Misconceptions 4-5
Gyroplane Stability Misconceptions
By Greg Gremminger
Misconception #4: “CLT does not require a Horizontal Stab!”
CLT has the issues described in “Misconception #1”. CLT gyros are not necessarily PIO or even buntover proof! For ANY prop thrustline configuration, passive DYNAMIC Stability in a gyro can only be achieved with an adequately sized HS. Dynamic stability is the issue involved in Pilot Induced Oscillations (PIO). PIO can lead to a buntover after a few cycles. CLT, in itself, does not provide dynamic stability!
Dynamic stability characteristics are a function of the inertias (Moments of Inertia) of the airframe and of the rotor, and of the interacting inertial reaction rates of the rotor and airframe. (Dynamic stability is not a function of the propeller thrustline!) The rotor and airframe inertias mostly determine the natural pitch oscillation rates of the whole system. All bodies, including aircraft and gyros, if they are statically stable in some condition, have natural tendencies, upon disturbance, to oscillate around that statically stable condition. If that natural pitch oscillation rate is so quick (short-period) that a human pilot cannot react with the proper amplitude and timing, PIO can be a likely result. With no HS, the pilot must be the “active” stabilizer to “dampen” the pitch oscillations. Especially if the reaction rates of the airframe and rotor “resonate” or build on each other, the pitch oscillation rate, once excited or started by a disturbance, can be much faster than the pilot can correct (“dampen” to zero). With wrong stick reaction inputs, the pilot can readily exacerbate the amplitude of the rapid natural oscillations until they culminate in a buntover. The gyro configuration itself must naturally avoid or effectively dampen such quick oscillation rates. This is not easy to do or determine “on paper” or by some simple guidelines such as a “cook book” size of the HS.
Dynamic stability characteristics are also a function of the HS. The HS provides “passive” (the pilot or autopilot doesn’t have to do anything!) damping of the pitch oscillations. “Damping” is required on all aircraft, because all statically stable aircraft will have a natural oscillation at some rate. The HS is what automatically and naturally “dampens” or reduces those oscillations to zero – without pilot action! Typical pilots may be able to “dampen” slower rates of oscillations, but, this requires active reaction (workload) by the pilot. Especially for airframe/rotor inertia combinations that produce very quick natural oscillations in response to a disturbance, the HS must provide aggressive “damping” to very quickly reduce those oscillation tendencies to zero – BEFORE over-controlling PIO reactions are excited in the pilot. The pilot can not be expected to provide the “active damping” required to avoid PIO at higher oscillation rates! PIO is a dynamic stability issue. Dynamic stability characteristics are not a function of the prop thrustline or of the RTV location. A HS is the only proven way, and at least the simplest “passive” way, to provide the damping required to automatically and passively reduce the natural oscillations to zero.
It is simply a fortunate convenience of nature’s laws that a HS can both provide the static “balancing” to affect static stability, and the dynamic damping for dynamic stability. Two benefits for the price of one!
Misconception #5: “The propeller thrustline determines a gyro’s stability!”
This is the mother of misperceptions that leads to most of the other misperceptions about CLT and prop thrustlines. The real issue with gyro stability is not the propeller thrustline (relative to Vertical CG) - that is just a part of the real issue. The real issue with STATIC stability is the location of the CG forward of the RTV. This alignment is the result of several static moments acting on the gyro airframe: airframe drag (as a result of airspeed), airframe lift or down-lift (as a result of airspeed), HS down-lift (as a result of both airspeed and propwash), AND propeller thrustline (as a result of engine power). All of these forces acting on a moment arm around the CG of the aircraft determine the overall static stability of the gyro by establishing the static airframe attitude and the position of the RTV aft of the CG. To consider just one or two of these static moments, without considering the effects of all of them, misleads to wrong conclusions about a gyro’s stability.
Static G-Load stability requires that the sum of all these moments acting on the airframe results in the CG remaining at or forward of the RTV. G-Load static stability is the prominent issue with buntover risk.
Static Airspeed stability requires that the CG be forward of the RTV, AND that the HS be providing down-lift to balance that forward CG. Airspeed static stability is a secondary issue with buntover risk as it prevents wind gusts from displacing the RTV from it’s stable location aft of the CG..
Dynamic stability has nothing to do with the propeller thrustline! Dynamic stability is the result of rotor/airframe inertias and the damping effectiveness of the HS. Dynamic stability is the prominent issue with PIO, which can lead to a fatal buntover (static G-Load instability) after a few cycles.
Static instabilities can lead to pilot over-control and PIO because they present significant and often wrong direction pitch attitude transients that can excite the pilot into over or wrong control inputs. The point is, propeller thrustline is not the only issue – and stability/safety over-confidences based on a “cook book” thrustline solution can lead to poor decisions of what and how and when to fly your gyro. It is difficult to boil these issues down to casually observed “cook book” recipes. The only true way to determine if these critical stabilities are proper is to flight test the result. To pass judgment without flight testing results is to fall prey to the misleading conclusions of this misconception.
