Ground Lesson 9: Performance, weight & balance and flight computers

• Predicting performance
• Your ability to predict the performance of an airplane is very important
• It allows you to
• Takeoff distance
• If you can safely clear obstacles in your departure path
• How long it will take to reach a destination
• Fuel requirements
• Landing distance

• Most of this information can be found in the POH
• Aircraft performance and design
• When developing performance charts, the manufacturer's make certain assumptions about conditions of the airplane and ability of the pilot
• The pilot
• Expected to follow normal checklist procedures  
• Perform each of the required tasks correctly and at the appropriate time
• The airplane
• Assumed to be in good conditions, with a properly tuned engine and all systems operating normal
• With the aid of these assumptions, the manufacturers develop performance data based on actual flight tests
• Chart presentations
• All performance charts apply to specific aircraft's
• You should only refer to the aircraft's POH for the model you are going to fly
• Generally present their info in either table or graph format
• Table format
• Find row and column that most closely match the conditions, and read the value
• Graph format
• Has more variables built into it, making it faster and accurate
• Factors affecting performance
• Two main factors are weight and wind
• The heavier you are, the more lift you need → giving you less power for thrust
• Wind can help or also hinder performance 
• Airplanes taking off or landing in strong wind have reduced ground roll
• In cruising flight, the groundspeed and time en route vary depending on the direction and speed of the wind
• Atmospheric conditions can decrease air density, Increasing the apparent altitude
• As pressure decreases, there are fewer air molecules in a given volume, so air density decreases
• When air is warm, it contains fewer air molecules than colder air
• Air containing water vapor is less dense than dry air
• Since aircraft performance decreases with altitude, it follows that decrease in air density due to temperature, pressure, or humidity will also reduce performance 
• When the air is less dense, the wings must move thru the air faster to produce enough lift for takeoff
• Lower air density also reduces engine power, since the engine must take a larger amount of air for combustion
• Takeoff and landing performance
• Depends on several factors you can measure and calculate in advance such as 
• Weight
• Wind
• Runway conditions
• Aircraft weight and configuration 
• To generate sufficient lift for flight, a heavily loaded airplane must accelerate to a higher speed than the same airplane with a lighter load.
• Since the acceleration will also  be slower, more runway will be needed
• You can see the effects of weight on the takeoff/landing distance chart
• Surface winds
• takeoff/landing distance can be influenced by direction and speed of wind
• Runway configurations 
• Aircraft performance data generally specify a paved and level runway with a smooth dry surface
• If any of these conditions are different, you must adjust the takeoff/landing distance
• Climb performance
• The POH lists airspeeds for a variety of climbing flight conditions 
• Two of the most important are best angle-of-climb airspeed(Vx) and the best rate-of-climb airspeed(Vy)
• Climbing speeds 
• The Vx is usually used for obstacle clearance immediately after takeoff
• Should be used to gain the maximum amount of altitude in the minimum horizontal distance
• Vy is usually used after you cleared all obstacles during departure
• Gives the best altitude gain in a given time
• Cruise performance
• Manufacturers of light airplanes provide cruise performance charts to indicate rate of fuel consumption, true airspeed, range, and endurance.
• Must compensate for nonstandard conditions
• Using performance charts
• Charts are provided for determining takeoff, landing, climbing, and cruising information

Weight and balance

• Importance of weight
• Almost every aspect of performance is influenced by weight of the aircraft and its content
• An overweight aircraft has a
• Longer takeoff run
• Higher takeoff speed
• Reduced angle and rate climb
• Reduced cruising speed 
• Shorter range
• Higher stalling speed
• Longer landing roll
• Loading an aircraft too heavily can dangerously decrease its performance, and increase risk of structural damage if you encounter turbulence or make a hard landing
• Aircraft manufacturers do extensive testing to establish safe limits for aircraft loading
• Importance of balance
• You can check the balance conditions of an airplane by locating its center of gravity(CG)
• Imaginary point where the aircraft would balance if suspended
• The location of this point is critical for the aircraft's stability and elevator effectiveness
• Improper balance of the airplanes load can result in serious control problems 
• You can avoid these problems by locating the CG before each flight and making sure it stays within limits
  
