Author Topic: Taking the "Electric Leap" (Read 1585 times)

Andrew Borgogna · « **on:** June 28, 2010, 11:44:07 AM »

My friend Larry Renger (AKA Designman) recently scaled up is Sky Sport 30 (available from RSM Distribution) from 1/2a to .15 size. I am currently building the test version and during a conversation with Larry we decided to make it electric. We both fly electric R/C planes and really like being able to go to local parks and fly without being hounded by neighbors.

Anyway I have started collecting parts for the power system. I have a CC 36 amp ESC, it is the same kind I use in my Jenny Biplane and it seems to work very good. I have a timer I got from Eric Rule at RSM and now I am looking for a motor that will give me the equivalent power of a .15 to .20 size IC engine. (Note: the correct usage of "motor" and "engine"

.) Since Larry is the MIT graduate rocket science man I asked him to do the numbers on current, voltage and efficiency ratings.

Here are the numbers Larry came up with. Any comments would be greatly appreciated.

Given: .15 size stunt engines produce about 1.5 hp per cubic inch, or .225 hp
Electric motors are running about 80% efficient
We should use only 80% of a battery's capacity in one flight
80%x80% = 64%

So: .225 hp is 168 watts
We need about a 6 minute run, or .1hour resulting in 16.8 watt hours to be expended
dividing that by the 64%, gets us 26.25 watt-hours
Finally, using a 3 cell Li Poly at 11.7 volts, we need a 2250 MaH battery pack.
The motor would be 168 watts/80% or 210 watt throughput
210 watts/ 11.7 volts gives a current drain of 18 amps.

Thanks in advance
Andy

CircuitFlyer · « **Reply #1 on:** June 28, 2010, 03:56:41 PM »

You might be a little low on the motor size. The OS web site lists the .15LA engine at .41hp.

Mark Scarborough · « **Reply #2 on:** June 28, 2010, 04:18:14 PM »

But please keep in mind, we do not run our motors at the HP peak, more like close to the torque peak, but that generates a lot less HP at the lower rpm. I would guess that the .225 figure is actually pretty accurate for a stunt type run on a .15, if anything it might be high

Dennis Adamisin · « **Reply #3 on:** June 28, 2010, 04:22:12 PM »

Yee haw, another convert!

Can you share a little more about the size of the airplane? span & area, expected weight, things like that. The reason I ask is that the 15-19 size is (just my opinion) one of the toughest sizes to deal with - getting the right mix of power and weight, even the physical strength of the motor to deal with the expected prop.

I have no quarrel with the mathematics, but I think a more pragmatic approach would be to check out the "List your set-ups" thread for similar sized birds - if nothing else it would help prove (or refine) your numbers..

Andrew Borgogna · « **Reply #4 on:** June 28, 2010, 06:06:54 PM »

Dennis
The wing span is 42" and the area is 350 square inches. Weight will have to wait until I finish the plane, since I am building the first article I don't have one to weight. I am guessing (read hoping) it will come in somewhere between 20 to 25 ounces. That estimate is based on a Tunder Tiger .15 not electric. I have more information when I get the battery and the motor, but (and it's a big "but") I can't get them until I feel more confident on what I need to get.
Andy

Dennis Adamisin · « **Reply #5 on:** June 28, 2010, 09:20:56 PM »

I think the "List your Set-ups" thread has a lot of Super Clowns & Ringmasters in that general size range that can give you a baseline.

I'd suggest starting with a 9x4.5 APC Pusher, a 2810 (internal dimensions) or 35-30 (external dimensions) motor with at least a 4mm motor shaft (smaller ones flex too much) the motors will weigh around 3 oz. For 3S you will need the kv to be in the 1300-1400 range, for 4S you can use a kv of 900-1000.

You can pic & choose your packs based on their size, shape & weight. 3Sx2800 should be safe, 4Sx2100 has the same energy either pack should weigh about the same...

