Helicopters are the ultimate flying machines - but they are not only
expensive to operate: They are LOUD!

Someone told me, this happens cause the rotors become faster than
Mach1, generating that much noise.

Questions:

1. Is that right? How fast do the rotors get?

2. Any known possiblities for bigger/less fast rotors and therefore
more silent solutions?

Sorry for not using the right expressions. I am German and not a
native English speaker.

Regards from Buenos Aires

Raus

thorstenrauser...

1. Your friend is correct, according to this page,
on Wikipedia.com:

"A helicopter cannot fly as fast as a plane because
 the forward-going edge of its rotor moves much faster
 than the helicopter. It is very easy for this edge to
 exceed the speed of sound. When supersonic,
 conventional rotors begin to get increasing drag. It
 is theoretically possible to have spiraling rotors,
 similar in principle to variable-pitch swept wings.
 These could exceed the speed of sound, but no
 materials are strong enough and flexible enough to
 construct them."
http://www.wikipedia.org/wiki/Helicopter

The above page has an excellent, extensive explanation
of helicopters in general.

2. As for quieter solutions, the NOTAR® anti-torque system
has represented a major breakthrough:

"Both the MD 600N and the MD Explorer are equipped with
 the NOTAR® anti-torque system, which replaces the
 conventional tail rotor and provides exceptionally
 safe and quiet performance. Boeing's NOTAR
 system-equipped helicopters -- the MD 600N, MD Explorer
 and MD 520N -- are certified by the Federal Aviation
 Administration (FAA) as the world's quietest helicopters."
http://www.boeing.com/news/releases/1997/news_release_971210n.html
(From the Boeing website).

Another page from Boeing, which supports this, says:

"Industry experts accept compliance margin (the difference
 between the sound a helicopter makes and the maximum
 noise permitted for its weight by government aviation
 agencies ) as evidence of a helicopter's quietness
 regardless of its size," he said. "Based on having the
 largest compliance margin, the (6,250-pound, FAA
 Category A) MD Explorer is projected to be the quietest
 helicopter in the world."
http://www.boeing.com/news/releases/mdc/97-21.html

A good explanation of the Notar® system is here, on the
Eastern Atlantic Helicopters website:
http://www.easternatlantic.co.uk/info.htm

The police helicopters in my community have begun to 
use these helicopters, and I can tell you from direct
experience that they are much quieter.


Searches done, via Google:

"quiet helicopter"
://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-8&q=%22quiet+helicopter%22

NOTAR® anti-torque system
://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-8&q=NOTAR%C2%AE+anti-torque+system&btnG=Google+Search

helicopter tail rotor tip speed
://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-8&q=helicopter+tail+rotor+tip+speed


Please do not rate this answer until you are satisfied that
the answer cannot be improved upon by means of a dialog with
the researcher through the "Request for Clarification" process.

sublime1-ga

---

Request for Answer Clarification by thorstenrauser-ga on 12 Dec 2002 08:45 PST

Hi sublime1


Sorry, but since I could not find the answer to my first question in
your reply, I used the search terms

helicopter sound rotor speed

The displayed match http://www.everything2.com/index.pl?node=helicopter

says something else: The rotors CANNOT exceed the speed of sound
(?!?!?)

(thing) by thebigmacd (2.9 mon) (print)   ?    Sat Apr 13 2002 at
19:16:07
 

The rotor of a helicopter is not technically a "horizontal propeller".
A propeller is a set of rigid blades with a wing cross-section, each
of which is twisted and tapered along its length such that the most
thrust is produced near the center (this reduces stresses on extremely
high-speed props). A helicopter rotor is a set of slender flexible
wings of uniform thickness and angle that are kept flat by centripetal
acceleration forces. A helicopter's rotor rotates slowly relative to a
propeller because the tips cannot exceed the speed of sound. If the
tips exceed the speed of sound, the rotor blades cause destructive
turbulence and shock waves. The result can be massive vibration and/or
structural disintegration.

http://www.eng.auburn.edu/department/ae/labinfo/AAL/SSAR.html (solid
state rotors)
http://carol.wins.uva.nl/~leo/lego/rotor.html (basic rotor structure
and principles)
http://www.howstuffworks.com/helicopter.htm (complete helicopter
explanation)


According to this, my friend was WRONG. Or did you mean the tail
rotors exceeding the speed of sound?

Again, (how) is helicopter noise related to rotors exceeding the speed
of sound?

---

Clarification of Answer by sublime1-ga on 12 Dec 2002 10:18 PST

thorstenrauser...

My understanding of the quote from Wikipedia.com:

"the forward-going edge of its rotor moves much faster 
 than the helicopter. It is very easy for this edge to 
 exceed the speed of sound."

is that the COMBINED speed of the forward motion of the
MAIN rotor PLUS the forward speed of the helicopter could
easily exceed the speed of sound. Thus, while the record
for helicopter speed is about 248.5mph
[ http://216.239.53.100/search?q=cache:aKMqlrUW1gQC:www.helis.com/howflies/maxspeed.htm+helicopter+speed&hl=en&ie=UTF-8
]
or 402 kph, the forward-moving tip of the rotor would
only need to be 502 mph in order to combine with the 
forward speed of the helicopter and exceed the speed 
of sound, or ~750mph.

On this page, from Aviation Today, a rotor tip speed
of 800 ft/sec, or 545mph, is cited:

"..the larger rotor was operating at a tip speed well
 over 800 feet per second. Combining this with a
 forward speed of 120 knots yields an advancing-tip
 Mach number of 0.9—well above the level where
 compressibility becomes significant."

"Shock waves are formed and mini-sonic booms are
 projected forward approximately in the plane of
 the rotor, which of course, tilts downward."

"..It is interesting to speculate how long this
 would have lasted had Bell stayed in its original
 home of Buffalo, New York. Compared to Fort Worth,
 Texas, Buffalo would have meant performing flight
 tests at much lower temperatures, where the Mach
 numbers were even higher."
http://www.aviationtoday.com/reports/rotorwing/previous/1000/10noise.htm

The speed of sound varies with temperature and 
humidity:
http://www.measure.demon.co.uk/Acoustics_Software/speed.html

120 knots is only 138.192mph. If the same helicopter
were traveling at 205mph (well under the record speed),
the combined speed of the 'copter and the rotor tip
would exceed the speed of sound.


Searches done, via Google:

helicopter speed
://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-8&q=helicopter+speed

"helicopter rotor" "sonic boom"
://www.google.com/search?num=50&hl=en&lr=&ie=UTF-8&oe=UTF-8&safe=off&q=%22helicopter+rotor%22+%22sonic+boom%22

helicopter "rotor tip speed"
://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-8&q=helicopter+%22rotor+tip+speed%22

knots mph converter
://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-8&q=knots+mph+converter

speed of sound
://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-8&q=speed+of+sound

Let me know if you need further clarification.
sublime1-ga

I read a book once that described the use of "stealth" helicopters.
Can't remember if it was Tom Clancy, Deen Koontz, Clive Cussler, or
some other author whom I've only read one book from.

I'm fairly certain it would have been Clancy, though. Of course,
that's a novel, so it might not be true.