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Old Jul 20, 2009, 01:50 PM
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Quote:
Originally Posted by zagiflyer67

Before going to fast here, where does the 50 Watts come from? Is that a default value?
Hi Scott,

This is actually empirical data gathered over the course of many years by the observations of many, many hobbyists.

The 50 watts is a very broad figure, representative of no plane in particular, yet indicative of a trend that has withstood the test of time.

Planes can fly on as little as 10 watts per pound depending on their aerodynamic attributes. Another plane with different attributes may need 75 watts per pound to barely squeak along. One of the most popular beginner planes today, the HobbyZone Super Cub, flies superbly, with power to spare, on 36 watts per pound.

But you see, the very fact that I can quote that figure for the SC means that it will be added to that broad base of data collected by hobbyists when we speak of how many watts a typical beginner's park flyer needs to be able to fly in a nice fashion.

Data is all important; which makes the advertising hype of motor vendors like HC to be so much useless tripe.

Chuck
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Old Jul 21, 2009, 08:22 AM
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Thanks Chuck. Now I will start there. I have one plane almost done and need to put a power system in that originally called for a SP480 but I want a brushless and lipo (3S) setup. I have a motor I was using for my Super Miss from Hobby Lobby and would like to see if I can use that one since I have it.

-Scott
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Old Jul 21, 2009, 08:25 AM
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Originally Posted by zagiflyer67
Thanks Chuck. Now I will start there. I have one plane almost done and need to put a power system in that originally called for a SP480 but I want a brushless and lipo (3S) setup. I have a motor I was using for my Super Miss from Hobby Lobby and would like to see if I can use that one since I have it.

-Scott
Hi Scott,

Don't waste your time experimenting... try this.... it's the only thing I use to pick my combos... and it's free...

WebOCalc 1.0.5 written by RCG member FliesLikeABeagle

Chuck
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Old Jul 21, 2009, 09:42 AM
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Humm,

I had forgotten about Webocalc. FLB, sorry for not being more supportive of your efforts here. Don't know why this one does not come to mind more often.

It is a good "what if" tool. You can change different factors and it will show you how the prop has to be changed to accomodate the change.

Thanks Chuck for bringing this up again.
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Old Jul 21, 2009, 10:42 AM
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Originally Posted by aeajr
Humm,

I had forgotten about Webocalc. FLB, sorry for not being more supportive of your efforts here. Don't know why this one does not come to mind more often.

It is a good "what if" tool. You can change different factors and it will show you how the prop has to be changed to accomodate the change.

Thanks Chuck for bringing this up again.
Hi aeajr,

Beagle has been incorporating new features into WebOCalc.... it now even suggests the size and kV values of the motors that would work best in the plane. This means a beginner (or pro) could punch in the weight, wingspan, and wing area of the plane and hit the help button and WebOCalc will suggest the rest... I love that program...

Chuck
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Old Jul 21, 2009, 11:14 AM
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I thought it looked new and improved.

When I originally looked at it, maybe 6 or 9 months ago it was, OK, but not enough to make it a common tool that I would use.

I have been playing with it today. LOVE IT!

There is now a link on my desktop. It will become a daily used tool for my own use and when trying to help others. Thanks for bringing it back to my attention.

FLB, nice job!
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Old Jul 21, 2009, 11:36 AM
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Originally Posted by aeajr
I thought it looked new and improved.

When I originally looked at it, maybe 6 or 9 months ago it was, OK, but not enough to make it a common tool that I would use.

I have been playing with it today. LOVE IT!

There is now a link on my desktop. It will become a daily used tool for my own use and when trying to help others. Thanks for bringing it back to my attention.

FLB, nice job!

Check out http://www.rcgroups.com/forums/showthread.php?t=1082678

I'm using it right now to pick a motor and prop for a Cessna...

Chuck
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Old Jul 23, 2009, 05:36 PM
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Quote:
Originally Posted by aeajr
When I originally looked at it, maybe 6 or 9 months ago it was, OK, but not enough to make it a common tool that I would use.

