Help : motor /esc problem

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Re: Help : motor /esc problem

New postby retiredVTT on Mon Dec 10, 2012 12:57 pm

For a plane that size..(12.5 lbs) I would go with two 6S packs.

A fully charged 6S pack (4.2 x 6) will have a voltage of 25 vts. So you will be starting out with 50 volts...exactly what the ESC is rated at.

A minute later the voltage will likely be 45 volts...hopefully it will hold at this value for several minutes.

If you prop for an amp draw of 45 amps, the system will see 2000 watts which represents about 160 watts per lb which will fly that plane in an aerobatic mode.

You will have to play with different props to obtain the required amp draw.

A 5000mah pack passing a 45 amp draw only represents a 9C discharge (45/5=9) so if the packs are rated at 25C there should be no appreciable heat generated in the pack. (Tip: always buy the highest C rated packs you can justify...high C = low internal R)

Granted the 6S 5000 packs are going to raise the overall weight of the model somewhat, so some of these figures will be changed.

..good luck..!

Bill
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Re: Help : motor /esc problem

New postby timberman on Mon Dec 10, 2012 4:04 pm

Could you let me know a battery size just for basic flight not needing 2000watts, I probably couldn't handle it anyways. Just enough for the 12.5lbs plane basic flight or do you do it with the prop changing? 5 or 6 ?
Could I go lower mah and higher c rate? Sorry, I still trying to understand all the terminology
Where is the cheapest place to buy lipos?
thanks again
Dennis
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Re: Help : motor /esc problem

New postby retiredVTT on Mon Dec 10, 2012 4:27 pm

No problem about the questions....we have all been there..!!

Most flyers are buying the larger lipos from HobbyKing in HK or HobbyPartz in USA. The shipping time is likely better from the US although carriers are getting very restrictive about shipping lipo cells. So this time of year is probably the best time to order since it could be five weeks before they arrive.

The length of flight time in minutes that you will get from a pack depends on it's size obviously....a 2200mah pack will deplete itself before a larger pack. However the bigger packs are heavier...and weight is a big factor in how electric planes perform.
Therefore a slick four pound pylon racer that only has to go three laps around a circuit might get away with a smaller 2200mah pack but will have to land after three minutes.

A plane like yours could maybe fly sucessfully with a 4000mah pack of either 5 or 6 cells (with 2 packs in series) but the way to do it would be to fly only for 2 or 3 minutes and keep the plane high and close so that if the packs did drop off you still can get down.

If they don't drop off you land and check to see what's remaining in the packs. HobbyKing sell a $24. meter that actually gives a reading similar to a fuel tank, that tells what percentage is left in the pack. A very valuable meter. If after three minutes the meter says you have 68% left in the pack...you know that you could go to five minutes a flight next time, as long as you are flying a similar style. Obviously wide open flying will drain the packs faster.

So..to answer your questions...a lower mah will lower your flight time. A higher C rating will allow the pack to pass a higher current without destroying itself.

Here is an example of how to pick a pack size:

In theory you multiply the pack size by the C rating to give the amperage that the pack in capable of delivering...BUT..to be safe you lower the current draw to 75% of that first figure.

So a 2200mah 40C pack would provide 2.2 amps x 40 = 88 amps but to keep it healthy reduce the draw to 88 x.75 = 66 amps.

Now for the interesting part...

The above pack (2200mah) means that it was designed to pass 2.2 amps for ONE HOUR before being depleted.
But we need much more than 2.2 amps...say we need 10 times 2.2 amps...(22 amps)
We can get 22 amps but we need to reduce the time period by a factor of 10....therefore the draw will be 22 amps for 6 minutes.
Double it again and we can (maybe) draw 44 amps for the grand total of THREE MINUTES.

Now you see why a higher amp draw (more power) requires a much larger battery pack.

Interesting stuff ain't it...!

In your case...with 4000mah packs and an "average" draw of say 20 amps....theory says you might get 12 minutes of flight time. That sounds like a lot to me...so likely with a 13 pound plane, the average draw might be higher than 20 amps.

