Blackhawk tail rotor

[volcanobreath1979]

On 4/20/2008 at 9:35 PM, CharyouTree said:

 

 

Having learned a lot since my previous post in this thread...

 

The mechanical mixing unit (MMU) provides yaw to pitch coupling, to compensate for the lift of the tail rotor.

 

So right pedal causes a decreased t/r lift vector, nose pitches up, and the helicopter drifts aft. The m/r tilts forward to compensate.

 

(I cheated, and used one of my books that deals specifically with left pedal inputs, but it's just the opposite for a right pedal turn)

Can you please amplify? With the main Rotor rotating anti clockwise, the tail rotor should ideally be positioned on the left side of the fuselage to provide anti torque for directional control. On the black hawk, it's on the right (despite main rotors rotating anti clockwise) 

[iChris]

On 4/24/2021 at 5:20 AM, volcanobreath1979 said:

Can you please amplify? With the main Rotor rotating anti clockwise, the tail rotor should ideally be positioned on the left side of the fuselage to provide anti torque for directional control. On the black hawk, it's on the right (despite main rotors rotating anti clockwise) 

Numerous factors define the final tail-rotor design, like rotor diameter, tip-speed, blade area, number of blades, blade twist, fin surface area, the direction of rotation, Pusher or Tractor, etc. 

The least unfavorable compromise is the designer's primary task. Most conventional tail-rotors are Pushers mounted on the left side. Induced velocity below the tail-rotor is higher than above it; therefore, it reduces net thrust if the tail-rotor is blowing toward the fin.

The exceptions to the Pusher are the Sikorsky UH-60 and Bell, 212, 214, 412. Also, the Bell UH-1, 204, and 205 were initially Pushers, and some were recently converted to Tractors. In 1954 Bell designed the XH-40, the prototype of the UH-1,  AH-1, and subsequent first production Bell 204 and Bell 205 were all Pushers.

 Sikorsky designers chose to tilt the UH-60  tail-rotor shaft so that part of its thrust could help counter CG issues by lifting the helicopter's rear end. The Tractor configuration was chosen to provide clearance with the fin without using a longer driveshaft.

 Bell chose the wrong direction of rotation when in 1954, it designed the XH-40, the prototype of the UH-1 series. Bell's solution 15 years later was to flip the tail-rotor installation from the left side of the fin to the right, using the same hardware. To Bell's good fortune, the tail-rotor blades had no twist, allowing for the change.

Quote

Over the past 45 years, helicopter development teams have discovered that the tail-rotor rotation should have the blade closest to the main rotor going up. A graphic demonstration of wrong and proper rotation was done during the development of the Lockheed Cheyenne attack helicopter. The figure shows the improvement in pedal position obtained when the rotation was reversed. 

In left sideward flight, the pilot ran out of left pedal between 14 and 18 knots as he tried to fly with the tail-rotor in the vortex-ring state. When the tail rotor's direction was reversed by redesigning the gearbox, the problem disappeared. (The effects of the proximity of the pusher propeller to the main rotor had been discounted by comparing test results with the Pusher on and off.) The change improved even right sideward flight. Just why the tail rotor is sensitive to the direction of rotation in the proximity of the main-rotor wake is not clear.

 

Edited May 1, 2021 by iChris

[Disguise Delimit]

Quote

Bell chose the wrong direction of rotation when in 1954, it designed the XH-40, the prototype of the UH-1 series. Bell's solution 15 years later was to flip the tail-rotor installation from the left side of the fin to the right, using the same hardware. To Bell's good fortune, the tail-rotor blades had no twist, allowing for the change.

The problem was that the front of the blade, as it rotated, was going down with the main rotor downwash, and was losing efficiency - the back blade was in clearer air, but the loss from the front blade was substantial.

When Bell flipped the whole thing to the other side, the front blade now rotated up into the downwash, gaining a lot of extra relative airflow. A simple fix and very worthwhile, known as the flip-flop tail rotor.

[iChris]

On 4/30/2021 at 6:54 PM, Disguise Delimit said:

The problem was that the front of the blade, as it rotated, was going down with the main rotor downwash, and was losing efficiency - the back blade was in clearer air, but the loss from the front blade was substantial.

That’s correct…

 Most U.S. manufacturers will turn the tail rotor clockwise when viewed from the helicopter's left side, taking advantage of the tip vortices coming off the main rotor. When the tail rotor turns in the same direction as the primary rotor vortices, it reduces the relative airspeed of the tail blades, and the available thrust is limited. When the tail rotor turns against the central rotor vortex, the performance increases because of the square-law connection between thrust and increased relative airspeed. 

 Two notable helicopters turn their tail rotor in the so-called wrong direction. They are the MD500 and Robinson R22. However, they both share another less conventional concept, for their time, NACA 63-415 asymmetrical tail rotor blades. More common were symmetrical tail rotor blades like those on the Bell UH-1, AH-1, 204/205/206, 212, 412, and Hughes 269/TH-55.

 Frank Robinson left Bell Helicopter in 1959 and joined the Aircraft Division for Hughes Tool Company, assigned to the U.S. Army's OH-6 Light Observation Helicopter and other Hughes 500 projects. Frank had already established himself as an authority on tail rotor design. Frank found the NACA 63-415 asymmetrical tail rotor blades exhibited noteworthy improvement in performance over the symmetrical blades. 

 Frank brought some of those design characteristics and gave birth to Robinson Helicopters (1973) and the R22. Frank reduced gearing in the tail rotor gearbox to save weight at the aft end of the tail boom. Consequently, due to the engine's reversed position (Front end facing aft), the driveshaft from the engine to tail rotor ended up turning the tail rotor in the wrong direction. Furthermore, asymmetrical tail rotor blades came with an inherent consequence, an undesirable twist or pitching moment. Frank countered the effect with a built-in coning angle designed into the tail rotor.

The overall compromises ended up cutting weight, and light helicopters like the OH-6 and R22 still provide better than adequate tail rotor performance. MD Helicopters seem satisfied, sticking with the wrong-direction design on their current MD500E/F series. Incidentally, the R44 and R66 tail rotors turn the right way.

 

Edited May 10, 2021 by iChris