I have been playing with it today. LOVE IT!
Glad you like it, Ed, and thank you!

And thank you, Ed and Chuck, for continuing to help newcomers to the hobby at every opportunity. I'm going to use an old-fashioned word that has been almost rendered extinct in our "me first, ethics be dammned" culture - what you guys are doing is noble.

-Flieslikeabeagle

P.S. I'm working on the next update to WebOCalc, which includes a few more bits of help: it will suggest an ESC amp rating, tell you the models wing loading, and so on, in addition to all the stuff already in the program. It's almost done, except for some last few gremlins that have been giving me fits for a while.
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Old Jul 23, 2009, 06:14 PM
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FLB,

One suggestion. Very often the pilot does not have wing area. And in some cases it is not all that important. You have a wizard to suggest other "standard" values. This might be a great place for a wizard.

Logic might flow like this.

Wing span - X inches or CM

Airplane type - Glider, Pattern, Slowfly, trainer, ????

Use - Trainer, Aerobatics, 3D ???

Not sure how many paramaters you would need.

For gliders for example, I have told my club mates to use 500 Sq In for a 2M glider and 1000 Sq In for a 3M glider. From the glider point of view the wing area is not important because the motor is only used to climb out to height. Stall speed and stall to top speed is not really an issue. In sport planes I can see where it would be.

Just some thoughts.

You really did a good job on WebOCalc. Nice work.
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Old Jul 24, 2009, 01:43 AM
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Ed, thank you for your suggestions. WebOCalc is one long learning experience for me, and continues to get better as I continue to learn more about the hobby, and also based on continued helpful feedback and suggestions from people like you. I wrote the first version of WebOCalc in 2004, not long after I entered this hobby - amazingly, that was almost five years ago now!

I am largely unfamiliar with glider culture, and have a lot to learn about it. I realize that you do not fly on the motor for long, but how do you decide on a suitable pitch speed for the power system? I would imagine it is still fairly closely linked to stall speed...do you try to climb at the maximum L/D speed, or is some other speed more optimal?

Older versions of WebOCalc asked for wing span and average wing chord. I found to my surprise that a large percentage of RC pilots did not know what "wing chord" meant. These days probably 97% of us are flying ARF's, and the wing area is usually printed somewhere on the box or in the manual - so I changed WebOCalc to ask for wing span and wing area, rather than wing span and (average) wing chord.

For those who know what average wing chord is, you can just multiply it by the wing span and you get a decent approximation to the wing area. If you're within 10%, it's probably quite good enough to get useful results.

As for what parameters we really need, what we're after is two things: how fast the model needs to fly, and how much thrust it needs to make. These translate into two dimensionless parameters, the ratio of pitch speed to stall speed, and the (static) thrust to weight ratio. For example, a model with a pitch speed three times its stall speed and a thrust 1.5 times its weight is going to be capable of all traditional aerobatics, including some of the power-hungry pattern manoeuvres, as long as the airframe can handle them.

One of the reasons why WebOCalc doesn't ask about airframe type is that for typical sport and pattern RC use, it turns out to be largely irrelevant from the power system point of view! While airframe drag is everything for computing cruise efficiency, we don't usually care about maximum cruise range with our models. Instead we care about climb angle, ability to perform aerobatics, ability to hover, etc.

With todays typical levels of performance, all of these things we care about require vastly more power than required by the airframe to maintain level cruising flight. The surprising result is that we can completely ignore the type of airframe (drag coefficient, etc) and it barely has any effect at all on the amount of power needed for typical sport or aerobatic flying.

Just to be sure we're all on the same page, I'm not suggesting model aircraft aerodynamic performance is never important - obviously, it is very important for good gliding performance, high speed hotliners, RC flight distance records, and so on. But for the typical sport or pattern RC plane, it is not much of a factor - if it were, we would long ago have gone to retracting undercarriages, wing fillets, circular fuselage cross sections, high wing aspect ratios, completely hidden servo linkages, and so on. Instead, we build abysmally draggy airframes with slab-sided fuselages, fat wheels hanging out in the airstream, and so on, and then vastly overpower them to guarantee all the performance we want despite the huge drag. It's a brutally wasteful and inefficient method, but it is simple, and so we keep doing it.

