From: David Lednicer <dave@amiwest.com>
Newsgroups: rec.aviation.rotorcraft
Subject: Re: Question for Nic Lappos
Date: Wed, 17 May 2000 14:48:23 +0100

  When I worked in the aero group at Sikorsky, story was that swept tips
were first put on the S-67 Blackhawk's blades for aeroelastic reasons.
When the aircraft flew with them, they were found to do what they were
intended to do, but it was found that they also helped improve
performance.  For this reason, they were incorporated in the
S-70/YUH-60  Black Hawk blade design.

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David Lednicer             | "Applied Computational Fluid Dynamics"
Analytical Methods, Inc.   |   email:   dave@amiwest.com
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From: nlappos@bellsouth.nospam.net
Subject: Re: Question for Nic Lappos
Date: 14 May 2000
Newsgroups: rec.aviation.rotorcraft

AKves wrote:
>Nic,
>  During the UH-60 transition at Ft. Tucker, the question of swept tips on the
>Blackhawk came up. The general instructor answer was,"Noise reduction." I
>crunched some numbers and found that the portion of the rotor blade just prior
>to the swept segment was very close to hitting transonic speeds on a standard
>day.
> Is the tip sweep an aerodynamic fix for noise reduction or a fix giving
>Blackhawk blades the ability to avoid compressability at the end of the blade?

Actually, your intuition is quite sound (pun intended!)  the transonic effects
are a big contributer to noise, so the swept tips help there alot.  However,
noise was not a requirement in the UTTAS design, so I can't say that the noise
benefit was the reason for the sweep, just a nice fallout.  The kind of noise
this effect causes is the slap of a Huey's blades.  The reason why a Huey has
that slap at lower speed is that the tip speed of  Huey is very high to
squeeze enough lift out of the two blades it has.  The tips of a Huey at 96
knots are as fast as the tips of a Black Hawk at 150 knots.

The delay of transonic drag rise is an important reduction in blade drag at
high speed, and that's why the sweep was put in, better high speed
performance - more speed and more range.

An added benefit is that the sweep actually untwists the blades at high speed,
which helps high speed performance as well.  At transonic conditions, about
Mach .95 to Mach 1, the center of pressure of an airfoil moves from 25% chord
to 50% chord.  When that happens on the swept tip, it moves the lift of that
section of the blade aft, which applies a down twist to the end of the blade.
The spar is compliant enough so that the blade goes from about 15 degrees of
twist to about 11 degrees, making it a bit better at high speed.

The twist of the Black Hawk blade is much more than previous convention to
allow much better low speed climb, because the vertical climb performance was
the most important requirement of the original spec.

Nick