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Saturday 7/7/18

2018-07-07, 21:07 by Gary M Jones

I was at the field today between 14:00 & 15:00 all on my own , good flying too. There is a dead sheep along the fence line towards the gate from the pits, I saw the farmer so reported this to her. I hope no one had plans for a BBQ Smile .

Farmer …

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Electric Powertrains.

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Electric Powertrains. Empty Electric Powertrains.

Post by Andy Sayle on 2010-07-08, 15:51

Afternoon guys,

Work is sending me to sleep, writing reports etc, so I thought I would do something to get the brain working a bit, and get the wallet shaking in fear! I have got some lipo packs that are gathering dust, and I quite fancy something to put them in, so I thought I would go through a quick set of calculations to work out what sort of model I could put them, then hopefully find a suitable model, and fully spec the powertrain from there. All to show you how easy it is to spec electric powertrains Smile

So, the packs I have are 5s lipo packs of 5000mAh (5Ah) capacity. They are rated to 20C max discharge, so that gives 5Ah*20C = 100Amps maximum discharge. Being 5s packs, their voltage ranges between 15v and 21v (depending on whether they are fully charged, or fully discharged). underload, they will hold approx 3.3v per cell, or 16.5v in total. So the max power these cells can deliver, is 16.5v * 100Amps = 1650 watts.

So, what sort of model could they power? well the first thing to look at, is power to weight. For a glider, I could get away with a power to weight of about 50-100 watts per lb. For a sporty aerobatic thing I could get away with 100-200 watts per lb. For a mental speed machine, or insane 3D ship, I would want more!.

So, I want mental. I like mental, mainly because I am rather familiar with being just so. So, I shall go for 300 watts per lb. So the approx. all up model weight will be 1650W / 300w/lb = 5.5lb, or 2.5Kg Napolean.

You with me so far? Good. Easy isn't it. Now a bit of help from you. Now I know the approx all up model weight, I can go and find something that fits the bill. As I said, I want mental, so has anyone got any ideas? Once I have the model, I can work out how to use that power. I.e. small prop turning at insane rpm, or big prop turning slowly.... Once the prop size is estimated, the motor/ESC comba can be chosen....

Over to you Smile Oh, there is one constraint, I am on a tight budget for this one, so the cheaper the better!

Andy

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Post by Mark Barnes on 2010-07-08, 16:30

The trouble you will have Andy with 5s packs is getting an aprox 2.5lb airframe big enough to put them in.

(2.5lb because i would guesstimate 2-2.5lb for the packs/motor/esc/servos/rx)

This will pretty much rule out F5D type mental gliders, im having a similar problem looking for what to do with 5 and 6s 4000mah there just sat there.

Ive been looking at 90mm EDF for them with around 190w per Lb or even a 120 EDF l39 like we spoke about and going 10s,

So i too will be intrested with peoples answers on this

M

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Post by Andy Sayle on 2010-07-08, 16:46

I was wondering about a scale sailplane, with a slightly less mental power to weight. Something that has a wicked climb with the taps fully open, but nice and scale.

The other option is I try something I have been meaning to do for a while, and do my own crude design wing speedster type thing.... A cheap general hack/winter model that is nice and basic, quick to build, but perfect for the packs I have.....

Hmmmm
Andy

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Post by Guest on 2010-07-08, 17:09

THE ANSWER IS SOOO SIMPLE!!!
Stick it in a shokie!

Guest
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Post by Mark Barnes on 2010-07-08, 17:18

For a scale glider have a look around for a 4mtr ASW like the one i got cheep last year, place i got mine from havent got them anymore but may be worth a call, rcartf was only 180quid

BB

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