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Old Dec 29, 2010, 04:17 PM
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Help with power setup for 2M e Chrysalis

Based on Don's website recommendations here's what I found. the MP Jets are no longer available. I found the speed 600 on Hobby-Lobby as a direct drive setup with prop SPEED 600 8.4V Motor/Prop Direct Drive Set, no data avail about the prop, I'm looking at the Phoenix 25A ESC. Now the questions, is the Phoenix ESC also a BEC or do I need one too if I choose not to have a separate Rx batt. Don't website suggests 7 cell (500-2000ma) but I can't find anything. What 3s lipo brand/config is compatible with this setup. Since I'm completely new to the electric model side of things I'm nearly clueless. I've read post after post about suggested combos for the Chrysalis but none that really explain what I need, where to get it and cost vs benefits. One that came close mentioned a low cost setup using the Turgeny motor but didn't tell me if I needed a BEC too. I always believe that keeping it simple is the best. Thanks in advance.
Wayne
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Old Dec 29, 2010, 08:09 PM
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I got an e-mail from Wayne on this today as well, sent him this reply:

Quote:
Wayne,

First of all, I would NOT recommend a Speed 600 today. Thirteen years ago it was the common setup for planes like the "Turbo 550" system sold for Goldberg's "Electra" (Goldberg's electric version of their Gentle Lady). It was a lousy, heavy, low efficiency ferrite "can" motor, typically powered by a grossly overweight 6 or 7 cell nicad pack, fitted out with those criminally awful Molex connectors, and a servo-controlled toggle switch instead of a proper ESC. It was a terrible powerplant, even then, but odds were if a beginner went into a hobby shop back then and asked for a motor system for a 2-meter sailplane, that's what they would sell them. We figured the plane better fly OK on that system, and that it would only fly better on something more sophisticated.

Today, brushed can motor systems like that are thankfully becoming a rarity. I should probably remove it from the drawing.

The motor you need is a brushless outrunner, 35-38 mm diameter, 5-6 ounces.

This one should be fairly similar to my MP Jet 28/20-7:

http://www.bphobbies.com/view.asp?id...7&pid=B1898547

On two cells and about a 13" folding prop you should be in good shape. Weight is a little low, you will need to keep the battery forward and watch the weight in the tail.

Other possibilities:

This one is a bit better in weight, Kv is a little low, so you would need a bigger prop, probably a 14" on 2 cells, maybe a little more:

http://www.hobby-lobby.com/25_size_b...33017_prd1.htm

Note, in this case "better" means heavier. The motor is just about the furthest-forward item in the plane. If you install a lighter motor, it takes a greater amount of weight further aft to get the right C/G. A lighter motor can actually make the plane's flying weight come out heavier!

Here's a Himax that's 5.7 oz and Kv=990. Himax motors are more expensive, but well made, and the company is very solid, not likely to dry up and blow away before the warrantee period runs out.

http://www.maxxprod.com/pdf/HC3522-0990.pdf

For an ESC, I'd recommend something like a Castle Creations Phoenix 35, which is what I have in mine at the moment. You will probably be pulling about 25-28 amps at full throttle, so a 35 am rating gives an adequate cushion above that.

Lots of possibilities, just get the Kv about right, and the weight about right.
Some further comments:

Brushed motors are largely on their way out these days for most of the larger applications. A brushed motor uses an armature with windings that spin, and a brass "commutator" at one end that spins with the windings and the rest of the armature. "Brushes", typically made of some sort of graphite compound (note, this is a block of graphite, not graphite fiber) ride on the commutator and bring the electricity from the non-spinning motor housing into the rotating armature.

The controller for a brushed motor is quite simple, all it has to do is supply voltage to the motor terminals. Throttling could be done by varying this supply voltage, but typically it's done by supplying pulses to the motor terminals. Note, the pulses are not phased with the motor rotation.

The down side is that these brushes add electrical resistance, arcing and electrical noise, brush and commutator wear, and a significant amount of mechanical friction, all of which have negative after effects.

