Nov 29, 2009, 09:06 PM Registered User Joined Nov 2009 4 Posts Discussion Path to sucess Hi to all, I am new to the forum, but I have been poking around for a while to get a grasp on my all new hobby. I have seen many suggestions, ratings and efficiency charts, but what I am missing is kind of a step per step cook-book of how to go about finding the correct power system. It should go something like this: Step1: Let's say my plane weighs about 2 pounds without power system. I guess it's save to assume that the power systems will not add more than a pound. Makes three pounds total. I think I remember to have read somewhere in this forum that you need at least 50W/pound, 100W/for a 3D model.. (fill in the blanks). Step 2: I choose a motor that has 150 watts continues power (which will give me some reserves for peak power) Step3: Now the Batteries, What is the difference in performance between choosing a high voltage, high RPM, small propeller system and a lower voltage, lower RPM, bigger propeller system if the resulting Watts are the same?? (Higher RPMs are more responsive/dynamic ???) Step4: The size of the propeller will determine the working point of the motor which should be somewhere in the 150W range (to stick with the example above). What voltage should be used for this working point? Peak battery voltage or let's say 30% less? Is this something I need to get out of the tables for the chosen motor or are there some rules of thumb for size and pitch? Step4: Choose a regulator (well if super peak efficiency is not required, just uses something that can sustain more than peak power of the motor). Step6: Last not least after choosing the battery voltage and the continues power the size (mAh) will give an idea of the possible flying time. Step7: Add all the weight and see if the assumptions in Step 1 were OK. If not, do it again. Any step above might be right or wrong, please correct and fill in the blanks where I missed something. I hope that I am on the right track here, maybe somebody can fill in some extras how to optimize efficiency in the different steps. Thanks a lot !!
 Nov 29, 2009, 10:23 PM Suspended Account Joined Jul 2006 22,990 Posts Here ya go... step by step... Beginner's Guide To Motor and Prop Selection Made Easy Follow along, it's a work in progress. If you have any questions, just let me know. Chuck
Nov 30, 2009, 02:36 AM
Registered User
Letchworth, Great Britain (UK)
Joined Jul 2004
13,396 Posts
Quote:
 Originally Posted by iosens ... Step3: Now the Batteries, What is the difference in performance between choosing a high voltage, high RPM, small propeller system and a lower voltage, lower RPM, bigger propeller system if the resulting Watts are the same?? (Higher RPMs are more responsive/dynamic ???) ...
You've got the basic idea, and I'm sure Chuck's link will give you a lot of detail.

But I would just correct one misonception in your post -- high volts does not necessarily have to mean high rpm . A high volts/low amps setup is generally considered to be more efficient than a low volts/high amps system putting out the same watts and, within limits, can result in you needing lower-current-rated components. If you decide to go this route from the outset, you can choose a motor with a low kv (rpm per volt) rating so that it's rpm will be the same as a lower voltage motor with a high kv.

Also, a larger prop swinging slower is reckoned to be more efficient than a fast small prop, so select a motor with a kv that will permit the largest prop that suits your model. Most reputable motor manufacturers will give a guide to what prop suits their motor with what battery, but you should also verify the actual amps draw on your system with a wattmeter before your maiden flight.
Nov 30, 2009, 04:21 AM
An itch?. Scratch build.
South Wales U.K.
Joined Mar 2003
16,473 Posts
One of the 'problems' with electric flight is the number of options and variables, see the picture below for a simple explanation.

Whereas the glow guys would say, "just fit the biggest motor you can in it".

# Images

View all Images in thread
 Nov 30, 2009, 11:45 AM Registered User Joined Nov 2009 4 Posts Thanks guys That cleared some things up. I'll chew on it and will get back ! Eriks
 Dec 01, 2009, 06:01 PM Registered User Joined Nov 2009 4 Posts NoFlyZones Tutorial Hi NoFlyZone Your tutorial is great, maybe at some time you can do some footnotes that explain some of the more complicated issues that are hidden between the lines. I am for instance surprized that the square root of the Wing load gives you the stall speed. Shouldn't the shape and profile of the wing have to do something with that, too? I would think that with the same load a profile for a soaring plane will fly slower than a symmetrical racing profile. Or not?
Dec 01, 2009, 06:11 PM
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Quote:
 Originally Posted by iosens Hi NoFlyZone Your tutorial is great, maybe at some time you can do some footnotes that explain some of the more complicated issues that are hidden between the lines. I am for instance surprized that the square root of the Wing load gives you the stall speed. Shouldn't the shape and profile of the wing have to do something with that, too? I would think that with the same load a profile for a soaring plane will fly slower than a symmetrical racing profile. Or not?
Hi Josens,

And you would be right in thinking that the profile of the wing would bear on the flight characteristics of the wing. The good thing for beginners though, is that it doesn't play into the calculations enough to throw the calculations off in any significant way. Sort of like, does it really matter if we say the stall speed of a particular plane is 15 mph, when in fact, it's really 16 mph?

The big thing about the tutorial is that it introduces you to the things that you may not have realized played such a significant role in selecting motors and props for our planes. Notice I stayed far, far away from complicated formulas and theory.... yet when your done, you have a very solid grasp of the things you need to know.

Here's an interesting tidbit for you, since you like to know the things between the lines, as you put it.

If we pick a top speed of about 3 times the planes stall speed... then the amount of power it takes to attain that top speed is 27 times the amount of power it takes to fly at it's stall speed! Neat, huh?

