Fleetwings 33

Missing the mark

By Tim Martin3 July 2024

Some aircraft designs make the cut, and others are forgotten. The Fleetwings Model 33 was one that remained a prototype only. Little information about it has survived. This article attempts to shed some light on its short life.

The sole prototype NX29033 of the Model 33 trainer. It was referred to as the 'XPT-33' within Fleetwings, but the type never received a true military designation.
Photo: William Engle Collection

The inter-war years was a period when things aeronautical were at the forefront of public imagination, so many business were keen to capitalize. It was not unlike the IT start-up craze in the late 20th century! One of the aircraft manufacturing enterprises that joined the fray in the United States was Fleetwings. Their success rate was limited, so they have passed into obscurity. They flew eleven different original designs between 1931 and 1945. One of these, the Model 33 trainer, serves as one example of a good idea that missed the mark.

Using experience gained from developing automatic welding equipment, Fleetwings Inc was established in 1929 to develop stainless steel aircraft structures. Fleetwings also made conventional components for other aircraft manufacturers, including ribs, control surfaces, flaps and tail surfaces. With the acquired knowledge, a stainless steel technology demonstrator aircraft was designed in 1931. This was a four-seat high wing cabin monoplane powered by a Wright R-760 engine. Designated as the F-101, it did not fly well and was not developed.

Sea Bird and Sophomore

In 1934, Fleetwings purchased the former Keystone Aircraft factory near the Delaware River at Bristol, Pennsylvania, and moved its operations there from New York state. At the new site they developed their first somewhat successful stainless steel design, the F-4/F-5 Sea Bird. Only six Sea Bird amphibians were built in 1936 through to 1939; however, two of these still survive in 2024. In 1939 the first stainless steel aircraft for the US Army Air Corps was produced. It was a low wing, tandem two-seat basic trainer given the model number 23 and the military designation XBT-12. Production of the resulting BT-12 Sophomore aircraft reached 24 examples before the Army decided it preferred the Vultee BT-13. This lack of success with stainless steel construction must have caused a change in thinking at Fleetwings since the Model 33 used aluminium, which had emerged as the industry standard.

Fleetwings' first attempt at a military trainer aircraft was the BT-12 Sophomore, which was still built of stainless steel sheeting on a tubular frame. A small batch of BT-12s was completed for the US Army, one of which was this USAAC 39-719.
Photo: AirHistory.net Photo Archive

Enter the Model 33

Designed in 1940, the Model 33 was a lightweight two-seat primary trainer that used more modern construction and had better performance than many of the old biplanes of the time. This was achieved by combining a low-drag airframe with a closely cowled engine, allowing for relatively low engine power. The instructor and pupil were accommodated in tandem open cockpits.

It was a cantilever low wing monoplane with a conventional tail unit and fixed tail-wheel landing gear. Light alloy was used for the structure plus skins on the fuselage, empennage, and the wings from the main spar forward. Fabric covering was used for control surfaces and the wings from the main spar aft. Power came from a Franklin 6AC six-cylinder horizontally opposed engine giving 130 hp. The Model 33 was aimed both at the civilian pilot school market and the Army.

This promotion in Aero Digest magazine imparts some of the high hopes Fleetwings had for the Model 33.

Testing and modifications

The first recorded flight of the prototype, NX29033, took place on 10 May 1941. For this, the aircraft had been moved from the Bristol plant to Willow Grove, Pennsylvania. An official first flight took place on 15 May. The next flight did not happen until 24 May, after a change to a more suitable propeller.

Most flight testing of the Model 33 took place at Pitcairn Field, home of the Pitcairn Autogiro Company. Here the sole prototype is undergoing a systems check.
Photo: William Engle Collection

A more significant problem showed up during spin testing at extreme aft centre of gravity position. The aircraft entered a flat spin that was impossible to recover from, even after 25 revolutions. The test pilot, William Engle, then deployed the anti-spin chute fitted and was able to regain control, saving his life and the aircraft. The solution was to extend the cord width of the vertical and horizontal stabilizers, and this was tested satisfactorily on 11 July 1941.

Test pilot William Engle found it impossible to recover from a spin except by pulling the anti-spin chute. The horizontal and vertical stabilizers were then enlarged. Here Engle is performing an engine run, while engineer Tom Knox and chief mechanic Milton Anderson look on.

More stubborn was the problem of the engine overheating. At this time, most aircraft had radial engines where all the cylinders were equally exposed to high volumes of cooling air blast. Four-cylinder opposed engines had been introduced in planes like the Piper Cub, but they were low-power and the cylinders projected into the air stream to guarantee sufficient cooling. For the larger Franklin six-cylinder engine, which came out in 1940, the science of close cowling and baffle design to provide cooling was not yet well understood.

NX29033 still with the original cowling and small stabilizers. Test instrument leads have been run from the engine to rear cockpit. Airflow tufts around the cooling air inlets in the cowling are also indicative of the overheating problems.

On 14 July 1941, NX29033 was flown to the Franklin engine plant at Syracuse, New York, for installation of a replacement engine. This may have been an attempt to eliminate the possibility of a 'bad' engine being the cause of the overheating. Cooling surveys were carried out at Syracuse from 21 to 25 July 1941, followed by a return to Pitcairn Field. Tests related to various features continued there until 20 October 1941 when NX29033 was flown to the National Advisory Committee for Aeronautics (NACA) airfield at Langley, Virginia.

Exactly how the assistance from NACA came into play is not recorded, but the eventual NACA report mentions a request from Army Air Forces Materiel Command, and adds that NACA had received 'several enquiries as to the cooling characteristics of this type of engine [i.e. six-cylinder horizontally opposed].' Thus, the Fleetwings prototype was presumably tested in the interests of the military and the industry as a whole.

Interestingly, NACA reported that NX29033 had no internal cowling baffles when received, and relied purely on ram air to cool the cylinders. This configuration was deemed to be the cause of the overheating, since the ram air missed most of the cylinder cooling fins. Looking back, it seems unbelievable that the engineers at Fleetwings and Franklin had not deduced this, but such was the state of the art at the time. The months of November and December 1941 were spent testing and making incremental design changes. This resulted in the extensive use of baffle plates to deflect the air flow around the cylinders, and in a cowling with a much greater inlet opening plus an air exit at the upper aft edge.

The final configuration showing the enlarged stabilizers and redesigned cowling, photographed at the NACA Langley facility.
Photo: AirHistory.net Photo Archive

Exit of the Model 33

In any endeavour, timing is everything. The NACA fixes were been tested just around the time of the US entry into World War II, but by that time, Fleetwings had already moved on. The Model 33 missed the mark because they could not get the engine cooling right in time. The Army did not wait for it, and ramped up production of existing models instead.

Thus the Fleetwings Model 33 had ended up on the back burner. On 15 December 1941, NX29033 was flown from Wilmington, Delaware, back to Pitcairn Field. From 21 January to 13 March 1942, several more test flights were carried out at Pitcairn Field. This was probably on behalf of the wooden Fleetwings PQ-12 drone, which initially used the same Franklin engine, and also suffered from overheating.

The final recorded flight of NX29033 was an aerobatic test at Lancaster, Pennsylvania on 23 June 1943. Total airframe time was 213 hours. It stayed registered to what was now Kaiser-Fleetwings until cancellation by the FAA in 1965, with its final fate unknown. No further examples were built.

The author would like to thank William Engle for the data and photographs that made this article possible, and for the access to the log books of his father, test pilot William H. Engle.