"Brushless" motors have stationary windings, and the rotating armature carries the permanent magnets. There is no rotating electrical connection, so no brushes and all of their problems and lost efficiency. The stationary windings get fed what amounts to a three-phase AC current that is perfectly phased to the rotation of the armature. The much more complex speed controller senses the rotation of the motor and figures out when to switch the current in each of the three phases to match what the motor needs. In the early days of brushless motors they had Hall effect sensors inside the motor to tell the ESC what the rotational position of the armature was at any given instant. Today we have brushless, sensorless controllers that sense the motor's armature position by monitoring the "back EMF" coming from the motor. As the magnets on the armature move past the stator windings, they generate electrical pulses that travel back up the wires to the ESC, which tells the ESC that a magnet has passed that winding. "EMF" is short for "Electro Motive Force", which is a fancy word for voltage.

The key thing in our case is that the ESC for a brushed motor is not the same as for a brushless motor. A Castle Creations "Phoenix" controller is a brushless ESC. It is absolutely not compatible with a brushed motor, such as a Speed 600. For an equivalent brushed motor like that you would needs something like a Castle Creations "Sprite 25", or some of the GWS controllers.

An "outrunner" is a type of brushless motor that has the windings in the middle, and the rotating armature assembly on the outside. That's right, the outside of the motor spins! This allows for a lower Kv, the RPM's per volt.

On a permanent-magnet electric motor there is a direct relationship between the voltage supplied to the motor and the RPM that the motor wants to turn at. If you bog it down with too much load (such as a prop that's too big), it will pull as much current as it takes (or as much current as it can get) in an attempt to spin at the RPM corresponding to the supplied volts times the motor's Kv. This can result in damaging levels of current and heat. It's the electrical equivalent of putting your stick-shift car in 5th gear and then doing a lot of stop-and-go residential driving. You're going to burn something out.

This is why the required size of your prop is related to the number of cells in your battery. A 2-cell Li-poly pack (about 4 volts per cell, although that droops a bit under load) puts out about 7.4 volts at mid-charge, while a 3-cell pack puts out 11.1 volts. The RPM of the 2-cell pack will have only about 2/3 as much RPM, and since the power absorbed by a prop is proportional to the fifth power of the RPM, that makes a huge difference in the amps and watts the motor will try to pull from the battery. The motors that Joe and I have in our two Chrysalis models have similar Kv's, but Joe uses a 3-cell pack and uses a 9" prop, while mine uses a 2-cell pack and a 14" prop.

By comparison, nickel-cadmium ("nicad") batteries, or nickel-metal-hydride ("NiMH") batteries put out about 1.2 volts per cell, or about 1.1 volts per cell for NiMH under high currents. Thus, a 7-cell nicad pack has a nominal voltage of about 8.4 volts, a little more than a 2-cell Lithium Polymer pack. My MP Jet motor pulls about 28 amps and 180 watts on a 2-cell 2000 mah Li-poly pack, and about 32.5 amps and 240 watts on a 2000 mah nicad pack. Part of the difference is because nicads tend to have lower internal resistance, so their voltage doesn't droop as much under load. Of course the 10.5 ounce weight increase for the nicad pack makes the 2-cell Li-poly more attractive despite the lower voltage under load.

"Inrunners" tend to be better at making lots of RPM and relatively low torque, exactly the opposite of what you need to turn a big, slow-turning, max-efficiency prop for a typical sailplane. These typically need reduction gearboxes to convert the high-RPM, low torque output of the motor into the low-RPM, high torque power needed to turn a larger propeller.

Inverting the motor design into an "outrunner" helps its ability to make more torque, so it can turn a larger prop at a lower RPM, without needing a gearbox. The electrical efficiency of an outrunner motor is generally a bit lower than for an inrunner, but the improved efficiency of the larger, slower prop, and the elimination of the efficiency loss due to the gearbox typically make up for that.
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Last edited by Don Stackhouse; Dec 29, 2010 at 08:19 PM.
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Old Dec 29, 2010, 10:44 PM
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e Chrysalis power configuration

