If I know the prop info, for example a 5X4, and the engine HP and RPM, is there a quick way to calculate prop thrust? I don't want to do Blade Element Theory or anthing fancy, just a quick guesstimation.

There are many softwares that calculate it. Here is one link where you can donwload it free. Use the zip one.
Scoubidou

http://freespace.virgin.net/barry.ho...e/goodies.html

Auroguy,

If you want to do the calculations some formula are

Thrust= 2.83e-12 * rpm^2 * Diam^4 (this is a dementional formula you need to use inches for prop Diam

EX: 5x4 prop 12000 rpm

Thrust=2.83e-12* 12000^2 * 5^4
Thrust= .255 pounds

The none dementional formula is

Thrust= Ct * q * RPS^2 * Diam^4

Ct= .0888 is what the thrust programs are using it ranges from about .06 for a low pitch prop to about .12 for higher pitch props.
q=.002378 for sea level
rps= revolutions per sec
Diam= feet

Thrust= .11 * .002378 * 200^2 *(5/12)^4
Thrust=.11 * .002378 * 40000 * .0301
Thrust= .315 pounds

Thank you both. I will use both of these.

DipStick -- I'm a senior in aerospace engineering and should know this, but thanks for educating me. Some days I learn a lot more here then in class... for a lot less tuition!

Aeroguy,

I envy you. Have you ever checked out the old reports in the NASA libarary?
http://naca.larc.nasa.gov/

Some of my favorite articles are reports 684, 378, 447.

http://naca.larc.nasa.gov/reports/1940/naca-report-684/ http://naca.larc.nasa.gov/reports/1932/naca-report-378/ http://naca.larc.nasa.gov/reports/1934/naca-report-447/

To calculate the Coeff of thrust I do the following. Calculate the approx .75 percent blade angle. I take the ratio of the

Blade angle=pitch/diam*20+2 (this is approx)

You can also do the blade angle=arctan(pitch/2*pi*(.75r)
r=radius pi=3.14


Coeff of thrust= blade angle*.0025 + .05

In the example
blade angle= 4/5*20+2
blade angle=18 degrees

Coeff of thrust= 18*.0025 + .05
Coeff of thrust=.045+.05
coeff of thrust=.095
If you take .095/.0888 (.0888 is what the program use for the thrust coeff) you can change the cf in the program to 1.067.

The only other trick I use is the tip speed correction, for every 10th of mach number from Mach .4 I correct by .025 percent when the blade angle is under 17 degrees. Here the blade angle is over 17 degrees so I would stay with .095 but to show you what I mean I well do an example.

tips speed on the example would be
5/12*3.14*200=262 fps or about mach .24 (262/109)
so .24-.4=-.16
and -.16*10*.025=-.04 percent reduction

.095*.96=.0912 would be the coeff of thrust if the blade angle was less than 17 degrees

Anyway that's about everything I know on how to calculate thrust. I know compared to what your doing this is pretty simplistic but it does seem to give pretty good results.

Like I said I just wanted a quick and easy way, so this is great. I'm really just trying to estimate if an engine can hover itself and how much more weight I can add before it can't. My senior design class is making a V/TOL MAV (Vertical Take-Off and Landing Miniature Aerial Vehicle). It's supposed to be around 100 grams (3.5 oz), so I'm looking at small throttleable gas/glow engines as well as electric motors and seeing if they will do the trick.

FYI, my major is a lot of hard work but also fun, and I'm graduating in Dec. Now all I have to do is find a job!

Aeroguy,

The reason that I do all the calculations for Ct is that the Thrust programs available do not account for the Blade Angle of attack very well unless the pitch/diam ratio is around .6 to .7. Most of the time the values are over by 10 to 30 percent. Something like a 12x4 prop will give way over inflated thrust calculation using the program. I guess to keep it very down and dirty you could just use pitch/diam relationship to adjust the Cf in the program.

EX. 4/12=.33 - .65= -.32
if you divided this by 2 (-.32/2=-.16)

The program also does not take into account body and wing interferance so I always take .10 percent away for this. So on the 12x4 prop I would reduce the cf by .26 or cf=.74.

About the smallest glow engine I've run is a .049 with a 6x3 prop if I remeber right 10,000 rpm probably about 5 oz of thrust. By the time you add the prop and fuel I think this was around 2.5 oz to 3 oz so probably to big for your application.

Good luck on the project and good luck on finding a job. I have alway hated job hunting!

Aeroguy,

Buddy if all you are looking for is to calculate Static thrust then why not put a spring behind the tail wheel.

I've been using it for years to test different prop efficiencies. A thrust to weight ratio of about 1.5:1 is a minimum for 3D aircraft, however 2:1 would be the ideal.

I am most likely to generate a lot of abuse regarding my simple method, but it has always worked for me.

Good suggestion but I am in the design phase and therefore don't have any engines or parts, much less a whole model!

Hi,
Quick and dirty prop thrust? How about adding engine thrust?
There is a very simplistic way for comparative testing and ball park
force amounts.
Tie a string on your airplane tail wheel. Add to that a fish scale and anchor
the scale.
Run the engine and note scale readings.
Viola! Theust amounts for both prop and engine.
Good luck!

Hal

Physics dudes,

Could you not just calculate mass airflow and convert that to thrust?

Schmleff

I don't believe what I have just read. Calculating engine thrust is just that, a calculation. It depends on all parameters being ideal in order to get close to a believeable number. If you're trying to calculate the quantity of water a cone shaped beaker will hold, forget all the bullsh*t formulas and just pour it full of water and then pour that into a graduated beaker. Don't make something impossibly difficult that is excruciatingly simple.

Start the danged engine, hook the plane to a $20.00 digital scale that you purchased from Wal Mart, which is anchored to something immovable, rev the engine, let it stabilize, read the scale. If you don't get the thrust you want, change the engine out to a larger one until you arrive at the magic numbers you desire. I'm sure there are many guys out there who would let you borrow an engine to test your thrust with.

That's the problem, I don't know anyone in Iowa who has an engine I'm interested in. In fact, if I had an engine, we have a test stand in the wind tunnel that would calculate the thrust precisely in 20 minutes.

If I were interested in finding out how much thrust a particular engine would generate I would look all through RCU and see what size engines produce what thrust at what altitude and with what propeller and then I would go BUY the one that fits my needs best.

Remember, thrust is a subjective parameter affected by propeller type, make, size and other considerations I can't think of, engine type, make , size, fuel, 2 cycle, 4 cycle, gas, glo, etc., etc., etc, altitude, moisture in the atmosphere, temperature and several other things I also can't think of right now.

To calculate a thrust would, at best, only be a temporary solution which would change by the next day. Ok, so you say you want a ball park figure, Look here in RCU and you can get all the ball park figures you want without spending any money or time or wearing out your calculator or computer. These guys have made a lifes ambition out of recording engine thrust numbers so dummies like me don't have to worry about it.

fish scale.