Climb Performance

Introduction:

  • Climb performance is a measure of excess thrust, which generally increases lift to overcome other forces, such as weight and drag
    • This is true for most aircraft, although some high-performance aircraft can function like rockets for a limited time, utilizing thrust to lift away from the earth vertically, with no lift required
  • Excess power or thrust, terms that are incorrectly used interchangeably, allow for an aircraft to climb

Power vs. Thrust:

  • Power and thrust are not the same, despite their use as such
  • Power is a measure of output from the engine, while thrust is the force that actually moves the aircraft
    • In a piston aircraft, power is converted to thrust through the propeller
    • In a jet aircraft, the engine produces thrust directly from the engine
  • When you are moving the throttle controls inside of the aircraft, you're controlling the engine, and that is why they are referred to as power levers
  • Therefore, the best angle of climb (produces the best climb performance with relation to distance, occurs where the maximum thrust is available
  • The best rate occurs where the maximum power is available)

Propulsion vs. Drag:

  • The relationship between propulsion and drag is such that it takes a certain amount of power/thrust to overcome drag both on the high end (the faster you go) and also on the low end (the slower you go)
  • This is noticeable during slow flight, where you find yourself adding extra power to overcome all the increases in drag that are necessary to sustain lift
  • If you fall "behind the power curve," however, you're in a position where you cannot generate immediate performance by simply increasing power
    • The increase in power must first overcome the increased drag, and then the expected performance will occur
  • You can learn more here: https://www.aopa.org/news-and-media/all-news/2013/november/pilot/proficiency-behind-the-power-curve

Best Angle vs. Best Rate of Climb:

  • Ultimately, it is because of excess power (or thrust) that an aircraft climbs
  • For the purpose of the initial climb, however, we are concerned with our aircraft's performance to get away from the ground
  • Certain conditions will call for a specific climb profile, generally the best rate (Vy) or angle (Vx) of climb

  • Best Angle-of-Climb:

    • Max excess thrust results in the best angle of climb
    • Occurs at L/Dmax for a jet
    • Occurs below L/Dmax for a prop
    • Reduced distance to climb to the same altitude as Vy, but reaches that altitude slower

  • Best Rate-of-Climb:

    • The best rate of climb, or Vy, maximizes velocity to obtain the greatest gain in altitude over a given period of time
    • Vy is normally used during climb after all obstacles have been cleared
    • It is the point where the largest power is available
    • Occurs above L/Dmax for a jet
    • Occurs at L/Dmax for a prop
    • Provides more visibility over the cowling
    • Increases airflow over the engine while at high power
    • Provides additional buffer from stall speeds
    • Takes more distance to reach the same altitude as Vx, but reaches that altitude quicker
  • Airplane Flying Handbook, Best angle of climb verses best rate of climb
    Airplane Flying Handbook
    Best angle of climb verses best rate of climb

Factors Impacting Climb Performance:

  • There are several factors which can impact climb performance:

    • Aircraft Weight:

      • One of the most basic considerations with regard to aircraft performance is weight, as it is a principle of flight
      • The higher the weight of an aircraft, the more lift will be required to counteract

    • Temperature:

      • Ambient air temperatures impact an aircraft's performance based on their physical properties
      • Engines don't like to run hot, and if they do, then reduced throttle settings may be required
      • Temperature is also a leading factor in determining the effect of air density on climb performance
      • Consider utilizing a cruise climb once practical to increase airflow over the engine

    • Air Density:

      • Air density, and more specifically, density altitude, is the altitude at which the aircraft "thinks" it is at
      • Performance does not depend on the physical altitude, but rather the density altitude, and the higher the temperature, the higher that altitude
      • As the engine and airframe struggle to perform, expect changes to characteristics like a reduced climb attitude

    • Winds:

      • Headwinds increase performance by allowing wind flow over the wings without any forward motion of the aircraft
      • Tailwinds do the opposite

    • Aircraft Condition:

      • Smooth, parasite-free wings produce the best lift
      • Anything to interrupt the smooth flow of air or increase drag will require additional forward movement, or thrust, to overcome

    • Icing:

      • Increased drag will require increased power, and therefore, during the climb, may result in decreased climb performance

Determining Rate-of-Climb Requirements:

  • Used to determine the rate of climb for a given departure/climb out

    • Formula:

      • Ground Speed (GS) (knots) ÷ 60 * Climb Gradient (Feet Per Mile)

    • Example:

      • Ground Speed = 75 knots
      • Climb Gradient Required = 200 feet per mile

    • Calculate:

      • 75 ÷ 60 * 200 = 250 feet per minute climb rate required

Conclusion:

  • Climb performance is governed by FAR Part 23, depending on aircraft weight
  • Pilots may always deviate from climb numbers for factors like cooling or the ability to locate and follow traffic
  • Remember, when flying under instrument conditions, minimum climb gradients are expected unless a deviation is communicated and authorized as applicable
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References: