Understanding G force

LongRoadBob

LongRoadBob

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I find I am a little uncertain about some aspects of G-force.

Would it be correct to say that G is 1 when flying straight ahead, but any time one banks or climbs G > 1 ?

Similarly, coordinated and correctly executed descent, is still G = 1?

When we say "the wings are loaded" we are not necessarily meaning "there is lift" but rather there is excess lift? When the wings are loaded, we also are talking about G being greater than 1?

I was just paging through book on aerobatics (for fun...I'm nowhere near thinking about that now) and saw that in a loop correctly executed there would not be negativ G forces present anywhere in it, yet that also said that one could feel "light" in the seat during some parts.

I just am really unsure how to think og G forces, specially negativ G. Top of a climb-descend arc I would think might be in negative G (I mean they do this to simulate the weightlessness of space). but again. Something fundamental I am missing here.
 
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Clip4 said:
You airplane is normally operating a 1G. When you change direction you experiance Positive or negative Gs.
Usually positive.
Here is 2 simulators I really like for students

http://media.aero.und.edu/interactive-trainers/forces-in-a-turn/

http://media.aero.und.edu/interactive-trainers/weight-cg-effects/
Click to expand...

D'oh...I "knew" that...(G=1) just wasn't thinking. Thanks, corrected to 1 in my original post.
Also thanks for the links. Specially liked the second one. boomarked the links and after work will check out more of that site, looks really well done.

The main reason I'm looking to understand this better is because G forces can raise the stall speed. I feel like I need to understand when they are other than 1, because it seems like it is not nec. intuitive at all.

Doing 45 deg. turns also, really noticable. I hadn't yet heard that tightening the stomach muscles can also help.

Also, "loading the wings", I get the idea it isn't just lift, but meant more than 1 G? How does one then, if needed, unload the wings?
 
LongRoadBob said:
D'oh...I "knew" that...(G=1) just wasn't thinking. Thanks, corrected to 1 in my original post.
Also thanks for the links. Specially liked the second one. boomarked the links and after work will check out more of that site, looks really well done.

The main reason I'm looking to understand this better is because G forces can raise the stall speed. I feel like I need to understand when they are other than 1, because it seems like it is not nec. intuitive at all.

Doing 45 deg. turns also, really noticable. I hadn't yet heard that tightening the stomach muscles can also help.

Also, "loading the wings", I get the idea it isn't just lift, but meant more than 1 G? How does one then, if needed, unload the wings?
Click to expand...

Play with that forces in a turn simulator, you will figure it out.
 
Increased G force making stall speed go up is similar to added weight making the stall speed go up. The wings are trying to support more load. A smooth 45* bank is no big deal but pull elevator to tighten up the turn in that 45* bank and stall speed changes with the amount of pull you’re applying. That’s probably the most common place G will bite a pilot in a small plane. If you pay attention to the seat pressure on your butt you can feel the state of G.
 
This is where your understanding of the forces of flight will begin to transition away from the "training wheels" explanations that are presented during initial training on to an actual understanding of how and why an airplane flies.

First, forget thinking that airplanes know (or care) about the orientation of the horizon. The airplane can experience G=1 at literally any attitude, up or down, upright or inverted, because "G" is simply a force vector that is measured in a direction perpendicular to the wings (in general terms, it is opposite of the lift vector, the force the wing generates in the direction from your butt up through your body to your head as you sit upright in the aircraft).

Second, remember that wings generate lift (and, correspondingly, stall) based on angle of attack, and that AOA is only connected to "stall speed" at one single point on the wing's performance curve. AOA can be generated, again, at any attitude, meaning that it can produce lift and stall at any attitude and a wide variety of airspeeds.
 
LongRoadBob said:
Thanks, still thinking too though about descent/ascent.
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Most airplanes cannot climb or descend steeply enough to significantly alter the G load seen during descents/climbs. It will still effectively be 1G.

See the pic below on the right. If you were flying a plane powerful enough to climb at a 50 degree pitch attitude and the "weight" line pointing perpendicular to the earth is always 1G, via basic trig, the "component of lift" line equals 0.64 in length, or 0.64G experienced by the wings of the aircraft.

GA airplanes climb more like the pic on the left, maybe a 20 degree pitch attitude. At this attitude, the component of lift equals 0.94G - practically 1G.

climblift.jpg
 
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LongRoadBob said:
I was just paging through book on aerobatics (for fun...I'm nowhere near thinking about that now) and saw that in a loop correctly executed there would not be negativ G forces present anywhere in it, yet that also said that one could feel "light" in the seat during some parts.
Click to expand...

Feeling “light” in the seat doesn’t necessarily mean negative G’s. It just means less than 1 G. 1/2 positive G means you would feel like you weigh half what you really do. Light in the seat. If you felt significant pressure on the seatbelts and shoulder harness, that would be an indication of negative G’s.
 
A stall at anything above 1G is called an accelerated stall. This is something that regularly kills people: the guy that buzzes the runway or a friend's place and pulls up hard, somtimes adding a bank to the pull-up. Stall speed rises by the square root of the increase in load factor, so if the airplane stalls at 50 Kt at 1G, it will stall at 71 Kt at 2G and 87 Kt at 3G. The airplane's nose falls and often a very brief spin develops, and it's all over. The guy has no idea what he did wrong.

In fact, this is how Va is figured: at 3.8G, the typical FAR23 standard category structural limit, the Va will be 1.95 times Vs. Note that Va gets lower at weights below gross, since stall is lower.

Remember that the wing only cares about angle of attack, not airspeed.
 
LongRoadBob said:
I find I am a little uncertain about some aspects of G-force.

Would it be correct to say that G is 1 when flying straight ahead, but any time one banks or climbs G > 1 ?

Similarly, coordinated and correctly executed descent, is still G = 1?

When we say "the wings are loaded" we are not necessarily meaning "there is lift" but rather there is excess lift? When the wings are loaded, we also are talking about G being greater than 1?

I was just paging through book on aerobatics (for fun...I'm nowhere near thinking about that now) and saw that in a loop correctly executed there would not be negativ G forces present anywhere in it, yet that also said that one could feel "light" in the seat during some parts.

I just am really unsure how to think og G forces, specially negativ G. Top of a climb-descend arc I would think might be in negative G (I mean they do this to simulate the weightlessness of space). but again. Something fundamental I am missing here.
Click to expand...

Watch that video of Bob Hoover pouring himself a glass of iced tea during a roll. That's one positive G all the way through.

The Warrior I trained in, maybe they all do, had a placard that said "No Zero G Manuevers" or something like that, so pushing the nose over to make yourself lift off the seat was prohibited. The reason a loop might not give you any negative Gs is the same reason you can spin a weight on the end of a rope over your head. If you spin in fast enough you'll feel that weight pulling on the rope all the way around (Gs). If you spin it slowly enough, it will reach the top of the loop and then fall onto your head (It will eventually go from a non-zero G to zero G and accelerate back to earth). Positive G = forced into your seat, negative G = pulled away from your seat. If you did an outside loop you'd feel negative Gs.
 
Hacker said:
This is where your understanding of the forces of flight will begin to transition away from the "training wheels" explanations that are presented during initial training on to an actual understanding of how and why an airplane flies.

First, forget thinking that airplanes know (or care) about the orientation of the horizon. The airplane can experience G=1 at literally any attitude, up or down, upright or inverted, because "G" is simply a force vector that is measured in a direction perpendicular to the wings (in general terms, it is opposite of the lift vector, the force the wing generates in the direction from your butt up through your body to your head as you sit upright in the aircraft).

Second, remember that wings generate lift (and, correspondingly, stall) based on angle of attack, and that AOA is only connected to "stall speed" at one single point on the wing's performance curve. AOA can be generated, again, at any attitude, meaning that it can produce lift and stall at any attitude and a wide variety of airspeeds.
Click to expand...

Silly me, I always thought Gravity acted towards the center of the earth, not perpendicular to my wings . . . How does that work in steep turns, when I'm banking towards 60°?

Fortunately @RoscoeT has a picture showing the correct relationship between your plane, lift and Gravity.

This is all basic high school physics. It's really not hard at all . . . . .
 
Part of the confusion stems from the fact that people (and, unfortunately, sometimes the FAA and various textbooks) are using G-load interchangeably with load factor. G-load is measured with respect to the entire airplane, while load factor is the ratio of lift created by the wings to the weight of the airplane. Since it's a ratio of two forces, it's not measured in "G" — it doesn't have a unit.

In a steady state climb at a constant speed, the G-load is 1, as would be described by Newton's first two laws. However, since a component of thrust acts upward, counteracting the weight of the airplane, the wings can generate a tiny bit less lift, so the load factor is slightly less than 1.
 
Hank S said:
Silly me, I always thought Gravity acted towards the center of the earth, not perpendicular to my wings . . . How does that work in steep turns, when I'm banking towards 60°?
Click to expand...

"Gravity" and "G" (as measured in an aircraft) are not the same thing. We are talking about "G" as a force vector.

I'll turn the question back on you, and ask if your definition is all-encompassing, how I can do a 7G (as measured by a cockpit G-meter) level turn in an F-15E? Better yet, how does a cockpit G-meter read 1 (or more than 1) at the top of a loop?
 
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Hacker said:
This is where your understanding of the forces of flight will begin to transition away from the "training wheels" explanations that are presented during initial training on to an actual understanding of how and why an airplane flies.

First, forget thinking that airplanes know (or care) about the orientation of the horizon. The airplane can experience G=1 at literally any attitude, up or down, upright or inverted, because "G" is simply a force vector that is measured in a direction perpendicular to the wings (in general terms, it is opposite of the lift vector, the force the wing generates in the direction from your butt up through your body to your head as you sit upright in the aircraft).
Click to expand...
That is a great way of putting it.
 
Hank S said:
Silly me, I always thought Gravity acted towards the center of the earth
Click to expand...

Gravity does, but acceleration is independent. :) Have you seen the rotating space station in the classic movie 2001?
 
G-force? All you need to know is if you're not at the double rate beeper at the merge you're a kitten.gif
:D
 
When you are standing on the surface of the Earth, you are moving in two directions at between 800-900 mph, but still experiencing 1G.
 
Dan Thomas said:
A stall at anything above 1G is called an accelerated stall. This is something that regularly kills people: the guy that buzzes the runway or a friend's place and pulls up hard, somtimes adding a bank to the pull-up. Stall speed rises by the square root of the increase in load factor, so if the airplane stalls at 50 Kt at 1G, it will stall at 71 Kt at 2G and 87 Kt at 3G. The airplane's nose falls and often a very brief spin develops, and it's all over. The guy has no idea what he did wrong.

In fact, this is how Va is figured: at 3.8G, the typical FAR23 standard category structural limit, the Va will be 1.95 times Vs. Note that Va gets lower at weights below gross, since stall is lower.

Remember that the wing only cares about angle of attack, not airspeed.
Click to expand...

Or a pilot get's a little stally in the pattern, lowers the nose like he's supposed to but the ground gets too big in the windscreen so he panics and yanks back on the yoke causing an accelerated stall and spin. Accelerated stalls are much more real-life than power-on/power-off stalls because they can happen in an instant.
 
Any stall can happen in an "instant." That instant is when the wing exceeds it's critical angle of attack. You may not recognize a high AoA in accelerated flight and be surprised by a stall, but the same is true for any stall where you are unaware of your AoA immediately prior to departure. That's why the FAA likes AoA indicators in airplanes.
 
Witmo said:
Any stall can happen in an "instant." That instant is when the wing exceeds it's critical angle of attack.
Click to expand...

Assuming you don't pull the wings off before critical AOA is reached.
 
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