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Old Sep 11, 2012, 07:08 PM
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NE Ohio
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SkyFly Max

Thanks for responding to all my questions. I'm a bit skeptical about the limit too. I find they are a lot stiffer and beefier than any of the GWS RD,RS,SF props and not nearly as beefy as the APC thin electrics. I have some GWS HD or DD props with 100k/D limit. I really think that is a more reasonable limit for them.

Anyways I did a stock SkyFly Max and according to the calc it came out perfect with the stock 8x4 prop. The time, battery,current draw all seem to be very accurate even though this is another one that uses a 1300 kv motor instead of what the calc comes up with. This is a rear prop though that can't fit anything bigger than 8.25 diameter. The calc did a nice job.
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Old Sep 11, 2012, 10:17 PM
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I have some GWS HD or DD props with 100k/D limit. I really think that is a more reasonable limit for them [Great Planes props].
Sounds good to me!
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Anyways I did a stock SkyFly Max and according to the calc it came out perfect with the stock 8x4 prop. The time, battery,current draw all seem to be very accurate even though this is another one that uses a 1300 kv motor instead of what the calc comes up with.
If you use the wizards, WebOCalc tries to guide you towards the most efficient power system it can find, which usually involves big propellers and relatively low Kv.

But, as discussed with psguardian a few posts ago, if you put in your own numbers (i.e. you don't use the wizards), WebOCalc tries to do the best it can, given the numbers you put in.

That's why it was able to find the stock power system once you put in that specific Kv and prop size limit, even though the prop is a bit small and the kv a bit high.

Given that you can't go bigger than 8" on the prop diameter, it sounds as though the stock power system is well chosen, and well matched to the airframe. Good job by the manufacturer!
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The calc did a nice job.
Thanks, it's always nice to know it's working!

There's a funny thing about writing software like this. In the early stages the programmer has the answers and the program acts pretty dumb. As the software is refined and the algorithms improved, you find the software is starting to get a little less dumb, and it starts to make decent choices most of the time.

If you stay on top of it, every time the program does make a mistake, you dig into it to find out why, and eventually find a way to fix that particular weakness. So it won't make that particular mistake again.

And if you keep this up, the program continues to improve, and then suddenly one day you may find that the software is apparently smarter than the programmer. It finds possibilities that the human brain overlooks or does not come up with.

It's always a little spooky when this happens, when a few thousand lines of computer code suddenly starts to act as though it has some intelligence of its own.

That happened to me with WebOCalc several versions (and several years) ago. It reached a point where it regularly made better choices than I could make manually. I stopped trying to out-think it, and started just using it to pick my own motors, props, and ESC's.

Of course nothing is perfect, and WebOCalc has its limitations. But it really does work pretty well 99% of the time. That's the only reason why I feel comfortable encouraging people to use it. I know the program will come through for them, and find a power system that will perform to their satisfaction. And that means they won't be wasting money on parts that really won't work properly for them.

-Flieslikeabeagle
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Old Sep 12, 2012, 11:22 AM
Sure, I can fly after sunset!?
United States, MI, Novi
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In other words, if you do everything right, you still lose 40% of the thrust-per-watt by halving the propeller diameter.

-Flieslikeabeagle
Now THAT is an AH-HA moment! I wonder how many RTF or ARF planes out there are designed with this in mind ... few?

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But the people into rubber power and endurance flying (and solar-powered electric flight) know all about it - you'll never find a tiny inefficient prop on those models!

-Flieslikeabeagle
This winter, I went to a rubber power FF event held in a soccer arena. I was very surprised at the size of the props on those things. I was even more surprised that they "spun" at well under 60 rpm.
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Old Sep 13, 2012, 10:33 PM
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I wonder how many RTF or ARF planes out there are designed with this in mind ... few?
"Few" puts it well. I'm not sure I've seen any, to tell the truth.

In the time I've been in the hobby, the trend I've seen has been away from lightness and efficiency, and towards heaviness and inefficiency. In 2006 most electric models were built lightly - they had to be, since many were designed in the days of weak NiCd batteries and weak brushed motors. Since then the cost of brushless motors, ESCs, and lithium batteries has gone down, the current ratings have gone up, and now it's possible to simply overpower a heavy airframe and inefficient small propeller by sheer excess wattage.

So we have a flood of laser-cut plywood ARF's that look nice but weigh two to three times what they should. Even the foamies have become much heavier, as manufacturers moved away from lightweight (but unattractive) Styrofoam towards denser (but prettier) EPO and other foams, and have added more and more weight through additional scale details.

The thing is, you can certainly get these heavy models to fly with enough power, but they don't fly the same way - they fly hot and heavy, too fast for their scale appearance. We now have many semi-scale WW-I biplane models that are so heavy they charge through the air like WW-II fighters, semi-scale WW-II fighters that zoom about like contemporary small executive jets, and semi-scale general aviation light aircraft that fly like WW-II fighters.

In Paul Ciotti's book "More With Less" he writes "In a world without constraints, the need for efficiency vanishes, and in many cases, so does beauty too." I'd say Mr. Ciotti hit the nail on the head.

That book, by the way, is a biography of Paul MacCready, telling the story of him and the super-efficient flying machines he created.
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This winter, I went to a rubber power FF event held in a soccer arena. I was very surprised at the size of the props on those things. I was even more surprised that they "spun" at well under 60 rpm.
Very cool! I've only seen those sorts of models on video clips, never in person.

That mid nineteenth-century propeller theory I mentioned shows that the most efficient way to make thrust with a propeller is to very gently move a very large volume of fluid. And that's exactly what those enormous, slow-revving FF propellers do.

I used to wonder why fish had evolved separate "propellers" (their waving tails) while birds didn't, instead using their wings for both flight and propulsion. Then, when I found out about the efficiency advantage of big propellers, it hit me: by using their wings for propulsion, birds are using the biggest - and therefore most efficient - "propeller" possible! Fish, on the other hand, are dealing with a much denser fluid medium and much lower energy demands (they don't have to generate lift), so they can get away with "propellers" that are pretty small.

And then there are hummingbirds. Their tiny and inefficient wings require so much power that these birds have to live on the most energy-rich food to be found in nature: nectar!

-Flieslikeabeagle
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Old Sep 15, 2012, 01:52 PM
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I'm going to take a tiny detour in my answer, if you'll pardon me for it.

It has become well known that scientists are the worst people at catching crooks who pretend to have extra sensory perception, telepathy, etc. That's because scientists expect the experiment to be honest - it doesn't occur to them that the experiment itself is rigged, that the observations are lies.

Con men and professional magicians are much better at catching these crooks who claim ESP - because both good con men and professional magicians are good at tricking other people, they recognise trickery and cheating when they see it.

You, psguardian, must be an honest man, because like the scientists studying the lying crooks, you've assumed honesty on the other persons part; that the data on the Hobby King website is honest and accurate.

Unfortunately, historical evidence shows that this is an incorrect assumption. In my experience with Hobby King motors, the only numbers you can somewhat trust are the published Kv and the motor weight - and even those may have significant errors.

The "performance" numbers, like internal resistance and maximum current, are usually complete nonsense. Most HK motors would fry like an egg long before they got anywhere near the stated maximum current or output power.

Lacking any trustworthy data, some of us have come up with usable approximations. 75% peak efficiency is one of them. Another is that most of these motors can handle a maximum of about 3 watts per gram of motor weight. (So an 80 gram motor will probably be maxed out at 240 watts).

These are only approximations, and any given motor may be worse (which happens more often than you'd think!), or perhaps a little better (rarely if ever).

From what I've learned, a crudely machined, sloppily wound motor with big air-gaps, poor balancing, low-grade bearings, thick laminations made of low-grade steel, and weak magnets is unlikely to do much better than 75% efficient. That's the way these budget motors are made.

To do better, you need precision manufacturing, very tight air gaps, expensive low-friction bearings, high-strength magnets, precise windings with thick (and hard to wind) wire that completely fills all the available space, thin motor laminations, high-grade steel, etc. All these things cost money and labour, so these motors are never going to sell as inexpensively as the Turnigys and so on.

As long as we know what we're getting for our money, there's nothing wrong with using the budget motors. Just be aware that in exchange for the lower cost, we also get lower efficiency, higher weight, and reduced power output compared to higher quality (but more expensive) motors.

My philosophy on this was to use the cheap, heavy, and inefficient motors on those models that didn't suffer too much from these characteristics: low-performance models (Cubs, etc), heavy models (warbirds, etc), and so on.

There are some programs (like Christian Persson's DriveCalc, http://www.drivecalc.de/ ) that include long lists of motors with painstakingly measured data, including efficiency curves. You may find it interesting to look at some of them.

-Flieslikeabeagle
John,

Word of caution. I would be very careful about making any negative comments about Hobby King products. Whether it is backed up by facts or not, saying anything negative about HK is a recipe for drawing undo attention to yourself. Trust me, I just went through a ton of drama and it is no fun at all. There seems to be a very divisive pro HK crowd around these days and I don't know why.

Frank
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Old Sep 17, 2012, 01:14 AM
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United States, CA, Marina Del Rey
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Just a note to thank everyone who contributed to my power system.

I've just ordered the Hacker A40-12L V2 and a Castle Creations Phoenix Ice 75 ESC.

Edit: glitch in order--changed to Ice 75.
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Old Sep 17, 2012, 09:49 AM
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John,

Word of caution. I would be very careful about making any negative comments about Hobby King products. Whether it is backed up by facts or not, saying anything negative about HK is a recipe for drawing undo attention to yourself. Trust me, I just went through a ton of drama and it is no fun at all. There seems to be a very divisive pro HK crowd around these days and I don't know why.

Frank
What advertisers name do you see at the top of the page every time you log in to RCG?
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Old Sep 17, 2012, 09:11 PM
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I've just ordered the Hacker A40-12L V2 and a Castle Creations Phoenix Ice 75 ESC.
I think those will work nicely for you. Every Hacker motor I've used has impressed me.

Personally I prefer the inexpensive Turnigy Plush ESC's (from Hobby King) to Castle ESC's. But Castle certainly has many fans, and hopefully by now they've fixed the stream of firmware problems that drove me away from their ESC's.

-Flieslikeabeagle
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Old Sep 17, 2012, 10:04 PM
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United States, CA, Marina Del Rey
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...hopefully by now they've fixed the stream of firmware problems that drove me away from their ESC's...
NOW you tell me!
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Old Sep 22, 2012, 04:50 PM
If I build it, it will fly
United States, NY, East Rochester
Joined Jan 2012
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I'm getting closer to ordering & have a few more questions.

I have found some bad mis cuts in my bipe so i'm going to hang it from my daughters ceiling (she really wanted a plane). My son gets another of my mis creations so it works out lol...

Taking what I've learned I increased the WS to get back to a lighter feel, flies like backyard flier/light park flier.

New bipe: 39" WS, 7" chord, rounded tips brings area to 540sq in. H. Stab 20"x6" swept LE & includes a 1" full width elevator, V. Stab 10"x6" swept LE & includes a 2" rudder. 2" squared Tubular fuse 33" from nose to TE of elevator.

Airframe weighs just shy of 300g. Robbing electronics from heli box: Turnigy ae-45a ESC/bec 43gr, 4x Hextronik MG-14 servos 56gr, Turnigy 8ch RX 18gr, Turnigy 3s 2200mah 40c 204gr. Total before motor/prop & landing gear = 621gr. After looking at a few other motors I have decided the Turnigy 2217-16t 1050kv really is best suited for me at this point. With mounts & wires installed 80gr, which brings me to 701gr. Ultra light wheels 25gr (not sure they will handle a 25-30oz plane but people say they will), so upto 726gr, still need the actual landing gear & a prop.

Looking for suggestions on where to find control surface wire a clevises that will hold them, and landing gear that is fit for a 30oz bipe.

In prop land is where my more 'on topic' question comes in. drive calc shows the actual kv of this motor as being 1105 @ 10.8v, putting this into WOC & matching efficiency as closely as possible I compare the results for a few likely props; 8x4TE, 8x6TE, 9X6TE, 9X4.5TE, 10X5TE... Drive calc shows higher rpm & thrust values at near the same amperage. So which one is closest to accurate?

~psguardian
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Old Sep 27, 2012, 09:50 PM
If I build it, it will fly
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Did this thread get too far back in everyones notification list?

~psguardian
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Old Sep 27, 2012, 10:02 PM
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United States, CA, Marina Del Rey
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We're here.

I'm pretty impressed with the appearance of the Hacker motor and Phoenix ESC.
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Old Oct 02, 2012, 10:23 PM
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<snip>
...putting this into WOC & matching efficiency as closely as possible I compare the results for a few likely props; 8x4TE, 8x6TE, 9X6TE, 9X4.5TE, 10X5TE... Drive calc shows higher rpm & thrust values at near the same amperage. So which one is closest to accurate?
Oooh, there's a long slippery slope to contemplate...

For one thing, accurate under what conditions? With a fully charged battery or halfway through a pack? In Denver, a mile high, or in Los Angeles, at sea level? In Death Valley in summer, or in Wyoming in winter?

There are so many factors that affect the performance of our little model aircraft. Full-scale pilots have to pay attention to all of them (substitute fuel weight for state of charge of a battery pack) to ensure their survival. Most hobbyists neither do nor can. We can't do anything about the fact that our batteries start discharging the moment we throttle up, and keep discharging until the aircraft is back down on the ground.

That said, as far as I know Drive Calc is an attempt to very accurately model electric motors, and even include some of the effects of the ESC. I'm not sure what Christian Persson (creator of Drive Calc) is using for a propeller model, or what propeller data he's using in the model, but I've heard other people comment on how accurate the program is.

WebOCalc is an attempt to get usefully accurate results while keeping the program as easy to use as possible. In the interests of ease of use, many simplifying approximations have been used. The goal is to get within plus/minus ten percent of reality, which is more than good enough for hobby flying.

At one time (before Drive Calc existed, AFAIK) I started work on something similar to Drive Calc. My creation was called Powercalc, and an early alpha version of it is still on my website. I was already developing WebOCalc at the time, so the plan was that WebOCalc would be extremely simple to use and accurate enough to get us in the ballpark, while Powercalc would be much more complex and a little more accurate as a result.

Something funny happened along the way, however. I found out that (in my opinion) programs like Powercalc were not all that useful, and I stopped work on it. That's because these sorts of programs focus only on the powertrain, and therefore only answer the question "What will this particular motor, ESC, battery, and propeller do?"

The trouble is, this is the wrong question to ask. There are thousands of motors and batteries out there, and hundreds of propellers and ESC's. I have no interest at all in how 99.999% of those millions of combinations might perform. The 0.001% of combinations that might actually be relevant to this particular model are hard to find, and when you find them, you STILL don't know how the model is actually going to perform with them! You may accurately know current draw and static thrust, but neither of those tells you a whole lot about how the model is going to fly!

When I'm building a model and planning its power system, the question I actually want answered is a completely different one: "What motor, ESC, battery, and prop will make this model fly the way I want?"

That is the question WebOCalc tries to answer. Instead of accurately showing you what the wrong combination of parts does, it shows you what combination will actually fly your model the way you want it to fly.

