Turn a flexwing
If a flexwing is to turn
efficiently, the angle between trike and wing needs to be 90
degrees. In other words the same as it is in level
flight. It follows that the most efficient and smoothest turn
will be where you have chucked the trike out by just the right
amount to keep the 90 degrees throughout the turn. The
'chucking out' is of course done by centrifugal force swinging the
trike to the outside of the turn like a ball on a string.
In order to make the trike swing out
there are two things you need to do (as well as banking the wing
that is). Firstly it helps if you gain a little speed by
entering a shallow dive, then once you have initiated the turn by
pushing the bar left or right, you need to push the bar slightly
forward. Oh, and at this point tweak the power up just a touch to
push the trike round the curve. This takes a bit of practice
until you can time it right and apply just the right amount of
forward pressure and power for the steepness of turn. But get
it right and it feels lovely.
It's not so bad as you think.
When (if) you can get close to the ground the turbulence usually
dies away quite a bit as the air sticks to the ground and you find
yourself floating reasonably smoothly in ground effect. If you
can feel it's going to be rough as you come in then come in with
some power - maybe 4000 revs until very late. This has two
advantages - you are driving through the crap, and you have instant
power to climb away and do a go round if you are not happy.
And the key thing to remember is you can go round as many times as
you like. It may be embarrassing in the club house afterwards
to have gone round 4 times, but not half as embarrassing as wrecking
your aircraft.
Make sure you know what the optimum airspeed
is for your wing. For the Blade it's 55mph. That means
you have maximum control over the aircraft's behaviour at 55.
If you come in at 48 it will be much less responsive (same sort of
argument applies to 60mph). The thing that will catch you out
is a huge gust tipping your wing so that one tip catches the
ground. When you are 50 ft above the ground you can heave the trike round as much as you like to get her lined up and level, but
you can't do that 15 feet from the ground.
Better to make more
gentle corrections and if you are getting tilted too much then apply
power, climb away and go round. Eventually you will make an
approach between gusts that lets you get down to 7 or 8 feet with
wings level and you can let her settle on to the runway.
(Remember to kill the power if you have used a powered approach.
It seems best to have
a heavy trike (passenger or ballast) and to try and fly at the
wing's design speed - in the case of the Blade this is 55mph.
If you have the washout set at normal (I don't, because you can
increase speed with it reduced), your pitch will stabilise more
quickly and without so much manual input.
Washout is the
amount of upturn on the ends of your wings - think of it like the
upturned feathers on a pigeon's wings or those of a soaring bird of
prey - the more upturn the more stable the flight, but the slower
the bird will fly (you won't see swifts or swallows with
washout). when you are learning you need the trike to sort
itself out as quickly as possible even if this means sacrificing a
bit of cruising speed. As you become more experienced you find
your brain making the corrections automatically but it can still be
quite tiring.
If it's thermally induced turbulence, you
can try climbing to the cloud layer as long as it's broken.
Above the clouds you will generally find it as smooth as silk - if a
little cold.
If it's wind turbulence coming over hills etc, it's a bit more
problematic. You should be able to climb above it.
Brian Milton (who flew round the world in a
trike) said that he used to fight to keep the trike level all the
time, but in the end just went with the flow and the trike always
sorted itself out.
The more you do it the more relaxed you
become. This means pushing yourself to take the trike out in
conditions where you are currently reluctant.
Bear in mind that your aircraft is designed to
withstand 3 times the forces that can be induced by
turbulence. However you as the pilot can theoretically
overload the structure through violently trying to keep the plane
level. This can only really happen if you end up in cloud in a
thunderstorm - so don't. Thermal turbulence might feel bad but
isn't dangerous. Turbulence caused by strong winds in the lee
of a range of hills, can be more violent and more dangerous if
downdrafts exceed the maximum climb rate of your
aircraft.
You can land a
flexwing in surprisingly strong winds. It's just once you are
down that the problems begin.
