Global Aviator South Africa - May 2018

54 Vol. 10 / No. 5 / May 2018 Global Aviator

Helicopter do's and don'ts

reason for this is that the lift vector

now has a horizontal component. In

other words, the power which was

being used simply to counteract the

weight of the helicopter now has to

do that, but also move the helicopter

forward. This is perhaps a more

generally accepted reason, but some

experts mention both effects, and the

real reason is possibly a combination

of the two phenomena. Anyway,

the student really doesn’t have

time to think about the reasons for

this, because another aerodynamic

effect follows on soon afterward.

The next thing that happens as

the aircraft moves forward is that

it begins to roll to the right, so that

a little left cyclic is required. This is

due to different amounts of lift being

produced at different points along the

rotor blades at very low airspeeds,

although the exact theoretical reason is

somewhat more complicated than this.

In practice inflow roll is a very

small effect, if it occurs at all. Until

I became an instructor I was never

aware of having experienced it, and I

still find it quite hard to demonstrate

it to students, since in the Robinson

R22 at least, you have to accelerate

very slowly for it to happen at all.

The reason for that is partly

that other aerodynamic effects

soon begin to take over.

The next effect – and this one is

really noticeable – is called ‘flapback’.

When the cyclic is moved forward,

the ‘rotor disc’ (the hypothetical

disc the rotors make when they

are rotating) tilts forward and

down, and this is what causes the

helicopter to move forward.

However, owing to dissymmetry

of lift – and again, the details are

quite complex – the disc tends to try

to flap back to its original position.

This means that although a forward

movement of the cyclic initially

causes the helicopter to speed up,

that acceleration will very quickly

cease unless the pilot continues

to move the cyclic forward.

So constant forward pressure

on the cyclic is required, or, as the

instructor will probably urge the

student, “push through flapback”. In

fact, flapback takes place throughout

the speed range of the helicopter, and

this is why slight forward pressure

on the cyclic is always required if

the helicopter is not to slow down.

The next aerodynamic effect which

happens is called ‘translational lift’. At

around 12- 15 knots of airspeed, some

of the air which is being sucked down

into the rotor system is blown away

horizontally, and this means that more

air is available to help the helicopter

climb. In other words, it is free lift,

and this is one of the few occasions

when you get something for nothing.

However, at this point in the

transition we don’t want the helicopter

to climb. Helicopters accelerate close

to the ground if at all possible, and

only take off vertically or nearly so

if they absolutely have to, in order

to avoid trees or other obstacles.

There is a good reason for this.

If the engine were to fail at any

point, the pilot wants to be able to

put the helicopter into auto-rotation

and land safely. This would be very

difficult if the helicopter were to be

high and slow, so you accelerate to a

safe flying speed close to the ground,

then allow the helicopter to climb.

Therefore when translational lift

occurs, the pilot pushes the cyclic

forward to convert lift energy into

forward airspeed, while keeping close

to the ground. At around 45 knots,

which is a safe flying speed, he or she

eases back on the cyclic and allows

the helicopter to climb, then pushes

the cyclic forward a little to select

the usual climb speed of 60 knots.

At this point the helicopter

will tend to yaw left due to the fact

that the tail fin is more efficient at

higher airspeeds. It will also tend to

roll left. Therefore both right cyclic

and right pedal will be required.

That is the transition completed,

and the helicopter is now in forward

flight. Reading this for the first

time, you might well think that this

transition to forward flight requires

incredibly fast reactions and the ability

to do several things at the same time.

Indeed, when I first learned all this I

wondered how you could possibly do

it in practice. However, the transition to

forward flight is one manoeuvre which

is actually far more difficult to describe

than to do. In practice, the pilot brings

the helicopter to a hover, looks well

ahead, then moves the cyclic forward.

He or she then adjusts all the

controls as much or as little as is

required until the helicopter is in

forward flight. It is possible to do this

without much real knowledge of the

aerodynamic factors involved, and

in fact, until I became an instructor

I had only a vague idea of how it all

worked ... and I suspect I was in good

company! In practice, transitions

from the hover to forward flight

are quite easy, and most students

At this point the helicopter

will tend to yaw left due

to the fact that the

tail fin is more efficient

at higher airspeeds

The process of slowing down from forward

flight to the hover is called a transition.