Pete,
Short answer:
The U2 and coffin corner are at altitudes and airspeeds the Europa can neve
r achieve... Therefore not relevant to mountain flying in a Europa. Or an
y flight operation of a GA light propeller driven aircraft for that manner.
The physiological limit of 25,000 feet without pressure breathing oxygen
or cabin pressure and 50,000 foot limit without pressure suits, kind of li
mits the Europa and its owners from fearing any of these effects.
I prefer to believe the aerodynamicists on control design. Flutter is ofte
n discussed and considered to be only the control surface moving by the non
aerodynamicist, when the torsional rigidity of the wing, or control linkag
e, or tab attachment, is the actual effect. Pilots feel vibration and repo
rt it as flutter. From where, they don=92t know, it has to be found out. T
AS is a component of the dynamic pressure or q = (1/2 density x V squared
) and the effect of q on the wing and the air speed itself can be explained
below. A Europa in orbit at 25000 mph has no flutter problems as it has no
air molecules. Without air molecules there is no air mass. No air mass,
no aerodynamics. Mass of the air (viscosity, temp, etc. ) must always be c
onsidered. Not just speed.
Long answer:
Re: Coffin corner.
Coffin corner is operating between critical Mach number and Stall equivalen
t airspeed. It is true that at 90 and 100 thousand feet, a subsonic slow f
lying aircraft (even a glider in a dive I suppose) with proper propulsion (
uplift to get there and then dive) can get so high it is near the speed of
sound even though it is near stall. Air liners at their operational limit
can be there also as they cruise at .86 Mach or so. Supersonic designs are
exempt from this. They just stall below transonic region at high altitude
or fly supersonic and not stall at all at very high altitude.
As you all know, Mach number is simply V/Va or the ratio of TAS speed divid
ed by the speed of sound.
The speed of sound is defined by the square root of specific heat of air x
gRT with Temperature being the largest component of change. So its really
cold at from 35,000-100,000 feet and the speed of sound is fairly constant
in that range at about -55C or about 575KTAS. Air density of course is ex
tremely small. So to get indicated airspeed high enough to fly (not stall
due to lack of density), by default you must be getting closer to the Mach
at the extremes of the atmosphere (from 45,000 you can feel the difference
but much be much higher to see them close on a straight wing airplane).
Example: 90 KIAS required to fly at the stall at 80,000 feet with a jet po
wered drone, the true is 170 TAS, the mach is going to be about .3, so ther
e is no technical coffin corner at 80K anywhere near the stall speed.
Critical Mach is the Mach number where the normal shock wave forms on the a
ircraft (normally the wing and fuselage juncture and the thickest part of a
thick wing). In a U2 or T-33 that occurs about .86 Mach 45,000 feet, the
indicated was 218 KIAS and 495TAS. Stall was about 120 KIAS. The cabin al
titude was right at 25,000 so we couldn=92t go higher. Every subsonic airp
lane has a different reaction to the large separated flow behind the normal
shock wave. The U2 and T33 had hydraulic controlled ailerons so no flutte
r or buzz possible on the wing. But the elevator control is severely affec
ted by the separated flow of the shock wave. (Eventual loss of elevator co
ntrol and nose pitch down.)
The flexible wing design of some airplanes (drones, gliders, and even some
powered planes) have very poor torsional rigidity (they bend leading edge u
p and down easily with a vertical gust load or due to load deformation). I
n straight and level non accelerated flight, if a normal shock wave forms o
n the wing, the separated flow near mid chord, may cause the wing to twist,
which changes the position of the shock wave, which changes the twist and
a flutter will occur in the wing and the non boosted aileron will follow al
ong, balanced or not. Turbulence exacerbates this problem due to the wings
elastic effects. If only considering Mach crit, as the wing twists LE up
(due to normal shock or turbulence), the lower side of the aileron will hav
e pressure (due to normal flow) and the upper side not due to separated flo
w and that can force the aileron upward, causing the wing to twist more and
divergence begins until torsional rigidity finally reacts and the wing twi
sts hopefully back down, changing the shock wave position to the lower surf
ace and it all starts again. The stick will move of course so aileron flut
ter is suspected when it is actually forced wing flutter. With aileron con
trol input by the pilot at the same time, that twist effect can be quite la
rge in flexible aircraft inviting very interesting effects. If operating a
jet powered drone near the coffin corner, the pilot can=92t just pull up t
o slow down (he=92s at the stall) and can=92t lower the nose to pick up air
speed (he=92s seeing aeroelastic or Mach effects limiting control function)
hence the name coffin corner. Only by deploying speed brakes and reducing
power going to a lower altitude will recover the aircraft. Because the IA
S at stall is a high TAS at very high altitude (50-100K), some have determi
ned the affect of adverse aerodynamic affects to be because of TAS alone.
