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spacestout -> RE: Hobbico Electristar Select EP Trainer RTF (4/5/2007 9:33 AM)
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To itchycods, I started to answer yesterday and got thru a paragraph of calculations and then had to leave. Just as well. My answer is that you need to find out if your batteries are good or if the problem lies elsewhere. A watt meter is good for this. I cut my harness and added a single Deans connection. Then I can plug the two batteries into their harness and in to the watt meter, and the output of the watt meter into the ESC. At full throttle on a new charge and stock prop, you should expect to see around 28A, dropping off a bit after a minute. Half throttle would be about 14A. You should be able to sustain this for a good 10 minutes (a little full throttle down to half throttle). Full throttle would produce something like 3.2Ah/28A=.11hoursX60min/hour=6.8 minutes (you would probably hit the cutoff before your battery lets you use all 3200mAh). So, your voltage starts at nearly 16.8 and drops to 15.5 after a minute. Then it stays there for 6-10 minutes and then starts to drop quickly as you get to 3V/cell or 12V/pack. LiPos drop from their peak voltage quickly over a minute, then stay fairly stable for most of the time, and when about out of juice, drop off quickly. If you don't see this type of curve your battery may have problems. I went to fly a plane which checked out fine on runup. The battery cutoff shut it down on climbout. Even though the battery appeared charged, it discharged too quickly under load. I undertook a program of charging the battery and then discharging it at 8A and repeating. It eventually worked correctly. Enough. Investigate, then go to your LHS with your findings to get better expertise.
To ro347:
Make sure that the APC is the E version, and not the slo fly. So, a 12x6E. They are thicker.
My first attempt with APC was just to deepen the hole on the prop. Then I realized that the widest point on the back of the prop is true for balance and the
thinnest ring which comes with APC fits in there and is the right size for the 'star. The rest of the hole to the front can be drilled in an inexact fashion as it doesn't true the prop. That is done by the back insert and the washer against the front face. The key is that the hole drilled to the front needs to be a little bigger that the motor shaft, but not as big as the hole for the back insert.
I used for the first time tonight a Prop Reamer. A cheap hand tool from your LHS which enlarges the holes.
In summary, ream a hole all the way thru the prop, bigger than the shaft, but smaller than the insert ring shoulder. Insert the thin ring in the back, slide the prop on, and put the screw and washer into the front. It should tighten flat.
Funny, the 12x6 still draws less current than the original 11x7, but charts show it delivers more thrust. A bigger disk moving air. It's like being in a lower gear. I like it. The plane climbs steeper. I have a 13x6 to try next. I just need to stay under the 35Amps at full throttle, and then not run at full throttle all the time. I'm told that every make of prop will be different. You have to experiment.
As you said you are new, a watt meter is useful. Place it between the battery and ESC ( some mods have to be done on the harness) and it will tell you how much current you draw, the voltage, and it computes the wattage. Motors and ESC's all have a continuous current rating, and a burst rating. You can do the surge rating for 15 seconds or so, and then you'd better back off. It's all about not overheating things. The electristar has continuous current capability of 35A, with 50A possible for short bursts. The ESC can handle 45A. Our motors try and spin at so many RPM per volt (the Kv rating). If our motor is rated at 1200Kv, and 10Volts are applied, it tries to spin at 12000rpm. Now, if you have a prop trying to take too big of a bite, it will draw too much current and overheat things. That is why, when experimenting with props, that you need to have a watt meter. Make sure you, at your altitude (thinner air needs more prop to get the same bite) don't draw too much. If you can find the specs on your motor, you can look up things like internal resistance, no throttle current, and use calculation SW such as at aveox.com. Entering the motor specs, altitude, voltage (what your batteries are putting out minus some losses thru wiring and the ESC) you can try different props and you will get a good approximation on thrust (how well it will climb) and pitch speed (how fast it will go. Always a trade off).
I'll try and post some measurements on the 13x6.5 prop.
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