Wise words indeed
Reprinted from Engineering Matters Volume 1 Issue 9
September 2008
HOW EASY IS IT TO CAUSE STRESS?
We understand stall speeds, but what of cruise speeds and the effects of
turbulence and over-exuberant control inputs?
Two young student pilots were propping up the clubhouse bar, pondering over
a question in a sample PPL airframes paper they had both being doing to
while away the time, their flying slots having been cancelled due to the
weather turning sour a couple of hours previously. The question was over how
strong an aircraft had to be to cope with 'g' loads in flight. Casting his
mind back to school physics lessons and a dimly remembered Newton's laws,
one hazarded: 'If the total weight of the Cessna including fuel, crew and
baggage is 1500 pounds, then if it has to cope with a 4g acceleration then
the wings have to carry four times 1500 pounds i.e. 6000 pounds, right ? So
I guess we tick box A?'
The other, who had spent the morning with his nose buried in a dog-eared
copy of the club's Airframes and Engines text book, rejoined - 'Yes, but
you've
forgotten the safety factor. Aeroplanes have to be able to carry at least
50% more load than the pilot might want to use, to give an extra margin of
safety. That means your Cessna's wings have to be good for an extra 3000
pounds, giving a total of 9000 Lbs. So tick multiple choice box B'.
The two pilots marvelled over the fact that by their reckoning their humble
club Cessna, which weighed less than an old-style Issigonis Mini, had to be
able to carry the weight of two transit vans. 'It just shows', said one,
'how enormously strong these aeroplanes are and how hard pushed you'd have
to be break one in flight'.
'Not so fast, young man' rejoined the grizzled CFI, who, sunk deeply into a
barely-recognisable armchair in another corner of the clubroom, had been
gloomily working out the effect on the club's turnover of yet another
weekend of cancelled lessons. It really had been a terrible summer. 'First
of all, that extra 50% safety factor wasn't put there for the likes of you
two to play with, once you start going into that territory then you're going
to be damaging my aeroplane for sure, even if the wings do stay attached -
which is doubtful. You'll be coming back with the whole airframe
overstressed and only fit for scrap. Even if there are no obvious external
signs like puckered skins or bent wing spars, carrying on flying an
aeroplane that has been overstressed means it may collapse later when some
other poor mutt is flying it.'
'And another thing, most of our 'planes have been around longer than you two
lads, and have been slogging the circuit for decades - much longer than the
designer probably had in mind when he drew up the thing - been repaired a
few times too, if you care to have a look in their logbooks over there...
riveted joints are prone to corrosion you know... despite the best efforts
of our maintenance chaps, these airframes can't be as strong as the day they
left the factory. That's part of the reason airframes are designed with the
extra 50 % safety factor - to allow for degradation in service. And of
course, designers like to have the factor there to give a little leeway in
case they have made a mistake or two in their calculations - slide rule
slipped, or they multiplied by 'pie' instead of 'alpha', too busy thinking
about lunch...!'
'Six thousand pounds sounds like a lot of load to put on a little
aeroplane's
wings, and it is - three tons give or take a bit.. Not bad considering each
of a Cessna's wings only weighs a hundred pound or so, which just shows what
efficient structures they are...have to be, if you built 'em like the Forth
Bridge you'd never get off the ground. Aeronautical engineers have to pare
off every bit of unnecessary weight. If weight wasn't a consideration, the
safety factors
would be much higher, like in most other industries. Ironic isn't it, that
in an aircraft, where collapse of the structure almost inevitably has fatal
consequences, we have lower safety factors than in ground-based vehicles
where failure would most likely just mean having to take the bus home?'
'How easy is it to overstress them? Well, you know there's an interesting
little fact buried in the design rules that apply to almost all light
aeroplanes, microlights and gliders, which is that the backward force the
pilot would have to apply on the control stick grip in flight, to make the
aeroplane reach the 'g' load where it starts to suffer structural damage,
must not be less than fifteen pounds. This is intended to ensure that pilots
can't overstress aeroplanes inadvertently. But think about it, fifteen
pounds is a force so low that you can just about hold it with your little
finger - you can manage more if you are in training from carrying the
dratted plastic bags of shopping away from the supermarket. So only the
force of one little finger may stand between you and a bent aeroplane..'
The students were deflated. Surely, even taking into account all this, 4g
was a lot, much more than you ever need in a simple Cessna. Surely there was
no reason to worry about it providing you just flew normally - after all,
these aren't aerobatic 'planes.
