Aerodynamics / Principles of Flight

Four Forces of Flight - Lift, Weight, Thrust, & Drag.  In straight and level flight, thrust = drag, and lift = weight.  

Thrust acts parallel to longitudinal axis.

Drag is the rearward force acting parallel to relative wind

Weight opposes lift, acts vertically down through CG

Lift opposes weight, acts perpendicular to flightpath through Center of Lift (CL)

Sum of opposing forces in straight and level flight = 0.  However, these forces are not nice and perpendicular to each other during different phases of flight.  

TERMS --  Chord - Line from leading edge to trailing edge, Camber - overall shape and curvature of the airfoil.  Dihedral - angle formed between wing and where it attaches to fuselage (line parallel to lateral axis).  Angle of Attack - Angle between chord line of wing and relative wind.  Critical Angle of Attack - point at which an airfoil will stall, regardless of airspeed.  Center of Gravity - point at which aircraft would balance.  Center of Pressure - point where lift is acting upon the airfoil.

When thrust decreases, angle of attack must increase.

Lift can be explained by Bernoulli's Principle - area of lower pressure above the wing than below, and Newton's 3rd Law - airflow deflected off bottom of wing has equal/opposite reaction forcing airfoil "up"

Weight is always a downward force toward the earth, acting through the CG.

Drag opposes thrust.  Types of Drag.

-Parasite Drag is drag associated with aircraft design and its protrusions

Types of Parasite Drag -- 

Form Drag - Drag from overall shape of the aircraft, engine nacelles, antennas etc.

Interference Drag - intersection of airstreams that restricts airflow (wing root and fuselage) creates eddies/turbulence.  It is like traffic trying to merge onto the 405.  They must give way to other traffic.

Skin Friction Drag - Contact of moving air with surface of the aircraft.  Molecules slow down over "rough" surface of the wing.  Boundary layer plays a role.  Airflow over top may become turbulent by reaching center of wing due to molecules getting "stuck" at surface.

Induced Drag - Byproduct of lift.  As airspeed is lower, AOA must be increased to maintain lift.  This places the components of lift more rearward (opposite thrust)

Parasite and Induced drag occur inversely to each other.  The point at which they are least is called L/D Max and is represented by best glide speed.  Lift/Drag Ratio is determined by dividing CL by CD.

Wingtip Vortices

High pressure attempting to reach low pressure above the airfoil finds path of least resistance at tips.  Spills over and creates cyclone of downwash.  This creates rearward lift component (induced drag).  This is of particular concern when flying behind larger aircraft generating these forces - particular during takeoff or landing.  These vortices cause wake turbulence which can cause induced roll. Land beyond prior aircraft or takeoff prior.

Ground Effect exists just above the surface and causes a reduction in induced drag and therefore a plane can generate lift at lower airspeeds and a lower angle of attack than out of ground effect.  

Aircraft Axes - lines that pass through the CG

Stability - aircrafts ability to correct for alterations to its equilibrium

Static - Initial tendency back to equilibrium

Positive Static - return to original state

Neutral Static - remain in new condition after disturbed

Negative Static - aircraft continues away from original state

Dynamic - Response over time (oscillations)

Positive Dynamic - oscillations decrease over time

Neutral Dynamic - oscillations do not change over time

Negative Dynamic - motion of displaced object increases and becomes more divergent

Longitudinal Stability

Most aircraft designed so Center of Lift (also Center of Pressure) are aft of the Center of Gravity.

Horizontal Stabilizer is like 'upside-down' airfoil counteracting nose-heavy aircraft from pitching down.  It is set at slight negative AOA.  A faster airspeed generates more downwash on the Horizontal Stabilizer.  Therefore, when power is increased, the nose has a tendency to pitch up and when power is reduced, the nose pitches down (if the plane is stable and loaded within limits).  

Lateral Stability

Dihedral - If aircraft roll is disrupted by gust or other factor, the lower wing will have a greater angle of attack, producing more lift and correcting for the rolling error.  The plane is rolling, but not turning (slipping)

Sweepback - Combined with dihedral aids in lateral stability and directional stability.  The low wing is more perpendicular to the relative airflow, creating more lift and also has less drag then the wing that is up, thereby allowing it to "catch up" to the other wing and return to its original state.

