In 1928, Olof Ljungstrom of Detroit, Michigan submitted a pair of unusual aircraft proposals to NACA which featured boundary layer removal via a primitive form of jet engine, in which a piston engine drove an axial flow turbine blower inside the aircraft. The power plant is reminiscent of that used by the Caproni Campini N.1, which first flew August 27, 1940. However, the Ljungstrom design did not feature a combustion chamber and was less advanced than the N.1, which was not a true jet itself but a “motorjet” or ducted fan type. Ljungstrom appears to have been employed as an engineer in the aircraft division of the Ford Motor Company, as W.B. Mayo, the head of this organization, sent the proposals to Edward P. Warner, Assistant Secretary of the Navy for Aeronautics, on Ljungstrom’s behalf.
Ljungstrom’s “Proposed New Airplane Design,” dated February 16, 1928, was a gigantic seaplane inspired by equally enormous tranoceanic seaplane designs put forward by Rumpler and Junkers in this period. In his proposal, Ljungstrom noted that airplanes which took off and landed on water would probably have more propeller failures than land planes. He suggested eliminating the propeller and producing the propelling air thrust by an axial flow turbine blower, located with its driving unit inside of the wing section.
For the sake of simplicity, the air intake and outlet openings were made in the form of slots extending partly or fully across the span of the wing. The outlet or thrust slot could be dimensioned to give practically the same area as the corresponding propeller and which would probably give the highest efficiency, though this required experimentation to verify.
The intake or suction slot was located on top of the wing and dimensioned to give an increased air velocity across the wing within certain limits which would probably result in considerably increasing the lift without a marked increase in the drag.
Ljungstrom’s giant seaplane was somewhat on the order suggested by Rumpler; payload could be carried in the wing, as suggested by Junkers. The aircraft featured 4 floats, each 24 m long and 2 m wide. The inner pair of floats supported a pair of tail booms which carried the horizontal stabilizer, 26 m in width. The inner wing section was unswept, while the outer wing panels were swept aft. The Ljungstrom seaplane was 9 m tall, 40 m long, and had a span of 90 m. The wing area was 1,262 m² and the thrust slot was 55 m long and 2 m high, equaling 1,190 ft², which corresponded to 10 propellers of approximately 12 ft in diameter. The suction slot was the same length but probably somewhat narrower so as to increase the intake velocity.
Ljungstrom also proposed a variation in which conventional propellers were placed on the trailing edge of the wing and driven by air-cooled engines buried inside the wing. These engines were to be cooled by air sucked in through slots on the upper surface and projected through slots on the lower surface. Additional characteristics of the design are summarized on page 4 of the proposal.
W.B. Mayo subsequently sent the Ljungstrom proposal to Warner, who sat on the government’s Aeronautical Patents and Design Board. He forwarded it to NACA on March 22. G.W. Lewis, Director Of Aeronautical Research at NACA, issued a memorandum evaluating the design on April 19. Lewis considered the Ljungstrom design to belong to a class of which it was difficult to state definitely that they were impracticable and whose merit could only be determined by actual trial. He noted that the aircraft was of a size which was far beyond anything which could be expected to seriously interest the military services for some time to come. Designs of this magnitude had been proposed in the past but none had been constructed up till that time.
The use of propeller to produce a suction over the wing and thus assist in increasing the stalling angle by virtue of the removal of the boundary layer also increased the air friction losses in the stream entering and leaving the propeller. NACA believed that the proposed arrangement would result in such great losses in this manner that the ultimate efficiency of the aircraft would be no better than, if as good as, a similar aircraft having propellers arranged in the more conventional way. Lewis noted that the possible diameter of these propellers would be rigidly limited since it would be determined by the dimensions of the wing and that consequently propellers of much smaller diameter than was most efficient would have been required.
Ljungstrom’s alternate power plant arrangement also did not impress NACA, which rejected his proposal for consideration of an award. Ljungstrom was informed of the unfavorable recommendation in a letter dated April 27 by Warner. On May 1, Ljungstrom sent a Departmental Communication to Mayo responding to Warner and the NACA evaluation; he did not understand the mention of an award and stated that he merely wanted NACA’s opinion concerning the new type of airplane. He then commented on specific aspects of NACA’s critique, defending and clarifying various aspects of his design.
This memorandum was forwarded by Mayo to Warner, who responded on May 8. Warner clarified the procedures of the Patent and Design Board. He also noted that “…it is difficult for a governmental department to enter into extended development work on a device of an altogether new type and as yet in a somewhat abstract state. It might very readily be that development could be better carried on by a private institution, such as the Massachusetts Institute of Technology.” That same day, Warner referred Ljungstrom’s memorandum to NACA suggesting that it be reconsidered, in the light of Mr. Ljungstrom’s memorandum, without reference to any particular design, but merely in light of aerodynamic theory.
On June 5, 1928, Ljungstrom produced a second proposal for an “internal propulsion aeroplane.” This was a small, shoulder wing monoplane that shared the same propulsion principle as his seaplane design of February 16, with only the engine and axial flow turbo blower/blowers being placed in the fuselage, which terminated in a large outlet for the propulsion air instead of locating the outlet in the trailing edge of the wing.
