Douglas A/B-26 Invader

On Mark Development

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Page 1. On Mark Engineering Co.- A pictorial feast

 On Mark Engineering Co. (OM), Van Nuys, California  

Specializing and licensed by Douglas Aircraft in A-26 Invader conversions and manufacture and sale of parts.

Writen by Richard E Fulwiler

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 - unpressurized.
On Mark Marksman A - pressurized.
On Mark Marksman B - pressurized.
On Mark Marksman C - pressurized.

On Mark Marketeer - Serial No's

Prototype serial number: 44-35326, N2889D, N40Y, N401Y

Serial number: 41-39161, N1470V, N317V, N317W, N26RP

Serial number: 41-39215, N5292V, N4000M, N200M, N142ER

Serial number: 41-39497, N71Y

Serial number: 41-39516, N237Y

Serial number: 43-22258, N1469V, (N36B?),   N94445

Serial number: 43-22444, N4050A

Serial number: 43-22602, N9990Z

Serial number: 44-34390, N6836D

Serial number: 44-34423, N9594Z, C-GHLI

Serial number: 44-34550, N7769C, CF-CCR, N355Q, HK-1247W, HK-1247P

Serial number: 44-34616, N2890D, N300V

Serial number: 44-34713, M5538V, N706ME, N36BB, N26MR, N26WB

Serial number: 44-34738, N9861C, N600D, N808D

Serial number: 44-34754, N66662, N666
Serial number: 44-34755, N67839, N256H, N3035S

Serial number: 44-34769, N67162, N500M, N29711, N500MR

Serial number: 44-35201, N8025E, N137WG, N437W

Serial number: 44-35326, N2889D, N40Y, N401Y

Serial number: 44-39495

Serial number: 44-35562, N707TG, N7079S, N9176Z, N7079G

Serial number: 44-35681, N60Y, N160Y, N168Y, CF-VPR

Serial number: 44-35698, N5001X, N800V, N58071, N67623

Serial number: 44-35708, N5530V, C-GXGY

Serial number: 44-35710, N7705C, OO-INV

Serial number: 44-35913, N3522C, N303WC

Serial number: 44-35938, N4203A, N510X, N516X, D-BACA

Serial number: 41-39467, N6844D

Serial number: 44-34765, N67160, D-CAFY

Serial number: 44-34766, N67807, N1243, N910G, N9150, N26BK 

Serial number: 44-35617, N7660C, N600WB

Serial number: 44-34774, N67163, N163Y, N917Y



On Mark Marksman - Serial No's

( See clarification below by Richard E. Fulwiler )

Prototype #1 (A) Serial Number: 43-22416, N1394N, N100Y, N140Y, N40XY, N190Y


Serial number: 41-39221, N9636C, N3035S, N256H, N26GT

Serial number: 43-22416, N1394N, N100Y, N140Y, N40XY, N190Y

Serial number: 44-34415, N60042, N5002X, N900V, N46358, N46598 " Blue Goose "

Serial number: 44-34526, N9178Z, N827W, N551EH, N400V, N7977, N26AB

Serial number: 44-34567, N9412Z, ZS-CVD

Prototype # 3 (C) Serial number: 44-34761, N67158, N400E, N60XY, N60XX

Serial number: 44-35698, N5001X, N800V, N58071, N67623

Serial number: 44-35870, N1471V, N320, N99426








Additional details


Note 1. Bulkhead modifications

Note 2. Rudder modifications

Note 3. Conventional lateral wing spar versus ring spar

Note 1. Bulkhead modifications by Richard E Fulwiler:
(See photos below)
Looking into how the Invader airframe developed as various companies undertook modifications to increase space and performance, I Checked several links and found data on the B.A.M.R.S. A-26 restoration, information which can be found via this link : 
I have accessed their photo files many times because, during the restoration process, many photos were taken documenting the disassembly and exposure of internal components and structure. To me this is important because it shows why certain skin seam lines overlap where they do because the underlying structure provides a point of attachment, and what that structure looks like.
As an example, compare the aft bulkhead ( above two photos ) where the empennage attaches on the standard fuselage B.A.M.R.S. A-26 photo to Graham's N99426  ( post Andrew ) Marksman photo. It shows how much the standard fuselage was modified by On Mark to raise the cabin top and provide for pressurization by the solid aft pressure bulkhead. It is fortunate for us that both photos were taken from close to exactly the same vantage point.



Note 2. Rudder modifications on On Mark Invaders
The actual difference in rudder design between the A-26B/C models and the On Mark Invaders is in the chord of the rudder - it was widened about a foot, in an attempt to lower the VMC ( the minimum single engine control speed ) with one engine operating at take-off power and the second engine windmilling - rather critical on any twin engine aircraft on take off, which on the early model A/B 26s was 140 kts).
Air Spray looked into the possibility of adapting the K model rudder to their B/C aircraft.
For the most part, it was a straight forward swap.
Simply remove the original B/C rudder, swap positions on the rudder attachment brackets ( Top bracket moved to the lower position and bottom moved to the top position ) and bolt on the K rudder. However, the FAA insisted that Air Spray also had to install the two rows of vortex generators that had been installed vertically on the right side of the vertical stab in front of the rudder.
Unfortunately the vortex generators were not available and they would have had to have them manufactured to match the original drawings, so it didn't go forward with the modification.



The two shots above show the obvious difference in rudder size, the top detail showing an A-26C and the lower shot an On Mark B-26K
Note 3. Wing spar modifications
The On-Mark Marksman had three major modifications from the stock A/B-26 B/C
1. A "ring" spar installed in bomb bay, replaced the original spar carry thru structure ( which was stronger than original and allowed more bombs to be carried )
2. A wing spar reinforcing kit which placed a 1/4" plate on the top and bottom of both spars, running from just inboard of each nacelle, running through the nacelle to about 3' outboard of the nacelle The modification wasn't that noticeable and didn't cure the problem - it was eventually discovered they were actually breaking just outboard of the fuselage, usually on the left side rear spar).
Note: On an similar note but not directly associated with On Mark and as we're talking Wing spar mods
A short term modification and used on both Douglas and On Mark Invaders, came in the form of a steel reinforcement cable which looped around a cleat fitted to a Titanium stress plate modification on the inboard section of the wing ( as seen below ) and was fixed within the fuselage, inboard of the main spar.
Although a little rough around the edges, this short term solution allowed Invaders that had suffered spar failure to undertake ferry flights in order to have the broken spar repaired or replaced.
Flight parameters were severly limited, but the mod was an FAA approved device and helped companies to recover aircraft that would have otherwise been grounded.
This interim mod was designed by engineers at Air Spray and believed used on N7079G when she suffered an in-flight spar failure.


Above, Air Spray Tanker 32 wing spar reinforcement for ferry flight

Other differences
The most visible modification, was the installation of 300 gallon tip tanks, which effectively doubled on board fuel but aggravated the spar problem because of their extremely long movement arm. If you had a surplus Marksman ( and not a civilianized version ) the only thing you had to do was remove the tip tanks and install a set of the original wing tips to make it look like a B/C.
But the most noticeable differences in the K model is the engine (higher horsepower -2500 vs 2000), the prop (still a Hamilton Standard but slightly broader and squared off tips and the rudder - the chord is 1 ft wider and there are a couple of rows of vortex generators running up the right side of the vertical stabilizer just in front of the rudder.