I don't see many people gearing outrunners these days, though I see a fair number of setups like this from a few years ago.

I want to convert my saito 72 3d model (laser 3d) to electric. It is currently 4.7 lb AUW. Looks like I can get 750W easily out of the AXI 4120, but the pitch speed is much higher than I need with that motor, so I am wondering if I could find a lighter, more efficient setup with a smaller outrunner geared 2:1 for example.

I am planning on using an A123 pack.

5 lbs plane and the 4120 should be ok. Not great performance but should be good. What do you mean that pith speed is much higher? If you wanna drop the pitch then increase the diameter to keep the amps up. Care to clarify?

For any given prop diameter, using a gearbox and upping the pitch to make the current draw the same can be more efficient since the prop speed is lower so you get less frictional losses.

I am fairly certain the 4120 would be tonnes and tonnes of power for a 5lb plane. There are people that fly 10 lb trainer like planes on that motor and say they have tonnes of power.

mullen - its all about getting the right combination for the purpose. I just got my Kyosho AgWagon up ... only on 3S with a 2826/12 turning a 14x8. Not much power but more than enough for scale flying. I only got like 350W max. But she looks great in the sky.

If you're interested in geared outrunners, see the Cyclon Elite series. They have a built in planetary gearbox. Very light and powerful, and they swing huge props. Only downside is the gearbox maintenance. See Horizonhobby.com/Atlantahobby.com they sell them now.

I would just say this: Cyclon, as 3DownLow mentioned is the obvious choice. You will not believe the difference between an outrunner like the AXI and the way the Cyclons deliver their watts People who have not tried should not try to talk you away from it... But I guarantee you will be amazed and pleased.

If you pick the Elite 18, and power it with 6S - 7S A123 it will nearly rip your plane apart
My first meeting with Elite`s lead to investing in more Cyclon!
But it can`t be "backmounted"... And large diameter propeller is needed to get all there is.
Gearbox maintenance: if the gear isn`t dry after the two first checks and relubs the maintenance intervals can be prolonged.

I felt the need to use a "spider mount" to support the rear of the gearbox. If you plan to 3D this motor with a large prop ,the front mount or a cage mount with the rear spider mount would be recommended. The torque imparted from a large 3D prop and the stressful gyrations we sometimes put our planes thru(who me?) can do bad things to a front (spider) mount. Just a word of caution for those hardcore 3D folks who want to try these motors. My 2 cents.

Good advice 3DownLow.
In addition set the ESC for soft-start if the plane is not strong enough. The torque when throttling is huge, and might destroy lausy constructions.
A friend of mine who use the Elite18 in a motorglider have set soft-brake for 2 seconds before 100% brake is applied. This is to avoid destroying the plane due to excessive torque

It is a pity so few people have discovered the nice power from those motors...

As far as I know, you shouldn't use brake function on gearboxes at all. This may cause binding. Don't want that! I turn off the brake function on all my geared birds. Outrunners are a different story. But the Cyclone Elite series uses a gearbox so I disabled the brake on my Cyclon setup too. My 2 cents

I do not see the need for a gearbox when using an out-runner engine. The great thing about outrunners are that they can be made to have a fairly low RPM/Volt, e.g., 515 RPM/V for the AXI 4120/18. That should be low enough for most applications. If you need lower RPMS it is better to use an in-runner engine with a planetary gear box.

On my particular application I needed the lightest, most powerful setup and for the weight/power the Cyclon worked. I also use Neu inrunners too, but they come out slightly heavier. The only bad thing about most outrunners is their weight. Also when making large 3D foamies, gearing an outrunner will get you the right prop size while keeping the cell count down and the cost as well.