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Question about newton third law and NASA article

  1. Nov 18, 2014 #1
    I was confused by a NASA pdf document about flight physics. I provide quote from NASA document instructor's flight manual, flight testing Newton's laws. Document is categorized DFRC-X41-1.

    Did NASA use some fast-and-loose language right there? Imprecise definition of drag using newton third law?

    When thrust is more than drag, then acceleration occurs. Correct or not? When thrust and drag are equal, then aircraft remains at constant speed, unchancing speed.

    Drag exists because aircraft pushes against airmass and airmass pushes against aircraft. Correct or not?

    No drag exists in outer space. Rocket in vacuum has thrust, surely rocket can accelerate in outer space. Correct or not?

    In outer space vacuum, rocket has thrust. Newton third law states that fuel gases push upon rocket body. Rocket body push upon fuel gases. Basically. No mention of drag and supposed relationship between thrust exists in this case of outer space vacuum rocket.

    In practical terms outer space vacuum contains hydrogen atoms (?) so it's not a pure vacuum. But surely there must be some sensible reason behind this NASA logic? Can you guys help me understand newton third law with respect to the normal aircraft in atmosphere, and also vacuum rocket with thrust.

    I was also wondering about another unrelated matter about flight physics, related to the compressibility phenomenon with certain WW2 aircraft. One such aircraft was P-38 Lightning. I was wondering what exactly prompted the flight surfaces to become so ineffective with this aircraft for example. The true cause was not understood during WW2, except that they installed a stopgap measure of an airbrake, to slow down a diving fast aircraft.

    I remember watching a history channel show about the compressibility phenomenon of a P-38 aircraft. The documentary noted that the propeller blades became transsonic, which caused some kind of disturbant airflow to the flight surfaces. (such as elevator tabs in the tail)

    Could the compressibility be explained in somehow more layman's terms? Why it happens at certain speed, as opposed to slower speeds that normally aircraft fly at ?
    Compressibility tended to happen at high speed dives, during air combat. This made it difficult for a P-38 pilot to pull enough elevator to pull up from a dive. This tended to cause pilot deaths to crashes.

    Soemtimes the thicker atmosphere at low altitudes allowed regaining of control for elevator tabs.
  2. jcsd
  3. Nov 18, 2014 #2


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    The Newton third law pairs aren't quite right. Assuming a propeller, then the propeller exerts thrust on the air, and the air exerts and equal and opposing force onto the propeller. The wings exert a downwards and somewhat forwards force on the air, and the air exerts and equal and opposing force onto the wings.

    The "balance in forces" describes an aircraft in horizontal flight at a steady speed. Thrust doesn't have to equal drag, and lift doesn't have to equal weight. If thrust is greater than drag, the aircraft accelerates forwards. If lift is greater than weight, then the aircraft centripetally accelerates upwards relative to it's own frame of reference (such as a banked turn).

    As for a rocket in space, assume a frame of reference where the rocket is initially at rest and that there are no external force (such as gravity). The center of mass of the rocket and it's fuel never moves with respect to that frame of reference. When the engine is operating, the pumps and engine exert a force onto the burning fuel mixture, and the fuel mixture exerts a force onto the engine and pumps. The fuel accelerates in one direction, and the rocket accelerates in the other direction. The fuel effectively has a terminal velocity relative to the rocket, and this velocity is used in equations for determining how the rocket will move. If the initial mass of rocket and fuel is nearly all fuel, then as the fuel is nearly depleted, the rockets velocity relative to the original frame of reference can exceed the fuel's exit velocity relative to the rocket.

    Two issues. One is that the control surfaces cease to function normally, the other is mach tuck. Wiki article:

    Last edited: Nov 18, 2014
  4. Nov 18, 2014 #3
    So NASA was wrong and I was right according to the quotation?

    Aircraft suffers drag upon its body. Drag increases as the speed increases.

    Drag seems to be based on surface area of the body somehow.

    Isnt that the reason behind the principle of an opened and effective parachute?

    When the parachute is packed inside a bag, surface area seems small.

    When the parachute surface extends open drag increases quite a bit...?

    I suck at physics but it seems that sometimea even NASA makes mistakes too :D
  5. Nov 18, 2014 #4


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    NASA has a well known problem with explaining Newtons 3rd Law correctly. See for example:
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