Mass & Balance

[David Webb]

A test of your knowledge of physics.

A widebody jet is straight and level. There are no seats in the aeroplane; they have been replaced by 1000 wooden perches on which are sitting 1000 parrots.

A sudden noise alarms the birds, they all get airborne and fly around inside the aeroplane.

Question. When the birds become airborne, what happens to the all-up weight of the aeroplane?

[Mister Geezer]

Ask the dispatcher......

[Gassbag]

Just think about all the corrosion from the Guano!
What's the answer then. I think it could be what happens is: Nowt. The mass of air displaced by the birds flapping is equal to their own mass?
Bill Oddie told me that once in a Tequila dream, or was it Mechila Strachan??:rolleyes:

[tom_higginson]

The plane will become lighter.

Think of when you are in a lift. If you jump up and down the lift jolts. This is your weight being placed on and off of the lift.

Gassbag
About the mass of the air as the birds flap. Air is alot lighter than parrots.

[foghorn]

Well, I'm convinced that the balance of the aircraft doesn't change, not unless someone places a big bucket of Trill at one end of the cabin.

But does the all up weight (or mass, as JAR would properly like us to say) change if the perched parrots take off? No.

Whether the parrots are flying in the cabin or sat on the perches the All Up Mass stays the same. This is because the parrots are still inside the aircraft (the all up mass of an aircraft actually includes the mass of the air within it, therefore includes the mass of any parrot flying within said air).

Therefore Gassbag is correct. Even about the Guano.

Quad Erat Demonstrandum

cheers!
foggy.

[Tinker]

I agree with 'nothing', the weight remains unchanged, the downward force of the birds flapping their wings acts upon the aircraft creating a downward force on it equal to the mass of the birds. (looking for a catch) I would also hazard a guess that it may increase due to the birds having to over come drag but i'm giving myself a headache now :D

(or being pedantic, the weight is constantly going down as the aircraft burns fuel).

[two speed prop]

The plane will become lighter.

The birds weight=mg, but they are balancing their weight by the lift they produce by flapping their wings.

Imagine if the birds were outside the fuselage, e.g. on the wings. No one would argue that their weight must be included in the AUW. But if they were to fly off the weight of the aircraft would suddenly decrease. It makes no difference if the birds are inside the fuselage.

TSP

Obviously they would have to have a bloody good grip!

[The Greaser]

Seems logical that the aircraft gets lighter - otherwise everytime a jumbo flew over we would all get crushed to death!

[Troy McClure]

Mass cannot be created or destroyed. Basic rule of non-nuclear physics (negating nuclear reaction type e=mc2 effects, which I'm sure is beyond the average Norweigan Blue).

Lift on birds' wings has to equal their weight, or they'd accelerate in a vertical direction (which would make it a slightly different case - cf jumping in lift, where f=ma also comes into the equation), though if they were all flapping up and down, the accelerations would average out.

Mass constant, acceleration constant, w = mg, therefore weight constant.

[mixture]

This is a common question (it usually arises and gets subsequently analysed over a pint of beer) ......:D

Clearly none of you are helicopter pilots .... or if you are, remind me not to fly with you :rolleyes: :p

(For the record I'm fixed wing not eggbeater)



Newton's third law states :

Every action has an equal and opposite reaction

Therefore, in the case of a hovering bird (or helicopter) :

Lift = Weight
(Thrust = Drag to be pedantic, but I've assumed the birds are hovering)


The fluid (i.e. air) that the bird now occupies has been displaced (ref. Archimedes for a nice example) .... but the aircraft is a sealed environment (ok ... so we've got AC Packs drawing in bleed air but I've assumed they are recycling air).

Therefore there is no difference in weight.


......... bows and exits stage left ;)


You didn't mention what the prize was for the correct answer ...:cool:

[Gerund]

Troy.........

Hoist by your own petard!

Yes, w = mg.

The problem mentioned 'weight' and 'physics'.

Weight is not the same as Mass; as you rightly say it is related to 'g'. Earth's gravity varies with the distance from the centre of the earth.

If the birds are flying above their perches they will weigh less, below and they will weigh more, and all over the place - a bit of averaging will be required....constantly!

We all 'weigh' less at the top of Mount Everest.

So the mass of the aeroplane will remain constant, but not the weight.

However, the difference in weight will be very tiny. (Half a feather??).

A bit of a trick question.

Well, that's my two penny worth!

[foghorn]

My answer above is based on the assumption that, as is the case in everyday and aviation usage, WEIGHT is MASS (and is measured in units of mass).

Not scientifically true I'll grant you.

foggy.

[two speed prop]

The weight decreases as the birds are supporting their own weight.


However, By raising off the perch they are merely displacing the air from above them to below them. So the total amount of matter within the aircraft and hence the mass does not change.

But as Foghorn said, in aviation mass means weight.

[Spit15]

The total amount of matter (ie. mass) present in the a/c remains constant, ignoring fuel burn... (imagine it as a glider). However the instant the birds are airborne then there is no longer a direct reaction between the a/c and birds.... therefore although the total mass remains constant within the a/c, the load experienced by the a/c reduces.


Think of the birds as balloons resting on the perches which are then inflated with hot air so they rise and hover. The perches no longer have to support the balloons as the balloons have become "weightless" in the atmosphere of the a/c. Therefore the weight of the birds / balloons is no longer experienced by the a/c.


Consider yourself standing on the ground, you are supported by the earths crust. Then you jump into the air and the earths crust ceases to support your weight, although the total mass of the system (you + earth) remains constant.


... confused..? so are all the parrots!!


- If the mountain appears too high... then try approaching from a differrent side!

[Keith.Williams.]

We can shed more light on this question if we change the scenario slightly. Image the aircraft is climbing from msl at 250 Kts TAS and 5000 fpm when the parrots take off. After 8 minutes the aircraft is at 40000 ft, and still at 250 kts TAS (just ignore the very low CAS for now).

Each of the parrots has acquired 40000 foot parrots of potential energy (these are of course not the same as parrot feet of which each parrot will have only two, regardless of height above msl). Each parrot will however still have the same kinetic energy (1/2parrot x TAS squared). Was that extra potential energy derived from jet A1 or from parrot food?

Now consider what happens if the aircraft then gently enters a descent at 5000 fpm still at 250 Kts TAS. When it gets back to msl the parrots have lost all of their potential energy but still have the same kinetic energy. By what means did the parrots get rid of the potential energy?

[FlyingForFun]

Keith,

Excellent answer! I've known, intuitively, that this was the way it worked for a long time (this isn't the first time this question has been asked!), but I've never been able to explain it scientifically, nor have I seen anyone else able to do it until now!

It's also interesting to compare to an open cockpit, in which case, if the parrots were to take off, the mass (and weight) of the aircraft would decrease. Your explaination, if extended to the open cockpit case, also desribes why this is so.

(Re. the mass vs weight argument: weight = mass x acceleration. So, assuming that the acceleration (read g. loading) of the aircraft is the same, weight and mass can be considered equivalent for this problem. Arguments that one changes and the other doesn't, therefore, are invalid.)

FFF
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[Deputy Dog]

I think its like a can of soup with say peas in it (or a tin of beans for that matter).

If the can is sat on the scales with all the peas resting on the bottom of the can, it still has the same weight if you shake the can thus suspending all the peas. The soup supports the weight of the peas. The air in the cabin supports the weight of the parrots if they flap enough ! If one suddenly dies mid flap and falls on the floor, the plane (can) still has same weight.

If now you were to open the can of soup, take the lid off and shake it then weigh it I expect the can will weigh less (and you will be covered in soup).

Could be wrong


:confused: :confused: