You don't have to exceed VNE to see a high enough true airspeed that
could
cause flutter.
VNE is indicated airspeed, not true airspeed. As you go higher and the
air
gets thinner, your IAS can remain the same -- and well within the
green arc
-- while your true airspeed goes higher.
With flutter, it is the true speed of the air passing over the
airframe that
is critical -- because it has to do with the frequency of the vibrations.
Just like an engine has certain critical frequencies, it will be
susceptible
at certain engine speeds.
If you're flying at 12,000 feet and you encounter a gust, you should
be okay
in terms of structural integrity as long as you are in the green arc.
That's
because even though your true airspeed is higher, the dynamic pressure
your
airframe is seeing is the same as it would be at a lower speed and higher
air pressure. So the increased angle of attack caused by the gust will
not
break your airplane, as long as you are in the green arc.
However, this does not protect you against flutter. If you encounter that
same gust while going at the same speed, well within the green, it
will not
break your airplane, but it could set your tail vibrating. And because
you
are going at a higher true airspeed, the frequencies may not damp out as
they would at lower true airspeed.
Have a look at that Van's article; it has a first-hand report from an RV4
pilot who was going high to climb over some clouds. With his IAS well
in the
green, he got a scary shaking in his tail as he executed a 360 to avoid
towering cumulus. And this guy should be a pretty good stick -- his
day job
is driving an F-16.
So even if you stay well within VNE you could still get flutter if
your true
airspeed is high enough.
Regards,
Gordon.
So what you are saying is one cannot relie on the markings of
one's airspeed indicator. Does the FAA know about that?
Perhaps he built the airplane himself and he did not balance
things properly. Either that the RV4 has insufficient margin
of safety in the tail flutter which I find hard to believe
given all the people flying them and the total absence of
flutter induced crashes. Another possibility is the tail was
painted by somebody that did not rebalance the surfaces.
Paul Lamar ...No rotor no motor.
My take on Vans published Vne limits was that it was only partially
related to flutter- the real limiting factors involved structural
strength margins given industry standard parameters of planning for
~50ft/min downdraft/up drafts in turbulent air at the various
airspeeds. The 9series RV's have longer, more efficient Roncz-style
wings designed to withstand 4.6 Gs; they perform very well with less HP
but structural limits cap the top speed. The choice of the 160HP-max
engine pretty much limits performance below Vne at higher altitudes,
unless they are turbocharged. FWIW, I have a friend who was flying off
his initial hours in a new 9A that popped all of the rivets on the
elevator trailing edge in a 230 (dont know kt or mph) indicated airspeed
dive from 9000 ft altitude- I assume flutter was the reason. He had no
warning as the plane was flying beautifully before he felt the sudden
vibration.
The short wide wings on the RV 7and 8's have a higher 6G rating and are
therefore rated for larger engines and higher top speeds. I believe the
RV8 has a top speed/Vne around 230, although the cruise speed specs are
only a few mph higher than the 9's.
Mike Parker
These are all cruise and top speed issues. You don't have to open the
throttle
all the way if you don't want to. However the HP is there when you need
it.
Namely getting off the ground under all conditions expeditiously and
climbing
over clouds and mountains. It is analogous to a car with a lot of power.
Who
drives at 150 MPH? But you need the HP margin at 60 MPH to pass a truck
quickly on
a two lane road.
Paul Lamar
Mike,
The reason for the limits has to do with flutter not structural strength.
That's because air loads are a function of indicated airspeed -- or more
precisely dynamic pressure -- not the actual speed of your plane through
the
air.
Structural strength will not be compromised as long as you stay within the
indicated airspeed limitations: within the green arc for gust loads, and
below maneuvering speed for full control inputs, etc. -- regardless of
your
altitude or true airspeed. The Vne that is marked at the top of the yellow
arc is indicated airspeed.
If structural strength were the only concern you could go to 40,000
feet --
if your plane could get that high -- and fly along at a true airspeed of
400
mph and as long as you were in the green arc on your airspeed indicator,
you
would not have a problem with structural issues. Gusts up to 50 ft/sec
would
be fine because that's the load the plane was designed to take.
