Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Aircraft velocity question

  1. Nov 20, 2014 #1
    if any object is in aircraft so it will have velocity same as that of aircraft ,right ? and say in case it is thrown out of aircraft in between will it retain that velocity provided that no external force act on it?
     
  2. jcsd
  3. Nov 20, 2014 #2
    If I understand right, the object has zero velocity in it's own reference frame, until it's dropped out of the plane, in which it would have the same velocity of the plane.

    Much like a cup on a dashboard of a car; it's not moving from it's spot on the dashboard until you slam the brakes or have another external force acting upon it.
     
  4. Nov 20, 2014 #3

    Orodruin

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    Yes. However, there is typically air outside of the aircraft, meaning that the object you throw out will be subject to air resistance.
     
  5. Nov 20, 2014 #4
     
  6. Nov 20, 2014 #5
    so ,when it is thrown out of aeroplane it would have same velocity as that of aeroplane,is this because of inertia?[/QUOTE]
     
  7. Nov 20, 2014 #6
    From my understanding of Newtons first law of motion, that appears to be correct :)
     
  8. Nov 20, 2014 #7



    Practical example of the aircraft with things being dropped from it includes bombers in WW2. You are defensive machine gunner responsible for defending American bomber against German attacking fighter aircraft.

    leading-from-behind.jpg

    The phenomenon really made it difficult sometimes for defensive gunners to shoot accurately at enemy aircraft. Look at the picture and decide if it looks counter-intuitive or not!!! oo)

    Defensive turret gunner had to calculate "lead shooting" because normally a moving target requires lead shooting (I'm sure this applies even with stationary shooter, and moving target example of a duck hunter with a shotgun for example)

    But the bomber always had to keep going forward. That was the tactic of the day. Tactic called for staying in formation with the bombers and not making evasive moves. (they would cost lot of fuel, and bombers would not reach distant bombing targets and still make it back).
     
  9. Nov 20, 2014 #8

    when you throw something out of the aircraft, it implies directly that external force IS acting on it.

    if you drop something from aircraft, like a bomb, then you release the bomb, and allow gravity to take over the effect.

    initially the bomb carries certain amount of forward velocity, but you have to realize that the aircraft is no longer attached to the bomb anymore. When the aircraft is no longer attached to bomb, aircraft speed cannot keep the bomb from moving forward similarly as the aircraft. Aircraft velocity force no longer affects bomb. The bomb does have momentum though.

    The difference is that the direction of the forces changes to my understanding, when you compare dropped bomb from aircraft, into a bomb inside the aircraft when aircraft is moving forward.



    It's a ballistic projectile with no forces effecting it except gravity and drag. Also, the bomb would produce certain amount of lift also, because of its bodyshape I would imagine, at least initially when the bomb is "sideways" instead of pointed downwards. This is how sometimes GPS guided bombs have guiding "flight surfaces" which can aim the bombs, and bombs are encased into aerodynamic shells, (JDAM). Also different so-called gliding bombs have been invented in which the purpose is the maximise the flight time of the bomb. (flight time forwards)


    When you drop the bomb, aircraft velocity forward, (force vector) no longer affects upon the bomb.

    However, because the bomber was moving, and it dropped the bomb, it stands to reason that momentum was conserved in the bomb.

    I'm not terribly sure how one should interpret the effect of forward flying bomb, from the result of bomb being dropped from forward flying aircraft. There does exist force for short time period somehow, when the bomb is dropped from aircraft.

    But clearly, when the bomb is droppped, the aircraft velocity no longer has any meaning to the bomb's movements. You could slow down in the aircraft, after you drop the bomb, it would not affect the bomb's velocity anymore.

    The dropped bomb pushes against air, air pushes against bomb, this creates drag.

    Dropped bomb pulls toward earth and earth pulls toward dropped bomb, this creates gravity.

    Dropped bomb should now tend to accelerate toward ground, because gravity would be the principal force.

    Then the drag keeps increasing, because speed has increased massively at this point. The speed is downwards though at this point. (towards core of earth) Gravity is the force vector now, I suppose.

    When speed increases, because gravity is more than drag in dropped bombs (aerodynamic shape of bombs), then this means acceleration has occured.

    But because speed increases, it should be remembered that drag increases when speed increases. Eventually there comes point when drag and gravity are balanced. Then acceleratin stops and bomb keeps falling at the same speed to earth's core.

    Earth is also dropping slightly toward the bomb (but the effect of earth's movement or acceleration toward the bomb is truly miniscule to be honest)
     
    Last edited: Nov 20, 2014
  10. Nov 20, 2014 #9
    terms like retain velocity... feels little bit imprecise when discussing problem of dropping something like a bomb from aircraft, on earth, with drag.
    http://www.physicsclassroom.com/mmedia/vectors/pap.cfm

    When you drop a bomb from aircraft (imagine that bomb is sideways in under the fuselage)

    The bomb has inertia characteristic

    The aircraft has momentum forward also. When you drop the bomb, the aircraft thrust vector no longer affects bomb. But the momentum forward is conserved in the bomb, even when bomb is dropped.

    But I should mention that the bomb does feel drag even in this sort of configuration. Horizontal momentum is conserved true enough, but when bomb flies horizontally, drag opposes bomb. This is the part where the above physics link was mistaken. The effect is small, but there does exist drag upon the package, when the package flies horizontally due to the momentum vector horizontally. Furthermore the above physics link is mistaken in the belief that projectiles are only those objects in which the affecting force is gravity. This notion is an arbitrary definition of projectile. Bullets on earth, are affected by drag and gravity, and they are projectiles also. That's my opinion on the subject if you ask me. US Department of Defense also agrees that projectiles are affected by both drag and gravity. ;)

    If you had a case, where there is no air resistance, then it seems that the bomb would fly horizontally with constant velocity.

    But gravity would also effect the bomb, I think. And gravity presumably would change the direction of the bomb.
     
    Last edited: Nov 20, 2014
  11. Nov 21, 2014 #10

    A.T.

    User Avatar
    Science Advisor
    Gold Member

    It becomes less counter-intuitive if you consider the velocity of the fighter relative to the bomber, which is not south, but south-west.

    But the picture is also misleading, probably because it is not correctly scaled. What it shows would apply if the fighter was still further north (which probably was the normal situation when the gunner started firing).

    In the situation shown in the picture however, the gunner should aim at the fighter if the planes have the same speeds, and in front of the fighter if the fighter is faster than the bomber (the likely case). So the picture is physically wrong, but it gives the right practical advice for the start of the fire. Real life was more complex anyway because the fighters were changing direction.
     
  12. Nov 21, 2014 #11
    Usually the fighter was almost always quite a bit faster than bomber. Reasoning was that the German pilots figured that speed would give them a measure of safety (at least in their mind!)

    Fighters would be making diving strafing passes at the bombers. At speeds up to 650- 700 km/h true airspeed.

    Let's say that Fighter is moving at 650 km/h and bomber is moving at 450 km/h
     
  13. Nov 21, 2014 #12

    A.T.

    User Avatar
    Science Advisor
    Gold Member

    450 km/h is nearly the maximal speed for the bombers like B-17, B-26. Their cruise speeds where around 300km/h. I think that speed ratios of up to 2:1 are realistic.
     
    Last edited: Nov 21, 2014
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Aircraft velocity question
Loading...