Inertia and stuff

[BigJETS]

I posted this question under the weight and balance topic going on in Tech Forum. I think it should be its own topic.....

What effect does overhead baggage have on center of gravity. I mean, would an AC benefit greatly if overhead weight was eliminated? It seems to me it could at least create a more stable AC during certain manuevers--maybe even enhance performance. I have to wonder what we could see if overhead luggage and the bins altogether were eliminated. Or possibly, would this have an adverse effect on AC turning/banking etc? Also could someone describe the cargo holds of AC. Are there any straps or restraints used in storing baggage in a hold? What if AC were more solid on the inside like a hard boiled egg as opposed to unrestrained like a soft egg and how would that effect the AC in flight? --------------------------------------------------------------------------------
A hard egg will spin and spin because it is relatively solid. there is no friction created inside the egg to act against its rotation. If you spin a soft egg on a surface, it will not spin and spin because it is not really one part--its not solid. It lags against itself to a rapid stop. It must be the same for a plane full of pax and cargo. My question is, would there be a benefit from extreme lockdown of cargo. Its only a theory, not really possible to counter the forces by severe restraint that would be required, but just curious how different an AC would handle.
A good way to test this would be to put a large bottle, filled with say 25 tons of water. Fly it around and record the performance. Then take the water off, refuel to previous level (liquid fuel blows the whole "solid" idea out of the water), and load 25 tons of lead weight to the AC and lock it to the airframe in a balanced fashion so as it becomes part of the AC. How differently would the AC respond on the same flight?



------------------
Rotate, Please! airliner/in-flight photo site (http://members.tripod.com/bigjets/flightline/)

[CONES R US]

Smaller aircraft have restraining netting in sections along the interior of the cargo holds to keep the baggage where it's put - so long as the loaders do up the nets. Larger aircraft such as Airbus' have the bags loaded into bins, which are then loaded into the holds.
Cabin baggage is a neccesary evil, and I don't see how airlines could do away with it, but it would certainly speed up the embarking and dis-embarking of pax.
As to the C of G being affected, it depends on the type of aircraft. The 146 has a high wing and so a high C of G and the total max cabin baggage would not amount to a significant portion of the total aircraft weight. Even in a 747, the proportion of the total weight in cabin baggage would not have that great an effect on the overall mass of the aircraft, but the weight would be above the C of G.
It's not something that has given me much cause for thought, and probably not too many other people I would suspect.

------------------
NO FAULT FOUND

[BigJETS]

I respect that Cones and appreciate your response. I feel it is cause for thought as it could save money. Could you just comment on the experiment I described (water bottle, solid weight), if only to satisfy my own curiosity? Its not so much a C/G issue I guess and it would only be important during changes in velocity and attitude.

[somefokker]

Interesting thoughts on C.of G and cabin baggage. What do you think of the notion of a double decked airliner like the Airbus A3xx. Does that exagerate the problem even more?
I wonder what effect replacing traditional gyroscopes with ring laser gyros has?
Anyway all this pondering has me in trouble with the wife as I've just broken a couple of eggs spinning them around the kitchen table. Best I go to the pub

[BigJETS]

I didnt think about the new Airbus. It does seem like it would be affected with more weight above the wing.
Another thought...Fuel in the wings. It seems like Ive heard of a mechanism that blocks the movement of fuel. I dont know how this works. Does it restrict fuel from spilling downward as a plane banks? It seems that this could hinder a manuever if not. Fuel in one wing settling toward the Fuselage while the other toward the low wingtip. I would assume that if a mechanism was not in place, that an aircraft would not turn as crisp as if there were a way to keep the fuel within each wing, in a relative position to each tank. The AC would respond quicker to direction.

[John Farley]

Hi Big Jets

As I see it you have asked two questions

1 What effect does vertical movement of the C of G have on aircraft handling

2 What effect does cargo (or any other weight) that moves about a bit in flight have on
handling

Taking the second question first, if large weights (large that is as a percentage of the total weight of the aircraft) were allowed to shift in flight the results could be considerable. Because of that everything that could be considered as large is strapped down and cannot shift at all. Even a minor crash (like an overrun into boggy ground where the aircraft slowed down abnormally quickly) would be disastrous as all the lose stuff moved forward. (just the same as your lose briefcase on the back seat of your car becomes a missile going past your ear if you stop suddenly). As to fuel in tanks, they have what are called baffles, which are no more then fore and aft and spanwise partitions with smallish holes in. The holes are large enough to allow the fuel to transfer at a rate that the engines may require but stop the whole tank contents sloshing about and changing the cg. BUT, you should not think that in a turn the fuel runs down to the bottom wing tip (anymore than you fall to the side of the cabin if you are walking down the aisle when a turn happens). In a properly balanced turn (where the rudder and ailerons are correctly used together) there is no residual side force across the aeroplane. The only time you should get such a side force is during certain aerobatic manoeuvres, but that is another subject.

Regarding the first question, the effect of raising the centre of gravity by adding a lot of cabin baggage in overhead bins is once again a matter of degree. Namely – how much is the cg shifted. My guess is that in most ordinary airliners you would be pushed to measure the amount. Consider the numbers – first with the bins empty. A jumbo weighs in at say 400 tons and so if you balanced it on its side in the hangar there would be 200 tons above the cg and 200 below. Think of it as a see-saw with 200 tons on each end. Now you add the baggage say 400pax x 25 lb each which amounts to 4.4 tons, if that 4.4 tons was added as high as possible (ie at the end of the see-saw – which means at the top of the fin, you would now have 200 at one end and 204.4 at the other. You would not have to move the pivot (cg) far to make the see-saw rebalance. In actual fact you don’t of course have overhead bins that high, they are actually only slightly away from the pivot and so the 4.4 tons of baggage hardly affects the balance (or CG position) at all.

Make any sense?

JF

[BigJETS]

Makes sense JF and thanks for responding. I still wonder how it might relate to the egg.
Nevermind eggs--I hope I dont sound like a nut.

------------------
Rotate, Please! airliner/in-flight photo site (http://members.tripod.com/bigjets/flightline/)

[near enuf is good enuf]

About fuel movement in the tanks, the baffles are (BOEING) one way valves.
The fuel can flow from outer to inner as fuel is used but cannot move back out the wing i.e. in manouvres.

------------------
So that you may not be the martyred slaves of Time,
get drunk, get drunk,
and never pause for rest!
With wine, poetry, or vitrtue,
as you choose!"