fnp

How exactly does prop pitch affect things and how does one decide on the pitch. Thanks fnp

crashingagain

Prop pich affect forward speed, thrust, and amp draw. For example, lets say you are flying a plane with a 10x4.7 prop, it will fly slower than a 10x8 but will produce more thrust at the expense of drawing more amps. Generally prop choice will come down to plane type and flying style. If you want to fly slow while maintaining stability you chose a big diameter/ low pitch prop. Same thing with 3d birds where you really need thrust over those controll surfaces. Now if you want to race you might choose a smaller high pitch prop to keep amps down and to increase foward speed. That pitch number like 4.7 indicates how far forward the plane will travel in one prop revolution.

Real world example on my brushless formosa 10x4.7 gws slowfly prop for slower aerobatics and cruising around
8x7 apc-e prop for tearing up the skys

fnp

Following what you were saying i looked at the gws site and i am finding the opposite true. on the 350c-b motor with 8*4.3 prop on 8cells the amp draw is 8.9A and 15.06oz of thrust Increasing the prop pitch to 6 i.e a 8*6 prop increases the current to 10.9A and the thrusr is 15.4oz in the prop advice table on the same page they reccomend the8*43 for speed and the 8*6 for more thrust. I am very confused can you please clarify this.Thanks for your help

mohr_dave

My Feedback: (6)

FNP,
Here is the deal as I have come to understand it from boats, planes and the RC world.

Basics: Pitch: this is the equivalent distance the plane would travel forward in one revolution of the propeller (not the motor) if there were no losses due to friction or anything else. Not the real world;- )

A higher pitch will provide more thrust, in this case you will have more air going over the surfaces. The reasoning behind the GWS recommendations are that the lower pitch will allow the motor to spin at a higher rpm. So you will (in theory) be able to travel faster than running the motor more slowly with the higher pitch. But, with the higher pitch, you don't need to spin the motor so high to keep the plane in the air.

You can also increase the diameter without changing the pitch and get an equivalent pitch change increase of 1. For example, in theory, a 9x5 is about the same as an 8x6. Yes, it gets complicated quickly if you let it.


For the gas planes I run, it's pretty easy, the engines come with a recommended size and you start with that and go from there. With the electrics, I have read the manufacturers recommendations for the cells I am using and then used a tach, a small digital spring scale to see what the thrust is at max rpm, and a current meter in series to see what the current draw is. This will give you an idea of the most efficient and powerful setup for the different props. It will also give you the time you can fly for a given battery pack at wot.

Does this help or just make things more complicated? Hopefully the first. Try the rpm vs. thrust vs. current reading a couple of times and you will get the hang of it. There are also thumb rules out there for how much thrust you need for a given model in relation to the type of flying you do. If you want to go vertical, I believe you need 1.1 oz thrust for each oz of weight. (overcoming gravity)

Matt Kirsch

My Feedback: (21)

As you can probably tell by now, changing the pitch of the propeller can have many different, CONTRADICTING, effects... It's not cut-and-dried by any stretch of the imagination.

Normally, we change pitch at the same time we change diameter. Increase pitch, reduce diameter to keep the overall propeller load approximately the same. The effects are much easier to understand in this case if you work with both factors instead of pitch alone.

Large diameter, low pitch propellers are good for lots of thrust (acceleration), but not lots of speed. The opposite is true of small diameter, high pitch propellers.

Finding the right propeller is more of an art than a science, really. You need to find the propeller that makes the plane fly the way you want it to fly, without overloading the motor.

fnp

I guess I am going to make a test setup hopefully that will increase my understanding further.

Thanks a lot.

fnp

mohr_dave

My Feedback: (6)

fnp,
You don't necessarily need to go through and make an elaborate test stand. Once you have your motor and battery and a couple of props on-hand, you can make an intermediate "harness" for the current meter, with all of your electronics still in your plane, and put the scale on the tail of your plane, with the other end secured to something heavier, or solid. As you put the throttle up, you will see the scale deflect and give you your thrust, look at the meter and check your current, and then put the tach in front (or behind) your prop and verify rpm. Just remember that you need a "good" light source for the tach, not flourescent lighting. A good flashlight, light bulb, or natural light if you can.

Good luck!

Dave

Campy

My Feedback: (1)

Download MotoCalc http://www.motocalc.com/

You can try it free for 30 days. You can play "what if" with the program and it will tell you what you need to know.