Some observations and possible implications for today

COURTESY: ASRA.org

The 1930s were the heyday of the gyro­ plane (then usually called the autogiro). In 1988 Peter G. Brooks published his land­mark work Cierva Autogiros, the Development of Rotary-Wing Flight, and this article is principally based on information contained in that book, together with later references from several other helicopter aerodynamics textbooks.

Brooks usefully classified machines from that early era as either ‘early autogiros’ or, later machines, as ‘direct control’. Early autogiros can be regarded as tail-dragger aeroplanes with a rotor in place of an upper biplane wing.

Most readers will be aware that the very earliest experimental autogiros had rotors that were incapable of flapping and which disastrously rolled machines over because the lift being developed by the advancing rotor blade as it accelerated down the runway resulted in a rolling moment that could not be counter­ acted by any control inputs.

Most readers will also be aware that the first major breakthrough occurred when Don Juan de la Cierva in 1922 incorporated hub hinges that allowed each blade to independently flap upward with variations of lift.

This immedi­ately stopped the tendency of the machine to roll over but did result in the rotor ‘disc’ or rotor plane wanting to flap-back instead (the rotor disc tilting back) as takeoff acceleration took place.

The flap-back tendency was not as daunting as the earlier rollover tendency and gradually Cierva and his engineers came to terms with it by ceaseless experimenting with hub spindle angles, flap hinge offsets, and arrangement of control surfaces.

…the very earliest experimental autogiros had rotors that were incapable of flapping and which disastrously rolled machines over…

The most important thing to note about these early autogiros is that the hub spindle (the rotor axis) was usually rigidly fixed at a set angle in the airframe and control inputs were made using conventional aeroplane con­trol surfaces. Pitching was controlled by a horizontal stabilizer and elevators at the tail and rolling was controlled by conventional ailerons on the wings.

These “fixed spindle” types are instantly recognizable because they usually have either a full aeroplane lower wing or with at least one type dispensing with the wing and having large all-moving airfoils shaped like outrigger paddles on the end of steel tubes sticking out of the fuselage sides (Cierva C6 -see below).

These ‘early autogiros’ were therefore con­ trolled exactly like aeroplanes with the pilot controlling the fuselage angle by rolling or pitching moments developed by the control surfaces. As the fuselage tilted it took the fixed hub spindle with it and aerodynamic forces on the rotor blades (free to flap) virtually instantaneously made the rotor disc fol­low the fuselage.

Satisfactory control was achieved but the major shortcoming with this arrangement was that although the rotor disc could keep the machine airborne down to incredibly low speeds, control effectiveness vanished at these very low speeds. Takeoff and landing accidents with this configuration appear to have been relatively commonplace, although only one fatality ever seems to have been recorded.

The second major breakthrough in auto­ giros was ‘direct control’. This is simply an arrangement where the pilot can tilt the rotor spindle within a limited range forward to back, side to side, and this is completely independent of the fuselage. This had the immediate effect of eliminating virtually overnight the wings and outrigger paddles previously needed for roll control.

The clas­sic example of this type was the Cierva C-30, the most successful autogiro of the 1930s (pictured). Interestingly, sizeable horizontal stabilizers were almost universally retained by various manufacturers but were no longer used for pitch control.

Hinged elevator surfaces were no longer vital and were either dispensed with completely or sometimes retained with drastically reduced surface areas to provide pitch trim, and some manufacturers appear to have simply retained aeroplane tail surfaces and locked the elevators.

The second major breakthrough in autogiros was ‘direct control’.

A noteworthy fashion briefly emerged be­ tween manufacturers where they fixed one horizontal stabilizer with the curved airfoil surface uppermost with the opposite number’s curved airfoil surface facing down – the theory being that these opposing surfaces would reduce engine torque effects because the slipstream from the prop would make those opposing airfoils would produce a roll input to counteract the torque.

One can only assume that this arrangement must have been found to have had at least some effect, because why else would they have bothered? No accessible papers are available on the subject and it could well have been more a fad or fashion or compliance with customer expectation being the driving force.

However, the fact that direct control gyro­ planes in the 1930s universally retained very substantial control surfaces will now become the major focus of this article. It should also be noted that at least some direct control autogiros of the late 1930s possibly in­corporated an offset-gimbal, which is univer­sally adopted in all modern light gyroplanes.

Cierva patented the arrangement in 1932 but current sources do not allow for it to be determined whether the feature gained widespread use. The book Cierva Autogiros contains a series of 3-view configuration drawings and for the purposes of this article it is assumed that these diagrams are fairly good depictions of the layout of the original machines.

Remembering that it cannot be determined how many (if any) of the late 1930s direct control autogiros incorporated an offset­ gimbal arrangement, the following observations should be considered as applicable to direct control but probably unlikely to involve an offset-gimbal as well.