That takes the TAS out of context and is a bit of a stretch to the actual p
otential cause of their aeroelastic problems and the dampening that increas
ed density may afford. It takes air to have aeroelastic problems.
Re: Aeroelastic concerns:
An aircraft built to be non rigid must be flown and tested very carefully a
s any force outside of straight and level flight will have very serious eff
ects. At very high altitudes, many high lift laminar airfoil designs have i
ssues with the boundary layer separation and control effectiveness degradat
ion occurs or wing twist due to the center of pressure shift the airfoil wa
s not designed for (that is at weird Reynold=92s numbers ((a dimensionless
number of 1.4 x density x V x length / viscosity of air that determines the
boundary layer separation point))). At these extreme conditions, aeroelast
ic effects and boundary layer separation are pronounced in these flexible f
liers operating near the stall in the cold thin air of higher altitude such
as the Perlan Project. We have all seen the wing flutter test video of gl
iders encountering wing flutter, aileron reversing due to wing twist, etc.
The effects of these problems are exacerbated at altitude and high TAS. T
he q is low and the TAS high so some believe the density is irrelevant. It
is not. Temperature is also a problem, especially with the modulus of ela
sticity of carbon fiber. These types of aircraft are very light, but very
flexible fliers and have to be thoroughly tested and instrumented due to th
e problems associated with aeroelastic and structural effects (as well as a
ileron deflection induced by separated flow and the affects of the low OAT)
. Or they have to be built heavy but rigid (i.e. the Europa). Aircraft de
signed for the long haul of normal operations tend to be quite stiff to avo
id these problems and have long operational lives.
Finally on aileron flutter. From Dommash - Airplane Aerodynamics:
=93Precautions must be observed to prevent destructive flutter or undamped
short-period oscillations of control surfaces. These precautions are relat
ed to mass balance about the hinge axis, static contour and changes of cont
our under air loads, and tightness of tab linkages.=94
=93Mass balance is balance the tab about the hinge axis. There is no rotat
ive tendency to induce flutter from mass-inertia effects if the control is
balance precisely on the hinge line.=94 See the Europa build manual.
Contour Effects: =93All control surfaces should be flat sided or concave.
=94 Convex surfaces (flexible fabric that balloons under pressure ((GB rac
ers of 1930s)) become unstable and begin an undamped oscillation. See buil
d manual on contour.
Note: =93Spanwise torsional stiffness in a control surface may also cause
flutter.=94 Long or solid hinges and stiff construction are important in c
ontrol design. The Europa ailerons have sufficient stiffness and are reaso
nably hinged to a rigid wing structure not prone in its flight envelope to
aeroelastic effects.
So contour your ailerons to the foam shape. Balance them. Keep the hinges
and linkages tight and maintained, pad the aileron quick disconnects with
no slop, and flutter will not be a problem at any operational limit of the
Europa, to include mountain flying up to 25,000 feet for sure. Don=92t fly
over 25,000 unpressurized for any extended period, as it is a serious phys
iological limit to most people even on oxygen without pressure breathing.
Fly your IAS within limits of the flight manual/POH and enjoy yourself.
If you want to design drones or manned gliders operating at 100K, that is a
different type airplane flying in a different world. To date, only highly
funded experimental aircraft fly near that regime.
Best Regards,
Bud Yerly
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s 10
From: Pete<mailto:peterz@zutrasoft.com>
Sent: Tuesday, January 9, 2018 12:38 PM
Subject: Re: Europa-List: Are Vne and Va IAS or TAS?
Not to be argumentative, but as i understand it, Bud's explanation ignores
the issue reported/described in the other articles, namely that at altitude
there is less flutter damping for the same reasons Bud explains wrt the st
ructural limits. So for flutter specifically it is TAS that is the determi
ning limit, and not IAS. Taken to the extreme, the U2 sometimes operates n
ear "coffin corner" where flutter speed is very near stall speed (IAS is in
dicating near stall, but they are so high and the air so thin that they are
near the risk of flutter). It is also why the http://www.perlanproject.or
g/<https://eur02.safelinks.protection.outlook.com/?url=http%3A%2F%2Fwww.p
erlanproject.org%2F&data=02%7C01%7C%7C9d0911445d9649fb76b808d55787d448%7C
84df9e7fe9f640afb435aaaaaaaaaaaa%7C1%7C0%7C636511163254423256&sdata=J07iN
vPuNgRaxf2BJeDJWAPgVCtpCC9rgT4PiYXpZzU%3D&reserved=0> glider is heavily i
nstrumented to detect the onset of flutter as they go higher and higher in
their record attempts.
