Rotor Track and Balance – Blade Balancing – USBF Tool Description
USBF Tool Description.
Universal Static Balance Fixture (USBF)(Brochure)
The USBF Static Balance Tool is a one-man operated system. It allows all rotor blades, even a CH47 blade, to be handled by a single person by using a dolly assembly provided. The USBF is now approved for use on ALL US Army helicopter types.
The USBF is a product of extensive US Army research after the first Gulf War when deploying US Army helicopters, CH47 helicopters in particular, were taking many test flight hours to balance helicopter blades after being re-assembled on deployment.
CH47 Chinook Blade on USBF & Blade Dolly
The research also shed important light on what causes “rogue” rotor blades. Blades that refused to traditionally fly with other blades on the same rotor head. Blades which traditionally, the helicopter community (operators, engineers & aircrew) widely accepted as a matter of something that we simply had to live with and accept the prohibitive cost and waste of valuable time of.
The normal methods of rationalising this huge problem was to simply randomly swap blades until one set would finally dynamically balance and fly together. Often, perfectly good and serviceable blades would be shipped back to an overhaul facility or OEM for repair. The reality being, that no repair was required and operators have and continue to incur huge, unnecessary costs by doing this. This led to “sets” of blades eventually. And the acceptance of “rogue” rotor blades in the industry. Some Manufacturers actually insist on only selling blades in “sets”. There were various local methods, techniques which many individuals would preach as “cures” of these problems. Things like :
- Matching flying hours on rotor blades
- Similar serial numbers
- Believing that one blade flexes more than another…Blades are designed to flap. They have to flap to equalize the dissymmetry of lift. If they didn’t flap, the rotor would behave like the only truly rigid rotor system in the world….the Australian Aboriginal Returning Boomerang.
A truly rigid rotor system that could not flap, would be unflyable due the unbeatable laws of physics and aerodynamics. A truly rigid rotor system (eg Boomerang) would have a fixed flight path defined by aerodynamics. Namely, of climbing (due aerodynamic Flap-Back which stops when forward speed stops), coupled with a turn towards the retreating blade due to dissymmetry of lift, ie more lift on the advancing blade side than the retreating blade side.
That is how a Returning Boomerang works. And that was invented (stumbled across??) many centuries before Leonardo De Vinci came on the scene!
In this flapping process, the blades each and individually, flap at varying energies depending on what local airflows each individual blade is seeing. So they will, by their very design and function, flap at different rates and displacements. When RTB is performed, we are only seeking to try and establish a “reasonable” track Tip Path Plane at the beginning of the process to try and establish a “start point” for the RTB process to commence. Once the dynamic balance has commenced, the Tip Path Plain may end up quick different in order to establish a “Smooth’ rotor set.
Damaged Fin due Flapping Blade at Low RRPM
- Believing that physical mass makes the difference. (no it doesn’t – its the DISTRIBUTION of mass that matters) eg look at a centrifuge training machine.
- Believing that a band new blade was the problem when fitting it to an existing in-service blades that have undergone span moment arm migration.
- Flying and Selling only “Sets” of Rotor Blades: Some Manufacturers actually insist on only selling blades in “sets”. They do this in order to prevent or minimise the problem of “rogue” rotor blades and as means of trying to closely control the Span Moment Arm of their blade population on the open market. If Manufacturers allowed the use of the Digital Static Balance Tool on their blades at overhaul and repair centres and even at operator level, there would be no requirement to restrict sales to “sets”. All blades would be made easily adjustable back to original design Span and Chord Moment arms by adjusting accurately the respective weights.
Now all these things can be cured by using a USBF to adjust and reset the Span Moment of every blade back to the original design spec by adjusting the tip weights – which are designed to perform exactly that function.
Until recently, the Universal Static Balance Fixture (USBF) has been limited to use in Global Defence Forces as the airworthiness approval to allow its use on the helicopters was limited. This was due to Original Equipment Manufacturers (OEMs) of civil helicopters limiting its approval by operators and private blade overhaul shops to use this incredibly quick and accurate balancing piece of equipment.
