Archivio Storico-Tecnico

Ambrosini S.7 in the Air

By Posted in - Archivio storico-tecnico & Articoli e Testimonianze on February 14th, 2013 0 Comments

Da Flight n. 2222 del 24 Agosto 1951 – Dell’editore

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Our photographer, who secured all these pictures, remarked with good reason that the S.7 is as pretty an aircraft as one could see in the sky. A bubble enclosure and retractable tailwheel could make it almost perfect.

 

Flying an Italian Operational Trainer: Low Power but High Performance

W

HAT better way could there be of starting a week than to drive to Redhill bright and early on a Monday morning, and from there to climb South-coastwards in a lively and responsive single-seater? To a sports-car enthusiast, even the knowledge that the Ambrosini S.7 has an Alfa-Romeo engine under its tight cowlings added zest to the occasion.

It will be remembered that two of these Italian Alfa-powered trainers were entered for the Daily Express Race, and it was No. 92, to be flown by Dott. Ing. G. A. Ferrari, which I piloted just before it returned to Italy. The accompanying photographs were taken, incidentally, from the Miles Aries, which, at over 1,500 ft/min, outclimbed the Ambrosini, though it could not match its top speed on the level.

In a class of its own, the S.7 is described by the makers as an aerobatic and military trainer. It was designed in wood for ease of construction by the parent company, or by other concerns under licence, and both single and tandem two-seat versions are being produced. Some forty have been completed for the Italian Air Force and as many more are on order.

The majority are monoposto and are used by experienced pilots, at present on staff appointments, to practice on and keep their hand in.

In conception the S.7 is not new, and, in fact, it is regarded by most people over here as belonging to a past generation of trainers—if it is a trainer at all in the accepted sense. Nevertheless, it is attractive, and for its power has an interesting performance. We have no counterpart here and would now probably classify it as a private-owner type for competition work or touring, with one or two seats to choice. Of the few British pilots who have been lucky enough to fly the S.7, one at least has aptly described it as a baby Mustang.

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Just the sort of morning to be pushing an aircraft out of the hangar: The Ambrosini at Redhill airfield.

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Explanation of the controls and instruments before starting-up. Note  the angle and size of the flaps.

In view of the recent debate in Parliament, during which the lamentable aircraft position—Tigers back—in the R.A.F.V.R. schools was mentioned, one cannot help reflecting that the S.7, or a type like it, is exactly what such units would like to have for ex-Service pilots to practice in. It carries all the necessary equipment and has a representative and worth-while performance on the power of a Gipsy Queen 30.

In the R.A.F., pilots are instructed to carry out an external check of their aircraft before getting in for a flight. In the more rugged atmosphere of private flying, some light aircraft are lucky if they get any check at all between annual C.s of A. However, old habits die hard, and I gave the Ambrosini a careful once-over before climbing in. What I saw was very pleasing, beginning with a tight-cowled shapely nose, the lower panels of which hinge-up to give unimpeded access to the six-in-line engine. One reason for the machine’s complete freedom from dirt and oil is the provision of a tiny tank, bulkhead-mounted, at the delivery end of the breather pipe. Oil can thus be drained away on the ground instead of blowing back over the underside of the fuselage, there to collect flies and dirt. This was one of many unusual gadgets and fitments on the S.7.

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Control and manoeuvrability are of a very high order on the S.7, so for this shot it could be tucked with confidence close “inside” the photographic Aries.

The landing gear can be deservedly described as fine, wide and handsome, and the machine sits well on the ground. The landing light is linked with the port leg, and folds with it. On this particular machine the wings are beautifully finished and the gun and camera gun ports carefully faired over. The gun, incidentally, would be a 7.7 mm S.A.F.A.T. type with 200 belt-fed rounds.

No trimming tabs of any sort mar the symmetry of the control surfaces, and so close-fitting are the root fillets mounted on the tailplane that no one would detect their freedom to move with it for alteration of incidence—the Method employed for fore-and-aft trimming. The tailwheel is neatly spatted and has a castoring lock interconnected with the stick, of which arrangement more will be said later. The flaps are large and have a big angular movement—marked in degrees on the flap nose, as illustrated overleaf.

