There are essentially two styles of park jets when it comes to how the motor is located. The most popular seems to be the mid mount "prop in slot" style and coming in second, the tail mount where the motor is mounted at the back with no hole in the middle of the fuselage. Pictures below show the two types.
More difficult to balance the plane with the motor weight right near the very back. From most build logs I have read, it also requires some extension wires between the motor and ESC in order to get enough weight forward to balance the motor location. This adds a bit of complexity and weight to the build. Performance wise, there is no plane structure blocking airflow coming off the prop, but the entire length of the plane is blocking air getting to the prop. Additionally, you won't have any moving control surfaces in the prop wash, so no advantages of any "thrust vectoring" you may experience with mid mount planes. The longitudinal moment ends up being quite long in order to counter balance the motor. I have read build logs where folks have reported almost 24" difference between the front of the battery and the motor which puts a lot of weight distributed over a very long portion of the longitudinal axis, making pitch maneuvers slower. Greater weight overall, with more 3D structure the whole length of the plane and no hole in the middle, the amount of foam is greater as is the weight.
More scale look, no big hole in the middle of the plane, allows for more 3D structure over the length of the plane. I have read and watched video that performance may also be a bit more scale without the thrust vectoring. As mentioned above, no obstructions to the airflow from the prop.
Mid mount "prop in slot"
Non scale look, big hole in the middle of the plane and really no 3D structure rear of the prop slot. Airflow to and from the prop are affected by airframe structure. Prop size may also be prohibited more than with tail mount planes due side structure of the fuselage.
Normally, at least one or two sets of control surfaces (elevons and rudders) in the prop wash, allowing for quicker maneuvering and more advanced aerobatics at slower speeds. Easier to place the electronics and battery closer to the motor and CG to get better balance, shorter longitudinal moment of inertia, makes for much more effortless and efficient movement in all three axes.
Since I have all my park jet experience with mid mount "prop in slot" planes, I will focus on this primarily, but as I discuss behaviors with respect to planes with motors too far back, you hopefully will see how those relate to tail mounted planes and what behaviors which might be present affecting the plane's performance.
As Stephan and I started modifying the RC Powers Mig-29 V3/V4 working unknowingly at the time towards the first Mig-35A, one of the major things we started playing with was moving the motor forward to shorten the moment of inertia by being able to concentrate the CG (center of gravity), CM (center of mass, primarily the battery location) and CT (center of thrust or motor mount location). Not that I am trying to shamelessly plug the Mig-35, but if you are interested, you can read our original background document here.
What we started to find along with other modifications we were making was that the plane felt much better balanced, moved more quickly and effortless in all three axes with the CG, CM, CT so centrally located and condensed in a smaller area of the plane. This meant much smaller control surface deflections to get the plane to move meaning more precise flying, reduced turbulence with small deflections and better overall stability. With other planes I have tinkered with since then, we started to see some numbers come forward that translated to excellent performance no matter the air frame.
We started to discuss the motor mount location being located at a percentage of the overall length of the plane. We discuss the wooden motor mount location as the datum point rather than the prop location as different motors and shaft sizes would make for inconsistent measurements between us. I took this picture from the planview of the RCP Mig-29 V1 to hopefully demonstrate what I mean.
Also to note on the diagram is the large black dot, of which the very center is the CG point. The rectangle highlighted in orange is where a 2000-2200 mah battery would normally be placed to get best balance. From the front of the battery to the motor mount, it covers only 20% of the length of the plane and is all very centrally located and condensed in the longitudinal axis of the plane.
So based on some testing I did with my modified RC Powers F-18 V3 (this link will take you to my detailed test summary), the RC Powers Su-30 V4, Parkflyers International Su-35, RC Powers Su-27 V5 and F-22 V5, I started to see a correlation between the motor location and the plane's overall handling and performance. I took several measurements and found that the best motor location is between about 55-65% of the plane's overall length. It is very difficult to get much further forward than that due to plane structure and still being able to place battery, receiver, ESC to balance the plane properly.
Much further back than that and I started to notice that my moment of inertia became much longer to balance the plane which slowed the pitch performance down slightly and made the nose feel a bit heavier when maneuvering. What happens as this moment of inertia between the very front of the battery and the back of the motor gets longer is that it is like trying to balance a yard stick on your finger instead of a 12" ruler. As the nose starts to move around, it can get a bit sloppy in pitch maneuvers like loops where the nose will wander a bit more than it will with a plane a tighter moment of inertia.
I have flown planes where the motor has been 70% or further back from the nose of the plane which required the battery to be sometimes 12-18" ahead of the motor mount in order to balance this difference. This very long moment of inertia made the plane feel sluggish in the pitch and when pulled up in the vertical as it slowed down, the nose would get sloppy and wander around on it's own, bringing on stall quicker than normal.
So, after all that scientific stuff, I know that sometimes without having access to the plans you may not be able to tell where the motor is as a percentage of the length of the plane, but it is certainly something to consider. Too far back, the moment of inertia gets longer, the plane can be more of a challenge to balance and maneuverability suffers. It is certainly something to consider if you have already built the plane and want to tighten up it's balance and maneuverability by modifying the motor location on a second build. Be aware that as you move the motor forward (which on most designs is what you will need to do in my experience), you will also be shifting the CG forward somewhat, so be prepared to balance it at a point forward of the stock CG. From my experience with the Mig-35 and the modified F-18 V3, if you move the motor forward by 1" lets say, be prepared to move the CG forward about 1/2", at least until you get the maiden out of the way and can really start dialing in your new plane.
In the next article in this series, I will discuss weight distribution, much of which I have touched on already in a couple of articles, but I will focus more on why it is so important and how you can perhaps adjust your builds to optimize your plane's performance by where you place your electronics other than the motor and battery.