Recently, a leading helicopter manufacturer has given approval for operators and overhaul centres to apply for use of the USBF in maintaining the span moment arm of their blades by adjusting tip weights.
CH47 Blade, USBF & Dolly
UH60 Blade, USBF & Blade Dolly
As332 Blade, USBF & Blade Dolly
USBF Tool
USBF Tool
USBF – The Tool Description
Tool Description: The blade carriage assembly is approx 20ft long and breaks down into two segments allowing ease of transportation. There is no other support structure or infrastructure required. It is a fully self contained balancing tool. It can be simply assembled on the hanger floor and balancing commenced.
Its portability makes it ideal for maintaining this static balance capability at various locations should the operator require long term deployments of helicopter assets to maintain operations. However this should not be required if the operator adopts the routine blade management program suggested on this site.
The whole tool consists of:
1. Gantry – approx. 20ft and reducible to portable size
2. 3 x electronic load cells– measures mass and distribution of mass
3. Load bar & support pads– which supports the blade on top of the load cells
4. Computer and software – to calculate the span/chord moment arms and calculate how much weight (by part number) to add or subtract from the appropriate station (tip or chord). It will provide a permanent record of all readings and adjustments for every individual blade by serial number. This allows easy monitoring of the adjustments, mass, and mass distribution changes through a blades life. The computer can be configured for ANY blade type once mapped. This ensures a non-variable mathematical model of the ideal master blade for that particular model or type of blade. This ELIMINATES the need for ever having a physical master blade with all the variables that a physical master blade introduces.
Every time the USBF is powered up and the load beam is installed, the entire tool and load cells are calibrated electronically through a Built In Test sequence which automatically calibrates the tool. No need to send away for calibration as physical master blades require.
The computer provides the interface between the operator, the tool and the blade. The display provides a means of inputting the blade type/model and serial number. It will show each step that the operator is required to perform to complete each blade balance after successfully performing the BITE and calibration.
5. Blade loading/handling Dolly– easily allows one man to handle any blade up to at least a CH47 size. Makes moving a blade from racks to the USBF and through the weighing process effortless. It has its own built in mini forklift function to allow for easy lowering of any size blade gently on to the load cells and to lift them off again on completion and return to the blade rack or aircraft.
6. How it Works: Self contained load cells and computer weigh the blade, calculate, display and print out the solution. Since the ideal CofG for any helicopter blade is a pure mathematical result of mass and moment arm, the individual blade CofG is compared with the stored OEM ideal mathematical CofG, both Span and Chord. The error is calculated and adjustment to the span CofG tip weights provided.
7. Load The Blade: The blade is loaded on to the load beam by using the portable Dolly (mini fork lift) provided. The Root Referencing pin is inserted before the blade is loaded to assist guiding the blade into opposition.
Loading a CH47 Blade
UH60 Blade, USBF & Dolly
As332 Root Reference Pin
8. Set the Root Ref Pin: The root of the blade is referenced using the Blade root reference pin provided (special pin for each blade type that the computer is configured for). Fine adjustment of this Pin accurately tells the computer where the root is and provides an accurate measurement of the span to enable calculation of the span mass distribution.
S70/UH60 Root Reference Pin
CH47 Root Reference Pin
Leading Edge Ref Pins
9. Index The Blade Leading Edge: Then the leading edge is referenced by manually pushing the leading edge reference pins. This tells the computer where the leading edge is located in reference to the load beam. It allows the computer to accurately calculate the distribution of blade mass. It enables the accurate chord-wise calculation of mass distribution.
10. Measure & Adjustment: When the computer is instructed, the mass is measured on all 3 x load cells. The computer then calculates the total mass and its span and chord distribution, It uses a similar three load cell measuring technique as an electronic aircraft weighing system. But much more sensitive and far more accurate. The system is corrected for local gravity effects to ensure the highest degree and consistency of accuracy
It will also provide an adjustment by part number to be added or subtracted from the tip weight packages if required. Each blade type has its own ideal design standard span moment arm. For example, the UH60 ideal Span spec is 35,418inchlb. The UH1 …28,886inchlbs, the CH47…65,369inchlbs etc …
The USBF computer’s digital solution provides two different bands of adjustment tolerances. One is called “Field Requirements’. The other is called “Depot Tolerance”.
