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Old Jan 09, 2007, 12:31 AM
jascamera is offline
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Question

Why, EDF's power seem to be measure for Watts?


I am somewhat new to EDF, just got a Het F-18 couple of weeks ago. Flys sweet. What I want to ask is, why everyone seem to measure how fast, how powerful by how many watts a power system pulls. Isn't it more correct to measure how many grams, ounce of thrust is actually produce, rather than how many watts is used?
I thought if a EDF have an efficient motor and ducting and duct fan can produce the same amount of thrust at a lower wattage than as system with less effecient motor, ducting and Duct fan system at a higher wattage?
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Old Jan 09, 2007, 12:52 AM
roadhogtri is offline
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Alot of guys are running similar systems on the F-18, therefore know how each system performes. You're right about power to weight ratio an effeciency, but most measure watts just to get a ballpark figure .
Old Jan 09, 2007, 01:51 AM
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Watts is a measure of Power, more power the faster the fan turns, making more thrust & speed.
Efficiency in motor means it will run longer not faster
" in ducting means it will make more thrust on less power (watts)
Light weight means it will accelerate quicker, not go faster

Bottom line is more watts (power) is faster
Old Jan 09, 2007, 03:08 AM
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We measure mainly in watts cus we are lazy... and also cus that's what the watt meter reads in,,, Watts......

Gene
Old Jan 09, 2007, 05:55 AM
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Also because the difference in fans (efficiency) is small as is the difference in motors (efficiency). Now all the fan people and motor people are going to flame me. I acknowledge that a specific fan with a specific motor in a specific application can be better than another. But I am talking generalities and that includes "because we are lazy"

Hurricane Larry
Old Jan 09, 2007, 06:34 AM
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If it helps as a rough rule 1 W of power gives you 2g of thrust. So 1000W give 2 kg of thrust.
Old Jan 09, 2007, 07:29 AM
JetPlaneFlyer is offline
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Quote:
Originally Posted by Richie Simpson
Watts is a measure of Power, more power the faster the fan turns, making more thrust & speed.
Efficiency in motor means it will run longer not faster
" in ducting means it will make more thrust on less power (watts)
Light weight means it will accelerate quicker, not go faster

Bottom line is more watts (power) is faster
I canít help but pick up on a few of these points...

Quote "Efficiency in motor means it will run longer not faster"
IMHO this is not true ... An efficient motor will convert electrical energy to mechanical work more efficiently... That means for a given electrical power input in Watts, it produces more torque or spins faster (or both) than an 'inefficient' motor. If the electrical power input in Watts is the same then length of run will be the same regardless of motor efficiency. You can only increase run length by reducing the Watts or by increasing the capacity of the battery.

Quote: "Light weight means it will accelerate quicker, not go faster"
This would be 'almost' true for a car or any vehicle supported on wheels, however it is not the case for an aircraft in flight... Induced drag is proportional to lift and lift (in level flight) is equal to weight. Therefore more weight = more drag and more drag = lower flying speed.

Quote: Bottom line is more watts (power) is faster
This is way over simplified... there are lots of factors to be considered, motor input power is one variable that effects top speed but it's probably not the most important... Drag for instance is proportional to the airspeed squared therefore reducing drag is a far more effective way of increasing top speed than increasing power.

Assuming that your model will be fast just because it uses up lots of Watts is like assuming your car will be fast simply because it consumes a lot of gas
The 'real' bottom line is electrical Watts do not push an aircraft forward, thust is what we are really after. As mentioned previously I suspect Watts is used simply for convenience on the assumption that it is reasonably proportional to thrust... This assumption may or may not be the case depending on other design variables.
Last edited by JetPlaneFlyer; Jan 09, 2007 at 07:58 AM.
Old Jan 09, 2007, 08:16 AM
perttime is offline
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Quote:
Originally Posted by Richie Simpson
Bottom line is more watts (power) is faster
Not really. More power may give you more speed or more thrust. Just like with propellers: depending on what propeller (fan + inlet + thrust tube) you use, you get more speed and/or more static thrust.
Old Jan 09, 2007, 08:34 AM
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Or, more watts on a given fan will give you more speed and thrust.

~Luke
Old Jan 09, 2007, 12:04 PM
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Thrust output can change with different exhaust diameters as well. Watts is probably the best measure, and not everyone has a thrust stand to measure with, and thrust can change when installed in a model, due to intake layout, size, smoothness, etc.
Old Jan 09, 2007, 01:08 PM
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Actually horsepower of motor alone wont say car is fast, but watts are like a measure of power to the wheels, since the motor see's the load the impeller creates, so it is a very good measure of just how fast your plane will go. I couldnt count how many planes I actually measured or cared how much thrust it had, but I always check the "installed" watts.


Barry
Old Jan 09, 2007, 09:06 PM
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EDF rules... :)
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Quote:
Originally Posted by jascamera
I am somewhat new to EDF, just got a Het F-18 couple of weeks ago. Flys sweet. What I want to ask is, why everyone seem to measure how fast, how powerful by how many watts a power system pulls. Isn't it more correct to measure how many grams, ounce of thrust is actually produce, rather than how many watts is used?
I thought if a EDF have an efficient motor and ducting and duct fan can produce the same amount of thrust at a lower wattage than as system with less effecient motor, ducting and Duct fan system at a higher wattage?
The reason why watts are used as the measure of power thru a fan or propellor is the system is electrical and the watt is the measure of power thru an electric motor(amps X volts = watts), it is directly convertible to horsepower as 750watts = 1hp.
Ducting can be disastrous to the airflow to and away from the fan where you can loose up to 30% of thrust with poor ducting and 5% to 10% with good ducting, rarely do you loose nothing as compared to no ducting, simply because of drag from wetted area.
This is all covered in high school and college physics books, not spelled out directly but all the concepts are there.

Eric B.
Old Jan 10, 2007, 02:11 AM
JetPlaneFlyer is offline
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Eric,
I think the original question was asking why everyone quotes output power from the battery (in Watts) rather than quoting thrust (in Kg / pounds or whatever)...
I think he has a very good point because quoting power in Watts in the wires between battery and motor neglects all the inefficiencies and ducting losses in the total system, which as you point out can vary considerably from one system to another...
Ultimately it does not matter a jot how much power we put into the motor, it is how much thrust that we get out that governs performance. I still think my analogy to car gas consumption is valid... If I claimed my car was fast simply because it used lots of petrol (gas) then people would think me insane.

By the way; a 'Watt' is a measure of 'power'... Not just electrical power but any power. It is equal to one Joule of energy per second (in an electrical system it is also equal to Volts x Amps as you said)

Steve
Old Jan 10, 2007, 02:30 AM
perttime is offline
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Quote:
Originally Posted by JetPlaneFlyer
a 'Watt' is a measure of 'power'... Not just electrical power but any power. It is equal to one Joule of energy per second (in an electrical system it is also equal to Volts x Amps as you said)

Steve
Yup. It is a part of the metric system where everything is connected.
Old Jan 10, 2007, 03:04 AM
monkamarm2000 is offline
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but unless you have a wind tunnel to eek out your percent's here and there your still gonna be in good shape looking at watts.

Barry


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