What are the aerodynamic reasons behind the sikorsky S92/uh 60 curved rotor tip?

[chris pochari]

From my observation probably 80% of rotor blades are straight all the way down but a few helicopters like the S 92 and UH 60 to name a few have curved rotor tips I tried searching for an answer but I couldn't find one.

Thanks

Chris.

[DizzyD]

There are several design advantages to swept tip rotor blades. The primary one that comes to mind is that such blades reduce normal tip Mach number and consequently drag due to compressibility at the transonic tip.

[twinhueyman]

The swept tips on non-M model S70/H60s provide 3 main advantages - management of transonic and supersonic airflow at the rotor tips to delay mach tuck during high speed flight, dynamic washout bending of the blade (more washout at high pitch angles, less at less pitch, and reduction of negative pitch in outboard portion of blade at very low pitch angles), and reduced noise through better tip vortex shedding patterns and improved vortex interaction by following blades.

 

The Anhedral tips on S92s and 60Ms presumably have more characteristics but I'm not 100% on them. They predominately help hover performance so I'd imagine it's better management of downwash.

 

Mike

[avbug]

Airflow velocity over the rotor is higher at the tip, where the velocity of the blade is highest. The tip will experience the fastest airflow the soonest, as it's traveling farther, faster through the atmosphere.

 

Mach effects, or transonic airflow, occurs a little way below the "speed of sound" (mach) to a little way above it. Mach effects vary with the aircraft, but the essential concept is that while subsonic airflow is considered incompressible, the same is not true of transonic and supersonic airflow.

 

Airflow dams up in front of the wing and along it, forming one or more areas of compressed air (shock waves). These areas affect other airflow, diverting it or moving the center of pressure (and lift) on the wing. The position of the shock waves over the airfoil vary with the speed of the airfoil, and mach is a function of air temperature. Shock waves and compressibility mach effects are measured at a right angle to the airfoil, or in other words, air approaching the leading edge and passing chordwise to the trailing edge.

 

By sweeping the airfoil, the airflow is no longer chordwise, front to back, but partially spanwise; it approaches the leading edge of the wing, travels along the span (outward, toward the tip), and exits the trailing edge at a more outboard point than where it crossed the leading edge. This changes the mach effects and reduces the movement of the center of lift and pressure on the airfoil. It minimizes the effect. The airfoil has less lifting and less useful characteristics at low speeds, but is more controllable and less prone to excursions, twisting, and other functions of compressibility or mach effects at high speeds.

 

Raked or swept leading edges are found on helicopters, propellers, and of course fixed wing high speed aircraft.

[superstallion6113]

When I worked on UH-60Ms at Sikorsky, an engineer explained that a big benefit of the anhedral tips was the reduction of blade tip vortices in a hover, giving increased hover performance over the A/L H-60 rotor blades. Though I think a good bit of that improvement was lost due to the Hontek coating that was applied to the leading edges of the blades.

Edited April 10, 2017 by superstallion6113

[Eric Hunt]

Anhedral is where the tip bends down, and isn't related to sweep. As Buggy said in many words, the transonic effects are related to the speed across the chord, so by sweeping the tip, the speed can be increased because the spanwise flow delays the chordwise flow from reaching the transonic affects.

[superstallion6113]

Anhedral is where the tip bends down, and isn't related to sweep. As Buggy said in many words, the transonic effects are related to the speed across the chord, so by sweeping the tip, the speed can be increased because the spanwise flow delays the chordwise flow from reaching the transonic affects.

 

 

I agree with the reason for sweeping the blade tip back. Was just adding in some info on the anhedral tip as well since UH-60M, S-92A, and other blades as well have both features on the blade tips.

[500E]

superstallion6113

How bad was the loss with the Hontek leading edge coating ?

Did it last long ? how hid it preform erosion wise, & lastly was it field replaceable in a reasonable time frame .

[superstallion6113]

superstallion6113

How bad was the loss with the Hontek leading edge coating ?

Did it last long ? how hid it preform erosion wise, & lastly was it field replaceable in a reasonable time frame .

 

 

The stuff did great in the desert in dry, arid conditions. Unfortunately it could not endure rain. The stuff would erode and flake away quickly. At my last job working on a fleet of 6 UH-60M helos we had the coating removed on all the blades on 6 aircraft, because it was flaking off too quickly, and couldn't afford the down time to reapply the coating. These 6 aircraft had less than 200hrs total time on them when we did this, brand new aircraft. Kits are available from Hontek, but it required a solid two days of down time before the aircraft could be flown.

 

As for performance loss, I can't recall what it was. I only remember the U.S. Army Airworthiness Release for the UH-60M stating a degradation in hover performance for aircraft with the Hontek coated blades.

Edited April 11, 2017 by superstallion6113