Botmite Construction Details & photos:

Hacks & Mods


Aircraft Stability  - Basics


Aircraft Proving Grounds

Aircraft Stability

gs Shear Forces

   Aircraft Stability is practically a matter of importance to radio control (rc) and autopilot or robotic flying in typical calm, windy and difficult conditions.  Unstable aircraft may fall into a spinning stall or make flying in windy conditions a sorry failure. Stable aircraft, weight balanced, require less pilot skills. Recovery of unstable aircraft may be impossible in basic turns or flight attitude changes. Aircraft stability is of prime interest, its achievement means a crash is less likely and mistakes are easier to correct.  Control and handling are easier and possible with a stable aircraft and pilot skills. Test pilots make their living pushing aircraft to their limits to discover aircraft handling and stability.

Where or what sources contribute to aircraft stability or aircraft instability? The question of aircraft flight characteristics is a subject of fundamental research, theory and experimentation.  In many ways we are limited by what we know works.  Testing new ideas for emerging discovery challenges our perceptions.  
   Wing dynamics is a basic understanding of how forces interact with an airfoil. Wing shape, configuration, aspect ratio, and structural design all interrelate to interact with air flow - be it laminar, turbulent, burbling or flowing. Knowing the limits of theory is useful in designing experiments for wind tunnel testing not understood by application of number crunching theory alone. Wing interaction with a fluid atmosphere consists of much unseen and easily misunderstood forces - this is where empirical research offers the next quantum leap of ability to expand on what materials research has delivered.

Center of Gravity (CG) - from nose to tail.  The focus here is on Lateral Stability. This helps the aircraft fly straight and level.  This means the nose does not pitch up or down and is trimmed accordingly. Where center of gravity may really matter is turning the aircraft.  Because unstable CG may force a loss of control and possibly a crash.  Flying close below a puffy cumulous cloud gives a roller coaster effect regardless of lateral stability. Flying above those clouds may yield smooth stable air.   

Directional Stability  - means the propeller spin is offset a few degrees to the side with a spacer and the aircraft remains pointed consistent with the yaw vector.  Typically by slightly offsetting the axis of the motor away from the propeller torque, the direction of the aircraft remains stable. On some aircraft with twin motors, the props may be spin opposite to each other to cancel direction drift.

Turning Stability - means given sufficient airspeed the aircraft can turn with or without wind and speed losses to hold a turn and bank rate of turn. Many times the aircraft pitch is turned slightly upward while holding a banked turn to offset airspeed losses.  

   Note to self: Exercise care in sharp turns! A 60 degree bank doubles the g forces on the wing and pulling abruptly inward can sharply increase those g forces to something like 10 gs or more -- this would exceed botmite wing design.
   If the plane weighs 7.5 pounds, a force of 10 gs would put 75 pounds at the junction of wings to fuselage. So, each wing would have to carry 37.5 lbs.
     One way to easily increase wing strength with negligible increase in weight:  add struts from the landing wheel supports to each wing. (See:
LGear - 3 for details)  To avoid wing deflection or wing bending, a strut to the leading edge and trailing part of the wing distributed over a broad wing surface area. 

Drag Forces

   Drag typically presents a negative force upon the aircraft; in some instances drag is useful to slow aircraft; in others drag costs energy and resources.
     Drag forces present opportunity like a legal dilatory tactic to extend flight envelope, maneuverability and aircraft design. Drag can be used to stabilize, slip, land or modify an airfoil - on the fly.


Aircraft As Flown

   Airspeed Minimum or Stall - means enough power must be applied or a descent maintained or the airplane will free fall lacking lift from the wings. Turning the aircraft downwind may also require more power or suffer loss of control. 

GPS speed is ground speed not Airspeed - this means you can not rely on this ground speed.  Airspeed means relative to the wind of velocity of air passing the wing. 

Weight and Balance - how much weight an aircraft can carry is limited by wing loading, structural design and available power. It is critical that whatever mass is added to the aircraft that it is placed to maintain balance of the structure.

Calcs versus As-Flown

   One should note calcs yield a point where the aircraft see-saws about.  But the aircraft As-Flown is more likely to shift this fulcrum or pivot point within a range due to other dynamic forces being exerted upon this mechanical structure.  A loose LiPo battery can exacerbate flying stability even further.


Pre-Flight Inspection

     Every time, before you fly, inspect the aircraft, test all controls and make sure all batteries and power sources are full and ready.  Range test or distance check the radio controls to know their limits in advance of flying.

Pre-Flight Pilot Skills

     Computer software or firmware flight training time is well spent and typically much cheaper than replacing aircraft or having to deal with a crash. Train for worst case conditions and events.  The British Navy has a slogan: Rough seas, skillful sailors make. And, on occasion, practice what you learned to keep it fresh, ready for use. You should have a fairly well developed expectation interest so you can handle surprises and situations without incapacity, shock or indifference. 

Kilogram Force Per Square Decimeter (kgf/dm2) is a unit in the category of Pressure. It can be converted to the corresponding standard SI unit Pa by multiplying its value by a factor of 980.665.
Eppler Code > PROFIL - programming info

[Home] [Photos] [Electronics] [Sensors]