Subject: Sixth in the Series--The Curtiss P-6 Hawk

From: [email-masked] (Joseph F Baugher)
Date: 28 Aug 92 16:34:37 GMT

>From Joseph F Baugher <[email-masked]>

The P-6 Hawk series resulted from the installation of the new  600  hp
Curtiss V-1570 Conqueror liquid-cooled engine in what were essentially
P-1C airframes.  The variant which is best remembered today is the  P-
6E,  which  IMHO  was  one  of  the best-looking biplane fighters ever
manufactured.

The Curtiss V-1570 Conqueror engine was a evolutionary  descendent  of
the  Curtiss  D-12 which powered the P-1 Hawk.  The direct ancestor of
the Conqueror was the unsuccessful Curtiss V-1400 engine which powered
the P-2.

The first aircraft to carry the P-6 designation  was  the  fourth  P-2
(Ser  No  25-423),  modified  to race for the Army in the National Air
Races of 1927.  It was the first  Hawk  to  be  bitted  with  the  new
Curtiss  V-1570 engine which later became known as Conqueror.  Because
of the use of the new Conqueror engine, the Army gave the  airplane  a
new  designation--XP-6.   Stripped  of  military  equipment, it placed
second in the unlimited event of the 1927 National Air Races.

For its principal entry in the  1927  National  Air  Races,  the  Army
ordered  that  extensive modifications be made to a stock P-1A (Ser No
26-295).  It was fitted with  a  V-1570-1  Conqueror  engine  and  was
equipped  with  a  set of PW-8A-type untapered wings complete with the
skin-mounted radiators.  It bore the company designation of Model 34Q.
The  Army  redesignated this aircraft XP-6A No 1 because of its use of
the Conqueror engine.  It took first place in the 1927 race at a speed
of  201  mph.   However, the XP-6A crashed during preparations for the
1928 National Air Races.

The success of the  Curtiss  Conqueror  engine  in  these  two  racing
aircraft  led to an Army contract for a service test quantity of 18 P-
6s placed on October 3, 1928.  These aircraft were assigned the serial
numbers  29-260/273  and  29-363/366.  These  aircraft  were given the
company designation Model 34P.  The Y-for-service-test designation had
just  been  adopted  at  this  time,  but  it  does not appear to have
actually been applied to these planes,  although  they  are  sometimes
recorded as YP-6s.

One of the innovative features of the new Conqueror-powered P-6 was in
its  cooling  system.  The water coolant of the earlier P-1 series was
to be replaced by Prestone,  a  trade  name  for  an  ethylene  glycol
(HOCH2CH2OH)  mixture.   Prestone  was  a product of the Union Carbide
corporation, and had an advantage of having a very high boiling  point
and  a  very  low freezing point.  By using Prestone instead of water,
Curtiss was able to reduce the surface area of its  radiators  by  one
third.   In  addition,  since  less  coolant  was  needed,  the use of
Prestone rather than water resulted in the savings of about 50  pounds
of weight.

However, since the new Prestone-cooled Conqueror engines were not  yet
ready,  aircraft  39-269/273 and 39-363/366  were delivered in October
1929 with water-cooled V-1570-17 engines as P-6s, so that  they  could
be  gotten  into  service  as  rapidly  as  possible.  The rest of the
aircraft  in  the  order  were  completed  later  as  P-6As  once  the
Prestone-cooled V-1570-23 Conqueror engines were finally ready.

The  P-6  was  generally  similar  to  the  P-1  in  construction  and
appearance.  However,  the  P-6   differed  from the P-1 in having its
fuselage rounded out to match the fatter engine  cowling  required  by
the  Conqueror  engine.   A  series  of  stringers  were  added to the
fuselage sides to round out the cross section. In addition,  the  rear
fuselage  was  deepened  in order that it could faire cleanly into the
bottom of the radiator.  The result was an airplane which had  a  much
deeper  and  broader fuselage than did the P-1.  Maximum speed was 178
mph at sea level, 171 mph at 10,000 feet.   The  P-6  could  climb  to
10,000  feet  in  6.6 min.  Service ceiling 27,200 feet, and range was
260 miles.  Weights were 2450 lb. empty, 3310 lb gross.  The  armament
of  the  P-1  was a pair of 0.30 cal machine guns mounted in the upper
fuselage decking inside the V-cylinder blocks and synchronized to fire
through  the  propeller  arc.  Deliveries of the P-1 to the USAAC were
made between October 1929 and December 1930.

The  first  nine  P-6s  (Ser  Nos  29-260/268)  were  completed   with
Prestone-cooled  V-1570-23  Conqueror  engines once these engines were
ready.  These aircraft were redesignated P-6A once they were delivered
to  the  Army.   Weights were 2389 lb. empty, 3172 lb. gross.  Maximum
speed was 176 mph at  5000  feet.   Initial  climb  was  1910  ft/min.
Service ceiling was 27,300 feet.  Armament was the same as that of the
P-1.

Two additional P-6s resulted from conversions  of  the  Curtiss  P-11.
The  P-11  was  basically  a P-6 airframe modified so that it could be
powered by the new  600-hp  Curtiss  H-1640  twelve-cylinder  two-bank
air-cooled engine.  Three examples of the P-11 had been ordered by the
Army for tests.  However, before any of the P-11s  could  actually  be
completed,  the  H-1640  engine  had  proved  itself  to be completely
unsatisfactory, and the first two P-11s (Ser No 29-367 and  368)  were
completed  as P-6s with water-cooled Conqueror engines.  The last P-11
(Ser No. 29-374) was fitted with a 575 hp Wright R-1820 Cyclone radial
engine and was redesignated YP-20.

During the service test period, various minor changes in the  radiator
shape  were made and some of the machines were fitted with three blade
propellers in place of the original two-bladers.

XP-6A No 2 was the designation given to P-6A Ser No 29-263 diverted to
test work.  Despite its X-prefix, it was not a prototype.

XP-6B was the designation given to the last P-1C (Ser No 29-259)  when
it  was  delivered  on  July  18, 1929 with a 600 hp V-1570C Conqueror
engine.  It was intended  to be used by  Captain Hoyt in a  long-range
flight  from the eastern USA to Alaska.  The aircraft came to be known
as "The Hoyt Special". The fuselage lines and the cowling were similar
to  those  of the later P-6 and P-6A.  Long-range tanks were built in.
The flight attempt took place in July of  1929.   However,  the  XP-6B
only  got  as far as Valemont, British Colombia, where it made a crash
landing.  After repair and deletion of the long range  tanks,  it  was
returned  to  service  as the sole XP-6B. Maximum speed was 197 mph at
15,000 feet.  Service ceiling was 32,000 feet.  Weights were 2698  lb.
empty, 3483 lb. gross.  Initial climb was 1730 ft/min.

P-6C was a new designation that had  been  initially  assigned  to  46
Y1P-22s.  [Y1  was  a  prefix  used from 1931 to 1936 to indicate that
these service test aircraft had been purchased with "F-1" funds rather
than  from  regular  USAAC  appropriations.  This distinction from the
regular Y-for-service-test  prefix  was  only  budgetary  and  had  no
technical  significance.]   The P-6C designation was later canceled in
favor of P-6E for the same planes.

The  XP-6D   was   a   new   prototype   created   by   installing   a
turbosupercharged  V-1570-C  Conqueror  in  P-6A  Ser  No 29-260.  The
turbosupercharger was mounted externally on the right-hand side of the
fuselage.   Sea  level  top  speed  increased  to 172 mph and speed at
15,000 feet increased  to  197  mph.   The  modifications  were  later
removed,  and the aircraft reverted to the configuration of a standard
P-6A.

All of the surviving P-6s and all the P-6As except 29-267 were  fitted
with  turbosupercharged  Conquerors in March and April of 1932.  These
were redesignated P-6D and were assigned to  Langley  Field,  Virginia
for  service trials.  The only outward difference between these planes
and the XP-6D was in the use of three-bladed propellers.

The path to the famous  P-6E  variant  is  sort  of  complicated,  and
involves  several  modifications  and quite a few redesignations along
the way.  It can be said to begin with the third P-6A (Ser No  29-262)
which  was  redesignated  XP-22 when used to test new radiator and oil
cooler installations for the  700  hp  V-1570-23  engine.   The  final
modification  to the XP-22 produced an entirely new nose, with the oil
cooler  and  radiator  located  on  the  belly  situated  between  the
undercarriage  legs.  A three-bladed propeller was used.  Machine guns
were lowered to  troughs  on  the  fuselage  sides  under  the  engine
cylinder  banks  rather  than between them as on previous versions.  A
new single-leg undercarriage was also installed.   Spats  were  placed
around  the  wheels.   These new features were eventually removed from
the XP-22 and were installed on the YP-20.  They were replaced by  the
original P-6A equipment and the XP-22 reverted to a standard P-6A.

