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How much turbulence is too much?



 
 
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  #1  
Old August 19th 03, 06:51 PM
Marty Ross
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Posts: n/a
Default How much turbulence is too much?

In reviewing the Warrior II's performance numbers in preparation for my
recent trip across California, I realized that Va would be much lower than I
had always considered it (given that it's just me inside, with gas burning
off towards reserve only). The Va that's written on my lapboard is 111
KIAS, however that's figured at gross weight (2325 lbs). It goes down all
the way to 88 KIAS (at 1531 lbs.), so I was surprised to see how slow I need
to go, according to normal procedures, when it gets turbulent in the Warrior
(e.g., I should slow down to mid-90's)!

-----------
(from the Warrior's POH, sec. 4.37: "Turbulent Air Operation"):
"In keeping with good operating practice used in all aircraft, it is
recommended that when turbulent air is encountered or expected, the airspeed
be reduced to maneuvering speed to reduce the structural loads caused by
gusts and to allow for inadvertent speed build-ups which may occur as a
result of the turbulence or of distractions caused by the conditions."
-----------

I specifically note this POH's references to "good operating practice used
in all aircraft" and "the structural loads caused by gusts" as they relate
to another question (of turbulence causing increased airframe stress) on
this thread.


"David Megginson" wrote in message
...
Ray Andraka writes:

How does one know if the turbulence is great enough that you should
slow down to Va to keep from structurally damaging the airplane?


If you have to ask, slow down.

Seriously, if I'm just bumping a bit (light chop), I don't worry. If
I'm actually getting uncommanded altitude or orientation changes
(i.e. a sudden 10-degree bank, or a sudden 200 foot altitude change),
I slow down to Va.

I also use some common sense. If I'm flying at 2500 ft AGL under a
clear sky in the afternoon (in the flatlands), I know it's just
thermals and a bit of mechanical turbulence: no big deal. If I'm
flying under cumuloform clouds and they're starting to build (and
there's reported ACC, TCU or CB), I assume that the turbulence could
get bad without warning, so I slow down as soon as I start bouncing
around.


All the best,


David

--
David Megginson, , http://www.megginson.com/



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  #2  
Old August 19th 03, 08:19 PM
Gary L. Drescher
external usenet poster
 
Posts: n/a
Default

"Marty Ross" wrote in message
news
In reviewing the Warrior II's performance numbers in preparation for my
recent trip across California, I realized that Va would be much lower than

I
had always considered it (given that it's just me inside, with gas burning
off towards reserve only). The Va that's written on my lapboard is 111
KIAS, however that's figured at gross weight (2325 lbs). It goes down all
the way to 88 KIAS (at 1531 lbs.), so I was surprised to see how slow I

need
to go, according to normal procedures, when it gets turbulent in the

Warrior
(e.g., I should slow down to mid-90's)!


Yup. One reason the W&B (or at least the W) is important, even if you know
you're well within bounds, is so you can estimate the Va that corresponds to
your payload. I make sure to have the flight's Va written on my kneeboard
notepad.

--Gary


-----------
(from the Warrior's POH, sec. 4.37: "Turbulent Air Operation"):
"In keeping with good operating practice used in all aircraft, it is
recommended that when turbulent air is encountered or expected, the

airspeed
be reduced to maneuvering speed to reduce the structural loads caused by
gusts and to allow for inadvertent speed build-ups which may occur as a
result of the turbulence or of distractions caused by the conditions."
-----------

I specifically note this POH's references to "good operating practice used
in all aircraft" and "the structural loads caused by gusts" as they relate
to another question (of turbulence causing increased airframe stress) on
this thread.


"David Megginson" wrote in message
...
Ray Andraka writes:

How does one know if the turbulence is great enough that you should
slow down to Va to keep from structurally damaging the airplane?


If you have to ask, slow down.

Seriously, if I'm just bumping a bit (light chop), I don't worry. If
I'm actually getting uncommanded altitude or orientation changes
(i.e. a sudden 10-degree bank, or a sudden 200 foot altitude change),
I slow down to Va.

I also use some common sense. If I'm flying at 2500 ft AGL under a
clear sky in the afternoon (in the flatlands), I know it's just
thermals and a bit of mechanical turbulence: no big deal. If I'm
flying under cumuloform clouds and they're starting to build (and
there's reported ACC, TCU or CB), I assume that the turbulence could
get bad without warning, so I slow down as soon as I start bouncing
around.


All the best,


David

--
David Megginson, , http://www.megginson.com/





  #3  
Old August 19th 03, 08:23 PM
Marty Ross
external usenet poster
 
Posts: n/a
Default

You seem to use percentage power as a primary input. How do you judge that?

In order to target a desired airspseed, I try to recall my performance chart
and just pick the appropriate RPM, without thinking about "percentage
power"; I'm interested to know if you find that thinking about "percentage
power" has some advantage.

