Originally Posted by
megan
The utilisation of the lift phenomena can show up in unexpected places. There is not a helicopter pilot in the world who doesn't under stand what the phrase "running out of pedal" means. For the incognisant it is when the pilot has demanded a level of power from the main rotor that the tail rotor is unable to counter, so the helo begins an uncontrollable yaw. The pilots of UH-1C gunships flown in Vietnam didn't have the power to sustain a prolonged hover when loaded for bear. To get from the revetment to the runway involved a series of jumps. Pilot would pull into the hover and start hover taxiing, the rotor would begin to bleed RPM, when reaching the point that you ran out of pedal, chop the throttle, thus regaining pedal control, perform a hovering autorotation, wind throttle back on after landing, rinse and repeat. On reaching the runway a running take off would be made, at times the two back seat crew would run along side to ease the load and jump back on board when the aircraft reached translational lift. An enterprising individual some where realised that the rotor wash passing over the tail boom generated lift in a horizontal direction and could be tailored to aid the tail rotor. Strakes fitted to the side of the tail boom trip the airflow into turbulence, effectively stalling that side of the tail boom. Now standard fit on some types.
PS: Just after we left the UH-1C was upgraded to UH-1M standard by simply replacing T53-11 engine with the T53-13, bliss, you could hover.
Good stuff Megan.
While I was doing the slat trick on the B737, I got asked to help a UH-1H/T700 that was doing some interesting work over water, at low speed, and with one very large mass appended at the end of the sling. The guys were having some excitement when operating below ETL, at high power, lots of barf bags needed, and expensive splashes were issues... lots of torque, same ol' TR system... I had previously done some testing on a B206BIII and an OH58 with different blades... and they had been able to come and see what we were doing on wings and props. The UH1 with the big donk gets lots of available torque, but the TR is standard. We placed a mod on the MRB LE and that worked well, but they wanted as much TR authority as could be gained. While i have modded TRBs before, I expected that they would get enough out of 2 foam rubber strips on the port side of the boom, at the 7 and 10 positions, and a couple of rows of foam rubber VGs on the RHS. The 3 remaining options, tabbing the MRB, putting CVGs on the TRB and tabbing the TRB were not needed.
The trick with tabbing a propeller, rotor or fan blade is doing that and not killing yourself at the same time. Most of the outcome is benign, remarkably so, but I recall being taught that adding mass to a TE could define bad days, and having half a dozen torsion and bending modes happening seemed like a bit of an issue to put anything in there that would alter the moment of inertia of the area with some of the highest strains... seemed like a good way to cause a super quick failure of the skins, or to cause the bonding to fail instantly, or to have the tab blow apart, or all 3 options. Upside was that fluids don't care what a device is made of, just if it will cause the desired flow modification, and for most tabs, that's a set of 3 transverse vortices and a couple of stagnation points. 30+ years on props gave some confidence on the rotor to try that out, and golly, if it isn't quite an interesting outcome.
While rotors suck as far as dynamic stall effects go, they really do, they are also rather sensitive to the addition of any disruption to the boundary layer, which just doesn't seem fair; there is no silver lining. Nevertheless, things can be improved, although lordy, the Enstrom and its 0015 section is darn hard to make happier in the hover without getting serious. Still, a rotor head that permits you to take your hands off the cyclic and fold them has some drawcards. Don't try that on an R-22...
anyway,
for some background, for a certain Brand X rotor, a vanilla 63105 club, the following are recorded data outcomes... It is consistent with the 2 D RANS CFD analysis done well before and published in AIAA J. Aircraft, and is also consistent with an EN done in early '90s (IIRC) looking at the BO-105 MR in autorotation, and certain modifications to same; using Wayne Johnsons CAMRAD II code.
Putting a tape on the LE increases the stall Nr of the blade, and takes more ergs to hover. Representative stall clean at 82%, 68 units of torque, add tape to the last 3 ft of the blade, and the stall Nr goes to 85%, and torque to 72u.
Place a conformal VG tape (a weird variant of a zig-zag tape although doing a different resultant flow...) on the same location, and the stall drops to 77%, and torque goes to 65u... add 3 feet of tab mid-span, and the stall Ne goes to 68%, and (seriously) the torque drops to... 34u. that is thirty-four, relative to a NACA clean section, 68... Look closely at the effect of an LET and it can be seen that the outcome is geometry dependent, a matter that caused major delays with development of LETs anywhere on planes, but in the right spot, it is possible to both get massive changes to drag at the same time as lift gets a large boost, of around 0.4 to 0.6 CL. Works on props, works on rotors, and it worked on the static test of the fan blade. LETs can impact the trim condition for auto rotation, and that was, without doubt, the most concerning flight that I ever flew, and that did include a parachute and a means to remove blades and not heads from the helo.... In comparison, playing with props and fans is much less exciting.
To prove the point on the effectiveness of the LET being actual, and not observer bias apart from the instrumentation of the performance and engine instruments, and straining the rotor root and pitch links, the flight controls were measured and recorded as well s displayed in real-time. The "standard" NACA clean rotor gets to stall Nr about the time the pedal hits the stop, For the conventional HS8663/8671 linear blade tape, a stall occurs as LTE has commenced, gets interesting. For the conformal VG tape, around 25% pedal remained at stall. for the tabbed blade, the pedal required was less than 50%, and turns in the anti-torque direct had essentially no constraints, other than ear plugs for the blaring underspeed (underspend?) warnings. From idle, the collective could be raised fully up, the throttle increased, and the rotor would accelerate. (don't do that at home, certainly not with anything that uses TT straps...) increasing the Nr to 68% resulted in liftoff, hover and some weird and uncomfortable flight. Rotors should be at normal Nr at all times, but from a standpoint of weird science, not sure that any blade at full collective will accelerate from idle without a tab. It suggested that the reduction in torque demands with a tabbed blade are quite real. They are repeatable and are consistent with published works with AIAA and on NASA NTRS. Just happens that tabs can be done.
The V-22 and the MH-47 are the poster children of systems in need of a bit of tabbing... look at the video of the U.S.S. Green Bay bingle... But, as much fun as rotors are, the most interesting application, other than ships props, C-130 props etc, are fan blades... I bought a jet just to do that, and circumstances intervened, but darn if that isn't more interesting than the effect on a rotor.
Just to be clear, fan blades are fancy fixed pitch props, they are a little more, once the overall effect of intake geometry has occurred. Thereafter, the TSFC collapses as TAS changes. The TSFC for a pure jet more or less is constant, nearly so, whereas the TSFC of the fanjet degrades by a factor of at least 2 to M0.8/FL350 from SL/static. That is where the parametric effects of tabs gets interesting. If anyone has a spare CFM56, TF-30, JT9, RB211, GE90... (or TFE731... mine are spoken for which is a necessary nuisance)
End of the day gotta go stop a large cat from snoring. Maine coons are an acquired taste
Aerodynamic cat plan form:
