Misconceptions 8-9
Gyroplane Stability Misconceptions
By Greg Gremminger
Misconception #8: “A CLT or low prop thrustline gyro cannot PPO!”
This is actually true, but only by definition. A true CLT or low prop thrustline cannot be “pushed over” by the prop. But that does not mean it cannot bunt over! A PPO is just one form of a buntover that is described as the prop thrust pushing it over into the buntover. A CLT or low prop thrustline gyro can still buntover – although it might be less likely and under different circumstances, it can still bunt! A dynamically inadequately stable gyro is still subject to PIO (if it has inadequate HS effectiveness). On inadequately stable gyros, a rapid nose-down rotation from either pilot input or suddenly reduced power can re-position the RTV suddenly forward of the CG and result in a buntover – especially if other airframe drag or lift moments cause the airframe to rotate nose-lower than normal flight. Don’t abuse any gyro because you are told it cannot buntover because it has this or that component or configuration. The only true assurances can only be given after thorough flight testing by a professional. Even then, don’t trust that your gyro cannot bunt!
Misconception #9: “A PPO happens when the rotor drag goes to zero and the high prop thrustline pushes the nose over!”
This is an over-simplified depiction of a Power Push-Over that confuses the real issue. The term “Power Push-Over” actually confuses the issue and creates misconceptions of gyroplane configurations that might be more likely to PPO! This visualization is often used simply because the true physics of the issue might not be readily understood by the “non-engineer” masses! I believe it is important to a gyro pilot’s safety to actually understand the real physics of what is going on, so that a pilot might not have over-confidences when those confidences might not be warranted.
Because of the popularity of this PPO description, many people have assumed that only a high thrustline gyro can bunt over. This is not true – any gyro can bunt! A PPO is a popular visualization description for one type of buntover, but it does not prescribe all types of buntovers! The buntover is not the result of a loss of Rotor Drag, it is from a reduction or loss of Rotor Thrust. (Rotor Thrust is a vector combination of both rotor drag and rotor lift) A buntover or PPO is not wholly the result of a mis-balance of prop thrust and rotor drag when rotor drag is lost. A buntover occurs when the RTV is forward of the CG (G-Load unstable) and the rotor THRUST is reducing rapidly. The reducing rotor thrust (from a down gust or a forward pitching spindle) causes the nose and spindle to pitch further nose-down. This further reduces the rotor thrust, and the cycle progresses rapidly until something really bad happens – buntover, precession stall or rotor strike!
So, the real issue with a PPO, or any form of a buntover, is whether the RTV is, or can be, forward of the CG under any flight condition or dynamic reaction of the airframe. The inference of this PPO misconception is that any “high prop thrustline” gyro is likely to PPO. A high propeller thrustline – that is not balanced by an appropriate HS balance in the propwash - WILL have the RTV forward of the CG (G- Load unstable), and can be susceptible to PPO. But, a properly designed “high prop thrustline” gyro, employing an effective HS to solidly hold the RTV aft of the CG in flight, and thereby avoid the underlying CG/RTV relationship that might otherwise make a buntover possible. On such a machine, where the CG is assured (by proper “balance” of prop thrust with the HS) to be forward of the RTV (G- Load stable), a change in Rotor Thrust causes the airframe (and therefore the rotor) to rotate in pitch so as to reduce the G-Load disturbance and return the G-Load to the normal 1 G – the definition of Static G- Load stability!
A properly designed HS on a slightly high prop thrustline configuration can “balance” both the prop thrustline moment and any airframe drag or lift moments (functions of airspeed) to avoid speed/power combinations or any transients of speed or power or wind from rotating the airframe so as to allow the RTV forward of the CG at any time – thereby actually removing the physical mechanism of ANY type of buntover!
The point is that this over-simple PPO misconception, intended for the technically less savvy, can invoke over-confidence in one gyro configuration while dismissing another configuration that might actually present a higher degree of buntover immunity!
Sidebar 1:
Following are a few other often misrepresented concepts. Because I feel that a pilot who truly understands and appreciates the technical factors in gyro flight will make better and safer gyro flight decisions, I would like to present some related concepts here that are often mis-stated or mis-represented:
• A PPO or buntover is the result of a STATICALLY UNstable gyro – usually, G-Load static instability. PPO is not a DYNAMIC instability issue; it is a STATIC instability issue! For a G- load statically unstable gyro, when a G load transient is encountered, the RTV forward of the CG causes the airframe to pitch worse in the worsening direction. The nose-down pitch rotation can diverge rapidly into a buntover. This is the divergence of a negative STATIC stability reaction. A STATICALLY stable gyro will be highly resistant to buntovers.
