Q. Do I need to complete a Form 337 for a dynamic propeller balance?
A. Not always! If the manufacturer has an established written procedure for the dynamic balance of the propeller, no 337 is required. If no such guidance is available, you may use the information in FAA Advisory Circular 20-37E (or subsequent) and the FAA-approved document “ACES Systems Guide to Propeller Balancing” as reference material. Remember that this is in lieu of any available guidance from the manufacturer which would always take precedence. The requirement for the 337 is spelled out in AC20-37E.
Q. Is ACES equipment approved by the FAA?
A. No! It isn’t required to be approved by the FAA. The analyzer you use, ACES Systems or otherwise, is only a tool much as a wrench or screwdriver. As such, the equipment itself needs no approval. The FAA approval is required for the hardware and associated methods of application needed to accomplish the dynamic balance. To say ACES Systems’ equipment (or any other equipment) is FAA approved is misleading. To say a balance job done with ACES equipment (or any other equipment) is FAA approved is also misleading. The only portion of the job that requires an FAA approval is the actual hardware and associated methods of application used to accomplish the task. ACES published an FAA-approved document which spells out these requirements of hardware and methods. The “ACES Systems Guide to Propeller Balancing” is provided with each propeller balancing kit sold by ACES Systems. You may also download a copy from this web site. You may use the guide with any balancing equipment and be assured that the balance job is then FAA-approved.
Q. I balanced my customer’s propeller down from .5 IPS to an acceptable level of .06 IPS with my ACES Systems’ Probalancer sport, but I could still feel vibration in the yoke and rudder pedals. What did I do wrong?
A. Nothing! When you balance the propeller you are balancing at a specified frequency, the RPM you selected as a balance speed. In doing so your balancer displayed the once-per-rev synchronous vibration associated with that frequency. Anything vibrating outside that frequency isn’t displayed in the propeller balancing program. Other components may be vibrating at different frequencies. You may also be feeling a resonate vibration in the airframe structure that has been excited by the propeller or other fundamental frequency. If you perform a vibration or acoustic survey and read the overall vibration you may see a level (IPS) higher than that being read from the propeller assembly. Having an operating frequency chart of common components for the aircraft will greatly simplify the task of pinpointing the source of the vibration. A full graphics spectrum survey can be accomplished using an ACES Systems’ Model 2020 Probalancer, Analyzer or Model 4040 VIPER Analyzer.
Q. I own another brand of balancer with all the sensors recommended to do a balance on my aircraft. Will my ACES Systems’ Model 1015 Probalancer Sport need new sensors to fit it?
A. Yes! The Model 1015 Probalancer Sport Analyzer use an IMI 608AII accelerometer. The analyzer can only recognize this sensor. This configuration was engineered to give our customers an economic balancer with the highest accuracy possible.
Q. My tachometer signal is erratic or drops out when balancing. What do I do to correct it?
A. If problems are experienced using the Phototach while balancing high-speed props with the reflective tape further than 14 inches from the center of the prop shaft, refer to the following steps for tape placement and adjustments.
- First, measure the distance from the center of the propeller shaft to the location you intend to place the reflective tape.
- In the chart below, select from the RPM column the first speed greater than the speed at which you intend to balance.
- From this RPM number, proceed across the chart to the right until you come to the first number larger than the distance measured in Step 1 above.
- From this point, follow the column up to the top to the minimum tape width required for your application. As an example, use the following parameters the distance from the propeller shaft to the intended tape location measures 25 Inches and the balance speed IS 2300 RPM. Select 2400 from the RPM column since this is the first speed greater than your intended balance speed of 2300. From this number, follow the row across to 26.5,which is the first number higher than your intended tape location of 25 inches. From 26.5 folow the column straight up to the top–2 inches. This is the width of tape required for accurate readings at the intended distance and RPM level. (If your reflective tape is only 1-inch wide, place two 1-inch strips oftape side by side to create 2 inches.)
Q. I get exceptionally low vibration readings. Is my analyzer broken?
A. NO! In these situations the analyzer, is rarely the problem. Cables are the most likely culprit. Cables can be damaged if pinched in doors and windows, always check for pinches, cuts, and abrasions prior to using the cable. Bent or damaged pins may cause problems with normal operation. Route cables away from all hot areas and electrical equipment. Duct tape or wire ties are excellent for securing the cables. Check all connectors for evidence of damage.
Q. My balancer gave me a different weight for the final balancing solution. Is this right?
A. Yes! The analyzer will use the last weight added as temporary weight and calculate the total weight needed to correct the imbalance. This is similar to calculating the CG for you airframe. When the length of the arm for the weight placement is changed the weight will be come more, or less effective as the distance increases or decreases, respectively. This is to say, as you place weight farther from the propeller shaft, less weight is required to correct the imbalance, and vice versa. AC 43.13 governs acceptable methods and procedures for permanent balance weight placement. If you wish to calculate the permanent weights manually you can do so by using the following method or the chart shown below.
