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Sunday, July 1, 2007

Ask Ray Prouty: Tail-Rotor Vortex Ring State

Ray Prouty

READER JEFF FOZARD RECOUNTS uncommanded right turns at low speed ("Loss of Tail-Rotor Effectiveness," May 2007, page 9). The cause was probably a tail rotor in vortex ring state, a condition we normally associate with the main rotor.

When a smoker blows a smoke ring, it propels itself with a constant velocity proportional to the strength of its "circulation." A hovering rotor makes its own rings (tip vortices) that propel themselves down in the same way. Their circulation strength can be related to disc loading.

When the pilot descends vertically by reducing collective and using partial power, his descent speed can approach the vortices’s self-produced velocity. This happens at about 800 fpm for the Robinson Helicopter R22, with a disc loading of 2.5 psf, and about 2,000 fpm for the Sikorsky Aircraft CH-53E (disc loading=14).

Since they are descending at the same rates, the vortices and helicopter get entangled. Erratic angles of attack at the blade elements result, causing lift changes and buffeting.

More importantly, average thrust decreases 20-30 percent. The thrust loss is not due to high angles of attack (which might cause stall), but low angles of attack caused by vortices in the rotor’s plane that create downflow through the rotor disc. They induce surrounding air to be re-ingested into the disc.

This is an unstable situation. If the descent rate increases, thrust falls off even more and the helicopter descends even faster. Increasing collective is ineffective; the tip vortices get even stronger, increasing the downflow and down you go!

The cure, of course, is getting forward speed to leave the vortices’s influence behind. You need a descent angle of about 30 deg. to fully succeed. Don’t do this and you continue down vertically, soon outrun the vortices, and enter a stable region leading to vertical autorotation — at about 2,800 fpm for the R22 and a hair-raising 6,500 for the CH-53E.

It should come as no surprise that a tail rotor can also get into vortex ring state. Depending on its disc loading, the critical velocity is 15-30 kt as generated in a right hover turn or by left sideward flight (on American helicopters, of course).

The U.S. Army’s desire to hover over a spot in a 35-kt wind and maintain good control while pointing in any direction produced great interest in this phenomenon, with some surprising results.

A model helicopter mounted on a turntable in a low-speed wind tunnel permitted simulation of flight at all wind azimuths. The tail rotor was mounted on a separate support to investigate its performance at several positions with respect to the main rotor. Mounted far behind the main rotor, it suffered a thrust loss in left sideward flight due to the vortex ring state. But a position close to the main rotor produced no thrust loss — for reasons I do not understand.

It appears the primary benefit of the main rotor proximity occurs when the tail rotor rotates so the bottom blade is going forward. The tail rotor on Lockheed’s AH-56 Cheyenne compound helicopter rotated the other way. It was impossible to maintain steady left sideward flight at 15 – 20 kt. This was cured dramatically when a gearbox redesign reversed the direction of rotation — for reasons I do not understand.

You can see evidence of similar fixes on other helicopters. Bell Helicopter mounted the tail rotors on the left side of the single-engine UH-1 and AH-1; they turn the "wrong" way. This led to some piloting problems in low-speed maneuvers. Bell fixed this on the twin-engine versions by switching the tail rotor to the other side, reversing its direction of rotation. (You can see the same fix on the Mil Hind. Compare photos of the prototype with the production version.) New H-1s Bell is developing for the U.S. Marine Corps have tail rotors again on the left, which is aerodynamically the best position. A gearbox change has them turn in the correct direction.

A number of helicopters have the "wrong" tail-rotor rotation, including the Hughes OH-6 and its descendants. Apparently, there were not enough complaints like Mr. Fozard’s to justify a redesign. The R22 has the same. Frank has told me he saved a few ounces and moved the center of gravity ahead a bit with this choice. The R44 has the correct rotation.

When the Hughes Apache was designed, all this was known. The tail rotor spun in the correct way and just a few inches separated the rotors. Did that preclude problems? No!

The Apache was tested by flying in formation with a pace car. In left sideward flight at about 15 kt, it became increasingly difficult to maintain heading. Pedal activity was nearly stop to stop as the test pilot tried to compensate for erratic tail-rotor thrust. A recent Army veteran current in the Cobra (the chase aircraft), he got into that, with its wrong-way tail rotor, to repeat the tests and just sailed through-for reasons I do not understand.

This Apache prototype had its horizontal stabilator atop of the vertical tail. This "T" proved aerodynamically and dynamically unsatisfactory; the configuration was changed to that of the Sikorsky Black Hawk. To properly clear the stabilator, the tail rotor had to be raised 35 in., putting it closer to the main rotor. This completely solved the problem-for reasons I do not understand.

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