# Why do Jet fighter pilots experience a greater G-force?

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1. Dec 2, 2015

### Alex Needleman

Hi! I'm a high school student, aspiring to pursue a career in Astronautical Engineering. I always try and ponder on questions about everyday aerodynamics and physics. So, here's a question from you from an aspiring scholar.

It is known that, at high velocities, pilots in super-sonic jet fighters, or just standard fighter jets that can achieve incredibly high speeds, experience an increased G-force pressing against them in the cockpit. Why does this happen? I've read it in many science-fiction novels as well, about how inertia and increased G-force due to great velocity can be, well, harmful to a human being.

The main reason I ask this question is because I believe (I hope, at least) that one day we will be able to reach relativistic speeds with manned spacecraft to venture the stars. Would such forces act the same way in space? And if so, would there be a way to balance the forces without affecting the motion of the spacecraft? A force field of a sort, maybe?

Alexander Needleman.

2. Dec 2, 2015

### Orodruin

Staff Emeritus
These forces are fictitious forces caused by the plane/ship/craft accelerating and therefore someone at rest with respect to it will be in an accelerated frame. It is not a priori related to velocity. If you travel in a jet aircraft at constant velocity you will feel no different from what you do on the ground (assuming you dont suffer from fear of heights or similar).

If you have been flying in commercial aircraft or even gone with a car you should already be familiar with this. The time you really feel different is when the vehicle accelerates.

3. Dec 2, 2015

### A.T.

G-force comes from acceleration (relative to free fall), not from speed. Pilots flying straight at Mach 3 experience the same 1g as you do sitting at your desk.

4. Dec 2, 2015

### SteamKing

Staff Emeritus
Flying in a supersonic fighter during high-speed maneuvers is a very different experience to flying in a commercial jet liner. In order to evade an enemy attack or to gain an advantage on a flying target, fighter pilots often engage in high speed dives and turns. It's often when pulling out of a high-speed dive that the g-forces build to such an extent that the pilot could lose consciousness, unless special means are taken to prevent such an event. If a fighter pilot is cruising along with doing such maneuvers, there will be no extreme-g events during the flight.

5. Dec 2, 2015

### FactChecker

The highest G forces are during a turn. At the proper flight condition, a modern fighter can sustain 9 Gs in a turn. The force of the air against the wings causes those high Gs. 9 Gs is about the most a pilot can stand with a G suit. That is an upward turn that causes downward pressure against the seat. The pilot will pass out due to lack of blood to the brain unless he has been trained and has a G suit. Turning down where the pressure is to lift off of the seat needs to be kept much lower.

6. Dec 2, 2015

### A.T.

Yeah, it's easier to squeeze the legs to prevent blood from going there, than the head.

7. Dec 2, 2015

### Staff: Mentor

The g-force experienced in a turning aircraft (or car, or just about anything else moving on a curved path) is just plain ordinary garden-variety centrifugal force. It's the same force that keeps a string taut when you tie a weight to the end of the string and swing it.

In the simplest case the object is following a circular trajectory and the g-force will be given by $v^2/r$ where $r$ is the radius of the turn and $v$ is the speed. Jet fighters fly faster and make tighter turns (larger $v$, smaller $r$) than commercial aircraft so the g-forces are larger.

Because you're thinking about spaceflight at relativistic velocities, you might find it amusing to try calculating the diameter of the circular path a spaceship travelling at .5c would have to follow to expose the occupants to not more than one g.

8. Dec 2, 2015

### rcgldr

The F-16 with it's heavily reclined seat is one of the few jet fighters designed to allow the pilot to experience a 9 g turn, and with somewhat limited duration before that becomes an issue. Even with afterburners on, a 9 g turn creates enough drag that terminal velocity of the F16 in a 9 g turn is around 400 knots. I'm not sure if the new fighters like the X35 have the reclined seats.

It's not just fighter jets, the Red-Bull aerobatic timed obstacle course events using tall pylons involve speeds around 200 knots and yet they pull 9+ g turns (the rules penalize if 10 g's is exceeded), the duration is shorter due to the slower speed.

http://en.wikipedia.org/wiki/Red_Bull_Air_Race_World_Championship

Aerobatic aircraft like the Extra 300 can handle up to 8 g (two peple on board) or up to 10 g (one person on board).

http://en.wikipedia.org/wiki/Extra_EA-300

9. Dec 2, 2015

### mrspeedybob

What's the reasoning for using a seated position at all? I fly prone. My aircraft is only rated to 5 g, I've never pulled more then 2.5 so I really don't know what problems I'd run in to, but blood flow to the brain shouldn't be one of them.

10. Dec 2, 2015

### A.T.

11. Dec 2, 2015

### mrspeedybob

Easily solved with a wire or rope supporting my helmet. This is not my rig, but you get the idea...

On a different aircraft a chin rest could work.

12. Dec 2, 2015

### A.T.

13. Dec 2, 2015

### SteamKing

Staff Emeritus
How many times have you got into a dogfight and had to bail out of your aircraft?

Can you imagine trying to design an ejection system for a pilot in the prone position?

Do you find it easier to scan the sky above and behind you from the seated or the prone position?

It takes less aircraft for one (or two) pilots in the seated position than with both prone, each placed either tandem or side by side.

14. Dec 2, 2015

### A.T.

And even ignoring all those points, it's simply more comfortable to spend hours sitting compared to lying in prone position. That's why most people watch TV sitting on the sofa. Hang-gliders have the pilot in prone to reduce drag. But that is not an issue for planes with a closed cockpit.

15. Dec 2, 2015

### Alex Needleman

It is indeed amusing. So, hypothetically speaking, if a spacecraft reached .5c in a perfectly straight trajectory and held its velocity constant throughout, would the g-force experienced by, say the pilot or passengers, still be equal to 1?

16. Dec 2, 2015

### Staff: Mentor

No. Perfectly straight constant velocity (if in space and beyond the reach of gravity) would be no g-force at all - the passengers would be in free fall, floating weightless. The g-force only appears if you're accelerating, which is to say changing speed or direction or both. The $v^2/r$ formula I quoted is the expression for the acceleration (all from change of direction, none from change of speed) of an object moving in a circular path.

17. Dec 2, 2015

### Alex Needleman

I see. That is something to consider, thank you for your insight. :)