hovering thrust calulation


by reebrah
Tags: calulation, hovering, thrust
reebrah
reebrah is offline
#1
Dec5-12, 06:05 PM
P: 10
1. The problem statement, all variables and given/known data
basically I am doing work that requires me to calculate power for given thrust of a fan which is hovering.



2. Relevant equations
We are given the diameter: 0.8m, thrust output of fan: 250kg, rho=1.1455 kg/m^3, and eta which is the fan efficiency = 0.8


3. The attempt at a solution

I found this equation in 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!
Phys.Org News Partner Science news on Phys.org
SensaBubble: It's a bubble, but not as we know it (w/ video)
The hemihelix: Scientists discover a new shape using rubber bands (w/ video)
Microbes provide insights into evolution of human language
BruceW
BruceW is offline
#2
Dec6-12, 06:17 AM
HW Helper
BruceW's Avatar
P: 3,339
Your equation looks fine (although, It depends on how the fan efficiency is defined).

One thing which is not right is that you've put thrust in units of kg, but thrust is a measure of force, not mass.
reebrah
reebrah is offline
#3
Dec6-12, 06:34 AM
P: 10
Quote Quote by BruceW View Post
Your equation looks fine (although, It depends on how the fan efficiency is defined).

One thing which is not right is that you've put thrust in units of kg, but thrust is a measure of force, not mass.
Theres also another eq. Ive found on wikipedia which replaces the eta^2 with a constant of four. How can i confirm this is correct? Also you are right about the thrust units.

BruceW
BruceW is offline
#4
Dec6-12, 09:49 AM
HW Helper
BruceW's Avatar
P: 3,339

hovering thrust calulation


The constant of 4 is assuming that no energy is lost as heat when the air is accelerated. You can derive the equation by considering the volume of air which passes through the fan in some time interval. It is the very simplest model you could imagine, since it neglects the effects of viscosity, turbulence, etc

For your homework, are you meant to be explaining why a particular equation is the correct one? Or were you just asking about the equation because you were curious?
reebrah
reebrah is offline
#5
Dec6-12, 10:34 AM
P: 10
No i am just trying to find an accurate way to model power consumed given an amount of thrust and radius of fan. Are their any other factors i should be worried about?
BruceW
BruceW is offline
#6
Dec6-12, 07:21 PM
HW Helper
BruceW's Avatar
P: 3,339
Your equation is fine. As I said, you just need to be careful what you use as your fan efficiency, since different people might define it differently. For example, you could keep the constant of 4 in the equation, as well as the fan efficiency, in which case, the fan efficiency would be defined differently.


Register to reply

Related Discussions
Helicopter hovering General Physics 9
UFO or UHO - Unidentified Hovering Object? General Discussion 0
Electric field calulation Advanced Physics Homework 4
Object Hovering In St. Paul Sky General Discussion 18
Is hovering helicopter torque ever exactly = 0 ? Aerospace Engineering 8