Obviously anyone can
get caught out by worsening conditions on a cross-country. If
you are reasonably sure that you have flown into localised strong
winds, you can always go back to your starting point where
presumably you took off in sensible conditions. Of course you
don't always know if the wind has started howling - it can be a
perfectly fine day, but the wind might have picked up to 40 mph
without you realising it. If you have a GPS you can spot this
straight away as your speed over the ground will vary dramatically
compared to your airspeed (beware though, this might not be the case
if you have hit a strong crosswind, but in this case you will find
your machine crabbing markedly into the wind). If you are
flying an old XL and you can't afford a GPS you can tell if it's a
40 mph headwind from your shadow on the ground ... it will be going
backwards!
Obviously if your
speed over the ground in say a Blade has dropped to under 20mph you
may have a problem. You can do one of two things Firstly try
and find out from your destination what the wind speed at ground
level is, it might be markedly less. Secondly you can turn
round - you'll make it home in no time at something like 85
mph. If on the other hand your ground speed has gone up to the
high 80's or 90s, you have more of a problem, as it's going to be
quite a fight getting back home and that is where the stronger
weather is coming from.
So you get to your
destination and there is a 35mph gale blowing. What do you
do? Firstly try and get some information on the radio.
Secondly look at the surrounding area. Is it flat ground where
the wind is coming from, because if so it's likely to be steady even
if it's strong. Now make some exploratory circuits at about
1200 to 1500 feet AGL (I'm assuming this is a microlight site rather
than Gatwick). Is there a runway that's directly into wind -
if so great. If not is there grass or a disused runway that is
directly into wind. The important thing here is that you do
not need much - remember your landing speed in relation to the
ground might be only 20 mph. You could be down and stopped in
30 to 40 metres.
Once you are on the
ground DO NOT TURN. Keep pointing into wind, and if you have
enough runway keep your wing level (it is most stable in this
position). If you are landing on a short bit, pull the bar in
as soon as you are rolling in a straight line. You will be
amazed how quickly this stops you.
Finally keep sitting
like this on the runway into wind until either the gale blows itself
out, or at least two people spot your plight and scurry out to walk
your wing in. Do not try to taxi in across a 35 mph wind - you
will end up with some expensive repairs.
The great thrill of
our type of aircraft is their short field take-off and landing
ability. What fixed wing aircraft can arrive overhead the
field at 2000 feet and spiral down to land in a bumpy grassy field
only 80 or a hundred metres long.
Well firstly coming in
steep from directly overhead is not necessarily a good idea in a
short field, as you may be travelling too fast as you arrive - and
this means that you will float on before you touch down. A
relatively shallow approach maybe 10% slower than you would normally
come in is best - any slower and you could run into difficulty if
you need to go around, or change direction (the wing is really quite
difficult to control at say 10% above stall speed). Aim to
touch down 1/3 of the way into the field, any less and you might run
into hedge problems if you hit any sink. As soon as you get
down, pull the bar hard in and the drag created by the down-pointing
wing will slow you down quite markedly. Use your brakes - as
hard as you like if they are rear wheel brakes but with great care
if it's a front wheel primitive kind like the Pegasus XL.
What factors might
cause you problems. Firstly very light winds will mean your
speed relative to the ground will be high and your consequent ground
roll longer. Secondly grass is better than tarmac - long grass
will slow you down quite quickly. If you are going to the
field to camp, and you have got a mate in the back and all the gear
for two, and a crate of beer, you may end up in the field beyond the
one you intended. Remember also that you need to get out of the
field later, and factors that were helpful in slowing you down
during landing, will also slow you down during take-off.
Assuming you have
already done what you can to improve the aerodynamics of your
aircraft, such as scraping all the dried on mud off, and dispensing
with the idea of panniers. Remember too that the prop's task
is to screw you through the air. If it has to work hard
overcoming excess drag on its own surfaces it won't be able to
provide the maximum propulsion for the aircraft (so keep the prop
clean).
What we are talking about here is the way you fly, and
the key is smoothness. If you want to get from A to B using
the minimum of fuel you need to do everything gradually. So
it's better to reach your cruising altitude by climbing steadily at
200ft per minute rather than holding her at full throttle and
climbing at 1000 ft per min (even a 912 uses 22litres per hour at
full throttle). Then fly at that altitude at the speed your
wing was designed for. On a modern flexwing that means the
trim control should be just beginning to bite.