I've only seen engine performance charts that give percentage power from
pressure altitude and rpm. Do you really do that calculation in order to
set desired percentage power from RPM, or are you just estimating?

"David Megginson" wrote in message
...
"Marty Ross" writes:

In reviewing the Warrior II's performance numbers in preparation for
my recent trip across California, I realized that Va would be much
lower than I had always considered it (given that it's just me
inside, with gas burning off towards reserve only). The Va that's
written on my lapboard is 111 KIAS, however that's figured at gross
weight (2325 lbs). It goes down all the way to 88 KIAS (at 1531
lbs.), so I was surprised to see how slow I need to go, according to
normal procedures, when it gets turbulent in the Warrior (e.g., I
should slow down to mid-90's)!


Or, to put it differently, you should throttle back to 55-65% power.
While it's not exact, on the Warrior II 55% power will tend to give
about 90 kias, 65% power will give about 100 kias, and 75% power will
give about 110 kias, whatever your density altitude (the IAS/CAS does
decrease slightly with altitude, but by only a few knots).

I specifically note this POH's references to "good operating
practice used in all aircraft" and "the structural loads caused by
gusts" as they relate to another question (of turbulence causing
increased airframe stress) on this thread.


Exactly: typically, Va will be double Vs. The Cessna 172P, for
example, has a lower Vs than the PA-28-161 (about 44 kias/50 kcas, vs
50 kias/57 kcas), so its Va is lower as well.


All the best,


David

--
David Megginson, , http://www.megginson.com/



  #4  
Old August 19th 03, 09:30 PM
Robert Moore
external usenet poster
 
Posts: n/a
Default

Ray Andraka writes:
How does one know if the turbulence is great enough that you
should slow down to Va to keep from structurally damaging the
airplane?


"David Megginson" wrote
If you have to ask, slow down.


"Marty Ross" wrote
In reviewing the Warrior II's performance numbers in preparation
for my recent trip across California, I realized that Va would
be much lower than I had always considered it (given that it's
just me inside, with gas burning off towards reserve only). The
Va that's written on my lapboard is 111 KIAS, however that's
figured at gross weight (2325 lbs). It goes down all the way to
88 KIAS (at 1531 lbs.), so I was surprised to see how slow I
need to go, according to normal procedures, when it gets
turbulent in the Warrior (e.g., I should slow down to mid-90's)!


Somehow I thought that the speed for maximum gust intensity was
Vb, not Va. Is Vb not the top of the "green arc"?
Va means "acceleration loads" as in pilot induced loads through the
use of the a/c controls.
Vb means "gust loads" as in weather induced loads.

Are pilots slowing un-necessarily?

From FAR Part 1:
VB means design speed for maximum gust intensity

From FAR Part 23:
Section 23.341: Gust loads factors.
(a) Each airplane must be designed to withstand loads on each
lifting surface resulting from gusts specified in 23.333(c).
(d) Design speed for maximum gust intensity, VB. For VB, the
following apply:
(1) VB may not be less than the speed determined by the
intersection of the line representing the maximum positive lift,
CNMAX, and the line representing the rough air gust velocity on
the gust V-n diagram, or VS1vng, whichever is less, whe

"Marty Ross" wrote
I specifically note this POH's references to "good operating
practice used in all aircraft" and "the structural loads caused
by gusts" as they relate to another question (of turbulence
causing increased airframe stress) on this thread.


Despite what the POH states, this is not the procedure used in
all aircraft.

Bob Moore
ATP CFI







  #5  
Old August 19th 03, 11:11 PM
David Megginson
external usenet poster
 
Posts: n/a
Default

"Marty Ross" writes:

You seem to use percentage power as a primary input. How do you
judge that?


For a fixed-pitch prop, you can judge it using the RPM directly, or
the IAS indirectly. Both need to be confirmed: you need to check the
RPM with an optical tach (mine indicates about 25 rpm high), and you
need to check your actual IAS at the proper power setting to see what
your plane does, in case it's draggier than it's supposed to be. For
my Warrior, 110 kias is pretty close for 75% power. If you want to be
more specific, I reverse-engineered these numbers from the true
airspeeds in the POH Performance section for 75%, and this is what
you're supposed to get:

0 ft DA: 113 ktas, 113 kcas, 115 kias
1000 ft DA: 114 ktas, 113 kcas, 115 kias
2000 ft DA: 116 ktas, 113 kcas, 115 kias
3000 ft DA: 117 ktas, 112 kcas, 114 kias
4000 ft DA: 119 ktas, 112 kcas, 114 kias
5000 ft DA: 120 ktas, 111 kcas, 113 kias
6000 ft DA: 121 ktas, 111 kcas, 113 kias
7000 ft DA: 123 ktas, 111 kcas, 113 kias
8000 ft DA: 124 ktas, 110 kcas, 112 kias
9000 ft DA: 126 ktas, 110 kcas, 112 kias

Speeds might be lower, of course, if they plane has old paint or is
badly rigged. I think I'm 2-3 knots slower than these numbers, but I
need to confirm now that I've had my tach rechecked.