THE STALL TO CRUISE SPEED RATIO AND ITS POTENTIAL EFFECT ON STRUCTURAL
INTEGRITY
Behind the storyline above lurk some really important issues, and dangers
that are becoming increasingly important with the newer, faster breed of
microlight and VLA aircraft and the more challenging types of flying now
regularly being undertaken. Faster speeds bring more potential for high 'g'
problems. To calculate the 'g' that can be pulled inadvertently in an
aeroplane, divide the speed the aircraft is flying at by the aircraft's
stall speed in that configuration and then square the result - so flying at
twice the stall speed means you might pull four 'g', four times the stall
speed equates to a mind-numbing 16g. Whereas the older types of traditional
homebuilt such as Luton Minors and Currie Wots had a relatively slow cruise
speed of barely twice the stall speed, and were therefore largely proof
against being overstressed in flight, today's machines such as the RV range,
Europa and so on have the capability of cruising at more than three times
the stall speed and could therefore relatively easily be overstressed in
flight - flying at three times the stall speed means that up to 9g might be
reached with too much 'back stick'. If the airframe is only designed to cope
with 4g then it will most likely not survive.
VA - MANOUEVRING SPEED
To stay out of trouble with the airframe, you have to fly with three safety
speeds in mind. The manoeuvring speed Va (pronounced 'vee-aye') is the
maximum airspeed you can fly without risking structural damage if you carry
out abrupt manoeuvres. Confusingly, that's not to say you mustn't manoeuvre
at speeds above Va, it simply means that if you do then you must be careful
not to pull too much 'g', to avoid overstressing the aeroplane. If you fly
at less than Va then no matter how much you pull (or, for that matter, push)
on the stick, the aeroplane will stall before it reaches the maximum
manoeuvring 'g' which it has been designed to carry. You'll almost
invariably find Va quoted in the aeroplane's flight manual in the
'limitations' section, on the Permit to Fly or in the manufacturer's data.
Sometimes this speed is referred to as 'maximum speed for full control
deflection'..this is a bit misleading because it rather implies that if you
fly at a speed a bit above Va then you will be OK providing you use a bit
less than full deflection, which is not necessarily the case. Depending on
the stability and control power of the aeroplane, and in particular its
centre of gravity position and trim setting, it may be possible to reach
high g levels without the stick being far from neutral. In an unstable
aeroplane, you might even find that the stick has to go forward of neutral
just to stop a steep turn 'tightening up' on you. Not that PPLs normally get
a chance to fly such unstable aeroplanes - but it can happen, especially on
older types, or if they are mis-loaded with an extreme aft cg.
When flying at speeds above Va, the risk of overstress and structural
failure is there, whatever reason you manoeuvre. Not all manoeuvres are
planned, and it may be the spontaneous response to some external cause which
leads you into danger - for example the Zenair pilot who was flying a low
pass over a farm strip at high speed when he spotted wires close ahead,
pulled up sharply to clear the wires - and caused a structural failure of
his wing attachments, with consequences fatal to himself and his passenger.
The Zenair, like many VLA and microlight aircraft, has light stick forces
and would have needed only a 25 Lbs pull on the stick to cause such a
catastrophic structural failure. High speed, light stick force and exuberant
flying make a dangerous blend.
VNO - MAXIMUM ROUGH AIR SPEED
The second safety speed to be aware of is the normal operating limit, Vno
('vee-en-owe'), which is the maximum speed the aircraft is designed to be
able to cope with in gusty or turbulent conditions without being
overstressed. It is based on an intensity of so-called sharp-edged gust
which is slightly arbitrarily assumed to be 50 feet per second, in other
words the whole aeroplane is assumed to have to transition straight from one
lump of air which is static into another which is going up at 50 feet per
second - like a high-power thermal. Putting it in simple terms, the faster
you are going when you slip from one airmass into the next, the bigger the
jerk required to accelerate the aeroplane from level flight to a 50 foot per
second climb. To hit the vertical gust at high speed gives you a hell of a
jolt, as you can imagine. Go too fast and the jolt will overstress the
aeroplane.
Of course in actual bumpy air you are usually encountering pretty much
random gusts in all directions, but the 50 foot per second model has been
found to give equivalent loads, based on the highly detailed instrumented
results of some brave RAE and NACA pilots who were sent up to explore
turbulence of increasingly severe magnitude, just after the last war. Some
of these pilots didn't come back, having found (like many glider plots
before them) that the violence inside a thunderstorm was more than their
airframes could cope with.
Vno is generally a few knots faster than Va. Again, you will find Vno stated
in most aeroplane flight manuals, and it is the bottom end of the yellow arc
(the cautionary range) on the ASI. If in doubt, use twice the stall speed.
For the pilot, the message is that unless the air conditions are smooth,
with negligible turbulence, you should not fly at a speed greater than Vno
otherwise you will risk overstressing the aeroplane if you hit a strong
gust. Slow down to give yourself a more comfortable ride, and save your
aeroplane's structure. Hitting a severe gust at high speed will cause 'g'
levels as high as pulling the stick hard back - but you may not be aware of
the danger because of the effect is an instantaneous jolt rather than a
sustained acceleration that can be felt through the seat of the pants, arms
like lead etc.
We are not talking academics here; there have been several accidents in the
last decade with structural overstress through hitting turbulence. In one
case, the pilot who was flying near vertical cliffs on a windy day appears
to have made the fatal mistake of increasing speed on encountering the
turbulence, to get away from the area of rough air - and lost his wings. The
pilot of another aircraft, flying in company with the first, chose to slow
down - and survived, but with some airframe damage.
VNE - NEVER EXCEED SPEED
The final safety speed is the most well known, the never-exceed speed Vne
('vee-en-ee'). This speed is indicated by the short red radial line and the
top end of the yellow arc on the ASI. This is the airspeed that the aircraft
is designed to cope with (usually, but not always, necessitating a dive) but
only in calm, turbulence-free conditions. The airframe is normally designed
to be able to cope with a much lesser intensity of gust at Vne, usually
equivalent to only a 25 feet/second sharp-edged gust. This is to cater for
the fact that even on an apparently turbulence-free and calm day there is
always a risk of suddenly encountering an isolated piece of mild turbulence
such as a stray thermal, or the remains of the wake turbulence from some
other aircraft which has since moved on. Encountering a 50 foot per second
gust (i.e. a severe one) at Vne would most likely cause a collapse of the
structure.
The other limiting factor is that Vne is usually the highest speed that the
aircraft is guaranteed by the designer to be free of flutter problems - he
will most likely have proven the prototype to a very slightly higher speed
than Vne (normally just 5%) to show that there is some safety margin, and to
provide for minor differences between one aeroplane and the next, the effect
of wear and changes in the friction levels in the control system with age,
and variations in the airspeed indicator errors. As high-speed flutter can
tear an airframe apart in fractions of a second, this is not a phenomenon to
be risked by ever going above Vne, outside of a proper factory test
program - or one authorised specifically by CAA, BMAA or LAA - not for
nothing do test pilots get paid to do this sort of thing - they have to wear
a parachute, and usually have jettisonable doors fitted to improve their
chances of escape.
Apart from the fact that modern light aircraft and microlights often cruise
at three or even four times their stall speeds and are therefore vulnerable
to overstressing, the streamlining of the airframe and close attention to
cockpit seals which are required to achieve this high performance causes a
further risk, which is that the pilot has fewer visible and audible cues to
warn him that he is flying fast. Flying older aircraft, you find that
increasing the airspeed much above normal cruise means a steep dive and a
roaring wind noise from the air whistling through all the leaks in the
cockpit canopy - or around the windscreen of the open cockpit. You would
have to be deaf as well as blind to miss the fact that an aeroplane like
this were being flown faster than normal - which is why in these types of
machines it is not a big deal if the ASI fails in flight. It is quite easy
to fly, manoeuvre and land these aeroplanes just using the visual and
audible cues as a measure of correct airspeed - and even if the approach is
flown a mite fast, to be on the safe side, the high drag means they won't
float very far so a safe landing can be made.
With a streamlined well-sealed aeroplane, by contrast, their higher
lift/drag ratio means that in the cruise the nose only has to drop a few
degrees to let the speed slip quickly past Vno and even past Vne. The lack
of wind noise and drafts in the cockpit makes it impossible to tell your
speed that way. Throw in a bit of bad visibility robbing the pilot of a
proper horizon, extra workload with navigation due to having to divert,
reaching behind for that flight guide or what have you, and the
possibilities for inadvertent overspeed become very real in these slippery
modern aircraft. Approach and landing with a failed ASI (all it takes is a
little water in the pitot pipework) is also much more difficult with these
machines - especially if they haven't got very effective flaps. The more
powerful the flaps, the more they reduce the lift/drag ratio which in turn
means that the change in glide angle becomes much greater (more perceptible)
for a given change in speed.
CONCLUSION
Fly too slowly and you may stall - fly too fast and there are equal or
greater perils. To fly safely, understand your aircraft's flight envelope
and speed limitations and, with modern slippery aeroplanes in particular,
the importance of taking into account turbulence when deciding your cruise
speed. If the ride feels uncomfortable, you are probably going too fast for
the conditions.