The reason is that your airspeed indicator does not measure the velocity
of
the air going past the wings, it measure dynamic pressure, which is a
function of air density and velocity.
You could have a lot more velocity, but at if the air density is lower the
dynamic pressure will stay the same. The actual sum total of pressure
exerted by the air molecules against your wing will be the same. Thus the
structural loads will be the same.
For the record, dynamic pressure is known as Q and is defined as Q +
1/2pV2
(That's V squared, where V is velocity and p is air density.)
Dynamic pressure plays a big part in the Lift Equation: L = 1/2pV2 x S x
Cl,
where S is wing surface area and Cl is coefficient of lift.
So your aircraft structure will still be sound even at very high true
airspeeds, as long as the indicated airspeed is within its proper
limits --
because the dynamic pressure is what determines air loads.
But flutter is a different thing altogether. Flutter is a vibration and
like
all vibrations it is sensitive to frequency. The frequency in this case is
the air molecules passing over the airframe surfaces. It has nothing to do
with airframe strength, but everything to do with whether some part of the
airplane will begin to resonate at a certain frequency of air molecules
passing over it. (More precisely it will take some sort of initial
disturbance to get the airplane part vibrating, and then the speed of the
air plays a role in whether the vibration is allowed to keep building or
is
damped out.)
So the bottom line is that there could be a situation where you are well
within Vne, or even the green arc, and yet if the actual velocity of the
air
going past your airplane is fast enough, some parts of your airplane could
be susceptible to vibration.
This does go against the grain of what we are used to because we have
always
been taught that Vne is the limit that must be obeyed. But if you give
your
airplane the ability to go higher than it was designed to go, it will also
be able to go at a faster true airspeed, which means you could get
flutter,
even if you stay within indicated airspeed limits.
However, your airframe will not see any higher air loads as long as you
obey
the indicated airspeed limits, so structural strength will not be an
issue.
Of course it's scant consolation that your wings remain firmly attached,
but
your tail falls off due to shaking.
I have attached the Van's article. It's worth a read.
Regards,
Gordon.
I am not disputing any of this. I stand by my original statement
of not to exceed vne. The one point vne should be replaced
by a vne curve showing vne as a function of altitude.
The implications are wide spread. This may mean it is useless
and a waste of money turbo charging your RV whatever.
This could have a significant impact on the sales of the RV10 which
would normally be a good candidate for turbo charging and high
altitude cruising.
Specifying a max HP for the airplane to prevent flutter at
high altitudes is dumb. People are more likely to make aerodynamic
improvements that allow a greater speed. This is the primary path
of people like Dave Anders and Tracy Saylor use to increase
the speed of their RVs.
Van could however make changes in the tail surface that would
insure flutter did not occur up to say 26,000 feet and the same
vne used at lower altitudes that IS set by structural limits.
Stiffness to weight ratio is a large factor in flutter.
Aluminum has a much lower modulus of elasticity to density
ratio than carbon fiber. Van could use carbon fiber tail
surfaces to improve the flutter margin. The RV 10 has a major
part of its fuselage already made from composites.
Paul Lamar ...No rotor no motor.
Mark W wrote:
Well presented articles on Vne. Thanks guys.
One other thing working againstyou with increasing altitude is damping.
Although the airflow is providing the flutter exciting force, the flow field
would never be uniform enough the all the air is at the exciting frequency,
so some will be damping the system. The reducing density at altitude is also
reducing the damping effect provided by the air.
Of course a kit manufacturer has to be conservative with Vne. If you take a
good look at the workmanship of some homebuilts (especially the tail
sections which is often made first before the guys have developed any skills
or an eye for quality), then you appreciate that Van's have to set these
parameters to a pretty low common denominator. In addition, for a homebuilt,
the flight testing of the prototype consisted of a rudimentary program based
of one non conforming airframe, two tops.
Dave Ander's superbly built fastback RV-4 with balanced carbon fiber tail
feathers has little in common with a bottom of the line slapped together
RV-4. The variation in manufacture between homebuilts is one reason why
there is no type certificate.
Mark
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