NOTAR® Maintenance — No SweatBy Greg Napert September 1999
NOTAR (No Tail Rotor) helicopters were introduced in 1991 as a revolutionary way to provide anti-torque to helicopters while reducing noise and vibration associated with conventional tail rotors.
Three models have been introduced commercially since 1991: the MD500 Series, MD600 Series, and MD900 Explorer.
Although the new systems appear to have fewer moving parts, NOTAR helicopters actually still have nearly as many similar components including a fan, tailrotor gearbox, and driveshaft. These components, which produce high volume/low pressure air for the NOTAR system still require inspection and maintenance.
The inspection and maintenance requirements, however, are significantly reduced when compared to conventional tail rotors. This is mostly due to the use of revolutionary new materials and designs which have reduced the number of friction bearings and vibration characteristics of the rotating components.
The result is a fan gearbox which is essentially "on condition," and a fan assembly and tail boom which is warranteed for 3,000 hours.
In order to properly maintain the NOTAR system, it is important to first understand how it works.
How NOTAR works
According to MD Helicopters, the NOTAR system uses a fan-driven air circulation system within the tail boom to control the directional heading of the helicopter. The tail boom functions as an anti-torque device.
The directional control pedals in the pilot compartment control the blade angle of the fan assembly, the vertical stabilizers and the thruster. Air flow from the fan assembly through the horizontal slots in the circulation control tailboom blends with the downwash from the main rotor and "hugs" the contour of the boom. This provides the majority of the anti-torque force required in a hover by creating lateral lift that is referred to as "Coanda" effect.
Directional control is then provided by a variable thruster assembly and moveable vertical stabilizers. The variable thruster assembly consists of a stationary thruster cone with two opposing openings, over which is fitted a rotating thruster cone, with a single opening on one side. The rotating thruster cone can be rotated around the stationary thruster cone allowing varying amounts of fan produced air to exit either side — thus providing directional control, particularly in a hover.
Moveable vertical stabilizers (called stabilators), whose position is relative to pilot pedal input, provides directional control in forward flight. This enables the fan to be off-loaded and reduces overall power demands.
According to Tom Strocks, customer support representative for MD helicopters, "The tailrotor output portion of the main transmission normally would drive the tail rotor, but on the NOTAR, it drives a gearbox, which in turn, drives the NOTAR fan. The fan gearbox jumps the fan speed to 5,388 rpm. This fan runs at a steady rpm and the pitch of the blades is changed depending on how much air you want it to move."
The fan contains thirteen blades, and its pitch is controlled through input from the pilot's pedals. Strocks explains that the fan is actually similar to the Fenestron design that Eurocopter incorporates in their shrouded fan tailrotor. However, instead of being mounted in the tail, it is mounted in the rear of the cabin and pushes air aft.
There are actually very few moving parts on the fan assembly, bearings are reduced with the use of a tension/torsion strap that holds the blades in position and spring loads the blades to a neutral position. The straps are made with two, heat treated, passivated stainless steel spools, and a Kevlar™ core, all encased in a urethane elastomer.
The blades are each connected to a counterweighted pitch horn, which is in turn connected to a pitch plate. As the pitch plate is moved fore and aft, it simultaneously changes the pitch of all of the blades.
Strocks explains the pilot's pedals actually control three systems simultaneously. They control the vertical stabilizers, the pitch of the fan blades, and the thruster output. The vertical stabilizers are controlled differently depending on which model helicopter you're talking about. The 500N, for example, has one of the vertical stabilizers connected to the pilots rudder pedal — the other is controlled with the Stability Augmentation System (SAS). Both stabilizers are connected to the rudder on the 600 series, there is no SAS. And on the 900 series, each stabilizer is connected to the rudder pedal, and has a built in stability augmentation system.
The amount of air which is allowed to exit the tail boom at hover vs. at takeoff is controlled through rigging and is connected to the rudder pedals. When the pedals are in neutral, you have 25 to 27 degrees of pitch on the blade. So in a hover condition, you have enough air pumping through the boom for stability of the helicopter. If everything is rigged properly, the pilot should be able to hover without even having to touch the rudder pedals. When the pilot pushes a rudder pedal to turn the helicopter, the fan blade pitch is increased — which increases the amount of air exiting the tail boom and thruster.
