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  1. #1
    TManiaci's Avatar
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    Motor - Prop Selection ???

    Okay, first forgive me for not searching these forums (for hours) trying to find my answers.

    My question is simple (I think). How do you figure out this mix of electric motor, battery source, ESC and propeller to get what you want?

    My goal is to put together a propeller with a brushless outrunner motor to make 30+ oz's of thrust. I look at motors, props, etc., but none of these sites seem to pull it all together and tell me what I need to know to make the right combo. It seems that it is easy to overload a motor by putting on too much prop or under-rating the ESC and/or battery.

    I have a TP1500 and a TP1320, so to start I would like to use these batteries if I can. I am building an Ultrafly Extra 300, which I see by reviews is sorely underpowered. The optional motor Ultrafly offers to go with the gearbox also are reported to be weak. The plane promises to weigh in at 22 oz's, so I figure I should shoot for 30 oz's minimum thrust to get decent vertical performance.

    I have an AXI 2208/34 running on a Jeti 8 Amp with the TP1320, but that is said to only turn out 17 oz's thrust with the 10x3.8. So, is there some magic formula to compute all this into simple terms to spec a proper system?

    Please help... [sm=rolleyes.gif]

    Thanks in advance...
    TMan

  2. #2
    Moderator Matt Kirsch's Avatar
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    RE: Motor - Prop Selection ???

    There's no magical formula, I'm afraid, just a different way of thinking than you're probably used to. Many motor manufacturers provide these thrust ratings because they can't come up with any other simple way to rate their motors. Unfortunately, they're as meaningless as the HP ratings on glow engines...

    First off, it's okay to plagarize. Find a plane that's similar to what you want to power, and copy its power system. That's the easy way out. For example, this ultrafly Extra 300 sounds like it's similar in size and weight to the Horizon Hobby's Mini Funtana. There are plenty of options out there for good 3D power systems on the Mini Funtana.

    To really understand what's going on, start by remembering that it's all about the power. Forget about thrust, because if you give the plane enough power, and a reasonably-sized propeller, you'll have plenty of thrust without worrying about it.

    Electric power is measured in Watts. Powering an electric plane requires so many Watts per pound to achieve a certain level of performance. I hesitate to call it a "power to weight ratio" but it kinda sorta is. For a 3D plane, 150 Watts per pound will provide all the thrust you'll need for most any maneuver you can think of, regardless of the size of the airplane. A 22oz airplane is 1.375lbs, so you need around 200 Watts of power.

    From here with AXi motors, it's pretty easy. Measure the prop clearance you've got for takeoff and landing, and plan on the biggest possible prop. Next, go to www.modelmotors.cz and look through the charts on the available motors for one that takes at least 200 Watts (input power) and spins a prop that's close in diameter to what you figured for ground clearance. Read from the chart to get your motor, prop, Amps, and battery.

    Check this out: http://www.modelmotors.cz/index.php?id=en&nc=produkty_vypis&kategorie=m_neodym_ac&id_rady=axi_28&id_produktu=axi_2808_24&nazev_rady=Series%20AXI%2028&hmotnost_rady=(78%20-%20161%20g) This is the AXi 2808/24. With a 10-cell NiCd battery and a 9.5x5 prop, it'll draw around 19 Amps for an input power of 209 Watts. Except for converting NiCd to LiPoly, it sounds like the perfect power system to me.

    Converting the NiCd pack to LiPoly, we know we need a similar voltage to the 10-cell NiCd, and the capacity to handle at least 19 Amps. A 3S LiPoly is VERY close in voltage to a 10-cell NiCd pack (11.1V vs 12.0V nominal, under load is much closer). Unfortunately, your 1200 and 1320 packs won't handle the 19 Amp load, so you'll either need to put two 3S 1320 packs or 3S 1200 packs (no mixing sizes) in parallel if you've got 'em, or you need a 3S LiPoly of around 2000mAh in capacity.