Any more and the wing will
be creating too much drag despite its greater lifting power. You do have
to balance this with the revs your engine runs at if it's a two-stroke.
It can be critical when the engine starts running purely on the main jet,
as opposed to the needle jet.
You should also regularly check the profile of the wing battens -
they flatten with use, making the wing fly faster but not providing
the lift it should.
Remember that (at
least in the northern hemisphere) the wind veers as you go
higher. Imagine one is flying north from Peterlee to Eshott and
there is the usual wind out of the west. It's better to
fly at about 1000ft rather than 3000 ft, because higher up the wind
is more likely coming out of the north west. Obviously on the
return trip one can make use of this tailwind if Newcastle ATC will
let one cross their zone at 3000 feet. GPS is the only way of
finding these different airflows by monitoring your groundspeed and
bear in mind it doesn't always work out.
More aircraft get
damaged in crosswind take-offs than deserve to be. And usually
it's not just the aircraft of the guilty party as he often goes
skittling into all the parked air craft outside the clubhouse.
What usually happens
is you are sitting on the runway and there's maybe a 12 to 15 mph
crosswind. At this point you are glued to the ground by the
weight of the stationary aircraft. As you start to move slowly
forward, the weight of the machine is still dominant, so at first
you follow the nice line you intended. But as the wing
gains lift, there is less grip for the tyres, and all of a sudden
the whole machine weathercocks into wind, and the take off track can
change by a remarkable number of degrees (now you are aiming
straight for the clubhouse with insufficient airspeed to climb over
the assembled barbecue, trikes and picnicking pensioners)
The answer is
relatively simple (apart from don't take off in strong
crosswinds). The key is to hold the trike firmly on the ground
for longer than you normally would, and well beyond the point when
she wants to fly. Do this by holding the bar in as you
accelerate down your chosen track. When she is travelling
maybe 10 mph faster than you would normally lift off, rotate the bar
just a touch to unstick her. You will find that although you
weathercock quite smartly round into wind, your forward momentum
should keep you on your original track, and anyway you have enough
speed to climb smartly away.
The key here is to line
up your direction of travel with the centre of the runway. This might
mean your trike is pointing
The key here is to
line up your direction of travel with the centre of the
runway. This might mean your trike is pointing substantially
away from the centre line. Obviously the more you are pointing away from your line
of travel, the greater the potential for disaster. When the
leading rear wheel touches down there is a tendency for it to tuck
under and the trike to roll. Even if it straightens up OK
there is a lot of twisting action on the trike, and it's why you
should always give the trike a good checkover after a strong
crosswind landing.
So what can you do to
make it easier on aluminium, skin and bone? If you have got a
wide runway, then land on a diagonal to the centre line (ie more
into wind). If you don't get your touchdown point exactly
right you can always do a go-round.
If you have to land
crabstyle, the technique is to land with the front wheel slightly
higher than you might normally (a touch of extra power helps do
this) and to make the back wheels just kiss the ground. If you
can hold it like this a fraction longer than you normally would,
you will find that the trike will just click round to
line up with the runway, and you are home and dry.
C - controls
H - helmet
I - instruments
F - fuel T - trim
W - wind
A - (other) aircraft
P - power
1. Apply full
power, and immediately push out the bar as far as it will go.
This will take the weight off the wheels and reduce the drag from
the grass and mud, and the high angle of attack of the wing will be
providing maximum lift at the lower speeds before the wing is ready
to fly.
2. The above is
crap, and the high angle of attack will create loads of drag which
added to the drag from grass and mud will mean a long take-off
run. Much better to keep the wing in a neutral (low-drag)
position so that it is ready to provide maximum lift as soon as you
are going fast enough to fly.
(or a whipstall or a
tailslide)
Your aircraft manual
and the textbooks tell you that it is illegal to do a 'wingover',
but no-one tells you what one is. So how do you know whether
you are doing anything illegal.
A true wingover is a
90-deg. climbing turn followed by a 90-deg. descending turn
resulting in a 180-deg. change in direction. Trikes can't perform
true wingovers because bottom rudder is needed at the top of the
climbing turn to keep the aircraft coordinated. Trikes and hang
gliders perform "wangs" (see drawing below).
