Ground Stability
Part 2
By Greg Gremminger
We all tend to focus on the flight stability of gyros. Flight instability events such as buntovers or Pilot Induced Oscillations have been the traditional headline safety and fatal accident issues with gyros. With the advent of the “Big Tail Way Back” configuration, those stability issues are really a thing of the past for gyros that employ that concept. However, not all accidents are related to just FLIGHT stability. It is important also to have strong ground directional stability in order to avoid dangerous and damaging accidents upon takeoff or landing. Unfortunately, ground roll‐overs are still occurring, even with some “new generation” gyroplanes. Ground roll‐overs cause severe damage but can also cause severe occupant injuries – including death! It is a bit disappointing that some Magni “clones” have copied the important parameter of dynamic flight pitch damping, but fail to recognize the importance of ground directional stability also built into the Magni gyros.
Like all tricycle gear airplanes, Magni employs a strongly castering nose wheel. All pusher gyroplanes land and takeoff as “tricycle” landing gear aircraft. The advantage of tricycle aircraft is that they tend to straighten out automatically when the nose wheel is touched to the ground – either on takeoff or upon landing. For a tricycle landing gear to function properly though, the nose wheel must be able to caster – freely align itself with the direction of motion of the aircraft. For example, especially when landing with a crosswind, or during full power takeoff acceleration, the necessary cross‐control rudder deflection may align the (rudder pedal coupled) nose wheel in a different direction than the direction of movement of the aircraft. If the nose wheel is unable to freely align with the direction of aircraft motion, the deflected nose wheel will “dart” the nose to one side when it touches to the ground. Even low center of gravity gyros, such as the Magni “clones” can easily roll over when the nose is suddenly deflected away from the direction of motion. This sudden nose “dart” may often excite the pilot into control reactions that exacerbate the problem.
Many gyros, for reasons I have yet to understand, use a nose wheel without any caster. Picture the castering wheels on a grocery cart! The touch point of the wheel to the ground surface must be behind the vertical steering pivot axis of rotation of the nose wheel strut. To work properly, the angle of the nose wheel strut must be closely vertical or perpendicular to the ground – picture the grocery cart again! To make matters worse, many gyros actually cant or angle the nose wheel strut severely forward. This probably looks good, but further prevents the nose wheel from straightening out when touched to the ground. Actually, when weight is applied on the nose wheel, as upon landing, a cant forward can cause the nose wheel to deflect away even further! Some producers even recognize this issue and require in their training and flight manuals to hold the nose wheel off the ground until the gyro is well slowed down or almost stopped. (I actually sold one of the first USA Magni gyros to a customer who had recently rolled over his other‐brand expensive gyro for exactly this reason!)
The potential for ground roll‐over upon a nasty sideways or drifting touchdown is one concern here. But, the restriction to not touch the nose wheel at higher speeds on the ground actually limits some of the operational and performance benefits of a gyro. For instance, the standard FAA short field takeoff procedure for gyros (similarly for most airplanes), and for the Magni gyro, is to lower and hold the nose to the ground once the rotor RPM is adequate to lift the nose off the ground, hold the cyclic well forward to reduce the rotor disk Angle of Attack and minimize rotor drag, allow the gyro to quickly accelerate to best rate of climb airspeed in a shorter ground roll, and then rotate and climb immediately at the best angle of climb airspeed, Vx, when it is reached on the ground. This procedure shortens the rolling distance on the ground by minimizing rotor drag after takeoff Rotor RPM is reached, and avoids the need to fly some distance in ground effect to build airspeed to Vx for best angle of climb after liftoff. When the nose wheel cannot be re‐touched to the ground at the higher airspeeds – on takeoff especially when full power actually requires more rudder deflection – the rotor disk cannot be leveled to minimize drag and lift, ground roll is longer, acceleration limiting rotor drag is more, and rotor lift initiates a takeoff at airspeed well below Vx.
The strong caster of the Magni nose wheel avoids the potential for a “nose dart” that could cause a narrow wheel base gyro to roll over; and allows full application of short field takeoff procedures in even strong crosswinds. In my over 3000 hours of flight instruction provided in our Magni gyros, I have experienced some very severe sideways or drifting landings with students. We often train even new students in hefty crosswinds. Sometimes the ground stabilizing reaction of the castering nose wheel straightening out the aircraft can be startling to the student, but we have never had a roll‐over tendency with any student. Students often make multiple landings in a single landing attempt, often exciting rudder control reactions that could exacerbate the situation – and still no real roll‐over tendencies. The point is, ground stability issues might not be the cause of such severe and fatal accidents as flight instability has been in the past, but ground roll‐overs are still serious and dangerous. Magni gyros address all types of stability – including ground stability.