-Flieslikeabeagle
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Old Jul 27, 2009, 08:06 AM
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I have a question about the results. It seems to me that some of your input values are actually guesses. Like the Maximum prop size. Do you get this from centerline of prop mounted on plane to ground? I have no idea what prop size I want to run. I also do not understadn why you pick Lipo 3S it only gives a 10.8 battery voltage when it is 11.1V? I put in the information on a plane I got from North East Sailplane the Accord II, all up wght. = 22 oz., wing span = 37", wing area = 325 sq. in., and prop size 9. Every thing else I used was through the Wizards. I am not sure what rpms are correct so I picked 1000rpm because I also do not know much about gearboxes. The results were an 8X6 prop (APC-TE) @ 8677 RPM @ 49.6 mph, 22.1 oz of thrust with gear ratio .93 or I can use a 9x6 prop @ 7738 rpm @ 44.2 mph with 24.6 oz. thrust with a 1.05 gear ratio. Would the gear ratios be considered a 1:1 since they are close meaning direct drive is okay? Or is it better to go with a motor Kv that will need a gear box? I am not sure what is best direct drive or geared motors. Also, I looked at hobby lobby and they never seem to have gearboxes for outrunner motors with the ratios WEBCALC picks.

Sorry to sound like a dunce but I have never had to find the right motor systems before. I usually just used what the manufacturer told me to use. Now that brushless are a mainstay and Lipos are the prefered I need to understadn and know how to change out motor systems plus generally wanting to understand if I ever want to scratchbulid planes.

Thanks everyone for your help. This is such a great hobby and fustrating at times but every enjoyable.

-Scott
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Old Jul 27, 2009, 02:11 PM
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Quote:
Originally Posted by zagiflyer67
I have a question about the results. It seems to me that some of your input values are actually guesses. Like the Maximum prop size. Do you get this from centerline of prop mounted on plane to ground? I have no idea what prop size I want to run. I also do not understadn why you pick Lipo 3S it only gives a 10.8 battery voltage when it is 11.1V? I put in the information on a plane I got from North East Sailplane the Accord II, all up wght. = 22 oz., wing span = 37", wing area = 325 sq. in., and prop size 9. Every thing else I used was through the Wizards. I am not sure what rpms are correct so I picked 1000rpm because I also do not know much about gearboxes. The results were an 8X6 prop (APC-TE) @ 8677 RPM @ 49.6 mph, 22.1 oz of thrust with gear ratio .93 or I can use a 9x6 prop @ 7738 rpm @ 44.2 mph with 24.6 oz. thrust with a 1.05 gear ratio. Would the gear ratios be considered a 1:1 since they are close meaning direct drive is okay? Or is it better to go with a motor Kv that will need a gear box? I am not sure what is best direct drive or geared motors. Also, I looked at hobby lobby and they never seem to have gearboxes for outrunner motors with the ratios WEBCALC picks.

Sorry to sound like a dunce but I have never had to find the right motor systems before. I usually just used what the manufacturer told me to use. Now that brushless are a mainstay and Lipos are the prefered I need to understadn and know how to change out motor systems plus generally wanting to understand if I ever want to scratchbulid planes.

Thanks everyone for your help. This is such a great hobby and fustrating at times but every enjoyable.

-Scott
I will try to address a few of your questions. Never hesitate to ask questions.

1) max prop would be determined by ground clearence if you are going to be taking off from a runway. This is called ROG or rise off ground. Size given is the diamater of the prop. So, for a plane that has 5" of clearence with the tail up, that would be 10". You would probably want a max size of an 7-8" prop, leaving 1-2" of clearence.

