Fast helicopters

wrote in message
...

The fully articulated hub design is susceptable to ground resonance
when landing. By increasing the rotor speed to just under the speed
of sound, that is, so nothing goes faster than the speed of sound even
at max forward velocity, the risk of ground resonance when landing is
significantly reduced. There are other advantages to speeding up a
fully articulated hub, but not flying apart when you land is a big
one.

Dennis.
You clearly don't know anything about ground resonance. It has little or
nothing to do with the blade tip Mach number.

"mm" wrote:

You clearly don't know anything about ground resonance. It has little or
nothing to do with the blade tip Mach number.

I don't pretend to be a fully articulated hub expert, but this is not
something I made up. It comes from an FAA inspector who makes his
living flying all sorts of helicopters. He IS an expert. Are YOU an
FAA inspector? Are you qualified to say that he is mistaken?

Think about the word "resonance". Do resonance and frequency go hand
in hand? By increasing the rotor RPM, do you increase the frequency
of rotation? Of course you do, its the same thing. If you sing a
perfect pitch into a tuning fork, does it also vibrate at resonance?
Yes. If you sing at a non-resonant frequency, does it still vibrate?
No. Are marching soldiers told to break step when crossing a bridge?
Yes. Why? Resonance. Think about this and what ground resonance
actually is.

Before you say this guy doesn't know anything about what he does for a
living, I suggest that you think about it a little while.

Dennis.



Dennis Hawkins
n4mwd AT amsat DOT org (humans know what to do)

"A RECESSION is when you know somebody who is out of work.
A DEPRESSION is when YOU are out of work.
A RECOVERY is when all the H-1B's are out of work."
To find out what an H-1B is and how they are putting
Americans out of work, visit the following web site
and click on the "Exporting America" CNN news video:
http://zazona.com/ShameH1B/MediaClips.htm

wrote in message
...
"mm" wrote:

You clearly don't know anything about ground resonance. It has little
or
nothing to do with the blade tip Mach number.

I don't pretend to be a fully articulated hub expert, but this is not
something I made up. It comes from an FAA inspector who makes his
living flying all sorts of helicopters. He IS an expert. Are YOU an
FAA inspector? Are you qualified to say that he is mistaken?

Think about the word "resonance". Do resonance and frequency go hand
in hand? By increasing the rotor RPM, do you increase the frequency
of rotation? Of course you do, its the same thing. If you sing a
perfect pitch into a tuning fork, does it also vibrate at resonance?
Yes. If you sing at a non-resonant frequency, does it still vibrate?
No. Are marching soldiers told to break step when crossing a bridge?
Yes. Why? Resonance. Think about this and what ground resonance
actually is.

Before you say this guy doesn't know anything about what he does for a
living, I suggest that you think about it a little while.

Dennis.

Yes, as a matter of fact I am qualified to say he is mistaken. I can
guarantee you that I know MUCH more about ground resonance than you or your
source. I challenge you to find a single reputable reference (text book,
technical paper, certification documentation, FAA training material, etc.)
that relates ground resonance directly to tip Mach number. Your source may
be the worlds ultimate helicopter pilot, but I don't think that this is much
of credential with respect to rotor aeromechanics and structural dynamics.

The original analysis of helicopter ground resonance was by Coleman and
Fiengold, NACA TN 3844. Read it, and will not find any mention of tip Mach
number.

Your tuning fork and marching soldiers examples are essentially meaningless
in this context.

I never said anything about tip mach number. I said "rotor speed".
The two are similar, but not the same. I'm not going to sit here and
argue with an anonymous person who thinks a "diskCopter" is practical.
Your facts about ground resonance may have been accurate 50 years ago,
but they are out of date.

Dennis.


"mm" wrote:


Yes, as a matter of fact I am qualified to say he is mistaken. I can
guarantee you that I know MUCH more about ground resonance than you or your
source. I challenge you to find a single reputable reference (text book,
technical paper, certification documentation, FAA training material, etc.)
that relates ground resonance directly to tip Mach number. Your source may
be the worlds ultimate helicopter pilot, but I don't think that this is much
of credential with respect to rotor aeromechanics and structural dynamics.

The original analysis of helicopter ground resonance was by Coleman and
Fiengold, NACA TN 3844. Read it, and will not find any mention of tip Mach
number.

Your tuning fork and marching soldiers examples are essentially meaningless
in this context.





Dennis Hawkins
n4mwd AT amsat DOT org (humans know what to do)

"A RECESSION is when you know somebody who is out of work.
A DEPRESSION is when YOU are out of work.
A RECOVERY is when all the H-1B's are out of work."
To find out what an H-1B is and how they are putting
Americans out of work, visit the following web site
and click on the "Exporting America" CNN news video:
http://zazona.com/ShameH1B/MediaClips.htm

wrote in message
...

I never said anything about tip mach number. I said "rotor speed".
The two are similar, but not the same.

Your original quote was "By increasing the rotor speed to just under the
speed
of sound,..." . When you talk about "rotor speed" and the "speed of sound"
in the same sentence, blade tip Mach number is the only rational way to
connect these two. How else can anyone compare "rotor speed" with the
"speed of sound", please tell us. Without my connecting these two
quantities this way for you, everything you said is completely meaningless
(as opposed to simply wrong).

I'm not going to sit here and
argue with an anonymous person who thinks a "diskCopter" is practical.