Andrew Borgogna · « **Reply #6 on:** June 29, 2010, 09:31:18 AM »

OK Dennis, now I am really going to show my ignorance to this aspect of the hobby. But here goes. Why the pusher prop instead of using a normal tractor prop? Are the motors more efficient running clockwise or is it to take advantage of the torque direction? Finally where is the magic decoder ring for all the letters used to define batteries? I come from a 40 year career in computer hardware and know circuit theory fairly well. But to me KV is kilovolts, somehow I don't think that's how it is being used here. Also in my world "I" is used for current "E" for voltage and "R" is resistance as in ohms law E=I*R. I also recognize "V" for voltage, but the "S" designation is new to me and it seems that "C" is used for current.

I really appreciate your help and will use your information soon when I get my motor and battery, I just would like to understand the terminology a little better.

By the way say hello to your brother Dave and your dad, we really missed them this year at VSC.
Andy Borgogna

Dean Pappas · « **Reply #7 on:** June 29, 2010, 10:38:18 AM »

Hello Andrew, and welcome.
Get ready to drink from a firehose! I'll start and let others chime in.

The reverse pitch prop is magic! It's just good old spiral airflow yawing the airplane in the correct direction. Remember that the RPM governor (another whole subject) actually jazzes the "throttle" in response to climbs, so now the prop induced yaw is in the happy direction. Line tension above 45 versus below gets better, as does tension on takeoff. Larry has stories to tell about that for sure.

Nobody uses the proper subscript in Kv: it means voltage constant, ususlly in RPM per volt. Yup, K-sub-v.
LiPo cells can be paralleled and charged/discharged safely so the S/P nomenclature was created to describe how many grpuops of paralleled cells were then hooked in Series to get a pack of the desired capacity.
These days, the cell manufacturers are making most of the sizes we want, so almost all packs are more simply described as #S where a single cell yields 3.6 or 3.7V under load, and droops to maybe 3.4V under load at the end of the flight.

Total energy consumption tends to be proportional to both line length and airplane weight. I find that 0.7 W-H per ounce on 70' is a good figure and often a bit high once a setup is dialed in.

Dennis' estimate of motor size is right as far as I am concerned, and there are several good choices in the 3-1/2 ounce range: both the Rimfire .15 on a 3-cell pack or an Eflite Power-10 on 3S or Park-480 on 4S should do well.

Next!

Dean P.

Andrew Borgogna · « **Reply #8 on:** June 29, 2010, 01:43:15 PM »

Dean
Thanks for the insight, the information you provided did clear up some of the smoke. Let's see if I understand "#S" is the number of cells wired in series, where "cell" is defined as 1 or more LiPoly cells in parallel.

"Kv: it means voltage constant, usually in RPM per volt". Still not to clear on this, how is this constant used to produce a better package. Dennis stated "For 3S you will need the kv to be in the 1300-1400 range, for 4S you can use a kv of 900-1000.". Are the ranges 1300-1400 and 900-1000 referring to battery current capacity? Where a 3S 1300 to 1400 would be a 3 cell(11.1v) 1300ma pack. If I am right on this, where does the RPM come to play and what is the "C" reference for? I think I need to read a book on this.

Maybe you could point me to a document somewhere that I can use as a primer for learning this stuff.
Thanks for all your help everybody.
Andy

Dean Pappas · « **Reply #9 on:** June 29, 2010, 08:43:52 PM »

Andy,
This forum is about as good as it gets ...Seriously

Dennis was referring to the Kv of the motor.
Let's assume that you will be running anywhere between maybe 8500 RPM with a 6" pitch prop to the mid 10,000s with a 4 or 4-1/2 " pitch prop. I'll pick 10,000 for this diatribe.

You want to run with the governor actually putting 75% or 80% of the battery voltage on the motor in order to maintain the desired RPM under maneuvering loads, even as the battery discharges toward the end of the flight.

The I-R losses in the motor windings will waste maybe 1 ~ 1-1/2 volts of the battery voltage in the form of heat loss. Let's say 1, for this example.

