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Old Oct 10, 2012, 05:23 PM
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Is it possible that we might be feeding a troll here?
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Old Oct 10, 2012, 05:46 PM
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Quote:
Originally Posted by ShoeDLG View Post
In chapter 4 of Principia, where he talks about propellers.
If I remember correctly, the Principia Mathematica was published in three volumes. Unfortunately, I do not have access to, nor have I read any of them.
Perhaps you could advise me in which of the volumes I may find the refererence to propellers?
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Old Oct 10, 2012, 06:00 PM
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Quote:
Originally Posted by Don Stackhouse View Post
Is it possible that we might be feeding a troll here?
You may be referring to me. This forum is titled "modeling science": Have I asked any question which is unscientific?

I have only asked for logical answers to things which may not be understood. If the combined knowledge of this forum is unable to answer my questions and my questions are considered "trolling" I will gladly depart.
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Old Oct 10, 2012, 06:53 PM
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Quote:
Originally Posted by TangoKilo View Post
I will not choose my control volume carefully
That approach should be fine... as long as you are equally discriminating about your results.

I think it's volume 2, chapter 4 where Newton discusses propellers... It's right after the section about downwind turns (which you really need to read in the original Latin fully appreciate).
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Old Oct 11, 2012, 04:47 AM
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Quote:
Originally Posted by ShoeDLG View Post
That approach should be fine... as long as you are equally discriminating about your results.

I think it's volume 2, chapter 4 where Newton discusses propellers... It's right after the section about downwind turns (which you really need to read in the original Latin fully appreciate).
Who was it who said,"the next best thing to solving a problem is finding some humour in it"?

Doesn't a control volume have to have dimensions? How can you define a control volume if you don't know and have no way of measuring the dimensions?
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Old Oct 11, 2012, 12:38 PM
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TangoKilo here are some dimesions.

The EDF unit is an E-Flite DeltaV 15 using a Lipo of 3 cells , 11.1v used in a BAE Hawk by E-Flite.

The E-Flite Specs are:
Diameter is 69mm
Shroud length is 58.3mm (2.3 inches)
Current = 29amps on 3S Lipo
Watts on 3S = 290 to 300
RPM = 31,000
Thrust (static) =1.7 lbs on 3S
Velocity = 71 mph

The air has density of 1.29 gm/L .This air mass is moving at 71 mph. This mass of air has both K.E. and momentum even though the EDF is on a test stand.

Let us ignore any aerodynamic friction (drag) and looses by the fan.
Can we say the Momentum of the air is imparted to the airframe while the plane is at a constant altitude and speed? The weight of the proposed airframe ready to fly with battery is 45 ounces.What is the ideal maximum speed of that plane?
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Old Oct 11, 2012, 02:37 PM
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TB, I am not sure what your figures have to do with the previous discussion. Shouldn't your post be on an EDF thread?

Anyway, looking at your figures, if your system is drawing 29A for 300W then the lipo voltage is dropping to around 10,3V under load.

I have converted your thrust of 1,7 lbs to 7,56N so the Watt per Newton figure is
around 40. That is pretty good, most results I have seen suggest 70 W/N is an efficient system.

As to your final two questions, the answer to the first is yes. The second I can not answer, maximum level speed is achieved when maximum thrust equals drag.
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Old Oct 11, 2012, 06:29 PM
Texas Buzzard
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Reply to TangoKilo

TangoKilo you did post this.
"TB, I am not sure what your figures have to do with the previous discussion. Shouldn't your post be on an EDF thread?" No Tango the original question on this thread was about an EDF. The question was actually a Physics question which belongs in Modeling Science. It was not some general question about EDFs.... it was specific. Tha data I posted for the EDF in question was from the manufactuer.

Other posters have given answers to QUESTIONS THAT WERE NOT ASKED. That is easy to do and the reason some students make a lower grade. They just didn't read and understand the question.

That said, you agreed that the momentum of the pumped air would be equal to the total momentum imparted to the airplane, so may we say that knowing the mass of the plane and it's velocity then from that data we can calculate the speed at which the air being pumped is moving?

Is this science (math) question or is it about our EDF's? Consider a jet fighter being accelerated during takeoff. The massive plane requires a large force in order to takeoff. For moving air to supply this force, considering the very low density of the gases, the gases (air for EDF's) have to have a very great velocity.

Perhaps you can write a more pointed question.
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Old Oct 11, 2012, 08:10 PM
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I lost track. What are we arguing about?
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Old Oct 12, 2012, 07:49 AM
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I thot his was a model forum for discussing issues with models and the science which was relevant to model aviation.
Obviously others have a completely different undertanding of what is and what is not relevant.
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Old Oct 12, 2012, 08:04 AM
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[QUOTE=Texas Buzzard;22976308]No Tango the original question on this thread was about an EDF.

TB, without getting into a petty argument, I have just re-read this thread and the first mention of EDF is in your post number 96.
If you wish to continue this dicussion on your "Ducted fan and velocity of air" thread, please transfer your post to that thread.
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Old Oct 12, 2012, 08:06 AM
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Originally Posted by Montag DP View Post
I lost track. What are we arguing about?
How propellers work.
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Old Oct 12, 2012, 09:22 AM
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They screw their way thru the air
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Old Oct 28, 2012, 02:26 PM
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Power conversion from battery output power to flight power

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Description:

The motor takes electric power from the battery . Some is converted into mechanical power, some is dissipated as motor heat.

The propeller converts shaft power into the power of moving air. Some of this forms a directed slipstream, which imposes a thrust force on the propeller. The remainder is dissipated in random air movement.

If the 'plane is held so that it cannot move, all the slipstream power is dissipated in the slipstream wake.

In flight, since the point of application of the thrust is moving, then useful work is done against the opposition of drag [producing an airframe wake], and gravity [producing an increase in altitude].
.................................................. .................................................. .............................

Mathematical presentation

Name: powcon2.jpg
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Description:

E = battery output power = motor input power [W] = terminal EMF [V] x current [A]

D = motor output power = prop input power [W] = torque [Nm] x rotation speed [radians/sec]

C = prop output power [static] = slipstream input power [W] = 0.5 x static thrust [N] x slipstream speed [m/s]

B = power expended against gravity [W] = model weight [N] x rate of climb [m/s]

A = power expended against drag [W] = drag [N] x flying speed [m/s]

[A+B] I refer to as flight power

motor efficiency = D / E

propeller efficiency [static] = C / D

slipstream efficiency = [A+B] divided by C = flying speed divided by the average of flying speed and slipstream speed [see attached graph]

overall efficiency = [A+B] divided by E = motor efficiency x prop efficiency x slipstream efficiency

You will sometimes see prop efficiency defined as flight power divided by motor output power [ = (A+B) / D ].
This parameter in no way describes the quality of the propeller because it depends partly on factors unrelated to the prop. In a static test, all propellers would show an efficiency of zero.
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Old Oct 28, 2012, 03:20 PM
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Yes -but static tests do show motor output at load - It is how rebuilt Lycomings ar e tested.
Load props will show loads at various rpm.
No math- - I cain't due math
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