    Don't get hung up on the prop size, as I know 9.5x5 is a tough find in the USA. You have some wiggle room in either direction, just like on a glow engine. You could go down to a 9x6, or up to a 10x3.8, and the power consumption will be about the same.

  3. #3
    TManiaci's Avatar
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    RE: Motor - Prop Selection ???

    Wow Matt, thanks a bunch... more in one post than I got out 100's I've read.

    Well, it's no wonder I was so mystified by all the hazy information I was gathering. I assumed this was a science, not hit-and-miss try-it-till-it-works kinda thing. I'm an engineer, so I am perplexed by the lack of collective mathematical precision all this should boil down to. Why is it not quantitative in some analytical way? I don't get it.

    In the glow world, we start with a prop turning at a specific rpm to give a fairly well defined thrust... that in-turn yields a horsepower requirement, then you select the engine to provide that rating... done. Seems the same routine should work for electrics?! There are 746 Watts per Horsepower... so you rate them by watts, but it's still power.

    So, help me to understand... why can't I use my existing batteries? They are all 3S 11.1V lipos... so don't they just limit flying time by being too small? Or, do these types of batteries somehow deliver whatever the motor/esc wants and that in-turn overloads them?

    Thanks again for your help!

    A humble student [8D] of a new art...
    TMan

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    Moderator Matt Kirsch's Avatar
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    RE: Motor - Prop Selection ???

    It's a science, but it doesn't have to be an exact science.

    You know, I think if you ask any of the glow converts here, that not a single solitary one has ever worried about RPMs or horsepower on a glow engine when powering an airplane. They choose their power system one of two ways:

    1. Use what's recommended on the box.
    2. Use the same engine as what's commonly used on planes of similar performance, size and weight.

    Regarding your batteries, size doesn't just affect flying time. Every LiPoly has a hard limit as to how much current it can produce without sustaining signifigant, permanent damage. It's called the "C rating," and is expressed in a multiple of the battery's capacity. The majority of LiPolys on the market are rated at 10C, meaning that they can handle 10 times their capacity in current. For your TP1320 and TP1500mAh packs, that means 13.2 Amps and 15 Amps respectively. Try to draw 19 Amps from either of them, and you'll fry them. You can use them if you gang two packs in parallel to share the load.

    It's the motor/gearbox/prop combination that determines how many Amps will be drawn. Everything else in the system is passive. The ESC only lets current through; it doesn't limit or regulate it, so you can easily draw 20 Amps through a 10 Amp ESC and fry it, for example. Same thing goes for the battery...

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    RE: Motor - Prop Selection ???

    Thanks Matt... you-dah-man!

    Okay, I get it... I understood the 10C was 10xmAH capy, but I was assuming the battery naturally limited it's output to that level as a function of internal resistance or whatever. My bad there.

    So, my son flys an AXI 2208/34 on a TP730 turning a 10x3.8... he is pushing it a little at WOT with that setup, right? He "could" burn up that battery if I interpret the charts right. I'm trying to make sense of the AXI charts. Should we downsize the prop to control the current delivery? (I assume this works)
    ~~~~~~~~~~~~

    So, clearly to equip this 22 oz bird adequatly, I need 200+ watts, so I need more battery (2000+ mAH w/ 10C capability). I'll look at the Mini-Funtana threads and see what they are using, that's a good suggestion and starting place. Been lloking at the mini-Funtana anyway, so the gear I get for this Extra would move there if I was dissapoiinted (good plan )
    ~~~~~~~~~~~~

    Now, on to my next quandry(s)... hope I'm not being a pain .

    Gearbox vs. Outrunner? What are the pro's and cons? I love the brushless outrunner for simplicity... why would I select a geared system?

    TMan

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    RE: Motor - Prop Selection ???

    try the 11x8 gws prop i swear by it ..... alot of people tend to use too small or too big try right in the middle = 11x8 gws prop
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  7. #7
    Moderator Matt Kirsch's Avatar
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    RE: Motor - Prop Selection ???

