AH-1Z First Flight Anniversary
Congratulations are in order to the men and women of the United States Marine Corps who fly and maintain the H-1 Upgrade aircraft.
AH-1Z's 07 December 2019, 19th anniversary of first flight: 6939 days, 1 accident, 2 fatalities (30 March 2019, Yuma Az.)
H-1 Upgrades Rotor Accomplishments Recorded from Otter's Rotor Structural Analysis and Flight Test Experience
22 November 2019
1. Identical (From Y’s to Z’s) Main & Tail Rotor Part Numbers on Rotor Components.
1st qualification of same part numbered main and tail rotors on 2 (two) different (Y’s & Z’s) airframes for a very discriminating Marine Corps customer. This fact contributes to the 84% common part numbers between the 2 (two) different airframes of Yankee’s vs. Zulu’s.
2. Main Rotor Components with 10,000 hour Fatigue Lives.
1st main / tail rotor qualified with 85% or greater of the rotor components having 10,000 hours fatigue lives or greater.
3. Bearingless Main / Tail Rotors.
1st viable (33 year endurance, relatively low life-cycle cost) BEARINGLESS main / tail rotors qualified, contributing to the total H-1 Upgrade MARINIZED design features. This fact allows for no requirement to lubricate main rotor components above the main swashplate duplex bearing and resulting in a main rotor parts count reduction of 75% below current articulated four bladed main rotor designs.
4. Marinized Attack and Utility Helicopters by Design
1st marinized helicopter rotor design with features incorporated at design initiation, November 1996. This effort directed at minimizing negative effects of corrosion through use of composites, corrosion resistant steels, appropriately protected aluminum, and employing corrosion protective sealants on fasteners. Also includes shipboard tie-down points, Main rotor brakes for quick rotor shutdown, blade folding for below deck stowage and EMI protected electric cables for resistance to powerful shipboard RADAR interference with weapon systems.
5. Eliminate Transmission Vibration Isolation
1st Bell designed helicopter with all main transmission vibration isolation eliminated and no frahm-dampers in the main rotor head. This contributes to the massive M/R parts count reduction. Previously Bell patented ‘Noda-Magic’, ‘Nodal Beams’, or ‘Focused Pylon Mounting’ systems are eliminated. Utility Transport UH-1Y ‘Yankee’ receives regular duty assignment from USMC as a Sniper Helicopter, a mission profile that requires a smooth and stable firing platform.
6. Eliminate Hydraulically Boosted Main Rotor Control Levers
1st Bell designed Main Rotor Control System eliminating all levers and control tubes between the main rotor boost actuators and the main rotor non-rotating swashplate. All three (3) actuator outputs are directly connected to receiving lugs on the lower surface of the non-rotating swashplate. Thus all mechanical mixing of controls is replaced by electronic signal mixing to the boost actuators.
7. Feathering and Flapping Flexure’s fatigue limit good for Hot–Wet environmental reductions
1st Bell Military Production Bearingless Feathering Flexure that is good for a required +8 +/-17 degrees of steady state feathering. This is a result of:
A. The new feathering flexure basic radial flange section.
B. The basic section feathering flexure length with blade attach located at 25% rotor radius.
C. Pre-twisting the main rotor cuff 5 (five) degrees to reduce the 13 (thirteen) degrees of steady feathering required down to 8 (eight) degrees.
The feathering and flapping flexure’s fatigue margins accommodate a required 21% reduction of the Room-Temperature-Dry fatigue allowables for Hot – Wet environmental conditions.
8. Main Rotor In-Plane Dampers
1st Bell Helicopter 4 (four) bladed main rotor dampers are made from 3 (three) materials functioning as dampers. Conventional external elastomerics with steel shims, internal proprietary liquid (good for operating temperatures from 125° F to -65° F) and a compressed gas in the internal cavity. This combination addresses large displacement – high energy damping associated with the first-in-plane natural frequency displacement damping, and low displacement – relatively low energy damping associated with ‘limit’ cycling oscillatory displacements. The dampers contribute to item 5 stability.
