If all the passengers ran to the back of a 'plane...

[V8LM]

The flight engineer on Concorde spent most of his time pumping fuel fore and aft, particular when transconic as the centre of pressure moved rapidly. There's a photograph in Brian Trubshaw's book of a Concorde sitting on its tail in a hanger after a 'fuelling error'.

I believe on the tank drops the pilot has to push the stick forwards slightly to counteract the pitch up movement as the weight moves back, and then back when it leaves.

https://www.youtube.com/watch?v=ORp9eJEoMpQ

Manouvres like this near to the ground are dangerous, especially if the tank doesn't leave at the right point so the pitch-up movement doesn't happen

https://www.youtube.com/watch?v=8PfXxQB_b-Y

Edited by V8LM on Sunday 14th February 09:11

[Ian Lancs]

V8LM said:

The flight engineer on Concorde spent most of his time pumping fuel fore and aft, particular when transconic as the centre of pressure moved rapidly. There's a photograph in Brian Trubshaw's book of a Concorde sitting on its tail in a hanger after a 'fuelling error'.

I believe on the tank drops the pilot has to push the stick forwards slightly to counteract the pitch up movement as the weight moves back, and then back when it leaves.

https://www.youtube.com/watch?v=ORp9eJEoMpQ

Manouvres like this near to the ground are dangerous, especially if the tank doesn't leave at the right point so the pitch-up movement doesn't happen

https://www.youtube.com/watch?v=8PfXxQB_b-Y

Edited by V8LM on Sunday 14th February 09:11

What happens in the cockpit - https://youtu.be/e9knSaT_4Tw?t=3m17s

[docter fox]

I'm just about to take off for New York, thought about organising a test running to the back but they have free champagne at the front so I'm not going anywhere!

[V8LM]

Ian Lancs said:

What happens in the cockpit - https://youtu.be/e9knSaT_4Tw?t=3m17s

Thanks. I'll revise that 'slightly' description.

[Pan Pan Pan]

Jayyylo said:

gazzarose said:

Always annoys be a bit when 11stone me gets the same baggage allowance as person of the larger persuasion.

It should annoy large people more as small people wear small clothes, therefore they can pack more in. Have you ever compared mens jeans to ladies skinny jeans? It's about 3-1 for size and weight.

Back on topic, no-one answered a previous poster about the army dropping tanks while on the move. How does that work if we're convinced a long toilet queue + the snack trolley will cause the plane to tip over?

When a heavy load such as a tank or armoured vehicle is dropped from a transport, it is rapidly drawn out of the aircraft by the drop shute (in some cases it can be slightly lifted off the deck of the aircraft by the action of the shute, before it has completely left the aircraft)
The load leaving the aircraft takes no more than a few seconds, then it is gone (after which the pilot trims the aircraft for its new (unloaded) flight condition) taking into account points that others have made about the size and configuration of the transport aircraft, and the fact that the pilot is ready to make the necessary control inputs to keep the aircraft in the correct attitude, it does not cause any problems.
If on the other hand the load was to be caught at the rear of the load deck for any reason, then the aircraft would be in a serious position, especially when the weight of the load starts to pull the aircraft into a nose high position. The OP was talking about a significant number of passengers moving to and `remaining' in the rear of an airliners cabin.

[IanCress]

docter fox said:

I'm just about to take off for New York, thought about organising a test running to the back but they have free champagne at the front so I'm not going anywhere!

This is why they put first class at the front of the plane. Nobody is going to leave a comfy seat, free food and champagne to try and tip the plane up.

Fact.

[jamieduff1981]

J4CKO said:

Pan Pan Pan said:

J4CKO said:

Isnt this more of a problem on the ground, in the air the lift of the wing and the tail stabilise the plane, it will make a difference but I dont think it would cause the plane to point skywards and stall, the back of the plane isnt the absolute rear, everyone would be between the wing and the tail, and 150 tonnes of people take up a lot of space, passengers can be dense, but they arent that dense so it wouldnt be like a 150 tonne weight hanging off the tip of the tail, some of that load is meant to be there anyway, i.e. the rear ten rows into which they would all have to clamber.

There is 20 tonnes or so of fuel in the middle of the plane, two massive engines, cargo, luggage etc, also, this wouldn't happen instantly like the Bagram cargo shifting, it would take time and be during level flight based on the question so wouldn't the flight computer just compensate or the pilot notice it and push forward on the stick to counteract ?

Scary thought, the Airbus model 911 !

An aircraft must be loaded so that the C of G remains within the fore and aft limits given for the particular type, both on the ground, and more importantly in the air. If the C of G goes outside the fore and aft limits for whatever reason, the aircraft may then go outside the limits that the control surfaces are able to correct. the further away from the c of g a weight gets, be it people, or cargo. the less weight is required to de stabilize an aircraft, and put it in a non recoverable condition. It does not have to be many people, nor do they have to be at the very rear of the aircraft. the key parameters are distance away from the C of G, and the weight of whatever is moving ahead of, or aft of the C of G limits.

I kno what you mean, but on the ground it hasnt got air travelling over the wing and tail at 500 mph, the weight will tip the plane back more easily on the ground as it hasnt got the air resistance, but I agree that its is still an important factor

It's not just static balance that matters but aerodynamic.

R/C models demonstrate it to more extremes than tends to happen fullsize.

As CofG migrates backwards, the proportion of aerodynamic surface area ahead and behind the CofG changes too. Ignoring balance for a moment, it can be observed that many aircraft end up with dorsal fins and ventral fins for yaw stability.

When the BAe Nimrod gained an inflight refuelling probe above the cockpit it also gained a set of finlets on the tailplanes. The Comet tail gained a massive dorsal and ventral fin when they added the bombay to make it a Nimrod. This is because an aircraft assumes the stability properties of a dart flying backwards if there's too much surface area ahead of the CofG and not enough behind.