    Indeed, your son is pushing it a little with that setup, both on the motor and at the ESC. According to the ModelMotors website, www.modelmotors.cz, the motor draws 8.8 Amps on a 3S LiPoly with a 9x3.8 prop. Current goes up proportionally with prop size, so be assured that you're drawing signifigantly more than 8.8 Amps. The battery should be just fine though. You can make an educated guess as to the current pretty easily by extrapolating from the existing data on the charts. I'd estimate in the 11-12 Amp range (ouch).

    Downsizing the prop is the best way to cure an overcurrent problem. Some people use throttle management if it's "close" (and some even if it isn't all that close...), that is, they limit their full-throttle runs, using it only in short bursts, but that's hardly the best solution.

    Geared vs. outrunner is mostly a personal preference, though many people are thinking along the same lines as you, and are choosing the outrunners for their simplicity and robustity (is that a word?). Overall, efficiency is close to a wash; geared motors' efficiencies are listed for MOTOR ONLY. They don't take the gearbox, which is only 80-90% efficient in and of themselves. The main advantage to a geared motor is flexibility. Outrunners are more like glow engines in that they're limited in the range of props they can spin without drawing too much current. On a geared system all you have to do is change the ratio and you've opened up a whole new range of props.

  8. #8
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    RE: Motor - Prop Selection ???

    Matt,

    I see on the Thunder Power site that the TP730 is rated for 12C (9 Amps), with burst capability to 16C (12 Amps). That, along with knowing he rarely runs sustained WOT flight is probably the reason he gets away with it. He probably has no less that 100 cycles on that setup. Never-the-less, we'll downsize the prop to put him into a safe zone. Thanks for the head's up...

    So, now I see the value in those amp/power measuring toys everyone seems to have. I assume that's the best way to test props and find the "optimum" load for a given setup. I guess I should get one of them.

    You have answered all my questions (for now). Thanks so much for your expert advise. I got more in one day here than I could from three hobby shops and reading dozens on threads on the subject (or related subjects) for the past week.

    I'll report my selection and outcome on the Ultrafly Extra 300 setup. I'm sure others will find this thread a usefull wealth of knowledge.

    Again, many thanks Matt...

    BTW: Do you work for RCU? How is it you "moderate" in th forums? Are you compensated in some way?
    TMan

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    RE: Motor - Prop Selection ???

    In what way are you going to downsize my prop?
    I hope that I am not going to sacrifice performance in doing this.

    I probably have about 50 cycles max. on the 730. I dont fly that much, it is always windy out in the boonies where I live.[]

    If I am pulling too many amps. with my speed controller, am I hurting it?
    ...Of course I meant to do that

    Tyler

  10. #10
    Moderator Matt Kirsch's Avatar
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    RE: Motor - Prop Selection ???

    Guys,

    Downsizing the prop will most definitely affect the plane's performance, which is why you need to be careful. Go too small, and you won't be happy. Use "baby steps." Diameter has more effect on the current draw than pitch, so you can drop from a 10x3.8 to a 9x4.7, for example, and end up with a lower current draw.

    tylerman, the problem with the way it's set up now is that you're exceeding the rated limits on pretty much everything in the plane when you go full throttle. Get a little heavy-handed, and it's going to cost you some $$$.

    How is the plane's performance now? If you're flying mostly in the bottom half of the throttle travel, you can afford to lose a little power on the top end.

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    RE: Motor - Prop Selection ???

    I have enough power for my tensor that I usually only fly 1/2 to 3/4 throttle(and below of course)--and I usually only use my full throttle to pull out of a hover or something of that sort--even then only for 15-20 seconds max. I will admit that a couple of times I may have gone full throttle for longer-But with my new knowledge I probably wont do that.

    I still dont understand that if I my batt. is rated for 12c to 16c bursts shouldnt I be okay?
    ...Of course I meant to do that

    Tyler

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    RE: Motor - Prop Selection ???

    Het Matt, another quick question for you...

    I see that many of the motors out there are rated by kV... other aren't. So, given our need to have a motor that pulls around 200 Watts, with a 2000 mAH power source at 10C (on an APC 9x6 prop for starters), how does this motor get converted to kV?

