Sponsored by REGGIANI S.p.A. Illuminazione - Milano, Italia
Both for studying Types pc.4 and Pc.5, as for Pc.1, I would need long windtunnel tests managed by myself, because it would be impossible to hand over other peoples such task, being absolutely unavoidable my presence as inventor.
In particular, it was not comfortable to ask the Government for use of aerodynamic plants used both by Government itself and all other Companies.
Thus, I obtained to build in my Company a windtunnel I could use even for other projects I had to work on. It was completed in Finalmarina, but I did not succeed in tooling nor in using it. I was obliged for this to modify the Pc.1 configuration, allowing propeller work by raising the whole seaplane bow with the device illustrated in British Patent nr. 318858 and in following Italian Patent.
"New kind of seaplane owned by PIAGGIO Company & Eng. Giovanni Pegna, in Genoa. The present invention has as object a new kind of seaplane, which has the charachteristics to have air propellers low on the floating line, so that their work should be impossible for lift-off without other two charachteristics, i.e. one or more sea propeller and two or more hydroplane fins, intended the first to give the seaplane sufficient speed for lifting on the latters sufficiently to allow air propeller action and allow subsequent lift-off. Figures 1, 2, 3, 4, 5, 6, 7, 8 represent some examples of realization of such a seaplane."
Hydroplane Fins Patent for Pc.7 (8Kb)
"Sea propeller could be actioned by an independent engine or by one of the air-propulsion engines, in the latter case through two clutched joints mating sea- or air propeller."
"Air propellers could be maintained in horizontal position while the aircraft is not sufficiently emerged by means of suitable locks on their axle."
"... Genoa, September 10, 1928".
During the discussons held ebroad about the priority on this invention, I verified that in England a patent had been registered in 1912 by Mr. Burney on the issue, whose I was obviously unaware when I imagined the Pc.7.
But, taking this choice, I moved the problem from the aerodynamic field to the Hydrodynamic one, which seemed to me more manageable.
In 1917, assigned to Froude tub in La Spezia, I had a series of experiences wit fins identical to those of fig. 6, 7, 8 of the Patent, derived from the previous, with fairly good results. Remembering this, I built a model which I tested towing it with a motorboat, with regular behaviour up to 6m/s. For transversal balance during first phase of bow raising, when the fins were totally underwater, I used the trick to put two inclined planes under the wingtips. I thought to avoid this problem eventually by fitting the hydroplane fins with small ailerons linked to the wing ones: this should have been effective and I managed the construction of another mpdel, without lateral planes, to be sent in Rome as "X Monoplane" for windtunnel tests.
Test results were encouraging. Continuing in my study I changed, under Gen. Crocco's advice, the low-lift, Curtiss profile with a higher-lift Munk, even with more resistence. I could nevertheless use a much smaller wing, with advantages in weight and torsional- and bending rigidity. So I finalized the Pc.7: the problem, so managed, seemed simple, but actually gave hard and unexpected troubles.
As I said, I moved the difficulties from aerodynamics to hydrodynamics. I had no big advantages indeed, and I found so difficult problems that, if construction of Pc.7 had not been started waiting for the finalization of hydrodynamical part, I could have switched back to Pc.1.
The reader could easily understand what a hurry pressed me, and why this could lead to necessity of not leaving what I reckoned good for what I supposed better.
In first towing experiences, up to 5-6m/s, the model behaviur was perfectly good, exactly as in my intentions. Bow raised up to allow air propeller movement, while stern partially emerged.
But, when I increased model towing speed, behaviour became strange and impressive. Suddenly it falled into water and proceeded as if it had no fins, or slipped sideways to make a complete roll.
The phenomenon was immediately identified, and the result was a sort of "cavitation", as I could compare it to the phenomenon happening to a water propeller above a certain peripheral speed. When model speed rised to a certain value the fins were almost completely emerged, and suddenly ater detached from their back and air replaced water, recalled by the surface of the latter. Since then, fins lift was generated by lower surface alone, with steep fall in lift coefficient estimated at 1/4 of the original one When the phenomenon was simultaneous in both fins, model was falling upright, aotherwise it slipped as I said.
