|12-13-2007, 02:22 PM||#11|
I am: Boolean21
Join Date: Sep 2002
Total Props: 56
Re: New to Electric - where to start ??? Help
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Not going to give you a lecture on electrics -- lots of that in the forums and various books you can get.
My recommendation as the best first step is to find out who has the same kit and see how their setup flies. Duplicate it.
How I got started -- got a solid airplane with a reasonable system, learned the ins and outs of operating and charging. Then went out and got my second plane and have worked that.
Now on several projects with various combinations.
One thing to standardize on is the battery setup that you're using.
No sense in having a dozen models all with different battery combinations.
I'm going to try to standardize on a few types (2s 350 for indoors, 3s 1350 for the prop-jets, and 4s 3300 packs for the big trainer and sports models.)
My 0.02 anyway (starting on the Ultrafly Hawk this weekend....)
|12-13-2007, 03:58 PM||#12|
Join Date: Sep 2006
Total Props: 0
So, assuming you test your setups on the ground first and verify current draw at WOT, then you know the worst case scenario that your given power system will pull, so matched system or not, you'll be OK in flight.
Your post has less to do with overpowered controllers I think and more to do with poor flight preparation. Absolutely, in your given example, if a person does not test their setup and so has no idea what their amp draw is, a more powerful controller will allow them to feed more and more current to the motor potentially destroying it and their battery. But this isn't because that person decided to use a more powerful controller, it's because they didn't test their setup.
There is, however, some good reasons to use bigger controllers. Differences in manufacturing, even among the same line of products, can spell problems if you match your setup right down to the last amp. I've seen both good quality (Castle creations) and cheaper branded controllers burn up just as nice as each other when people come too close to their rated current. On the same token of course, i've also seen the same controllers work way beyond their specs as well, it's a gamble one way or the other.
The other benefit of a stronger controller than you need is that it also gives you room to play with different props if you're not happy with initial performance.
|12-13-2007, 10:53 PM||#13|
I am: fred f
Join Date: May 2005
Location: Burlington Ont.
Feedback: 1 / 100%
Total Props: 2
I still say an oversized speed control is just a problem looking to happen.
You even state "it will give you room to play with different props" (go bigger)
How many of your new to electric customers buy a Watt meter along with their first power system?
I base my comments on real life observations, out of the 5 electric Heli and 12+ fixed wing electric flyers in my club only the Heli guys started out with watt meters. All the fixed wing guys who own one purchased it after smoking components! and they all smoked stuff edging up the prop size.
I fly indoor and outdoor up to 36" wingspan 3D so yes I want the lightest power system putting out the greatest power possible but will not add dead weight to a plane and for me an oversized component with unused capacity is just dead weight and money that could have been spent on useful stuff.
From a sellers standpoint I can understand your advise, more dollars for you and fewer returns as the customer is less likely to damage an oversize unit. Happy you, happy (repeat) customer
So back to the original point. Match the motor and speed control then prop to draw UNDER the max. I aim for the continuous rate. Simple easy and no component is pushed hard. If the flight performance is less than expected you have undersized your power system for the flying weight or wing area of your toy.
|12-14-2007, 11:02 AM||#14|
Join Date: Aug 2004
Total Props: 0
You're getting good info here, although because we're all sort of shooting from the hip, it's coming at you in a shotgun sort of way.
Let's try it from scratch. You're not a total newb, so you're familiar with how glow plane engine requirements work. A plane of a certain weight will need a particular sized motor, right? So a plane that weighs 3-5 lb will need a .30, a 5-7 lb will take a .40, a 7-10 lb will take a .60, and so on. You probably also know that if you're going 4-stroke, you need roughly 30-50% more displacement than a comparable 2-stroke.
So that covers the basic power plant, but then there's the question of propping. You probably know that for different styles of flying you're going to use different sized/pitched props... so for high-speed, you want a smaller faster-turning prop, but for 3D you want a bigger, slower-turning prop.
Well, it works the same way with electrics. Except that instead of measuring electric motors by their displacement, we measure their power output - Watts. Glow engines are ranked the same way, but they're so standardized, you don't think about it... the displacement is more-or-less equivalent to a given horsepower.
For any plane, you need to decide what sort of flying you want to do - obviously you'll need more power for an aerobat than you would for a slow-flyer. An easy way to do this is use the Watts/pound guide:
Using that, you can figure how much power you're going to need to get your plane to fly the way you want. You've said you're looking at an EDF - the power requirements are a bit different, since the ducted fan is less efficient than a free-prop (that's another discussion!) - so general rule of thumb, figure 50-100% more watts/pound for an EDF.
So now you know how many watts you're going to need to fly, you can pick a motor rated for that power. That motor should come with a spec sheet (if it's any good) that will tell you how many watts it can produce on various battery (voltage) and prop combinations. It will also tell you what the maximum amperage it can handle, and that's where you decide what ESC is appropriate for that motor.
As I said before, there's no standard on describing motors, so ignore all the nomenclature that the various manufacturers hang on them, and just look at how much power the motor will produce.
As for batteries, think of them as the % nitro... sort of... Basically the more voltage, the more zip you get out of the motor. Lipos are pretty simple, really. There's 2 numbers you need to worry about - pack voltage and pack capacity. Individual lipo cells have a standard voltage of 3.6v - just like regular household batteries have a standard voltage of 1.5. And just like household batteries, if we need a higher voltage, we put more cells together in series. And that's where the 1s 2s 3s nomenclature comes in - the number of cells in series. so a 2s pack has 7.4v, the 3s pack has 11.1 v, a 4s pack has 14.8v...
The capacity is just like the fuel tank size on a glow plane. The bigger the number, the more charge the battery will hold, but the heavier it will be.
But the capacity also has a bearing on how much current the battery can handle. Larger capacity packs will handle larger amperages. Lipo packs are rated for how much discharge they can provide. The discharge rate is a function of the battery's capacity (C). most packs will easily handle a current of 8 - 10 times the pack's capacity (8-10C). So a 10 C 2000 mAh pack will handle 20 amps. Larger C ratings let you get away with higher amp draws, but you'll deplete the pack faster too.
1) figure out what the plane weighs, and decide how many watts/pound you want in it,
2) find a motor that's rated for that wattage (maybe allow for a bit of overhead),
3) from that motor, find what it's max amp draw is and get an ESC rated for that (again, perhaps with a bit of overhead,
4) get a battery that will provide the proper voltage to provide that power level, and be able to handle the current draw.
With an EDF, your "propping" options are going to be more limited than with a regular prop, and the EDF unit you're getting should come with some motor recommendations. You should be able to take the specs from the recommended motors and translate those to other manufacturers motors.
Hope my little lecture helped... I'll step out from behind the podium now...
Eagles may soar,
but weasels don't get sucked into jet engines.
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