Designed to be the lowest-cost ($2,995), easiest-to-use optical blade tracker on the face of the Earth!
The DynaTrack optical tracking accessory for the DynaVibe GX3 sticks to the windscreen with a suction cup. This is more convenient than traditional hand-held designs and eliminates the need for a dedicated tracker operator in flight.
Blade Track: Target Blade is 0.05" Low
Signal Quality: 100% of blades were successfully acquired and analyzed
Vibration (IPS): 0.11 IPS lateral at 4 O'Clock
Conditions: Record up to 8 flight conditions per flight and review after landing
With the DynaTrack Optical Tracking Module mounted to the windscreen, both hands are free to fly the helicopter.
Data is acquired when the operator pushes a push-to-talk button that can be mounted to the collective or yoke.
Rotorcraft track and balance procedures vary by rotorcraft type and manufacturer recommendations. But generally speaking, main rotor track and balance procedures start on the ground with an initial dynamic balance and blade track. Once the initial track and balance procedure is complete, the process is repeated in a hover, then in forward flight.
When using the DynaVibe/DynaTrack system for track and balance procedures, the DynaTrack optical blade tracker is mounted to the windscreen with a suction cup and a tach is mounted to the aircraft close to the main rotor. One or two accelerometers are mounted on the airframe depending on the specific procedure bring followed.
The DynaVibe GX3's Rotorcraft mode first helps you set up the system, then allows several measurement conditions to be taken back-to-back, such as hover and multiple forward flight speeds. Blade track and up to two vibration measurements (i.e. vertical and/or lateral) are recorded simultaneously during each data set acquisition. This comprehensive set of measurements can then be used to determine what adjustments are needed between flights to track and balance the ship.
Helicopter main rotors are a unique challenge when trying to reduce aircraft vibration. Unlike propeller driven aircraft, helicopter main rotors operate under extreme varying air speeds and blade loading. Propellers benefit from the fact that the blade aerodynamics are relatively fixed by their short, stiff design.
For instance, the accompanying table shows that the aerodynamic balance of a propeller is relatively stable. The propeller chordwise and spanwise loading do not change significantly in normal operation. A propeller can typically be statically mass balanced prior to installation and dynamically balanced once installed to provide a smooth operating aircraft. The only time that the propeller experiences aerodynamic vibration is when improperly installed, after repair that causes aerodynamic changes, or when operating the propeller at an angle of attack.
|Static Mass Balance||REQUIRED||REQUIRED|
|Static Aerodynamic Balance||By Design||REQUIRED|
|Dynamic Mass Balance||REQUIRED||REQUIRED|
|Dynamic Aerodynamic Balance||By Design||REQUIRED (Hover and Forward Flight)|
Rotorcraft tracking and balancing is often challenging. The blades require both a static and dynamic mass balance, and a static and dynamic aerodynamic balance. Because of the large span of rotorcraft blades and the need to transfer load from one blade to the other, the blades are typically very flexible. Videos can show the extreme aerodynamic flexibility of rotor blades.
Because of the extreme flexibility of the rotor blades, the static and dynamic aerodynamic balance becomes much more important. Just like a propeller, the helicopter rotor needs to start with a good static balance. Equipment is available to hang the rotor-head and statically balance the blades before there are even mounted on the helicopter. This technique provides a good starting point for centering the rotor's mass prior to installation.
The static aerodynamic balance can be tuned prior to even starting the engine. The lead / lag or fore / aft position of the blades can be adjusted to manufacturer's specifications and pitch links can be adjusted to equalize the angle of attack of the blades. Any proper rotor smoothing procedure must start with a thorough static mass AND aerodynamic balance.
Once the initial static balance processes are complete, dynamic balancing can begin. Variations in manufacturing can cause a mass distribution difference between blades that generates unbalanced dynamic forces while operating. Therefore, even a perfectly statically balanced assembly can have dynamic mass errors that cause rotor vibrations.
Imagine that a string supporting a rotor assembly. The rotor blades are statically balanced and hang horizontally from the string. However, there are two mass errors, one on the top of one blade, and one on the bottom of the other blade. Theses mass errors could be caused by excess resin, bonding material, or any other slight difference in assembly or materials. Regardless, if the two masses offset each other (mass1 × radius1 = mass2 × radius2), the static balance will indicate that the blades are balanced.
Once the rotor starts, these two masses will start to try to align along the plane of rotation of the rotor. This will cause a vibration as the two masses force the blades to alter their paths to align the weights. This is on of the reasons that a dynamic balance is necessary. The DynaVibe can detect the vibration caused by this mass error and generate a correction to eliminate the dynamic mass imbalance.