I've NEVER said anything about the "diskCopter" at all in this discusssion.
(For the record I think the "diskCopter" is silly.)

Your facts about ground resonance may have been accurate 50 years ago,
but they are out of date.


We're talking about physics here, what part of that "went out of date".
Please tell us what part of the analysis "no longer applies". I'll bet you
don't even really know what ground resonance is. How about this, tell the
group just what is in resonance with what. What is the frequency of the
blade motions, what is the frequency of the body motions? How is it
prevented? Do you know the FAA or military requirements with respect to
ground resonance? Tell us how many helicopters you have done a real
stability analysis for. When you can do this, then please explain further
your comments about the "speed of sound" and it's crucial role in this
phenomenom, I'd really like to hear your detailed analysis.

I really don't care about what you know and don't know, I just get a little
annoyed with people on the internet who spout complete nonsense to the rest
of the world, and then rather than trying to learn something argue with
reality. You might mislead someone with your nonsense and cause some harm
some day. I'm sorry if tone of this conversation is less than polite, I
just don't deal well with nonsense.

Like I said, my source does know what he is talking about and you
don't. I am not the expert and I cannot adequately explain why the
rotor speed is important for preventing ground resonance. However, my
source told me that it is and he is far more credible than you. How
many of the newer fully articulated hubs have tip speeds of only 400
mph? I wonder why they sped things up.

The other poster did have a point regarding landing struts. Fixed
struts hitting the ground are high(er) frequency. Spring loaded
struts are low frequency. Its the frequency imparted into the body
and thus the blades during impact, not just the impact itself. Past
that, I'm not going to say any more.

I suppose in your next post that you will be spouting nonsense about
how it has something to do with gravity and the space-time contuinuum
or something.

Quite frankly, you are a troll. I am not going to waste any more time
with you. If you want to post that you are the helicopter god of the
world, be my guest. I won't challenge you. You are still posting
anonymously and expect us to believe that you are an expert. Get
real.



Dennis.




"mm" wrote:

Nonsense deleted

Dennis Hawkins
n4mwd AT amsat DOT org (humans know what to do)

"A RECESSION is when you know somebody who is out of work.
A DEPRESSION is when YOU are out of work.
A RECOVERY is when all the H-1B's are out of work."
To find out what an H-1B is and how they are putting
Americans out of work, visit the following web site
and click on the "Exporting America" CNN news video:
http://zazona.com/ShameH1B/MediaClips.htm

I don't mean to further yet another moronic discussion about yet
another pie in the sky (lol) rotorcraft, but MM, regardless of
who he is, is correct. Ground resonance has nothing to do with
rotor tip speed. Its mainly germane to helicopters which have
shock-struts as the main rotor rotational rate crosses the
resonant frequency(ies) of the struts. It tends to be exaggerated
and more dangerous in machines with four struts and three blades.
Nothing has ever caused my Jetranger to enter the onset of
ground resonance. Know why?; Two blades, No struts....Hmmm.

Mach number my eye, and who the heck cares about ground resonance
in a discussion on increasing Vmax?

Bart

wrote:
I never said anything about tip mach number. I said "rotor speed".
The two are similar, but not the same. I'm not going to sit here and
argue with an anonymous person who thinks a "diskCopter" is practical.
Your facts about ground resonance may have been accurate 50 years ago,
but they are out of date.

Dennis.

"Bart" wrote in message
...
I don't mean to further yet another moronic discussion about yet
another pie in the sky (lol) rotorcraft, but MM, regardless of
who he is, is correct. Ground resonance has nothing to do with
rotor tip speed. Its mainly germane to helicopters which have
shock-struts as the main rotor rotational rate crosses the
resonant frequency(ies) of the struts. It tends to be exaggerated
and more dangerous in machines with four struts and three blades.
Nothing has ever caused my Jetranger to enter the onset of
ground resonance. Know why?; Two blades, No struts....Hmmm.

Mach number my eye, and who the heck cares about ground resonance
in a discussion on increasing Vmax?

Bart

What a nice change, someone who does actually understands something about
the issue!

The reason that a JetRanger can not have ground resonance is that it does
not have lag hinges or other source of flexibility in-plane. If the rotor's
natural lag frequency is higher than the rotor speed, as is the case with
the Bell teetering rotors, it is impossible for it to have a ground
resonance instability. Rotors with this characteristic are called
"stiff-inplane". You can have a stiff-inplane rotor with more than 2 blades
(BO105, BK117) and they too are immune from ground resonance. (If look at a
BO105 or BK117 hub you will not see any lag dampers.)

The struts are associated with ground resonance, but not in the way that you
seem to think. They do not cause the problem; they are there to provide the
damping needed to stabilize the system.

If anyone is interested, we could have a nice little productive thread on
ground resonance. If you do care about this subject, though, please, ignore
that guy "Dennis". He really, really doesn't know what he is talking about
on this subject.

mm wrote:


If anyone is interested, we could have a nice little productive thread on
ground resonance.

Pardon me for sounding snide but if your next nice little productive
thread is like your last one then two other people will participate
besides youself and one of those will be Dennis who you claim
doesn't know what he's talking about.

Sounds like a barn burner to me.

If you do care about this subject, though, please, ignore
that guy "Dennis". He really, really doesn't know what he is talking about
on this subject.

AND YOU DO......!!!!!!!!!!!!!

Your not related to Badwater Bill are you...???????????