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Tailboom strakes reduce stress in Hueys and their pilots

Source: NAVAIR

13 March 2003



© Boundary Layer Research

NAVAIR PATUXENT RIVER, MD - Faced with increasing fatigue cracking in the tail booms of aging UH-1N Huey helicopters, H-1 program engineers here recently applied a commercially available aerodynamic device to help prolong the helicopter’s life as well as reduce pilot work load.


The device, a strake kit, developed by Boundary Layer Research, Inc of Everett Wash., consists primarily of an aerodynamically shaped strip of metal attached to left side of the Huey tail boom, according to Bob Moore, senior systems analyst with PMA-276.


“We were looking for something to fix our fatigue problems in the Huey tail booms in the short term,” Moore said. “We’ve got tail boom fixtures and other fixes in place, but we needed to reduce the stresses on the tail boom that cause the fatigue. We found this and saw it as something that would reduce the fatigue and help get us to our long-term fix for the tail boom.”


NASA, according to Moore, initially developed the technology. The technology was then bought by BLR and marketed to civilian single-rotor helicopter operators.


By manipulating boundary layer airflow produced by the helicopter’s main rotor as it passes over the tail boom, the strake kit improves the helicopter’s hover stability, climb and cruise characteristics, according to Dave George, spokesman for BLR.


Originally, the problem NASA discovered was that rotor wash passing over the tail boom of single rotor helicopters, like the Marine Corps’ UH-1N Huey, creates a fluctuating low pressure area along the advancing blade side of the tail boom. This results in a strong “suction” (or lift) force opposing the tail rotor thrust, thus requiring more power to the tail rotor to be effective.


With these increased opposing forces at work, affected helicopters experience increased component wear, greater fuel consumption, structural and performance degradation, increased pilot workload and a restricted flight envelope, according to BLR engineers.


And with older helicopters, the problems are even more manifest.


“We’ve been having consistent fatigue cracks,” Moore explained. “Between the two types of aircraft we support, the UH-1N for the Marines and the HH-1N for the Navy, we see a lot of hard flying. The UH-1N operates in the utility mode with lots of maneuvering, landings, etc. The HH-1N operates at higher altitudes, high winds and high gross weight.


“Both of them see a lot of stress,” he added. “And both of them are old.”


An aerodynamic process, called vortex shedding, is the culprit, according to BLR engineers.


The rotor downwash only flows smoothly down the sides of the tail boom for a short distance before curling into a vortex. The vortex gets bigger as it flows down until it is too big to “hide” under the tail boom and gets swept away. Meanwhile, the next vortex forms on the opposite side of the tail boom and the process repeats.


This alternating vortex shedding causes the tail boom to wag back and forth, and the associated vibration and oscillation causes increased fatigue and stress in both airframes and pilots trying to counteract the problem.


Enter the strakes.


The NASA research indicated that the strakes, when installed, would eliminate the problem by dramatically decreasing the vortexes and their effects by smoothing the airflow around the tail boom and eliminating the alternating sideways lift.


Moore, and his H-1 program colleagues who support fielded UH-1N’s and HH-1N’s, found out about the NASA research and the BLR strake kits and with the engineering support of Maj. Rick Bowen, the PMA-276 deputy program manager for UH/HH-1N Huey, they were soon testing a kit on a Huey assigned to Rotary Wing Test Squadron 21 here.


The kit, as applied, consisted of two strakes, each of which was formed from three 42-inch segments attached to the left side of the tail boom.


Rotary Wing’s Maj. Pat Lindauer evaluated the strake kit installation in 10 flight hours flown over the course of nine flights Feb. 5-14. His maneuvers included turns, approaches and departures, hovers, level, climbing and descending flight, and simulated engine failures.


“I was a little skeptical we would see all of the benefits [the BLR and NASA representatives] said we would see (lower power required to hover, lower tail boom loads, better pedal margins, more stable hover in yaw axis, etc.),” Lindauer said. “Because we did just a quick look, we were unable to measure some of the benefits and others were within the error of the standard aircraft instrumentation. However, the stab bar aircraft we did the test on was noticeably more stable in the yaw axis.”


The end result for the Fleet?


“I think the biggest benefits the fleet will see is improved flying qualities of the stab bar Hueys.”


Pilots at Marine Corps Air Station New River, NC have already agreed.


“The first time I picked up, I was over controlling the aircraft, but then it smoothed out,” reported Maj. James Judkins, maintenance officer for HML/A-167 at New River, and evaluator of the strake kit, after a recent flight in a strake-equipped UH-1N Huey. “It almost felt like I was flying in [a helicopter equipped with an automatic flight control system]. Hover performance seemed improved dramatically, and flying straight and level seemed to be easier than normal.


“I truly believe this modification is one the entire fleet should desire to get,” he added. “It’s an outstanding modification to the overall aircraft.”


With the initial indicators so good, the strake kits are on their way to the Fleet, according to Tom Boswell, program management support for the Huey deputy program manager.


“It follows that if you put this thing on the aircraft,” Boswell said, “and you need less tail rotor authority, it must mean you have less stress. We’re going to do a lot of qualitative testing to make absolutely sure we’re reducing fatigue and how much we’re reducing it by. In the meantime, we’ll have a formal Engineering Change Proposal approved within 180 days and then a Technical Directive will be released to the Fleet to authorize installation.”


Testing will also take place here to determine if the strakes will yield the same benefits when installed on an AH-1W SuperCobra.



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We have a few in the fleet, but I have not flown any of them. The guys who have Flown them are unconvinced. They say that it may help in some situations. There''s such a difference between aircraft that it''s really hard to tell if the Strake kit makes a difference or if one old gal lifts a bit more than another old gal. Maybe V-Ref has some serious strake time?

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yea the jury is still out on this one, some say it makes a **** of a difference, others not. the more noticable improvement is on the small chord t/r machines. i''ll stick with the general consensus of my fellow alpine buddies and say the aircraft is now just 15 pounds heavier  i can take it or leave it. no matter to me, i can''t hoover anywayz!!!


i am looking forward to the vertical fin mod where they are going to remove the skin aft of the fin spar and just have the ribs showing...now that will make a BIG difference in t/r performance and reduction in all up shp going to the back end...bring on dem bigger lags boyz.

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For what its worth I spoke to a few operators at Heli Expo about the strakes and the general feedback was that the strakes had limited use, and it all depended on what you were using the helo for.


On that note there was a fantastic looking 212 with every extra you could think of at the show. CTD might have seen it and could enlighten us all as to all the bells and whistles that it had. Have to admit it had the best looking paint job of anything there.


Heli Ops

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Sorry, I didn''t notice.


Funny, I didn''t look inside one helicopter while I was there, not even the Royal Saudi 412 that I was project pilot on in my former life, but had never seen completed. Maybe I should take the hint?


Have heard mixed reviews on the strake - I suspect they do add some margin of performance, but it would probably be difficult to notice unless you knew the same machine intimately before and after the mod.

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Talked to driver for Advantage 204 same a/c same job .

has about 400 hours strake time. He noted that the best

noted difference is amount of pedal margin. On the 204 with -13 eng and increaded alt STC this would be great. BLR has several test reports before and after types . The reports show a large reduction in fin spar loads due to less pedal input. Less pedal input less wear and tear on the boom. Sounds good to me.

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