A swing-over front cockpit enclosure is fitted, which will open on either side, using the bolts on the fixed side as hinges. Both bolts may be released at once for jettisoning in the air or removal on the ground. On this single-seat version the rear cockpit position is occupied by radio and a luggage locker, with batteries behind again. A non-transparent swing-over enclosure, similar to that in front, is fitted over the locker.

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The swing-over enclosures for the cockpit and rear compartment on the single-seat version of the S.7. An R.A.F. type F.24 camera can be mounted on the rear cockpit floor.

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Signor Guido Ferrari reveals the unusually accessible Alfa 115 engine in his aircraft. Cooling airflow is controlled by gills.

To the pilot learning the cockpit layout of a foreign aircraft everything seems elaborate and confusing. This is mainly because not even the dials of the flying instruments are familiar. In addition, the labels and instructions are sometimes difficult to understand, and may even be misleading to those who do not speak the lingo—for example, caldo for the cooling shutters on the Ambrosini means hot. My mind is quite unable to visualize climb in metres per second or convert them into more familiar units. There is something of a colour code apparent in the control system, but it is not fully carried out. Moreover, there seems to be a certain amount of complication of controls and dials which should be avoided in a machine of the type.

Concerning the central blind panel, the directional gyro also carries a bubble; the horizon bar is adjustable and can be caged; and the turn needle has a sensitivity control which permits Rate 1, 2 or 4 to be selected for a displacement of the square-ended needle by one width. Another device making for ease of blind flying is the provision of a datum mark at the edge of the A.S.I. dial. This is fixed, and the whole dial can be rotated to bring any desired speed against the marker. Should a prolonged approach be required at a 180 km/hr, for example, this speed is set against the marker and from then on the pilot need not read the figure but keeps the A.S.I, needle on the mark—horizontal at 3 o’clock.

Below the blind panel are the r.p.m. and boost indicators, and, below again, engine air temperature (in and out) and oil, fuel, suction and hydraulic pressures.

A peel-off reveals as neat a pair of wheel doors as could be found anywhere. Breather-oil is trapped; hence the spotless tummy.

The surrounding controls are: left floor, wobble pump and harness release; above it, fore-and-aft trim wheel and indicator, and throttle and pitch levers; left panel, undercart and flap levers, with lights and indicator respectively, and ignition switch; central, engine air gills and carburettor air; also fuselage fuel tank cock and engine cut-out toggle. On the right are the main wing-tanks fuel cock (and fuel gauge), primer and fire extinguisher, starter button and clock; starboard side, electrics; and, on the floor, parking-brake selector, gun and camera controls, main electrical services switches, hydraulic hand-pump, emergency selector and pressure release. The press-to-transmit button is on the throttle, and the stick carries an ingenious twist-for-safe firing trigger. The machine I flew carried four-channel V.H.F. with selector box on the port side; a range receiver, also to port; and over one’s shoulder on the same side could be seen the 17-gallon fuselage-fuel-tank contents gauge.

Regarding this fairly formidable selection in the cockpit, one may add that the tailplane incidence is variable for trim, and that this is achieved with the aid of a shaft extending rearwards and apparently worm-operated from the trimmer wheel.

A two-position selector controls the brakes for parking. When “park” is selected, the brakes stay on when applied with the toe pedals on the rudder bar. The castoring tailwheel is interconnected with the fore-and-aft movement of the stick, and is locked in all positions except stick hard forward.

No adjustment is provided for the pilot’s seat, but it is possible to adjust the reach of the rudder pedals. The sitting position would not be very comfortable on a long flight, for not only must the legs be straight out in front, but the whole cockpit is very small. It is certain that, for reasons of size alone, the cockpit would be unacceptable in a trainer in this country. However, performance (and appearance) would obviously suffer if it were made larger.

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Two more very pleasing views of the Ambrosini reveal the characteristic shapes of its nose and tail.

There is nothing about the starting-up procedure which calls for comment. With the requisite taps and switches on, it is a case of prime, switch-on and press. The Alfa idles smoothly and should be warmed-up with gills closed at 800 to 1,000 r.p.m. When oil temperature and pressure are correct the mags can be checked at 1,500 r.p.m. Before taxying out, it is also as well to check hydraulic pressure build-up.

