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In the early 1950s, Grand Central Air Terminal, Glendale, California,
was a hot-bed of ex-military aircraft conversion and modification.
This historic airport (which exists almost completely intact but
is in threat of demolition by the Disney Corporation which now owns the property) was America's first west coast transcontinental
terminal and over the years (it closed in 1959) had seen numerous historic aviation firsts.
Grand Central Aircraft Company executives came up with the idea
to modify an Invader to carry passengers. Several Invaders were completed when the employees split off and established On
Mark Engineering at nearby Van Nuys Airport. The company obtained a Supplemental Type Certificate (STC) and began to produce
a number of conversions. With conversion production getting underway in the late 1950s, the company created first the
Executive, becoming the Marketeer in 1957, which were the unpressurized variants. Most had the rear spar
carry-through (which limited cabin access) replaced by a strong and efficient circumferential "ring" spar of On Mark's patented
design. By removing most of the original interior bulkheads and adding the ring spar, provided
additional room for passengers in the new cabin space of the aircraft. The forward wing spar was not changed because of the
magnitude of the re-engineering that would have been required, and would have altered the aircraft's design geometry and excellent
flying qualities. With the wings mounted as with the original design, meant that crew members were left with the inconvenience
of mounting the flight deck through a crawlway along the right side of the cabin beneath the forward spar. The
customer would supply or either purchase a standard Invader which would be restructured in On Mark's spacious hangar.
The cockpit would be removed and rebuilt to include dual controls, upgraded instruments and radios, a lengthened
fiberglass nose for baggage and radar added, and, more importantly, the cabin was built to customer specifications, and added
a number of large windows. The standard A-26 curved windshield design would be retained, with the upper canopy clamshells
either painted or skinned over. Air stair doors were installed into the belly, or right side of the fuselage. Also offered,
were distinctive 165 gallon wing tip fuel tanks added to increase range, and upgraded P&W R-2800's.
Next came, perhaps
the best-known of the civilian Invader conversions, the On Mark Marksman series in
1961. The Marksman was the pressurized variant and required a special Supplemental Type Certificate (STC). The airframes were restructured
with a greatly altered fuselage, DC-6 / -7 canopy structure with heated windshields, radio/navigation/radar improvements,
ring spar, lavatory and food service consoles, soundproofing, air conditioning, increased area vertical tail, long nose, wingtip
fuel tanks, improved anti-skid brakes, deicing, and numerous other upgrades and systems included as standard. The pressurized Marksman
series were offered in three versions; Marksman A, B, and C. The Marksman A
had 2100 hp R-2800-83AM3 engines; the Marksman B had 2100 hp R-2800-83AM4A engines; whereas the premier Marksman
C had a raised cabin top that provided a 6 foot walk-through full length cabin, 2500 hp R-2800-CB-16 / -17s
and added internal wing auxiliary fuel tanks. Only 15 percent of the original fuselage remained, and dependant on customer
options, were designed to carry from 6 to 8 passengers.
The base price of the Marksman A was $257,430,
up to $361,492 for the Marksman C. The On Mark Marksman was regarded as the best
of the A-26 Invader conversions, providing up to eight business executives and a crew of two, an all-weather aircraft
that was able to cruise up to 25,000 feet at 325 - 365 miles per hour with a range of between 1200 and 2500
miles with normal reserves. At 20,000 feet, the pressurized, air-conditioned cabin was at a comfortable 7,500 feet.
On Mark also built a model and partial mock-up of the Model 450
which was a greatly modified pressurized aircraft fitted with Allison 501 D turboprops but, after a lot of deliberation, the
project was shut down. Oddly, there is no exact record of the number of converted Invaders built by On Mark but its thought
to be up to 50 aircraft. Also, the reign of the On Mark aircraft was short - a new generation of business jets led by the
Lockheed JetStar and Lear Jet quickly dominated the market and the On Marks were purchased in the late 1960s and 1970s by
drug runners who respected the type's long range and load carrying abilities. The following is a quick look at On Mark Invader
conversions.
On Mark - Marketeer
On Mark Marketeer:
Prototype serial number: 44-35326 ( c/n: 28605 ) Registered as N40Y ( 1957 )
Unpressurised version
of the executive transport produced
by On Mark Engineering on almost production-line basis.
After the airplane was stripped of its armament,
it was entirely re-engineered, re-framed and altered. All surface metal of fuselage, wing and empennage structures were inspected.
The aft wing-spar that passes through the fuselage was removed and was replaced by a ring of steel. Most
of the partitions within the fuselage were removed to open up the cabin area. Skin, stringers and ribs were altered,
added to and strengthened.
Large 'picture windows' were installed,
as was the side entrance. Thick Fiberglass insulation was packed in cabin and cockpit
walls to quiet the cabin.. When
changes in the structure were completed, the interior was finished off to
customer's specification that included a lavatory and a seating capacity for up to seven passengers
in clubroom comfort.
Extra fuel tankage in the wings, tip tanks, an auxiliary power unit (APU), advanced radios, radar,
and 2,500 hp (1865 kW) R-2800-CB-16 / -17 radials were often installed to customer specifications.
See feature by Richard E. Fulwiler
On Mark - Marksman A
On Mark Marksman A: Prototype
serial Number: 43-22416
( c/n: 18563 ) Registered as N100Y ( early 1961 )
Pressurized, air conditioned version of the executive transport produced by On Mark. Cockpit windshields and side windows were flat panels from Douglas DC-6
/ -7 airliners, while cabin windows became rounded-corner squares. Extra sound deadening materials were used to further quiet
the cabin. Powered by 2,100 hp (1567 kW) R-2800-83AM3 engines.
On Mark - Marksman B
Similar
to the Marksman A apart from the provision of R-2800-83AM4A radials.
On Mark Marksman C: Prototype
Serial number: 44-34761 ( c/n: 28040 ) Registered
as N400E ( late 1961 )
Vastly modified, premier version
of the pressurized Marksman series. Cabin interior volume was increased by raising the fuselage top to provide a 6' 1" full
length "walk-through" height. All "C" model Marksmans were powered by high output Pratt and Whitney R-2800-CB-16
/ -17 radials developing 2,500 hp (1865 kW) for take-off with water injection. Also standard were fully reversible, broad
chord Hamilton Standard propellers, and a "707" type anti-skid braking system. The Marksman C was the ultimate in executive
transport only for a brief time until executive jets became available.
The early radar-equipped Marketeers had fiberglass nosecaps
that were round-ended conical sections, but otherwise "B" model gun-nose profile. But when they went to the 103" nose, it
was a extended, sharply tapered, all fiberglass unit that mounted on fuselage station "0". All Marksmans and many of the Marketeers
were equipped with the 103" nose.
Prices for these aircraft
On Mark Marksman A - $257,430.00
On Mark Marksman B - $314,715.00
On Mark Marksman C - $361,492.00
Note: These prices did not include for radio,
electronics or sales tax
Early nose with radar (note the slight step in conical section)
103" Nose
Clarification by Richard E. Fulwiler
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