasonably clean plane.
* 50 W/lb = Trainer/Casual/scale flying
* 75 W/lb = Sport flying and sport aerobatics
* 100 W/lb = aggressive aerobatics and mild 3D, effortless loops from level flight.
* 150 W/lb = all out performance.
* 200 W/lb = Unlimited high-speed vertical flight.

Remember that Watts = Volts X Amps. This is a power measurement. In case you were wondering, 746 watts equals 1 horsepower. (If you've ever been near a horse you'll know that it's far less power than a horse can provide; the original measurement was supposed to equate to the amount a work a pit pony could do all day, and it was set by the makers of steam engines, who were understandably biased.)
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Old May 20, 2007, 02:54 PM
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jmendell's Avatar
Boston, MA
Joined May 2007
70 Posts
Quote:
Originally Posted by JMSTECH
What kind of FPV gear are you using?
RangeVideo 900mhz transmitter (12v)
KX151 cam (12v)
ThunderPower 11.1v 850mah LiPoly

No pan/tilt..
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Old May 20, 2007, 03:00 PM
Awesometown
jmendell's Avatar
Boston, MA
Joined May 2007
70 Posts
Quote:
Originally Posted by PeteSchug
I prefer LOTS of power. As long as you can stop the motor at low throttle, and the motors don't tear out of the mount at full power you are in a usable situation. The good part of that is that you can get to altitude very rapidly.

I've run half the capacity out of my pack just getting to an altitude where I could look around, so now I go for lots of power.

Don't worry about how long the batteries will last. As long as you can feed the motors at full power you will get normal run time since you will be throttled way back for normal cruise.

Of course you could get into a situation where you have enough power to rip the wings off or cause flutter if you go wide open in level flight or a dive, so you have to think about what you are doing.

Basically I am saying that you almost can't have too much power, and with normal throttle management you will still get lots of flight time.

Pete
Ok, I think I'll copy Crash9's setup except with my airframe, the only thing I'm worried about is the fact that the plane i got has literally half the wing area as a Twinstar II. When the motors are off it will fall like a rock.

Hmm.. I knew I should have listened to you guys on which airframe to use!

Thanks for the input guys...
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Old May 20, 2007, 04:16 PM
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Elmhurst, NY (Queens in NYC)
Joined Apr 2004
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Quote:
Originally Posted by jmendell
Ok, I think I'll copy Crash9's setup except with my airframe, the only thing I'm worried about is the fact that the plane i got has literally half the wing area as a Twinstar II. When the motors are off it will fall like a rock.

Hmm.. I knew I should have listened to you guys on which airframe to use!

Thanks for the input guys...
That may be a bit much for your plane. One motor like that might be plenty and one battery!

Hmm... you can stick the camera in the nose and use only the pusher motor. I'm kidding, but it is an interesting idea except that cameras are expensive and you don't want the first point of contact with the ground to be your camera!

Check out the 100 watts per pound for starters or maybe a bit more.

Pete
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Old May 22, 2007, 02:47 AM
Awesometown
jmendell's Avatar
Boston, MA
Joined May 2007
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Quote:
Originally Posted by PeteSchug
Check out the 100 watts per pound for starters or maybe a bit more.

Pete
Awesome - will do, thanks for the help fellas!
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Old May 22, 2007, 03:31 PM
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RobinBennett's Avatar
Crawley, West Sussex, UK
Joined Jun 2004
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Only a few years ago, 100 watts per pound was considered a fantastic achievement for electric power, now it's the minimum...

100w/lb is enough for very rapid (but not vertical) climbs and effortless aerobatics.
50w/lb is what you'd get in an electric trainer, enough for a gentle climb and trundling around in circuits.

So aim for 100, but if you get more than 50 it will fly OK.
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Old May 22, 2007, 06:38 PM
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Madrid, Spain
Joined Dec 2004
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With 50W per pound would fly, but each climb would be very very long. For planes around 1m it´s not much more expensive going to 80-100W or even 150W per pound, and the climb ratio can´t be compared. Much more fun and, if not longer flights, at least much more time to enjoy

Personally, I like 100-150W/lb (200-300W/kg), depending on how much overpowered I want it


Andres
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Old May 23, 2007, 08:14 PM
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Renton WA
Joined Oct 2001
825 Posts
It's important to also take into consideration what sort of motor you are using, a speed 400 at 100watts, spinning the push on prop isn't going to make anywhere near the same power that a axi gold 2212 26 with a 9X6 Apc can on 100 watts. There seems to be huge differences in efficency, esp with cheap outrunner motors. For most sport airframes, I like to have at least 60%, preferably 75% static thrust to weight ratio, with a prop speed at least in the 45-50 mph range.
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