In order to target a desired airspseed, I try to recall my
performance chart and just pick the appropriate RPM, without
thinking about "percentage power"; I'm interested to know if you
find that thinking about "percentage power" has some advantage.


It lets me know how much fuel I'm going to burn, how hot/hard I'm
running my engine, and whether my engine is running properly (if the
IAS and RPM are too far out of whack, something is wrong).

I've only seen engine performance charts that give percentage power
from pressure altitude and rpm. Do you really do that calculation
in order to set desired percentage power from RPM, or are you just
estimating?


A combination of both. You always need to know your density altitude
to set power with a fixed-pitch prop anyway, so I'll assume you've
already got that info handy. Then all you have to do for 75% power in
the Warrior II is start at 2480 rpm for sea level, and add 25 rpm for
every 2000 ft of density altitude -- it's an easy calculation to do in
your head.


All the best,


David

--
David Megginson, , http://www.megginson.com/
  #6  
Old August 20th 03, 08:41 AM
Marty Ross
external usenet poster
 
Posts: n/a
Default

Interesting (and thorough) analysis... No wonder your SAX parser worked so
well...

OK now I have another problem with this. Shouldn't this depend on the
propeller? For example, I believe the limitation on static rpm (for maximum
throttle setting) on the plane I fly is less than the 2480 number you
mention, so I would guess all of the numbers will be different for me.
However, my POH seems to show the power curves up to 2700 rpm, and does not
mention the type of propeller (maybe it's the "standard" prop?).

What's with your maximum static rpms? Can you really spin your prop up much
higher than 2480 (e.g., without damaging it)?

"David Megginson" wrote in message
...
"Marty Ross" writes:

You seem to use percentage power as a primary input. How do you
judge that?


For a fixed-pitch prop, you can judge it using the RPM directly, or
the IAS indirectly. Both need to be confirmed: you need to check the
RPM with an optical tach (mine indicates about 25 rpm high), and you
need to check your actual IAS at the proper power setting to see what
your plane does, in case it's draggier than it's supposed to be. For
my Warrior, 110 kias is pretty close for 75% power. If you want to be
more specific, I reverse-engineered these numbers from the true
airspeeds in the POH Performance section for 75%, and this is what
you're supposed to get:

0 ft DA: 113 ktas, 113 kcas, 115 kias
1000 ft DA: 114 ktas, 113 kcas, 115 kias
2000 ft DA: 116 ktas, 113 kcas, 115 kias
3000 ft DA: 117 ktas, 112 kcas, 114 kias
4000 ft DA: 119 ktas, 112 kcas, 114 kias
5000 ft DA: 120 ktas, 111 kcas, 113 kias
6000 ft DA: 121 ktas, 111 kcas, 113 kias
7000 ft DA: 123 ktas, 111 kcas, 113 kias
8000 ft DA: 124 ktas, 110 kcas, 112 kias
9000 ft DA: 126 ktas, 110 kcas, 112 kias

Speeds might be lower, of course, if they plane has old paint or is
badly rigged. I think I'm 2-3 knots slower than these numbers, but I
need to confirm now that I've had my tach rechecked.

In order to target a desired airspseed, I try to recall my
performance chart and just pick the appropriate RPM, without
thinking about "percentage power"; I'm interested to know if you
find that thinking about "percentage power" has some advantage.


It lets me know how much fuel I'm going to burn, how hot/hard I'm
running my engine, and whether my engine is running properly (if the
IAS and RPM are too far out of whack, something is wrong).

I've only seen engine performance charts that give percentage power
from pressure altitude and rpm. Do you really do that calculation
in order to set desired percentage power from RPM, or are you just
estimating?


A combination of both. You always need to know your density altitude
to set power with a fixed-pitch prop anyway, so I'll assume you've
already got that info handy. Then all you have to do for 75% power in
the Warrior II is start at 2480 rpm for sea level, and add 25 rpm for
every 2000 ft of density altitude -- it's an easy calculation to do in
your head.


All the best,


David

--
David Megginson, , http://www.megginson.com/



  #7  
Old August 20th 03, 01:06 PM
David Megginson
external usenet poster
 
Posts: n/a
Default

"Marty Ross" writes:

OK now I have another problem with this. Shouldn't this depend on
the propeller? For example, I believe the limitation on static rpm
(for maximum throttle setting) on the plane I fly is less than the
2480 number you mention, so I would guess all of the numbers will be
different for me. However, my POH seems to show the power curves up
to 2700 rpm, and does not mention the type of propeller (maybe it's
the "standard" prop?).


