Is this expression for the efficiency of a fan blade correct?
I'm making a ducted fan and seeing as these things take a long time to draw and a lot of money to manufacture I want to have some idea of the efficiency even if it's basic. I'm worried because it's a 12 blade fan which means the blade angles have to be quite steep in order for it to all fit together. Other EDF manufacturers seem to have done the same thing and theirs works fine but I want to check anyway because mine is much bigger.
If pic related is right then I can compute the coefficents of lift and drag on the chosen NACA 4412 airfoil using XFOIL and hence the efficiency. Vortex losses are being ignored because it's in a duct.
If Veeky Forums helps me out I'll post a video of it in operation. It's gonna sound amazing. Might rice it out as well with supersonic swept back blades and afterburners
Dylan Sullivan
Oh come on seriously fuck this board, Loads of people eager to spew bullshit about quantum mechanics or inter universal teichmuller theory but these same people somehow don't know enough physics to verify a simple efficiency expression? I'd use physicsforums if I was good at LateX.
Nathan Cook
Your result seems pretty correct, but I only ever did a minor in aircraft engineering so what the fuck to I know.
Keep in mind that this approach assumes that the air that hits the fan was in free stream before, but in reality it's not since there is a fan in front of it. Also, these formulas only hold when incompressibility is assumed, which is until about Mach 0.3 if I recall correctly.
All in all the accuracy of this calculation is dubious at best, you might get more realistic results if you put your model through a flow simulation. Solidworks does a pretty good job at this.
Aaron Stewart
>tfw Mach 2 at the tips This is haram right?
I can neither afford nor know how to use Solidworks.
Adrian Gutierrez
Learn LaTeX and go to physicsforums. Veeky Forums's only good for making engineers feel inferior to physicists anyway, lel
Julian Cruz
Where's a good LateX guide?
Zachary Fisher
>mach 2 at the tips this pleases me
Michael Anderson
Speeds like that put enormous forces on your blades, and I'm not even considering the heat and noise it will produce. To make your fan stable at those speeds requires some precision engineering. Also, the efficiency of your blade drops very quickly after you pass the sound barrier, so having half of your fanblade at supersonic speeds is probably not such a good idea. How do you even plan to make it go that fast in the first place?
Jacob Torres
Ok well thanks for nothing Veeky Forums
I'm gonna build it. Pic related works so why won't mine when it's the same shape?
Colton Russell
>How do you even plan to make it go that fast in the first place? A big ass motor. No but seriously a 8000 W brushless inrunner.
It really sucks balls when an entire project has to be abandoned because just one number fucks your shit up. I was really excited for this. Only hope is to find a motor with a lower RPM but I don't think I can.
Parker Edwards
Link to your motor? You need a very low KV motor.
What diameter were you planning to make your fan?
Joshua Martinez
You know what the real problem is? The voltage. The motor can handle 90 A max but at 8000 W that requires 89 V minimum. Since the RPM of a electric motor depends on the voltage that means the thing will spin too fast.
Since the motor takes AC power, you can always put it through a transformer to increase the voltage. This does decrease the current though.
Chase Wilson
>all kinds of theory around on this board >basically the low-hanging fruit of science >OR rainman levels of math that will never be used other than in a computer program
Came to the wrong place for applied science and math.
Chase Barnes
I haven't chosen an ESC yet
No no that would make it worse as I said before it's because there is so much voltage why the thing will spin so fast To stay under the speed of sound I would need no more than 50 V however to supply 8000 W (actually 9000 W to be safe) I would need 180 A which is double the maximum the page says that the motor can handle.
Cooper Flores
Well the motor will never take 180A if you run it at 50V. It will only take what it needs.
The page says that its max rated amperage is 90A, and its max voltage is 166V. The power at this voltage is 166*90=15000W, which is conveniently also its max rated power.
So putting this all together the motor will take about 27A at 50V, and will only put out 50*27=1350W of power.
Andrew Baker
So what I'm saying is, if you want to run your motor at 8000W, you need to find a motor with a lower KV, or you need to make some kind of transmission in between the motor and the fan to make it spin slower.
Nathan Gutierrez
>make some kind of transmission in between the motor and the fan to make it spin slower. Yes! Fuck Yes! Why was I so dumb? Why did I not think of this? It will hurt efficiency though. It's running at 8000 to 9000 W
Easton Mitchell
Let me email the guy, I think he can supply gearing. I can't be fucked to make it myself. I'm shit at it anyway, they never mesh good.
Oliver Powell
You are not entitled to an answer retard. Also, googling drag equation and doing some algebra is really nothing to do a fucking thread on.
Matthew Ortiz
Keep in mind that the KV value on the website is for when the motor is not under load. When you put a bigass fan on it the kv value drops about 30%, probably even more with such a high tip speed
Jaxson Rodriguez
>You are not entitled to an answer retard It's not about getting no replies, I make plenty threads all over Veeky Forums that get no replies and I don't care. It's making a legit science thread and seeing it getting ignored in favour of some racebait or "can schrodinger's equation send me back in time" that niggles me >googling drag equation and doing some algebra is really nothing to do a fucking thread on. Exactly what I am talking about. Someone posts a classical mechanics problem some arrogant undergrad student comes along and calls it "babby tier" despite him only managing 70% on his last mechanics exam. To cap it all off this same guy then jumps right into a General Relativity thread to act like an expert on things that he knows absolutely nothing about.
And FYI my problem is actually quite advanced I am just trying to simplify it as much as I can. As one of the few people to be helpful pointed out I incorrectly assumed a free stream and didn't take into account the Mach number.
Classical mechanics isn't just "here's the equation, plug n chug?" you have to think "is this equation valid?" "What other things must I account for?" "How many things can I account for until the model becomes uncomputable in reasonable time?" But of course for an intellect as great as yours airfoil analysis is beneath you, Only String Theory is worthy of your genius.
Anyway fuck that guy, thanks for your help I know, but it's the best estimate I can get without experiment.