    This rating method doesn't make sense to me. kV is not a power rating... duh. The motor we are looking at is rated at 1270 kV (Dymond Maxi 09 brushless outrunner, www.rc-dymond.com ). Yea, we are doing this one on a budget .
    TMan

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    RE: Motor - Prop Selection ???

    Tyler,

    The "burst" rating means just that, a short moment running over the rating. They are not clear ont he website or literature what time that represents, but the 12C rating is for 60 seconds... so the 16C is probably for just a few seconds at most. Also, you have a Jeti 8 amp ESC... even at 12C it's pulling over the Jeti's rating (9 amp load on a 8 amp ESC)...

    The fact that you have not complained about motor shut-downs (internal safety shutoff) probably tells us that you have been staying below the 12C rating most of the time. We both ran this same setup with the TP1320's on the Hellraisers, and they definitly were overloading the ESC considerably. I think we have just been lucky, or the Jeti is just a good quality robust ESC that can take overloads well.

    The other possibility is that we are not extending the endpoints of the throttle channel, and maybe we have never been running full throttle. I saw somewhere where a guy adjusted the upper endpoint on his Throttle Channel and got more power WOT. I thought the Jeti automatically adjusted for full range, but I really don't know for sure. We'll get one of those Watt Meters and check this out for sure soon. Maybe there is more power to tap, and we didn't know it?

    Maybe Matt can address this question too...
    TMan

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    RE: Motor - Prop Selection ???

    When I get a different selection of props and stuff, and when I find a good prop will it reduce the how much current it is drawing from my 730?
    ...Of course I meant to do that

    Tyler

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    RE: Motor - Prop Selection ???

    Tyler,

    I believe changing props to control your current load will be of some benifit, because you will not be overheating the motor and the ESC. However, the battery will not last any longer unless you were regularly overloading on your old setup (10x3.8 prop, AXI2208/34, Jeti 8A, 3S TP730 pulling over 12A peak WOT).

    You are running around 7-8 amps at 3/4 throttle now. With the proper propeller, you should pull around 8.75 amps WOT (12C, battery rating). That way you can run WOT without damaging any of your equipment. I got you a 9x4.5E and a 8x4E to try.

    The big difference is the vertical pullout at WOT... it won't be as agressive. You are just lucky you have not fried that battery already.
    TMan

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    RE: Motor - Prop Selection ???

    An update here… Have learned TON's since last postings, and I noticed I never posted the results from the tests on the motor selection.

    First, the Maxi 09 from Dymond Modelsports was not the best selection for 3D performance, but it does perform very well. We started with an APC 9x6, 3S 2100 12C Lipoly, and 25A ESC. This setup drew 26 Amps WOT, and we decided it was overloading a bit too much. Backed it down to a 10x5E, and now pulls 21 Amps. Performance was very good, but since then we have learned a few more things... hope this helps others to get past all the mud and fog to come to some level of understanding on this stuff.

    Anyone, please chime in if you find error in my understanding.

    Propellers:

    Slow Fly Props... The APC "SF" props (GWS, etc) are VERY different from the "E" series props. The SF props have very broad blades all the way to the end and skimpy connections to the hub in the middle. The E props are more like the big props for nitro planes, where they taper to the tip and have thick healthy sections at the center. SF props are for “slow fly” planes, where 3D is probably the primary flying mode and lots of prop wash is desirable over the control surfaces. They are designed to carry small loads and turn relatively low speeds (generally well below 10,000 rpm). Slow-fly props are apparently not very efficient compared to E props (power in to power out ratio).

    E props are designed for high speed, and better efficiency. They generally are smaller in diameter and applications tend to best use them turning really fast for sport/speed flying. They are a poor choice for 3D applications. They don’t produce a big wide blast of air. The faster they turn, the more efficient they become, until the blade tips approach supersonic velocity.

