Super Tigre .20 & Super Tigre .30

This multi-engine set up runs as smooth and strong as I'd hoped.

ST .30

The trick to getting the multi-engine setup to run is to use a dedicated ESC per motor.  Interestingly, there isn't much additional cost to outfitting the airplane this way.  In fact, summing smaller ESCs can be less expensive, and you get multiple power sources to the receiver and servos.

With low-friction brushless systems, there is surprisingly little overhead when running only one motor in the group.  I think the jury is still out on the best battery configuration.  It works fine with multiple batteries or a single battery.

What apparently doesn't work great is to attach a battery on only one ESC, and leave the other(s) unpowered.  When one ESC is unpowered (= nothing attached to the battery connector) it's idle motor acts as a generator being spun by the other motors and it sends power back to the ESC from the motor side.  The unpowered ESC gets quite hot when other motors are running.  If a battery is attached, even if the ESC is not being controlled by the receiver and thus the motor is idle, the voltage from the attached battery apparently doesn't allow current to flow back into the ESC from the motor/generator, and it does not heat up.

ST .20

Three ST motors with fit nicely on a 100mm shaft.  The shaft diameter must be exactly 4mm.  Building the motor is pretty easy with a good strong vice: 

1. Place a spark plug socket (from a ratchet set) firmly in a vice, vertically oriented.  The top opening should be about an inch above the vice jaws.  The top should be the end with the smaller hole.
2. Pry off the tiny black c-clamp from the rear of the ST .10 shaft.  The second or third (or fourth) motor bell to be mounted on the longer shaft can perform the function once served by the C clamp: to keep the propeller from pulling the bell forward and out of the motor.
3. Grab the front of the shaft and pull the shaft/bell assembly forward and out of the coil/mount section.  The strong magnets will try to hold it in place.
4. Place the shaft/bell assembly on top of the spark plug socket, with the shaft protruding down the middle of the spark plug socket.  The spark plug socket should form complete circular support underneath the bell's cooling holes, around the shaft.
5. With the motor bell centered on top of the spark plug socket (it will be hard to center due to the magnets wanting to pull it to one side), and while holding it firmly in place, lightly tap on the top of the shaft with a hammer, so it slides down and out through the bell.  You will need a punch or a nail to tap out the final portion of the shaft through the bell.  The shaft will fall out.
6. With the bell still on the spark plug socket, tap the new, longer shaft down into the front of the bell as far as necessary.  If the bell is a bit loose on the shaft, tap the shaft all the way through to start over.  Use a medium pliers and (lightly) squeeze the rear circular opening of the bell's shaft collar (on the magnet side).  It is made of pliable metal, so be careful not to squeeze too hard and deform it too much.   Using any Dremel-type roughing tool to lightly roughen the shaft only where it will meet the bell, will help as well.  Now tap the longer shaft into place through the bell.
7. Slide the motor coil/mount assembly back onto the new motor shaft.  Make sure the bell spins without slightly wobbling, or gently align it with your hands by bending the steel slightly with your hands until it spins perfectly true.   One motor is done.
8. Repeat as many times as necessary.
9. If you do a reverse motor orientation (like the middle motor of my .30 photo), be sure to reverse the red and black wires for that motor when it hooks into its ESC.
10. Test for proper spin direction, one motor at a time.