In a copter, Is there a formula for how much lift is generated by a prop, or are there normally stats for how much upwards thrust is generated for a prop at a certain RPM range? or is this just trial and error.

Now that I understand motors (or at least I think I do), I know nothing about props which go onto those motors?

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Try this for all things about model aircraft airfoils http://www.mh-aerotools.de/airfoils/ including a section about propellors.

Peter

thanks, I'll see if any of this makes sense.

although, having props that are too big never really hurt (all I have to do is add more weight to balance the thrust), I think...

Martin himself admits that his otherwise excellent JavaProp program uses a mathematical model that begins to break down at low advance ratios.  I recommend the site myself as an excellent primer for propeller aerodynamics, but sometimes, static thrust is something for which there's no substitute to actual testing.

At the grave risk of self-adulation, I recommend this thread, which covers the basics pretty well:

DIYD Thread on Lifting Propeller Aerodynamics

The raw data on most of the common hobby props can be found here:

http://www.ae.illinois.edu/m-selig/props/propDB.html

wow... I have a lot to learn.

thanks a lot.

P.S. is there any problem with blades that are too large? larger blades at slower speeds have the same lifting capacity, and if necessary, they can lift heavy loads

Theoretically you get more efficeint with higher aspect ratio.  Most thrust data backs that up, but like Brad said, statically operation isn't always as straightforward as props with an velocity.  All of the props we now use, are designed for airplanes which operate at a velocity.

so...

there is a difference between static props and those with velocity?

care to explain more?

Yes, there is much about helicopter aerodynamics that is not intuitive, even for someone well-versed in classical mechanics.  If fact, that question you just asked has all sorts of permutations.

For a fixed-pitch multicopter, rotational inertia is undesirable for control response reasons.  However, if the blade sections can't be driven to Reynolds numbers >>100K (via high RPM if the chord is short), then the resulting drag coefficient (Cd) will send your figure of merit (FM or relative efficiency) into the dirt.  So, it depends on what "too large" means in context.

I see an entrepreneurial opportunity for someone to make a line of light carbon-fiber lifting props in the 11" to 24" range with low solidity ratio and more optimal taper.  See Paul Pounds' paper, which is a great intro to the subject.

Attachments:

Virtually all model airplane propellers are designed to move forward at some velocity.  There is an important parameter in propeller aerodynamics known as advance ratio, which is the axial velocity divided by the tip speed or V/nD.  When there is no axial velocity, the most common propeller performance formulas don't work, hence the "actuator disk" analysis used for helicopters.  Generally speaking, a blade designed for static thrust would have a lower pitch and an airfoil section with a thicker, more rounded leading edge for higher angle of attack tolerances.

A methodology called "blade element analysis" looks at a section of a propeller and views it properly as a section of a wing.  All the standard parameters apply, such as Cl, Cd, angle of attack, etc.  Look at Martin Hepperle's site and play around with both JavaProp and JavaFoil if you want to get a feel for how these things all interrelate.

http://www.mh-aerotools.de/airfoils/index.htm

Take a look at eCalc:

http://www.ecalc.ch/xcoptercalc_e.htm?ecalc

While it does not show you its formulas, it will collect lift for a given setup.

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