I had to find something. and I began using two vertical or horizontal diaphragms on the fins, hoping to block the way to air recalled by hydrodynamical depression on upper side of the fins themselves. It's clear that, when such depression (in water, some 800 times more dense that air at same speed) is greater than 1Kg/cm2, the discussed phenomenon is about to appear. It is understandable that the above trick has been marginally effective, as one could argue by phisics of the fact.
I found the final solution on Dec 18, 1928, only eight months before the race. I read in my old notepad: "The aircraft must stand on front and back fins, then take off. Landing must be performed by touching almost simultaneously on those points. Thus, it is necessary to test back fins, upper and lower, together with front ones."
Such tests gave positive results: only a small speed transition from 30 to 60Km/h remained, where a light transverse instability insurged, but this did not worry me because it could be managed my small ailerons or by pilot's training, as I'll explain.
Flat rocks, launched tangentially on water surfaces (a very old play indeed), and the modern slipping hulls of "aquaplanes", are pratical examples of use of lift generated without circuitation.
Thus, when Pc.7 stands onthe fins, it could be compared to a normal seaplane without floats, except parts near and after redan. Inverted-V-shaped fins of Pc.3 should aim at replacing hydrostatic lift generated by usual floats with hydrodynamic one.
While I experienced the fins, with help of Eng. Gabrielli and with primitive means, gen Crocco ordered similar tests in Italian Royal Air Force's Froude tub on fins similar to those of Pc.7. Cavitation was immediately identified by him, and the best profile to be used was the plain-convex on.
I acknowledged Gen. Crocco's results in Dec. 1928, and at the same time he acknowledged my own experiences, the troubles and the solution I implemented. I did not profit of gen. Crocco's work because cavitation inhibited use of circuitation above 70Km/h.
Pc.7 is nevertheless practically handicapped for the following reasons:
A substantial improvement in take-off may be foreseen in the second design, through implementation of the above. Fortunately, the sea propeller, already included in the design, is very well suited to solve the lift-off problem.
Looking at typical diagrams, one could see that pilot should be able to operate the air propeller at very low speed, provided its thrust could be sufficient alone to power the aircraft, as it should happen in a non-racing seaplane.
Finally, some observation had been made about Pc.7's landing. This should not give particular problems: skates incidenceis some 3deg with respect to flying line at maximum speed, thus, landing at that speed, the skates should provide, even in that extreme condition, a positive lift wit remarkable efficiency, and the resultant force should be applied forward of center of gravity. Thus, the aircraft should not have tendence to capsize even in those conditions.
Landing at proper speed and on forward and back skates as intended, skates efficiency becomes 3 at least, thus any ditching danger should be avoided. Ditching could occur only in a low-attitude approach, which is highly unlikely for me.
Adimensional polars of Pc.7 and "X-Monoplane" (both referred to windg- and hydroplane fins surfaces) do not coincide, but the latter's are better than the first's. This is due mainly to fins worsening instead of wing profile modifications, because of the described facts. However Pc.7, even in its primitive design, which may be considerably improved, is absolutely (i.e. in adimensional form) much better than other racing seaplanes I am aware of.
I can consider Supermarine, Macchi, Gloster and Pc.3 as adimensionally equivalent, and, even with the errors i could have introduced, equivalent also in effective polar diagram at same total lift.
For the above consideration I reckoned to be allowed to say that world speed record is more result of engines improvement than of architectural developments in racing seaplanes.
I liked to use a four-bladed propeller, but the concept itself of Pc.7 blocked me in doing so.