In the meantime, the third P-11 (Ser No 29-374) had been completed  as
a  YP-20  with a 650 hp Wright R-1870-9 Cyclone radial engine in place
of the failed Chieftain  engine.  The  fin  and  rudder  were  changed
slightly  by  raising the division between the rudder balance area and
the top of the fin by  half  a  rib  space.   This  radial  powerplant
installation  ultimately  turned out to be unsatisfactory.  29-374 was
then fitted with the 700 hp V-1570-23 Conqueror engine, mounted with a
new  horizontal  tail,  and given the single-strut undercarriage first
tried out on the XP-22.  A tailwheel was used in the place of the tail
skid.   The  YP-20  was  then redesignated XP-6E, the prototype of the
most famous of the Hawk line of pursuit aircraft.

46 production examples of the XP-22, with the 700 hp V-1570C  (V-1570-
23)  Conqueror  engine,  had been ordered on July 8, 1931.  These were
initially designated Y1P-22.  However, since many of the parts of  the
Y1P-22  were identical with P-6 spares, the designation was changed to
P-6C in order to simplify the bookkeeping.  However, this  designation
was  changed  yet  again  to  P-6E  before  delivery. In the meantime,
features that had first been tried out on the XP-22 had been added  to
the  YP-20  airframe  and  it was decided that the production aircraft
should be  to  the  YP-20  standard  as  P-6E.   The  YP-20  was  then
redesignated XP-6E, which is considered as being the true prototype of
the P-6E variant.

Deliveries of the P-6E began in December 1931.   Serial  numbers  were
32-233/278.   The company designation for the P-6E version of the Hawk
was Model 35.  32-278 was held back at the factory for  conversion  to
XP-23.   32-233  became the XP-6H, and 32-254 became XP-6G, then P-6G,
and eventually reverted to P-6E.  Structural refinements  had  brought
the  empty weight of the P-6E down to 2715 lbs.  Gross weight was 3436
lbs. Maximum speed was 193 mph at sea level. Initial  climb  rate  was
2480 ft/min.  Service ceiling was 23,900 feet and range was 244 miles.

The P-6E was quite a good-looking airplane, and became the most famous
of the Hawk line of biplane fighters.  It is perhaps the best-known of
all the "between  wars"  Army  pursuits.   In  a  flyoff  against  the
contemporary Boeing P-12B, the P-6E was faster, but the P-12B was more
maneuverable.  The good speed of the P-6E was counterbalanced by  some
unsatisfacory  handling  characteristics  which  made  it  sluggish in
response to controls.  The 700 hp Conqueror engine  was  exceptionally
powerful  for  its  day,  but  it had many minor and some major faults
which needed to be corrected.

P-6Es served from 1932 onward with the  1st  and  8th  Pursuit  Group,
flown  by the 17th, 94th, and 33rd Squadrons based at Selfridge Field,
Michigan and at Langley Field, Virginia.  They were  kept  in  service
until  1937.  The  shapely  wheel  spats  for  which  the P-6E is best
remembered today were often replaced in service with a  set  of  open-
sided wheel fairings, especially in later years.  In Army service, the
P-6Es were involved in a protracted series of accidents which  claimed
no  less  than  27 of the 46 examples built.  The Army's P-6Es rapidly
became obsolete as the 1930s wore on. Instead of being given expensive
overhauls  when  they  were  called  for,  the  P-6Es  were allowed to
deteriorate and wear out in service.  One by one, they either wore out
and  were  scrapped,  or  else  they  crashed.   However, at least one
survived into 1942.

The XP-6F (Model 35C) was the designation given to XP-6E Ser No 39-374
that   was   sent   back   to   the  factory  for  installation  of  a
turbosupercharged V-1570F (V-1570-55) engine delivering  675  hp.   It
was fitted with an experimental cockpit canopy.  It was redelivered to
the Army in March 1933 as XP-6F.  The gross weight was nearly 400  lbs
greater than that of the standard P-6E.  Sea level top speed decreased
to 194 mph, but speed at  15,000  feet  was  an  impressive  225  mph.
Cooling  difficulties  with  the  engine precluded any careful testing
above that altitude.  However, the XP-6F  did  prove  that  as  speeds
exceeded   200  mph,  the  traditional  open  cockpit  was  no  longer
satisfactory.  The XP-6F later became an P-6F with a 775 hp  V-1570-55
engine.

