Turbulence

"Peter Duniho" wrote

If the air is bumpy, it always potentially could exceed the design
limits of the airplane. For that matter, you could be flying along in
completely smooth air and experience sudden and severe turbulence.
I haven't read Machado's book, so I don't know what he says and what
he doesn't. I would disagree that there's never any turbulence
outside of a thunderstorm that you need to worry about, or that
there's never any moderate turbulence in which newer planes might have
a concern. Those kinds of absolutes seem troublesome to me.

Quoted from Aerodynamics for Naval Aviators:

"The loads imposed on an aircraft in flight are the result of
maneuvers and gusts."
"As a general requirement, all airplanes must be capable of
withstanding an approximate effective +/- 30 foot per second
gust when at maximum level flight speed for normal rated power.
Such a gust intensity has relatively low frequency of occurrence
in ordinary flying operations."

BTW, the aircraft must withstand the 30 fps gust at Vno (top of
the green arc) even if the aircraft cannot attain Vno at maximum
power.
Weather induced gust loading establishes Vno, pilot induced
maneuver loads establishes Va.

Bob Moore

On Sat, 09 Oct 2004 00:04:33 GMT, Bob Moore
wrote in ::

Quoted from Aerodynamics for Naval Aviators:

"The loads imposed on an aircraft in flight are the result of
maneuvers and gusts."

There's some information on the subject he
http://ecfr.gpoaccess.gov/cgi/t/text...3.70.8&idno=14
§ 23.333 Flight envelope
(c) Gust envelope. (1) The airplane is assumed to be subjected to
symmetrical vertical gusts in level flight. The resulting limit load
factors must correspond to the conditions determined as follows:

(i) Positive (up) and negative (down) gusts of 50 f.p.s. at VC must be
considered at altitudes between sea level and 20,000 feet. The gust
velocity may be reduced linearly from 50 f.p.s. at 20,000 feet to 25
f.p.s. at 50,000 feet.

(ii) Positive and negative gusts of 25 f.p.s. at VD must be considered
at altitudes between sea level and 20,000 feet. The gust velocity may
be reduced linearly from 25 f.p.s. at 20,000 feet to 12.5 f.p.s. at
50,000 feet.

On Sat, 09 Oct 2004 00:04:33 GMT, Bob Moore
wrote in ::

Quoted from Aerodynamics for Naval Aviators:

"The loads imposed on an aircraft in flight are the result of
maneuvers and gusts."

There's some information on the subject he
http://ecfr.gpoaccess.gov/cgi/t/text...3.70.8&idno=14
§ 23.333 Flight envelope
(c) Gust envelope. (1) The airplane is assumed to be subjected to
symmetrical vertical gusts in level flight. The resulting limit load
factors must correspond to the conditions determined as follows:

(i) Positive (up) and negative (down) gusts of 50 f.p.s. at VC must be
considered at altitudes between sea level and 20,000 feet. The gust
velocity may be reduced linearly from 50 f.p.s. at 20,000 feet to 25
f.p.s. at 50,000 feet.

(ii) Positive and negative gusts of 25 f.p.s. at VD must be considered
at altitudes between sea level and 20,000 feet. The gust velocity may
be reduced linearly from 25 f.p.s. at 20,000 feet to 12.5 f.p.s. at
50,000 feet.

Gary Drescher wrote:


Minor correction: the wings snap due to excessive lift (force), not due to
excessive Gs (acceleration).

What?!!! You can't have force with out acceleration and you can't have
acceleration with out force. The most basic formula in physics is:
F = MA or Force = Mass times Acceleration. A net force gives you
acceleration.


--
Chris W

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Gary Drescher wrote:


Minor correction: the wings snap due to excessive lift (force), not due to
excessive Gs (acceleration).

What?!!! You can't have force with out acceleration and you can't have
acceleration with out force. The most basic formula in physics is:
F = MA or Force = Mass times Acceleration. A net force gives you
acceleration.


--
Chris W

Bring Back the HP 15C
http://hp15c.org

Not getting the gifts you want? The Wish Zone can help.
http://thewishzone.com

"Chris W" wrote in message
news:a9T9d.5690$cJ3.3838@fed1read06...
Gary Drescher wrote:

Minor correction: the wings snap due to excessive lift (force), not due
to excessive Gs (acceleration).

What?!!! You can't have force with out acceleration and you can't have
acceleration with out force. The most basic formula in physics is:
F = MA or Force = Mass times Acceleration. A net force gives you
acceleration.

Yes, of course. But staying below Vno is what keeps the wings from snapping,
by keeping the lift (force) from being more than the wing can bear. Staying
below Va is what keeps the aircraft's acceleration from being more than
other components can bear.

The difference is important, because it explains why you have to scale Va
(but not Vno) in proportion to the square root of your gross weight. For a
given airspeed, weighing less obviously does not put greater lift force on
the wings. But (again for a given airspeed), weighing less does increase the
acceleration that the lift force causes (F=MA; for a given airspeed, lift=F
is constant; so less M implies more A). That's why Va becomes a stricter
limit at lower weight, whereas Vno is unchanged.

--Gary

"Chris W" wrote in message
news:a9T9d.5690$cJ3.3838@fed1read06...
Gary Drescher wrote:

Minor correction: the wings snap due to excessive lift (force), not due
to excessive Gs (acceleration).

