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From: (Nick Lappos)
Subject: Re: mu -a  (Sikorsky ABC helicopter?)
Date: 11 Aug 1999
Newsgroups: rec.aviation.rotorcraft

In article <7ofcp9$681$>, ""gridiron\""n wrote:
>Doug Marker wrote:
>> We are seeking confirmation that any helicopter has ever broken the mu-1
>> barrier other than
>> in a wind tunnel test.
>> We have conflicting reports that the Sikorsky ABC may have done so but
>> the deeper we
>> research the more confused the Sikorsky story gets.
>> Firstly does anyone know for fact if it has been done ? (preferably with
>> a reference)

I flew the ABC for a bit, and have the facts.  It went 300 miles an hour (259
knots) using the J-60's as auxiliary thrust.  When blasting along, it was
actually in autorotation, with the rotor freewheeling as the jets pushed it
I have never heard of a supersonic rotor of any kind, wind tunnel or
otherwise.  Sounds quite improbable to me, considering retreating blade stall.

>The S-69 ABC had two rotors turning in opposite directions so that at
>all times there was an "advancing blade".  Of course Charlie Kaman, a
>whole lot of Russians and others had done this for years.

Doug implies that there was nothing really new about the ABC, since others had
flown co-axials before.  Actually, the XH-59A  ABC (Sikorsky designation
S-69) was very rigid, with extremely stiff blades, unlike the very low offset
Kamov designs (including the KA-50 Hokum/Werewolf).  The extra rigidity is
quite unique, and allowed the ABC to operate beyond stall on the retreating

With a more conventional Coaxial, stall makes the blades flap a bunch, and tip
clearance between disks becomes an issue.  Self mid-air collision can ruin
your whole day.

This ABC  post stall operation relied on the upsweeping blade on each side of
the helo to keep roll and pitch control (in a single rotor at stall, you lose
control because the down sweeping blade gives up the ghost).  ABC could pull
about 1/2 to 1.0 more G's than an equivilent helicopter, and could do so at
altitude.  The ABC demonstrator pulled 2.5 g's at 25000 feet!  As the waterboy
would say, "Not too shabby!"

>I don't know the speed they reached with the 69.

Cruise at 225 knots, dive at over 260 knots.

>into the S-72 which had a more conventional single rotor system but it
>also had the two big "pusher" jets mounted on the sides along with some
>pretty long wings for a helicopter.  I think the acronym was Rotor
>System Research Aircraft (RSRA)?  The idea was to have propulsion and
>lift that would support any type/size of rotor system, even one that
>would not lift the airframe. -vic
The RSRA was originally developed by Sikorsky for a NASA contract to build a
flying wind tunnel that would have the ability to adapt itself to several
rotor designs.  It was based on the fuselage of the S-67 Blackhawk, close
observers will note the similar tail cone and vertical fin design.
The RSRA became the X-wing, which held the promise of stopping its very stiff
rotor and becoming a high speed airplane.  The trick was to use the rotor
blades backwards on one side, achievable because the blades were oval in cross
section, and relied on air slots on the leading and trailing edges to control
the circulation around the airfoil.  A massive compressor on the aircraft
produced the air to feed the rotor.  The program was humming along but ran
into several snags that kept driving the cost up until it ran out of money.
Some of the difficulties were part of the more conservative attitude after the
Challanger shuttle disaster, with more concern about safety, even though the
S-72 had crew extraction seats.  When I last saw the airframe, it was parked
at Edwards, in shrink wrap.

Nick Lappos

"Been there, done that, the T shirt wore out a long time ago."

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