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Electric Stunt => Gettin all AMP'ed up! => Topic started by: Curare on February 14, 2013, 06:17:16 PM
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Hey guys, I've been scouring the forums for info on basic setup and trimming of aircraft, and while I feel I understand the basics there are a few things that are making me very nervous about the first flight with my new electric model.
Let me go through it, I'd appreciate any input.
The model is a scaled down Oriental (or a scaled up pinto), 350 squares and is topping the scales with battery at 32 oz. I've got an E-Flite park 480 on 910kv, which in an RC model is pretty close to a hot 15 or a 19 IMO. Swinging a 10x5 prop the calcs are showing a speed of about 40mph. This is in effect a low speed, high pitch setup.
Is this too slow? I have a slightly higher KV rated motor (same size) that will bump up the speed to 46mph. Should I swap out?
Given these speeds I'm considering my line length, obviously I want as much as I can get, but I think 60' is too much, should I start at 55?
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What battery are you using?
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3s 1800mah. according to the data that's a 5.3 min run. Suprisingly I jagged the balance point to within a mm!
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With a 3S battery you'll need about 1100Kv motor. With the 910Kv motor you'll need a 4S battery.
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With that size airplane I'd seriously look at a APC 9x6 or a 9x4.5 - which ever one matched up best with the motor kv per Crist's advice. That is also about the same size & weight as the electric Super Clown that flies great on a 3Sx3300 pack, 1300 kv motor, turning a 9x4.5 around 12k and flying on 60' lines.
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Yes 40 mph is way too slow unless you are flying a Bi-Slob. The gentleman I used to fly with and I both agreed that between 55-60 mph is the mark for most stunt planes. If kept super light with no wind, you might get by at 50 mph. A heavy plane will need to go faster. As far as lap times, they can be controlled by line length.
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Crist is right. Go with a 4 cell battery. You can then reduce the mah as well.
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Well this leads to a question I have had for some time. This motor was intended for R/C park flyers with big prop and slow RPM compared to what we use. I checked the spec and it says:
Needed to Complete
20A–35A brushless ESC
2S–3S Li-Po or 6- to 10-cell Ni-MH/Ni-CD
10x7–12x6 propeller
We all agree that a 4 cell will put this motor in a better place for control line, the question is will it stand up to a 4 cell battery. I think that as long as you keep the continuous power below 222 watts you will be OK. 222 watts power was derived from the spec stated continuous current of 20 amps times 11.1V (3 cell battery). The thing is it was said the plane weights 32 ounces, I like to have 150 watts/lb., which takes you up to nearer to 300 watts.
Denny you have forgotten more than I know, so what do you think. Below is the full E-Flite spec for the Park 480 910Kv motor.
Type: Brushless outrunner
Size: Park flyer
Bearings or Bushings: One 4 x 9 x 4mm Bearing, and One 4 x 10 x 4mm Bearing
Wire Gauge: 16
Recommended Prop Range: 10x7 to 12x6
Voltage: 7.2 to 12
RPM/Volt (Kv): 910
Resistance (Ri): .08 ohms
Idle Current (Io): .85A @8V
Continuous Current: 20A
Maximum Burst Current: 25A (15 sec)
Cells: 6–10 Ni-Cd/Ni-MH or 2–3S Li-Po
Speed Control: 20–35A brushless
Weight: 87 g (3.1 oz)
Overall Diameter: 35mm (1.40 in)
Shaft Diameter: 4mm (.16 in)
Overall Length: 33mm (1.30 in)
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Well guys, I've changed the motor out for a 1020kv version, and while I'd like to go 4s, I don't trust it, and what's worse, the battery won't fit. I'm treating this more like a diesel than a glow setup initially. These little park 480's are quite powerful little cans, and I would go as far as to say they're 250w rating is probably underrated.
I actually took the entire power system out of a 72" grob-style motor glider (who remembers the kyosho valenica?) which used to pull that thing around like rag doll.
I guess we'll see whether it works, or it doesn't :!
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Ok, flight report!
First things first, it flies, didn't do too much as it was only a test flight, and the timer was set for 60 seconds, just in case something went wrong. As it happens it did just as the motor stopped, a pretty vicious gust came through and blew her in on me, but thankfully I ran fast enough to catch it and survived a bouncy landing.
Now, lets get down to brass tacks. The model was flown on the 1020kv motor and a 3s pack, with an APC 10x7E on board. prior to test flying I'd measured it at 255w and 22.2 amps. It's about as high as I'd ever want to go with this setup.
Timing the flight (roughly, forgot my stopwatch) I think I was in the 4.8 sec/lap on 60' lines eye to eye. It pulls pretty well for its size.
I'm now thinking of dropping pitch to get my lap times about right, so I'm thinking a 10x6, a 10x5 may be too fine to be any good.
Any thoughts gentlemen?
Oh, and this is what it looks like.
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Curare ... well that can't possibly be your name, but hello anyway!
I'm afraid we might not have all the information we need to help you.
