Ducted Fan & Velocity of the air in tailpipe. - RC Groups
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Oct 06, 2012, 11:32 AM
Texas Buzzard
Texas Buzzard's Avatar
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Ducted Fan & Velocity of the air in tailpipe.


I used to watch F-84s takeoff carrying two 500 lb boms + 6 HVAR rockets fully fueled. They were able to takeoff. ( in the 1950s)

Having some experience in free flight and controline I marveled at how that jet engine could produce the needed thrust to make all that mass accelerate. I envisioned all the gasses escaping from the tailpipe and wondered what mass was being driven throught that tailepipe.

I knew some basic physics, i.e., Newtons 3rd Law, and it seemed that the very low density of the escaping gases would have to have a great velocity in order to produce the significant thrust needed.

If we were to compare the Momentum of the exhaust gases to the momentum of the airplane at first it seems that they would be equivalent to one another..... friction aside.

I have an electric ducted fan rated at 1.8 pounds of thrust by the manufactuer.
I know the density of air is 1.29 grams per liter.

What would the exaust velocity of the air have to be to produce 1.8 pounds of thrust?

You might want to rewrite this question......... ??? Please do not tell me to buy an anemometer.....ha, ha.
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Oct 06, 2012, 12:05 PM
Cognitive dissonance
kcaldwel's Avatar
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https://www.rcgroups.com/forums/show....php?t=1332150

EDFs are not the same as turbojets. (Turbojets add to the efflux mass through creating added gas by combustion and the heat increasing the volume and velocity.)
Last edited by kcaldwel; Oct 06, 2012 at 08:58 PM.
Oct 06, 2012, 12:40 PM
B for Bruce
BMatthews's Avatar
It's not only the velocity but the volume/mass that you're moving. The EDF is going to accelerate some volume related mass to some velocity to achieve the 1.8 lbs of thrust.

It's also a bit of a trick question since because the thrust is based on two factors one can take over for the other. You could have a lot of volume that is only accelerated by a little or a little volume accelerated a lot. Without pinning one of the numbers down to some reasonably accurate value for that EDF it's pretty hard to calculate the other.

One way would be to stick a large industrial size garbage bag on the outlet. Then switch the fan on and see how long it takes to fill the bag. Calculating the size of the bag and how long it took to fill that volume would give you a reasonably good value of the volume per second that is being moved. From that you could work with more equations to find the velocity of the outlet.

Or just stick your hand behind it and call it fast, darn fast or DAMN fast and call it a day....
Oct 06, 2012, 01:31 PM
An itch?. Scratch build.
eflightray's Avatar
Who needs calculations when you can just build a model and try it, it's a hell of a lot more fun.

I always feel the more you discuss whether a plane will fly or not, the more worried the plane gets.

Mig August.wmv (5 min 34 sec)
Oct 06, 2012, 01:50 PM
Registered User
Thrust = mass_flow_rate * (Vexhaust - Vflight)
Pideal = mass_flow_rate * 0.5*(Vexhaust^2 - Vflight^2)

where Pideal is the ideal or absolute minimum power or energy rate required by whatever is accelerating the air, either a propeller, fan, or jet engine core.
Oct 08, 2012, 01:49 AM
German Engineering.......
HugePanic's Avatar
"Please do not tell me to buy an anemometer.....ha, ha...."

Why not build one???

Some clear tubing, some water, ink and a ruler should do the job...
Oct 09, 2012, 05:46 PM
Registered User
[QUOTE=markdrela;22929313]Thrust = mass_flow_rate * (Vexhaust - Vflight)

Mark, how did you arrive at this equation?
Oct 10, 2012, 04:41 PM
Registered User
[QUOTE=TangoKilo;22956646]
Quote:
Originally Posted by markdrela
Thrust = mass_flow_rate * (Vexhaust - Vflight)

Mark, how did you arrive at this equation?
It's part of the continuity equation, a rewriting of Newtons second law, you can see here:

http://en.wikipedia.org/wiki/Mass_flow_rate

Near the bottom of the page, under usage.

The equation Mark gave ignores the acceleration term.

