
Nov 29, 2009, 11:23 PM  
Joined Jul 2006
22,991 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 
Dec 01, 2009, 07:01 PM  
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, 07:11 PM  
Joined Jul 2006
22,991 Posts

Quote:
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, 09:42 PM  
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 speedsquared  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, 10:05 PM  
Joined Jul 2006
22,991 Posts

Quote:
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, 06:32 AM  
Joined Jul 2006
22,991 Posts

Quote:
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 preengineering 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, 07:46 AM  

Quote:
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, 08:09 AM  
Joined Jul 2006
22,991 Posts

Quote:
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, 09:05 AM  
Joined Jul 2006
22,991 Posts

Quote:
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 

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