Misconceptions 1-3
Gyroplane Stability Misconceptions
By Greg Gremminger - 2002
Many of you might consider what I am about to say as “blasphemy.” But, I’m going to say some things about stabilizers, thrustlines and stability, which may not totally agree with some popularly espoused concepts on these subjects. Essentially, gyroplane stability technology has advanced, and is now becoming more understood, beyond the old popular and often misleading “cook book” concepts.
Most popular solutions to unstable gyros (and PIO and buntovers, etc.) are certainly valuable guidance and have improved the safety of the gyro sport. For instance, it IS a very good thing to have a large and effective Horizontal Stabilizer (HS). It is also a good thing to have a propeller thrustline that is reasonably aligned with the CG of the aircraft. However, these popular “cook book” solutions can be a bit misleading as to what they can do and how well they can do it. The true stability and safety of a gyro can not be insured by simply following one or more “cook book” solutions. In fact, some popular “cook book” solutions, or poor combinations of these solutions, can actually present other, even less apparent safety issues.
The purpose of this article is two-fold: To make sure gyro pilots won’t have misplaced confidence just because they use one or more of these popular “cook book” solutions. Pilots should still understand the limitations of their particular gyro configuration, with perhaps some unexpected traits, and the proficiency required to operate that gyro. The second point of this article is to correct some of the inferences about prop thrustlines and HS configurations that are not technically supported and can even influence much less than optimal control, stability and safety.
While presenting a number of downsides of popular misconceptions, be assured that a proper design can actually provide gyroplanes that exhibit few or none of these downside issues. This article is just to make you aware and avoid falling into “over-confidence traps”. When designed properly, as verified by actual flight testing, gyros can present stability and transient safety margins well beyond those capable on even fixed-wing aircraft.
Misconception #1: “Centerline Thrust (CLT) is best”
Although a much better idea than a highly offset and “unbalanced” prop thrustline, CLT is certainly not the ideal or only thing to do. “CLT” can certainly be a desirable condition, but designers should consider some issues:
• Most people consider any “high seater” gyro as CLT. Often these may really be “low prop thrustline” not a true “CLT.” A low prop thrustline can present some less than optimal flight characteristics itself – See Misconception #2 below.
• There are very few true CLT gyros! Burn off some fuel or carry a heavier pilot, and the Vertical CG (VCG) has moved, and is no longer aligned exactly with the prop thrustline. The rest of the gyro design should allow for these variations – not depend totally on “CLT”.
• A perfect CLT without a HS has only neutral static stability – not positive static stability. CLT, alone, does nothing for dynamic stability – PIO is the critical dynamic stability issue.
• A perfect CLT with a HS may not be Power Stable at the same time as Airspeed and G-Load statically stable. A perfect CLT may not, on its own, position the Rotor Thrust Vector aft of the CG. A down-lifting HS must be arranged to provide this nose-up moment to the airframe in order to achieve Airspeed and G-Load positive static stability – CG forward of the RTV. But the down- lifting HS (a very good thing!) is may not be compatible with a CLT that does not provide a balance to the nose-up moment from the HS. This means the airspeed can change as a function of power setting – not Power Stable. This may not be totally bad if not severe, but it can get novice pilots into trouble upon sudden power changes such as in a botched landing or engine failure – the nose attitude can change dramatically and excessively!.
• A perfect CLT with a HS that does not provide a down-lift moment likely has only neutral static stability in airspeed and G-load. Even though the HS may provide dynamic stability, the lack of static airspeed and G-Load stability requires additional skills and workload from the pilot. Lack of positive airspeed and G-load static stability are critical issues in buntovers and can initiate PIO!
• Reliance on assumed CLT for stability does not consider an airframe drag line offset (from CG), and other nose-down aerodynamic moments that likely might statically destabilize the gyro.
Misconception #2: “A Lower Propeller Thrustline is Better!”
