Figure 1-3. Airflow Under the Rotor.

TM 9-1270-219-13&P

Figure 1-3. Airflow Under the Rotor.

Similar

characterization

required

for

lateral

flight,

e. Since the probe is always aligned with rotor

though in this case the AADS always underreads as the

downwash the static pressure port does not suffer from

helicopter cannot fly sideways at a speed where rotor

pressure

errors

and

pressure

error/velocity

gradients.

b l a d e turbulence can be sensed by the AADS. Therefore,

Having correctly sensed the pitot and static pressures it is

the level of the airspeed correction required for combined

then

necessary

correct

this

data

for

fuselage

forward and lateral is determined by the AADS which

downwash effects in order to obtain accurate airspeed and

senses the downwash turbulence under the rotor,

airflow direction. These effects are removed by airspeed

characterization, to achieve corrections to the forward,

lateral and vertical airspeed parameters.

Airspeed

corrections

are

applied

for

any

combination of forward and lateral flight. For example,

forward errors for all forward airspeeds with zero lateral

f. Characterization is defined as the correction of the

airspeed

are

stored

the

form

curve.

This

system airspeed outputs for the effects of downwash and

discontinuity curve is stored in the Central Processor Unit

airflow distortion

around

the

helicopter

fuselage.

memory in the Electronics Processor Unit (EPU) as a

therefore

peculiar

any

one

Characterization

s e r i e s of polynomial equations.

helicopter

type.

i. By using a similar curve for forward airspeeds

g . The Airspeed profile of the AH-1S in forward and

with

knot

lateral

component,

possible

rearward flight is shown in figure 1-4. At airspeeds of less

i n t e r p o l a t e between the two to give the forward error for

than 32 knots forward, the AADS will underread while

a l l forward airspeeds with lateral components of between

f r o m 32 knots to 60 knots the AADS will overread, due

0 to 10 knots. By extending this process, a complete

to the turbulence generated by the tips of the rotor blade.

picture of the forward errors for all lateral airspeeds can

A t airspeeds above 60 knots, the AADS is clear of the

be constructed. With similar curves for lateral airspeed

rotor

downwash

and

characterization

required.

1-4