Why, EDF's power seem to be measure for Watts? - Page 2 - RC Groups
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Jan 10, 2007, 05:10 AM
Grumpy old git.. Who me?
JetPlaneFlyer's Avatar
Originally Posted by monkamarm2000
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.

A simple spring balance or set of kitchen scales is all that’s required to measure static thrust…
Static thrust is not as ‘perfect’ as measuring actual thrust as flying speed (which would require a wind tunnel or air blower of some sort) but it’s still a lot more accurate a measure of actual motive power than measuring Watts at the battery. A simple spring balance is also cheaper than the hardware required to measure electrical power too.
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Jan 10, 2007, 03:17 PM
Stilwell Shipyard
lslewis's Avatar


JetPlaneFlyer - Your beating this thing to death. He asked a question, why do we use watts instead thrust. The answer is simple, because it is easy nobody said accurate.

The little boy asked his mother, "what's lightning?", mother said, "ask your dad he is an engineer" little boy said "I didn't want to know that much about lightning!"

Smile JetPlaneFlyer,

Hurricane Larry
Jan 10, 2007, 04:27 PM
Grumpy old git.. Who me?
JetPlaneFlyer's Avatar
Larry... i am an Engineer... i cant help myself
Jan 10, 2007, 07:03 PM
EDF rules... :)
AirX's Avatar
Originally Posted by JetPlaneFlyer
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)

Yup I could have given all the conversion factors that are relevant but it did not answer the question he asked and what I gave, even though short and sweet, did. He actually asked two questions and I did not see any previous posts that addressed it properly except Ed and myself. So if you want to find fault in anything that I wrote answer the question first and help the newby then you can have at me....

Eric B.
Jan 11, 2007, 12:52 AM
Grumpy old git.. Who me?
JetPlaneFlyer's Avatar

This were the original questions:
Isn't it more correct to measure how many grams, ounce of thrust is actually produce, rather than how many watts is used?
The simple answer to this is 'Yes'.. in terms of flying performance it is more 'correct' to measure thrust.

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?
Again... simple answer is 'Yes' an efficient system can produce more thrust on lower watts than an inefficient one.

Are they simple and concise enough?
Last edited by JetPlaneFlyer; Jan 11, 2007 at 01:21 AM.

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