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Blackhawk tail rotor

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#21 67november


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Posted 11 November 2008 - 13:49

The idea for a key in all army aircraft came from an incident in the late 70s, or early 80s where a disgruntled student stole a UH-1 from Fort Rucker an flew it to the White House. At least that's the legend that everybody is told.

it was about 80/81.
it was a digruntled crew cheif that took the bird for a joy ride when the Army in it's infinate wisdom said that crew chief's would no longer have flight status ability. Wally, Gomer or bossman might be able to confirm that.
it happened a couple years before I joined. (84)
spinning wings are better than fixed.

#22 stickwiggler80



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Posted 15 March 2012 - 22:24

Why was the tail rotor system put on the right hand side of the vertical stabilizer?

The commitment to design a tilted tail rotor (2Odeg) demanded that the tail rotor be placed on the right side of the

vertical stabilizer or fin. This is called the tractor’ design. and is not the best side for a tail rotor. The induced

velocity on the downstream side of the tail rotor is higher than the velocity on the upstream side.

For this reason the optimum design is to have the tail rotor sucking air past the fin rather than blowing air at the

higher velocity past the fin as in the tractor design. The drag of the fin is higher if the tall rotor is blowing on it

which reduces the effective net thrust of the tail rotor. Ray Proutv depicts the two types of tail rotors and the power

penalty due to blockage.

For example, with the same separation distance between fin and rotor (X/A) of .4 and the same blockage ratio (S/A)

of .2, the power ratio penalty is 1.25 for the tractor and 1.1 for the pusher type. This example shows a 15% penalty

for the tractor design over the pusher design.

A tilted pusher tail rotor could have been designed for the Blackhawk. but the separation of rotor and fm would

need to be significant considering tail rotor flapping. The increased separation results in larger components and

heavier tail, thus more adverse aft CG effects. There is a penalty with the tractor tail rotor, but the beneficial effects

of the tilted rotor resulted in the Blackhawk design.

#23 James Francisco

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Posted 10 August 2019 - 16:01

I have never heard or read an answer to this question that was satisfactory to me. The short answer to this question has been to provide lift to the tail. Two questions should come from this answer. 1 how much of whatever lift is provided is lost in an autorotation? Answer - all of it. 2. Why does this helo need lift in the tail when other single rotor helps don't. Answer - this is my educated guess as a seahawk, blackhawk Sikorsky tech rep for over thirty years. The Blackhawk design requirements for payload and transport inside of a C130. The disc loading and weight lifting requirements determine the rotor disc diameter. The required disc diameter determine the rotor blade length. To make the helo fit inside the C130 the rotor center had to be placed forward of the ideal center of gravity to get the long rotor blades in the C130 cargo hold. The lift produced by the tail rotor added much complexity to the mechanical and automatic flight controls. Sad thing was the C130 load requirement was short lived.

#24 iChris


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Posted 17 August 2019 - 14:36

I have never heard or read an answer to this question that was satisfactory to me. The short answer to this question has been to provide lift to the tail. Two questions should come from this answer. 1 how much of whatever lift is provided is lost in an autorotation? Answer - all of it. 2. Why does this helo need lift in the tail when other single rotor helps don't. 


There’s been much written on the canted tail on the H-60, as evidence in this post; however, you must analyze the tail section dynamics in its entirety (tail rotor, tail pylon, vertical stabilizer, horizontal stabilator, microprocessor controlled stabilator-incidence angle, etc.). Numerous technical papers and article from NASA and others, two listed below.


“Some basics first: The canted tail rotor is tilted so that some of the rotor thrust is directed upward, which means it contributes to the total lift of the aircraft. The cant angle is 20 degrees, so the tail rotor thrust in the vertical axis is over 30% of its total thrust, while the horizontal axis retains about 94% of the total thrust, a small cost to pay for that lift.


The two benefits for the H-60 and H-53E are that the lift from the tail rotor help the CG of the aircraft. The 53E third engine was added to the 53D and was placed aft of the transmission, so the tail rotor was used to retain aircraft balance. For the H-60, the aircraft was designed to fit inside a C-130, and so was made low and longer relative to its required payload and volume. The tail rotor helped the designers retain good longitudinal balance.


For the other aircraft with canted tails, the S-92, the AW-139 and the Bell 525, the lift from the tail rotor helps payload. For the S-92, the canted tail rotor is worth between one and two extra passengers."


Nick Lappos, Technical Fellow Emeritus


You can also download Ray W. Prouty's article titled, Evolution of Sikorsky Tails, link below:

Center of Gravity & Evolution of Sikorsky Tails


Some try and make sense out of these designs. They often don’t make sense. From a designer’s quote below, it’s just the ‘least worst compromise’


“Many detailed decisions concerning the rotors and the stabilizing surfaces need to be made before the design is complete. The most sophisticated of the computer preliminary design programs contain logical procedures for making some, but not all of these.


Many have to be based on considerations that are impossible to computerize and depend on factors ranging from solid scientific fact to controversial aesthetic judgment.


In almost all cases, there are powerful arguments pulling the designer in opposite directions. Resolving these dilemmas so as to achieve the ‘least worst compromise’ is the designer's primary task.”

Edited by iChris, 18 August 2019 - 16:48.



#25 kona4breakfast


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Posted 17 August 2019 - 15:24

Everything's a compromise in engineering. If you look at graphs of the sine and cosine you'll notice that as you rotate a bit from zero degrees, you get a lot of sine (vertical thrust) without losing much cosine (horizontal thrust). The vertical is also at a very convenient location, all the way at the tail, which lets you add payload aft of the M/R, in the case of the hawk: fuel, in other designs, passengers or cargo. It costs some weight in terms of structure and complexity, but in order to meet the C130 transport requirement that's what Sikorsky decided they needed to do. They were forced to add the stabilator during flight testing because the M/R couldn't do enough to compensate for the problems induced by the canted T/R.

I told my mom I wanted to be a pilot when I grew up.  She told me I couldn't do both.

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