negative g's

Factual Questions
21

I’ll call your fussy, and raise you. My example was a 60 deg bank, in level flight, in a coordinated turn. I dropped the qualifiers to keep the word count down. Niggle away.

And, the conventional meaning of weight is the force on an object due to gravity. There are other forces at work, as I described. The weight of an object doesn’t change just because it’s moving around. The weight of my body remains 160 pounds(force) regardless of the motion of the airplane. It’s the other forces that add up so as to move me around the sky, forces from the wings, thru the fuselage, ultimately thru the seat of my pants. This makes it “feel” like I weigh twice as much.

As for g meters - what would you say they measure? Is it load factor, which is dimensionless? If so, what’s the “g” stand for? Is it acceleration (1g = 32.2 ft/s^2)? If so, why does it say “1 g” with the airplane parked in the hangar?

22

The real reason for distinguishing between negative and positive g’s is that the body has different limits for the two. As has already been stated, fighter pilots tend to red-out at around 4-5 negative g’s. However, fighter pilots can usually withstand about 8-9 positive g’s before blacking out. That means that it is much better to turn sharply up than to sharply drop in a plane.

This is of course why fighter pilots invert when going into a very steep dive. The g’s from the dive are in the “down” direction to them, so they can withstand higher forces.

23

You can experience +2g’s in an uncoordinated turn, too - it just won’t be through the seat of your pants. You’ll have crap sliding all around the cockpit (assuming you’re doing this in an airplane). T’ain’t a lot of fun getting smacked by loose change or clipboards or pens/pencils at 1g, but when everything weighs twice as much it’s more painful. It leaves dents.

True, it is not motion alone that affects weight - it’s acceleration, which can also be positive or negative, although the negative is usually called “slowing down”

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The weight of my body remains 160 pounds(force) regardless of the motion of the airplane.
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Nope. Under two g’s your body is exerting a force of 160 pounds upon whatever you are sitting or standing upon.

You got it - they measure acceleration. 1g is indicated on the ground because the Earth’s gravity imposes an acceleration of 1g on objects on the surface at all times.

Load factor is g force x the gross weight of the airplane.

24

Huh? An airplane parked in a hangar is not accelerating at all. Accereration is zero. If a g-meter measures acceleration, why doesn’t it say zero? [answer: it detects acceration, but the result is fudged by one in order to show load factor (that is, as I defined it earlier, and I’ll admit it was not a textbook definition - your definition here makes no sense)].

Let me assume you meant 320 pounds. In my earlier post I said the seat was pushing on me with 320 pounds. So what’s your point? My point was that my weight (due to gravity) is still 160 points. The force from the seat on my butt is 320 lbs. Therefore the load factor is 2. What is your assessment?

25

You aviators think you’re nitpicking? Just wait 'till you see the physicists. Yes, that airplane sitting in the hanger is accelerating, at a constant 1 g. Furthermore, an object in free-fall isn’t accelerating. By Einstein’s equivalence principle, which has been thoroughly verified experimentally, the gravitational “force” is actually a fictitious force, just like the centrifugal force or Coriolis force. Yes, by the way, us relativists do tend to get dirty looks from the other physicists when we say things like this.
As to “weight”, so far as I know, there’s no standard definition, so you have to be very careful to define your terms precisely. Does “weight” include fictitious forces other than gravity? Does it include buoyancy? In either case, you can get consistant results, but you have to make it clear what you mean.

26

No, hon - we really are all accelerating towards the center of the Earth at the rate of 1g (92 feet per second ^2 - me being old enough to have learned it English and not metric) We don’t go anywhere because of the solid nature of the planet, which halts our centerward progress. No fudging required or present.

Thank you for correcting my math without getting huffy. That’ll teach me to post at 4:30 am!

While there is a relationship between load factor and g force, they aren’t quite the same thing. Yes, under 2g’s load factor does, indeed, double. But that’s like saying that under 2g’s you weigh twice as much. You do. But your body mass is unchanged.

If you’re under 2g’s of acceleration and you’re sitting on your butt, yes, the “load factor” on your ass is 320lbs. But if part of your weight is supported by your arms then, while you are still under 2g’s, the load factor on your ass is NOT 320lbs, it’s only the actual weight supported by your backside.

27

Except that we’re all accelerating upwards, not downwards, precisely because the Earth is pushing up, and the value in American units is much closer to 32 fpss than to 92. But I’m sure that’s what you meant to say, Broomstick.

28

Back to the OP.

G’s in an aircraft can be measured relative to the aircraft or relative to a person. Since most people are sitting upright in an aircraft the two are usually the same. “Positive” G’s are toward the aircraft’s floor, and therefore toward a person’s seat. “Negative” G’s are toward the ceiling/head.

I’ve been told that anything over about 2 negative G’s hurts, even short of serious damage. That’s one reason you don’t see aerobatic pilots perform many outside loops.

A person is generally able to withstand more positive G’s than negative G’s. They can withstand many more if they’re lying on their back. I knew a guy who was very over-weight and in generally poor physical condition. He once had the opportunity to ride in a centrifuge and experienced 12 to 13 G’s through his chest without passing out, or even all that much discomfort, he said.

The majority of the perceived forces in an aircraft are due to a change in direction, IOW, centrifugal force. Just gaining or losing altitude won’t produce positive nor negative G’s. Instead, pulling the stick back, which starts a rotation of the nose “upward” around the wings, is what causes positive G’s greater than one G. Rotating the other way causes negative G’s.

If one uses the rudder to rotate the plane left/right one would feel lateral G’s. This is fairly rare, because it’s a very inefficient way to change direction. Most aircraft will skid sideways a great deal for every degree of direction change, bleeding off a lot of energy. Aircraft bank because it’s much more efficient to “climb” the aircraft on its wings around the turn.

As an aside, there was an experimental version of the F-16 that had extra control surfaces added onto the the underside of the aircraft that gave it a great deal of lateral authority. The pilots reported that it was very difficult to control the aircraft when smashed against the side of the cockpit in a 1.5 G flat turn.

The magnitude of the perceived G force is a combination of the radius of the turn and the speed. Two turns of the same radius taken at different speeds will give different G forces. Similarly, two turns at the same speed of different radii will produce different G forces.

Most of these concepts apply to roller coasters, race cars, or anything holding people and changing direction quickly and at high speed.

Recently, an Indy car race (actually, a CART race, but I don’t want people thinking go carts) was canceled because of the effects of long-term G forces. On some tracks these cars undergo 4 to 5 G’s vertical acceleration and 2 to 3 G’s lateral acceleration in the turns. Adding the vectors, the total G load approached 6 G’s. While aircraft usually undergo high G’s for several seconds at a time, maybe a dozen times in a given mission, these car drivers were undergoing 4+ G’s for 80% of each lap for over two hours. They were suffering from fatigue, tunnel vision and vertigo. http://espn.go.com/rpm/cart/2001/0429/1188368.html

There’s a very good discussion of how aircraft fly at http://www.monmouth.com/~jsd/how/htm/how.html In particular is a discussion how centrifugal force affects a plane http://www.monmouth.com/~jsd/how/htm/motion.html#toc355

29

With due respect to Chronos, I confess being blissfully unfamiliar with with the relativistic universe (but very curious). All of my discussion is rooted in Newton’s view of the universe.

Hon? Well, big fella, I haven’t made any assumptions about your sex or preferences, but unless you like males, you’re barking up the wrong tree. Don’t patronize me. My questions to you are intended to show you the inconsistency of your own explanations, and you don’t seem to see it. Don’t get the idea that I am learning anything from you.

The day at school when they talked about objects accelerating toward the center of the earth - do you remember that they were talking about freely falling objects? Things that are resting on the ground are not freely falling. An object at rest has no acceleration. A falling object accelerates at 32.2 ft/s^2.

If an object at rest is really accelerating at 1g (as you say), then how much acceleration would you say that an object in freefall is experiencing?

You might be interested in now load factor relates to the regulations for design of aircraft in the US. Try this link to the FAA.

30

As a matter of fact, I do like males, although I find it difficult to feed and maintain more than one at a time. I presume you’ll be relieved to know I already have one at my house so you’re safe.

If you are talking about the effect of Earth’s gravity at reasonably close to the surface of the planet (since gravity’s effects decrease with distance and increase with proximity) an object at rest with respect to the planet surface is under a constant acceleration of 1 g, but can’t go anywhere because a solid object (the planet) blocks the progress towards the center of the Earth. An object in free-fall - such as a skydiver - undergoes the same 1 g acceleration until reaching terminal velocity, the point at which the drag of the body falling through the atmosphere prevents greater speeds. At that point, the skydiver is no longer accelerating, merely maintaining speed. In a vacuum, with no air to impose drag and, asssuming in orbit, no planet to block the way, the object/person in question falls and continues to fall towards the center of the Earth. If the speed is sufficient, the object/person will overcome the force of gravity. If not, the object/person will either remain in orbit or eventually fall to the surface (on the way to the center).

In all of the above cases, the acceleration is 1 g (until reaching terminal velocity, if applicable), or close enough for the present discussion.

A continuous 1 g acceleration is indistinguishable from 1 g of gravity. It doesn’t matter whether you believe this or not, it just is. Physics is like that.

Oh, like I don’t have to read the damn regs enough as it is. Buddy, I don’t design 'em, I fly 'em.

31

I give up.

32

Not quite, Broomstick. If you’re in free-fall, then your accelerometer will read zero. Once you reach terminal velocity, your situation is the same as that of a person standing on the surface, and your accelerometer will read 1 g upwards. Standing on the surface, your accelerometer will also say that it’s one g upwards. If you’re in deep space, far from any massive bodies like the Earth, then your meter will tell you whatever the acceleration is produced by your rocket’s engines.

Basically, all that the Equivalence Principle (the foundation of Einstein’s General Relativity) is saying is that your accelerometer is correct: When you’re standing still on the surface of the Earth, you’re accelerating upwards, just like the meter says.

33

Chronos, you say you’re accelerating upwards with respect to what?

I would assume that the pebbles of earth beneath my feet are also accelerating upwards. Deep down inside the earth, where the accleration of gravity is stronger, I suppose the rocks there are accelerating upwards with greater acceleration. If so, why don’t they overtake me?

34

Actually, stuff near the core is accelerating less, not more. As to why this doesn’t cause stuff to pull apart, this is where the curvature of space comes in. I’m not sure how to explain more thoroughly without a whole bunch of complicated math, but as long as you look at a small region, you don’t need to worry about the curvature: If you took a big (several kilometers, to give us room to fly planes around inside) spaceship, and put really powerful engines on it to accelerate it at 9.8 meters per second squared, then to a person inside, it would be just like walking around on the surface of the Earth, or flying above it in a plane, or skydiving to it, etc.

35

Ah, yes, my mistake. The core of the earth is moving away from me, while the ground is zooming off into space. I know I’ll sleep easier tonight…

So why did you select 9.8 m/s/s here? And don’t you tell me 'cause that’s what gravity does at the earth’s surface. How do you know it’s gravity? Maybe those Chinese have put a big-ass rocket on the earth, and are shoving the planet out of orbit as we speak!

36

sips beer