Images courtesy of www.dssmicro.com
PROPELLER BALANCING PROCESS
- There are several distinct types of propeller assemblies which are candidates for Dynamic Balancing and this list includes almost all aircraft. The following is a list of the major types which can be balanced. A very small number of engine propeller combinations cannot be balanced.
These are the ones with propeller assemblies that have no way to add or remove weights on the rotating propeller. The old Hamilton Standard propellers on the world war II warbirds are examples of this type.
- The following is a list of the types that can be balanced, and this capability is characterized by having a means to add or remove weight on the rotating propeller assembly.
- Fixed pitch with a spinner
- Variable pitch with a spinner
- Fixed pitch Radial engines with a spinner or means to add or remove weights.
- Reverse turning Radial engines with a spinner or means to add or remove weights.
- Turbine engines. There are two main types, geared and direct drive. The geared type are very sensitive to propeller unbalance, since it causes premature wear in the gear section.
- BALANCING METHODS
Balancing a propeller system involves measuring the amount and location of the unbalanced weight while the propeller system is turning, thus the word “dynamic”.
The amount of unbalance is measured with an accelerometer which is attached to the engine near the propeller; usually on the front engine cross tie bolt on flat engines, or on one of the front crank case bolts on a round engine or a turbine engine. The accelerometer measures the motion of the front of the engine resulting from the propeller unbalance.
Determining the location of the weight causing the shaking as the propeller rotates is done with an optical pickup attached to the cowling or engine valve cover. A strip of reflective tape is placed on the back side of one of the propeller blades. This serves to provide an electrical pulse once each time the propeller rotates.
With this information about where the propeller blade is located when the accelerometer sees a peak in the vibration, a computation is performed which provides the “where at” information.
The acceleration level measured by the accelerometer provides the “How much” information.
Using this data the computer calculates the amount of weight to add and, based upon the normal location of the sensors, computes a trial location.
This information is used to install a small trial weight on the propeller assembly.
The amount and location of the installed weight is then input to the computer.
A second run is performed and with this new data, the computer then knows the amount of weight and the location to place the weight for this particular propeller assembly.
Additional runs are made to fine tune the balance.
During these runs, the actual propeller RPM is noted as shown on the Balancer and this is compared with the RPM seen by the pilot on his tachometer. From this, a correction to the aircraft tachometer is determined and provided to the pilot as a correction so that he may make correct his inflight RPM settings.
After balance is achieved the installation of the weights is made permanent.
A final run is made after final installation to verify the installation was performed correctly.
During the final run a spectrum analysis is performed after the final balance measurement.
This is a mode in the balancer where all the vibrational energy from 500 RPM to 10,000 RPM is recorded in graph form.
This is a very important measurement as it allows one to observe the various spikes of energy corresponding to RPM. The propeller unbalance, or what little remains, will show up as a spike at 1800 or 2000 RPM depending on which type of propeller, (Fixed or Variable pitch) is being balanced. Above that rpm, spikes will be seen which are related to 2 or 3 times the RPM which are caused by each propeller blade passing.
The critical spike here is any one that exceeds .6 IPS, and this will vary with engine types but will generally be in the 4000 to 7000 RPM range. Many years of experience has shown that if a spike is at or above the .6 IPS region then there is likely a serious engine problem which must be investigated.
CONSTANT SPEED POWER CHECK
if the propeller is a “Constant Speed” type,
then we must perform a power check to verify that the pitching mechanism is working properly. If a propeller shop has performed maintenance on the propeller then this verifies the quality of that work.
The power check is performed by increasing the propeller RPM with the throttle to 20% above the 2000 rpm where the balancing was performed ie 2400 rpm.
The propeller control is then used to drag the rpm back to 2000 rpm and reading is recorded.
The balance level should not change as a result of the additional load placed on the propeller during the power check.
If it changes by more than .2 IPS then the propeller should be removed and sent to the Propeller shop.
You may wonder how long a balance lasts. Well, it depends on the type of operation that the aircraft is subject to. Conditions that cause erosion of the propeller blades such as dirt or gravel runways can accelerate the erosion and the resultant change in propeller balance. Several things can affect the rate of erosion of the propeller blades. The type of aircraft will have a dramatic effect. A tail wheel type of aircraft causes the propeller to draw air down at an angle to the ground, thus avoiding picking up runway surface material and prolonging the life of the propeller balance. If a tricycle aircraft is operated in the same dirt or gravel conditions the propeller sucks greater quantities of material up from the runway thus causing increased nicking and wear on the blades.
A tail wheel aircraft can generally operate 300 hours in these conditions while the conventional aircraft may have significant damage in 150 hours which will require rebalancing the propeller.
Not finding what you want? Reach out directly through our Contact Us page.