For hand launched planes it might be determeind by other factors. For gliders that typically use folding props, it would take into consideration the space needed to fold the prop. If you have 8" from the nose to the wing and you put on a 20" prop then the almost 10" long blade will overlap the wing by nearly 2" when it folds. It will break the prop or damage the wing when you start the motor, so consider a max of 15" leaving a 1/2 clearence between the prop and the wing.

Remember the calculator asked for the Max size, not the size you were going to use. So use the max that works for your plane. It is just establishing options. It can't recommend a 20" prop if your ground clearence is 6" and you plane to ROG. If the max prop you can use is 8" than it will look for a motor/battery combo that works with an 8" or smaller prop. The recommendation might be for a 7" prop but it won't be for a 9" prop.


2) Voltage. The nominal voltage of an 3 cell lipo is 11.1. It's fully charged voltage is 12.6. But under load that voltage will drop and the voltage under load is what determeines how fast the prop spins. Some recommend using 3.3V per cell under load. Others suggst 3.6 and so on. They are just estimates of voltage under load because it "all depends". If you have a wattmeter you can actually watch the voltage of your battery pack as you run the motor.


3) Gearboxes - Typically a gearbox is used with an inrunner, not an outrunner. That is not an absolute. I have an outrunner on a gearbox but that outrunner is made for that purpose. Most outrunners are not made for use with gearboxes.

If the calculator says 1:1 or anything close to that, it means you don't need a gearbox. That would be direct drive. Most outrunners are used direct drive. If I was planning to use an outrunner in direct drive I would choose the prop that shows a gearbox of 1:1 or close to that. That might be .9:1 or 1.1 to 1, for example.

WebOCalc gives you a range of props so you can choose the kind of set-up you want. Maybe you want to use a gearbox because you have clearence problems with an outrunner. So pick the prop/motor/gearbox combo that works for your plane.

WeboCalc is also doing straight calculations, as far as I can see. It is not concerned about whether there are gearboxes of those actual ratios. So, if it says use a gearbox of 2.3:1 So there may not be a 2.3 to one but there may be a 2.4 or a 2.5. So go back and look at the chart. Would that change the prop recommendation?

You are talking a difference of 10%. Probably not enough to make a big difference. What if you increased the amp draw by 10%, would it over work that motor? About about the battery or ESC. If they all still work, than that is OK.


This is all about options and possibilities.

If we compare this to glow power, you could have a .40 plane but you could put a .30, a .40, .43, .47 on it and it would probably still fly. Or you could put a .60 on it and it would still fly. But you would have different performance, different balance issues and different mounting issues, depending on which you used. Some might have size pipes or some might have needle valve installation issues based on location. And you would probably have a different prop on each one of those choices.

Of course not all .40s are created equal. You have sleeve, ball bearing, and other variations. But it is a .40 plane so you know about what size motor it should take.

Look at WebOCalc's suggestions in the same way.
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Old Jul 28, 2009, 05:10 AM
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Scott, hang in there, we'll get your Accord off the ground. Picking an electric powertrain for a model is an inherently complex task. WebOCalc makes it as easy as I know how, but there is still a learning curve. As you point out, you need some background information to make intelligent choices.

In the long run, you will be much better off knowing that background information. However, in the short term, you can use WebOCalc to pick an effective power system even without that background knowledge. This is possible because WebOCalc contains a number of "wizards", software tools that tell you what numbers to enter into those boxes you're having trouble with. Read on, I'll walk you through some of them.

Also, don't forget to read the WebOCalc tutorial on my website (same place you get the calc itself). The same tutorial is also packed with WebOCalc if you download it - just click on the "Help" button at the bottom of the program to read it. The tutorial walks you through selecting a power system for one model, and shows you how all the wizards work. This should help you understand how to use the program.
Quote:
Originally Posted by zagiflyer67
It seems to me that some of your input values are actually guesses. Like the Maximum prop size. Do you get this from centerline of prop mounted on plane to ground? I have no idea what prop size I want to run.
Scott, did you notice the blue text that says "Run prop size wizard" right next to the box where you select your maximum prop size? If you don't know what prop size to use, click on this text. The prop size wizard will open up and suggest a suitable range of prop sizes to you.

