Yellow arrow points to 2.75" FFAR in flight (USAF Photo)
Early vs. Late O-2As (or "the big window")
After delivery of the initial order of O-2As (Ser# 67-21295 thru 67-21439), the USAF asked Cessna to make some modifications as a result of experience in the field. Most important of these was enlarging the window on the pilot's side for better visibility in turns to the left. The window also bulged out a bit, improving downward visibility out the left side. This change included an external emergency window release handle, allowing rescue crews to get immediate access to the pilot. According to pilots who have flown both airplanes, the bulging window is supposed to reduce the speed of the O-2A by as much as four knots. This reported decrease in performance may be airplane specific, rather than valid across the modification. The USAF Dash 1 does not indicate any change in performance for early vs. late models and no two O-2A's seem to have the same performance, anyway. (In talking to other O-2A owners, mine seems a few knots faster than others.)
The new window design required replumbing the fuel lines, so that the fuel selector switch for the left tank (feeding the front engine) was moved to the right side of the cockpit. You might think that having both the left and right fuel selectors on the right side of the cockpit could create some confusion, but it actually solved a different problem. The left fuel tank selector was now to the left, or forward of the right fuel tank selector. This now made it clear which engine was drawing fuel from that tank. The second delivery of O-2A's (68-6857 thru 68-6903) had the same layout of the earlier 145 planes, but all deliveries from 68-10828 and higher are with the larger, bubble window.
O-2A vs. C-337
It is often said that the O-2A was just a civilian Cessna Skymaster in Air Force gray. Although this is true for the O-2B it is NOT true for the O-2A.
The O-2A was special purpose built to USAF requirements to perform forward air control missions in a low-intensity conflict environment. It was expected that maneuvering stresses would certainly exceed that encountered flying in a civilian, peacetime environment; there was the requirement to carry and fire a variety of weapons; and – to be certain – there was the requirement to survive hostile encounters with unfriendly people.
Here is a list of some of the features that make the O-2A different from the civilian C-337 Skymaster:
Wings equipped with four hard points capable of mounting weapons pylons and 350 pounds of ordnance each or 550 pounds total among all four pylons. Allowable weapons weight was in addition to the 4300 pounds maximum weight for the civilian model C-337B. (Actual combat weight frequently exceeded 5000 lbs.)
Armaments panel and gunsight to use the weapons on the wings
Wing spar was reinforced and the wing mounting bolts were 25% thicker
Landing gear mountings were reinforced
Heavy duty brakes
Self-sealing fuel tanks
Military furnishings on the interior: seatbelts, armored seats, rifle mounts
Instrument and switches: Military standard. Power management instruments moved closer to pilot.
Observation windows above, on right side, and built into door (and door could be jettisoned in flight)
All electric instrumentation – no vacuum system
60 amp alternators (C-337 had 50 amp systems)
Fire detection system mounted on rear engine
Larger cowl flaps for rear engine: aided cooling on hot days and slow flight at low altitude
Different baffling to aid cooling without propeller spinners
Belly skids and lifting eyes to enable quick repair after a belly landing or for recovery after an unexpected landing away from the airbase
Lots of other little things which makes many similar parts between the civilian and military versions NOT interchangeable. E.g., Engine control cables and attachments are thicker or built stronger ($)
In addition to the O-2A, there were 31 O-2Bs. The O-2B was a C-337A built for the civilian market by Cessna, and re-fitted with loudspeakers and leaflet dispensers for psychological operations (PSYOP) use. In this case, that meant propaganda (or battlefield information.) The USAF purchased 31 C-337A's that had already been built by Cessna, but not yet sold. These had the standard civilian instruments and layout, interior, and none of the unique attributes of the O-2A. One quick way to tell the difference is that the O-2B, like its civilian counterpart, has propeller spinners on the front and back, while the O-2A does not. That is not universally true, as some O-2A's have been fitted with spinners as part of their transition to civilian life. (The spinners add about 4 knots to the cruise speed.) As of the beginning of 2014, there was one O-2B registered as airworthy by the FAA.
The USAF was not the only purchaser or user of the O-2. Thirty-five O-2’s were transferred to the Republic of Vietnam. Others had a somewhat longer service life. In 1969, the Imperial Iranian Air Force purchased 12 O-2As. Current whereabouts unknown, but the FAA Type Certificate allowing O-2’s to be registered in the normal category specifically EXCLUDES these Iranian aircraft. Other Air Forces that have flown the O-2 include Costa Rica, Dominican Republic, El Salvador, Ivory Coast ,Haiti, Namibia, Solomon Islands, South Korea, and Zimbabwe. Zimbabwe is an interesting case. Two former USAF O-2’s were delivered in the mid 1990s. As the Government of Rhodesia, however, the country purchased more than 20 French built 337 variants, with O-2 features such as hard points, observation windows, and a very interesting pair of machine gun mounts on top of the fuselage. These are reported to have been very effective counter-insurgency aircraft, working in cooperation with other air and ground elements.
Rhodesian Air Force Photo