    Motor Selection:

    The Maxi 09 motor we selected is a really strong little workhorse. But, we learned something about the “Kv Rating” I kept asking about. Kv translates to RPM’s per Volt (RPM/V). So, that means that 1200 Kv motor, running at say 10.2 volts, will be turning at 1200 x 10.2 = 12,240 rpm. Other definitions need to be clear to understand all this…

    Brushed Motor: A conventional motor, where brushes ride on a commutator on the moving stator with windings and stationary permanent magnets on the outer shell. Typically runs on conventional single phase DC voltage.

    Brushless Motor: A unique adaptation of giant industrial electric motors, where there are no brushes. The motor runs on three-phase AC power. There are 3 leads to these motors, and the Electronic Speed Controller (ESC) digitally produces this variable low-voltage 3-phase AC power from a DC battery power source. These motors run at far greater efficiency than brushed motors.

    InRunner: A motor, brushed or brushless, running thru a gear reduction assembly to reduce RPM and increase torque. Allows use of larger diameter props with greater pitch turning slower to provide better prop-wash over control surfaces. Also provides the opportunity to alter gear ratio.

    OutRunner: A “direct-drive” (typically) brushless motor design, where the prop is mounted directly on the motor shaft and the magnets turn about a stationary stator. Outrunners have typically far lower Kv ratings than Inrunners.

    So, for a typical “OutRunner” setup, you look to match the Kv rating to the type of aircraft or flying style. A lower Kv means greater torque and slower rpm to turn big fat low-pitch props for lots of air movement to wash over control surfaces, obviously that’s for 3D flying. A high Kv is selected to turn smaller high-pitch props fast for high-speed flight, where you rarely transition from sub-stall speed flight to WOT high-speed flight.

    For an InRunner setup, the same applies, except you use a higher Kv motor, and use the gear reduction to reduce the relatively high rpm to the desired revs for the application. The downside to InRunners is the inherent loss of efficiency with the extra bearings and gear friction. The upside, as Matt explained, is the wider prop selections you have available by changing the gear ratio. The InRunner also allows use of relatively inefficient and very inexpensive brushed motors. The biggest benefit to the InRunner in my mind is that if you choose the wrong Kv motor, you can correct it to an ideal setup with gear ratios.

    Continued…
    TMan

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    RE: Motor - Prop Selection ???

    Motor Selection, Continued:

    Watts, Amps, Thrust, Kv ratings, Props, Lipoly Batteries and ESC’s, how do you muddle thru all this?! First, let’s discuss batteries. Then we can pull it all together…

    Lipoly Batteries:

    Batteries were a real mystery at first, but it isn’t as complex as it first seemed. First, we now recognize that “Lipoly” batteries are the best commercially available power storage devices that we can obtain at affordable prices.

    The acronym Lipoly is from Lithium Polymer (aka, Lithium-Ion), and represents a very high-density energy storage technology developed mostly for cellular telephones. The energy density of a Lipoly or “Lipo” is more than twice that of NiMH (Nickel-metal Hydride) which is better than NiCd (Nickel Cadmium) by some significant margin. This translates to batteries that are now far less than half the weight with incredible current delivery capacity.

    This advance in battery technology is the single most significant reason for the explosion in popularity of “foamie” aircraft. Coupled with high efficiency brushless motor technology, the power-to-weight ration of electric powered aircraft has finally come to a practical level to make this hobby segment a possibility.

    Here are the basics about Lipoly batteries as I understand it now. Terminology is cryptic. Let’s decipher the simple “3S TP1320 12C” that I mentioned before.

    3S = 3 cells, and each Lipoly cell is 3.7 VDC, so a 3C is 11.1 VDC.

    TP = Thunder Power. Many batteries have grown nick-names.

    1320 = milliamp-hour rating, or “mAH” as it is typically noted. This is what it implies; you should be able to draw 1320 milliamps (mA) for one hour (or 1.32 Amps for one hour). It’s not entirely accurate in reality, because you can only safely discharge the battery to a specific minimum level, and beyond that the battery can be damaged. There are also efficiency factors and temperature variables to further influence the battery’s ability to deliver energy, as well as the charge “quality” and topping-off efficiency of the charger.

