Prop thrust and pitch

More diameter and less pitch=same power more thrust, but lower top speed. Your choice.

| I have a new AXi 2212/34 motor swinging a 12/8 APC E prop. It gave a | static thrust of about 18 ounce (fish scale, not very accurate) for a | 14 ounce plane. | | Would a lower piched prop, same diameter, say a 12/6 give more static | thrust?

No, it will give less static thrust. It'll also use less power, and heat up your motor less.

Now, a 14/6 might give you more static thrust ... or it might give you less. It depends on a lot of variables. A 14/8 will definately give you more static thrust, but it could burn out your motor/gearbox/esc/battery if they can't handle the extra load.

Putting a bigger or higher pitch prop on an electric setup will just about always give you more static thrust. (But you do run the risk of burning things out.) Note that this is not true for IC engines -- they like to run at certain speeds, and once you get out of those speed bands, performance drops greatly.

| I knew a lower pitch prop gives more thrust for take off

You missed half the equation there ... lower pitch AND LARGER DIAMETER prop.

A larger diameter and lower pitch prop will generally give more static thrust and a lower top speed than a smaller diameter higher pitch prop.

| and once in flight the higher pitch is better for speed.

Higher pitch/smaller diameter ...

| Which is the better compromise? Use the lower pitch prop or the | higher pitch one all the time?

Depends on what you're doing. Doing 3D? You want a larger prop with a lower pitch. Pylon racing? You want a smaller prop with a higher pitch.

Get Motocalc. It's your friend.

If Wan has a 12x8, then assuming he wants to draw the same power, going to something like a 13x7 or a 13x6 would actually give him more static thrust at the expense of airspeed. Given the same available power, a larger, lower pitched prop pulls harder at low speeds than a smaller, higher pitched one. Doug is correct in saying that a lower pitched prop of the *same diameter* will give less thrust but will draw less current.

Wan, if you're serious about designing your own electric, a program like MotoCalc is almost indispensable.

Morris

Morris,

Not wanting to sound ignorant, so I seached Hobby Lobby and Tower Hobbies and could not find a MotoCalc.

What does it do? Could I learn how to calculate the battery power, propeller, motor requirements for a given airplane and weight?

Wan

Sorry, I missed the site given by "Flint Fredstone" Accordingly the site is very thorough in describing on what the MotoCalc does. It doesn't cost much for what may be done with it.

Thanks, Flint. I am a builder who designs "by the seat of my pants" and just make a few sketches and modify as I build when problems crop up.

The above statement by Morris has me a bit confused. I am pondering how a lower pitched prop would give less static thrust? I liked the part where he said "It'll use less power, and heat up your motor less".

Wan

| Thanks, Flint. I am a builder who designs "by the seat of my pants" | and just make a few sketches and modify as I build when problems crop | up.

Then you'll either love or hate Motocalc. You'll love it because it will help remove much of the trial and error. You might hate it for the same reason :)

| > > > > No, it will give less static thrust. It'll also use less power, and | > > > > heat up your motor less. | | The above statement by Morris has me a bit confused. I am pondering | how a lower pitched prop would give less static thrust?

That was me, not Morris.

You were talking about a 12x8 and a 12x6 prop if I remember correctly. I did expand on my statement right after, but I can do so again.

All else being equal, a larger prop takes more power to spin at a given speed than a smaller prop. And similarly, all else being equal, a higher pitch prop takes more power to spin than a smaller pitch prop. This is really common sense -- it's doing more work, so it needs more power.

However, when somebody puts a lower pitch prop on a plane because they want more static thrust, the prop they put on is typically larger than the old prop, and you didn't mention that. Going from a 10x8 to a

12x6 prop may very well give you more static thrust with similar power usage. (To be sure, you'd run it through Motocalc or test it on a plane.)

| I liked the part where he said "It'll use less power, and heat up | your motor less

Well, with less thrust being produced, using less power and generating less heat seems fair, doesn't it? :)

Sorry, Doug. The thread is some times confusing to me as to who is who (whom?). I ordered some 10x8 props, but the LHS made an error and got the wrong type, not for electric. I then purchased a couple of 12x6's.

I installed one (12x6) on my airplane and it produced roughly about 24 ounces of thrust for my 14 ounce plane. I used a fish scale. It should go ballistic.

Can't compare it to the 12x8 I was swinging though, as I put a freshly charged battery in and the 12x8 was run on a partially drained babttery.

Still, I believe any true comparision should be made in the air. I spoke with the tech person at Hobby Lobby about the size of props for the 2212/34 AXI and he said they tried them out first before market. He also marveled at the power produced by the tiny motors available.

See? Even the large companies do some testing. He said (I hope I have the facts straight), I should try a 12x6, then other sizes. I will consider the MotoCalc.

Thanks, Wan