Three propellers were thus ordered for Pc.7, with steel hub and variable-pitch blades, produced by Standard Steel. One of them, even forcing the negative opinion of hat Company, was designed by me with very thin, plain-convex profile tip sections: I extended some concepts of ballistics, remembering that sharping projectiles spinner reduces resistance. Today ona could say that at sound speed no circuitation still exists, thus thin profiles are much better than conventional ones at that speed.
I ordered also three Caproni fixed-pitch propellers, each with different pitch, made by dural blocks.
Variable-pitch props were needed for various reasons, mainly to ease, with a proper pitch selection, first lift-offs.
Dural sea propeller, with two orientable blades, was designed starting from old experimental results of Froude tub in La Spezia, published on its Annals. Theory was consolidated, and the prop gave the intended results.
Being impossible to test this propeller directly on Pc.7, and being also necessary to have the maximum confidence in it before mounting on the aircraft, my Company built a motorboat (lenght 10m, width 2m, displacement 3000Kg) to test it. The motorboat hull was chosen among those I had previously tested tug, in order to have a resistance profile highly similar to that of aircraft model in Rome Tug.
Motorboat tests had the double aim to verify that propeller thrusts were the required ones, and that work necessary to change pitch were light, in order to allow the pilot to perform without fatigue.
First and most difficult obstacle was about the engine to adopt. This had to be fitted with clutches and transmissions for both propellers and with the device needed to stop air propeller in horizontal position.
Initially FIAT was interested, and associated its name to my Company's to christen Pc.7 as Piaggio-FIAT. I began using a FIAT 1000HP engine and FIAT-developed transmission. Some time later they resigned from collaboration, thus I contacted, in accordance with Regia Aeronautica, Isotta Fraschini which accepted the work. My famous and old friend, Giustino Cattaneo, engine designer at I.F., produced his better geniality to understand my ideas and translate them in mechanical jewels.
Watching at the blueprints, no particular difficulties seem to have been encountered in designing the machine, but I was obliged to use instead all cerebral resources of myself and my co-workers (Eng. gabrielli, Dr. Luotto, Mr. Arrigoni) to solve the thousnds of problems I eound avery day.
It's sufficient to mind that I had no antecedents to inspire, and that lack of space was sometimes dramatic. After frozing the main section of hull-fuselage, it was not possible to make changes any more. A number of problems had to be solved in a while, for example the carburettors intakes, engine exhausts, oil cooler: three sensitive parts which finally worked well, but had to be remarkably perfected, as I considered to do continuing testing after the race. I stress that I did not install the controllable ailerons on the fins, since there was no sufficient time, trusting in pilot's training to overcome the short instability transient in water.
Finally, Pc.7 floated on the fins, piloted by the late Dal Molin, as seen in the picture (finished by a poor photography).
Fig. 6 - Pc.7 Section View (12Kb)
Waterproof empennages were aerodynamically sleek, and their skin was in plywood. They were separated from fuselage and rudder hub served also for sea rudder.
Fuselage had two waterlocks, and floating was helped by two thin corrugated- aluminium boxes.
Fig. 7 - Pc.7 during take-off (19Kb)
Whole wing were submitted, both with filled coolers or not, to bending and torsional period measurement, in order to verify that no relevant engine RPM were multiple of wing period.
Wing coolers had 55.000 l/h flow: oil cooler was somehow "invented" by applying loophole intakes opening when air propeler was moving. Today, I would prefer cooling oil with main coolers water, with an hidden heat exchanger into the fuselage, and I would put air intakes above instead of on hull sides.
The aircraft, as soon air propeller was activated, raised bow as expected.
A big trouble happened immediately: the water propeller clutch, which worked very well on the test bed and in the motorboat, in the aircraft was suddenly inundated with oil and skidded. Tus, while the engine slowed down, aircraft plunged in water, even without damages.
Fig. 8 - Pc.7 three-view (5Kb)
Having the Pc.7 been unuseful for the race, nor it could have been used for a speed record, the project was abandoned both by my Company and Regia Aeronautica.
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