    12C = Current Delivery Rating, which is represented as a multiple of the AMP-Hour (not milliamp hour) rating of the battery. So, the rated “continuous” current draw for this battery is 1.32 x 12 = 15.84 Amps.

    This is a tricky little trap for novice electric buyers. We see in the market that the lower the C rating, the cheaper the battery. Not understanding the concept of the “current delivery capacity” and buying a bigger mAH battery with a low C rating can leave you wanting.

    My first purchase of a Lipoly battery was a Watt-age 1500 mAH. I figured out later that this battery is only 7C, which calculates to 10.5 Amps. Later, by luck only, I bought Thunder Power 1320 mAH batteries that have a 12C rating… they can deliver 15.8 amps! At first that was mystifying that a 1320 was WAY better than a 1500? Well, it’s all about the quality of the Lipoly cells, and their inherent ability to deliver current. Buyers beware! You need a calculator to buy properly matched batteries!

    There are other complications with battery jargon that I don’t see the novice concerned with that has to do with parallel and series gang-wiring of multiple cells for bigger aircraft. Buy the time your ready for that, you’ll already understand all that from your own learning.

    Continued…
    TMan

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    RE: Motor - Prop Selection ???

    Continued… The Control in E-systems:

    So, the last part of the “system” of motor-battery-propeller mystery is the ESC, or Electronic Speed Controller. These expensive little toys are simply a means to convert the variable throttle position on the transmitter to an output power to the motor via the receiver in the plane. They do far more than this, and are far more sophisticated than you may think.

    There are fundamentally two types, one for brushed motors (straight proportional DC output) and one for brushless motors (3-phase AC output). Other skews in this device are about the BEC, or Battery Eliminator Circuit.

    The conventional ESC provides one connection that serves two purposes… one is to supply power to the control system via the receiver connection on the throttle channel (that’s the BEC), and the other is to act as the input for the throttle signal from the receiver. The connection is a typical 3-pole servo-type connector on a lead, again, plugging into the Throttle Channel.

    A note here: It is not clearly documented, but you may supply power to the receiver, and hence servos, via any of the “outputs” on the conventional radio receiver. This was confusing to me at first, because we are getting power “from” the ESC and sending proportional throttle position signals “to” the ESC thru a single connection.

    As I can tell, the BIG ESC’s generally omit BEC power supply because the practice is to provide a separate power source for driving the servos and receiver. Those platforms rely on separate isolated power systems just like on fuel aircraft for safety and reliability.

    ESC’s are rated by their Amperage Capacity. This is the current load that is drawn by the motor from the battery thru the ESC. These devices do not limit the current, but they do generally have overload protection. So, you may pull more current than they are rated for, but when you do this, they will heat up and may trip thermal overload circuit protection. Most ESC’s, when overloaded or overheated, cut power output to the motor, but continue to provide power thru the BEC so the flight control systems are not lost.

    Good cooling is necessary for the ESC, particularly if you are pushing its limits. It is recommended that the ESC be located in a space where it is in a free flow of air. Do not wrap it in protective foam or place it in a confined space without good ventilation. Better quality ESCs can generally perform well above their rated limits with no apparent ill effects, but you should always size the ESC at least one step above your expectations for current load to be safe. I had, by ignorance, been running my Jeti 8 Amp ESC regularly at WOT (Wide Open Throttle) loads at 13.5 amps without ever hitting thermal overload or noticing excessive overheating. Lucky I guess, but a tough little bugger for sure.

    Many ESCs have programmable features like Braking, Soft Start, “timing” functions, etc. These provide modelers with options to suit their preference in how the ESC controls the motor. Read the instructions that come with your ESC for detailed instructions on programming. Every brand of ESC has various different techniques to program these features.

