I recieved one of your wonderful little MPS units for Christmas. Other than the switch to GWS props, is there any difference between the MPS-1 and the MPS-1A that would be noticable performance-wise?

From : Don Stackhouse

We'd been looking at some improvements for the MPS-1 for a while now. When we ran into increasingly serious problems with the reliability of the supply of the old white props we needed for the MPS-1, we finally decided to make the changes across the board. The new design gives us a reliable source for all major components, gives us a wider range of options we can offer our customer, and improves the reliability of the system, all without requiring a change in the price.

Performance wise the difference is very minor, essentially negligible. The two props are nearly identical in performance, with the new GWS 5-4.3 having a very faint edge if there is any difference at all. The gear mesh is perhaps a little better and a lot more consistent because of the improved tooling we developed for boring the gear hubs. We've also altered the fit in the propshaft bearing bores. Theoretically the mechanical efficiency and noise are better as a result, but that difference is very small.

However, the new prop mounting allows for using a modified GWS 6-5 or 7-6, and probably a few others. The new shaft is 1/8" longer, allowing room for a nut at the front of the prop, which is why we can now use these props where we couldn't on the MPS-1. This is also an advantage on pusher installations, where the elastic stop nut we use for a prop retaining nut provides a more reliable mounting.

The 6-5 will be very helpful for folks at higher altitudes, and also seems to be the propeller of choice on our new MPS-2A twin-motor unit. The MPS-2A will be available as an upgrade for existing Roadkill models, and as standard equipment on some of the new ones, such as our soon-to-be-released Ryan ST. Joe and his son Terry have also been flying MPS-2A's on a RK Series Corsair and an Me 109 with outstanding results.

The other major change in the MPS-1A and -2A is the motor frame. The MPS-1 gearbox used a front frame that seated on the bronze bushing at the front of the motor. While this is more directly related to the motor shaft and therefore theretically gives better control of the gear mesh, a few customers had trouble getting the bronze bushing to stay seated in its hole in the front frame. A dab of thick CA or epoxy between the motor can and the plywood mounting plate could lock the motor in place and eliminate the problem, but we still wanted a more fundamental fix. Joe came up with the idea of using identical frames at the front and back of the motor that would both grab the motor around the outside of the can, just like the rear frame on the old MPS-1. While this is theoretically less consistent than grabbing the front of the motor by the bronze bushing, in actual practice it seems to work just as well, and virtually eliminates the problem of the front of the motor slipping out of the frame and letting the gears pop out of mesh. The new system is solid and reliable, and easier to work with.

If you don't need the ability to change prop sizes on your existing MPS-1, I wouldn't worry about it. The Gunther props perform almost identically to the new GWS 5-4.3 props. If you tack the motor in place with a few drops of thick CA between the top of the motor can and the plywood plate, the reliability will be as good as the MPS-1A. We've discussed offering an upgrade kit for folks who want to convert an MPS-1 to an MPS-1A, but it requires replacing so much of the assembly that it just doesn't make sense to do so. For not much more you can buy a complete MPS-1A.

The MPS-1A is an improvement, but your MPS-1 is still a fine little powerplant that will reward you with excellent performance if you build and treat it with respect and common sense.

Don Stackhouse
DJ Aerotech