DYNAMIC IMBALANCE EXAMPLE: Balanced Static forces and moments make this a statically balanced assembly. However, the difference in the vertical distribution of mass makes this a dynamically imbalanced assembly.
Optical tachometer mounted on the mast of a Robinson R22. Reflective tape shown as the reflective target.
On the Robinson R22, the accelerometer is mounted vertically on the helicopter's console. Rocking about the lateral axis is sensed as vertical movement of the ship's nose.
The DynaVibe reports vibration magnitude (IPS) and phase angle (degrees). These readings are used to determine where weight adjustments are needed.
As the blades begin to experience the aerodynamic forces, the airfoils generate forces sufficient to alter their paths as they rotate. If the two blades have the slightest difference in shape, stiffness, or construction, then blade lift differentials are generated. This will cause individual blades to climb or fall relative to the other blades.
The different blade flight paths will cause each blade to have a unique center of gravity, essentially making the blades behave like they have a mass differential. The differences generate a force that causes a vibration throughout the rotorcraft. Therefore the blades must be matched aerodynamically while spinning. Once the blades are aerodynamically matched, the blades will then fly in a single plane, or in-track.
Aerodynamic tuning, or tracking, is first done in hover, with light wind conditions, so that the blades can be matched when they are both experiencing the same airspeeds. This is typically done with pitch link adjustments which alter the relative angle of attack of the blade.
PITCH LINK ADJUSTMENTS: Pitch link adjustments change the relative angle of attack of each blade. By adjusting pitch links (Robinson R66 pitch link shown above) using DynaTrack hover data, the blades can be brought "in-track" thereby balancing their relative aerodynamic forces.
The DynaTrack is conveniently mounted to the windscreen with a suction cup which allows for hands-free operation by the pilot. This is mounted on a Bell 206 Jet Ranger and aimed just insider of the trim tab.
Initial track readings can be taken light on the skids. Once in-track, transition to a hover and take track and balance (vibration) readings.
The DynaTrack accessory measures blade track and reports the result with respect to the designated "master blade". In this case the slave blade is 0.33 inches lower than the master. Adjustments should be made according to the helicopter manufacturer's recommendations in the helicopter maintenance manual. See below for references.
After the blades are aerodynamically matched (in-track) in a hover , the forward flight testing can begin. Forward flight is one of the most challenging aspects of rotorcraft because the blades are now experiencing vastly different air speeds depending on the rotational position of each blade and the velocity of the ship.
While in forward flight the advancing blade sees much higher airspeeds, therefore greater lift, than the retreating blade. The advancing blade therefore also generates drag than the retreating blade. One way this is reconciled is with the flapping hinge, typical of most two bladed rotorcraft. The flapping hinge transfers the excess lift from one blade to the other, minimizing the forces applied to the main rotor shaft. However, if one blade generates more lift versus airspeed, meaning it has a different coefficient of lift curve, then that blade will always generate more lift during its advancing portion of flight. This causes aircraft vibration in forward flight. Therefore, this aerodynamic mismatch must be equalized.
Foward flight blade lift equalization is done with trim tabs. By adjusting trim tabs on each rotor blade, the blade's coefficient of lift is be modified so that its lift versus airspeed behavior is matched with respect to the other blades. This will allow the blades to stay in-track and minimize vibration in forward flight.
TRIM TAB ADJUSTMENTS: The forward moving blade has a higher velocity than the retreating blade. Trim tab adjustments take advantage of this nonlinear velocity vs lift property in order to balance each blade's coefficient of lift (lift vs velocity) with respect to the other blades on the rotor.
Once the blades are in-track in a hover, transition to forward flight testing. The DynaTrack can be operated in-flight with a push-to-talk button that is attached to one of the controls.
The DynaTrack can take several readings during the test flight and the readings can be reviewed after the flight. Usually readings are taken at multiple flight speeds, including a hover, to get a complete sense of how vibration and blade track readings are changing with airspeed.
The combination of blade-track and up to two simultanious vibration measurements are used in conjunction with the helicopter manufacturer's polar charts to make adjustments in pitch-links, trim-tabs, etc. as shown in this chart for a Bell 47.
Our notes. Manufacturer Notes. Whatever it takes to make you successful.
All DynaVibe models can balance other spinning components, not just the main rotor.
The optical tach and accelerometer are mounted on the fan to isolate vibration from the fan imbalance.
The DynaVibe's propeller balancing functions can be used to balance helicopter tail rotors.