The Hydrofoil and Float Combination PDF Print E-mail
Written by Leon Kaplan   
Aug 14, 2008 at 10:32 AM

0027a Figure 1: Initial water-based configuration of the hydro-ski equipped Grumman OA-9. (EDO Corp.)
     In the January/February 1997 issue of Water Flying, J.J. Frey reviewed the several unsuccessful attempts made to improve float performance. Among these was the case of a Cessna 150 with a single hydrofoil installed between EDO 88A-1650 floats. Having been involved in the development of the hydrofoil with float (or hull) concept, it may be of interest to share some of my recollections in connection with the previous and subsequent associated technology, and suggest the next steps toward reducing seaplane takeoff distance and increasing rough water capability.

     When I began employment as a junior stress analyst at EDO Aircraft in 1947, one of the "hot" programs was the design and fabrication, under U.S Air Force sponsorship, of a "panto-base" hydro-ski modification of the Grumman OA-9 "Goose." The purpose of this cold war impetus project was to investigate the feasibility of operating a single aircraft off snow and ice as well as water. The initial water-based configuration, derived from towing tank model tests, is shown in Figure 1. The four-point arrangement consisted of a main ski, tail ski and wing tip float skis. The rigid streamlined section struts were replaced by oleo shock absorbers for snow and ice operation. The airplane was beached by utilizing a cumbersome and bulky cradle. The flight tests demonstrated the validity of the concept in snow and ice but, more important, proved that with pilot input of the available aerodynamic control, only a single main hydro-ski is required to achieve successful water takeoff and landing. The program then attracted U.S. Navy support, as the results also indicated the potential of the hydro-ski for achieving significant improvement in seaplane rough-water operations.

0027b Figure 2: Grumman JRF-5 with single hydro-ski. (EDO Corp.)

     Research and development then continued on utilizing the hydro-ski for seaplane operations only, and a single ski modification of a Navy Grumman JRF-5 "Goose" was undertaken. Although the hydro-ski was similar in size and shape to that of the OA-9 installation, it was supported from the hull by an aircraft landing gear shock strut housed within a telescoping fairing. The hull afterbody was also extended in order to prevent excessive pitch-up attitudes of the aircraft as the submerged ski unported onto the water surface. This configuration is illustrated in Figure 2. (The afterbody extension was removed during the flight tests.) My contribution to the project was in the stress analysis of the afterbody extension and the excessively long strut beaching gear necessitated by the extended hydro-ski.

0027c Figure 3: Grumman JRF-5 on twin hydro-skis. (EDO Corp.)
     With the successful completion of flight tests, yet another "Goose" modification followed, this time for a twin hydro-ski version. The twin ski design enabled ramp operation of the seaplane without a beaching gear. This was accomplished by incorporating small wheels into the keels of the shock strut supported hydro-skis, Figure 3, which were free to pivot during ramp and ground operation. Although I do not recall any specific involvement, I probably did provide some stress support. Flight tests were only moderately successful. Much unporting spray was generated and considerable pilot skill was required during this process, as one ski tended to rise while the other remained submerged. Further, the bending loads on the fixed-end cantilever shock struts induced binding, which prevented their movement.

     Having demonstrated the superiority of the single hydro-ski for hull seaplane application from the "Goose" tests, a Martin PBM-5 "Mariner" was then modified for this configuration (Figure 4). As the previous tests also showed the shock absorber action to be of little value, the hydro-ski was mounted on a retractable rigid strut below the hull, enabling the use of the aircraft's normal beaching gear. My hydrodynamic analysis debut was made here, for I performed the theoretical water impact calculations used for establishing the design loading conditions.

0027d Figure 4: Martin PBM-5 with single hydro-ski. (EDO Corp.)
     Although the U.S. Navy flight tests were demonstrating the hydro-ski equipped PBM-5 capable of operation in significantly higher sea states than the basic seaplane, there was a concern for the weight penalty. Fortunately, concurrent towing tank model tests at the National Advisory Committee for Aeronautics (now NASA) also achieved reduced rough water impact load factors and stable model behavior with a small "penetrating" hydro-ski. Based on this technical breakthrough, a hydro-ski of one-third the area of that previously installed was designed and mounted on the existing PBM-5 strut. Full-scale flights by Martin and the U.S. Navy confirmed the tow tank findings. My assignment in this program was to supervise the tow tank tests on a PBM-5 model, conducted at the Stevens institute of Technology, for the purpose of verifying the hydro-ski size, incidence and longitudinal location.

     During this time (circa l960), major strides were being made in hydrofoil boat technology. Previously, the speed of a hydrofoil boat had been limited to 35 - 40 knots because of instability induced by the onset of cavitation and/or ventilation in the flow about the hydrofoil cross section, which was usually similar to the airfoil section of an airplane wing. With the development of the supercavitating section hydrofoil at the Naval Ship Research and Development Center, efficient high-speed operation of a hydrofoil-supported craft was now possible. To cautiously exploit the concept for seaplane application utilizing the test bed Grumman "Goose," the U.S. Navy selected the inherently stable but heavy Grunberg system, consisting of twin bow hydro-skis and a main supercavitating section "V" configuration surface-piercing hydrofoil. Again, EDO designed and modified the aircraft which, because of propeller and engine damage caused by the heavy spray from the bow skis, was only briefly flown (Figure 5).

