Aerodynamics: Rolling Gs

Avweb has an interesting article on ‘Extreme Maneuvering’ about practical applications of the FAA commercial aerobatic maneuvers (chandelles, lazy 8’s, steep spirals) that mentioned “rolling G’s.”

A rolling G occurs when you maneuver an aircraft in more than one axis at a time, causing the airframe or wing to twist. The rolling G design limit is considered to be 2/3 of the normal G limit, according to FAR 23.

Although I performed the commercial maneuvers during my Commercial Airplane practical test and Citabria checkout, I wasn’t really aware of two things:

  1. airframe twisting from rolling G’s can more easily exceed a plane’s load limit. Those limits would be lower in older aircraft, possibly already damaged, than newer ones. It’s important to load the airplane one axis at a time.
  2. the commercial maneuvers can be used to reverse in a box canyon. I know a private pilot who crashed in a box canyon (he luckily survived) because he knew of no course reversal methods, so this is handy to know.

Chandelle (Climbing, Reversing Turn) Animation

Advanced Section

The asymmetric lift, resulting in a torque, caused by the ailerons travelling up and down simultaneously with yawing and pitching maneuvers is believed to have caused several airshow accidents in older airplanes, shearing the wing spar. Contributing factors are the acceleration rate of the control movement, airspeed above maneuvering speed, Va, and wing and fuselage harmonics. Sideslip also affects G limits.

It would be difficult to calculate actual rolling G limits without destroying several aircraft to build a mathematical model. There are a number of reasons for that, but primarily the problem is that dynamic torque must be calculated for multiple types of members, including spars, fuselage skin, and especially attach points. The latter is tricky because attach point hardware may be very strong in one axis, and very weak when loaded off-axis (or corroded.) Why is Rolling G dangerous?, Normal G limits vs Rolling G limits?
W: Chandelle,
Ice and Tail Stalls

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