Vertical Stability

Most straightforward.  If a sudden wind gust from the side begins to push a plane in one direction, the force of the wind on the vertical stabilizer pushes it back (yaw).  This is also known as directional stability.
Keel Effect - Top area of fuselage as well as rear area have more area.  This means more wind will hit the top and rear areas of the fuselage - encouraging both lateral and vertical stability.

Dutch Roll & Spiral Instability

In Dutch Roll the lateral and vertical stability are working, but out of phase with one another and creating oscillations of roll and yaw.

In Spiral Instability there is an overbanking tendency due to relatively weak dihedral in comparison to strong static stability.  Compared to dutch roll, spiral instability is easy to recover.

Stalls

Often misunderstood because the aircraft has stall speeds.  Though, a plane can stall at any airspeed if exceeding it's Critical Angle of Attack.  A stall is when an airfoil stops producing adequate lift due to separation of airflow over top of airfoil.

In straight-winged aircraft, stall occurs at root and moves out to tip.  This allows time for aileron control.  Center of Gravity also is important in stall recovery.  If CG is forward, recovery is easy because the nose already has a tendency to go down, reduce its AOA and therefore break the stall.  If an aft CG is present, the nose-down tendency is gone and may prove difficult to recover from.

Stalls occur in slow speed flight because a greater AOA is required to maintain lift.  They can occur when pulling out of a dive, however, due to centrifugal force and the relative wind continuing to counteract the plane's new direction.  Planes also stall at higher airspeeds when turning than in straight-and-level flight.  Again, centrifugal force is added to weight and a greater component of lift is required to counteract it.  This required increase in lift increases AOA.

A Spin occurs when both wings are stalled, but one is more stalled than the other, forcing a rotation during the stall.

Propeller & Aerodynamics

A propeller is like a rotating airfoil.  It is twisted so that the blade maintains a relatively constant AOA throughout its length since the tip has a greater distance to travel and therefore faster to go.  

Propeller efficiency is ratio of thrust horsepower to brake horsepower.  Propeller Slip is difference between geometric pitch and effective pitch.  Geometric Pitch is theoretical distance propeller should advance in one revolution.  Effective Pitch is actual distance.

Turning Tendencies - 4 main turning tendencies of aircraft include

Torque - Aircraft's propeller motion creates opposite turning (roll) tendency due to Newton's 3rd Law.  This action rolls plane to the left.

Corkscrew/Slipstream Effect - propeller slipstream encircles the plan and exerts a force on the fin/vertical stabilizer causing rolling force to the right while torque is causing force to the left.  Corkscrew force is most pronounced at high power/low speed (takeoff) when corkscrew is compacted.

Gyroscopic Precession - Gyroscopes/spinning discs all have precession - forces acted upon a point on that disc occur 90º ahead in rotation.  In this case, the propeller is our "disc."  This is most relevant to tailwheel aircraft when leveling for takeoff.  The pitching moment down becomes a yawing movement to the left.

P-Factor - When aircraft is at high angles of attack, the downward swinging blade is "scooping" more air and exerting more force than the upward/backward swinging blade.  This would result in a left yaw.

Load Factors

Load factors are measured in G's.  An increase in load factor results in an increase in stalling speed.  Load Factor Categories are as follows:

Normal:  3.8 to -1.52 Gs

Utility:  4.4 to -1.76 Gs

Acrobatic:  6.0 to -3.00 Gs

Not surprisingly, increase in angle of bank increases load factor due to Centrifugal Force and the increased back pressure required to maintain altitude.  When load factor is squared, stalling speed doubles.

Maneuvering Speed (VA) = Highest speed the plane can fly and have complete control deflection without structural damage.  ie: plane will stall before damage can occur.

High Speed Flight

At speeds of about 260kts, air is considered incompressible (its density remains nearly constant while pressure varies).  At high speeds, compressibility results in shock wave formation, drag increase, buffeting, and control difficulties because air is now accelerating at sonic speeds.

Critical Mach - Mach 1.0 is the transition from subsonic to transonic flows.

Compressibility effects occur when approaching critical mach because the air ahead of the aircraft is no longer "warned" of its arrival.  These air molecules become disrupted violently changing their pressure and temperature.  Immediately after the plane's passage, the air pressure increases, and subsequently drag increases.

Shock Waves - a factor in increase in drag can be delayed through the use of Sweepback.  Airflow hits the wing at an angle less than 90º resulting in the wing believing it is flying slower than it actually is.  Swept wings stall at tips before roots, however because the tip is behind the Center of Lift.