Installing The QCU Prop Drive
By: George Hurt

The intent of this article is to assist you in the proper installation and adjustment of your Challenger prop drive. BTW, this unit is more properly identified as a PSRU, a Propeller Speed Reduction Unit. Illustration 2 will show the proper assembly sequence.

Illustration #1

Since our engines operate mainly between 5000 and 6000 RPM, and a prop begins to lose efficiency above about 2800 RPM, we must have some method of slowing the prop down. That is the main function of the prop drive but it does a couple more things for us too. It places the prop hub far enough above the aft fuselage assembly to allow us to turn a 60" prop. Also, by being a cog belt drive, it softens combustion impulses which reduces loads on the crankshaft and drive components.

To begin the installation, be sure you have all the components on hand (fig. 1) along with the necessary tools and loctite (available at any good auto parts store). Begin by installing the drive tower to the engine (fig. 2) using the 8 (6 for Hirth) provided bolts and washers. QCU directions call for using Red Loctite on these bolts but since they bolt into an aluminum casting, I prefer Blue Loctite to protect the casting threads on disassembly. Be sure that the mating surfaces are clean and dry. Torque each of these bolts (fig. 3) down to 10 FT-pounds and then to 15 FT-pounds in a criss-cross fashion.

Figure 1

Figure 2

Figure 3

The main drive pulley that installs on the end of the crankshaft may or may not have a starter ring gear bolted to it depending on how the drive was ordered. This pulley has two face plates bolted to it using one length of bolts. If the starter ring gear is to be mounted to this pulley then a longer set of bolts must be used (fig. 4). If you have the pulley and ring gear separated, the installation instructions for the starter will detail how to mount the ring gear to the pulley.

Figure 4

Next, lock the crankshaft by inserting the locking pin (included in the Rotax Tool Kit) into the fuel pump pulse line fitting (fig. 5) and rotating the crank until the pin falls into its hole and locks the crank from moving. (NOTE: If you don't have the Rotax tool, you could use a steel dowel or rod, but it would need to be a very hard steel. If that rod ever got bent inside there, it may be extremely hard to remove. I have never seen one of the Rotax pins bend. Since it is almost surely a metric size one would have to be careful of using anything smaller than what is provided in the tool kit because of that possibility of bending. A a steel concrete form builders nail--the ones with a collar about about 3/8" below the head--should fit perfectly.) Then install the drive pulley onto the crankshaft end. This pulley will be installed with or without the starter ring gear depending whether or not you are using the QCU starter. The ring gear cannot be installed with the pulley installed on the crankshaft. Be sure both surfaces are clean and dry, do not use any kind of loctite or anything else on these mating surfaces, just clean and dry. Then install the 1/2" X 20 bolt using the thick washer and lock washer provided. Use Blue Loctite on this bolt and torque it to 50 FT-pounds.

Figure 5

To begin the assembly of the top pulley and adjuster, begin by lowering the adjuster bracket into the slots in top of the tower and then inserting the top pulley shaft into the adjuster and tower and installing the big flat washer and nut, this assembly is shown in fig. 7. At this point, tighten the big top shaft nut good and snug. Then check pulley alignment with a straightedge between the pulleys (fig. 6).

Figure 6

Figure 7

Begin belt installation and adjustment by installing the belt onto the pulleys. The top pulley may need to be lowered by using the adjuster bolt on top of the adjuster. Once the belt is installed, slowly raise the top pulley until most of the slack is gone from the belt. This operation is done somewhat by 'feel'.

To develop this sense of feel for belt tension, there are some things that you need to know. First is the difference between cog belts and V belts. V belts pull by the sides of the belt being squeezed by the tapering sides of a pulley. To do this, they must be run very tight. Cog belts pull by cogs on the belt and grooves in the pulleys much like a chain and sprocket on a bike. The cog belt on the QCU drive must have a small amount of slack once the cog belt and drive achieve operating tempature. The final check for proper belt tension must be done with the engine and drive at operating temps because the pulleys and drive tower are made of aluminum and have a tendency to 'grow' as they warm up.

Once you have the belt installed, you will want slack in the belt. As you raise the top pulley by screwing down the adjuster bolt, the big nut will have to be loose enough to allow the assembly to slide but when you begin to lock it down by the final tightening of the big nut, often the shaft will finally become absolutely straight with the tower and the belt tension will tighten up. You may have to adjust it several times to get it correct.

Once you have the belt tension adjusted, the big nut must be tightened. Factory specs call for 175 FT. pounds of torque but I always go far beyond that to prevent the shaft from turning. I use two large wrenches and normally with short 'cheater' tubes to get the torque high enough. You cannot simply torque the big nut, you must also hold the shaft stationary by having a backup wrench on the square end of the shaft. This article also shows another method. Illustration 1 shows one method of checking belt tension.

The cog belt will run with a wide range of slack in it but if you run it snug like a V belt, it will overload the bearings on the output end of the crankshaft and the prop drive bearings in the top pulley. Early bearing failure WILL result.

When tightening the big top nut, you must align one of the castles on the nut with a hole in the shaft to install a cotter safety pin. If you find that you cannot turn the nut any further to reach one of the holes, do not back the nut off to reach one. Instead, remove the nut and big washer and replace the big washer with one from another source. Usually, there will be slight differences in thickness of the washer allowing the big nut to tighten down in a different hole position.

Once you have the belt tension adjusted to where you think that it will have just a slight bit of slack at operating temps and the big nut is tight and has the cotter pin installed, you are ready to finish up the installation. First, back off the adjuster bolt a few threads, put some blue loctite on it and screw it back down just snug against the top of the tower. Then be sure the nut goes down against the tower with a bit of loctite on it. Hold the top of the adjuster bolt and snug the lock nut down good. Wrap safety wire around the top of the adjuster bolt just above the locknut, bring it down and wrap around the bottom of the bolt and finally to the loop end of the cotter pin (fig. 7). This safety wiring is not to keep anything from turning, it is there to keep pieces from going through your prop should the bolt break . Using a marker pen or something similar, put a reference mark on the big nut, across the big washer and onto the tower (fig. 7). If the big nut is not tight enough, over time the shaft assembly will begin to rotate slightly and you should make checking this reference mark a part of your preflight inspection.

The last step is to fly the airplane and get the engine and drive up to full operating temps. On landing, stop and immediately check the belt tension. There should be a small amount of slack in the belt. If the belt is snug with no slack, the adjustment will need to be reset. Belt tension should always be adjusted by raising, not lowering the top shaft so you will have to back off the adjuster and begin by raising it back up to a slightly lower position. If you only just lower the adjuster, often it will not stay well.

You may have to adjust the belt tension a few times over the first few hours to finally get it just right. With proper adjustment, these new belts and drives will last many hundreds of hours of good service.

Illustration #2