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Old Oct 08, 2013, 12:46 PM
The pied piper piper!
sodea63's Avatar
United States, FL, Pensacola
Joined Jul 2013
138 Posts
Help!
Avistar electric conversion issues

I recently aquired an ARF Hobbico Avistar .40 and wanted to convert it to electric.
I read a bit and from what I could find, I estimated the AUW was going to be around 5.5 pounds.
When I got a motor for it I went with a motor that had a max of 2650gms of thrust, 43amp(Hobby Mate 3748-850?).
The motor said it could be run on 3S to 6S so naturaly I assumed it needed the 6S to pull that kind of thrust.
Last night I got around to putting a battery connector and bullet connectors for the motor on.
After soldering them up, I put one of my 3S 2200mah batteries on it and powered it up.
I was very surprised that with the 3S it had enough thrust to lift the airframe (with no wing while holding the fuse) with ease.

Where did I miss the numbers? Did I over estimate the airframe by that much? Will the motor produce max thrust on a lower voltage (11.1 vs 22.1)?
Should I even bother with the 6S (2 - 3S in series) batteries?

I want it to have good power, (was shooting at 1-1) but do not plan on trying to 3D a high wing trainer.
This is my first time venturing into electric conversions and thought I had it worked out but obviously not.
Granted, to have errored on the side of too much power is better than vice versa.

~Steve


HB3748
Weight 169g
Battery 3S-6S Li-poly Battery
Maximum output 650W
Outside Diameter N/A
Can Length 48/6 mm
Total Length N/A
Shaft Diameter 5.00mm
Mtg. Hole Number & Size 3.0mm
Type of Cooling Wind cooling
No Load Current 0.6A
Max Current (A) 41A (14.8V)
Max Voltage (V) N/A
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Old Oct 08, 2013, 04:12 PM
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Doug Bartley's Avatar
Canada, ON, Owen Sound
Joined Oct 2008
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In every case, the propeller size and pitch determines the load and watts you will see. The power is determined by the voltage chosen and amps within safe perimeters.
Your motor may spin a 13/8 prop on 3s. On 6s to stay within safe amp draws, you may need a 11/5.5 or smaller.
These are only samples, not tests.
The only/ proper way to determine if the load on the motor is within its amps rating at the voltage rec is with a WATTMETER!!
Most (not all) mfg give a rec prop size or two, ie 13/8 to 11/5. Usually the larger prop would be for 3s and the smaller for 6s in your case. A wattmeter at home is the best way to know what works while staying within safe limits. Good luck Doug B
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Old Oct 08, 2013, 05:42 PM
The pied piper piper!
sodea63's Avatar
United States, FL, Pensacola
Joined Jul 2013
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Actualy for someone who knows how to use one, a good dvom is much better but that isn't what I'm asking.
I know it is within safe limits, I'm wondering if maybe the airframe is much lighter with electric or, maybe the motor puts out much more than I expected on 3S...
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Old Oct 10, 2013, 08:37 AM
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Canada, ON, Owen Sound
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You can't tell anything by holding a fuselage in your hands and running the motor.
A wattmeter or vdom will tell you the amps. In this case, 43a is suggested max. So 43A x 11v (good 3s lipo under load), with a prop sized not to exceed 43a equals 473.
Taken to 4s, again with a prop to pull 43a, equals 636 watts (14.8 under load).
Approx 30% increase in power without an increase in amps.
It goes up the same for for each voltage increase, but you must be very careful with prop sizes, 1" diameter or 1" pitch increase can suddenly put you way over the amp rating. A motor with 850kv as yours is will use a very small prop to stay within the 43a rating on 6s. fwiw Doug B
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Old Oct 10, 2013, 11:05 AM
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Quote:
I know it is within safe limits
Not till you see the numbers.
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Old Oct 10, 2013, 03:56 PM
The pied piper piper!
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United States, FL, Pensacola
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So, that's a no. Neither of you could venture to guess at why it feels like it is lifting the airframe with ease or where I missed my calculations?
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Old Oct 10, 2013, 04:09 PM
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You can do all the adding, subtracting, multiplying, long division, and fractions you want,
but till you see what the real wattage, voltage, amps. it is all just paper work.

We got the recommended motor, prop, esc, battery, on one plane it was low, on the next
it was real high. Like nearly double what they stated.

The math is a starting point, the watt meter is the final word.
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Old Oct 11, 2013, 06:45 AM
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United States, ID, Shelley
Joined Dec 2011
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Depending on the set-up, it's possible to get the same amount of power from either a 3S or a 6S power system, the higher voltage will just do it more efficiently.

For instance, at 500 watts (suitable for a 5 lb. aircraft with a 100watt/pound ratio)...
-a 3S battery will run @45amps, with a 4:1 voltage/current ratio (acceptable numbers).
-a 6S battery will run @22amps, with a 1:1 voltage/current ratio (better numbers).

The lower the voltage/current ratio, the more efficient the system.
There are advantages/disadvantages to each set-up, and as previously stated, the only way to know what the REAL numbers are is to run a watt-meter.

The 3S motor may be lifting the aircraft because your current set-up may in fact be providing enough power.
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Old Nov 26, 2013, 02:46 PM
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United States, TX, Plano
Joined Jun 2013
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I'm running my Avistar on a Scorpion 3020-12 on 4s with a 10x7E prop. On paper it's pulling 49amps, 689 watts, 2479g thrust. It flys much better than the glow setup. It's around 4.5 lbs AUW with a nano-tech 2650 4s battery. http://www.flyelectric.com/Scorpion3020-12.html
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Old Nov 27, 2013, 02:37 PM
The pied piper piper!
sodea63's Avatar
United States, FL, Pensacola
Joined Jul 2013
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Thanks ddemu and mu5tanggt. It seems I was both over-estimating weight and underestimating the power of that set-up on 3S.
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Old Nov 27, 2013, 04:25 PM
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Quote:
Originally Posted by ddemu View Post
The lower the voltage/current ratio, the more efficient the system.
Can you elaborate on this?
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Old Nov 27, 2013, 05:31 PM
The pied piper piper!
sodea63's Avatar
United States, FL, Pensacola
Joined Jul 2013
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Quote:
Originally Posted by Yoda466 View Post
Can you elaborate on this?
It very simple terms, the more current you need, the more voltage you should use to carry it.

If you are only running a motor/prop combo that pulls 10A, a 3S battery at 11.1v will have a .9:1 ratio. If you are running a combo that pulls 40A on the same 3S battery your ratio goes up to 3.6:1. A 6S battery at 22.2v will bring your ratio down to 1.8:1. The lower the ratio, the more efficient the components.
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Old Nov 27, 2013, 07:26 PM
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Quote:
Originally Posted by sodea63 View Post
The lower the ratio, the more efficient the components.
This seems to simply restate the original claim that I questioned. Don't get me wrong; I am am not disagreeing or arguing. But I don't understand the fundamental reason why this should be true. Thanks.
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