This applies equally to longitudinal stability as well as in yaw. The surface area of the wing is huge. Move the CofG back along it and the stability becomes negative. I.e. any upset from steady state produces a divergent response from the aircraft. It can get worse with speed.

An analogy with a car might be steering that doesn't try to self-centre after an upset. Hit a bump and the steering tries to wind on full lock by itself - etc ...

[Pan Pan Pan]

jamieduff1981 said:

J4CKO said:

Pan Pan Pan said:

J4CKO said:

Isnt this more of a problem on the ground, in the air the lift of the wing and the tail stabilise the plane, it will make a difference but I dont think it would cause the plane to point skywards and stall, the back of the plane isnt the absolute rear, everyone would be between the wing and the tail, and 150 tonnes of people take up a lot of space, passengers can be dense, but they arent that dense so it wouldnt be like a 150 tonne weight hanging off the tip of the tail, some of that load is meant to be there anyway, i.e. the rear ten rows into which they would all have to clamber.

There is 20 tonnes or so of fuel in the middle of the plane, two massive engines, cargo, luggage etc, also, this wouldn't happen instantly like the Bagram cargo shifting, it would take time and be during level flight based on the question so wouldn't the flight computer just compensate or the pilot notice it and push forward on the stick to counteract ?

Scary thought, the Airbus model 911 !

An aircraft must be loaded so that the C of G remains within the fore and aft limits given for the particular type, both on the ground, and more importantly in the air. If the C of G goes outside the fore and aft limits for whatever reason, the aircraft may then go outside the limits that the control surfaces are able to correct. the further away from the c of g a weight gets, be it people, or cargo. the less weight is required to de stabilize an aircraft, and put it in a non recoverable condition. It does not have to be many people, nor do they have to be at the very rear of the aircraft. the key parameters are distance away from the C of G, and the weight of whatever is moving ahead of, or aft of the C of G limits.

I kno what you mean, but on the ground it hasnt got air travelling over the wing and tail at 500 mph, the weight will tip the plane back more easily on the ground as it hasnt got the air resistance, but I agree that its is still an important factor

It's not just static balance that matters but aerodynamic.

R/C models demonstrate it to more extremes than tends to happen fullsize.

As CofG migrates backwards, the proportion of aerodynamic surface area ahead and behind the CofG changes too. Ignoring balance for a moment, it can be observed that many aircraft end up with dorsal fins and ventral fins for yaw stability.

When the BAe Nimrod gained an inflight refuelling probe above the cockpit it also gained a set of finlets on the tailplanes. The Comet tail gained a massive dorsal and ventral fin when they added the bombay to make it a Nimrod. This is because an aircraft assumes the stability properties of a dart flying backwards if there's too much surface area ahead of the CofG and not enough behind.

This applies equally to longitudinal stability as well as in yaw. The surface area of the wing is huge. Move the CofG back along it and the stability becomes negative. I.e. any upset from steady state produces a divergent response from the aircraft. It can get worse with speed.

An analogy with a car might be steering that doesn't try to self-centre after an upset. Hit a bump and the steering tries to wind on full lock by itself - etc ...

Your analogy to what happens in car, gives a good description of what can happen in an aircraft, if the weight moves too far away from the C of G fore and aft limits, and goes outside what the control surfaces can deal with.

[IforB]

Back in the days of yore when I was a young freight pilot, we often had sectors where we would have other crew on-board but an otherwise empty aircraft. My colleagues used to delight in kicking a football around the back and then to annoy us who were actually workeing, they would run up and down the aircraft to move the CofG around and cause the aircraft to pitch up and down.

There was never enough weight to move us outside of the safe envelope, but it was very annoying, especially if you didn't have a working autopilot. Cue plenty of retalliation over the tannoy.

[tritium]

Anything built in the last twenty five years for commercial passenger transport is going to be fine.

The horizontal stabilizer has enough movement to overcome any possible imbalance with for or aft of all the passengers crowding hillsborough style towards either end. Which they might plausibly do in the event of hijack or cabin fire.

What do you think aircraft design engineers do all day?!

[perdu]

tritium said:

Anything built in the last twenty five years for commercial passenger transport is going to be fine.

The horizontal stabilizer has enough movement to overcome any possible imbalance with for or aft of all the passengers crowding hillsborough style towards either end. Which they might plausibly do in the event of hijack or cabin fire.

What do you think aircraft design engineers do all day?!

You're new so we might have to give you a pass

They love to get excited

Do try not to calm them down too much

They might really start thinking about stuff.............

[Pan Pan Pan]

tritium said:

Anything built in the last twenty five years for commercial passenger transport is going to be fine.

The horizontal stabilizer has enough movement to overcome any possible imbalance with for or aft of all the passengers crowding hillsborough style towards either end. Which they might plausibly do in the event of hijack or cabin fire.

What do you think aircraft design engineers do all day?!

Possibly you also believe that car designers can design cars that will do a right angled turn at any speed the car can reach, just by turning the steering wheel?

[DrTre]

perdu said:

You're new so we might have to give you a pass

They love to get excited

Do try not to calm them down too much

They might really start thinking about stuff.............

What's happened to PH?

Edit: slight overreactionery flounce on my part but I'm a little baffled at the criticism in this post. It was a simple, slightly tongue in cheek, question that was meant to generate discussion. The basic physics aren't exactly difficult to work out, it wasn't ever meant to be about that.
Fair enough though, this isn't Quora.

Edited by DrTre on Sunday 21st February 11:08

[jmorgan]

I believe a certain Graham Chapman recounts such an attempt in his book. Pilot was not happy.