gene whitt

Y'all,
Im going to better looking minds in search of an answer that I was unable to
give to a student's question.

The Question:
Why does an aircraft have a Va at gross of 97 knots and a structrual cruise
speed (where the orange and green meet) of 122 knots?
The first related to control movement and the second
predicated on turbulence. If the potential for damage exists at both speeds
why such a wide spread between the two speeds?

Gene Whitt

icebound

I thought that: above your "Orange and green meet" (V-no) speed, there is
potential for damage in turbulence even with zero control deflection, and
that below Va is safe for full control deflection. Presumably just above
Va, damage would occur with full deflection... at somewhat higher spped,
damage would occur at half deflection... and at V-no damage could occur with
only slight deflection. ???


--
*** A great civilization is not conquered from without until it
has destroyed itself from within. ***
- Ariel Durant 1898-1981

andrew sarangan

This is my understanding of Va vs Vno.

Va assumes a sudden change in angle of attack caused by full control
deflection. The resulting load factor will not exceed 3.8G if you stay
below Va.

Vno accounts for wind gusts only and does not assume full control
deflection. Presumably, the effect of wind gusts is smaller than full
control deflection, and as a result Vno is faster than Va.

mhteas

Hm. Not sure I should presume to have a better mind, but one should
reach high for goals, so here goes...


Vno is solely due to the stress of drag forces with no G forces
involved. Ever stick your hand out the window at cruise speed?
There's a lotta force there that has to be supported.

Since Va is for control movement, it's going to incur a increase in
load due to G forces. This is an increase over and above the drag
forces present at Va. So, Va is loading in several ways and Vno is
only loading in one way.

-Malcolm

dale

Don't think so. The aircraft must be able to withstand prescribed gust
velocities up to Vno so there is some load factor consideration.

--
Dale L. Falk

There is nothing - absolutely nothing - half so much worth doing
as simply messing around with airplanes.

http://home.gci.net/~sncdfalk/flying.html

anothername

Va is frequently used as turbulent air penetration speed. Slowing down
helps, but there is no guarantee. And it is curious, because what
determines Va is control movement, not movement due to turbulent air.
Vo is determined by who knows what, but is related to turbulence
breaking the structure.
I agree, it doesn't make sense. Never has to me. It's out of date and
probably erroneous. But it seems to work, so we use it. Its really not
well defined or explained anywhere that I have seen. I think the FAA
should clear this one up. The FAA has a "standard issue" turbulence
criteria (hope God agrees). They are useful speeds to know, but the
definitions don't match with the way we use them, especially Va. I
guess it will breakup past Vo, but it will take more to break it up at
Va. But if you go slower than Va, it will take even more to break it
up. So Vo is really too fast for turbulence, Va is better and easier
on the airframe and allows full control movement if its needed to keep
the plane upright. Much slower than Va is dangerous due to getting
close to stall speed...

Anyway, if you get hit by the Minden wave, slow down to Va!

usenetreplies

Not true. There is a guarantee that if you fly Va or slower
(corrected for weight) and you hit an updraft, you will stall before
you exceed positive g-loading limits. Whether this is good is another
matter, but that much is indeed guaranteed.

Va is generally stall speed at max positive g-loading. How you attain
the g-loading is not really important. There are other criteria that
must be met at Va (for example, you must be able to perform a full
rudder deflection, either direction, from zero yaw rate. That means,
practically, that you can go to the stop once. You can't necessarily
go immediately to the opposite stop) but for light planes, this is the
biggie.

Note that this provides no protection against exceeding negative
g-loading, or against g-loading with a rolling component, or lots of other things.

The criteria for Vno (which is what I think you mean) are spelled out
in 14CFR23. Much insight can be gained by reading 14CFR23 - it
explains EXACTLY what the airplane must be able to do at Va, Vno, Vne, etc.

I do not concur. Vno is maximum speed at which a standard gust will
not cause the positive g-loading limit to be exceeded. The standard
gust is defined (in 14CFR23 of course). Va is maximum speed at which
ANY gust will not cause the positive g-loading limit to be exceeded
(you will stall first). Why would there NOT be a difference?


It works because the defined parameters are reasonable.

Look in Part 23.


It's already very clear.


The criteria have stood the test of time. Mostly.

If you go slower than Va, a vertical gust CAN'T make it break up. You
will stall first. Then you will probably overstress the plane in the
recovery...

No, it does NOT necesarily allow full control movement. You need to
read Part 23 to find out exactly what control movement it allows.

Michael

kdarling

It's because Vno assumes no control movements. Now, Va is an
interesting item. You have to watch out for what people base their
answers on.

The FAR 23 version of Va is a FIXED speed, based on a theoretical Vs1,
used only to help in construction design. In fact, they go so far as
to point this out in AC23-19 AIRFRAME GUIDE FOR CERTIFICATION OF PART
23 AIRPLANES (01-27-03):
"(c) The design maneuvering speed is a value chosen by the
applicant. It may not be less than Vs*sqrt(n) and need not be greater
than Vc, but could be greater if the applicant chose the higher value.
The loads resulting from full control surface deflections at Va are
used to design the empennage and ailerons in 14 CFR part 23, §§
23.423, 23.441, and 23.455.
"Va should not be interpreted as a speed that would permit the
pilot unrestricted flightcontrol movement without exceeding airplane
structural limits nor should it be interpreted as a gust penetration
speed. Only if Va = Vs*sqrt(n), will the airplane stall in a nose-up
pitching maneuver at, or near, limit load factor. For maneuvers where
Va>Vs*sqrt(n), the pilot would have to check the maneuver; otherwise
the airplane would exceed the limit load factor."

The Va that pilots are taught about, however, is really FAR 23 "Vo".
It is defined by 23.1507 and the placard is mandated by 23.1563.
"Sec. 23.1507 [Operating] maneuvering speed. [The maximum operating
maneuvering speed, VO, must be established as an operating limitation.
VO is a selected speed that is not greater than Vs sqrt(n) established
in Sec. 23.335 c).]
"Sec. 23.1563 Airspeed placards. There must be an airspeed placard
in clear view of the pilot and as close as practicable to the airspeed
indicator. This placard must list-- (a) The operating maneuvering
speed VO; [...]"

Doing some calculations, it appears that older Cessna 172 specs will
often state the older FAR 23 Va, which is the theoretical (and
dangerous) one. Moreover, even if you calculate the correct Va (Vo)
for the aircraft's weight, so in theory the plane should stall before
breaking anything, there is still the possibility of frontal wind
shears boosting your airspeed above Va.

Kevin
(Not an expert, but I did sleep at a Days Inn once)

julian scarfe

But it is, as you clearly point out later, only protection against vertical
gusts, not horizontal (+ vertical) gusts.

Moreover, I'm not sure that exceeding positive g-loading limits is the
dominant issue in making sure the aircraft doesn't break. While some parts
of the aircraft may not like exceeding those limits, you may find that it's
the absolute load on the main spar that's the real limit. At lighter
weights, you can considerably exceed the g-loading limits without exceeding
that absolute load.

I've always been reluctant to slow to low Va values in turbulence. As you
rightly point out, stalling in turbulence (because of wind shear, horizontal
gusts) is not nice.

Julian

anothername

I'd like to have an aeronautical engineer take a look at this
statement. Right now, based on the limited statics and dynamics
undergrad course I had in engineering, I'm not buying it. It's the ANY gust, I don't buy.