A propeller thrustline that is significantly below the gyro VCG should not be confused with Centerline Thrust (CLT). The significant nose-up moment from a low prop thrustline actually does dramatically improve the G-Load stability of the gyro - by rotating the in-flight attitude more nose-up to position the RTV further aft of the CG – but only when significant power (thrust) is applied. This is a great thing for G-Load static stability and for high speed flight stability and wind insensitivity! This is called “power augmented stability”, but it has much less effect when power is low. The problem is that this enhanced G-Load static stability is present only when significant power is applied. When lower power or idle or no power is present, the nose is not artificially held at such a high attitude and the RTV is not so far aft of the CG. In fact, other configuration issues may drag or push the nose down and the CG aft of the RTV – static G-Load instability, the root cause of buntovers. The issue this presents is that a different pilot proficiency is required to fly at high speed and low power safely, as compared with the stability and ease of control when power is higher on that same gyro.
A second issue with this configuration is that a low prop thrustline actually forces the HS to present an up- lift to balance the nose-high, high power condition. An up-lifting HS may not provide static airspeed stability, in other words, the airspeed continuously tries to diverge from the set or “trimmed” condition - and “active” pilot control is required to maintain the airspeed balanced at the trimmed airspeed. If airspeed is allowed or forced to change significantly above or below the “trimmed” airspeed, stick forces attempt to further increase that deviation. An airspeed statically stable gyro would always attempt to self- return to the “trimmed” condition.
A third issue with this configuration is that a change in power significantly changes the “trimmed” airspeed. That means that it takes forward stick pressure to maintain the same “trimmed” airspeed in a higher power climb as compared to cruise power at cruise “trimmed” airspeed. In other words, to climb at the same airspeed, forward stick pressure is required. The opposite happens with reduced power. This becomes a more serious issue when rapid changes in power are made – such as in landing or a botched landing. This also presents a possible buntover or precession stall issue upon rapid power changes (sudden reduction or loss of power) – the rapid nose down rotation from loss of nose-up thrust, coupled with the tail-up force of the HS, can cause the RTV to move rapidly forward of the CG and possibly precipitate a buntover or rotor precession stall. This may not be the priority buntover issue among gyros, in fact the augmented stability of such low prop thrustline gyros may have dramatically improved the safety of gyros overall – much more stable at high speed in gusty winds! But, pilots should appreciate the potential for these other, less apparent safety issues. At high speed, a sudden reduction of power might initiate a transient pitch reaction beyond which the pilot skills are adapted or experienced.
Misconception #3: “High Prop Thrustline is Bad!”
Sorry, a slightly high prop thrustline is the reliable way to achieve positive Airspeed Static Stability, positive G-Load Static Stability, and good Power Stability - at the same time. It is true that the “high prop thrustline” must be “balanced” by a properly configured HS, but, to reliably assure all three static stabilities, a gyro’s HS must have a down- load on it in flight. A down-load on the HS forces the CG to be forward of the RTV for G-Load static stability and balances the forward CG for Airspeed static stability. Just as in airplanes, Airspeed Static Stability requires a down-loaded HS. Low prop thrustlines require an actual up-lifting HS which presents a divergent or negative static airspeed characteristic. True CLT requires a near neutral lift HS for Power Static Stability, providing only weak Airspeed and G-Load Static Stability. A slightly high prop thrustline requires a down-loaded HS (reacting to propwash effects) in order to “balance” the slightly nose-down moment of the high prop thrustline. This down-loaded HS, also reacting to airspeed, should also “balance” the forward CG for static airspeed stability. The down-loaded HS (reacting to both propwash and airspeed) holds the RTV aft of the CG in flight – providing the important G-Load static stability. The inference of this misconception is that high prop thrustlines are more susceptible to Power Push-Over (PPO). This is only true if the high prop thrustline nose-down moment is not reasonably “balanced” by a nose-up moment of the HS reacting to propwash. This “balance” is easier and more efficient if the prop thrustline is not excessively high, but any high prop thrustline can probably be “balanced” with enough down-loaded HS in the propwash! The truly stable gyroplanes are those that have enough “balanced” HS down-load (reacting to both propwash AND airspeed) to hold the RTV steadily aft of the CG (CG forward of the RTV) – at all power settings and at all airspeeds.
