More Questions about coning forces & rotor noise!!

[Eric D]

If you start a descent from cruse by reducing pitch and maintaining
rotor speed, what happens to coning forces as compared to cruse?

I have noticed that during a decent in a R22 that you can get a
"Wop-Wop" noise from the main rotor blades. Changing cyclic or
increasing rotor speed a very small amount can make it go away, or at
least not heard from the cockpit. Can someone explain what is
occurring and why?

Thanks,

Eric D

[Bart]

Not sure how much its related to coning, but the whop whop sound is caused
by the blade slapping the vortex wake from the blade before. It happens
predictably within specifc range of descents and airspeeds.

Bart

"Eric D" > wrote in message
om...
> If you start a descent from cruse by reducing pitch and maintaining
> rotor speed, what happens to coning forces as compared to cruse?
>
> I have noticed that during a decent in a R22 that you can get a
> "Wop-Wop" noise from the main rotor blades. Changing cyclic or
> increasing rotor speed a very small amount can make it go away, or at
> least not heard from the cockpit. Can someone explain what is
> occurring and why?
>
> Thanks,
>
> Eric D

[Andrew Crane]

"Eric D" > wrote in message
om...
> If you start a descent from cruse by reducing pitch and maintaining
> rotor speed, what happens to coning forces as compared to cruse?
>
> I have noticed that during a decent in a R22 that you can get a
> "Wop-Wop" noise from the main rotor blades. Changing cyclic or
> increasing rotor speed a very small amount can make it go away, or at
> least not heard from the cockpit. Can someone explain what is
> occurring and why?

You're probably talking about when you descend 300-500'/min at around 80-85
kt. This is your blade chopping the vortex left by the previous one and can
be stopped by increasing your descent, speeding up or slowing down. It's not
down to coning forces.

Regards
Andrew


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welcome

[Shaber CJ]

Along these lines, what causes the loud burbling blade noise in a MD 500 in
turbulence. Is it just caused by a larger than naormal vortex burbble?

Craig

[Eric D]

> It's not down to coning forces.
>
> Regards
> Andrew

Hi Bart & Andrew,

Maybe I didn't word this properly or maybe I should have put it as to
separate postings. These are two separate questions. I didn't intend
them to be interrupted as part of the same question.

Seeing that the focus so far is on the noise question, and it seems
that it has to do with the blade colliding with the previous blades
vortexes from the posting responses so far. Is this something to
avoid? Does it cause harm to anything with the helicopter?

[Steve Waltner]

In article >, Eric D
> wrote:

> > It's not down to coning forces.
> >
> > Regards
> > Andrew
>
> Hi Bart & Andrew,
>
> Maybe I didn't word this properly or maybe I should have put it as to
> separate postings. These are two separate questions. I didn't intend
> them to be interrupted as part of the same question.
>
> Seeing that the focus so far is on the noise question, and it seems
> that it has to do with the blade colliding with the previous blades
> vortexes from the posting responses so far. Is this something to
> avoid? Does it cause harm to anything with the helicopter?

Blade slap or whatever you want to call it is not a problem for your
helicopter, but it is a problem for all helicopter pilots. It will not
cause any damage to the helicopter. The only problem is the noise
complaints that you can get by repeatedly flying over the same people
and making noise like this.

HAI ( Helicopter Association International http://www.rotor.com/ ) has
sponsored the Fly Neighborly program which talks about ways to fly to
reduce the noice impact of your flying so we can hopefully enjoy this
passion called helicopters throughout our lives.

Steve

[John]

When you do an auto rotation from a power on cruse condition you drop
the collective and at THE SAME TIME move the cyclic back and to the
right to reload the rotor system for the new rotor airflow condition.
If you do the transition correctly you NEVER unload the rotor system.
If you do NOT move the cyclic when you drop the collective then yes
you will unload the rotor and you will loose rotor RPM rapidly because
the airflow has to flow up through the rotor in an auto rotation and
it will not do this until the rotor is tilted back to allow this.
The coning may flop around some during the power to auto transition
but when a steady auto is established the cone angle is very nearly
the same as at hover because the horizonal and vertical forces on the
blades are almost the same. There is some difference in where the
center of lift is verses span distance on the blades between an auto
and powered flight but this also varies with the amount of twist built
into the blades.
John


On 2 Mar 2004 05:30:47 -0800, (Eric D)
wrote:

>If you start a descent from cruse by reducing pitch and maintaining
>rotor speed, what happens to coning forces as compared to cruse?
>
>I have noticed that during a decent in a R22 that you can get a
>"Wop-Wop" noise from the main rotor blades. Changing cyclic or
>increasing rotor speed a very small amount can make it go away, or at
>least not heard from the cockpit. Can someone explain what is
>occurring and why?
>
>Thanks,
>
>Eric D

[Mikko Pietilä]

On Fri, 05 Mar 2004 04:52:00 GMT, (John) wrote:

>When you do an auto rotation from a power on cruse condition you drop
>the collective and at THE SAME TIME move the cyclic back and to the
>right to reload the rotor system for the new rotor airflow condition.
>If you do the transition correctly you NEVER unload the rotor system.
>If you do NOT move the cyclic when you drop the collective then yes
>you will unload the rotor and you will loose rotor RPM rapidly because
>the airflow has to flow up through the rotor in an auto rotation and
>it will not do this until the rotor is tilted back to allow this.

We did dozens of entries to autorotation during a couple of Test Pilot
School Exercises with an instrumented aircraft (Gazelle). From the
instrumentation data gathered it was very clear that the aircraft
would enter autorotation very nicely if no longitudinal control input
was made as the collective was lowered.

I remember wondering a bit why this was the case (although I am an
engineer, and not a RW pilot myself, I had read your explanation above
in a number of texts) and making a couple of quick calculations;
When the collective is promptly lowered, the momentary low load factor
will very quickly result in a downward speed component of the
aircraft. This will of course then generate the flow up through the
rotor. ( If you do a 0g lever drop, which is not awfully aggressive,
you will obtain a 2000fpm rate of descent in about a second, from high
school math, right?) At least this was how I explained this to myself
then.

I do not know, what is the exact procedure for the entry to
autorotation in the British military, but the way the exercises were
conducted was to use cyclic to keep the aircraft attitude steady
during the entry (or in a few instances to do nothing with the cyclic
during power chop and entry to auto to observe the engine failure
cues). No deliberate back cyclic was applied in order to retain load
factor.

Mikko

(Disclaimer: The above is mostly my own observations. Please keep
doing your entries to auto as you have been trained and as your flight
manual tells you to. :)



>The coning may flop around some during the power to auto transition
>but when a steady auto is established the cone angle is very nearly
>the same as at hover because the horizonal and vertical forces on the
>blades are almost the same. There is some difference in where the
>center of lift is verses span distance on the blades between an auto
>and powered flight but this also varies with the amount of twist built
>into the blades.
>John
>
>
>On 2 Mar 2004 05:30:47 -0800, (Eric D)
>wrote:
>
>>If you start a descent from cruse by reducing pitch and maintaining
>>rotor speed, what happens to coning forces as compared to cruse?
>>
>>I have noticed that during a decent in a R22 that you can get a
>>"Wop-Wop" noise from the main rotor blades. Changing cyclic or
>>increasing rotor speed a very small amount can make it go away, or at
>>least not heard from the cockpit. Can someone explain what is
>>occurring and why?
>>
>>Thanks,
>>
>>Eric D