Chuck
 Dec 01, 2009, 08:42 PM Registered User Joined Nov 2009 4 Posts Interesting Hi NoFlyzone I am an engineer and I do understand your tutorial to stay away from formulas and instead apply "rules of thumb" to get to a "close enough" result. But - once I understand what makes things tick, I also what to know the details. Just like your example with three times the speed takes 27 times the power. 3^3 = 27 I know that the drag goes by speed-squared - where does the second power come from? Can you suggest a good book to dig in further? Maybe you also can explain the thrust issue a little bit deeper. Thanks
Dec 01, 2009, 09:05 PM
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Quote:
 Originally Posted by iosens Hi NoFlyzone I am an engineer and I do understand your tutorial to stay away from formulas and instead apply "rules of thumb" to get to a "close enough" result. But - once I understand what makes things tick, I also what to know the details. Just like your example with three times the speed takes 27 times the power. 3^3 = 27 I know that the drag goes by speed-squared - where does the second power come from? Can you suggest a good book to dig in further? Maybe you also can explain the thrust issue a little bit deeper. Thanks
Hi josens,

You don't need a good book, since these basic formulas can probably be found back in your introductory to physics classes you took back in high school or college. Pretty much all Newtonian classical physics here.

Now here's the thing, if you really want to get into this on a far deeper level than I understand, head on over to the Power Systems Forum, and jump in. There are some powerful smart people over there who eat, drink, and sleep this kind of thing. I don't want to start listing names, because I'm afraid of leaving out someone. We have guys here on RC Groups who are aeronautical engineers by profession, electrical engineers, and even physicists who will glad answer any question you have, on any level of complexity you choose to take on.

Some of these guys are scary good, and I don't deserve to be even mentioned alongside them.

What I do, is take what they say, and translate it for common people like myself, and beginners, so that we understand the gist of what they're talking about. Take a look see at that forum, and dive in....

As to your question about thrust, I'm not sure I follow what you're asking; but for good overall system efficiency, you want the largest prop you can swing that develops precisely the amount of thrust you desire for the style of flying you want, and you want to turn this prop at the minimum speed necessary to get you your desired top speed.

Chuck
Dec 02, 2009, 05:32 AM
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Quote:
 Originally Posted by iosens Just like your example with three times the speed takes 27 times the power. 3^3 = 27 I know that the drag goes by speed-squared - where does the second power come from?
Hi losens,

I just realized I never gave you an answer to this.... sorry.

You're right, of course, when you say that drag goes up by the square of the speed increase, so that when we increase our speed by a factor of three, the drag becomes 3 x 3.... or 9 times as much.

You wanted to know where the third factor of 3 comes from, though. If you'll recall from your pre-engineering days, power is defined as work per time, and since work = f*d, we have that power = (f*d) / t

We can rewrite this as power = f * d/t

Now, since you accurately stated that the force was 3x3, all we have to do is multiply this by the d/t, which is really velocity, to get that third factor of 3 you wondered about...

3x3 x 3fold increase in velocity = 27

So to wrap it up then, we have that it takes 27 times as much power to fly at 3 times stall speed as it took to fly at stall speed....

Chuck
Dec 02, 2009, 06:46 AM
Registered User
East Anglia, UK
Joined Sep 2002
29,710 Posts
Quote:
 Originally Posted by NoFlyZone Hi losens, I just realized I never gave you an answer to this.... sorry. You're right, of course, when you say that drag goes up by the square of the speed increase, so that when we increase our speed by a factor of three, the drag becomes 3 x 3.... or 9 times as much. You wanted to know where the third factor of 3 comes from, though. If you'll recall from your pre-engineering days, power is defined as work per time, and since work = f*d, we have that power = (f*d) / t We can rewrite this as power = f * d/t Now, since you accurately stated that the force was 3x3, all we have to do is multiply this by the d/t, which is really velocity, to get that third factor of 3 you wondered about... 3x3 x 3fold increase in velocity = 27 So to wrap it up then, we have that it takes 27 times as much power to fly at 3 times stall speed as it took to fly at stall speed.... Chuck
Actually, that's not strictly correct. You have left out induced drag, which dominates at lower speeds.

And tends to be more or less constant.

The p=f(v) equation is more like

P=aV + bV^3

where the 'a' term is the induced drag and the 'b' term is the parasitic.

Empirically, I've found that its takes about 6 times the power to get to 3x stall on a typically smooth sort of plane.
Dec 02, 2009, 07:09 AM
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Quote:
 Originally Posted by vintage1 Actually, that's not strictly correct. You have left out induced drag, which dominates at lower speeds. And tends to be more or less constant. The p=f(v) equation is more like P=aV + bV^3 where the 'a' term is the induced drag and the 'b' term is the parasitic. Empirically, I've found that its takes about 6 times the power to get to 3x stall on a typically smooth sort of plane.
Hi Vintage,

Wow, 6 times the power? So then you're saying that the induced drag would be equal to the parasitic drag throughout the speed envelope, correct? I never would have guessed that!

Thanks,

Chuck
Dec 02, 2009, 08:05 AM
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22,990 Posts
Quote:
 Originally Posted by vintage1 Empirically, I've found that its takes about 6 times the power to get to 3x stall on a typically smooth sort of plane.
Hi Vintage,

This one baffles me. You're saying it only takes 6 times the power to triple the speed of the plane... yet to triple the prop speed takes 27 times as much power (and realistically even more when accounting for motor inefficiencies).

I can't see how it would take 27 times as much power to spin the prop 3 times faster, yet only 6 times the power to triple the plane's speed.

Could you expand on this obvious major discrepency?

Thanks,

Chuck