Assessing Gyro Stability
Part 6
By Greg Gremminger
Continued from part 5:
What’s best?
We never seem to get an easy answer to this question! The answer depends on how the designer intends for each individual machine to be used and how it is intended to perform under various conditions. And, the answer depends on how well the designer met his/her goals. In my individual opinion, weighing the current safety issues and situations, the generally preferred sport gyro would be one that does not require pilot proficiency to assure stability.
To me that would be a gyro that applies the various aerodynamic and configuration tools to keep the airframe CG far in front of the RTV - at all speeds, but especially at higher airspeeds and power settings. The further the CG is held forward at higher airspeeds, the less sensitive and demanding are the control inputs of the pilot, and the machine is more self-correcting to wind gusts - and does not rely on pilot proficiency to maintain inherent stability.
As discussed in previous installments, such inherent stability, properly achieved, does not necessarily limit the maneuverability or controllability of the gyro.
How can I tell if my gyro is Safe?:
Without a lot of testing, you can’t be 100% sure! But with appreciation of the complexities and perhaps jeopardies discussed above, you may hopefully be at least aware when you might be flirting with reduced safety margins.
Actually, you can make some rough assessment of how well your machine achieves or maintains the CG location necessary for positive dynamic stability.
1) Typically, a normally loaded and hung gyro will fly keel level at its best L/D airspeed (about 45 mph). If your gyro is normally loaded and hangs ideally (2-3 degrees nose-down as measured on the pitch block of the rotorhead - more on the Hang Test in the next installment), you may be able to apply the following test and criteria:
In calm air and flying level and steady at 45 mph, have a ground observer carefully eyeball the keel of your gyro to a horizontal reference on the horizon behind the gyro. The keel should be level or angled slightly nose-up. If the keel flies noticeably nose-down (visually from a ground observer), even at 45 mph, all bets are off, the CG may be already forced aft of the RTV due to the Prop Thrustline relationship to the CG and CD. This is so because normally the keel is angled 9 degrees (nose-down) from the rotor head angle.
The ideal hang angle for a gyro (keel) is typically 11-12 degrees nose down. A typical rotor will fly at a 9 degrees AOA at 45 mph, so if the keel is level in flight the CG is 2-3 degrees forward of the RTV. Remember, for improved dynamic stability, the CG should be forward of the RTV - not just on the RTV. That is why that extra 2-3 degree nose-down is the ideal hang angle. This is not an absolute guarantee that all is OK, but it is a starting point. If the hang angle is different than ideal, other keel angle criteria would need to be determined.
This criteria might not be valid also if the rotor is a less efficient rotor. Most rotors will fly at no more than 9 degree AOA at 45 mph. A draggy rotor, flying at a higher AOA, would have its RTV angled more forward toward the CG, and the keel might need to fly more nose-up than level to maintain normal margin. For most quality rotors, flying at less than 9 degrees AOA, a level keel on a properly hung gyro should assure the CG is adequately forward of the RTV - at 45 mph!
2) In calm air and flying level, trimmed and steady at 45 mph, reduce power to idle - slowly the first time! A sudden reduction of thrust should not result in a sudden rise or drop of the nose or re- quire stick action to prevent the nose from pitching up or down. The nose of the airframe should only gradually lower to maintain airspeed. Gradually increase the rapidity of the power reduction to gage the actual nose pitch movement as a result of loss of thrust.
What does this indicate?
a) If the airframe nose pitches up or down (or tries to pitch up or down) as a result of a power reduction, this indicates that normal thrust is affecting the static longitudinal CG location relative to the RTV. At 45 mph, this is mostly an indication of a vertical offset of the CG to the propeller thrustline - since airframe drag and CD is less significant at 45 mph. This means that propeller thrust may be adding dynamic stability or detracting dynamic stability by causing the static longitudinal CG location to be more fore or aft than for the normal gyro.
b) If the nose rises sharply as a result of a sudden power reduction, this indicates that normal propeller thrust is forcing the static longitudinal CG aft (less than desirable). The reduction of propeller thrust allows the CG to “swing” forward to its more normal or typical gyro position. This indicates a less than normal stability margin prior to power reduction - even at this lower airspeed. This would suggest caution at even lower airspeeds in turbulent wind conditions because the dynamic stability is less than normal gyro stability at even this lower airspeed.
c) If the nose drops sharply as a result of a power reduction, this indicates that the normal propeller thrust is forcing the static longitudinal CG further forward than normal (a desirable condition). The reduction of propeller thrust allows the CG to “swing” aft to its more normal or typical gyro position. This indicates an improved stability margin under power - at this lower airspeed. This would be the typical response of an “offset keel” or “low thrustline” gyro. This does however suggest caution or at least awareness that the extra degree of stability margin is not present when power is reduced or not applied.
3) Repeat step 2 above at increasing airspeeds. A change in the amount of nose drop or rise upon reduction of power at the higher airspeeds indicates the additional effect of an offset of the propeller thrustline to the CD of the airframe. At higher airspeeds, the drag or CD off-set becomes much more significant and can result in better or worse dynamic stability margins because of the forced foreaft static longitudinal position of the CG relative to the RTV.
A sudden rise in the nose as the result of a sudden reduction of thrust indicates that that initial condition of high thrust and higher airspeed is forcing the CG into a less dynamically stable position prior to the reduction of power. The reverse is true for a gyro that displays a sudden nose drop upon sudden reduction of power - the machine is more stable under conditions of power and speed than it is under the condition of reduced or no power at those airspeeds.
4) Generally, a sudden rise or fall of the nose upon sudden power reduction indicates that under that condition of airspeed, there may be a power situation that will result in a less dynamically stable machine. This is a bit less important at the moderate airspeed of 45 mphthan it is at higher airspeeds. At lower airspeeds, the rotor AOA is naturally higher, with less pitch sensitivity and more safety margin before reversed airflow through the rotor can occur. But, any nose shift upon sudden power change indicates a less than perfect balance of the propeller thrustline offset with the aerodynamic HS moments and may indicate some combination of power and airspeed that results in reduced dynamic stability.
Generally, low propeller thrustline designs utilize the propeller thrust to force or hold the static CG further forward for improve dynamic stability under normal powered conditions and even at higher airspeeds (stability is improved by propeller thrust in that type gyro).
Be aware however, that, with the absence of that stability augmenting thrust, the dynamic stability and safety margins may be reduced and pilot proficiency under those conditions might not be adequate to avoid problems under those conditions - generally a fast descent under low power!
5) Traditionally, controlling a gyro is often described as a series of “jabs” and counter “jabs” to initiate and stop a pitch or roll movement. These are often unconscious control inputs by a pilot experienced in flying that machine. However, it is a sure sign of a degree of instability if a series of “jabs” is required for even moderate maneuvers or steady flight. The truly stable and forgiving aircraft, requires only small forces in the direction of intended movement to initiate and control that motion - such as in the typical fixed-wing aircraft. Note that the truly stable machine does not necessarily mean that machine is not highly maneuverable. If you are experienced in your machine and have mastered the “jabs” required to fly that machine, be aware of two things: One, this machine might be risky if flown by someone who has not developed the proficiency to fly that machine. Two, this machine may be much more difficult for even you to fly in gusty winds or at high airspeeds.
6) It is extremely important for all gyro pilots, experienced OR new to the sport, to realize that every individual gyro may certainly be different from other configuration gyros or other similar gyros with higher power engines. It is also important to realize that even the same gyro may behave quite differently under different power and airspeed conditions - requiring different proficiency levels than we may be tuned to and familiar with. For instance, a high time pilot might be very proficient and experienced at high speed under power - but may have very little experience in the same machine at high airspeed and low power. With awareness of that potential difference, the prudent pilot might approach that less familiar environment with appropriate caution in gustier wind conditions. We should all be aware that considering ourselves to be a “gyro pilot” does not necessarily mean we can proficiently pilot ANY gyro in all environments without adequate practice and familiarity in that different machine - the pilot “tuning” required can be as different as that required between a Cessna and a Pitts - even with seemingly minimal configuration or environmental changes! We should also be aware that a seemingly small change in speed or power, in some gyro configurations, can present the difference in control and stability margins between a “Cessna” and a “Pitts”! I know of few Cessna pilots who would confidently jump into the seat of a Pitts without more training - the stability margins are reduced!
Meeting these six tests is certainly no guarantee of dynamic stability or reason to lower your guard at the limits of your personal safety envelope. But, failing these tests certainly indicates reason for increased diligence in any conditions other than those in which you are extremely proficient and experienced. These criteria, or any of the technical discussions or theories above, may certainly be open for debate. As stated previously, these subjects are presented primarily to heighten our awareness that there are issues and factors involved in gyro flight that may present situations for which we are not adequately prepared. It is my sincere hope that these presentations might peak at least one pilot’s attention enough to avoid treading into an unknown and dangerous situation.