The XP-6G was P-6E Ser No 32-254 fitted with an unsupercharged V-1570F
engine.  When the experimental work was done, the plane retained the F
engine, but dropped the experimental prefix to become just plain  P-6G
and  finally  converted  back  to P-6E again when the standard V-1570C
engine was installed.

The XP-6H was the designation given to the first production P-6E  (Ser
No  32-233)  returned to the factory for installation of four 0.30-cal
machine guns in the wings.  Two guns were in the upper wing and one in
each lower wing panel.  All four fired outside the propeller arc.  The
fuselage guns were retained, resulting in a very heavily-armed fighter
for  its  day.  Gross weight went up to 3858 lb and top speed declined
to 190 mph.  Delivered April 20, 1933.  After the tests,  the  special
wings   were   removed   and  the  plane  reverted  to  standard  P-6E
configuration.

The last P-6E (Ser  No.  32-278)  was  retained  at  the  factory  for
coversion  to  XP-23  (Model 63).  This plane resembled previous Hawks
only in the wings.  The fuselage structure was a metal monocoque.  The
tail surfaces had different shapes, and a turbosupercharged and geared
G1V-1570-C Conqueror drove a left-handed 3-blade prop.  The  nose  was
much more pointed.  It was delivered  April 16, 1932.  It had improved
altitude performance, but the Army recognized that the biplane fighter
had finally reached the end of the line and no production was ordered.
The turbosupercharger was removed, and the XP-23 was redesignated  YP-
23.

P-6S was an export model with a P-6 airframe fitted with 450 hp  Pratt
and  Whitney  Wasp  radial engine.  Three were sold to Cuba and one to
Japan in 1930.  Company records are unclear about the  Japanese  Hawk.
In some photos, it appears with a Conqueror engine.

Hawk 1 was the designation given to civil and export versions  of  the
P-6.   A  company  demonstrator  built  in  1930 and registered as 9W.
Following a crash, it was rebuilt and registered  NR9110.  Because  it
was  demonstrated by the famous pilot James Doolittle, it became to be
known as the "Doolittle Hawk".  This airplane was later  sold  to  air
show  pilot Jesse Bristow and used for air show work until forced down
at sea between Florida and Cuba during an air race in January 1940.

Another Hawk 1 was a special demonstrator  built  in  April  1929  for
long-distance  flights.   It was  powered by  the Conqueror engine and
had extra fuel tanks fitted into  the  sides  of  the  fuselage.   The
original civil registration was NR636E.  After a crash, it was rebuilt
as a Hawk 1A with a 575 hp Wright Cyclone radial engine and  was  sold
in  August  1930 to Alford J. Williams, a famous air show pilot of the
1930s.  Registration was NR982V.  Since Al Williams flew for the  Gulf
Oil Company, his plane was known as the *Gulfhawk*. In August 1931, Al
Williams installed in the *Gulfhawk* a 575 hp Bliss Jupiter engine, an
American-built  version  of  the  British  Bristol Jupiter.  Following
another crash, NR982V was again rebuilt with a 710 hp Wright R-1820F-3
Cyclone engine.  The side fuel tanks were removed and the fuselage was
skinned with metal rather than fabric.  The engine was later taken out
and  transferred  to  Williams'  new  Grumman  *Gulfhawk  II* in 1936.
NR982V was then placed  in  an  aeronautical  trade  school.   It  was
retrieved in 1958 by movie pilot Frank Tallman, who installed a 600-hp
Pratt and Whitney Wasp engine in the airframe. It is  presently  owned
by the US Marine Corps Museum at Quantico, Virginia.

A P-6E currently resides in the Wright-Patterson Air Force Museum.   I
do  not  know  anything about the history of this particular airplane.
Perhaps  one  of  the  readers  of  sci.military  can  dig   up   some
information.

Sources:

  United States Military Aircraft Since  1909, Gordon Swanborough  and
  Peter Bowers, Smithsonian Institution Press, 1989.

  The American Fighter, Enzo Angellucci and Peter Bowers, Orion Books,
  1987.

  Curtiss Aircraft: 1907-1947, Peter M. Bowers, Naval Institute Press,
  1987.

  "The Curtiss Army  Hawks",  Peter  M. Bowers,  Aircraft in  Profile,
  Doubleday, 1969.

Joe Baugher       AT&T Bell Laboratories    2000 North Naperville Road 
		  Naperville, Illinois 60566-7033.   (708) 713-4548