Calculating thrust?

[lexi]

Ok first I need to confess that husband is a big dork, and like most men to embaressed to ask for help. So of course he has me ask for him.

How do you calculate the thrust a motor will get? Hes pretty sure it only depends on the prop and rpm's.

Personaly I dont get why it matters, but this is one these when I just say Ok Dear.
Thanks Lexi

[markdrela]

Ok first I need to confess that husband is a big dork, and like most men to embaressed to ask for help. So of course he has me ask for him.
How do you calculate the thrust a motor will get? Hes pretty sure it only depends on the prop and rpm's.

To calculate thrust, you must know as a minimum
* prop geometry
* airspeed

In addition, you must also know any ONE of the following sets of variables:
1) RPM
2) voltage, motor constants (Kv, Io, R)
3) current, motor constants
4) electrical power (voltage*current), motor efficiency
or maybe some other parameter combinations I can't think of right now.

You might try Motocalc or something similar, but these tools must make some crude assumption on the details of the propeller geometry, such as the blade chord distribution, blade airfoils, etc.

For the static thrust scase, one can also estimate the thrust from the power and prop diameter, using the formula for the power of a hovering helicopter:

T = [ (eta*P)^2 * 2 * pi *R^2 * rho ]^0.3333

T = thrust (in Newtons)
eta = prop hover efficiency = 0.7-0.8 is typical for unstalled low-pitch props
P = shaft power = voltage * current * motor efficiency (in Watts)
pi = 3.14159
R = prop radius (in meters)
rho = air density = 1.22 kg/m^3

Plugging in eta=1 will also give the absolute upper limit on the thrust obtainable for a given amount of power and a given prop radius.

[Downwind 1]

;) Here is two sites that have free download programs.that help me alot '
http://www.lcrcc.net/thrust_calc.htm
http://parkflyermotors.com/secure/shop/custom.asp?recid=7

[coosbaylumber]

I recently obtained a thrust calculator via the internet. One that can be downloaded and used a bit, then compare to information from a magazine, etc. It is called THRUSTHP. If used to be available onna web at a few places. It works within Windows 95 on up, but is simply a quick calculator, with an option to insert a few different propeller brands. Much like the initial one noted above, but home computer based, not internet based. And it will let you print out comparisions when finished.

Take a search for it, or I can e:mail it off to you.

Wm.

[hul]

find prop theory and a prop calculator here:
http://www.mh-aerotools.de/airfoils/
click on "propellers"

It'll let you calculate in flight thrust.

Hans

[adam_one]

You may also check out following:
http://adamone.rchomepage.com/calc_motor.htm
http://adamone.rchomepage.com/calc_thrust.htm

[lexi]

wow thanks guys, I wasnt expecting anywere near that many responces