Airplane, Car, Relative Motion ```Name: Rory Status: student Age: N/A Location: N/A Country: N/A Date: N/A ``` Question: If you are riding in a car and a radio controlled toy airplane is flying outside, parallel to the car and you fly it into an open window, would it just hover next to you? Replies: Under normal conditions, the airplane would not continue to fly but would settle in the seat next to you. The airplane requires a relative wind moving over the wing to create lift. While flying next to you, it has the same relative wind as your car. When it moves into the car, it would now experience a relative wind based on the air held in the car. It would not be sufficient to maintain flight. If the car were large enough (really big) the plane could accelerate in the inside air (moving as fast as the car and airplane) and develop a flying speed. It would then need to remain this speed within the car to continue flying. The key to creating lift with an aircraft wing is relative wind. If you find this interesting, you might want to explore wind shear and the effect on aircraft.... particularly landing and departing aircraft. Larry Krengel Rory, Sounds great - and a great plot for a story, but unfortunately not possible. The plane will travel relative to the air in which it is flying. Inside the car the air is relatively calm, being pushed along by the back window and protected by the windscreen. As soon as the plane is inside the car it will fly at speed RELATIVE TO THE AIR IN THE CAR and therefore relative to the car itself. I for one would not wish to be in the car when you try this stunt - it could get very messy and painful!! Nigel Skelton Tennant Creek High School Australia Rory, Here is my mental experiment. I thought of two things: Scenario 1 If the radio controlled airplane is flying at the same speed the car was traveling, it might be tempting to think that if it took a little jog through the window, it would just hover there. However, the little airplane still has to obey the laws of Physics including the whole Bernoulli thing, you know, lift. As long as the air is passing over the wings, the little guy has lift and can fly. Supposing it could make it through the car's slipstream without getting off course or beat up and get through the window, I think it would fall on the seat next to you. Inside the car, you are behind a windshield, this will prevent the airplane from gaining or maintaining lift. Now it might be momentarily fly forward into the windshield as it moves through the car's interior, but it will run out of room real fast. Senario 2 Now I though,t how might this happen? I think you would have to be in some sort of open-air carriage. A convertible has a windshield and I am not real sure how far the air is interfered with. It might work. Then I thought of something like a car going slowly, not highway speeds. A horse drawn carriage would work. I am pretty sure you could get the airplane to hover quite near in the carriage as long as the horse was not bothered by the noise of the plane. So those are my two answers to your thought experiment. If you ever do test it, yo willl have to let me know how it all turned out. ( Please don't try to drive and operate the RC plane, get a designated driver or pilot.) Happy Science, Martha Croll Rory No, if a toy airplane flying along the side of a moving car flew in the car’s window, it would be flying around inside the car. It would not just be hovering because the airplane; because The toy airplane only experiences the wind stream resulting from its flight regardless of whether the wind stream is moving or not. On a calm wind day the ground speed of the toy airplane outside of the car would simply be equal to its air speed: Its speed in the air. Inside the car, ( if the toy airplane is still facing forward in the car) the toy airplane’s ground speed would be its air speed in the car plus the speed of the car over the ground. Sincere regards, Mike Stewart The airplane needs air to flow over its wings to generate lift. The lift due to the wind flowing over the wings is what allows the airplane to fly. The air in the car is moving at (close to) the same speed as the car. Inside the car it would no longer feel the wind, and therefore would no longer generate lift. So no, it would not hover next to you. Also, as a practical concern, there is a lot of unpredictable air flow around the car - as the plan approaches the car, the buffeting winds close to the car would likely cause the plan to lose control and possibly crash. Hope this helps, Burr Zimmerman Rory - As you slid your RC plane through the window into the car, it might seem to hang next to you for a briefly as it started to fall and accelerate forwards. When you let an object go and it falls, does that seem smooth or abrupt to you? Both points of view are OK, but either way, that's about how fast the plane would start to move upon entering. What is changing, as seen from the airplane's perspective, is the airspeed around it. It goes from 60 mph to 0 mph in a second or less. So then the wings cannot have any lift, so