Forces in equilibrium when on ground?

[Chaptain]

Hi all,

I know this is a very basic question and more than likely one of definition/terminology, but I've come across it a good few times now. It was a question in one of the theory exams as well.

Are the four forces acting on an aircraft on the ground (stationary) in equilibrium?

I would've said no, assuming we are talking about lift/weight. Since (at least in zero wind conditions) the wings are not generating lift, what's the name of the force opposing weight in this scenario?

[Young Paul]

If we keep it Newtonian ... an object stationary in a state of unaccelerated motion has no net force acting on it. There is no lift - half cee ell rho v squared ess - v is zero, so lift is zero. Similarly for drag. The force that balances the weight of the object is a resisting force from the ground upwards - as you would observe if you rested it on something that wasn't able to generate that force - say, a shallow layer of ice on a pond.

[Wizofoz]

To agree with YP but maybe put it more simply- If something isn't accelerating, it is in equilibrium.

The force acting against weight is more colloquially known as "The Ground"...

[cwatters]

Trick question? On the ground there is an additional fifth force provided by the ground. So the four forces you are familiar with don't have to balance on their own.

The force provided by the ground would probably be called the "reaction force provided by the ground".

Actually it's six forces at least... I forgot friction between tyres and the ground so thrust and drag don't have to balance either.

[dixi188]

When an aircraft is stationary on the ground it is effectively part of the Earth so the forces on the Earth are the ones in equilibrium. ie. Angular acceleration around the Sun, (Galaxy, Universe) and Gravity.

Maybe also that elusive Higgs Boson, for all I know!

[Wizofoz]

Well, ACTUALLY those forces are still present on an Aircraft in flight.

We nominally assume both the Earths Surface, and the air an aircraft is flying through are inertial frames of reference, as it makes the Math much easier, when in fact they are no such thing.

Anything not moving or at a constant velocity reference the Earths Surface is in fact never in Equilibrium, but that would seem beyond the scope of the question.

[FE Hoppy]

Isn't the aircraft still accelerating at 1 g? Where does the energy to compress the tyres/oleos come from?

[Wizofoz]

It doesn't take energy, just force.

If it is accelerating, in what direction and by what magnitude is its' velocity changing?

[Guest 112233]

Wizfox

I've got one - Because the aircraft is on the surface of a rotating sphere, its in a constantly accelerating reference frame. Its constantly accelerating towards the center of the Earth.

Force on Gnd: Inertial Mass x acceleration due to gravity - The vertical component of reaction upwards due to the rotation of the earth (coefficient 1 at the equator - 0 at the poles)

Back to my Fox Hole. CAT III

[rudderrudderrat]

Hi CATIII-NDB,

Its constantly accelerating towards the center of the Earth.

I presume you mean the frame of reference is constantly accelerating towards the centre of the earth?
It only makes a tiny difference that the sphere is rotating.

[Guest 112233]

rudderrudderrat

I wonder just how small the differences are: both longitudinally and the even smaller transverse component ? too.

I suppose we are in the realms of micro gravity here.

Yes its the reference frame that's accelerating constantly toward the centre of the earth.

CAT III Edit: [I'm not being pedantic but imagine "Big imagine" trying to align a GPS at the North/South pole? - It would be impossible to get a fixed bearing because of the Earth's rotation. Now someone will prove me wrong from real life experience.]

[New edit: Sorry I'm wrong GPS relies on Geostationary Sats: They would be still fixed at either pole: ooops.]

[wiggy]

Well all that's helped the OP..........

IMHO in ATPL land the forces are in equilibrium and the aircraft's weight is opposed by the force of reaction of the ground.....

But I digress:

Sorry I'm wrong GPS relies on Geostationary Sats:

Sorry but no it doesn't....... ( because they're not).

[rudderrudderrat]

what's the name of the force opposing weight in this scenario?

When the Blue Whale (which was brought into existence when the improbability drive was activated) in "The Hitch Hiker's Guide to the Galaxy" saw the very large blue and white round thing, which was hurtling towards it at an increasingly fast rate, gave it the name "ground" and hoped it was friendly.

The reaction at the surface of the earth is, what we in the trade call, "ground".

imagine "Big imagine" trying to align a GPS at the North/South pole?

I assume you mean IRS or INS?
One reference direction could be towards the "pole star" (vertically up) and the other two at right angles tangentially to the earth's surface. Provided the last two references rotated at the same rate but in the opposite direction to the earth's spin, then you'd be able to calculate any deviation sensed by your reference system with respect to the earth.

[Guest 112233]

Re INS/GPS

Yes of course: I did mean INS; Inertial Navigation System.

And yes I realize that I should have said the poles along the axis of the earth's rotation. At these poles there's a rotation at a fixed point (in theory). And the pole Star is centrally overhead for alignment

Re GPS: Quite correct the Satellites are not geosynchronous.

As an aside, I do not think the earth's speed of rotation is constant (irrespective of Latitude) - Any stellarium users out there (you can see variations in the earths speed of rotation on screen)

I had no idea that such a "simple" question could yield such complex answers and analysis.

Thanks for all the corrections.

CAT III