# So why arent you weightless when flying in a plane?

1. Dec 23, 2012

### jaydnul

When ever your flying in a plane, doesnt it go by the same principles as, say, the moon orbiting the earth? Wouldnt the plane be in free fall and just accelerating forward to keep it up? I realize this is wrong but why?

2. Dec 23, 2012

### cjl

Aircraft don't fly by the same principles that keep the moon in orbit - they aren't going nearly fast enough. While in theory, it would be possible (ignoring aerodynamic drag), it would require the aircraft to go about 8 km/s, while in reality, they go about 0.2-0.25 km/s. What keeps them in the air is the force from the air over the wings, and as a result, they are not in freefall, but instead are basically in equilibrium - the weight of the aircraft is balanced by the upwards force from the air, and thus when you are inside, it feels the same as if you were sitting on the ground.

3. Dec 23, 2012

### mrspeedybob

The force holding the plane away from the earth is the air pressure difference between the top and bottom of the wing. This force is transfered to your body through the seat, therefore you feel the seat pushing up on you.

If the airplane were in orbit there would be no aerodynamic lift (no air). The force holding the plane in orbit away from the ground is centrifugal force, which is another way of saying inertia. The other force affecting your space plane is gravity. Since you are in orbit you have achieved a balance between inertia and gravity. Both inertia and gravity affect the plane and your body identically. Therefore, there is no difference in acceleration between you and the plane, and therefore no need for the plane to apply any force to you.

4. Dec 24, 2012

### Lsos

As has been said, you need to go around the earth about 17,000 mph in order to be in free fall. Airplanes, even the fastest ones, travel at just a small fraction of that.

That said, even that small fraction has an effect on your weight. I believe you (used to) weigh 1-2% less on a Concorde at full speed, for example, than on the earth.

5. Dec 24, 2012

### Redbelly98

Staff Emeritus
Without verifying your 1-2% values, I'll just mention that it would be less only if the plane is traveling eastward. Traveling westward should result in an increase, since then the plane would be traveling slower -- in a nonrotating frame -- than a plane that is sitting still w.r.t. the ground.

6. Dec 24, 2012

### Lsos

I've never actually verified the 1-2% values myself (until now), and I'm not sure where I got them from. But you do bring a very good point.

So I in fact did calculate the value now, and indeed just the speed of the Concorde only brings about a ~.5% change in weight. However, due to the fact that centrifugal force is a squared function of speed, the change DOES approach 2% when flying east (at the equator, of course).

7. Dec 24, 2012

### Integral

Staff Emeritus
A plane can be flown on a ballistic path which will provide a weightless ride.

Vomit comet

Years ago I was given a brief ballistic ride in a small private plane so it does not need to be a "special" plane.

8. Dec 24, 2012

### davenn

I'm pleased you brought that one up... saved me doing so.
Lsos forgot about that easily attained situation ....
one doesnt need to be travelling at 17,000mph to attain weightlessness

Dave

9. Dec 25, 2012

### Lsos

I didn't forget. In fact, I live on the 3rd floor so weightlessness is literally right outside my window (for a few moments). No need for airplanes whatsoever.

I wanted to discuss what happens to an airplane in steady-state flight, which is what I understood OP to be asking.

10. Dec 25, 2012

### Redbelly98

Staff Emeritus
That was my understooding too, if by steady-state you meant constant speed and altitude, i.e. a circular trajectory centered on the center of Earth.

Hmm, post number 1 of the thread said:
When ever your flying in a plane,..."​

11. Dec 25, 2012

### Staff: Mentor

Sure, but I rather suspect that none of us have ever been in a vomit comet. In either case, that sentence is written as an affirmative declaration. Paraphrase: we are NOT weightless when we fly in planes. Why not?

12. Dec 25, 2012

### Jupiter6

If the plane and passenger had no weight, it wouldn't be able to fly. The earth is ultimately what foots the load of an airfoil and it's passengers.

Apparent weight during a red-out in an airplane(while much more noticeable) is the same feeling you get while on roller coaster or speeding over a hump in a vehicle, or in an fast elevator that starts to descend or come to a stop while moving upwards.

13. Dec 25, 2012

### Redbelly98

Staff Emeritus
Russ: yes, put that way, that was surely the OP's intent. And it was answered in posts 2 and 3; the rest of the thread is just discussing nuances or related phenomena.

Jupiter6: Most (hopefully all?) of us do mean zero apparent weight when we say "weightless". It's pretty standard usage of that term.

14. Dec 25, 2012

### Jupiter6

I know. I just never cared for the term because it confuses people.

15. Dec 25, 2012

### Redbelly98

Staff Emeritus
No argument here.