Wizard aside, there are some reasonable rules of thumb you can use. One is to avoid small propellers as much as possible. Another is that for this sort of park-sized model, a good propeller diameter is around 1/4 of the wingspan, or a little larger, up to about 30% or so of the wingspan for typical models. Go much bigger than that, and you'll start to have lots of prop torque reaction - the model will twist counter-clockwise when you open the throttle.

With the 37" wingspan of your Accord, I'd try 10" or 11" as the maximum prop size. When I ran the prop size wizard, it suggested "Prop sizes from 6.7 to 11 inches should be suitable for this model. Note that the 6.7 inch propeller will be least efficient. The 11 inch propeller will be most efficient."

Some people have strong opinions about prop size, like the guy who told me he "Won't run a 60 size prop on a 40 size model, even if it performs better!" WebOCalc gives you a choice - pick your poison if you have a strong preference, or use the wizard to guide you, or use the rule of thumb.
Quote:
Originally Posted by zagiflyer67
I also do not understand why you pick Lipo 3S it only gives a 10.8 battery voltage when it is 11.1V?
Because a 3S lipo battery doesn't really give you 11.1 V. When it's fully charged and there's no load on it, it will put out 12.6 V. When it's fully charged and you put a full load on it (motor at full throttle), it will typically put out 10.5 V - 10.8 V, depending on various factors. When it's fully discharged with no load on it, it will typically put out around 10.5V - 10.8 V. And finally, when it's fully discharged with a full load on it, it will be down around 9V or so!

WebOCalc uses 10.8 V, which is about what most good 3S lipo packs will give you with a full charge and a full load on the pack. Note that once again, you can edit the value to match your pack - WebOCalc fills in a number for you when you select "Lipo" and "3S", but you can change that number if you like.
Quote:
Originally Posted by zagiflyer67
I put in the information on a plane I got from North East Sailplane the Accord II, all up wght. = 22 oz., wing span = 37", wing area = 325 sq. in., and prop size 9. Every thing else I used was through the Wizards.
Oh good, you did find some of the wizards. Did you use the prop size wizard to arrive at 9"? It suggests 7" - 11" for your plane.

What did you use for "Flight Mission"? This is based on how you intend to fly the plane. I'm using "Fast Unlimited Aerobatics" as I work through WebOCalc with your airframe numbers.
Quote:
Originally Posted by zagiflyer67
I am not sure what rpms are correct
The Kv wizard will give you suitable ranges for both geared and direct-drive options. Pick a value in the middle of the range, or if you have a specific suitable motor in mind, enter it's Kv.

I just ran the Kv wizard on your Accord, and it suggests "If you plan to use a direct-drive motor, motors with Kv between 790 and 990 rpm/V should be suitable." Your decision to try 1000 rpm/V wasn't a bad one, it's close to the upper suggested range, and you can commonly find motors with about this Kv.
Quote:
Originally Posted by zagiflyer67
so I picked 1000rpm because I also do not know much about gearboxes. The results were an 8X6 prop (APC-TE) @ 8677 RPM @ 49.6 mph, 22.1 oz of thrust with gear ratio .93 or I can use a 9x6 prop @ 7738 rpm @ 44.2 mph with 24.6 oz. thrust with a 1.05 gear ratio. Would the gear ratios be considered a 1:1 since they are close meaning direct drive is okay?
I'm not getting the same numbers as you - I must have picked something different than you did - motor efficiency? Flight mission? Something.

No matter, first let's talk about your numbers and how to interpret them. Then I'll run WebOCalc my way on your model, and show you what comes up.

This prop size/gear ratio bit is where you have to help WebOCalc a little by telling it what you want. WebOCalc doesn't know if you want a gearbox or not, and for complex technical reasons I won't go into here, I haven't found a good way to make the program work its magic if you take away the gearbox. So if you don't want to use a gearbox, you have a little more work to do with WebOCalc. Don't worry, it's not hard, and usually doesn't take more than a few seconds.