After reviewing various diagrams of 18 of the direct control types of the 1930s, some note­ worthy trends emerge:

1. all have substantial horizontal stabilizers;

2. when looking down from above and using the rotor hub as a reference point, if a line is drawn from the hub to the outer forward tip of either horizontal stabilizer, the resulting angle invariably falls between 23 and 30 degrees from the longitudinal axis, with the average calculated to be almost 26 degrees;

3. the average ratio of horizontal stabilizer span to chord is about 4 to 1;

4. the leading edge of the various horizontal stabilizers is arranged on the fuselage airframe at points ranging from 50 to 80 percent rotor radius, with the average appearing to be about 60%; and

5. many of the 1930s horizontal stabilizers were fitted with sizable endplates angled up from the horizontal between 20 and 70 degrees.

Using the diagram of a C-30 as one example, the 5 typical trends of 1930s autogiros can be seen. It appears that a safe inference can be drawn that manufacturers in the 1930s regarded sizable horizontal stabilizers on auto­ giros as being essential.

Helicopters take over

Of course, almost everyone in the gyro movement knows that WWII and the advent of the practical helicopter completely stalled autogiro development. Interestingly, the early helicopters from Bell, Hiller and Sikorsky had no horizontal stabilizers to speak of, although by the late 1940s and early 1950s smallish fixed horizontal stabilizers were finding their way onto production machines.

This trend continued with horizontal stabilizers in 2nd generation designs from the late 1950s to mid 1960s increasing noticeably in size. The usual placement of the horizontal stabilizer in that era was somewhere along the fuselage closer to the tail than the rotor, with the surface area modest to avoid hovering rotor downwash producing too much of a tail down or ‘pitch-up’ tendency.

Much later 3rd generation helicopters such as the Sikorsky Blackhawk have very substantial horizontal stabilizers, so large in fact that the Blackhawk stabilizer is hinged to tilt downward to avoid a dramatic pitch-up as the rotor downwash impinges on the surface during transition into horizontal flight.

The lesson appears to be that slowly but surely helicopter manufacturers have recog­nized the desirability of adequate and effec­tive horizontal stabilizers.

Emergence of light powered gyroplanes

Light gyrocopters can be traced back into the early 1940s work of Fritz Kunner, largely responsible for the Focke-Achgelis Fa 330 submarine-towed gyroglider (Ubootsauge, or U-Boat’s Eye) of 1942 and also to the work of Raoul Hafner in the U.K. and later work of Igor Bensen in the U.S.A.

Hafner is notable for his work on his Rotachute 1 (a 1-man rotary-wing parachute contraption), and also for his work at the Airborne Forces Experi­mental Establishment (AFEE) on projects such as the Malcolm Rotabuggy (an arrange­ment intended to allow a wartime Willey’s Jeep to be air towed).

The Rotabuggy, which had been previously car-towed on runways about 60 times, was on September 11 1944 aero-towed once and once only on “an hor­rific flight behind a Whitley bomber” (to 1700 feet and rapidly landed again because of severe vibration). Hafner’s work is notable because of his reliance on 2-bladed seesaw­ing or teetering rotors (now almost universal in light gyroplanes).

The talented and innovative Igor Bensen is most directly credited as being the father of the modern light gyrocopter, with many current gyroplanes still being of broadly similar in configuration to Bensen’s earliest powered machines.

Bensen’s particular legacy is in placing of engines on machines that had previously usually only been towed, and later adopting the offset gimbal to dramatically improve pitch stability.

Igor Bensen’s machines characteristically had a small horizontal stabilizer not far behind the rotor axis and perhaps more useful as a stone guard for the propeller. Pictured here is the famous Bensen B-8M Spirit of Kittyhawk currently on display at the Smithsonian Insti­tute’s Udvar-Hazy Center at Dulles Airport, Washington D.C..

Bensen’s counterpart in the U.K from the 1960s onward was undoubtedly Wing Com­mander Ken Wallis. Many of Wallis’s ma­ chines are noteworthy because of the com­plete absence of any horizontal stabilizer, or if fitted, of being even smaller than Bensen’s.

However, a close examination of the Wallis layout reveals that because the keel tube is angled upward significantly Wallis has more or less inherently achieved centerline thrust, and Wallis also chose to usually locate any horizontal stabilizer (if fitted) in line with the prop axis.

Are there possible lessons from the 1930s about horizontal stabilizers?

One emerging hypothesis in this article is that although sizeable horizontal stabilizers were universal in the 1930s that because most post-war light gyroplane development usually involved use of the highly effective offset-gimbal configuration for pitch stability, horizontal stabilizers were probably not as vital as they were in the 1930s.

It could be said that during the first 50 years of post-war light gyroplane existence many machines got along OK without having any significant horizontal stabilizers, but on the other hand the past decade has been characterised by rapid increases in both weight and installed horsepower and, in some cases, the potential for significantly increased opera­tional speeds.

Also, PPO (power pitch over or “bunting”) and PIO (pilot induced oscillation) events have been persistent through­ out the 50 year history of the post-war light powered gyroplane. The author suggests that there are compelling reasons why an effective horizontal stabilizer is highly desirable.

It can be shown mathematically that opera­tions at higher speeds by machines with no or with inadequate horizontal stabilizers will attract substantially increased risks of rotor instability and pitch divergence. The obvious advantage of a mathematical demonstration is that no-one’s life is put at risk in the process.