With a long nose in front, and a tailwheel undercarriage, visibility ahead is negligible. However, the engine is slender, and a little nose-swinging from side to side enables the pilot to see plenty for safe taxying. It feels most unnatural to stuff the stick forward and open the throttle in order to taxi in such an aircraft, but that is the way it is done on the Ambrosini—another case of the machine which looks nose-heavy on the ground but, in fact, is not.

The brakes on 92 were not particularly good when used differentially via the pedals, but they held the aircraft stationary for a full bore run-up when park position was selected. At almost 2,500 r.p.m., the maximum on the ground, the Alfa remained very smooth and noise was by no means excessive.

A standard take-off check sufficed, elevator trim being set just aft of neutral. The trim wheel moves freely when the aircraft is on the ground, but gets steadily heavier as speed increases in the air until it is practically immovable at about 250 m.p.h. (400 km/hr). Flaps are set according to circumstances—I used 15 deg. The response to the flap selector is extremely rapid—too rapid, in fact—and one’s hand must do no more than twitch in order to milk-down 10 degrees. The steady position is with lever mid-way; up and down selection is in the appropriate sense.

I took the long run at Redhill into a light wind, keeping the tailwheel locked, with stick in neutral position. I must confess that I was not completely at ease, either with the rudder or with the lack of forward view, although the S.7 kept straight enough. No doubt I was unduly apprehensive, but because of the relatively poor acceleration a long time seemed to elapse before we were established on the climb. Take-off power for the Alfa is 225 b.h.p., and the weight of the aircraft is 3,030 lb. These facts, considered in conjunction with the use of thin, fairly-small-area wings (power loading is 14.1 lb/h.p.) account for the long run. The aircraft unsticks at just over 70 m.p.h., but seems reluctant to climb until speed builds up to around 110 m.p.h.

The wheels come up quickly, and the flaps when raised from take-off position produce no appreciable trim change and very little sink. A good climbing speed is about 120 m.p.h. Until this speed is reached the aircraft gives the impression that it is labouring. According to the makers, it can climb initially with full throttle at 1,100 ft/min. My instrument gave an average reading of just under 4m/sec for the first 6,000 ft (at 2,450 r.p.m. and 110-120 m.p.h. forward speed). This represents a pleasant, practical sort of climb at 750 ft/min.

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Features of the compact cockpit are described in the text. No. 92 was appointed an “honorary Throttle Bender” while visiting this country for the “Express” Race hence the insignia.

Quick-action Undercart

As the wheels and their doors fold up, the sound from the engine changes in the cockpit and, in fact, it would probably be possible to hear whether the wheels were properly locked up. I thought that I was even able to hear which wheel went up (or down) first, and the “clunk” of the locks and change of indicator lights confirmed my guess at the timing. The movement of wheels up or down is quicker than the average.

The thing that impressed me at once, when wheels were in and speed had built up, was the aileron response. These controls have the crispness of a fighter rather than the lightness of, say, a Messenger or Chipmunk, and the rate of roll produced is high. The elevators are of the same order, but the rudder is somewhat softer, though powerful in its effect.

At a safe height I set about stalling with and without flaps and power. With everything up, the stall came with very little warning at about 70 m.p.h. I.A.S. It was quite straight until slight rudder was applied, and then awing dropped firmly, though by no means viciously. As it dropped, it could be picked up quite easily with opposite rudder, and one could sit for a minute or so doing what might be called an incipient falling leaf, or alternatively, a sort of rhumba. With half flap the stalling speed went down to about 65 m.p.h. I.A.S., and a little less with full flap. The symptoms were much the same in each case, and recovery was almost instantaneous. I allowed a turn of spin to develop to the left, and found that recovery began with controls no more than centralized. I had intended to do more spinning later, but when the time came I consulted my watch and realized I that I would be delaying Signor Ferrari’s E.T.D. for Italy.

Before sliding in beside the photographic Aries, I did some rolls, slow and barrel, at various speeds between 150 and 200 m.p.h., and found the S.7 very sweet and smooth to take round them. I even made a fairly successful stab at a hesitation roll to starboard. Encouraged, one to port was attempted with speed rather too low, but this had to be abandoned half-way round with a coughing Alfa and with the nose pointing to a nearby farmhouse instead of on the horizon. A mental note was made to brush up my aerobatics-one of the results of normally using a wholly likeable but non-aerobatic machine like Gemini G-AFLT.