Measure the spinner’s diameter then divide it by 2 to calculate the spinner’s radius. Measure the distance from your temporary weight location to the permanent weight location, and subtract that measurement from the spinner’s radius. The resulting measurement is the permanent weight’s arm. Multiply the spinner’s radius by the total weight required to balance the propeller. That will give you the net effect of the weight in gram inches. Divide the net effect by the permanent weight’s arm. The result is the total weight required to balance the propeller at the measured permanent weight location.
|Diamter of Spinner||14.0″|
|Divide by 2||/2|
|Equals Radius of Spinner||=7.0″|
|Minus Distance from Test Weight to Permanent Weight||-1.5″|
|Equals Permanent Weight Radius||=5.5″|
|Required Weight to Balance||25.0 Grams|
|Times Radius of Spinner||x 7.0″|
|Equals Net Effect||175 Gram Inches|
|Divided by Permanent Weight Radius||/5.5″|
|Equals Permanent Weight||31.8 Grams|
Q. Where do I get replacement parts for my balancer?
A. Contact the distributor you purchased your balancer from. If you purchased your balancer from an individual you can contact one of the distributors on the links page to procure replacement parts.
Q. Will the Model 1015 Probalancer Sport perform a vibration spectrum survey?
A. No! The Model 1015 Probalancer Sport was designed to only perform propeller balance.
To accomplish a vibration survey, you will need to acquire an balancer designed to acquire vibration surveys. Several balancers are available through different manufacturers. ACES Systems offers the Model 2020 ProBalancer and the Viper Model 4040 balancers. Both balancer’s offer vibration spectrum surveys, and the Viper Model 4040 offers an additional tool, transient vibration surveys, to assist in locating and recording vibration surveys. For information concerning balancers offered by ACES Systems, visit www.acessystems.com or contact your distributor.
Q. What are the dangers of unchecked vibration?
A. The Tacoma Narrows Bridge collapse is an excellent example of what could happen if vibration is not counteracted.
The original Tacoma Narrows Bridge was opened to traffic on July 1, 1940. It was located in Washington State, near Puget Sound.
Prior to this time, most bridge designs were based on trusses, arches, and cantilevers to support heavy freight trains. Automobiles were obviously much lighter. Suspension bridges were both more elegant and economical than railway bridges. Thus the suspension design became favored for automobile traffic. Unfortunately, engineers did not fully understand the forces acting upon bridges. Neither did they understand the response of the suspension bridge design to these poorly understood forces.
Furthermore, the Tacoma Narrows Bridge was built with shallow plate girders instead of the deep stiffening trusses of railway bridges. Note that the wind can pass through trusses. Plate girders, on the other hand, present an obstacle to the wind.
As a result of its design, the Tacoma Narrows Bridge experienced rolling undulations which were driven by the wind.
Strong winds caused the bridge to collapse on November 7, 1940. Initially, 35 mile per hour winds excited the bridge’s resonant vibration frequencies, with amplitude of 1.5 feet. This motion lasted 3 hours.
The wind then increased to 42 miles per hour. In addition, a support cable at mid-span snapped, resulting in an unbalanced loading condition. The bridge response thus changed to a 0.2 Hz resonant vibration frequency, with amplitude up to 28 feet.
The torsional shape was such that the bridge was effectively divided into two halves. The two halves vibrated out-of-phase with one another. In other words, one half rotated clockwise, while the other rotated counter-clockwise. The two half spans then alternate polarities.
A 600 foot length of the center span broke loose from the suspenders and fell a distance of 190 feet into the cold waters below.
The fundamental weakness of the Tacoma Narrows Bridge was its extreme flexibility, both vertically and in torsion. This weakness was due to the shallowness of the stiffening girders and the narrowness of the roadway, relative to its span length.
Engineers still debate the exact cause of its collapse, however. The prevailing explanation is called the “Aerodynamic Instability” theory.
Aerodynamic instability is a self-excited vibration. In this case, the alternating force that sustains the motion is created or controlled by the motion itself. The alternating force disappears when the motion disappears. This phenomenon is also modeled as free vibration with negative damping.
Airfoil flutter and transmission line galloping are related examples of this instability.
The wind did not strike the bridge perpendicularly, thus the bridge is initially at an angle-of-attack with respect to the wind. Aerodynamic lift was generated because the pressure below the span was greater than the pressure above. This lift force effectively placed a torque, or moment, on the bridge. The span then began to twist clockwise. Specifically, the windward edge rotated upward while the leeward edge rotated downward.
The span had rotational stiffness, however. Thus, elastic strain energy built up as the span rotated. Eventually, the stiffness moment overcame the moment from the lift force. The span then reversed its course, rotating counter-clockwise.
The span’s angular momentum did not allow it to simply return to its initial rest position, however. The reason is that there was little or no energy dissipation mechanism. Thus, the span overshot its initial rest position. In fact, it overshot to the extent that the wind struck the span from above. The wind’s lift force effectively placed a counter-clockwise moment on the span.
Once again, strain energy built up in the span material. Eventually, the stiffness moment exceeded the moment from the wind’s lift force. The span thus reverse course, rotating clockwise. Again, it overshot its rest position. The cycle of oscillation repeated, except that the span then had rotational velocity as it passed through the original rest position.
The cycles of oscillation continued in a repetitive manner.
Eventually, one of two failure modes occurred. The span experienced fatigue failure due to an excessive number of stress reversals, or the angular displacement increased in an unstable manner until the material is stressed beyond its yield point, and then beyond its ultimate stress limit.
These two failure modes are interrelated. Accumulated fatigue effectively lowered the yield and ultimate stress limits.
Effectively the wind excited the bridge to the point where it vibrated itself into two pieces. The Tacoma Narrows Bridge’s collapse remains the most well-know structural failure due to vibration.