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Old Feb 21, 2002, 08:01 AM
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Hervey Bay, Sunny Qld, Australia
Joined Jun 2001
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Watts per pound for a sports plane

I was reading about watts per pound somewhere and I have forgotten what the basic requirements were for a mild plane to a wild plane were. I have a low wing sport plane (like a SR X250 but more rounded and cuter {sorry SR Batteries: the X250 is a great plane too though}). I want it to be a good aerobatic flyer with ROG capabilities. It uses a graupner 2.33 : 1 gearing (like the x250) with the 7.2 volt motor (was to be a Permax 480 but I burnt it out ). With a Wattsmeter I was getting about (I think) 64 watts MAX only, which is a bit concerning as I feel it will be underpowered (my reason for the Permax 480), but I have to reinstall the sp400 anyway. I feel if I try to keep the plane as light as possible I might get away with it. So what's the deal with the Watts per pound???

Later
G
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Old Feb 21, 2002, 09:05 AM
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RPatrick's Avatar
Hurst, Texas USA
Joined Jan 2002
704 Posts
Here is the scoop, from the Electrajet review on Ezone:


25 watts-per-pound - airplane is typically hand-launched and won't do much more than just fly around
50 watts-per-pound - airplane is hand-launched, would ROG off of hard surfaces, and perform mild aerobatics
75 watts-per-pound - airplane would ROG off of a grass field, climb well, and perform nice aerobatics
100 watts-per-pound - airplane would nearly go straight up, can handle all the aerobatics

I think most people use the watts from the battery rather than the watts to the prop for this guideline.

RPatrick
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Old Feb 21, 2002, 10:30 AM
RIP Ric
Andy W's Avatar
Marietta, GA
Joined Jun 1999
43,312 Posts
Yes, watts in is generally used. For more efficient motors, of course, this means more watts out for the same input power, and better performance.
For reasonable (means good aerboatics) performance, 75Wlb with a brushless (or other efficient) motor works well, but for 'internal combustion'-like performace, look in the 100W+ range.
I wonder if you're using 8 cells? You should be running 10 with that motor, or 8 with a 6V motor.

For example: My X250 weighs 'about' 30oz (and flies very well at that weight) with a Kontronik 400/22 motor, 8xCP1300 and a 7x4 and 8.5x5 prop - the 8.5x5 gives unlimited vertical, although the peak current is well beyond rated capacity for this motor (at close to 40A), you rarely use it. The 7x4 is 'within specifications', and a good all around prop.. ROG in about as much time as it takes to advance the throttle beyond about 50%..
Point is, this combo is about 150W/lb peak.
..a
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Old Feb 21, 2002, 12:24 PM
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BURNOUT's Avatar
Westford,Ma
Joined Oct 2001
65 Posts
I just got my x250! What did you cover it with?? What kind of servos? Have not started building yet, just gathering parts!
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Old Feb 21, 2002, 06:33 PM
RIP Ric
Andy W's Avatar
Marietta, GA
Joined Jun 1999
43,312 Posts
Goldburg transparent red (all the LHS had) and gold monokote, and HS60's, per SR.
See it at my RC site (liked to from my profile)..
..a
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Old Feb 22, 2002, 03:40 AM
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Hervey Bay, Sunny Qld, Australia
Joined Jun 2001
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My powerplant is as such

Hi Andy
Just to let you know that I'm running a 9 x 6 APC light prop with the Graupner set up (with the 7.2v motor). I have tried 10 cells with this but it really heats up the motor (this is on a static test and NOT in flight, so my problem may be there) so I decreased it to 8 cells to be on the safe side. Do you think I should increase the cell count anyway? My planes wingloading is about 10 oz / foot (so lower than the X250). Also do you think that the heating up of the motor is due to the prop used? I've heard that everybody uses 10 cells with the X250 and they don't have any problems with the motor heating up (so I'm wondering why I'm having this trouble). Thanks for your time

Later
G

PS: nice planes from your website. I'm VERY JEALOUS of the amount you have (I only have 5 at the moment). They all look great. I have an Robbe Calibra Pro model that I've added a Robbe Pro Sydney 536/3 motor (10 cell) to and that generates 236 watts with a 50 amp draw (but this is only with a 7 cell battery, which is soon to be changed to a 10 cells as normal). The wingspan of the Calibra is 240cm instead of the 210cm so it should be interesting on the 1st flight.
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Old Feb 22, 2002, 03:49 AM
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Radek Z's Avatar
Pampigny, Switzerland
Joined Oct 2001
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The Watts guideline (25,50,75,100) is static or in flight?
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Old Feb 22, 2002, 04:22 AM
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Hervey Bay, Sunny Qld, Australia
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Hi Rzitek,
My watts reading were static (I'm not sure how to measure this in flight, though I'm sure there is a way).

Later
G
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Old Feb 22, 2002, 04:39 AM
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Radek Z's Avatar
Pampigny, Switzerland
Joined Oct 2001
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Well, I was more thinking what do I look at in MotoCalc when I look at the motor performance without the airrame.

BTW: I heard about a data loggers which can store rpm, volts in onboard memory during the flight. Not sure it can do amps.

This brings another question I had recently. I have seen nice amp meters in the electronic part scatalogues. They do not need connecting to the cables, they just need to be "clamped" around the cables. I think Graupner is selling one like that. This sounds great and very easy to use. Does anyone have any experience with this kind of meter?
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Old Feb 22, 2002, 05:13 AM
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Pampigny, Switzerland
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for those of you reading german ... this is what i meant
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Old Feb 22, 2002, 08:56 AM
RIP Ric
Andy W's Avatar
Marietta, GA
Joined Jun 1999
43,312 Posts
10 cells is appropriate, and what everyone (who has flown the SR suggested power system) else is flying with. Static tests are NOT an indication of how it will perform.. I used 9x5, and liked it, you may try that..

Don't be envious of my models - if you want a credit card balance like I have, you can have such a fleet also!
..a
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