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# ducted fan3

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## ducted fan

(OP)
Hello I'm looking for information all information is greatly appreciated

I need to greatest thrust using a combination of fans and blade size??? I would like to try and generate about a thousand pounds of thrust using the smallest diameter fans.
does using a series of fans in one duct generate more thrust in a smaller area ????? if so what size diameter fans could I use to generate a thousand lbs of thrust using any combination of fans to achieve my target thrust without going over 12" diameter????

thanks..

### RE: ducted fan

Well let's see if we can apply some common sense. To get a thrust of 4000N you need V*mdot =4000 (that is force =change of momentum/time)
and
mdot=rho*A*V (continuity equation)

So the crudest approximation is that 4000=V^2*rho*A

so V=sqrt(4000/1.2/.15^2/pi)

V=217 m/s

That is too close to the speed of sound to expect to get away without compressibility effects, which engine designers call Mach choking. Then add in realistic efficiency for ducted fans (60% at best), and the answer is more firmly no.

Cheers

Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

### RE: ducted fan

I think you're missing a "1/2" term and a "C" term, ie Lift = Cl*q*S, q = 1/2*rho*V^2, S = area, and C for a ducted fan is between 0.02 and 0.05 (the things you can easily find on the internet); all terms increase V. Online suggestion is max force is something like 160N (40 lbs).

and it's "ducTed".

another day in paradise, or is paradise one day closer ?

### RE: ducted fan

(OP)
Thank you for your information what is the smallest fan or fans that I can use to achieve 1000 lbs of thrust using any combination of fans????

thank you

### RE: ducted fan

(OP)
hahahah ducked fan idk thats funny I missed that one correct spell idk???

### RE: ducted fan

if 1' dia gives you 40 lbs, and you want 25x ... I think you can figure it out.

if you try to sequence fans, first you need to carefully design them (I'm assuming you're using fixed pitch) as the downstream fan is working in a different airstream to the first fan. 40 lbs thrust is limited by the tip speed of the fan (and not the deltaV of the airflow), according to the trusty internet but you are still limited by sonic airflow inside the duct. I'd suggest setting the downstream fan to be contra-rotating. Multiple fans is starting to sound like a turbine, so research how these are designed.

If this is a homebuilt, build something and test it.

another day in paradise, or is paradise one day closer ?

### RE: ducted fan

(OP)
It's so encouraging to see so many well educated individuals sharing their hard earned education
with such enthusiasm. I remember a lesson in school my science teacher said "The gift of knowledge is a privilege to be shared the ability to educate is a gift to be honored without prejudice"
I thank all of you for your serious reply's and well thought out advice.

You truly are a credit to higher education.

It was once said that Thomas Edison knew all there was to know about electricity then he met Nicola Tesla.

### RE: ducted fan

perhaps like how Trump knows everything about politics ?

another day in paradise, or is paradise one day closer ?

### RE: ducted fan

Oh the other equation that you need is the power=1/2*mdot*v^2. In the example above that comes to around 240 kW, or 320 hp.

All these equations apply to static thrust, if the aircraft is moving in the axial direction then the equations change slightly I think.

Cheers

Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

### RE: ducted fan

(OP)
Yes, I am very new here I am not an engineer I am a pipe welder and machinist my father was a machinist all my life he had his own shop for 30 plus years. I went to school to be a welder I have been a pipe welder for over 25 years... I'm not a student... I just needed some information on a new concept I am working on.

As soon as I finish my research I will build my prototype I have a full machine shop CNC. machines, mills, lathes and a flow jet cutter equipment I have used most of my life I can make just about anything

I only asked a couple questions I promise I am not here to bother anybody sorry for bothering you/

I hope nobody else asks me to read the policies again its been mentioned 3 x now.

yes I have read the policies something I did not do when I joined.

I thought this site was about professionals helping each other?????

### RE: ducted fan

btw, I think you'll find that 1000 pounds of thrust will require a not insignificant amount of blade, engine, and fuel.

The MQ-8 Firescout take-off weight is about 3x your desired thrust with a 27.5 ft diameter rotor.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

### RE: ducted fan

you're right, this site is for engineering professionals to help each other. As you've declared you're not an engineer, which means you're missing alot of stuff we assume. We don't know your application, so many of our responses are off-base ... part of the cost of doing business I guess. Much of what you've asked can be found with google searches, it is amazing what can be found. Much of this could require an engineering education to understand, much can be taken at face value. Building stuff and testing it is a way forward.

another day in paradise, or is paradise one day closer ?

### RE: ducted fan

One slight proviso. The efficiency of 60% I quoted is from a NASA windtunnel test, the only one I've seen. However you'll find many papers quoting up to 80% for a DF, based on CFD, and a chat with a real DF engineer suggests that this may be reasonable. The NASA DF had a parallel bore for the duct, which is a surprisingly poor choice.

Cheers

Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

### RE: ducted fan

The first thing that came to mind here is the idea of disk loading:

In particular, the relationship between disk loading and efficiency:

Most light-to-medium lift helicopters operate at less than 10 lbs per square foot, yielding an efficiency of over 12 lbs of lift (or thrust) per horsepower. A disc of 1' in diameter has an area of slightly less than 1 square foot (0.785 square feet) so 1000 lbs of thrust would imply a disk loading of almost 1300 lbs per square foot which would put the efficiency at less than 2 lbs of thrust per horsepower (6X worse). On the chart above, that's in between the zones of "Lift Fan" (like the F-35 joint strike fighter) and "Direct Lift" (like the Harrier.)

### RE: ducted fan

That graph ignores many subtleties, including the internal profile of the duct for a ducted fan. This provides a much better impedance match between the DF exhaust speed and the general atmosphere. In other words the profile can be modified to increase the effective disc area.

Cheers

Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

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