0027e Figure 5: Grumman JRF-5 with Grunberg hydrofoil system. (EDO Corp.)

     It was as a result of the Grunberg "Goose" tests that I presented the small penetrating hydrofoil concept to my bosses at EDO. The penetrating hydrofoil was essentially the logical next hydrodynamic study phase based on all the aforementioned investigations. The Goose/Mariner hydro-ski flight tests had proven the feasibility of the "pogo stick" configuration, an arrangement that would have been considered impractical before the full-scale aircraft modification program. The Grunberg Goose flight tests had proven the ability of an aircraft with a supercavitating section hydrofoil to stably unport, from slow speed with foil fully submerged, to high speed planing on the water surface until liftoff. From this viewpoint, the presented concept could be considered as merely replacing the low aspect ratio hydro-ski by a high aspect ratio hydrofoil. The positive response of EDO management resulted in a formal proposal to the Navy for further development by tow tank model tests.

0027f Figure 6: Tow tank model Grumman HU-16 with penetrating hydrofoil. (Stevens Institute of Technology)
     The dynamic model towing tank tests on a Grumman HU-16 "Albatross," also at S.I.T., were performed under my direction. The model towing apparatus and relative size of the hydrofoil are shown in Figure 6. The results indicated great potential for the penetrating hydrofoil concept in that, compared with previous hydro-ski tests on the same model, the foil area was less than one-third, and significant reductions were achieved in hydrodynamic resistance during take off and wave impact loads.

     Full-scale verification of these promising results was not, however, obtained on an Albatross. Instead, Thurston Aircraft Corporation conducted Navy-sponsored flight tests on a modified Lake amphibian, designated HRV-1 (Figure 7). Dave Thurston, (author of the book, "Design for Flying") had previously demonstrated that scaled hydro-ski tests on this 2300 lb. aircraft could, at much lower cost, reliably provide data comparable to that obtained using the 9,000 lb. "Goose" or the 50,000 lb. "Mariner." The Summary section of his report contains the following conclusions:

     "This test program has demonstrated that the single hydrofoil represents an improvement over both the basic HRV-1 hull and the hydro-ski in the following areas:

  1. reduced take off and landing runs under all sea conditions
  2. reduced impact load factors under all sea conditions
  3. lower immersed drag under all sea conditions, and
  4. improved handling in rough sea conditions.
In addition, the hydrofoil installation:
  1. is lighter than the hydro-ski installation, and
  2. reduces bottom pressures below values recorded for the basic hull."
0027g Figure 7: Lake Amphibian HRV-1 with penetrating hydrofoil. (Thurston Aircraft Corporation)

At this time the U.S. Navy decided that it no longer had a mission requirement for seaplanes and discontinued funding for research and development in this area.

     As reported by Jay Frey, the results obtained with a single penetrating hydrofoil jury-rigged to a Cessna 150 twin float seaplane were indeed "not very successful in that the entry speeds became very critical and if you were to touchdown at a couple of miles per hour faster than the stall speed, the aircraft would be tossed back up into the air with little or no control surface inputs."

     It should be mentioned that to minimize the cost of this commercial float program, no towing tank tests were conducted to verify and refine the hydrodynamic design. Nevertheless, the above statement indicates that with additional efforts at adjustment in foil size, position and/or incidence, the program would have been successful.

0027h Figure 8: U.S. Navy Mark 105 helicopter-towed minesweeping system. (EDO Corp.)

     My participation in the hydrodynamic and structural design of hydrofoil configuration continued with the U.S. Navy Mark 105, an EDO conceived helicopter-towed mine countermeasure system, which has been in fleet operation for over 30 years. It is to be mentioned that the development of the basic hydrofoil geometry was aided by boat-towing a one-quarter scale model in Long Island Sound, adjacent to the EDO plant. An early production version of the Mark 105 is shown in Figure 8. The "four-poster" surface-piercing hydrofoil arrangement ensures inherently stable behavior while foilborne in calm water and waves. A close examination reveals that the four forward and two aft hydrofoils are of an "inverted V" geometry. The lower subcavitating section foils are designed for fully wetted flow, consistent with the design operational speed. The forward upper supercavitating section foils are designed for fully ventilated flow, as required by the intermittent wave impacts of rough water operation. The two wheels below each of the outer floats enable shipboard and ground handling. This is accomplished, while the system is in the displacement mode, by lateral rotation of the foil support struts about the pivoted attachment to the floats.

     Based on the foregoing, the suggested next phase should be the application of the lessons learned by modification of a twin float seaplane to incorporate a hydrofoil system. For this, rather than a single hydrofoil mounted between the floats, a single, small hydrofoil should be attached to each float. This arrangement is expected to require less pilot skill to achieve unporting onto the foils, a characteristic forcefully demonstrated in observations of the Mark 105 behavior.

     Successful accomplishment of the hydrofoil and conventional twin float combination, while expected to result in major performance improvements, a worthy objective in itself, is but another (although major) step in the ultimate goal of seaplane float design. That ultimate goal might be attained in the suggested following step: a low drag seaplane float with lines primarily determined from aerodynamic considerations. Such a float would have no step, deadrise, keel, chine, forebody or afterbody. Also, no hydrofoil would be visible during cruising flight, for it would be housed within the floats streamlined contour and extended only at takeoff and landing.

     And a final suggestion. Don't forget the model tests!

Source: SPA Web Site, 1998