Fascinatingly enough, when I started work on WebOCalc, I didn't find a lot of information out there designed to answer that question for electric RC pilots and builders. There were a few very inaccurate rules of thumb that people were using. There was a commercial computer program that was hard to use and extremely inaccurate. And everything else out there pretty much focussed on answering the wrong question - telling you as accurately as possible what the wrong motor and battery and ESC and propeller would do if matched together!

If this isn't making sense yet, consider this analogy. You want to go racing, so you decide to start by building the perfect race engine. You buy an engine dyno, a few thermodynamics books, a bunch of machine tools and expensive engine parts, and set to work. When you're done you have a wonderful little 2-litre four cylinder engine that reliably makes 250 horsepower at 7500 rpm for hours on end.

And now, for the first time, you start to consider what chassis you're going to put that engine in, and what type of racing you're going to do. You stick that lovely little engine in a 1969 Chevy Chevelle, head for the dragstrip, and find out to your dismay that completely stock Honda Civics blast past you without even trying. Your shiny new race car is a total dud!

What went wrong? The problem is that your lovely little jewel of an engine is a very poor match for a the heavy Chevelle with its stiff gearing. It would be a great match to a very lightweight race chassis with much deeper gear ratios that let the engine rev into its power band. But in the Chevelle, it's a slug.

In other words, there is no such thing as a perfect power train by itself. No race team would ever choose an engine without knowing ahead of time what sort of vehicle it was going into, and what sort of performance was being asked of that vehicle. And no e-power builder should choose a motor, ESC, prop, and battery without knowing what sort of airframe they're going in, and what sort of performance is expected of that airframe.

-Flieslikeabeagle
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Old Oct 02, 2012, 10:38 PM
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<snip>
New bipe: 39" WS, 7" chord, rounded tips brings area to 540sq in.
Rounded wingtips are pretty, which is why they're on many beautiful aircraft from the Golden Age of aviation, back in the 1930's or so.

However you don't see rounded tips on most aircraft designed in the last sixty or seventy years, and that's because it turns out they usually carry a performance penalty.

In an oversimplified nutshell, a wing flies because the air pressure under it is higher than the air pressure above it. At the wingtip, the higher pressure under the wing tends to leak vertically around the tip and try to cancel the lower pressure above the wing.

This air leakage causes the infamous tip vortices that accompany all heavier-than-air flight, causing increased drag, reduced lift, and endless headaches for both aircraft designers and air traffic controllers, who have to keep flying aircraft far enough apart so the wingtip vortices from the one in front don't tear apart or cause loss of control of the one behind. Light aircraft like Cessnas have to stay a long, long, long way behind heavy monsters like Boeing 747's to avoid being destroyed by the wake turbulence left behind them.

The same air leakage also causes the tips of the wings to provide less than their fair share of lift. And that's where rounding the wingtips hurts: usually the rounded contour adds weight and wing area without adding any significant amount of lift, and on top of that often there is an additional penalty because the rounded wingtip may actually increase air leakage around the tip, increasing the size of the tip vortices. That's why you don't see Piper Cub wingtips on Airbus A320's.

All of which is a longish way to say this: don't count too much on those few extra square inches from the rounded wingtips to add lift to your bipe! Enjoy the improved cosmetics, but don't expect aerodynamic benefits from them.

Incidentally, if you want to see what efficient model airplane wingtips look like, take a gander at a Multiplex Easy Star or Easy Glider. The efficient wing design is a big part of the superb glide performance of both these models.

-Flieslikeabeagle
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Old Oct 02, 2012, 10:41 PM
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I'm pretty impressed with the appearance of the Hacker motor and Phoenix ESC.
Now I have a mental picture of IoNslo sitting there, all hunched over, fondling the Hacker motor in his hand, and hissing "My precioussss!" every now and then.

Not that I've ever done anything like that myself or anything.

-Flieslikeabeagle
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