The Manoeuvres of Inverted Flight - SE5a
Find below extracts of a lecture documented in the proceedings of the 59th session of the Royal Aeronautical Society 1928. This details the first systematic investigation into inverted flight, manoeuvring into/out of inverted flight and inverted spinning. Flight tests were performed on an SE5a, Sopwith Camel, modified Sopwith Camel, Sopwith Snipe and BAT Bantam.
I have extracted sections relevant to the SE5a.Half Loop into Inverted Flight
Before inverting himself by the half looping method Bulman tried a series' of normal loops, watching the airspeed indicator all the way round. Whether the loop was commenced at 100 mph or 120 mph the speed at the top did not exceed 50 mph. He finally commenced his half loops for inversion at 115 mph, waited till the aeroplane was just over the top of the loop, pushed the control stick right forward and throttled his engine down. The engine appeared to fire for 10 to 15 seconds after inversion; but if the inverted glide was prolonged the propeller stopped, even though the airspeed indicator showed 80 m.p.h. In any case the aeroplane stalled immediately on inversion, and the stick had to be held firmly forward while the nose dropped and the aeroplane gained speed. The greater the speed at the top of the loop, the less was the stalled drop; under average conditions the nose only fell 15° to 20° from the horizontal.Half Roll into Inverted Flight
Bulman and Scholefield both performed the slow and quick half roll for inversion. They commenced the slow half roll at 75 to 85 mph at about half throttle, but throttled right down on inversion; for if the throttle was left open the engine restarted with a violent jerk during the recovery. Bullman gave rudder and aileron in the desired sense of rotation, and after the wings had passed, the vertical pushed the control stick fully forward. As he approached the inverted position he took off aileron and rudder. Schofield differed by pulling up the nose slightly at the commencement of the manoeuvre; and instead of continuously applying rudder until nearly inverted, gave rudder during the initial stages, centralised it as the wings passed through the vertical, and then gave it again in the same sense. He also found that he could apply it in the opposite sense after the wings had passed the vertical, which corresponds to Gerrard's experience on the Snipe. The aeroplane did not approach stalling at any time, and Bulman, though he experienced a small amount of sideslip, found remarkably little tendency to yaw off his course. ScholefieId however found that if he did not allow some yaw he experienced considerable sideslip. All the control movements in this manoeuvre, allowing for the firmness required with the SE5a, were comparatively slow and gentle (see Fig. IV.).
Bulman commenced the quick half roll in a similar way to the ordinary roll, by giving aileron and rudder in the desired sense of rotation and pulling the control stick back, with the exception that he made these control movements rather more gently. When nearing the inverted position he pushed forward the control stick, took off aileron and centralised the rudder. At the commencement of inverted flight he found that the aeroplane was as much stalled as when looping into the inverted position. This could be mitigated to some extent if the half roll could be commenced with no more backward movement of the control stick than was just necessary to induce the rolling motion. In manoeuvres such as the quick roll, which seem to involve auto-rotation, the elevator, in controlling the angle of incidence, is the important factor in governing the motion.Half Loop out of Inverted Flight
During the half looping recovery the SE5a tends, if the pilot is not careful, to lose a considerable amount of height in the inverted dive. On the SE5a the pilot has the' control stick further forward to maintain inverted flight than on an unstable or neutral aeroplane. The necessary backward movement of the control stick is therefore considerably greater, for in all the types investigated, the final position of the control stick for swinging the aeroplane round is fully back.Half Roll Out of Inverted Flight
Although the slow roll for recovery, if skilfully performed, need only involve a comparatively small loss of height, the nose of the SE5a tends to drop so much that the pilot may lose as much as 700ft in his initial efforts. When he is flying at low speeds inverted the control stick is nearly full forward. To commence the roll he pushes it along the dashboard to give aileron in the desired sense, and gives rudder in the usual way. As the wings pass the vertical the aeroplane will sideslip downwards, against its rudder. At this point the control stick must be pulled back along the side of the cockpit and, together with the rudder, finally centralised when the aeroplane has come round to normal flight. In unstable aeroplanes the corresponding control movement amounted to a circular sweep on one side of the cockpit; in the SE5a this sweep is elliptical, with its major axis fore and aft of the aeroplane. The difference in movement arises from the necessity of using the elevator more coarsely. After the wings have passed the vertical the aeroplane, in addition to side-slipping, wants to yaw against the rudder. In the case of a right-hand roll from the pilots point of view, it yaws to the left, and vice versa. The tendency of the SE5a to yaw in the slow half roll for recovery seems more marked that in that for the attainment of the inverted position. Bulman suggests that when rolling into the inverted position, the pilot has the advantage of the engine with its consequent slipstream effect on the rudder until the last moment, whereas during recovery the slipstream effect is absent. If, by allowing the nose to drop, the pilot gains sufficient speed to compensate for this, the symmetry of the manoeuvre is lost (see Fig. VI)Inverted Flight
The important factor in settling these characteristics is the position of the C.G. relative to the wings. The more stable an aeroplane is in normal flight, the more difficult is the pilots task of maintaining the inverted position, and vice versa. It is therefore relatively easier to fly unstable aeroplanes like the Sopwith Camel and Snipe inverted than stable aeroplanes like the SE5a.
If the engine is off the gliding angle is relatively poor and the aeroplane loses height more rapidly than he expects. Added to this the aeroplane stalls inverted at a higher speed, sometimes as, much as 30 percent in excess of its stalling speed in normal flight.
For inverted flight on the SE5a, which is longitudinally stable in normal flight, the best position for the tail adjustment is two-thirds forward. Fully forward it would obviously do the maximum to assist inverted flight, but the excessive nose-heaviness that it causes in normal flight introduces an awkward condition at the commencement of inversion and during the final stages of recovery.
The Bat Bantam, due to its heavy loading, stalled inverted at as high a speed as 73 m.p.h., about 3 m.p.h. in excess of the SE5A
Contrary to expectation the SE5A proved relatively amenable to the controls in inverted flight; its main difference from the unstable types lay in the position (about three quarters way forward) in which the control stick had to be held to counteract its powerful self-righting properties. Scholefield found no difficulty in aileron technique to counteract a dropped wing, but was considerably puzzled by the use of aileron to carry out banked turns; As was explained previously, the bank necessary to produce an inverted turn is the reverse of that for a normal turn. Apart from this the controls had to be used as a whole more coarsely and vigorously on the SE5a, a feature that to some extent applies to its behaviour in normal flight. Although it was possible not only to fly the SE5a inverted, but even to stall it inverted, its self-righting properties were such that, as far as the investigation was able to show, no mishandling of the controls would result in the development of an inverted spin. If it be granted that the "Bat Bantam," with its extraordinary controllability, is an exception, the suppression of the risk of an involuntary inverted spin has generally to be paid for by a certain loss of ease in inverted manoeuvres and by the extra force which is necessary to keep the nose of the aeroplane from falling.
Bulman found that, with tail adjustment two-thirds forward, he could stall the SE5a inverted at 70 mph. Just prior to the stall it wobbled laterally, and, frequently dropped the right wing. Though my experience relates to a different example of this type, I found that it was more often the left wing that dropped. In both cases the aeroplanes were, as far as the pilot could tell, in correct lateral trim. With the tail adjustment in the above position the aeroplane felt nose heavy both in normal and inverted flight; and although in the inverted stall the characteristics of instability were searched for, they could not be detected, being masked perhaps by the lack of elevator control. It is interesting here to note the difference between the SE5a and the modified Camel; both of them longitudinally stable aeroplanes with powerful self-righting properties in inverted flight. The relatively long fuselage and effective elevator control of the SE5a enabled it to be stalled inverted in a similar way, apart from the greater control force necessary, to the unstable aeroplanes.Inverted Spinning
Finally I tried an inverted spin on the SE5A. I set, the adjustable tail at its maximum incidence, thus producing considerable nose, heaviness in normal flight. I made the usual control movements and the aeroplane entered an inverted spin in much the same way as the modified "Camel." The rate of spin was less smooth, with a noticeable "kick". The SE5A showed no tendency to fall into the inverted spin; in fact the pilot had to be determined with the controls to produce it at all. Once produced it was, in spite of the "kicking", perfectly definite and consistent. The recovery-was more direct than on any other of the aeroplanes examined.
The conditions favourable to the inverted spin required that rudder should be applied when the control stick was in a far forward position, or conversely that the control stick should be pushed forward when the rudder was across. When carried out intentionally, the inverted spin had always been produced with control stick forward, rudder given in the desired sense of rotation and aileron against it, by analogy with the normal spin. In normal spinning, however, it has been found that the ailerons are the least important factor in exciting or governing the motion, and that aeroplanes cap readily be spun with ailerons neutral or given in instead of against the desired sense ,of rotation. It seemed natural that this should be true of the inverted spin.