• PIO is a DYNAMIC stability issue – rapid natural oscillation rates of a gyro that are inadequately damped so that the pilot reactively exacerbates the DYNAMIC reaction. PIO is not a STATIC instability issue. During a PIO, before the actual buntover, the gyro is actually statically stable – pitch is oscillating (severely) around the statically stable condition. An adequately DYNAMICALY stable gyro will be highly resistant, if not totally immune to PIO.
• A dynamic PIO rapidly progresses to an amplitude where the gyro RTV/CG relationship becomes statically unstable - CG aft of the RTV. At this point, the dynamic oscillation stops and the static divergent forward rotation progresses to a buntover. In other words, dynamic instability is not what bunts it over; there can be no oscillation around a stable condition when that stable condition no longer exists. It is the statically unstable condition of that moment that diverges the nose-down rotation into a full buntover.
Sidebar 2:
Here is another perspective on PIO:
As described herin, PIO is most likely on an inadequately dynamically stable gyro – one that has a rapid natural, short-period oscillation rate with inadequate damping. If a gyro has a high natural oscillation rate, damping is absolutely required.
If a gyro does not have a “passive” dampener or stabilizer, the pilot must constantly be “actively” stabilizing the aircraft. With experience, this can eventually be automatic or subconscious pilot control inputs, but the pilot must always be “balancing” the gyro around a steady-state condition (airspeed, attitude, G-load, etc.) This is not unlike the constant work it takes to balance a yard stick upright in the palm of your hand – you can do it, and with practice it is not difficult, but it does require constant “work” by the “pilot.” In other words, for a statically unstable gyro to fly with relative stability, the pilot must be the “active” stabilizer (a human autopilot!) – constantly working to maintain the steady condition.
In some ways it can be said that this entire gyro/pilot system is indeed statically stable, even though the gyro itself is not inherently statically stable by itself, and would diverge into extreme conditions if the pilot were not in the “active” stabilization “control loop.” The pilot is the “stabilizer!” So, as in any STATICALLY stable condition, the gyro/pilot DYNAMIC stability becomes the issue – the gyro/pilot’s reaction to a disturbance of wind or G-load or over-reactive pilot input. In the absence of any other “passive” stabilizer, it also falls on the pilot to provide “active” DYNAMIC damping! The experienced pilot can probably dampen slower rate disturbances or oscillations. But, the human pilot is just not able to react with proper timing or degree when rapid rate natural oscillations might occur.
So, PIO can be envisioned as a stabilizer (the pilot) that is adequate at the common slower rate oscillations or disturbances, but is inadequately designed to handle the more rapid oscillatory rates that are true and fatal PIO events. I suggest that the old argument that training or experience can recognize and correct PIO is simply not true. If a human is the only stabilizer on a machine that has a rapid natural oscillation rate, the human pilot is simply not adequate to stop such oscillations and will actually exacerbate the rapid rate oscillation amplitude – PIO. It may be true that an experienced pilot, with perhaps greater sensitivities to seat-of-the-pants indicators, may be less likely to venture into conditions (of speed or power or wind) - where gyro/pilot stability “feels” dangerously inadequate!
Try this fun experiment: Balance a yardstick (or meter stick) upright in the palm of your hand. Notice, with a little work, you can keep the stick upright. This might be the analog of a gyro pilot’s work to “stabilize” the slow natural response tendencies of a gyro. Now, do the same thing with a shorter, 1-ft ruler. Notice the human “pilot” will have difficulty “stabilizing” this faster reacting, short-period, PIO- type instability. This somewhat characterizes what happens when the pilot “stabilizer” experiences a situation where the pilot skills are inadequate. Now, clip a weight to the top of each of these sticks, and repeat the exercise. Note that the higher Moment of Inertia (MOI) “gyro” becomes somewhat easier to “stabilize” – characterizing the differences between higher MOI and lower MOI gyros.
The point is it is much safer to rely on the installed “passive” stabilization that an adequately effective horizontal stabilizer can provide, and not trust solely in the pilot’s abilities to totally provide “active” stabilization through all flight conditions. Someday, that flight condition of wind transient, speed or pilot reaction might exceed the capability of the “human stabilizer!” “Passive” stabilization also reduces or can completely remove the pilot attention and workload otherwise required to statically stabilize the gyro and constantly correct for disturbances. This means more fun with less work and a whole lot less anxiety!
Have a safe flight – Greg
