HISTORY: Helicopter Rotor Smoothing
Over the last thirty years a variety of methods and techniques have been used to reduce vibrations produced by the main and tail rotors of helicopters.
Main Rotor Tracking Methods
Theoretically, main rotor blades should all fly in the same plane and maintain equidistant angular spacings during flight. Pitch links and tip tabs can be adjusted to compensate for blade differences to keep the blades in line at all forward speeds. Rotor tracking systems have focused on providing information that can be used to adjust the pitch links and tabs to coax the blades to fly "perfectly".
The earliest technique for rotor tracking was flag tracking, where the tip of each blade was marked with colored chalk or crayon and a white strip of cloth mounted to a pole was pushed into the edge of the rotor blade path. The marks on the cloth gave a measure of the blade track.
In the 1960's an opto-electronic method of measuring rotor track height and lead lag was developed and patented by Chicago Aerial. They built small single lens systems that could be mounted to the aircraft and measure track in flight, but the system they sold the most of was a large ground based dual lens/sensor system that was very accurate but could only measure track on the ground.
Strobe Light Tracking
In the late 60's, early 1970's Chadwick-Helmuth adapted a strobe light and retro-reflective tip targets to allow blade track and lead-lag to be measured in flight. This technique required the operator to manipulate a dial and visually locate a group of targets in space and remember their relative locations. This method required significant operator skill and training, often making results obtained by military units unreliable. In the early 1980's the US Army and Helicopter manufacturers made it clear they were looking for a system that could measure blade track consistently and accurately without highly skilled human operators.
Electro-optical Tracking Revisited
In the 1980's Stewart Hughes revived the method originally developed by Chicago Aerial and applied up-to-date electronics and packaging methods to make this technique practical for in-flight tracking. Helitune developed another method using a line scan video camera. Scientific Atlanta entered into a licensing agreement with Stewart Hughes to utilize their technology for blade tracking. Chadwick-Helmuth introduced a system in the early 90's that utilized the method patented by Chicago Aerial to measure track optically without using a strobe. In 1995, DSS introduced an optical tracker using a unique method patented in the 1970's but never produced commercially. The DSS technique uses a hand held electronic camera that has several advantages over the methods currently in use. These are 1. No tip targets or tape or painting blades is required. 2. The camera is hand-held, eliminating mounting problems. 3. Very little power is required, meaning aircraft power is not used. 4. The data is machine readable, making the results reliable and repeatable with unskilled users. 5. Using daylight, the system is totally passive.
Tracking using Vibration Sensors
Users found the track conditions of the rotor directly related to vibrations in the airframe. Experimentally it was found that the vibration information could be used to adjust pitch links and tabs to produce minimum vibrations at all forward speeds. After this process was complete, the blade track could be measured optically and surprisingly the blades were not in perfect track! This lead to a quandary... do we want perfect track or minimum vibrations?
Is Tracking of any value?
In the process of using these tracking methods and measuring the vibrations that resulted, users found that "perfect track" rarely produced minimum vibrations. Various theories have been proposed to explain this effect. One theory is that each blade has a slightly different shape, twist, flexibility etc. and only by putting them slightly out of track can these variations in lift be compensated. Another theory is that each blade produces a "turbulent wake" that the trailing blade must fly through. If alternating blades are set to fly high and then low, each blade will have "calmer air" to fly through resulting in smoother flight. This effect is more pronounced on aircraft with four or more blades on the main rotor.
Tracker is Useful for Finding Rotor Faults
This fact has lead some manufacturers to conclude that blade tracking is of little value and the only purpose of rotor smoothing should be to minimize vibrations without regard to blade track. This approach has a few drawbacks however. First, a "bad" blade that must be put way out of track to minimize vibrations can only be detected if a tracking system is used. Second, blade track and lag information may make finding some problems with the rotor much easier. For example a bad damper may produce a subtle transient vibration effect during turns, but a lead lag measurement will show the damper problem as a blade obviously unstable in angular position.
Rotor Balancing Methods
Early methods of balancing helicopter rotors were limited to static "bubble" balancing of rotor heads and "weighing" blades to be sure the rotor was symmetrically loaded. In the 1970's maintenance personnel began attaching vibration sensors and using spin balancing techniques that were common in industry on large industrial blowers. Using a strobe flash to establish the phase of the vibration along with a meter reading of the amplitude of the vibration, charts (nomograms) could be used to determine where to add weight and how much.
Early Complex Algorithms
With Helicopter rotors, there is interaction between mass imbalance and blade track at hover. If one blade is flying high and producing more lift, it will also lag its normal position due to higher drag force. This induces an effective mass imbalance due to the blade being out of position. Due to this and other interactions, users developed procedures or "algorithms" that allowed the rotor to be smoothed by performing steps in a particular order. For example, first: track blades with pitch links on the ground, second: track the blades with pitch links based on hover and forward flight track, third, adjust tabs based on track data or vertical vibration data in forward flight, fourth, spin balance the main rotor at hover. These procedures required a good deal of skill and accuracy from the maintenance personnel. Once again the military did not get consistent good results with "average" military personnel trying to execute these complex algorithms. The military and helicopter makers asked for automated computerized methods that eliminated the need for highly skilled users.