As a result: a 3S pack really gives you 8 volts, a 4S maybe 10 volts and 5S maybe 12 volts across the internal RPM/voltage generator of the motor. This is not the same as the voltage across the battery terminals: this is the voltage that determines the RPM. That's what determines the necessary Kv. So if you wanted to turn maybe 10,000 RPM with a 4S setup, then you want a Kv of around 1000. (10,000RPM / 10 Volts) Anywhere from 900 to 1100 works well on 4S.

C-rating is for another day.

Start meandering through the older threads ... there are quite a few gems in there.

Dean

Andrew Borgogna · « **Reply #10 on:** June 30, 2010, 10:46:19 AM »

Dean
Sorry for seeming to be so dense on this. I got an email from Rudy Taube yesterday and he explained the whole Kv thing, I am good with it. I think I have what I need to get some hardware.
Again thanks
Andy

Dean Pappas · « **Reply #11 on:** June 30, 2010, 12:32:18 PM »

Great!

Kirk_Binns · « **Reply #12 on:** June 30, 2010, 04:25:39 PM »

Been reading these posts w/great interest....

One of the US-based distributors I get my R/C electric stuff from lists most of his motors with a average thrust rating in ounces (given a specific prop/ESC/Lipo combo). For R/C it makes great sense and easy to choose motors for the type of machine you fly (i.e. 40 oz machine w/50 ozs of thrust for a given prop size will give you some good vertical performance). My question would be, for vintage stunt or simple sport flying, what would be a good thrust to weight ratio?

Larry Renger · « **Reply #13 on:** June 30, 2010, 04:35:20 PM »

Judging from just pointing the nose up on stunters, I would guess 1.2 to 1.5 thrust to weight is needed. You need to be able to accelerate straight up after a hard corner in several maneuvers. I don't recall ever seeing hard data on this, but perhaps it is buried in some of Wild Bill's writings.

Dennis Adamisin · « **Reply #14 on:** June 30, 2010, 05:26:32 PM »

Andrew
I cannot beleive I let myself answer your pefectly reasonable questions in jargon! Sheesz I hate when people do that to me and now I did it to you. UGGGHH! I am soooo SORRY. Thankfully Rudy & Dean (and others) have waded in to help you. I really do not have a lot to add to what they have said.

I think if you just keep in touch here and collectively we can all get you over the finish line.

Dean Pappas · « **Reply #15 on:** June 30, 2010, 08:56:55 PM »

Hello Kirk,
As far as thrust to weight is concerned, I have no idea.
More importantly, the governor's action during static conditions completely muddies the waters and renders any comparison to previously established wet standards useless.
Additionally, E-setups often run larger diameter props thatn equivalent wet setups ... once again, comparisons are difficult.
Peak in-maneuver power levels run around 10 Watts per ounce, with an average around 6 or 7.

later,
Dean P.

Andrew Borgogna · « **Reply #16 on:** July 02, 2010, 01:34:39 PM »

Thought you might like to see some pictures of the Sky Sport 350. This is Larry Renger's latest design, as you can see the motor of choice is the Rim Fire 10. It seems to meet all the specification requirements stated in this thread, I will be using a Castle Creations 36 amp. speed controller and a timer being marketed by RSM Distribution. If all goes well this plane will be available as an electric or gas engine powered plane. I will let you know how the project progresses.
Andy Borgogna

Larry Renger · « **Reply #17 on:** July 02, 2010, 03:55:01 PM »

Yer doin' me proud, Andy! Looks good!

Dave Adamisin · « **Reply #18 on:** July 02, 2010, 08:48:00 PM »

Hey Andy. Welcome to the circle electric. This is a great thread with a lot of helpful people. I've been an engine guy my whole life and I can tell you this electric thing is just sooooo technically sweet. By the way, Big and I missed you guys too. You're going to like the backwards prop. Imagine pull on the lines at the top of the circle at less than rat race speeds.....

Larry Renger · « **Reply #19 on:** July 02, 2010, 09:44:59 PM »

As we discussed this morning, be sure your battery is centered to give the same lap times upright and inverted and cushion it to avoid the dreaded electron foaming!

I think I am going to LOVE electric!

Author Topic: Taking the "Electric Leap" (Read 1585 times)

Dave Adamisin