Canted tail rotor pylons, why?

[Flag Track]

Some of the larger Helos have 'em, what's the reasoning please?

Flag T

[212man]

Primarily - I believe - it's to expand the allowable Centre of Gravity range. The downside is that it introduces a yaw/pitch coupling so requires a mixing unit in the control system.

[SASless]

Sikorsky finally figured out Tandem Rotors rule....for a reason! Ask any Chinook pilot....we'll remind you!

As 212man says...increased CoG reasons...more lift in the aft end of the helicopter.

Standard single rotor configurations gain nothing for the power used to offset Main Rotor torque...whereas the canted tail rotor design does benefit in that regard.

[jimjim1]

SASless said:-
...increased CoG reasons...more lift in the aft end of the helicopter.

Bbbut, surely this would need something to decouple the horizontal and vertical components of the tail rotor thrust to be useful. e.g. A cyclic control on the tail rotor.
Is that what is done?


SASless continued:-
Standard single rotor configurations gain nothing for the power used to offset Main Rotor torque...whereas the canted tail rotor design does benefit in that regard.


Hmmm.
Surely:- The power used to offest main rotor torque will need to be horizontally directed. Any power used to provide lift will be additional. As far as I can see, no free lunch here:).

Hmmm.

<Noise of thinking machinery grinding>

Work done = Force x Distance.

Since a hovering helicopter is stationary, why does it require power to maintain a hover?

[delta3]

The difference between putting a hook in a concrete wall or in thin air...

If you put the hook in thin air, the distance part of the equation changes because unfortunately the air is not stationary.

d3

[delta3]

If you would put the TR say at a (probably extreem) angle of 30 degrees you need 14% more power to create the same anti torque but you create 30% lift. If the TR is sufficently efficient (in comparison to the main rotor) you get a (small) free lunch. It is however not the prime reason, that is CoG I would think.

m2c, d3

[JohnDixson]

Delta 3 has the math right, and also has the other thought that made the concept so attractive to "some" at SA in the early 70's, i.e., "something for nothing".

Prior to the UH-60 design, it was tested on two other models: the S-61R ( USAF model ) and the CH-53A. Of course we had added various mechanical coupling actuators which had electrically variable gains to arrive at an optimum "compromise".

But one doesn't usually get something for nothing and this has been the case with canted tail rotors as to getting all of the coupling and tail rotor height right. I believe that the Comanche did the best at making it transparent, fully utilizing the flexibility afforded by the fly-by-wire control system. The S-92 is in the process of doing the same thing.

Nonetheless it does provide a bit of opportunity for the double-headed dump truck drivers to have a shot!

I do very specifically recall a memorable AHS meeting in Connecticut wherein Sergei Sikosky had invited Marat Tishenko, the famous Chief Engineer from Mill Helicopters and a man who was personally involved in their most successful designs, to give a presentation to a group which was mainly SA people. After the presentation Sergei introduced me and I asked Dr. Tishenko whether Mil had ever looked into the idea of canted tail rotors? The good Doctor grinned and said with a twinkle in his eye, " But we always wondered why you did this!"

Thanks,
John Dixson

[S70]

In addition to increasing the available C of G range Canting the Tail Rotor moves the bottom of the tail rotor tip path plane higher from the ground. In view of the additional flight control complexity and resulting icreased weight I wonder what drove the Engineers to adopt this configuration. Was it:
1. Increasing the C of G range,
2. Providing adequate tail rotor authority and still meeting a Design Spec (Mil or other) regarding Tail Rotor/Ground Clearance?

Perhaps John can provide some insight.

[JohnDixson]

S-70, as I wrote, Delta 3 had it correct in that the main reasons were:

Aircraft balance with an aft CG design. A secondary, related benefit is a shorter aircraft, although one might ask which comes first.
The aircraft hover performance benefit.Tail rotor tip clearance to ground was not a factor in any of the SA canted tail designs. The design approach to that question in the first such machine, the UH-60, was to select a tail wheel over a tricycle configuration, and we would have done that, canted tail or not. In the CH-53E and S-92, both tail rotors were sufficiently high to allow a tricycle and in the Comanche, the tail fan was ducted.

Nor was the selection of a canted tail decision on the four models influenced by tail thrust requirements, the primary one being a sideward flight capability of 35 kts for the first three* and I'm unclear on the spec for Comanche, but it was at least in the 80 kt range.

*The FAA only requires 17 kts as I recall but SA and most military req'ts call out 35.

Thanks,
John Dixson

[IFMU]

I heard a story about the UH60 competition once (way before my time). The Boeing entry supposedly was missing about 400 lbs from the required spec weight for HOGE. Their smart people analyzed the Sikorsky design and decided they missed it too, by about 400 lbs. So, if both companies failed that point, it wouldn't count against either one of them. They missed the canted tail rotor, and about 400 lbs of thrust in their analysis. Whoops!

-- IFMU

[JohnDixson]

IFMU,

You have a good memory: I do recall that we asserted that the canted tail provided 400 lbs of hover performance ( simply the vertical component of the tail rotor thrust ).

But in fact there were two events during the fly-off that illustrated more than a 400 lb hover performance difference between the two candidate ships:


For training the operational Army pilots with sling loads, the Army had made up (7) 55 gallon drums filled with concrete and some odd iron and each weighed 1000 lbs. We trained on the same day as Boeing at Shell Field over toward Enterprise Al. With full fuel ( at first ) we and our student picked up seven cans, took off and did the syllabus. Did that for each student. Boeing then got the cans and started with seven, but couldn't get them off the ground, then tried six, again couldn't get them off the ground, and finally got five off the ground, barely ( I was looking very closely! ).
At the end of the operational evaluation at Ft. Campbell, the Army wanted to perform an external lift of an infantry vehicle called a Gamma Goat that the infantry had high hopes for. Weighed 7100 lbs. Two Army pilots picked it up in our machine with a full gas tank, flew it around for pictures and put it back down. The Boeing machine then picked it up and did the same. However, the Boeing crew had done a pretty aggressive strip job, took out all of the interior and troop seats and all but one comm radio, and had only ( we were told by the Army people ) 500 lbs of fuel on board.Bottom line was that there was a huge difference in hover performance between the two. There was more than just the canted tail involved here though: the Boeing machine had a five foot smaller main rotor diameter (!) and a one foot smaller tail rotor diameter.

As I recall the Boeing production proposal to the Army addressed the hover performance shortfalls with larger rotors, but of course this meant a big redesign, a re-qualification and re-flight test of the whole main and tail rotor and drive train system. SA was good to go as is.
Thanks,
John Dixson