Pitch divergence is where the any upset of the rotor from gusts or turbulence tends to either not recover or become worse if the pi­lot doesn’t react. A typical response might be where a gust or turbulence results in the front of the disk momentarily rising and the gyro correspondingly slowing slightly.

If the gyro won’t speed up again or in the worst case continues to slow down without any control or power inputs, then the likely explanation is that the rotor is not pitch stable and is tending to diverge (from the trim speed).

Speed and Stability

The normal operational environment of the gyroplane is flight involving forward air­ speed, and any increase of forward airspeed has an immediate effect on the rotor ‘disc’ because of the difference in relative airspeed on the advancing and retreating sides leads to an incremental and predictable nose-up tilt of the disc. The unconscious action of the pilot of an accelerating gyroplane or helicopter is to simply incrementally put the stick forward slightly to avoid any climb.

The amount of rotor disc pitch-up change is entirely predictable for articulated rotors, and is consistently reckoned in the academic literature as being ‘about 1 degree per 10 metres per second increase, independent of the airspeed’. What this translates to is that any articulated rotor disc will want to tilt back a further 1 degree for every 19.5 knots or 22.5 miles per hour increase in forward airspeed.

Once a high speed is reached, however, a sig­nificantly enhanced risk emerges that pitch divergence may occur with any disc upset caused by turbulence or sudden pilot input. Usually a pilot will simply regard the pitch as becoming more and more sensitive as the speed increases.

In a landmark academic work called Helicopter Dynamics (1976), A.R.J. Bramwell undertook a mathematical treatise of heli­copter pitch response at an ‘advance ratio’ of 0.3. The advance ratio is simply the ratio of the machine’s forward airspeed compared to the calculated rotor tip airspeed for a given rotor rpm.

These calculations are equally applicable to helicopters and gyroplanes, because they deal with the dynamic response of a rotor. Bramwell’s particular calculations were adopted by John Seddon in his influential 1990 book Basic Helicopter Aerodynamics and the results incorporated in a particularly compelling table within chapter 8 of that book. The table below is a simplified adaptation of Seddon’s work.

The table shows that a rapidly increasing pitch-up tendency can occur on a tailless rotorcraft whereas a rotorcraft fitted with a mid-sized horizontal stabilizer (on a moment arm representing about 40 to 50 percent rotor radius) will experience a stabilizing dampening effect.

Of course, such a divergence would not ordi­narily be allowed to develop to any alarming level by any sane pilot. The mathematical demonstration assumes a “stick fixed” situa­tion, whereas in reality a pilot would be consciously or subconsciously making control adjustments. The pilot would probably notice that the rotor is becoming difficult to manage at high speeds and a handful to keep steady.

What this table represents is a mathematical demonstration of the tendency of any rotor to become pitch unstable (or ‘diverge’) in the absence of any aerodynamic damping (usually only achievable with an effective horizontal stabilizer).

Whether a given ro­tor has an offset-gimbal, or has offset flap­ ping hinges, or is a classic teetering rotor is outside the scope of the Bramwell’s and Seddon’s calculations. The table does, how­ ever, usefully make a valid point about basic articulated rotor divergence tendencies with or without horizontal stabilizers.

Readers should also be aware that there are other influences that will undoubtedly come into play – for instance, once a pitch-up commences it is now well established rotary-wing orthodoxy that if an imaginary line drawn perpendicular to the central axis point of the rotor tip-path-plane passes in front of the machine’s center of gravity, that the pitch-up rate will be aggravated, whereas if the imaginary line passes behind the ma­ chine’s center of gravity the pitch-up rate or tendency will be subdued (or even damped out by inertia alone).

These are dynamic (or inertial) effects inde­pendent of aerodynamic influences. Also, none of Bramwell’s or Seddons help­ful calculations make any allowance for the effect of separate propeller thrust directed at, or above or below, the machine’s center of gravity.

Conclusions

Hopefully this article will raise awareness of the historical importance of horizontal stabilizers and also contribute to an understanding of why they have not enjoyed their former “must have” status most likely because of the effectiveness of the offset-gimbal.

At the risk of being criticized from some quarters, the author suggests that current gyro orthodoxy strongly endorses the de­sirability of having an effective horizontal stabilizer fitted. ‘Effective’ meaning that it should be placed as far rear of the rotor as the structure of the machine allows, to maximize its moment arm, as well as being moderately sized (propeller “stoneguards” do not qualify).

Exactly what “moderately sized” should be is a matter of constant debate but perhaps a very good starting point would be closely looking at the horizontal stabilizers of the ELA, Magni, MT-03 or the Xenon (all factory machines with excellent operational records) and mimicking their relative sizing and placement making allowance for how much smaller and lighter your machine is.

The author also strongly suspects that the trends in horizontal stabilizer identi­fied in this article as applicable to 1930s au­togiros are probably not directly applicable to modern light gyroplanes because modern offset-gimbals provide substantial pitch stability.

Therefore, any horizontal stabilizer designed using those 1930s trends is prob­ably likely to be oversized, although how much oversized cannot easily be determined, it is also not known whether a horizontal stabilizer designed for a modern gyroplane using those identified trends would potentially suffer any performance degrading charac­teristic such as being overly stable.