What?!!! You can't have force with out acceleration and you can't have
acceleration with out force. The most basic formula in physics is:
F = MA or Force = Mass times Acceleration. A net force gives you
acceleration.

Yes, of course. But staying below Vno is what keeps the wings from snapping,
by keeping the lift (force) from being more than the wing can bear. Staying
below Va is what keeps the aircraft's acceleration from being more than
other components can bear.

The difference is important, because it explains why you have to scale Va
(but not Vno) in proportion to the square root of your gross weight. For a
given airspeed, weighing less obviously does not put greater lift force on
the wings. But (again for a given airspeed), weighing less does increase the
acceleration that the lift force causes (F=MA; for a given airspeed, lift=F
is constant; so less M implies more A). That's why Va becomes a stricter
limit at lower weight, whereas Vno is unchanged.

--Gary

"Gary Drescher" wrote in message
news:nCT9d.150772$wV.28126@attbi_s54...
for a given airspeed, lift=F is constant;

Sorry, I should have said: for a given airspeed, and at the maximum
coefficient of lift, lift=F is constant. (The limits Va and Vno are
calculated to be the airspeeds that offer protection even at the angle of
attack that achieves the maximum coefficient of lift.)

--Gary

"Gary Drescher" wrote in message
news:nCT9d.150772$wV.28126@attbi_s54...
for a given airspeed, lift=F is constant;

Sorry, I should have said: for a given airspeed, and at the maximum
coefficient of lift, lift=F is constant. (The limits Va and Vno are
calculated to be the airspeeds that offer protection even at the angle of
attack that achieves the maximum coefficient of lift.)

--Gary

Marco
Be careful doing what others say to do, one day it may cause problems for you.
Some people who fly only go by rules they make up for their airplane .. ie ..
they see nothing wrong with flying thorugh clouds while VFR, put tape over gas
tank filler hole because they lost their gas cap, perform an instrument
approaoch in IMC with a handheld GPS and no approach plates, fly into known
icing and so on.

Personally, If I expect turbulence I will slow down a bit or as I got in the
habit of doing at this mountain near kingman, I would pass the mountain on the
upwind side to avoid the turbulence it causes. I have been in what I called
severe before, when I got done bouncing I was pointed 90 degrees off course, it
was just bone jarring bumps, felt like a hundred of them in a matter of a few
seconds.

My suggestion is to fly the way your used to flying, as you get more time, you
get used to the bumps and will start to tell when you may experience them and if
they are an issue. If you want to slow down, slow down, nothing wrong with it,
its your flight. Its better to slow down then assume some type of turbulence is
less then it is and cause damage to your plane or make your passangers not want
to fly with you again.


Marco Rispoli wrote:

One of the things that TERRIFIED me for the longest time was turbulence.

In fact, the very reason why my wife bought me a "Discovery Flight" at the
very beginning (and got me hooked to flying) was to get me to see that there
was nothing to fear from planes, after me and her had a really bad (for me)
flight back from Italy.

Ok ... after a little less than 100 hours of flight I have had my dose of
turbulence in flight ... while at the controls of a plane.

It is reccommended that if you encounter turbulence you slow down below Va
(manouvering speed), so if you encounter an especially stiff gust the plane
will stall before pulling enough Gs to snap a wing. That's what the book
says ... and I used to follow it to the letter, to the point that even the
minimum turbulence would make me slow down to below Va.

I remember once, during a mock checkride with a senior instructor we
encountered some turbulence and I instinctively pulled the power to slow the
plane below Va.

The intructor's reaction was instantaneous and unyelding:
"The hell are you doin'?" (Heavy Staten Island accent).
"Slowing the plane below manouvering speed" I say, my tone of voice like
that of a guy that has just been asked why the sun comes up in the morning.
"Why?"
"Ah .. " I stammer surprised by the question ... isn't it obvious why?
"Because we are in turbulence?" it was supposed to be a statement but it
came out as a question ... a sheepish question at that.
The Instructor roars in laughter, for an instant louder than the engine.
"Dude, this is not turbulence. Put that power back in. I want to see that
needle at 2400 RPMs, in the RPM green arc." End of the argument.

It was light turbulence with occasional moderate. It was one of those cold
late winter days when winter is about to give up to spring but doesn't quite
like the idea ...

To me it felt like we were getting slammed. To him this wasn't even a
tickle. Obviously our concepts of turbulence differed dramatically.

So, question to the more seasoned and weathered pilots out the

If I am clear air (VFR), and I am not close to mountains (so I don't have to
worry about mountain waves), and I am flying around in my little cutie Piper
Cherokee 180 do I even need to worry about turbulence?

I am just about done reading Rod Machado's IFR book and the bottom line is
that you need to be inside a thunderstorm in order to start worrying about
turbulence. In most conditions, just flying around VFR in your everyday CAT
(Clear Air Turbulence) doesn't warrant fussing about it, and all you really
have to do is just stay in the green arc. That's all.

According to what the book is saying, only older planes really need to worry
about Va in moderate turbulence. Modern planes (certified after 69) can fly
right through it, as long as you stay in the green arc.

I don't plan to fly into thunderstorms any time soon ... so, do I even need
to worry about manouvering speed if I am just flying in one of those gusty
fall or spring Northeast VFR days?

Is staying below the green arc good enough?

--
Marco Rispoli - NJ, USA / PP-ASEL
My on-line aviation community - http://www.thepilotlounge.com