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View Full Version : Car on a Trailer/Plane in a boxcar

Tog
2008-Dec-25, 09:08 AM
I the spirit of Plane on a Conveyor Belt, and recalled from years ago by the Van Full of Budgies, I've got a question about a plane in a truck trailer (boxcar just sounded better).

Most everyone has seen the stunt where a car is driven up onto a moving trailer. If the trailer is going 30 units of choice per hour, and the car is doing 33, the car will overtake the trailer at 3 units per hour, but as soon as the car hits the trailer the tires will be going 33 relative to the trailer, and thus accelerate to 63 relative to the ground, assuming the driver does not back off of the throttle. Is this right? It seems right, but is it?

In the same line, what if a remote controlled plane were flown into the open back of a box trailer? Let's say the trailer is doing 50 kph and the plane is doing 60. The plane would gain on the trailer at 10 kph, but what would happen when the plane crossed the threshold where the air in the trailer was not moving relative to the trailer? Assuming of course that the plane were able to pass through the turbulence coming off of the back of the trailer.

I've always figured that the plane would have a sudden acceleration, but as I think about it now, I think the plane would drop to the floor of the trailer since the airspeed would drop from 60 kph to 10. At least until it got back up to speed or hit the front of the box.

JohnD
2008-Dec-25, 10:00 AM
Unless you saw the original '69 Italian Job, this will be nonsense.
If you did, it's brilliant but hallucinatory.

Happy Christmas
John

princemyheart
2008-Dec-25, 11:09 AM
The plane is travelling relative to the trailer. There is no contact with the trailer, so no change once in the trailer's airspace.

The air would of course be turbulent and effect the performance of the plane.

The car's wheels coming in contact with the ramp [if front wheel drive] transfers the road speed to the surface of the ramp and therefore relative to the ground accellerates accordingly... ??

grant hutchison
2008-Dec-25, 11:34 AM
The car, which is initially travelling at low velocity relative to the truck, isn't going to suddenly accelerate and smash into the front of the trailer at high relative velocity: for that to happen, it would need to pick up a lot of energy from surface traction in a very short distance. (Such a car would be able to accelerate zero to sixty in a trailer-length, which would be a sight to see.)
Instead, you get immediate wheel-spin when the drive wheels hit the ramp, and need to declutch and get into first gear before the engine stalls. Then you need to power it up the ramp and hit the brakes before you drive into the front end of the trailer at (relatively) low speed.
As I recall, the driver of the modified bus used in the original Italian Job was protected by a reinforced wall and a lot of padding, and was shifted forwards a couple of inches by the low-speed impact from behind.

Grant Hutchison

mugaliens
2008-Dec-25, 11:34 AM
...the car will overtake the trailer at 3 units per hour, but as soon as the car hits the trailer the tires will be going 33 relative to the trailer, and thus accelerate to 63 relative to the ground, assuming the driver does not back off of the throttle. Is this right?

When the car hits the ramp, the wheels may be going 60 mph relative to the trailer, but the car is only going 3 mph relative to the trailer. The wheels rapidly decelerate, while imparting a slight acceleration to the car, perhaps 1 mph.

However, the latter is quick absorbed by the fact that the car is going up a ramp. In practice, the driver must mash down on the accelerator to move up the ramp. For cars with automatic transmissions, it's a bit of a shock, and for manuals, the driver essentially overtakes the other vehicle around 10+ mph, mashing the clutch a split-second before the drive wheels contact the ramp, then rapidly shifting into first for the trip up the ramp.

In the same line, what if a remote controlled plane were flown into the open back of a box trailer?

The moment it reached the mass of air moving along with the trailer, it would lose lift and dive to the floor of the trailer, probably breaking a prop in the process.

I've always figured that the plane would have a sudden acceleration...

You're right, as the zero drag and maximum bite of the prop would result in it rapidly accelerating forward compared to the ground.

...but as I think about it now, I think the plane would drop to the floor of the trailer since the airspeed would drop from 60 kph to 10.

That too! Both factors would be at work.

At least until it got back up to speed or hit the front of the box.

Precisely - if it got back up to speed rapidly enough, it could recover and fly through the box. Until it goes BLOOEY! when it hits the front end.

IfUnless you saw the original '69 Italian Job, this will be nonsense. If you did, it's brilliant but hallucinatory.

Happy Christmas
John

That's awesome! Exceptionally well done.

Here's what the original looked like (http://uk.youtube.com/watch?v=j0nXDOr1r6A&feature=related).

Tog
2008-Dec-25, 11:58 AM
Instead, you get immediate wheel-spin when the drive wheels hit the ramp, and need to declutch and get into first gear before the engine stalls. Then you need to power it up the ramp and hit the brakes before you drive into the front end of the trailer at (relatively) low speed.
Grant Hutchison

Okay, that's about what I figured. On the old Knight Rider they did this at least once per episode and there was always a puff of tire smoke when the car transferred to the ramp. I never thought the actual car would have an instant acceleration, though in re-reading what I wrote it does come off that way. Just that the wheels would have a reaction to trying to travel at 33 when the rest of the car is doing 3.

Thanks for the replies.

joema
2008-Dec-27, 01:54 PM
...the stunt where a car is driven up onto a moving trailer. If the trailer is going 30 units of choice per hour, and the car is doing 33, the car will overtake the trailer at 3 units per hour, but as soon as the car hits the trailer the tires will be going 33 relative to the trailer, and thus accelerate to 63 relative to the ground, assuming the driver does not back off of the throttle. Is this right?...
The TV show Mythbusters investigated this in the 2007 episode "Big Rig Myths", the segment title was "Knight Rider Ramp".

They found at both 30 mph and 55 mph, you could safely drive a car on and off a ramp to a moving semi trailer.

http://en.wikipedia.org/wiki/MythBusters_(season_5)#Knight_Rider_Ramp

JohnD
2008-Dec-27, 11:32 PM
As I recall, the driver of the modified bus used in the original Italian Job was protected by a reinforced wall and a lot of padding, and was shifted forwards a couple of inches by the low-speed impact from behind.

Grant Hutchison

Grant,
I'm quite sure you are right about the driver protection, and accidents in practice will have been invitable. But they seem to have got it right for the takes used in the film. While the red and white cars' entry to the bus could have been studio shots with a moving background, the blue car is shown from a car mounted camera - and it doesn't hit the white car!
See: http://video.google.com/videosearch?q=Italain+Job+69&emb=0&aq=f#q=Italain%20Job%2069&emb=0 19:40 to 20:14 minutes.

All the Minis are suddenly very much cleaner as soon as the bus doors close!

John

nauthiz
2008-Dec-27, 11:56 PM
However, the latter is quick absorbed by the fact that the car is going up a ramp. In practice, the driver must mash down on the accelerator to move up the ramp. For cars with automatic transmissions, it's a bit of a shock, and for manuals, the driver essentially overtakes the other vehicle around 10+ mph, mashing the clutch a split-second before the drive wheels contact the ramp, then rapidly shifting into first for the trip up the ramp.

There's also the detail of whether the car is front or rear wheel drive; that determines when the downshift would need to be made.

I'm having a hard time figuring out if this would be a good idea or not in an all wheel drive vehicle. I'm guessing it depends on details of the drivetrain such as whether it has a traction control system or not.