    Many ESC also have a low-battery warning system to notify the pilot when it’s time to land. This is provided so the pilot does not lose control of the aircraft by complete power loss, since the ESC powers both the controls and the motor. Various ESC controllers do this differently. Some make the motor surge, some beep, some shut off the motor and require pulling the throttle back to idle and then back up to restore motor power. Most reserve some juice for a controlled landing under limited power. Again, refer to the ESC instructions to understand how your particular ESC manages low-battery conditions.

    Continued…
    TMan

  19. #19
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    RE: Motor - Prop Selection ???

    Continued… Pulling it all together, sorta.

    Well, I don’t profess to be an expert on any of this, but those are the basics for the hardware on these toys as I have learned it. Forgive me if there are inaccuracies, but understand I am trying to present this for a novice’s perspective and skipping over some technical details to keep it simple and meaty. Now we can go back to the question that started me on this learning journey.

    We decide we want to go electric for whatever motive, and we start getting all juicy over a particular plane, we now have to face the hardware selection head-on. Like Matt suggested, just go look up similar aircraft and see what they do, and duplicate it. The other choice is to put in the setup recommended by the manufacturer. Good ideas, but it didn’t take long to discover that most of the guys out there are not that well educated, and most all the manufacture’s recommendations perform well short of “awesome”.

    It’s obvious to me that manufacturers recommend “minimum” reasonable setups so that potential consumers are not too shocked by the cost of the equipment to put up the bird. This is definitely a bigger-costs-more hobby. You can easily spend 20-30% more for a good notch up to get that desirable “unlimited” performance.

    It seems there that it is common to overload equipment to save money. Seems we always plan with a budget, and then push it to ruin when we want more power. Then we spend more to upgrade or repair it later. My thing here is to learn how to avoid that circular trap. As I think many will agree, I would rather spend a little more now and buy the right goodies than replace/upgrade it later and buy twice. I have done this several times already in my short time in this segment of the hobby, and I don’t “plan” on doing it any more.

    Okay, now let’s get a few issues on the table so we are not misled. We have a fundamental problem with this process that presents contradictory goals. We want incredible power, but a light aircraft flies much better. Powers means mass, get over it and deal with it. I suppose as these technologies advance, this will be less of an issue. Soon you will see 20C and 30C Lipoly batteries. However, we still drop mAH ratings to keep the weight down, even though we will have the big current capacity necessary on smaller batteries. Bad news is, when we do this, we sacrifice the other variable benefit, flight time.

    So, now we have to deal with some balance of price, power, weight and flying time. I won’t debate that here, but you get the idea, it is a quandary. The point is, you sacrifice one for the other in every case. Until the “next generation” battery and/or motor technologies become affordable mainstream items, this is the game we must play.

    So, I think it’s prudent here to focus on 3D flying requirements. I think it’s the most demanding need to find the right mix of hardware, since the flying style is highly diverse. Experienced modelers in other segments of the hobby might argue.

    Now for the confusing part. This is where I had to dig to gain some understanding. First, we have selected a model. There is generally a published weight for the aircraft with recommended hardware. My experience is brief, and limited to 12-22 oz aircraft. In most every case, the plane weighed more than the published weight by 1-2 oz’s when I was done with it. So, let’s factor that in right up front.

    Second, as Matt suggested early in this discussion, to get good 3D performance you need 150 Watts per pound (9.4 Watts/oz). Power directly translates to thrust by the prop. Power, by definition, is the energy required to move a mass a certain distance in a certain timeframe. After using his benchmark, I have found that 9.4 Watts/oz represents good performance and you still get “decent” performance as low as 8.0 watts/oz. to get “great” performance, you need over 10 Watts/oz.

    I measure my assessment of very good as a thrust to weight ratio better than 1.6:1 and “decent” as 1.3:1 to 1.6:1. IMHO (In my humble opinion) anything under 1.3:1 is not adequate for 3D flight. This is an opinion, not a recognized standard. Conversion of Watts-to-Thrust is the part of this whole thing that is not very well defined. Hence, the general guideline is given in watts, and the conversion to thrust is dependent on too many variables to define exactly without complex formulae that include air density, prop efficiency factors and a bunch of other stuff.

