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Old 01-06-2013, 07:57 AM   #11
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Re: Prop question

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It can become very difficult, especially with smaller craft. There have been lots of solutions tried so far and some look pretty cool.

The NACA style duct is effective and not as obvious as some. If you ignore the looks, the back half of plastic spoons make great inlet and outlet scoops and if not already the correct color can be painted to match.
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Old 01-09-2013, 12:36 PM   #12
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Re: Prop question

Originally Posted by Cougar429 View Post
With brushless motors there is a very narrow efficiency range for each Kv
no, but for the other stuff I would l have to agree

Electric motors do have a peak efficiency speed, but the efficiency doesn't vary as much as an engine.

Engines have much lower efficiency and much smaller power band than motors.
Until recently we just didn't have the price/computing power/energy density to fly model aircraft (reasonably) with electric motors

Engines, at best, only get about 35% efficiency, there is just so much energy in gas and alcohol to waste it didn't matter.

Brushless motors are running in the 90% efficiency range now. (keep in mind we are still burning coal to charge batteries, but we are only talking efficiency of the airborne power system, right?)
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Old 01-09-2013, 10:31 PM   #13
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Re: Prop question

Perhaps I was not as clear as possible. What I meant was the brushless motors are most efficient at a certain RPM/V. If at any power setting the motor is not allowed to reach that speed then the efficiency falls off dramatically. This can easily be noted as the current gets turned to heat, rather than torque.

Obviously, hitting that peak RPM/V is impossible due to the fact you are never running the motor in a free condition, (ie, using it to power other mechanical devices such as a rotor system, driveline or prop). It is when you load it down TOO MUCH that what I described occurs.

As for efficiency ratings. In the real world readings I calculated I have never been able to get numbers to match unless I plug in approx 82-85%. To be honest I have not done much of that in the last couple of years, so perhaps some of the new designs are much better. I do know of some that are real honkers, (NEU for one), but you pay more for that quality.

With motors this is true throughout the RPM range. For internal combustion engines a whole host of factors come into play, (combustion chamber design, compression ratio, intake and/or exhaust tuning, etc.). This is why they have a narrow peak efficiency range. There have been a lot of improvements in just the last few years with roller cams coming into general use and high-pressure direct injection, (to name just a couple). This has allowed a much wider peak starting at much lower RPM's. Still, this is as you say, way less efficient than electrical motors.

p.s. Don't even get me started on the addition of methanol............
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