Gaining an Edge
Rotor blades operate in a pretty tough environment. Whirling around at considerable speeds, they have to deal with dust, sand, rain and lots of other things that want to wear them down.
This is particularly true of a blade’s leading edge, and even when metals like titanium are used to protect that leading edge, it’s bound to wear out. Plus, replacing the leading edge is neither easy nor cheap, even if the disruption caused by having to take blades off and ship them away is ignored.
One solution to this problem is the Hontek sprayable erosion-protection coating system offered by kaman. In recent years, this coating has been applied to more than 2,500 Sikorsky uH-60 Black Hawk rotor blades operating in desert environments like Iraq and afghanistan (see p.9, Vertical 911, AAAA 2012).
A more common solution is the sacrificial, replaceable blade tape. unfortunately, while such tapes do protect the blade, many of these have the hidden side effect of reducing performance — by a measurable amount. the cause of this performance loss has been determined to be the trailing edge of the tape: even at one millimeter of thickness, the abrupt step from the tape to the normal blade surface generates a small vortex that rolls down the blade, disrupting the airflow and increasing drag. Since the engine has to overcome drag, this small increase results in reduced hover performance.
Another disadvantage of these tapes is that they normally have to be replaced along their entire length when they wear out, and this is not easy to do. I recall having to remove wornout blade tape on a wet and cold day in england many years ago, after the rain had finally eroded the tape and the resulting vibration from the disrupted airflow had become quite unbearable. Fortunately, a pocketknife was available to cut it off, but both blades had to be cleaned afterwards, and that’s not easy to do with no stepladder!
The overall situation with blade tape — the trade-off between protecting the leading edge and the associated performance loss and maintenance issues — has stayed like this for a long time; until recently that is. What follows is the remarkable story of a man who took his very detailed knowledge of aerodynamics and combined it with technology to produce a blade tape concept that not only protects main rotor blades, but even improves their performance over unprotected blades.
An Idea With Legs
Over the years, I’ve been approached (and still get approached) by a lot of people with aviation ideas; some have potential, but most are merely novel thoughts with little practical application. Normally, asking a few detailed questions quickly reveals whether there’s any substance behind the concept, and whether the proposer has any idea of the complexities facing him or her. So, when I was first approached by Peter Ireland about his concept, I was skeptical… but Ireland’s answers to my increasingly difficult and detailed questions convinced me that his ideas were worth listening to.
In fact, Ireland — an airplane and helicopter pilot who has flown for, among others, the royal New Zealand air Force, The Royal Australian Air Force, Qantas and Korean Air — had already obtained a patent for his concept, which in itself was a good indicator that he was not some aviation version of the wild-eyed Dr. Emmett Brown from Back to the Future. then, he started talking about vortex generators on blade tape, and that really got my attention.
At this point, I must confess that I know more than most about vortex generators (which are protrusions that delay flow separation and stalling on an aerodynamic surface, thus improving its effectiveness). In fact, I have a company that produces a special type of vortex generator to reduce aerodynamic drag on trucks (which is a whole other story in itself).
Curious about Ireland’s concept, I started asking more questions about the tape design. He, in turn, told me about the data he had collected on his own Robinson R22 helicopter.
Now, I would expect any helicopter pilot who knew what he or she was doing to be able to measure a change in something like manifold pressure via the cockpit instruments, but Ireland also talked about fuel flow. a fuel flow gauge is not an instrument normally fitted to an r22, so his mention of this measure was intriguing. He then went on to talk about differences in collective position, and at first I thought he might have used something rudimentary like a tape measure for his data, but no, Ireland was talking about percentage of collective position. Here, I thought, was someone who probably knew what he was talking about, if he was measuring all of these variables to this degree of fineness.
When Ireland began to talk about tail rotor pedal position and cyclic positions, I knew that he was doing things correctly — very correctly. (In fact, to later obtain the data he needed to get a supplemental type certificate, Ireland had to instrument the blades on his r22 to measure stress and strain. His novel way of doing so without a slip ring would take too long to write about, but, suffice it to say, it was little short of brilliant engineering!)