Don,
Thanks for the detailed explanation on the motor, batt and prop relationship. I still have so much to learn. I wasn't aware that the Speed 600 was a brushed motor or I wouldn't have even suggested the Phoenix ESC, I knew at least that much that you have to match brushed vs brushless.
I appreciate all the help. I know this forum is rich in learning, I just have to find all the appropriate threads and read them. I'm trying to get up to speed quickly, no pun intended...
Happy New Year to all.
Wayne
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Old Dec 29, 2010, 11:24 PM
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How about this combo

Don,
How about this combo: http://www.nesail.com/detail.php?productID=3331
Multiplex BL480-3G-37A ESC Combo on a 3s lipo with a 9x6 prop.
Wayne
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Old Dec 30, 2010, 09:35 AM
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The motor Kv of 4180, divided by the gearbox ratio of 4.4 equates to a Kv at the prop of 950, which is in the ballpark. It's quite long, which moves the C/G of the motor further aft, which could cause CG problems for the airplane. Also, that length forces everything else in the nose further aft, compounding the problem. We also don't know what the motor weight is, but I suspect it might be a bit lighter than the plane is designed for, so again, a potential C/G issue.

Also, with a motor that long, it's possible that the downthrust angle will tip the aft end up far enough to cause it to interfere with the underside of the hatch, although it looks like the motor is probably slender enough to avoid that.

For sport flying, I personally favor a 2S setup, and a 13" or 14" prop. Don't really need the extra cost and weight of the third cell.

That ESC is fine.
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Old Dec 30, 2010, 09:52 AM
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Quote:
Originally Posted by Don Stackhouse View Post
The motor Kv of 4180, divided by the gearbox ratio of 4.4 equates to a Kv at the prop of 950, which is in the ballpark. It's quite long, which moves the C/G of the motor further aft, which could cause CG problems for the airplane. Also, that length forces everything else in the nose further aft, compounding the problem. We also don't know what the motor weight is, but I suspect it might be a bit lighter than the plane is designed for, so again, a potential C/G issue.

Also, with a motor that long, it's possible that the downthrust angle will tip the aft end up far enough to cause it to interfere with the underside of the hatch, although it looks like the motor is probably slender enough to avoid that.

For sport flying, I personally favor a 2S setup, and a 13" or 14" prop. Don't really need the extra cost and weight of the third cell.

That ESC is fine.
Don,
I found a source for your recommended Himaxx setup in www.RCtoys.com that seems reasonable so I'm going to go with that along with your servo recommendations, a Thunder Power RC G4 Pro Lite V2 20C 2100mAh 7.4V 2 Cell LiPo 2SPL2 2100 Lipo, Phoenix 35A ESC, Spektrum 6110E Rx.
Will your recommendation for the separate 4 cell 270mah NiMH pack power Rx and servos? or do I power them off the BEC side of ESC?
Do you have a good source recommendation for the prop you suggested? What pitch, spinner size etc? Sorry for all the questions.
I will be calling today to order the plane.
Wayne
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Old Dec 30, 2010, 10:21 AM
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I like the Graupner "Precision" CAM props. Mine has a 14-9.5, but if you don't have a Wattmeter to make sure all the parameters of your system are within limits, it might be good to start a little smaller, maybe around 13". These props consist of a hub and spinner (the "Precision" ones use a plastic spinner with an aluminum backplate, make sure you get the correct collet size to fit your motor's shaft diameter), and a set of folding blades, which you can buy separately. You can swap blades to experiment with different sizes. I typically get them from Hobby Lobby, but I'm sure there are other good sources as well.

The nose can be carved and sanded to fit a variety of spinner sizes from about 35mm to 42mm, although the 42mm results in about the smoothest nose shape.

The radio battery pack I use in mine is a stock JR brand 270 mah square pack. That seems to be enough capacity for my purposes, but what works best for you depends on how long you want to be able to fly between charges, vs. the extra weight. As far as shape goes, mine uses the nose for the Rx, ESC and radio battery, with the big motor battery centered on the C/G under the wing. A square pack for the radio battery works well for me. However, in your case you might have a different arrangement that lends itself to a different shape. It's pretty much the same concerns that you deal with when selecting a pack for a pure sailplane or gas model, the required capacity for the flying you want to do, and the shape required to fit the place where you want to put it.
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Last edited by Don Stackhouse; Dec 30, 2010 at 11:04 AM.
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Old Dec 30, 2010, 11:26 AM
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On more comment regarding the geared motor in post # 5:

Using a geared inrunner can give you better electrical efficiency than a direct-drive outrunner with the same output RPM. However, that has to be multiplied by the gearbox efficiency, which on small, low power gearboxes like this might not be all that great.

