
#1
Dec512, 06:09 PM

P: 10

Basically I am doing work that requires me to calculate power for given thrust of a fan which is hovering.
Variables known: diameter of fan: 0.8m, thrust output of fan: 250kg, rho=1.1455 kg/m^3 which is the density of air at 35Celcius , and eta which is the fan efficiency = 0.8 I tried this equation I found on wikipedia: P^2 = (T^3) / ( (eta^2)(rho)A) using this equation I got 7322w of power to attain a thrust of 250kg...This seems a little off. What is wrong here? does anyone know if this is even the correct value or even correct equation? thank you and appreciate it! 



#2
Dec512, 07:03 PM

P: 546

You can always use this equation which is valid for cases when the initial velocity of the air is 0.
Here is the link: http://en.wikipedia.org/wiki/Thrust#Thrust_to_power and in case you are feeling lazy: [itex]P^2 = T^3 / {4 \rho A}[/itex] 



#3
Dec512, 07:08 PM

P: 10

True but in this equation the eta^2 is replaced with a 4. This would then further decrease the power estimated from this relation. Anybody else know anything for hovering thrust?




#4
Dec512, 07:21 PM

P: 546

Hovering thrust calulation
Visit this link, specifically check pages 281. I think it has the equations you are looking for. The book also has a nomenclature section at the front of the text. Hopefully this helps:
http://books.google.com/books?id=SgZ...page&q&f=false 



#5
Dec512, 07:40 PM

Engineering
Sci Advisor
HW Helper
Thanks
P: 6,344

Did you forget that thrust is in Newtons, not "kilograms"?




#6
Dec512, 07:50 PM

P: 10

I think that might just solve my problem lolz




#7
Dec512, 10:51 PM

Engineering
Sci Advisor
HW Helper
Thanks
P: 6,344

At least it will give you a bigger number. 7kW is obviously way too small for a quartertonne chopper with a teeny little 0.8m dia rotor



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