9. No Prototypes or Remaining Test Aircraft.
1st new clean-sheet rotor design helicopter flight test qualification effort with 0 (zero) prototypes being used and 0 (zero) test aircraft remaining after post qualification testing. 4 (four) out of 5 (five) test aircraft were maintained in production configuration and DD-250 delivered to Marine Corps Customer as production aircraft. Upon completion of the Flight Test Program, March 2006, the four DD-250 aircraft departed PAX early morning, formed up and flew cross-country to MCAS Camp Pendleton, Ca. with just fuel stops and one overnight stay. The 5th aircraft (Z-1), with older steam gage (vs. digital) instrument panel, was used as ballistics test article and provided the required ballistic tolerance test data, generated at China Lake, California.
10. H-1 Upgrades Flight Test Location and Personnel.
1st Bell Helicopter, U.S. Military complete qualification flight test effort to take place 1500 miles away from Bell’s Flight Test Center located in Arlington, Texas. Marine Corps Customer had awarded Flight Test Program to low bidder DynCorp, after which, during preliminary flight test setup, it was concluded by the Marine Corps Customer, that the expertise DynCorp had claimed in the Flight Testing Field was non-existent. Marine Corps subsequently informed Bell Helicopter that if Bell wanted to complete the sale of H-1 Upgrade helicopters (Y’s and Z’s) to the Marine Corps, Bell was going to have to help DynCorp conduct the flight test qualification. Bell quickly assembled a ‘skeleton’ flight test team consisting mainly of the original Design Team (due to reluctance of existing flight test personnel to relocate to PAX River for four (4) years). Two (2) very qualified and helpful Bell Helicopter flight test engineers, Mr. Mark Robinson and Mr. Eric Gibson did indeed relocate from Texas to Maryland for the flight test qualification effort. Thus Bell’s skeleton flight test team set out to qualify the Marine Corps’ 1st bearingless rotor at a test site 1500 miles to the East while simultaneously instructing and teaching DynCorp how to conduct a flight test program. This particular flight test program ultimately included four (4) unique modification programs to the airframes requiring fixture jigs to be transported to PAX from Texas and laser leveled and secured for close tolerance airframe modifications. The transport of equipment and/or components to/from Texas was a two (2) day trip one way. While dealing with the unions at Bell Fort Worth it was necessary to overcome union resistance (adding to schedule delays) at Bell due to non-union work being conducted at PAX River, Md.
11. Deep State’s (Two Bladed, Articulated M/R, Focused Pylon Attach Patent Holders) Insurance Policy.
And if items 1 thru 10 and the new issues being resolved weren’t going to be difficult enough to overcome, some additional complicating factors were introduced just as flight test was beginning and of course the rotor design had been fatigue tested and frozen. Bell management, including the Fatigue Group management, introduced 4 (four) significant design criteria changes as follows: (all introduced in the name of helping the predicted weak link main rotor yoke flapping flexure survive the demanding flight spectrum, yet all (100%) of the already introduced design criteria changes were going to make the fiberglass main rotor yoke flapping flexure work harder as detailed below.)
A. Increase G.W. from 17,500 lbs. to 18,500 lbs. (5.7% increase and will generate approximately 3% more flapping displacement with 3% higher oscillatory beam shear loads; measured, with the predominate failure of the flapping flexure being mid-plane shear delamination).
B. Change the static elevator to fly-by-wire elevator to minimize airframe drag at Vh. (Will increase oscillatory beam shears by approximately 3%; measured).
C. Change the main rotor mast attachment clamp plate on the upper surface of the yokes from circular to racetrack. (Increases the oscillatory beam shears by 3%; measured).
D. Increase the main rotor RPM 3% from 287 to 296. This will effectively stiffen the flapping flexure and will result in increased oscillatory beam shears due to flapping being a displacement driven parameter with the beam shears resulting from the displacement. (Increases the oscillatory beam shears by 3%; measured).
This should theoretically have destroyed the originally designed, marginally acceptable, flapping flexure of the fiberglass main rotor yoke. But (Thank Goodness) toward the end of fatigue testing and prior to fabrication of the flight test article main rotor yokes, rotor structures engineers made a physical change to the flapping flexure that contributed to an approximate 20% improvement in fatigue capability. 20% is a huge number in the Fatigue world with improvements of 2 or 3% being a ‘Big Deal’. So the flapping flexure has been able to handle all the loading it has seen so far and should continue to do so for a long time.
Ernest Powell
NASCC
22 November 2019
Merry Christmas and Happy New Year OOH - RAH!