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Helicopter in a box....

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Helicopter in a box....

Old 3rd Dec 2004, 02:32
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Helicopter in a box....

Dear fellow PPruners,

Myself and my colleagues have been wondering about the following conundrum, and your help would be greatly appreciated...

Picture this - A Bell 206 in a huge cardboard box. The box, with the aircraft inside it is sitting on a massive pair of bathroom scales which show the weight of the helicopter plus that of the box. Now, the pilot is already inside the aircraft inside the box and lifts into a hover.

What do the bathroom scales show now???

We really want to know!

After several discussions with various people, we cannot come to a conclusion. Some people think the scales would show the same weight, others think they would show less.

Regards,

Fay-Slag
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Old 3rd Dec 2004, 02:46
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Assuming the box was airtight, then the weight of the box would include, the box, the Jet Ranger, and the air. It would still contain all of the above whether the JR was on the skids or in the air, ergo the weight would not change.
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Old 3rd Dec 2004, 03:03
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but why will the scales read the same? - is it because the downwash equals the weight of the helicopter...or is it because the helicopter sucks the roof of the box down equivalent to the weight of the helicopter.....


or is it a bit of both?

many thanks,
Fay Slag
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Old 3rd Dec 2004, 06:16
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Helicopter on skids = weight of helicopter supported by bottom of box.

Helicopter in air = Helicopter supported by moving an equal mass of air downward.

Scale doesn't know or care how the helicopter is being supported or if air in box is being moved.

Hope I'm making sense to someone but me!
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Old 3rd Dec 2004, 06:28
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Why would the scales change? It's the same question as the trucker with the load of canaries. When he pulls onto the scales, he's overweight whether the canaries are flying or not.
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Old 3rd Dec 2004, 08:37
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Warning: rambling answer from an insomniac physics major contained within.

If the box is sealed, the helicopter and box form a closed system. Physically speaking, the question becomes not what the system weighs, but what the scale will measure.

If the helicopter is resting on the bottom of the box or hovering, the center of mass of the box + helicopter system is stationary. Thus, there is no net force and the force on the box from the scale is equal to the weight of the system. If the helicopter is descending, the center of mass is as well. The net force is downward (the gravitational force is greater than the upward force from the scale), and the scale will read low. The opposite is true if the helicopter is ascending.

The trucker with the canaries, however, will find his weight changing as they take off and land (unless his truck is sealed, and then the canaries won't be flying very long!).

I'll stop there because I should be studying for my thermo and electro exams instead of reading PPRuNe
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Old 3rd Dec 2004, 09:56
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Smile

Perhaps people should be 'thinking outside the box'.
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Old 3rd Dec 2004, 11:47
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What on earth has cog got to do with it? If CoG affects the reading on the scales, does that mean that the scales would read different if the pilot were sat down or stood up?

In a steady hover, the air is accelerated by the blades and the mass of this accelerated air causes a reaction force equal to the mass of the helicopter. The bottom of the box decelerates this air, causing a reaction which is measured on the scales. I agree that the reading on the scales will change if the aircraft climbs or descends within the box, since the aircraft will be accelerating, but once in a steady climb, the scales will return to a steady reading. In other words, the scales would read the mass of the helicopter plus box, unless the aircraft was starting/ stopping a climb or decent. What is more, the hover height will not affect the scales (ie independant of ground affect).

I wish Santa would bring me a Jet Banger in a box, or even better a twin squirrel....... he can keep the scales!
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Old 3rd Dec 2004, 12:17
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The center of mass of a closed system can be considered a point where all mass is concentrated. Any external forces that act on the system (like the scale pushing up from the earth in opposition to the weight of the box) effectively act upon the center of mass. The position of the CoM within the system (which is not necessarily the CoG, though on Earth I suppose it would be) is irrelevant in this case. What counts is the vertical component of its velocity

The rotor downwash has no measurable effect on the outside of the box, since energy cannot enter or leave a closed system.

Can't say I'd mind a heli for Christmas either, though
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Old 3rd Dec 2004, 13:32
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C of G could change - if you moved the helicopter from one end of the box to the other, but the total weight on both scales wouldn't.

If you entered autorotation or did a low-G pushover, for a moment the box would show less weight.

If you flew long enough, the box would show less total weight, as some of the mass of your fuel would have been converted into energy (heat and noise). Probably wouldn't be able to measure it on a bathroom scale though.

What if the box were on the back of a truck and you drove it through a TFR while hovering the helicopter inside the box? Is it a bust?
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Old 3rd Dec 2004, 14:01
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As to the truck driver....if he let some air out of the tires....would that not reduce the air pressure being exerted upon the exterior of the tire thereby reducing the weight on the scale....in that the scale measures the pressure exerted against the platform?
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Old 3rd Dec 2004, 14:02
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Just for fun, why not take a radio controlled helicopter, put it in a large box on some scales (or probably easier to hang from a spring balance) and try.

G
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Old 3rd Dec 2004, 14:38
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Hey all,
I'm new to this forum, i'm an air cadet and fly with a Volunteer Gliding School at RAF Halton. I wondered if any of you know of similar problem solving questions like this one which i could give to the cadets at my squadron so as to get them thinking! Any help would be appreciated!
Thanks Matt
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Old 3rd Dec 2004, 15:04
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Find your offshore oil rig....

Bell 296B-III helicopter

Leg length: 90nm

Indicated Airspeed: 90 knots

Track: 180 degrees magnetic

Variation: 5 degrees west

Deviation: +5 Degrees for 180 degrees heading

Wind: 270/10

Visibility: 5nm


Remarks....No GPS, No NDB on rig, only fuel is on the rig. Mag compass and clock for navigation.

Weather is forecast to remain VFR for the duration, no frontal passage forecast. Sky clear. Departure point is the shoreline...rig is in open water.

How would you plan the flight???
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Old 3rd Dec 2004, 18:01
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Not very PC but all this talk of boxes reminded me of a question.

If with a male crew an aircraft has a cockpit would it with an all female crew be called a box room?
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Old 3rd Dec 2004, 18:42
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Hey Sassy!!

Is that one of those new 3 engined Bells?
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Old 3rd Dec 2004, 18:52
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OOPS!

One cup of coffee shy of enough this morning....sorry! I meant the old proverbial Jetranger Short. Freudian slip I fear!


Per the request....all female crew sits in the "Box Office".
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Old 3rd Dec 2004, 21:19
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Matt,

Are you looking for aviation specific stuff, or just physics riddles in general? The question in this thread is typically posed in physics classes as the "bird in a bottle experiment," so you could try googling that for a starting point.

Here's another one that gets people (Robert Oppenheimer famously embarassed himself with this one). Imagine you're in a small boat in a swimming pool. With you in the boat is a large boulder. You toss the boulder out of the boat and into the pool. What happens to the water level in the pool?
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Old 3rd Dec 2004, 21:47
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Ok, how about this then..............


A hot air baloon, in the same kind of box (somewhat taller though). The air baloon is FLOATING inside the box with neither an ascent or descent.

What do the scales weigh, and why?

Cheers,

Fay
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Old 3rd Dec 2004, 22:10
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don't know what the scales will show but I do know that the value of the box will go up when the balloon rises....
due to inflation of course!

I thank you!
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