The velocity term gives you a linear relationship, from maximum attainable thrust at 0 airspeed, down to zero thrust at whatever the exhaust velocity itself is.
Oct 10, 2012, 05:23 PM
Registered User
[QUOTE=nristow;22965239]
Quote:
Originally Posted by TangoKilo

It's part of the continuity equation, a rewriting of Newtons second law, you can see here:

http://en.wikipedia.org/wiki/Mass_flow_rate

Near the bottom of the page, under usage.

The equation Mark gave ignores the acceleration term.

The velocity term gives you a linear relationship, from maximum attainable thrust at 0 airspeed, down to zero thrust at whatever the exhaust velocity itself is.
Well, I see F = d(mv)/dt and I see F = m.a +mdot.v

but I don't see Thrust = mass_flow_rate * (Vexhaust - Vflight)

Can you briefly explain how you have rewritten Newton's second law?
Oct 10, 2012, 07:25 PM
Registered User
Thrust is a force, so substituting thrust in the wiki equation and ignoring acceleration gives you

Thrust = mdot * v

where mdot is the mass flow rate. The units work out, [kg/s * m/s = kg*m/s^2].

As I've said above, the velocity term is a sum since you don't make any thrust when our velocity is the same as the exhaust velocity, and you assume that thrust tapers linearly.

Does that help at all?
Oct 10, 2012, 07:41 PM
It wasn't me...
DanSavage's Avatar
Quote:
Originally Posted by HugePanic
"Please do not tell me to buy an anemometer.....ha, ha...."

Why not build one???

Some clear tubing, some water, ink and a ruler should do the job...
He's talking about an anemometer and you're talking about a manometer. Two different beasties.

An anemometer measures velocity and a manometer measures pressure.

But, you are right. A manometer is quite simple to build. I built one as you describe to calibrate my ASI I use to measure efflux velocity of my fans.

If one wants to measure efflux velocity of a fan, a very cheap and easy way is to buy a used ASI off eBay. I paid $35 for mine and it measures up to 250mph.
Last edited by DanSavage; Oct 10, 2012 at 07:49 PM.
Oct 10, 2012, 10:52 PM
treefinder
springer's Avatar
Quote:
Originally Posted by eflightray
Who needs calculations when you can just build a model and try it, it's a hell of a lot more fun.

I always feel the more you discuss whether a plane will fly or not, the more worried the plane gets.
Go Ray, loved the vid! Especially the commentary!
Oct 11, 2012, 05:17 AM
Registered User
[QUOTE=nristow;22966584]

Thrust = mdot * v

nristow, yes, I agree. F = dm/dt * v

I would advise you not to mention that equation on the "Propeller efficiency" thread. I was shot down as a troll for doing so.

Quote,
As I've said above, the velocity term is a sum since you don't make any thrust when our velocity is the same as the exhaust velocity, and you assume that thrust tapers linearly.

If I understand you correctly, you are saying that the exhaust velocity is the maximum speed at which your model can move and at that speed no thrust is being produced. If there is no thrust what force is overcoming the aerodynamic drag?

It is true that the sum of thrust and drag equals zero but that does not mean there is no thrust.
Oct 11, 2012, 04:34 PM
Registered User
[QUOTE=TangoKilo;22956646]
Quote:
Originally Posted by markdrela
Thrust = mass_flow_rate * (Vexhaust - Vflight)
Mark, how did you arrive at this equation?
Integral Momentum Theorem applied to a control volume surrounding the propeller.
Pretty standard stuff.
http://web.mit.edu/16.unified/www/SP...es/node86.html
section 11.7.2
Oct 12, 2012, 09:19 AM
Registered User
[QUOTE=markdrela;22974919]
Quote:
Originally Posted by TangoKilo
Integral Momentum Theorem applied to a control volume surrounding the propeller.
Pretty standard stuff.
http://web.mit.edu/16.unified/www/SP...es/node86.html
section 11.7.2
Mark, yes I have already read that and many other similar works.

I still can not accept the concept that thrust reduces as "Vflight" increases.
What happens if "Vflight" = "Vexhaust"? Zero thrust?

Can it truly be that aerodynamic drag, increasing as the square of the velocity, is being reacted by thrust decreasing with velocity?


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