Computer Based Algorithms
In the 1980's hand held computers were programmed to perform the rotor smoothing algorithms. In some cases, the engineers programming the new computer systems wanted to "leapfrog" the algorithms that were currently in use. A new concept became popular that theorized all interactions between pitch link, mass balance and tab were linear. If this linearity were valid a single flight to gather data at several flight regimes would be all that is necessary to make all required adjustments. Another popular concept was that all helicopters of the same type were sufficiently similar to allow a single computer math model to be used without adjustments for each individual aircraft. Equipment based on these two concepts was developed and introduced into the market by Chadwick-Helmuth in their 8500 system and Scientific-Atlanta in their RADS system.
Single flight computer based rotor smoothing algorithms based on these two concepts have proven to be unreliable. They work fine so long as the aircraft is close enough to normal conditions for linearity to hold fairly well and so long as the aircraft is a fairly good match for the math model contained in the program. The problem is: How often are these two conditions met? Experience has shown these conditions are only met 50 to 75% of the time. When these conditions are not met, the algorithm fails and cannot provide any further assistance in rotor smoothing. The user is stuck. Some vendors feel this method is still viable... we do not. If your wrench only fit 50 to 75% of the time would you be satisfied?
Algorithm Unable to Verify Move
Compounding this problem is that a single flight method does not lead to any method of verifying that the changes recommended were executed properly. This is because the single flight method requires the user to make several adjustments at once. Due to the interaction of these adjustments it is virtually impossible to determine if the adjustments were done correctly.
Example: On Huey and Jet Ranger main rotors, blade sweep is used to accomplish chord wise mass imbalance of the main rotor. Field data has shown that there is tremendous "stiction" in this adjustment. Quite often when the adjustment is done, no mass shift can be measured. Only by isolating this adjustment can we verify that it has been done in a manner that overcame the "stiction".
Improved Fault-Tolerant Algorithm
The DSS rotor smoothing algorithm is the single move method. Each flight will only result in instruction to make a single adjustment. The next flight will be used to verify that the change matches the math model, if it does not match but is within a tolerance, the math model will be corrected to match the current aircraft. If it does not match and is radically different from the expected result, the user can double check that the move was as specified and if so the move can be taken back out to see if the original data repeats. This algorithm will allow the math model to "learn" the particular characteristics of the current aircraft. Any errors in user execution of the change will be caught. When the rotor smoothing is completed the math model can be stored and labeled with the tail number of the aircraft being worked.
Another advantage of this method will be the first flight may only be pulling the aircraft up into hover for 30 seconds. Forward flight will not be attempted until hover vertical and lateral vibration is low. This will avoid the safety hazard of trying to fly an aircraft that is badly out track or balance in forward flight conditions.
The DSS algorithm will work reliably on all rotors and will require lower skill levels on the part of the operator when compared to earlier methods using paper charts. The math model will be improved with each use to the point where it will match the aircraft in use precisely. This will save time in future track and balance operations compared to repeatedly trying to balance an aircraft with a "fixed" math model of a "typical" aircraft.
The DSS algorithm is well grounded in the scientific method used in all research. Using this method all conditions are held constant except one. The true effect of changing one condition can then be accurately assessed. Only by using this method can real learning take place.
Another advantage of the DSS approach over the "single flight" method has to do with development cost and time. Development of a "single flight" program requires hundreds of flights on many different aircraft of the same type. This is required to arrive at a "reasonable" average math model of this type of aircraft. Since this method cannot "learn" from normal use, all this data must be gathered and averaged before the program can be written. This process is time consuming and costly. During this process, which may take up to a year or more, the program for this aircraft is unavailable. Even when the process is complete, if the math model does not match the users particular aircraft the program will not work. The DSS approach does not have this flaw. Our algorithm will only need to know basic mechanical facts like the number of blades, number of tab stations, type of rotor, if tabs are adjustable etc. and can then be used immediately to smooth any helicopter rotor. In fact only one "program" will exist for this method. The data the program learns for each aircraft will be stored in a separate data file for that aircraft. For example: after working one Huey, a user can use this data file to work a second Huey for a good head start, but each Huey will be slightly different, and the program will learn from the process. This leads to the advantage of the DSS method for working helicopter types that are new, or that are few in number, making development of a "single flight" program uneconomic.
Another advantage of the DSS approach is when each aircraft of a particular type is significantly different from its sister aircraft. We have found this condition to exist with the Robinson R44 for example. If the algorithm does not "learn" each individual aircraft, the algorithm will fail.
Another problem with several helicopter track and balance systems on the market is their limited basic capabilities. They have become so highly specialized that they have left out several basic features found in most general purpose balancers and vibration analyzers. Some systems require a special program in order to take a simple balance measurement. Some systems cannot recall a saved spectrum back to the screen for analysis. Some systems can only print to a built-in thermal printer, limiting the quality of the hard copy record. Some systems have only one choice for the number of lines in the spectrum analysis. Some systems have little or no spectrum averaging capabilities. Some systems cannot even make reliable and repeatable balance measurements during slight gusty winds or minor turbulent conditions due to poor measurement methods and averaging techniques. The MicroVib is not only a powerful helicopter track and balance system, it retains all the basic capabilities of a general purpose balancer and vibration analyzer that other more expensive systems have left out.
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