Hovering thrust calulation

reebrah

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!

Aero51

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]

reebrah

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?

Aero51

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

AlephZero

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

reebrah

I think that might just solve my problem lolz

AlephZero

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