The other problem with gearboxes is the noise. I can't speak for this particular gearbox, but most of the gearboxes I've heard in this sort of application and size range emitted a loud, piercing whine similar to that of a dentist's drill. Very annoying. A direct-drive outrunner just has a low-pitched hum from the prop, much easier on the ears.
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Old Dec 30, 2010, 11:33 AM
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Originally Posted by Don Stackhouse View Post
The other problem with gearboxes is the noise. I can't speak for this particular gearbox, but most of the gearboxes I've heard in this sort of application and size range emitted a loud, piercing whine similar to that of a dentist's drill. Very annoying. A direct-drive outrunner just has a low-pitched hum from the prop, much easier on the ears.
+1

I love the quiet climb of an outrunner setup. That is the way electrics are supposed to sound.

I do have a couple of geared setups, but they are in the ~1kw range with noses too small to fit an outrunner. Nice Hacker gearboxes, so they are fairly quiet, but the whine is there.

Pat MacKenzie
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Old Dec 30, 2010, 06:59 PM
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Adamone motor calc results

Don,
Using the Himaxx 3522, Phoenix 35, Graupner folding 13x7 and Thunderpower 2s 2100mah lipo, adamone's motor calc shows:
188.9 input power, 151.2 output power at 6.8V, 27.8A 80% efficiency, static rpm 5876, static thrust at 44.5 oz, full throttle duration of 4.77 min, static pitch speed of 39mph, app max level speed of 43mph.
Does that look good to you? If so, then I'll order that configuration.
Wayne
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Old Dec 30, 2010, 07:17 PM
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That all looks reasonable. Pretty similar to the numbers for my setup.
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Old Dec 30, 2010, 07:26 PM
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Quote:
Originally Posted by Don Stackhouse View Post
That all looks reasonable. Pretty similar to the numbers for my setup.
Thanks, looks like we have a combo....Can't wait to get the kit. I build real slow and take my time, measure twice and cut once....Besides I can only work on it every other weekend due to my work schedule...Still should have plenty of time before spring gets here in New Mexico. The place I fly from is at elevation of 6500', a pasture near our land in the edge of the Mangas mountains in southwestern New Mexico.

Again thanks Don, I'll be pickin your brain again later I'm sure.
Wayne
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Old Dec 30, 2010, 09:54 PM
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At that altitude you might do well with a little more prop pitch than the program predicts for sea level operations. The required prop pitch is related to the true airspeed, which increases as the air gets thinner at higher altitude.

However, it's not a big deal to get some other blades and do some experimenting.
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Old Dec 31, 2010, 10:38 AM
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Quote:
Originally Posted by Don Stackhouse View Post
At that altitude you might do well with a little more prop pitch than the program predicts for sea level operations. The required prop pitch is related to the true airspeed, which increases as the air gets thinner at higher altitude.

However, it's not a big deal to get some other blades and do some experimenting.
Don,
Thanks I was wondering about the effect on prop performance. I still have more to read to get all the basic aeronautical knowledge and terms in my head.
Wayne
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Old Dec 31, 2010, 11:39 AM
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For an airplane, the propeller is analogous to the transmission, drivetrain and wheels of a car. The pitch of the prop is analogous to the gear ratio of the car's drivetrain. If you are operating the car at a higher speed, you need a higher gear. If you are operating the plane in thinner air, and therefore a higher true airspeed, you need a higher prop pitch.

There's an on-line calculator for true airspeed here:

http://www.newbyte.co.il/calc.html

It looks like at your altitude your true airspeed is about 10% higher than at sea level, so that means you need about 10% more pitch. Instead of a 13-7, you would need a 13-7.7 prop. A 13-7 would not pull as much watts at your flying site as it does here. Going to a 13-8 would pull a little more. However, you need to verify that it doesn't pull enough more to exceed the parameters of any of your components, such as the amps limits on your ESC.
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