Are you running on an RPM governor?
At what RPM?
Are you simply running a standard ESC at full throttle?
Which brand and type ESC?
If you are running without a governor (and we don't really recommend this but it does work) then speed is adjusted with pitch and power consumption is adjusted with diameter.
Regards,
Dean Pappas
P.S. I would like to keep things in this forum on a "real name" basis, so please sign your posts or add a signature block to the bottom. It's an option in your profile.
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Ahh, sorry Dean, I didn't think to add all that stuff.
Here goes though.
I'm currently running a KR timer/governor. It's set to be a few hundred rpm off peak so I have some headroom on voltage as the flight progresses.
AS for a tach reading, i'll have to pull my trusty old tach out to tell you that.
Speedy is a 25 Amp old Castle unit (had it for years). If I recall correctly it was a phoenix 25.
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Hi Greg,
A few hundred ain't enough, as you'll find out eventually.
Not only is headroom consumed in battery voltage drop, but it is necessary to allow the RPM to recover quickly when maneuvering loads try to bog the motor down.
The guideline is for the setpoint RPM to be somewhere between 70% and 80% of the theoretical no load max RPM, or Kv * Vbatt. In your 3S case, that would be 11.1V, and with your motor that would be 11,300 * 75% or 8500 RPM. That sounds like a 5" pitch prop on your line length.
Now you can see why so many chimed in that you need a 4S pack, so that you'd actually run on close to a fully charged 3S voltage.
The advantage of running with minimal overhead is a small decrease in battery consumption. That would mean a pitch reduction and increase in RPM setpoint.
At the flight's end, the battery shouldn't be below 3.4V per cell, so that sets a ceiling on the useable RPM range.
I hope that helps just a little bit; please keep us up on your progress.
Regards,
Dean P.
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Will do, thanks for the help. BTW, I still think the maya is one of the best looking pattern models ever designed/built/coveted.
As for running 4s, this is why I think I painted myself into a corner. E-flite doesn't reccommend using 4s on the park series of motors, so there's a question mark as to whether it can do it or not.
Also the aircraft balances (maybe a touch more aft than I'd like, around %20) with the existing pack and gear. I also in my stupidity designed the fuselage crutch/ battery tray, once everything else went in, to accomodate batteries a little over 4". I could go for 4s, but would need to go for a corresponding reduction in capacity which i'm not keen on.
Funnily enough, due to the low KV's of the motor and being governed by Keith's great timer/governor, it took some head scratching to figure out why I couldn't reduce the rpm initially. AS it turns out, max rpm for the governor is way above the max rpm for the engine, so it took quite a reduction in governed rpm before I could notice a change in acutal RPM. My flight RPM is a few hundred below that. Now that we've disussed how much headroom i acutally need, I'll probably reduce it a bit more once I get a handle on my lap time.
I'm going to have another crack at it today (it's morning here) and see how I go, will report back later.
Thanks again.
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Hey Greg:
I have never heard of any phenomenon* that limits the voltage for a brushless motor other than the speed that the manufacturer thinks it can turn. In other words, if you never run it fast, a 4S pack should be A-OK.
You mention that you don't want to run a 4S pack with the commensurate reduction in capacity. But remember that if you run four cells at 3/4 the mAh rating, the energy contained in the pack will be held constant. So in the sense of the amount of charge it can hold, yes, the capacity would go down. But in the sense of the energy it holds it'll be constant -- and it's Joules that gets you around the circle, not Coulombs.
* Well, until you get into the 100's of volts -- which you are most certainly not doing.
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Very true, according to my new favourite calculator, http://www.ecalc.ch/motorcalc_e.asp (http://www.ecalc.ch/motorcalc_e.asp), ecalc I should be getting around 23 amps with my current setup 3s-1020kv-10x7e, and a prop-speed of about 65mph full throttle, (note this is only a comparison!) If I plug in a 4s pack I need to drop the prop diameter and pitch down a 9x5E for a prop speed of 58mph.
Flight time at full throttle on 3s is 5 mins, and on 4s is 4.2 mins on a 1600mah pack, which is similar in size and weight to the existing pack, which is a loss.
Would I be right in assuming that a low-speed high pitch prop is not as desirable as a high speed, low pitch prop? Why?
I know I might be suffering from a touch of confirmation bias here but looking at some of the low speed (glow/diesel) setups, there's some big pitch, and 11x7 is a regular. The difference is P/D ratio is not that great between an 11x7(0.636) and a 10x6 (0.6). Even a 10x7 is around 0.7 and I've heard of guys running 11x8.5 composite props on low revving enyas.
Sorry guys I know this has probably been covered a myriad of times and I've scoured the site for as much information as my feeble mind can absorb, now it seems I need to be spoon-fed!
In other news, it was blowing a gale this morning, so no updates.
:(
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I did run into a problem with the KR timer trying to run a 4C pack on a 1400 KV motor. The start up sequence for the KR timer is to run the motor wide open for a few seconds and then throttle back to the set rpm. The 4C/1400KV combo exceeds the upper RPM limit of the KR timer causing it shut down. It simply would not run more than a second. I am not sure what constitutes the upper rpm limit for a KR timer. Does anyone know?
D>K
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I think the upper limit for the KR timers is somewhere in the vicinity of 12.5K (I'm sure Keith can be more accurate!). Given the fact that you're running a 1400kv motor on 4s, that puts your full throttle RPM at around 18K (assuming ~90% efficiency), which is a bit high!
Now, I had an issue on mine initially where it'd rev up and then just before it would reach it's goverened RPM it's screech and squeal, annoying both my partner and the dog. A small reduction in RPM removed that.
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Very true, according to my new favourite calculator, http://www.ecalc.ch/motorcalc_e.asp (http://www.ecalc.ch/motorcalc_e.asp), ecalc I should be getting around 23 amps with my current setup 3s-1020kv-10x7e, and a prop-speed of about 65mph full throttle, (note this is only a comparison!) If I plug in a 4s pack I need to drop the prop diameter and pitch down a 9x5E for a prop speed of 58mph.
Flight time at full throttle on 3s is 5 mins, and on 4s is 4.2 mins on a 1600mah pack, which is similar in size and weight to the existing pack, which is a loss.
Would I be right in assuming that a low-speed high pitch prop is not as desirable as a high speed, low pitch prop? Why?
I know I might be suffering from a touch of confirmation bias here but looking at some of the low speed (glow/diesel) setups, there's some big pitch, and 11x7 is a regular. The difference is P/D ratio is not that great between an 11x7(0.636) and a 10x6 (0.6). Even a 10x7 is around 0.7 and I've heard of guys running 11x8.5 composite props on low revving enyas.
Sorry guys I know this has probably been covered a myriad of times and I've scoured the site for as much information as my feeble mind can absorb, now it seems I need to be spoon-fed!
In other news, it was blowing a gale this morning, so no updates.
:(
Hi Greg,
Try fudging the cell count in your simulator #s now equals 2-1/4S and 4S now equals 3S (the 75%) and rerun da numbas. We just don't run at wide open throttle on the governor.
later,
Dean
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Ahh, smart, I was wishing I could have a throttle percentage or something, but that makes a lot more sense!
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Would I be right in assuming that a low-speed high pitch prop is not as desirable as a high speed, low pitch prop? Why?
Yes. Because a high-speed low-pitch prop tends to make the airplane accelerate more strongly when it's going slower than the pitch speed of the prop, and decelerate more strongly when it's going faster than the pitch speed of the prop. The result is that your airplane flies at a more constant speed.
I know I might be suffering from a touch of confirmation bias here but looking at some of the low speed (glow/diesel) setups, there's some big pitch, and 11x7 is a regular. The difference is P/D ratio is not that great between an 11x7(0.636) and a 10x6 (0.6). Even a 10x7 is around 0.7 and I've heard of guys running 11x8.5 composite props on low revving enyas.
Well, yes. But if you look at the modern piped stunters, they're turning fairly high RPM with fairly flat (3.75-4.5 inch pitch) props. The pipe acts to govern the engine RPM, the prop does the same thing as an electric prop.
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Will do, thanks for the help. BTW, I still think the maya is one of the best looking pattern models ever designed/built/coveted.
Oh yes, Thanks a bunch, Greg. What can I say ... it was always a little bit Stunt!
I have one kit left, cores all sheeted and waiting for E-classic Pattern. I hope the fuse hasn't twisted up with age; it was an early glass/foam sandwich. Larry Philips (RIP) and I played with both foam and wood in glass sandwiches back in the Fall of '89.
take care,
Dean
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Well, I went for a few laps yesterday and the results are... interesting.
As it turns out in a 12K breeze I was getting consistent 5.5 second lap times, on the 10x7 and 3s. I'm starting with short flight times, but will open it up slowly.
I'm now considering my next move, but will get a feel for everything (temps etc) running on 3s, before I do anything drastic. My gut feeling is to fly it like it is, and build something bigger, and use a well proven existing setup. Watching Russel Bonds Bandolero (Firecracker) at last years Nats started all this, might be a good place to start!
Thanks for your help, it's very much appreciated.
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Oh yes, Thanks a bunch, Greg. What can I say ... it was always a little bit Stunt!
I have one kit left, cores all sheeted and waiting for E-classic Pattern. I hope the fuse hasn't twisted up with age; it was an early glass/foam sandwich. Larry Philips (RIP) and I played with both foam and wood in glass sandwiches back in the Fall of '89.
take care,
Dean
Dean
This talk of the Maya brings back old memories. That was one of my faverate pattern ships from years ago. I was reviewing a gift of old Flying Models magazines last year, found this great photo ....enjoy. Finding the photo lead to my latest stunt design the FinessE. I think the lineage shows. SORRY, didn't mean to steal the thread.
Regards
Dave
ama41041