Okay, thehn. For convenience, most people use direct-drive and outrunners today. This is not always the ideal technical solution, but it usually is the cheapest and most convenient one. So let's say we want to go direct-drive, which corresponds to a gear ratio of exactly 1.0:1.

Notice that your 8x6 prop choice (at a 0.93 gear ratio) is actually spinning a tiny bit FASTER than the motor at the current draw you entered into WebOCalc (gear ratio less than 1.0). This tells us that if you connected the same prop direct-drive to the motor, it would spin a little slower. That in turn means it will need less power to spin it, so the current draw from the battery would drop a little if you were to use this prop in a direct-drive setup.

How do you know how much it will drop? The simple answer is that you decrease the number you entered in the "Desired current per motor (A)" box a tiny bit, and click "Calculate" once again. In a couple of tries you'll get your gearbox ratio of exactly 1.0:1, and then you're done.

What if you decided to use that 9" prop, the one at a 1.05 gear ratio, and you want to run this one direct-drive? Same reasoning in reverse: this time the prop is turning slower than the motor (1.05 ratio). If you couple it direct-drive, it will turn a little faster. This will require more power from the motor, so the motor will draw a little more current than the value you entered in the box. How much more? Tweak the current up a hair, click on "Calculate", and in a couple of tries you should arrive at your exact 1.0:1 gear ratio. That's the current draw you can expect from this motor and prop, used direct drive. (Of course the answer is never going to be exactly right - expect 5% - 10% error in WebOCalc's results. Calc programs of this sort never do much better, because there are too many unknowns (air temp, humidity, altitude, age of your battery pack, minor variations in prop manufacture, etc, etc, etc.)

We just went over how to use these props with the specific motor you already picked out (1000 rpm/V). But there's another option - change the motor.

Back to the 8" prop - we know it "wants" to turn faster than that 1000 rpm/V motor to draw the amount of current we originally specified, because of that 0.93 gear ratio. Why not just use a faster-turning motor? If you used a motor with a Kv a little higher than 1000 rpm/V, you'd end up with a perfect match. So if you do a little hunting around and find a suitable motor with a Kv of, say, 1050 rpm/V, you can put that number into WebOCalc and hit "calculate" again. If necessary, tweak the current draw a little up or down as necessary - in a try or two you'll have your 1.0:1 gear ratio. (E-Flite makes a Park 480 motor with a specified Kv of 1020 rpm/V.)

Same deal with the 9" prop...if you find a motor with a slightly lower Kv, try entering its Kv into WebOCalc to see if it would be a good choice to power your model. E-Flite makes another Park 480 variant with a Kv of 910 rpm/V.
Quote:
Originally Posted by zagiflyer67
Sorry to sound like a dunce but I have never had to find the right motor systems before. I usually just used what the manufacturer told me to use. Now that brushless are a mainstay and Lipos are the prefered I need to understadn and know how to change out motor systems plus generally wanting to understand if I ever want to scratchbulid planes.
Another reason to know how to do it yourself is cost - very often the manufacturer's suggested combination costs way more than an alternative combination that will work just fine for you. Or perhaps you want to fly the model differently than the manufacturer intended - I often buy aerobatic models intended for 3D, but I have no interest in 3D and fly precision aerobatics with them instead. That means I don't need the 3D power system recommended by the manufacturer. Instead, I need something better suited to precision aerobatics.

I congratulate you on being willing to learn something that is new to you. Many people avoid ever having to learn anything new, and it only holds them back. You do not sound like a dunce to me - you sound like a beginner in a new field of knowledge, and that is NOT a put-down: being a beginner is an exciting, wonderful, brain-enhancing stage we all go through many times in our lives. Enjoy being a beginner, it only lasts a short while, and then you have to find a whole new field of knowledge you know nothing about to re-visit this exciting state!