Then came the data comparisons. Ireland said that performance in the hover with the traditional blade tape installed was four to five percent worse compared to clean blades. (It is interesting to note that such performance degradation is never mentioned in any flight manual supplement.) When he installed his BladeGuard “conformal vortex generator erosion protection system,” however, the hover performance was around nine percent better than with clean blades, by all measures: at the same weight and height above the ground, his helicopter exhibited lower manifold pressure, lower fuel flow (which is an indirect measure of power) and lower collective position. For simulated hovering engine failures, the time from failure to touching the ground was increased. also, he found rotor r.p.m. took longer to decay after shutting down the engine at flat pitch. In forward flight, meanwhile, tested at the same cruise speed and density altitude, the helicopter again required less manifold pressure, less fuel flow and a lower collective position. Finally, rate of descent in autorotation was lower than with the clean blades. In other words, all of the data was pointing in the same direction. and, when Ireland explained the physics of how his new blade tape worked, the positive results all made a lot of sense to me.
The Right Kind of Vortex
The major difference in Ireland’s design — and the object of its patent protection — is the trailing edge of the tape. that edge is cut into a vortex generator pattern (although there’s more to the patent than just that).
Of course, vortex generators have been around for a long time, and you’ll see them on the wings of most airliners. In that application, they are designed to throw air up into the boundary layer. they create some drag, but for a large airliner wing, they work in the low-speed conditions that they’re supposed to, while having a minor performance penalty the rest of the time. You don’t see these same types of vortex generators on helicopter rotor blades, though, as the aerodynamic conditions in which rotor blades operate differ from those for seized-wing aircraft.
What makes Ireland’s tape unique is that its vortex generators are conformal to the leading edge of the blade, and rather than projecting air up, they suck the air back to the surface of the blade, behind the leading edge. So, instead of creating vortices that roll down the blade — as the performance-degrading blade tapes do — these vortex generators produce a series of vortices that spin at 90 degrees to the airflow and bring the air back down onto the surface, improving lift. (this effect can be clearly seen from the patterns left when Ireland’s helicopter was hovered in a dusty environment.) By delaying flow separation, the vortex generators increase the effectiveness of the rotor blades, accounting for the performance increases Ireland has measured.
Ireland’s tape also offers advantages from a maintenance perspective.
As previously mentioned, most blade tapes must be replaced over their entire lengths, even when — as is often the case — only the very outboard portion is worn. They also lack reliable indications of how much life they have remaining, so they are often replaced only when they are actually worn through (as evidenced in my own example stated earlier). Putting tape on can also be a pain, as any bubbles that get trapped under the surface can upset the airflow and provide a point for the tape to start lifting, especially because of rain.
To solve these problems, Ireland incorporated numerous small slits in his tape that permit any bubbles to be easily pressed toward a slit and dispersed. Alignment marks on the tape, meanwhile, make it easy to align, and users are provided with information on how to recognize when it’s time to replace the tape. to top it all off, the tape comes in short lengths, so only the worn pieces have to be replaced. Numerous trial installations have shown all these features to be effective. Ireland has been able to obtain worldwide patents for his concept, and has started a company called Edge Aerodynamix to market the resulting products. He has also already obtained an StC in his home country of Australia for his BladeGuard blade tape on the R22; the product has the added bonus of being an approved “alternative means of compliance” there for the current airworthiness directive that addresses blade delamination in r22 main rotor blades (see p.40, Vertical, AugSept 2011). Ireland will be pursuing an StC for the r44 next, followed by StCs for other popular helicopters (preliminary tests on a variety of other machines have shown benefits similar to those for the r22). Lastly, Ireland told me that a kit for tail rotor blades is under development, and that early tests have shown significant performance improvements there, too. As for his products’ planned availability in other countries, since StCs from australia are normally quickly accepted by most other countries, Ireland is hoping there should be no need for additional certification work once each australian StC is approved. The only thing to note with Ireland’s BladeGuard, however, is that no flight manual performance benefit is being claimed. To do so would take a lot of flight-testing, and that would only increase the price of the product and its time to enter the market. So, the performance benefits I cite here are not official numbers. even so, it’s nice to see someone take a fresh look at an old problem, and find a creative — and effective — solution.