• Terminology
• Empty weight
• aircraft itself 
• Avionics
• Unusable fuel
• Gross weight
• Sum of the empty weight and useful load
• Max gross weight
• Maximum allowed weight to fly
• Useful load
• People 
• Cargo
• Fuel
• Payload
• Load that pays → passengers, cargo, baggage
  
• Ramp weight 
• Airplane loaded for flight prior to engine start up
• Takeoff weight
•  Maximum weight at which the pilot is allowed to attempt to take off
• Landing weight
• Maximum aircraft gross weight due to design or operational limitations at which an aircraft is permitted to land

• Center of gravity 
• The theoretical point where the weight is concentrated
• CG moves when weight shifts
  
• Datum
• A fix from where measurements are taken 
• Principles of weight and balance
• Arm of an object
• Horizontal distance from the datum to any point
• Total moment / total weight = total arm
• Moment of an arm
• Is the result of a multiplication made between an object and weight
• Force acting on an object weight from a distance
  
• Total moment
• All moments from an object added together
• Calculating the position of the CG
• Moment divided by weight
• Cg allowable range
• A range of position within an object where Cg is allowed
• Lateral
• Left or right of the vertical plane through the mast
• From nose to tail
• Left is “-“, right is “+”
• Longitudinal
• Forward or aft of the datum line
• Forward is “-“. aft is “+”
  
• Weight and balance management 
• Adding/ subtracting objects changes the weight and balance calculations
• Cg change of an object
• Change of equip. , payload, fuel, changes the cg of the aircraft
• During flight, fuel is consumed, passengers are dropped, cg will shift
• Computation method
• Multiply weight and arm to get moment
• Add weights and moments
• Divide total moment by total weight
• Table method
• Uses a series of tables provided by the manufacturer to eliminate the multiplication and division
• Moment table provided for each of the most common payload areas
• Front seat
• Rear seat
• Usable fuel
• Baggage area
• The manufacturer has taken many weights and multiplied them by the arm for that location
• Graph method
• Allows you to use values between increments published in a table
• Loading graph used to find the moment for the loads you intend to put into the airplane
• CG moment envelope tells you if your proposed loading is within the weight and balance limits

• Weight-shift formula
• Weight moved/weight of airplane = distance CG moved/ distance between arms
• Effects of operating at high total weights
• Wing must fly faster or at higher AoA to generate additional lift
• Takeoff roll is longer - landing distance is longer
• Angle and rate of climb is reduced
• In cruise, range is reduced and speed is lowered at any given power setting
• Closer to stalling angle, and stalls at higher speed
• Flight at various CG positions

Flight computers

• Mechanical flight computers
• Flight planning and enroute navigation require solutions to several different types of mathematical problems 
• The mechanical flight computer makes these common calculations easier 
• Has two sides
• Computer side
• For ratio type problems, such as time, speed, distance, fuel, conversions 
• Wind side
• For wind drift calculations
• Time, speed, and distance
• If you have two variables , you can find the third
• A scale is for distance, B scale represents time in minutes
• Speed index always points to the speed
• Airspeed and density altitude computations
• As air density decreases, indicated airspeed will be lower than true airspeed by about 2% per 1000 feet of altitude
• Use windows marked with pressure altitude/temperature 
• For TAS calculation, use the scale on the right of the DA window
• To calculate DA, use the same scale, and set the pressure altitude opposite the OAT, read the DA in the center window
• Wind problems
• Correcting for wind drift
• Pointing the nose slightly towards the wind will keep you on course
• Conversions
• Multi-part problems
• Sometimes you may have to solve a series of related problems to to obtain a particular solution
• Example: to find the fuel required for a proposed flight, you must first find the true airspeed, then ground speed and time enroute, then fuel required
• Electronic flight computers