Directional control of the tail boom is achieved through the rotating thruster. The rotating thruster is a two-piece assembly, which consists of a stationary thruster with cascades (airfoil shaped vanes) with two opposing openings and a rotating portion installed over it which rotates to either side.
Rigging the anti-torque system involves rigging the vertical stabilizers, the fan, and the rotating thruster. A splitter assembly (bellcrank) at the top of the helicopter accepts the input from the rudder pedals and splits it into three outputs: for the fan, the thruster, and the vertical stabilizer(s).
Rigging starts at this bellcrank. The rudder pedals are placed in the neutral position, and the bellcrank is pinned for each item to be rigged. The bellcrank requires pinning at different locations depending on what you're rigging. Essentially, each system is rigged to the rudder pedals and is adjusted for neutral, full right rudder, and full left rudder. It's important to follow instructions in the appropriate maintenance manual for rigging each item.
Items to remember for rigging the NOTAR system include the following:
After adjustments are made to the fan controls, ensure blade is at 26 degrees ± 1 degrees. Note that the re-rigging of the thruster and empennage surfaces is not required when the tail boom is removed or reinstalled. Note that every 60-degree turn on the bearing retainer is equivalent to a 0.6 degree change in the fan blade pitch angle.
In order to properly accomplish thruster control rigging, the fan assembly, tailboom, rod assembly, and stationary thruster cone must be installed. Also, if the stationary and rotating thruster cones are not properly aligned, hover performance is significantly affected.
With respect to rigging the thruster control, there are two important items to remember:
First, the NOTAR control system must be re-rigged after replacement of control rods, linkages and components, or if helicopter operation reveals a rigging deficiency.
Second, do not adjust the control rod end beyond the witness hole, or tighten jam nuts until after final adjustment.
Vertical Stabilizer Assembly
Rigging In order to rig the vertical stabilizers; the fan assembly, tailboom, rod assembly, and vertical stabilizers must be installed.
During the rigging procedure, it is important not to adjust the control rod end beyond the witness hole, and not to tighten the jam nuts until after the final adjustment.
Pilot's Pedal Rigging
The marked pin holes on the splitter assembly are as follows:
L = Full left pedal rigging hole
R = Full right pedal rigging hole
M = Mid pedal rigging hole
When rigging the pilot's pedal, the fan assembly must be connected. It is also important to remove the Pilot and Copilot pedal assemblies to prevent damage to the window glass during rigging.
If the optional dual controls are installed, adjust the copilot's pedals to the same limitation as the pilot's pedals. Both sets of pedals may be adjusted at the same time.
In addition to 100-hour and annual inspection requirements spelled out in the maintenance manual, there is relatively little to be done to the NOTAR system.
Although rare, one of the more common maintenance items that needs to be addressed is vibration of the fan assembly.
According to Strocks, "Vibration typically is not a problem unless there is FOD damage or if the fan assembly has been removed and reinstalled for some reason."
Strocks says the fan is easily balanced with a dynamic balancer of your choosing. You simply use the balancer to determine the amount and location of imbalance, and then each blade has its own balancing stud to which weight can be added or removed in order to achieve balance.
"Once the fan is balanced, it typically stays balanced so well there is no need for a periodic balance as is required on the tail rotor. Balance is on condition only," says Strocks.
"The blades themselves are a thermoplastic and are very durable. It is possible, but rare, to damage the blades, as the intake is located at the top of the fuselage."
However, Foreign Object Damage is possible. "One set of blades that we received recently, was damaged by a mechanic who left a shop rag in the area. The rag was ingested and it tore up the blades considerably. Tip clearance should be maintained within limits. This is done with a titanium felt tip seal which is cut to size on installation of the fan assembly to produce a predetermined clearance."
Strocks adds, "In terms of the remainder of the NOTAR system there really isn't much maintenance to perform. The composite tail boom is relatively maintenance free, and the vertical stabilators are very simplistic in their operation."
The remainder of the maintenance technician's responsibility is tracking the life limit of the major components. A few of the components have finite life limits, which means you remove and discard components after a specific period of time.
"By the way," explains Strocks, "the warranty period on all NOTAR components is 3,000 hours, which is very high for the industry. Based on our experience, the only item that even comes close to requiring maintenance before this time might be the fan tip seal. And even this would only be required if the operator is running the helicopter in a harsh environment where sand, and what not, will cause excessive erosion of the fan tip seal. Corrosion has also been a problem in coastal environments."
"All in all, it's a very simple system to maintain," says Strocks.