Notar makes

the grade

Tail rotors are a necessary evil on single-rotor helicopters.

Aerospatiale made them less dangerous with its fan-in-fin

fenestron

but now McDonnell Douglas plans to abolish the tail

rotor entirely.  raham W arwick  reports.

very act ion has an equal and

opposi te react ion. In s ingle-rotor

hel icopters this basic t ruth

makes the fuselage rotate in the opposi te

direct ion to the rotor unless prevented

from d oing so by the ant i- to rque tai l rotor .

A tai l rotor ma y be necessary, bu t i t is a

l iabi l i ty . Hovering in confined spaces

carr ies the r isk of the tai l rotor s tr iking

something, while i ts scything blades are a

menace to those on the ground and, in

combat , a target for smallarms f ire .

Now McDonnel l Douglas Hel icopte rs

has decided to seek commercial cer t i f ica

t ion for a no-tai l -rotor (Notar) MD500, to

pave the way for i ts tai l -rotorless MDX

light helicopter in the early 1990s.

Th e decision is based on successful

Notar f l ight tests on a modified OH-6,

whic h first flew in 1981. W hile th e test bed

tours the USA demons t ra t ing the benef i t s

of tai l -rotorless f l ight , MDH is working

towards commercial cer t i f icat ion of the

Notar concept by the end of 1989.

The cur ren t -produc t ion MD500E and

hot -and-h igh 530F were bo th in t ro

duced by Hughes in 1982, the 500E

powered by a 420 s .h.p. All ison 250-C20B

an d the 530F by the 650 s .h.p. C30 varia nt .

Two new higher-performance versions are

planned by MDH, a 520L to be cert i f i

cated in December this year , and a hot-

and-high 530K to be cert i f icated in June

1989.

Th e two varia nts wil l bot h feature

increased-diameter main and tai l rotors

which increase l i f t ing capabil i ty and oper

at ing al t i tude, and decrease noise. Based

on the 500E, the 520L is powered by the

new 450 s .h.p. All ison 250-C20R, while the

530K retains the C30 pow erplan t of th e

exist ing 530F.

The ta i l - ro tor less MD500, des igna ted

the 520N, wil l be based on the new 520L,

with i ts C20R engine and increased-

d iameter main ro tor . MDH plans to bu i ld

two Notar-equipped prototypes, the f i rs t

of which will fly in May 1989, wit h certifi

cation set for late 1989.

The tai l -rotorless hel icopters wil l be

pro to types because MDH is no t ye t

comm it ted to p roduc ing the 520N, bu t th e

differences between prototype and pro

duction aircraft will be small, it says.

Wh ether the comp any proceeds in to 520N

production wil l depend on market

response, however.

MDH delivered 69 MD500s in 1987,

down from 76 in 1986, bu t ended the year

with i ts largest order backlog—28

aircraf t—in recent years . This level of

sales would not be enough to launch the

520N, however. The company is looking

for a s l ight ly bet t er m arke t , and bel ieves i t

wil l material ise in the next 18 months.

Already there are s igns.

So MD H is be ing cau t ious about in t ro

ducing Notar , a l though i t bel ieves the

concept to be the most exci t ing tech

nology in recent years .

The Notar tai lboom is , in effect , a

circular-sect ion circulat ion-control aero

foi l . As main-rotor downwash f lows over

the boom i t meets higher-veloci ty air

exi t ing from slots running the length of

the pressurised boom's s tarboard s ide.

The mixing of these f lows causes the

downwash to adhere to the boom on that

side while i t separates on the other . This

has the same effect as camber on an aero

foil, generating a sideforce which opposes

main- ro tor to rque .

Th e Notar tai lboom is pressuris ed to

about 0-51b/in

2

  by a fan driven from the

engine gearbox. Fan p i tch is varied to keep

boom pressure, and therefore s lot exi t

velocity, fairly constant at about four

t imes the downwash veloci ty.

Circulat ion control provides about

two- th i rds o f the an t i - to rque mom ent

26 FLIGHT INTERNATIO NAL 27 February 1988