That said, it is comforting to know that Ivan has tested Vd with stick exci
tation to 8000 feet, so i would assume that this could be used as the derat
ing baseline when going to higher TAS's.
Cheers,
Pete
A239
On Jan 9, 2018, at 7:08 AM, William Daniell <wdaniell.longport@gmail.com<ma
ilto:wdaniell.longport@gmail.com>> wrote:
Bud thanks, sets my mind at rest.
William Daniell
LONGPORT
+57 310 295 0744
On Sun, Jan 7, 2018 at 11:09 PM, Bud Yerly <budyerly@msn.com<mailto:budyerl
y@msn.com>> wrote:
Will,
I'm still on the road but here goes.
Short answer:
Q is force, whereas True is just speed.
Long Answer:
Think of it this way. The mono cruises at 175 TAS at 18,000 with a 914. T
he indicated is only 125 ish. You cannot pull enough G to achieve 6 Gs bef
ore you stall or have a vertical gust break the plane. Is the mono above VN
E, at 175 TAS, NO. Heck you are not at Vno or about 131 KIAS which is your
green arc for gust factor(turbulent speed).
That gust factor is what you need to be aware of in the turbulence of the m
ountains.
"Aircraft Performance" by Domash explains it.
So does the FAA. For finding high speed affects, the USAF F104 VN diagram
is on Wikipedia and shows how Mach affects figure in for high TAS.
TAS is important in turn rate, radius, navigation, and determining your Mac
h and Q velocity. But it is your Q (dynamic pressure), aka IAS, that affect
s, flutter, structural deformation, and your stall and not to exceed speed
s. This means what you read on your airspeed indicator is what you need to
know for the plane. TAS and Ground Speed affect your pilotage which is a
different topic.
Again, in mountain flying, you need to know your turn diameter when valley
flying, high altitude patterns (wider pattern necessary), lead turns to a r
adials etc. (especially In high speed aircraft) and in light aircraft in ve
ry high elevations. Engine performance vs airspeed bleed off becomes a fac
tor as well.
Regards,
Bud Yerly
Custom Flight Creations
From: William Daniell
Sent: Friday, January 5, 4:07 PM
Subject: RE: Europa-List: Are Vne and Va IAS or TAS?
Bud
Does this apply even at the upper altitude range ....say 13k or 15k?
Will
On Jan 4, 2018 22:19, "Bud Yerly" <budyerly<mailto:budyerly@msn.com>@msn.co
m<mailto:budyerly@msn.com>> wrote:
Yep Graham,
Airplanes only feel air pressure, not the velocity of the molecule alone.
Dynamic pressure is =BD Density times Velocity Squared or IAS (actually y
ou have calibrated then equivalent) is what the airplane feels. Those RV g
uys got all hung up on this and confused everyone.
Bottom line, what you read on the airspeed indicator counts. TAS is import
ant (actually Mach number) as the skin heats up due to friction which is a
different ball of wax. I was always a slow speed aero guy to match my mind
.
Regards,
Bud Yerly
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EiPs%3D&reserved=0> for Windows 10
From: owner-europa-list-server@matronics.com<mailto:owner-europa-list-serve
r@matronics.com> <owner-europa-list-server@matronics.com<mailto:owner-europ
a-list-server@matronics.com>> on behalf of Pete <peterz<mailto:peterz@zutra
soft.com>@zutrasoft.com<mailto:peterz@zutrasoft.com>>
Sent: Thursday, January 4, 2018 5:49:37 PM
Subject: Re: Europa-List: Are Vne and Va IAS or TAS?
Hi Ivan, just to confirm, Vd IAS @8000ft DA?
Thanks again!
Pete
On Jan 4, 2018, at 5:21 PM, <ivanshaw<mailto:ivanshaw@btinternet.com>@btint
ernet.com<mailto:ivanshaw@btinternet.com>> <ivanshaw<mailto:ivanshaw@btinte
rnet.com>@btinternet.com<mailto:ivanshaw@btinternet.com>> wrote:
All our company aircraft were tested to Vd, 10% over Vne. And not just take
n to the speed but then tested [short stick and rudder raps] to see if any
flutter mode could be excited at Vd. I have performed these tests at/up to
8000ft . We have never experienced any flutter mode. I also tested the tail
plane underbalanced and over balanced with the same results. To my knowled
ge we have not had any reported flutter incidence on the entire fleet. As y
ou mentioned Pete did exceed Vd on a few occasions.
Ivan
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