The air entered the slot located on top of the wing, as close to the trailing edge as practicable, with velocity equal to the speed of the plane and flowing to the blower through venturi shaped channels. The blower increased the air velocity corresponding to the slip stream velocity approximately 15% or whatever slipstream velocity was found most efficient. The venturi shaped outlet from the blower delivered the propulsion air to the atmosphere.
Air entering the inlet slots was properly guided for the purpose of removing the boundary layer in the slots, with Channel “A” connected to the slots through perforations which were also connected to the suction side of the blower. Ljungstrom references Ludwig Prandtl’s theory concerning “removal of the boundary layer” in the design of this system.
Ljungstrom considered it advisable to take the bulk of the air in through the wing on top of the fuselage and only take in so much air towards the wing tips as could be carried by the tubes without any material change of air velocity relative to the intake velocity. This would also prevent an excessive amount of circulation around the end of the wing from below to above.
The outlet area for the propulsion air corresponded to the propeller area for equivalent engine horsepower and air speed. The vertical rudder was placed in the outlet to test the practicality of this location; if not, it would have been moved to the outside.
Ailerons were omitted, at least initially. Means for regulating the air in the tubes coming from the wing tips would raise or drop the wing by increasing or decreasing the boundary layer removal. As the blower was an open one air would flow through the same freely even when the engine was at a standstill which would probably permit gliding with power off.
As seen from the accompanying photos, the cabin was located in the extreme front of the aircraft, the engine under the wing and the blower somewhat behind the wing connected to the engine by means of a shaft expression. The engine would be made movable in axial direction for the purpose of balancing weights as Ljungstrom was not in the position to calculate the moments, not knowing anything about the center of pressure of this new wing system.
No dimensions were given for the aircraft, but Ljungstrom’s May 1 memorandum mentions the development of a single engine test plane with a wing spread of 26 ft; this is likely the same aircraft described in his June 5 proposal. No three-view of the design was included with the proposal, though a reasonably accurate one could probably be constructed from the scrap views in the report and photos of the model.
NACA’s evaluation of the second Ljungstrom proposal, dated September 18, 1928, was even less favorable than the first. NACA acknowledged the advantages of boundary layer removal, but for all conditions below the usual stall angles there was a decided increase in resistance. At best, the greatest benefit which could be obtained from the best designed system of boundary layer removal would be a greater maximum lift and a lower landing speed. These would be obtained at the sacrifice of maximum speed due to the increased resistance at the lower angle of incidence. This had not been considered to be a very practical arrangement up till that time. It was believed doubtful whether the subject system would operate as proposed. NACA believed that the intake slot as indicated would be altogether too large and, regardless of the effect of the blower, would have a distinctly detrimental effect upon the action of the wing.
The Committee believed that the considerable increase in lift without a marked increase in drag would not be obtained by the Ljungstrom design. It was not clear how the lift distribution indicated in Figure 3 of the proposal would be obtained, and was believed to be unjustified. NACA was skeptical that the distribution of the air flow inside the slot could be controlled in a simple manner by guide vanes to maintain the even distribution of the air flow, particularly in the case of engine failure.
It also thought that placement of the rudders just inside the outlet slot would introduce many uncertainties. NACA believed that the positions of the slot openings shown in Figure 7 could be improved upon and that in particular the location fairly close to the leading edge would be better.
In general, NACA believed that adaptation of Ljungstrom’s propulsion principle to small craft would be extremely dangerous. In the first place, the disturbance of air flow by the slot would seriously reduce the lift in the event of engine failure, assuming that with the engine running the lift could be maintained. This reduction in lift, with the correspondingly large increase in drag due to the forced burbling of the wing and the apparent high wing loading of the machine, would mean an excessively steep gliding angle and a very high landing speed.
Furthermore, the assumption that the lift could be maintained and thrust produced by the slot and blower arrangement seemed to be optimistic. It was believed that the transfer of air from the slots to the blower, in so far as could be determined from the figures shown, would result in large losses and would have the same general effect as having the blower in a tube and drawing air through a comparatively small orifice. In addition, the diameter of the slipstream or jet appeared to be about half that which would be found in the corresponding conventional propeller. Consequently, there would be a considerable reduction in the propulsive efficiency of the arrangement as compared with that of a propeller.
The omission of ailerons would have introduced new dangers since the failure of the engine would mean a loss of lateral control. From experiments which had been made on the effect of slots, it was believed that a slot which would have to be as large as that involved in the proposed device would, regardless of the pressure used, have a detrimental effect on lift.
NACA did not consider the Ljungstrom device practicable and advised against granting him an award. In a letter dated September 27, Secretary Warner informed Ljungstrom of the unfavorable evaluation of his design. No further correspondence concerning Ljungstrom or his internal propulsion aircraft has been found in NACA files; how much further he pursued the concept at Ford before abandoning it also isn’t known. Ljungstrom apparently went on to focus on improving automobile engines; a Google Patents search reveals four patents awarded to him in this area between 1935 and 1939. If any readers have further information on Ljungstrom or his remarkable internal combustion aircraft proposals, please contact the editor or comment below.
All images from NARA Archives II, College Park, MD, RG 255