The Field requirements are a broader band of tolerance applied to the designed ideal Span Moment Arm. This is designed to be applied for field deployments where quick, but accurate adjustments are required. The “Depot Tolerance” is a much tighter tolerance designed to be applied if time permits and/or to be applied in a Depot level maintenance facility where much tighter tolerance is demanded to ensure a blade has maximum life potential in service.
This is what the traditional physical master blade has tried to replicate. Unfortunately the population of physical master blades around the world have quite a wide range of variation between their individual span moment arms themselves.
This leads to a very large range of Span Moment Arm between blades coming from different Physical Master blades of the same blade type spread around the world.
This phenomena results in freshly overhauled blades and new blades often having difficulty in being dynamically balanced on the same helicopter, ie the “rogue” rotor blade is born. This is one of the causes that is added to the mix of “Rogue rotor Blades”. Again, by eliminating physical master blades, we reduce yet another potential cause for rogue rotor blades.
Hence, once again the USBF is such an important tool to eliminate rogue rotor blades though the entire helicopter fleet.
The adjustment is made and the blade re-weighed to confirm the blade is now within acceptable static balance tolerance.
Example USBF Solution printout
By performing this tip weight adjustment, the Static Moment Arm of the blade has been now returned within the original design specifications, identical to a new blade. This now frees up the FULL dynamic balancing authority so any dynamic adjustments that may be require due to worn bearings, components, misalignment, shimming, etc.
CH47 Tip Weight Adjustments
AS332 Tip Weight adjustments
Current Military types for USBF:
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Capable of balancing any Blade.
The USBF can balance any of the blades listed above. These have already have approval.
The USBF can balance ANY main rotor blade type or model. But if the blade type is not listed above, OEM permission would need to be obtained before the USBF could be used for the procedure. But the tool would be capable of doing it.
For further information please download the Free description.
Virtual Master Blade
Virtual Master Blade – For Tail Rotor Blades and Tip Caps
The Virtual Master Blade (VMB) is in effect a mini USBF. Its name describes perfectly what it (and the USBF) does. It is a virtual Master Blade.… for tail rotors & Main Rotor Tip caps.
It uses three load cells to determine blade mass and the distribution of that mass. Exactly the same as the USBF. It then compares it to the selected blade type which then compares the result to the stored engineering data for the ideal blade for that type/model in its software as a digital map.
It then provides the precise part number and amount of tip weight to remove or add to return the blade span-wise mass distribution back to within the original manufacturers limits.
Something previously that could only be done by using ridiculously expensive, physical master blades and limited to strict, air-conditioned and environmentally controlled rooms occupying large parts of workshops and severely limiting who could actually adjust these rotor blades.
The Virtual Master Blade (VMB) is a bench top unit which is approved for use on:
1. UH60/S70 BlackHawk Tail rotor blade:
UH60 Tail Rotor Paddle on VMB
UH60 Tail Rotor Paddle on VMB
2. UH60/S70 BlackHawk Main Rotor tip caps:
UH60 Main Rotor Tip Cap on VMB
UH60 Main Rotor Tip Cap on VMB
3. AH64 Apache Tail Rotor blades:
AH64 Tail Rotor Blade on VMB
4. Virtual Master Blade (VMB) Tool:
Virtual Master Blade Tool
Virtual Master Blade – Table of Tail Rotors Approved for Use.
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Capable of balancing any Blade.
The VMB can balance any of the blades listed above. These have already have approval.
The VMB can balance ANY Tail Rotor blade type or model. But if the blade type is not listed above, OEM permission would need to be obtained before the VMB could be used for the procedure. But the tool would be capable of doing it.
