Current Press Release

Helicopter transmission overhaul

Oil shear technology ensures helicopter transmission test stands operate continuously, without adjustment, while achieving precise positioning for desired percentages of load.
Elizabeth Engler Modic (Edited by)

Professional Aircraft Accessories (PAA) in Titusville, Florida, specializes in the repair and overhaul of landing gear, accessories, instrumentation, pressurization, radio, radar avionics, and airframe components. Already well entrenched in fixed-wing aircraft, expanding into the helicopter market was a natural extension, and the request for quote (RFQ) for overhaul and recertification of OH-58 Kiowa helicopter transmissions provided a perfect opportunity.

There was only one caveat: a very condensed timeframe. PAA officials found that oil shear braking technology helped them meet the technical requirements of the project and the fast response needed for initial certification testing.

Short-fused project takes off

While the system was designed for vertical operation the brake was mounted horizontally to allow direct torque measurement.

With the RFQ approved, PAA officials had four months to design and build a dynamometer test stand and overhaul three transmissions to get their project verification audit from the Army. Given the short timeframe, the project engineering team opted to use non-regenerative technology. Jerry Leach, director of production engineering and planning, led the team designing the system.

“Designing a system to do what we wanted with regenerative technology would have been more efficient, but it would have taken 4x longer and cost at least twice as much,” Leach says. “We contracted out key components of a system we could build in-house and decided to dump power into the system and then load it via braking. This is very effective, but it builds up a lot of heat which must be exhausted.”

Power is supplied via a GE electric motor and variable frequency drive (VFD) combination with 700hp and 3,600rpm. The specification calls for testing at 40%, 60%, 80%, 100%, and 112% of load for various timeframes. Precisely controllable load is supplied by a TB 83 oil shear brake from Fairfield, Ohio-based Force Control Industries. At 112% of load, Leach cites 8,200 lb-ft torque. The test takes about an hour, during which there is very little temperature rise.


Dry brakes employ a sacrificial surface, usually a disc or pad, to engage the load. With no good way to remove the heat caused from engagement between the disk and plate, these surfaces must absorb the heat. The extremely high temperatures will eventually degrade the friction material. As the friction surface wears away and begins to glaze, the coefficient of friction falls, causing torque fade. This causes positioning errors which require adjustment or replacement of the friction surface.

Oil shear technology plays a major role in ensuring that, in the case of the helicopter transmission test stand, it can operate continuously without adjustment and still achieve precise positioning for the desired percentages of load. A fluid film flows between the friction surfaces and is compressed as the brake is engaged. The fluid particles in shear transmit torque to the other side. Since most of the work is done by the fluid particles in shear, wear is virtually eliminated.

In addition to transmitting torque, the oil dissipates heat due to a fluid recirculation system. Given the size of the system and the temperature rise, a cooling system was also provided.

Along with torque transmission and heat removal, the fluid continually lubricates all components, extending their service life. Oil shear technology provides a cushioned engagement that reduces shock to the drive system – further extending service life. Unlike dry brakes and clutches, the totally “The plumbing required to move that much fluid, at temperatures from 50°F to 175°F to accommodate expansion and contraction, was challenging,” Leach says.

Additionally, 1,200A electrical service had to be put into the facility before everything could be wired and tested. Getting it all in place was the first hurdle, but it wasn’t the last challenge for the project.