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Posted: 10/14/2004 5:30:16 AM EST
There seem to be several factor in determining this -- the diameter of the rotor, the number of blades, the rpms, the pitch. Does anyone know the forumla to calculate the thrust or lift of any given propeller?
Link Posted: 10/14/2004 5:30:47 AM EST
divide by 3.14159265
Link Posted: 10/14/2004 5:32:01 AM EST
mmmmmmm pie
Link Posted: 10/14/2004 5:32:53 AM EST

Originally Posted By Cleatus:



divide by 3.14159265




Wow. Could you BE any more wrong?
Link Posted: 10/14/2004 5:36:40 AM EST
[Last Edit: 10/14/2004 5:39:51 AM EST by CAR-10]
I've got a book at home called Aircraft Accident Investigation by Richard H. Wood & Robert Sweginnis. It has a similar formula. Even though I've used it plenty of times, I don't know it off hand.

Why do you need this formula.
Link Posted: 10/14/2004 5:42:04 AM EST
Doing a fluid dynamics problem?

Yes there are many variables. No, I don't have the formulas I developed for boat propeller thrust with me.
Link Posted: 10/14/2004 5:54:11 AM EST
Link Posted: 10/14/2004 6:06:20 AM EST
With enough thrust, I get to see this:



Without enough thrust, I don't get to see that. That's about all I know.

vmax84
Link Posted: 10/14/2004 6:16:20 AM EST
Link Posted: 10/14/2004 6:17:35 AM EST
Link Posted: 10/14/2004 6:18:11 AM EST




Originally Posted By fight4yourrights:

Originally Posted By Cleatus:



divide by 3.14159265




Wow. Could you BE any more wrong?




oh yea!!!! thanks ...its 2 pie r squared!!!!
Link Posted: 10/14/2004 6:19:29 AM EST
[Last Edit: 10/14/2004 6:21:50 AM EST by Gunner1X]

Link Posted: 10/14/2004 6:20:40 AM EST
Link Posted: 10/14/2004 6:24:13 AM EST

Originally Posted By Hullabaloo:
There seem to be several factor in determining this -- the diameter of the rotor, the number of blades, the rpms, the pitch. Does anyone know the forumla to calculate the thrust or lift of any given propeller?



Does anyone have the feeling that
college has started...we have gotten
a TON of homework-type questions.

I don't have a problem with it
because it reminds me why I am
glad that I never have to go back
to school again.


Link Posted: 10/14/2004 6:26:59 AM EST

Originally Posted By DoubleFeed:
To give you an idea of just how complex this might become, here are some formulae.
Force = mass x acceleration
Mass flow: F = (mass flow in mass units per second) x (Velocity 2 - Velocity 1)

There are more, but they are getting really long.





Is that in anyway similar to the equations used to calculate the Thrust/Sqeeze ratio?
Link Posted: 10/14/2004 7:01:11 AM EST
Link Posted: 10/14/2004 10:11:06 AM EST
It's not for school, but I'm building a custom r/c helicopter from scratch. Not a conventional helicopter with a tail rotor and such, but it will have two main props rotating in opposite directions.

One site I found that gives a formula for calculating thrust is here.

Instead of crunching numbers, it may just be easier to slap a big engine on it and hope it flies.
Link Posted: 10/14/2004 10:22:00 AM EST
The calculation is very complicated. Thrust is not evenly distributed across the rotor disk. It is affected by chosen airfoil, angle of attack, rotor speed, rotor diameter, tip speed, etc.

Thrust is static conditionwill be different than thrust during climb or forwardflight.

As your airfoil increases in span, efficiency goes up, smaller (ie rc models) suffer from very small airfoils, scale sized models do not exhibit direct scale performance of the big boys.

If you want it to fly the first time, stick a big engine on it.

Link Posted: 10/14/2004 10:50:14 AM EST
Yeah, it seemed real complicated once I started to research it.

I basically just wanted to try calculate what kind of payload this thing would be able to handle, and what size prop would be best.
Link Posted: 10/14/2004 10:52:57 AM EST
[Last Edit: 10/14/2004 10:54:37 AM EST by AeroE]
The equation from momentum theory is adequate for your needs. Then follow this rule: "Make the rotor blades as long as possible, as long as possible." Just make the rotors and powerplant installation similar to other models - don't make it a science project, just build the machine and experiment.

If I remember tonight, I will look up an equation to use for computing the static thrust - momentum theory does not work for this.
Link Posted: 10/14/2004 11:02:00 AM EST
"Instead of crunching numbers, it may just be easier to slap a big engine on it and hope it flies."

This is outstanding thinking.........worked for this plane, except it took two engines........you may two as well.



Hope this helps.

vmax84

Link Posted: 10/14/2004 11:02:32 AM EST
The Bootstrap Approach to Aircraft Performance
(Part One — Fixed-Pitch Propeller Airplanes)
(Part Two — Constant-Speed Propeller Airplanes)

www.avweb.com/news/airman/182410-1.html
www.avweb.com/news/airman/182418-1.html
Link Posted: 10/14/2004 11:15:53 AM EST
Go with the "big engine" theory........it's tried, trusted, and it WORKS!!!!



vmax84
Link Posted: 10/14/2004 11:18:58 AM EST
Ooops, forgot this thread was about props, not jets. Sorry.

vmax84

I guess I just love the old Phantoms.
Link Posted: 10/15/2004 4:00:36 PM EST
[Last Edit: 10/15/2004 4:07:44 PM EST by AeroE]
Here are some equations I promised-

Static Power Required for the static thrust from a rotor disk:
P = M(Lift^1.5)/[2(air density)(rotor area)]^0.5

M is the figure of merit and varies from 1/0.70 to 1/0.80 (1.43 to 1.25)

To calculate the velocity of the wake behind the rotor:

Ve = 2(P/M)/(lift)


Will these work for models? Don't know, but there could well be (i.e., prolly are) scale effects that will bias the first equation.

Post pics, but don't be pokin' nothin'.
Link Posted: 10/15/2004 4:02:47 PM EST

Originally Posted By DoubleFeed:

Originally Posted By fight4yourrights:

Originally Posted By Cleatus:



divide by 3.14159265




Wow. Could you BE any more wrong?

He has pie on the brain, which incidentally, a spinning propeller could resemble pie, with enough imagination. Just don't try licking it.



Or sticking your dick in it.
Link Posted: 10/15/2004 4:03:38 PM EST

Originally Posted By vmax84:
"Instead of crunching numbers, it may just be easier to slap a big engine on it and hope it flies."

This is outstanding thinking.........worked for this plane, except it took two engines........you may two as well.

img82.exs.cx/img82/9053/phantom.jpg

Hope this helps.

vmax84




Is that a Belgian RF4?
Link Posted: 10/15/2004 4:07:41 PM EST
there is a text called "fundamentals of flight" it has a military TM number but it is available commercially. read it...learn


not only blade area, chord, angle of attack etc etc one must consider factors that degrade lift such as retreating blade stall, disymetry of lift and P-factor.
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