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Author Topic: How are models usually weighed?  (Read 502 times)
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MrTin
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« on: June 25, 2015, 12:33:03 PM »

Sounds like a dumb question and maybe it is, but I see people talking about weights and it's usually without motor and battery.  My motor is OK for up to 500 grams.  Is that with or without motor and batteries?
My model weighs 504 grams WITH motor and battery, so I'm guessing it should fly, yes?........

Thanks,
MrTin
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Pops
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« Reply #1 on: June 25, 2015, 02:16:43 PM »

Yes, it's always a question about total weight, ready to fly.

You can say that when a motor is rated "from- to-", it means that below the "from-" weight, this motor is considered to be overkill, (more like a motor with an airplane, than an airplane with a motor, so to speak...) but the more you approach the "to-" weight, the more sluggish and underpowered the model model will perform. Then again, this is also depending on what type of airplane you have. With the same motor, a trainer with lots of wing area and can fly slower and better than a scale model with less wing area even though the models weigh the same. This is because of wing loading; total weight divided by wing area. The higher the wing loading, the more speed (=more power) the plane needs to stay in the air. The wing profile (lift) is also a part of the equation.

Hope this helps you, may all your landings be in one piece! Smiley

Brg

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slipstick
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« Reply #2 on: June 25, 2015, 02:21:29 PM »

The only useful weight is the AUW (All Up Weight) i.e. the weight ready to fly.

If a motor "is OK for up to 500 grams" then that's the weight that's meant. But even at 4g over it should probably still stagger into the air provided you have the right prop/battery etc. The choice of those is at least as important as the motor itself. It's the prop that flies the plane, the motor just turns it Wink.

Steve
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MrTin
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« Reply #3 on: June 25, 2015, 05:49:15 PM »

Thanks for that.  I think the only way is to just do it and see what happens.  The overwhelming usage of prop with that motor seems to be a 7x5.  So I'll order one with a 3S Lipo and see what happens.  I tried to test the CoG today and I think it'll need a lot of weight to get that right, so I might try lengthening the nose, rather than add dead weight.  I've seen that done successfully.

Cheers,
MrTin
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p40qmilj
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« Reply #4 on: June 25, 2015, 06:19:46 PM »

 Grin  its all about wing loading.  the commonly accepted wing load for rubber planes is .5 gram/inch squared so if you have a wing which is 25 inches span  4 inch chord, you will have 100 sq inch wing area and that can handle up to 50 grams with ease.  i have flown up to 70 grams on this wing area but but of course they need the power to stay airborne etc. just my 2 cents worth
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TimWescott
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« Reply #5 on: June 25, 2015, 06:40:19 PM »

With battery and motor.  Or engine and fuel.  Unless it's control line aerobatics for the purposes of the pull test -- then it's battery and motor, or plane + engine less fuel.  Go figure.
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Dave Andreski
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« Reply #6 on: June 25, 2015, 07:07:36 PM »

Sounds like a dumb question and maybe it is, but I see people talking about weights and it's usually without motor and battery. 

Thanks,
MrTin

Tin,
You have an example of people talking about weight w/o motor and battery?
Dave
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MrTin
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« Reply #7 on: June 26, 2015, 05:52:36 AM »

Many times I've seen someone, in answer to a "will  it fly" type of question, ask "what is the weight of your model without motor and Lipos?"  I thought it rather odd at the time as the motor obviously has to lug itself and its power pack around.
I dare say the up to 500 grams statement is fairly arbitrary, hence the notion that "suck it and see" will be as good as any.  If not, then I'll have to get a more powerful motor.
On that score, is there an easy way of working out instantly what a motor's power is likely to be as there is with IC engines?

Cheers,
MrTin
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