    Now, we CAN translate Watts, as it is the product of Amps and Voltage. So, as an example, 12.2 Amps x 10.4 amps = 127 Watts.

    We know we have a plane that let’s say weighs 15 oz’s, and we want “great” performance (10 Watts/oz). We want to fudge the weight an ounce (5-10%) for a cushion and a conservative calculation, then 16 oz x 10 Watts/oz = 160 Watts.

    So, now we can calculate the Ampere Load this represents, assuming the battery voltage. A fresh 3S Lipoly actually measures 11.4-ish volts, and rapidly drops below 11 volts when loaded to near it’s amp capacity, then decays steadily from there. Again, to be conservative so we overstate the real current, we use the fresh battery voltage 11.4V) to “predict” current.

    So, in our example, the current load is 160 Watts / 11.4 V = 14.0 Amps. So, we now know our minimum requirement for the ESC in this example. We also know have all we need to select a Battery. Again, simple math and your pocketbook help you decide. A good 15C battery costs a lot, and won’t live long, but will keep the plane light:

    14A / 15C = 0.93 Amp-Hour capacity, or 930 mAH

    So for extreme 3D with light wing-load and short fly-time, select a 15C battery at or slightly above 930 mAH.

    A nice medium is a 12C battery.

    14A / 12C = 1.17 Amp-Hour capacity, or 1170 mAH.

    For better fly-time and moderate weight penalty, select a battery here in the 1300 mAH range.

    In every case, you may go upscale on the current capacity (mAH rating) and extend flying time at the expense of weight, and hence a vertical performance. In this instance, an extreme might be to select a 2000 mAH battery. Fly long periods, and great wind stability, but a little more sluggish 3D performance unless the bird is very agile to begin with.

    Next is motor selection…
    TMan

  20. #20

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    RE: Motor - Prop Selection ???

    TMan:

    I have a 32oz stik (below). I am using an AXI 2808, with a 25 amp bec, 3S 2000 mAh pack, and a 9X6 prop. It has good vertical, but that is WOT and I burn about 20 amps doing that which really pushes the pack. I have started to use 3S 2P 3100 mAh that has more weight, but the abuse is a bit less on the pack. I still get very aerobatic performance, but a little less vertical due to the weight.

    Tom

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  21. #21

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    RE: Motor - Prop Selection ???

    TManiaci....you are teaching so much in such a short space. Thanks for the effort

  22. #22
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    RE: Motor - Prop Selection ???

    Tom,

    What is the C rating on your battery? The motor specs say that 9x6 is a bit much, but if you limit WOT runs, it's probably okay. Which 2808 are you running (2808/16,/20, /24)?
    TMan

  23. #23
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    RE: Motor - Prop Selection ???

    Thanks HBailey... I hope it helps some avoid the mistakes I have made.
    TMan

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    RE: Motor - Prop Selection ???

    Maniac:

    I am using 10C-12C 3S 2000 mAh packs. Yes, the 9X6 is on the high side and will pull 20 amps WOT (I believe the 2808/24's max draw is 22 amps/30 secs), which I almost never do. I generally fly from 1/2 to about 2/3rds trottle. So, at 2/3rds I pull about 16 amps. I have the 2808/24; I wanted the lower amp draw. Works very well. The pack never gets above 140 F, even on 100+ degree days (although I am a little extra careful on those days), and is generally in the 120 - 130 range when I test it after my flights.

    A more agressive, heading towards 3-D, flying style would give my setup quite a workout.

    Tom

    ORIGINAL: TManiaci

    Tom,

    What is the C rating on your battery? The motor specs say that 9x6 is a bit much, but if you limit WOT runs, it's probably okay. Which 2808 are you running (2808/16,/20, /24)?

  25. #25
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    RE: Motor - Prop Selection ???

    Tom,

    Sound like you are one of the rare ones that have it under control, and knows your limits. Kudos, and Good Flying!
    TMan


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