Anyway if I were buying the packs I would probably go with either NANO packs from HK or GEN ACE packs from HobbyPartz and get at least the 4000mah size with a 40C rating. Sometimes you will see bargains in a pack size that for some reason a manufacturer has a barn full of.

cheers,

Bill
Last edited by retiredVTT on Mon Dec 10, 2012 5:17 pm, edited 2 times in total.
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Re: Help : motor /esc problem

New postby astroflyer on Mon Dec 10, 2012 4:32 pm

dennis, you need the MAH (capacity) to get the flight time. I would go with 5000 for that plane. lower the mah and you lower the flight time. it's like going to a smaller fuel tank.....

you need the 10-12 cells to get the voltage x amp draw (watts) to get the zoom zoom.

powering electric planes can be confusing but simply said it is the watts per pound of airplane that will tell you reliably how it is going to fly. so with any given motor you add a propeller and battery pack and power it up with an amp meter attached. the wattage (amps times volts) is what you are looking for. divide that wattage by the weight of the airplane to get watts/lb.

80 watts/lb or so means trainer power.
100 watts/lb gentle scale flying like a cherokee or similar type plane
150 watts/lb would mean scale fighter type flying...your plane for instance
200 watts/lb or better and you're getting into real performance territory, usually where my son resides. LOL

you can get total watts by either drawing a lot of amperage or by upping the cell count for a lot of voltage. Remember it's amps times volts equals watts. You can get the final number by juggling either one BUT with bigger heavier planes a higher cell count (more voltage) is better. also, getting enough watts by higher amperage (rather than voltage) will mean your battery dies sooner and gets pushed harder. In a high wattage high performance environment as in your type of plane or the bigger helis, higher cell count is desirable.

you should get new packs, six cell packs of 5000 mah with at least 25C rating. A higher C rating means the packs last longer as they don't work as hard or heat up as much providing the needed amp draw.
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Re: Help : motor /esc problem

New postby retiredVTT on Mon Dec 10, 2012 5:19 pm

HaHa..!
Dennis, this is your lucky day...as you don't have just one "ole silver haired fart" trying to assist you....but TWO..!

Bill
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Re: Help : motor /esc problem

New postby astroflyer on Mon Dec 10, 2012 5:26 pm

Yessir, very accurate description bill. :D although the haired part is a bit sketchy. :D
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Re: Help : motor /esc problem

New postby retiredVTT on Mon Dec 10, 2012 5:34 pm

Here are a couple of lipo links;

http://www.hobbypartz.com/77p-sl5000-6s1p-40c-6666.html

http://www.hobbyking.com/hobbyking/stor ... _Pack.html

..drum roll please, for the latest and greatest from HK ...just a 5S pack but wow..look at the C rating..65/130
http://www.hobbyking.com/hobbyking/stor ... _Pack.html

Another Tip: when ordering from Hobbyking...stay on the site for a few minutes and they come back with a price reduction sometimes. While I was looking at the latest pack at $106...they reduced it to $100. Hey that's a $12. saving for two packs...!




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Re: Help : motor /esc problem

New postby timberman on Mon Dec 10, 2012 9:39 pm

thanks for all the help and examples they did help me understand it easier.
Now it looks like I just have to start saving my pennies , and a lot by the looks of those .
Your help is greatly appreciated. I would hate to of put all the work into building it to loose it .I'm very happy how it looks and turned out.
thanks again
Dennis
ps the ole ones are the wise ones.
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Re: Help : motor /esc problem

New postby retiredVTT on Mon Dec 10, 2012 10:21 pm

Yeah, something to do with "been there ,done that, got the T shirt.."
However, we all learn something every day.

Today I found out that I can buy an external hard drive for my DVR (TV) so I can record many more episodes of Ice Pilots NWT after the internal drive is filled..! So I gotta lead my grown kids over to Best Buy prior to Xmas..!

Glad you picked up some knowledge from all those posts Dennis ....soon it will be your turn to pass it on.

Here's one thing we didn't tell you.....those costly lipo packs do not like to be run down more than 80%. It weakens them and if too low they might not recharge. A properly programmed ESC usally will look after that by gradually cutting power to the motor and forcing you to land. Fully charged each lipo cell will be around 4.2 volts. When discharged about 80% the pack voltage should be no lower than 3.65 vts. to remain robust and rechargable.
Another item is that all cells in a pack should remain very close in value to each other. A 3S pack that reads 3.92, 3.91, 3.59 is headed for the trash because that one cell will weaken the pack drastically.
That would be the first thing to do with your present packs...find out if the cells are balanced, both before charging and after charging.

If you look through the HEFA "University" Forum website, there is a lot of good stuff printed there that might help you out.

cheers,

Bill
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