it stalls and falls. And the air-drag disappears, so the propellers' steady thrust gradually starts to accelerate the plane forwards. It is not clear whether the propeller's thrust is more at 60mph airspeed or 0 mph airspeed, but the drag just went from 100% of thrust to 0% of thrust, so even with no change in thrust, there is suddenly plenty of unbalanced force to cause acceleration. I the car were huge enough for a plane to fly around inside, say a blimp with an open hole in the side, then the plane would eventually be flying 60mph within the blimp, and burst out through the nose of the blimp. After that the plane would find itself in 120 mph airspeed, and accelerate backwards due to excess drag, until it was flying ahead of the blimp at 60mph matching speed.. My point is that changing airspeed suddenly ends the equilibrium of thrust and drag that the toy had before. The plane will accelerate (positive or negative) to reach a new equilibrium at 60 mph perceived airspeed (or whatever speed you were flying at). If the RC toy was helicopter instead of a plane, it might do a bit differently in the vertical direction. but the onset of forwards acceleration would still happen, unless the helicopter adapted rapidly. Modern RC helicopters might just adapt that fast, because they all have on board electronic gyro-sensors adjusting their attitude and thrust faster than the operator can, and they could easily be given accelerometers too. So then there would be some size of forwards lurch upon entering, after which a good hover could commence. Jim Swenson Rory, Einstein might say that this is a good gedunken (thought) experiment. If you think of the reasons why the plane might hover, you might state the fact that the plane is traveling the same speed as it was outside of the car and so it should continue to hover right next to you. However, if you start to explore the reason why a plane can fly in the first place, then you will discover that if you were lucky, the plane would fall nicely into your lap and you would be able to fly your plane another day. It would not in fact hover because its ability to fly has been removed. Planes fly because of the difference in pressure between the air above the wing and the air below the wing. The shape of the wing is such that it is pretty straight and flat on the bottom of the wing and it is curved on the top of the wing. As a plane moves forward, the wing "cuts" into the air and half of it must go above the wing and half below the wing. The half that goes above the wing has to travel farther because of the curvature. Without getting too technical, this causes a lower pressure on the top of the wing and allows the higher pressure below the wing to push the plane up. It is the shape of the wing and the fact that wind is traveling so fast over and under the wing that allows it a plane to fly. If your model plane was to fly into your car through the window, it would stop the air from traveling so quickly passed the wing. If you were in a convertible car the wind would still be there and so it would still be able to fly. Matt Voss Rory, The plane would fall to your lap. The wings of a plane do not “know” how fast the ground appears to move. Flight is based on how fast the air around the wings appears to move. If the plane moves fast through the air, or if the air moves fast past the plane, then the plane flies. After entering the car, the plane and the air around it are moving together. From the plane’s “point of view”, the air is not moving. A plane can fly while moving backwards relative to the ground, provide the air is moving backwards much faster than the plane. A simpler example is a kite. If the wind blows strongly, the kite flies in the moving air without moving along a ground. If there is no wind, you can fly a kite from a moving car. The kite flies while moving quickly through the air. Dr. Ken Mellendorf Physics Instructor Illinois Central College Good question. The airplane gets its lift, the force that keeps flying from the speed of the air moving over its wings. The air inside the car would be going at the same speed as the car and the airplane. When the airplane came in the window it would be moving at the same speed as the air inside the car. No air would move over the wings. It would lose lift and crash. A similar thing happened a number of years ago for real. A Delta jet was landing at Dallas-Fort Worth airport. A strong wind from a thunderstorm hit the plane from the rear and stopped the air moving across the wings from in front. Even though the plane was moving at a pretty good speed, the wings acted as if it were standing still. The plane lost lift and crashed. Even though the plane was moving at a pretty good speed, the wings acted as if it were standing still. The plane lost lift and crashed. The same thing would happen to your plane as it came in the window - it would crash Hope this helps. Bob Avakian OSUIT, Okmulgee, OK Click here to return to the Physics Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (help@newton.dep.anl.gov), or at Argonne's Educational Programs