Electric Stunt > Gettin all AMP'ed up!
Tom Dixon Tutor Too - electrified!
John Rist:
The way I pick an electric motor for a gas to electric conversion is make note of prop size and RPMs for the original gas engine. I then go to the Cobra web sight and look at the prop data charts for possible electric motor choices. Keep in mind that if running in the governed rpm mode you will be running the motor at less then max RPMs and max current draw so a prop just outside the "safe" range will probably be OK. SO I pick a motor/cell count that will turn a prop size and RPM similar to the original gas setup. For instance the setup I am using on my S1 Ringmaster is:
Motor, Cobra 2221/10 KV 1500
Battery, Hyperion 3S 1600 70C
ESC< Trunigy Plush 30 amp
Prop APC 9x6 EP
Timer, KR. Initial RPMs set to 9500
According to the prop chart the Cobra 2221/10 on a 3 cell LiPo will turn a APC 9x6-E @ 11,378 RPMs. It is in the yellow category meaning "The prop can be used, but full throttle should be kept to short bursts. (80 to 100% power)" So with the RPMs set at 9.5K and lots of cooling (profile ship mount) it has worked well. I picked this motor on the fact that a Fox 35 turns a 9x6 prop at around 9 to 10K. y1
Teodorico Terry:
Steve,
I have been thinking about this model as well but my approach might be a little different; generally speaking, electric motors do a better job generating torque than a glow engine (4-Strokes included). With this in mind, I don't shy away from using a somewhat larger prop. The increased diameter can eat up power very quickly if you try to spin it too fast; the answer is to increase the pitch. I know that this goes against the current wisdom.
I currently fly a SIG Fazer which I built as a C/L model. The original model has a very short and stubby wing so I increased the span slightly by adding rounded tip and went with a narrower cord flap. I also increased the tail moment by 3 inches. In the end, the model came in at 56-58 oz ready to fly. The wing area is north of 600 squares but I do not remember the exact number. It is electric powered and with everything hanging out and the fat wing the model is quite draggy. Once the motor is off it will not glide very far unless you whip it.
I did quite a bit of experimentation with props and RPM settings. The smallest prop I tried was a 12x7 and the largest was a 13x10. In the end, I discovered that the reverse rotation prop provided better line tension but is also limited my choice of props. The most efficient prop was the 13x10 but I settled on a 13x8 since I could not find a reverse rotation 13x10.
For the motor, I am using a Hacker A10XL with a Kv of 900. Power is provided by a 4S pack of 2600 mah capacity. The ESC is a Castle set-up to run on governor mode and the timer came from Brodak. With the 13x8 prop I set the RMP in the 7200-7300 range which is quite low for proper governing but it seems to work reasonably well. On the ground, the current draw is in the 24 amp range while in flight it can peak at around 30-31 amps. The only thing I would do differently is use a pack of larger capacity, probably in the 3000 mah range. Even with the larger prop the plane will corner reasonably well (given my lack of skill).
For the Tutor I think that an Eflite 25; the one with a Kv of 870 is a reasonable choice. Given its Kv you will probably want to stay with a 4S pack. For props you could go with the popular 11x5.5 and opt for high RMP or maybe try a 12x8 at lower RPM. The Hacker has a somewhat larger outer can (diameter, larger diameter -> better torque) than the E-flite; I am not too sure that the Eflite would like the 13x8 prop. The thing to keep in mind with electrics is that some experimentation might be needed to find the set-up that works best. That is part of the fun.
Good luck,
Teo
PD I have been flying electric R/C since 2007 which probably skews my thinking away from what is common practice in C/L. I fly pattern for the most part (rather poorly I might add) and I am getting back into C/L after a 25 year break. Electric off course.
Dwayne Donnelly:
I can't remember who posted it here but a simple formula to figure out what size motor you need is 150 watts per pound of plane, so say you are shooting for a 52 oz. plane rtf = 3.25 lbs = 487.5 watts of motor.
Hope this helps ;D
Teodorico Terry:
Hello,
I have seen the 150 w/lb rule posted before, I think that the initial recommendation was for 100 w/lb and as systems got better the numbers went up. In my experience, the power required is actually more of a function of the speed you would like to fly at and the prop you intend to use for a given motor and battery combination.
For the Fazer I fly, the nominal power is in the 370W range in level flight. This propels the plane at about 53.5 mph which on 65ft lines gives you a relatively slow lap speed of 5.8 sec or so. When I pull into a wingover, the current draw goes up to 31 amps or so (ESC has data logging) which means that the power has gone up to 460W or so. That is almost a 25% boost in power due to the governor trying to keep the RPM constant. The model weighs 56oz or 3.5lbs which means that in level flight I am at 105 W/lb and 131 W/lb while performing a wingover. I would agree that I am slightly under powered but mostly due to the fact that the plane is so draggy and it could have been lighter. This is using a Hacker A30-10LX, CC ESC, 13x8 prop and 4S 2600 mah pack.
While trying to set-up the model, I played with different props trying to reduce the lap time to about 5.5 sec. What I found out was rather interesting, generally speaking, to get the same lap time, the power requirement was roughly the same. The larger diameter prop was about 5% more efficient than the smaller prop of the same pitch. Also, for the same pitch, the smaller diameter prop had to spin a little faster (makes sense if the level flight power was the same for a given lap time). The difference became more noticeable when maneuvering, the larger diameter prop did not appear to lose as much speed in a corner and it appeared to recover the speed more quickly. This is just an observation.
Going back to the original question, I would probably follow what others have done (i.e. Circulas 46) as a starting point keeping in mind that some experimentation might be needed.
Good luck
Steve Berry:
My kit finally came in (post office was slow this time). Now I have a feel for just how big this one is. I would say at least the size of the Forerunner, and the plans confirm the area to be 599in^2. I think if it was glow, the perfect engine would probably be either the .46LA or a ST G-51. Since I'm going electric....
This one is meant to be quick to build and get me back to flying, in a hurry. With that being the case, I'm very seriously considering either the RSM Sport or Competition glow .46 equivalent system, as opposed to building my own system. Everything is already done, and all I have to do is just mount & fly. I believe, given the size, the competition system may be better since it appears to have the stronger motor. Only other things to worry about would be a charger and spare batteries. It should be a good starting point for me so I can get familiar with the electrons. From there I can play & experiment with other components to have the confidence to build my own system.
What are y'alls thoughts on which system to go with, and which charger should I go with? The cost difference between the 2 systems isn't an issue, so I think it comes down to quality of components and how easy is it to tinker with later (switching to a different motor, adjusting timer & RPMs, playing with props, etc). I don't want to go cheap on the charger - since I'll be using it for years, a quality charger is in order. I just need to know which one, and if it needs a power supply.
Oh, before I forget. For those that have the experience, should I go with one of the 3D-printed mounts from Okie Air (leaning that way) or the BH-2022 Universal “E” Electric Conversion (for Profile Fuselages & Front Mounted Motors) by Brodak? Cost is the same, installation looks to be about the same. I personally think the printed mounted to be the cleaner looking installation (looking at you Dennis Nunes and your Circulas 46 IIe), but I'm welcoming any and all input on this matter.
Steve
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