Is it
safe?........depends on pilot skill, technique, judgment, experience,
etc.
click
thumbnail to expand
And here's a
description of the other two big 'no-no's
a whip stall is an exuberant stall. Instead
of continuing as long as possible to fly straight and level with the
throttle shut, you push the bar out as you shut the throttle, so
that you enter the stall in a climbing attitude. The angle through
which the wing has to recover is therefore
greater.
As you increase the effect, the rotary
inertia of the wing/trike starts to kick in, tending to make the
whole thing rotate beyond the point at which the wing would
otherwise start flying again.
The eventual consequence is a tumble, such
as on the video clip that did the rounds a few months
ago.
If you do a sufficiently exaggerated stall
entry, the aircraft may actually slide backwards before
rotating.
A whip stall is a deep
stall induced by an accelerated entry. The CAA definition of a
standard stall entry is a reduction of speed by 1 knot/second. A
typical whip stall on a flexwing is entered by speeding up well
above a fast cruise, and shoving the bar hard forward onto the front
strut. Dont try it until you have tried several more gentle entries
gradually increasing in severity. The severe cases are best left to
test pilots with parachutes, since the deeper the stall, the more
severe the nose down rotation speed; at its extreme, it can either
result in a tail slide or a tuck or both.
A tail slide is
most definitely to be avoided on all flexwings. If you end up in an
extreme nose up situation (probably with power on) and the wing
stops flying, there is a chance it will slide backwards before
rotating nose down. If it is extremely well pitch-damped there is just a chance that the rotation is slow
enough for a tuck not to result (I know of one case where this
happened). Best avoided.
Here is a superb
diagram published by Pegasus which details what happens in a tail
slide and gives you two clues as to how to get out of one. Mind you
if you have to think about getting out of one it's probably too
late. This diagram was published at the time of the fatal
accident in the USA in Oct 00, but may date from some time earlier.
click
thumbnail to enlarge
A true spiral dive is defined as a
continuous descending turn in one direction where the speed is
increasing. Obviously in a flexwing once this airspeed gets up
to 75 or 80 mph, it's getting dangerous. There is also the
real risk of hitting your own wake turbulence so it's best to know
how to get out of it. On the other hand it's difficult to know
how you get into one of these accidentally. The key is to remember to get out of the turn
first before thinking about controlling the speed. If you try
to do it the other way round you are likely to tighten the turn and
drop the inner wing even more.
So let's say that you
are diving clockwise at about 70mph. First shove the wing out
to the right. You don't have to be too aggressive.
Gentle but firm pressure will eventually cause the aircraft to shoot
out of the dive at a tangent - but still pointing down and probably
accelerating. By now the wings should be level, and you should
be heading in a straight line. Now all you need to do is get
out of an ordinary dive. You shouldn't have any power on (I am
assuming that you took the power off as soon as you realised you
were in a spiral dive), so all you need to do is make sure your
forward pressure on the bar is gentle - so that you don't swoop the
trike up into a stall. If you hold this slight forward pressure, you
will eventually slow up and can resume normal straight and level
flight.
For some reason
let's say you have stalled your aircraft in level flight - maybe you
were trying to get it into a very small field and you decided to fly
it very very slowly. You have to hope that you aren't too
close to the ground as even with the standard technique you are
likely to lose 50 feet before you can recover.
The first thing you
notice is a buffeting through the bar - this is a thump thump of
back pressure. At the same time the aircraft becomes pretty
unresponsive to the controls - so don't bank whatever you do.
If the trike is about to stall in level flight, entering a turn will
undoubtedly cause the lower wing to stall, and pitch you into a
steep side slip.
At the point of stall,
pull the bar back (just sufficiently to reduce the angle of attack
of the wing below the critical stall angle), while simultaneously
applying full power. Initially the nose will drop in a fully
developed stall, but as the air speed increases to a safe figure
raise the nose positively into the (full power) climb attitude. If a
wing drops at the point of stall then recover as above, but roll the
wings approx level before raising the nose into the climb attitude.
When at a safe height from the ground return to aircraft to level
flight.
