New Main Rotor Gear Box
| For the JetExec main rotor gear box, KISS Aviation chose a Speedway Engineering Mini-Stock Sprint Car Quick Change rear end. This Speedway rear end uses a custom (Dana 30 dimension) 41:10 ring and pinion plus a set of 24:23 quick-change (QC) spur gears, for an overall 3.93 reduction ratio. I knew when I first considered the KISS kit that I wanted to eliminate the QC gears and housing. With my seat in the recommended position, the QC gear box would extend through the seat back into the cabin 2" or more. It would be too close to my head (a safety and ergonomics concern), spur gears are notoriously noisy, especially right next to your ear, and the housing interfered with the forward cyclic cable routing and also made it more difficult to install a decent firewall and sound dampening. I really wanted a commercially proven, off-the-shelf replacement for the QC rear end, but nothing was available. So I found a 3.91 ratio (43:11) DANA 30 ring and pinion manufactured by Precision Gear, then designed a new MRGB housing to replace the Speedway drive. | |
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Here is a comparison of my MRGB with the standard KISS/ Speedway QC drive. Upper right is the QC spur gear housing that, on the standard JetExec, projects forward through the seat back. This new MRGB eliminates the spur gear housing and pair of spur gears, the long steel input shaft and 4 sets of bearings. Even with a thicker housing and steel bearing support flange, it still came out 12.7 lbs lighter than the Speedway housing. The new MRGB is sustantially stronger, quieter and lighter. |
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I'm really pleased with the way the new gearbox turned out. It fits in the same space, and still uses the Speedway end bells and the KISS aluminum spool, input flange/40 tooth pulley and sprag clutch. (Note: You can click on these pictures to view in higher resolution.) |
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The new housing is only 9" outside diameter. It requires less space than the original Exec 162F chain drive and oil bath, and doesn't interfere with the forward cyclic control cable routing, and it is definitely smaller, lighter and quieter than the Speedway Q.C. sprint car rear end. |
| 3/1/2021 Update So, I had to upgrade the gears in the gearbox! Several JetExec builders out there, including myself, experienced ring or pinion gear failures. The automotive-quality Dana 30 gears used in the standard kit gearbox (and I used OEM Dana 30 gears in mine as well) were not robust and suffered early fatigue failures. A few other JetExec owners and I spent about a year working with gear engineers, metalurgists and manufacturers to develop improved gears for the JetExec. The new gears have a better steel alloy, tooth design, machining, heat treating and finishing and are made in the US by a leading gear manufacturer. These new gears are specially designed and manufactured for 'infinite' fatigue life for our JetExec application. If you are a current JetExec owner with original Speedway gears, you really must do this modification. Further information about the ring and pinion gear problem and solution may be found on the RotorWay Owners Group forum here.
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These are the new gears I had made for my MRGB. They are custom-machined, heat and cryo-treated, finish-ground, deburred, shotpeened, REM ISF-finished and magnafluxed. The additional heat and cryo treatment steps improve the martensitic grain structure of the hardened steel. Shotpeening compresses and improves tensile strength at the gear tooth roots. Deburring and improved surface finishing reduce potential stress risers. All of these steps combine to significantly improve bending fatigue resistance and life of the ring and pinion gears. In addition to the above improvements, I changed from a 43:11-tooth ring and pinion to a 39:10-tooth ring and pinion. Larger teeth on the 39:10 ring and pinion make it another 10% - 13% stronger. |
Last Updated: March 18, 2021
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