Expert: Paul Soderman - 1/12/2011

Question
Paul
I wanted to know how large of a role aerodynamics can play in compensating from the receiving or loss of mass mid-flight (1/16 total aircraft mass and up to 1/6 just to get some sort of idea of change). Larger, more powerful aircraft have the benefit of more surface area than the smaller build of planes and are typically more stable in (upright) flight as well.

It makes sense that a plane's shape is intended to allow enough lift to be generated so that the plane will be able to maintain altitude and allow optimum control over the direction of it's flight; however I would expect the design might include compensation for expected (deployable) payload weight as well.
I'm not sure how it's done, but I'd like to know how flight stability is maintained in aircraft (especially small aircraft) after deploying heavy equipment (asymmetrically, balanced, or externally tethered). Modern planes seem to perform better under variable weight, however I'm guessing that it is due to computer/sensor-based flight management systems rather than relying on the aircraft's shape to make compensation easier for the pilot, but I'm not sure.

Another question: which tends to be more susceptible to destabilizing due to a mid-flight change in balance/weight, high or low speed aircraft?

Perhaps I've taken too many things into consideration, however thank you for reading this and I will appreciate your taking the time to answer as well.

Answer
Gabriel
Your question falls in the area of aircraft stability and control, a subject of great interest to me in graduate school.  Whenever possible, engineers design aircraft to be statically and dynamically stable.  That is - they can be flown hands off and will return to level flight aerodynamically after being perturbed by turbulence for example.  To do that the aircraft will usually have a tail that will provide a restoring nose-down moment if the wing lift is increased that causes a pitch-up moment for example. Tail design, wing sweep, twist and dihedral all enter the stability equations.  When stability is not possible or even desired such as in some fighter aircraft, the designer resorts to computer controlled stability through the autopilot system or the pilot flies the aircraft hands on full time.  Modern transports with fly-by wire control systems employ computer augmented stability.

The loss of mass in flight is an easy problem that was worked out by bomber designers years ago.  When the weight drops the aircraft tends to climb but the change in pitching moment is nose down (C.G. aft of the lift vector) so the aircraft tries to descend.  The pilot or computer can easily adjust angle of attack via the elevator and control the aircraft.

A mass increase such as in-flight refueling is a bit more difficult to manage simply because it forces the aircraft to a greater angle of attack leaving less room for stall margin, and it requires the tail to do more work when a gust hits the aircraft, which may be beyond the tail's design limits if the aircraft is overloaded.

A bigger problem to aircraft stability than weight change is improper location of center of gravity.  If the aircraft is loaded wrong before takeoff, the tail and elevator may not have enough power to control the aircraft after it is airborne.

The question about aircraft speed and stability is difficult to answer.  Usually, a high speed aircraft has plenty of control power due to speed.  But it is also possible that things happen too fast and pilot-induced oscillation can occur.  In that case the pilot cannot restore the moments fast enough and can actually exacerbate the situation.  On the other hand, a low speed aircraft may not have enough control power to react fast enough to an upset and might fly into the ground before it has time to recover.  So, your question really depends on the aircraft and mission.

I hope I have followed your train of thought.
Paul