Aerobatics is hard
enough as it is, both mentally and physically, without having to
overcome an evil handling aircraft. To maximise performance and
minimise time spent learning, a well set up aircraft is a must.
The human brain and
body is amazing at adapting and learning things, but often pilots
learn bad habits in order to overcome handling issues. Once you fly
a well sorted aircraft, or improve yours, you have to unlearn those
habits! How much easier would the pilot progression be if the
aircraft was set up correctly from the beginning?
Modern monoplanes are
extremely tunable and two similar looking aircraft can handle very
differently, much like race cars. And just like in motor racing,
some pilots are better, or take more care, setting-up an aircraft
than others. It is difficult to find good info on the subject.
There is virtually nothing written down that I can find specific to
aerobatic aircraft. Often we must learn the hard way, through lots
of thinking and experience, and listening to those that know. This
would be easier if we understood Russian or French!
When you first purchase
a dedicated competition aircraft, get a knowledgeable person to help
you set it up, or get them to set it up for you, before you spend
time and avgas compensating for handling problems. An aerobatic
aircraft should be 'easy' to fly; not too heavy or light in pitch
with good harmony between controls. Controls should be powerful,
but not twitchy or overbalanced. You should feel comfortable and
secure in the aircraft, not on edge and unbalanced. Try and relax,
and loosen the grip on the seat cushion as well as the stick.
In roll the aircraft
should respond to ailerons the same to the left and to the right,
apart from the known slipstream and torque effects. And,
importantly, the aircraft should roll the same from inverted as when
upright! It is very common to find that this is not the case – try
it yourself. It is obviously more difficult to fly good high-rate
rolls when the characteristics are changing significantly as you go
round, requiring you to make inputs to keep it all on track.
I won't be going into
spade set up here, but it is a bit of a dark art about which there is
much to know. Rather, I wish to explain a little discussed
phenomenon....CG effects on rolling.
The effect of Centre of
Gravity (CG) on pitch feel is pretty well known – the more aft the
CG the lighter the stick force per G, and the less stable in pitch
the aircraft is. But the CG position also has a secondary effect
when rolling.
When upright S&L
the horizontal tail must have a down load on it to balance the
aircraft (whether this force is trimmed out or you hold it with the
stick, it is still there). Think back to your PPL theory with an
aircraft drawn in profile. The aircraft mass at the CG drawn as an
arrow down, Lift up through the aerodynamic centre and a smaller down
load on the tail. Just like a balanced see-saw with the pivot being
the Lift arrow, or vector.
Now, imagine this
aircraft force diagram is rolled suddenly to 90 degrees of bank.
The aircraft mass still pulls down towards earth, but the wing and
tail 'lift' forces continue to act perpendicular to the flying
surfaces; that is, horizontally, not vertically. The see-saw is not
balanced any more. What will the nose of the aircraft want to
do.....?
What was a down force
on the tail, is now a horizontal force wanting to 'pitch' the nose in
the direction of roll, and the Lift force, rather than supporting the
aircraft weight is supporting nothing! Unbalanced forces =
acceleration, said Newton, so the aircraft must want to displace and
pitch 'up' in the direction of the roll.
Try it yourself next
time you fly, or think back to what happened during your waggles into
the box as a novice. Did the nose come off line, and by the time
three waggles were done you were 30 deg off axis, and all before you
even started any aerobatics! The cure? A bit of forward
pressure/movement on the stick as you waggled. This reduced wing
angle of attack/lift, reduced tail 'down' load and thus helped you
hold your aim point. Inverted the physics is the same, so you can
work out those inverted wing waggles for yourself.....
Greater problems occur,
and are less easily cured, in the dynamic environment of a rapid
roll. There is more to learn here.
Getting back to PPL
aerodynamics, and the reason for this article, how does CG position
affect the download on the tail? Thinking like a see-saw, the
closer the CG (mass arrow downwards) is to the Lift (upwards), then
the less download will be required on the tail to balance
(aeronautical engineers please forgive me the simplification).
So it follows that an
aft CG, and thus reduced tail load, will also reduce the unwanted
pitching that occurs when you roll. It will also reduce the forward
stick needed when inverted. This will make rapid rolls much easier
as you won't have to manipulate the stick fore and aft as much to
stay on axis. You still have the tilted lift vector to cope with
though – can't do much about that.
Now, we can't go to
extremes here and move the CG too far back, as the aircraft will
become unstable and NO elevator input will be required to pitch the
aircraft. Think about this, and you should realise it is a BAD
THING.
What is a reasonable CG
position? Personally, I think for comp aeros in a purpose built
monoplane you want it at least in the aft half of the recommended
envelope, say 2/3 – ¾ of the way back. Usually, this is
somewhere around 26-28% of Mean Aerodynamic Chord (MAC). You may
want to creep up on this if new to high performance aircraft,
starting with the CG a bit further forward, and as experience grows
move it back as you become comfortable with the lighter pitch forces.
Don't delay too long though, as you want to start building the
right muscle and visual memory as soon as possible.
I recommend re-weighing
and calculating the CG position whenever you get a new aircraft or
make changes such as a lightweight starter, battery, prop or
alternator. Don't rely on factory figures or those in the Flight
Manual. They may still be legally OK but years out of date. Don't
assume, find out for yourself so you KNOW – it's your time, money,
and life. We are working in the aft end of the envelope, an inch CG
movement can be 2% of MAC change, with a significant affect on
stability if you go too far.
You don't need an
Approved Weight Control Authority to do the re-weigh. You don't
need to change the Official W&B figures, you just want to check
for yourself what the actual figures are. Borrow or hire some
reliable scales (car clubs, SAAA chapter), research how the job is
done, or get someone knowledgeable to assist. Weigh the aircraft
empty, then with acro fuel, and finally with both acro fuel and you
kitted to fly. That way you will be able to calculate the actual
moment arms for the fuel and pilot, so if you alter either of these
you will still be able to calculate what the actual CG is. A
spreadsheet makes the whole job very easy.
Roll On!