A loop or two and rolls off the top were pleasantly easy to execute with the S.7. The powerful effect of the rudder enables the pilot to come out of the half roll very precisely, right down to near-stalling speed. I glanced at the A.S.I. going up, and on the top of the loops, and found that there was much less fall-off in the speed than might have been expected. Apparently the speed for the S.7’s loop and the size of loop could both be varied within wide limits without spoiling the manoeuvre.

On the way back to Redhill from somewhere near Lewes I trimmed carefully in normal cruising conditions at about 300 km/hr (186 m.p.h.) and tried flying hands and feet off. It was necessary almost at once to keep the nose straight, but wings and nose held steadily on the horizon for as long as I liked to leave them. One way to describe the lateral stability would be to say that the ailerons seem to be a shade on the positive side of neutral. The nose was pushed down until speed built up by about 30 km/hr and again the aircraft was left to fly itself. It was pleasant to see the nose very gently come up, until the speed was dropping below the 300 km/hr mark, and as gently settle into the level position again. As the airfield came into sight I put the nose down at about 4,000 ft and dived until the A.S.I, showed 480 km/hr (300 m.p.h.). The S.7 made no fuss, and both the ready acceleration and absence of wind noise spoke of aerodynamic cleanness. The controls remained reasonably light but it was at this point that I discovered that the incidence trimmer wheel was almost “solid.”

No particular top speed limit is placed upon flap and wheel lowering so far as I could ascertain—it is just a case of being reasonable. I set about making a normal circuit and fairly wide approach as soon as I had got the speed down after the dive.

Once the wheels were locked down the drag they produced necessitated a considerable increase in power to hold height. I made a long shallow turn onto the final approach and flicked down 15 deg of flap half-way round. This made the approach a little steeper. Finally, I used full flap, which flashed down on selection, greatly increasing drag but making practically no trim change.

Down to Earth

On the first attempt I undershot by quite a lot, and to keep the speed up to 90 m.p.h. and prevent the sink being too rapid full cruising power was needed. The S.7 sits down very firmly and nicely and keeps quite straight after touch-down. The stick is, of course, back, so the tailwheel is locked.

On a second circuit and bump I felt more at home when getting airborne, but again found the run very long, and the first few seconds in the air felt leaden. This time I made a steeper approach, but again nearly undershot when full flap came down. The aircraft descends very quickly, but its attitude on the approach is rather flat. It is as well to get a good clear landing path sorted out, because there is little enough to be seen ahead once one is down.

For an overshoot it is necessary to raise the flap by at least 30 deg from the full-down position. I wast old that the S.7 won’t go round again with flaps and wheels down. This fact, the stick-forward taxying, the lack of forward view on the ground, the stall with practically no warning, and the feel of the initial climb after take-off are all characteristics which, while they might be acceptable to a reasonably experienced pilot, and might be met individually in an operational aircraft, are still not good in a trainer. However, they do not prevent one being left with very pleasant impressions of handling a thoroughbred.

In conclusion, I may add that if the cockpit harness is as efficient as it is simple—and I was given no cause to believe otherwise—it is worth further examination in this country. Normal shoulder-straps meet at a quick-release attachment which is in turn cupped dog-lead-fashion to a nicely plated chain anchored centrally at the front edge of the pilot’s seat. Adjustment of length is effected by clipping on to the appropriate link.

DATA FOR THE AMBROSINI S.7
225 h.p. Alfa-Romeo 115ter Six-in-line Engine
Maximum s.l. speed: 223 m.p.h.
Stalling speed: 71.5 m.p.h.
Initial rate of climb: 1,100 ft/min
Service ceiling: 17,200 ft
Take-off distance: 273 yd
Take-off time: 15 seconds
Range at 3,000 ft and 165 m.p.h: 515 miles
Fuel capacity:
Wing tanks, total: 25 gallons
Fuselage tank, single-seater: 17 gallons
Wing area: 137.8 sq ft
Wing loading: 22lb/sq ft
Power loading (max. continuous): 14.1 Ib/b.h.p.
Weight empty: 2.365 Ib
Useful load: 666 Ib
All-up weight: 3,031 Ib
Structure load-factor: not less than 12
Max. dive speed (compliance with I.C.A.O.): 370 m.p.h.
Wing thickness (max. at root): 12 per cent