Prop thrust and pitch

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Wan - 11 Jun 2004 09:15 GMT

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?

I knew a lower pitch prop gives more thrust for take off and once in
flight the higher pitch is better for speed. But we don't have
variable pitched props.

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

Plase give this your thoughts,
Wan

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The Natural Philosopher - 11 Jun 2004 12:44 GMT

> 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
[quoted text clipped - 12 lines]
> Plase give this your thoughts,
> Wan

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

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Doug McLaren - 11 Jun 2004 16:34 GMT

| 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.

Signature

Doug McLaren, dougmc@frenzy.com
The way some people find fault, you'd think there was some kind of reward.

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Morris Lee - 11 Jun 2004 19:05 GMT

> | 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.

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

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Wan - 12 Jun 2004 03:52 GMT

> > | Would a lower piched prop, same diameter, say a 12/6 give more static
> > | thrust?
[quoted text clipped - 13 lines]
>
> 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

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Flint Fredstone - 12 Jun 2004 05:23 GMT

http://www.motocalc.com/

> > > | Would a lower piched prop, same diameter, say a 12/6 give more static
> > > | thrust?
[quoted text clipped - 23 lines]
>
> Wan

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Wan - 12 Jun 2004 13:00 GMT

> http://www.motocalc.com/
>
[quoted text clipped - 28 lines]
> >
> > 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.

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

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

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Doug McLaren - 12 Jun 2004 17:33 GMT

| 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. <snip>
|
| 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? :)

Signature

Doug McLaren, dougmc@frenzy.com
"One in a million odds happen eight times a day in New York." -- Penn Jilette

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Wan - 12 Jun 2004 22:44 GMT

> | 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
[quoted text clipped - 33 lines]
> 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

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Model Forum / Radio Controlled / Air Models / June 2004

Prop thrust and pitch