If you have a PA-28-161 (or any other plane recent enough to have a
standard POH), look at section 1 "General" to see what propeller the
POH uses for the performance tables. Under subsection 1.5
"Propellers", I have this information:

(a) Number of Propellers 1
(b) Propeller Manufacturer Sensenich
(c) Model 74DM6-0-60
(d) Number of Blades 2
(e) Propeller Diameter (inches()
(1) Maximum 74
(2) Minimum 72
(f) Propeller Type Fixed Pitch

If you don't have that propeller, I'll hope that you got revised
performance tables with your STC or field approval, but I don't know
enough about the regs.

What's with your maximum static rpms? Can you really spin your prop
up much higher than 2480 (e.g., without damaging it)?


Yes -- it's rated up to 2700 rpm in the type certficate, but obviously
it cannot get up there in a static run. At sea level ISA, with the
60-inch-pitch propeller, minimum static RPM (i.e. sitting still on the
ground) is 2330 and maximum static RPM is 2430 -- for the propeller to
spin any faster, the plane has to be moving.

With a 58-inch-pitch propeller (also in the type certificate), the
static RPM range at sea level ISA is 2365-2465.

In my Warrior II (or a 172 from the same period), you have to spin the
propeller faster to get rated performance at higher altitudes -- at
8000 ft density, 75% power is 2665 rpm, for example.


All the best,


David

--
David Megginson, , http://www.megginson.com/
  #8  
Old August 20th 03, 02:54 PM
Mike Rapoport
external usenet poster
 
Posts: n/a
Default

I've never seen or heard of Vb being published for light aircraft.

Mike
MU-2


"Robert Moore" wrote in message
. 8...
Ray Andraka writes:
How does one know if the turbulence is great enough that you
should slow down to Va to keep from structurally damaging the
airplane?


"David Megginson" wrote
If you have to ask, slow down.


"Marty Ross" wrote
In reviewing the Warrior II's performance numbers in preparation
for my recent trip across California, I realized that Va would
be much lower than I had always considered it (given that it's
just me inside, with gas burning off towards reserve only). The
Va that's written on my lapboard is 111 KIAS, however that's
figured at gross weight (2325 lbs). It goes down all the way to
88 KIAS (at 1531 lbs.), so I was surprised to see how slow I
need to go, according to normal procedures, when it gets
turbulent in the Warrior (e.g., I should slow down to mid-90's)!


Somehow I thought that the speed for maximum gust intensity was
Vb, not Va. Is Vb not the top of the "green arc"?
Va means "acceleration loads" as in pilot induced loads through the
use of the a/c controls.
Vb means "gust loads" as in weather induced loads.

Are pilots slowing un-necessarily?

From FAR Part 1:
VB means design speed for maximum gust intensity

From FAR Part 23:
Section 23.341: Gust loads factors.
(a) Each airplane must be designed to withstand loads on each
lifting surface resulting from gusts specified in 23.333(c).
(d) Design speed for maximum gust intensity, VB. For VB, the
following apply:
(1) VB may not be less than the speed determined by the
intersection of the line representing the maximum positive lift,
CNMAX, and the line representing the rough air gust velocity on
the gust V-n diagram, or VS1vng, whichever is less, whe

"Marty Ross" wrote
I specifically note this POH's references to "good operating
practice used in all aircraft" and "the structural loads caused
by gusts" as they relate to another question (of turbulence
causing increased airframe stress) on this thread.


Despite what the POH states, this is not the procedure used in
all aircraft.

Bob Moore
ATP CFI









  #9  
Old August 21st 03, 05:30 PM
Tarver Engineering
external usenet poster
 
Posts: n/a
Default


"David Megginson" wrote in message
...
"Marty Ross" writes:

OK now I have another problem with this. Shouldn't this depend on
the propeller? For example, I believe the limitation on static rpm
(for maximum throttle setting) on the plane I fly is less than the
2480 number you mention, so I would guess all of the numbers will be
different for me. However, my POH seems to show the power curves up
to 2700 rpm, and does not mention the type of propeller (maybe it's
the "standard" prop?).


If you have a PA-28-161 (or any other plane recent enough to have a
standard POH), look at section 1 "General" to see what propeller the
POH uses for the performance tables. Under subsection 1.5
"Propellers", I have this information:

(a) Number of Propellers 1
(b) Propeller Manufacturer Sensenich
(c) Model 74DM6-0-60
(d) Number of Blades 2
(e) Propeller Diameter (inches()
(1) Maximum 74
(2) Minimum 72
(f) Propeller Type Fixed Pitch

If you don't have that propeller, I'll hope that you got revised
performance tables with your STC or field approval, but I don't know
enough about the regs.


Ever since the Governor of South Dakota was killed by an improperly selected
Propeller, the rules have become very stricct in those changes.


 




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