Ducted Fan Thrust

[468DW]

Hi.
I have been searching to find out some information regarding Ducted Fans.
I am interested to know how much thrust can be obtained by one.
I do understand that this will depend on how much power and RPM the engine is putting out.
I am interested for a project that will be used for ground running only.

Regards
David

 

[GregLocock]

David, try one of the search engines (we have two). Also, have a look in the FAQ in my sig.





Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[468DW]

Hi Greg.

Thanks for your reply.

I did the searchs. But Not much luck.

Regards
David

 

[GregLocock]

google found this in a flash.

thread2-58711

I strongly recommend you look for the NASA paper

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[touch84]

Under the same power the thrust depends on the fairing (casing) because the trhust is the sum of two factor:
1)the thrust due to the rotor (propeller) ( and for a given rotor you can't change it)
2)the thrust due to the fairing.
So you can choose the fairing: converging, cylindrical or diverging.
Fairing thrust is positive for diverging fairing because it acelerates the flow on the propeller disk and because the wake section is greater, so the flow of mass is greater and exit velocity is greater consequently the thrust is greater.
T = mp*(ve-v)
mp = is the flow of mass
ve = is the exit velocity of the flow
v = is the asintotic velocity of the flow (upstream).

Fairing thrust is also positive for cylindrical and low rate converging fairing.

Fairing thrust is negative for converging fairing because it slows down the flow and because the wake section is smaller.

So a ducted fun with diverging fairing is able to give greater thrust (in comparison with a free propeller with the same diameter) at low velocity, while the converging one let us to use the ducted fun at higher number of Mach because it slows down the velocity on the propeller disk and so it retards the critic conditions.

I hope to be helpful,
excuse me for my english, but I'm Italian.

 

[touch84]

However overlooking the loss due to the viscous effect on fairing under the same power you can calculate the thrust at fixed point (asintotic velocity = 0 ) in this way:

T = rho*pi*(D^2/4)*lambda*ve^2

where

rho = density of air
pi = 3.1415
D = diameter of propeller disk
lambda = ratio between velocity on propeller disk and exit velocity
ve = exit velocity

for a diverging fairing lambda > 1
for a cilindrical fairing lamba = 1
for a free propeller disk lamba = 0.5

the power absorbed is:

P = rho*pi*(D^2/4)*lambda*(ve^3/2)

so you can calculate the thrust by power (at fixed point):

T = cubic sqrt (4*lamba*rho*(pi*D^2/4)*P^2)

this is tha maximum thrust because remebering that an equivalent form of thrust is

T = mp*(ve-v);
and at fixed point v = 0;