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.
Paul,
I should correct my previous post with regard to the example I gave of the
hypothetical small plane flying at 400 mph at 40,000 feet. I was giving an
exaggerated example to illustrate the point about dynamic pressure, but the
truth is that at such a high true airspeed, compressibility effect will come
into play. It was a bad example.
Compressibility effects, also known as Mach number effects, start to come
into play at true airspeeds above 250 mph and actually cause the density of
the air around the airplane to change.
What happens when an airplane is flying through the air below 250 mph is
that the air molecules go around the plane pretty much undisturbed. Above
that speed, the airplane's energy actually compresses the air right around
the plane's surface somewhat as it hits it, making it denser. This means
that the air loads can indeed be greater than they would be by simply taking
into consideration dynamic pressure alone.
Admittedly compressibility is just taking effect at 250 mph and its effect
at that point is really negligible. However at 400 mph it will be enough to
noticeably increase air loads, so structural strength does become an issue
at some speed level.
For the purposes of Van's airplanes though, compressibility is not going to
be an issue, because even at turbocharger altitudes, true airspeeds will not
be much above 250 mph -- so it is still true that air loads will be the same
at higher true air speeds as long as the indicated airspeed is the same.
This is not the case with Columbia's 400 though. It is just fast enough to
get into the speed where compressibility starts to have an effect. So the
reason that the FAA has lowered Vne at high altitudes probably does have to
do with structural strength rather than flutter. The Columbia is also
certified to the higher strength limits of the Utility Category, so this
makes it even more difficult.
I agree with you that it seems rather clumsy for Van's to address flutter
issues by limiting the power of the plane. However, at least they are
providing some guidance -- there are probably kits out there that have given
no thought to this whatsoever.
The other issue that bears mentioning is that people who modify their planes
to go fast do not necessarily have to run into flutter, even if they
regularly fly in the yellow arc, or even exceed Vne -- provided they fly
fast at low altitude.
Again it's because of dynamic pressure. For the sake of illustration, let's
say we have a plane with a Vne right at 200 mph. If you are flying near sea
level and exceed that speed by say 20 mph, your true airspeed and indicated
airspeed are both 220 mph. At this speed it's not likely you will get
flutter.
However, since you are exceeding Vne, you could have structural strength
problems if you encounter a gust or make large or sudden control inputs.
Now let's say you take that same plane up to 20,000 feet and you are flying
along at 180 mph indicated, well below Vne and within the green arc.
Here your true airspeed will be about 250 mph.
At this point, you will not have structural worries because the air loads
will be exactly the same as if you were flying along at an indicated
airspeed of 180 mph at sea level (where your true airspeed would also be 180
mph.)
However, because air molecules are passing by your wing at 250 mph, your
flutter margin is much less, and has possibly even evaporated.
The upshot is that the guys who go fast, even perhaps exceeding Vne, but do
so at low altitudes, will have less chance of getting flutter than the guys
who go high but keep within the Vne.
The guys who go with turbocharging and then fly high had better reduce their
Vne as they climb. A placard like that on the Columbia 400 or Pipistrel
should be considered mandatory. Even if you reduce your Vne by a couple of
knots for each thousand feet, you will still be covering ground more quickly
than at going at a higher indicated speed at lower altitude -- and you will
do so at lower fuel burn.
So I guess it's up to the individual. If you like to fly high and are
oxygen-equipped and have the discipline to adjust your Vne as you go, it is
possible to fly with the proper safety margins for flutter. Personally I
think this is okay, but I guess Van is taking the safe approach and telling
people to just not do it.
Regards,
Gordon.
PS: In my previous post I made a typo in the dynamic pressure equation. It
should read: Q = 1/2pV2
Or dynamic pressure equals 1/2 times air density times velocity squared.
I had a plus sign instead of an equal sign in there.
Could be Van is not selling turbo charged engines :)
If so he is making a big mistake.
Paul Lamar ...No rotor no motor.
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