Just to finish up my example: I picked "Fast Unlimited Aerobatics", props up to 11", and a Kv of 1000 rpm/V. The bigger props made more thrust, but needed gearing down. So I tried a Kv of 910 rpm/V, knowing I could use that Park 480 motor. Bingo - I hit the jackpot - exactly 1.0:1 gear ratio with a 10x7 APC "E" and 15.6 amps current draw. Predicted performance is excellent. See attached screenshot.

-Flieslikeabeagle
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Old Jul 28, 2009, 05:23 AM
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Quote:
Originally Posted by aeajr
Remember the calculator asked for the Max size, not the size you were going to use. So use the max that works for your plane. It is just establishing options. It can't recommend a 20" prop if your ground clearence is 6" and you plane to ROG. If the max prop you can use is 8" than it will look for a motor/battery combo that works with an 8" or smaller prop. The recommendation might be for a 7" prop but it won't be for a 9" prop.
Great reply, Ed!
Quote:
Originally Posted by aeajr
WeboCalc is also doing straight calculations, as far as I can see. It is not concerned about whether there are gearboxes of those actual ratios. So, if it says use a gearbox of 2.3:1 So there may not be a 2.3 to one but there may be a 2.4 or a 2.5. So go back and look at the chart. Would that change the prop recommendation?
Quite right, Ed. WebOCalc is not wedded to any particular manufacturer, and owes nobody any secret allegiance. So it doesn't know or care what gearbox ratios are made by brand X or brand Y. What WebOCalc does do is tell you what it thinks is the "ideal" gearbox ratio for the particular set of numbers yo have entered. As Ed says, this is rarely critical - and you can easily find out what effect it would have, anyway. If WebOCalc suggests a 2.3:1 gearbox and you have a 2.33:1 gearbox, you know the prop's going to be turning a tiny bit slower than WebOCalc intended; therefore it will draw a little less power and make a little less pitch speed. So try turning down the motor amp draw, and perhaps the pitch speed you entered, by a smidge; then click "Calculate" again. Pretty soon you'll be looking at that exact 2.33:1 gearbox ratio you happen to have.

And once again, when you try it in real life, you will probably find the actual current is off a few percent from the WebOCalc prediction. My experience is that the error is completely negligible as far as actual flight performance is concerned - it still flies the way WebOCalc says it will.
Quote:
Originally Posted by aeajr
If we compare this to glow power, you could have a .40 plane but you could put a .30, a .40, .43, .47 on it and it would probably still fly. Or you could put a .60 on it and it would still fly. But you would have different performance, different balance issues and different mounting issues, depending on which you used.
<snip>
Of course not all .40s are created equal. You have sleeve, ball bearing, and other variations. But it is a .40 plane so you know about what size motor it should take.

Look at WebOCalc's suggestions in the same way.
Very nicely put, Ed, and this is exactly the spirit in which I intended WebOCalc to be used. The program isn't intended to be an oracle, telling you about the One True Power System, the one that "rules them all". The reality is that there never is such an ideal, perfect system - they all have tradeoffs of one sort or another, including cost, weight, efficiency, and so on.

What WebOCalc is intended to do is to help you very quickly home in on a suitable power system for your model that meets your tastes, your intended flight mission, your budget, your preferences. The program will let you do this and be confident that the model will fly, and fly the way you want it to. You won't waste money on components that don't do the job, you won't waste money on oversized components that only add weight, and you'll be steered away from poor combinations that are mis-matched to the needs of your particular airframe. In short, WebOCalc is intended to take the pain out of power system choices, so you can go ahead and have more fun flying the darn model instead.

-Flieslikeabeagle
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Old Jul 28, 2009, 08:04 AM
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Elbridge, New York
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Flieslikeabeagle and Aeajr,

Thank sfor the input and will keep plugging away at this. You explanations are very helpful. I now have a few good ideas for a power system for the Accord II. Hopefully in a couple of weeks I will be able to give it its first maiden flight. I will keep you posted and probably stil have more questions.

-Scott
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