Negative G ratings- What do they represent?

[916Ducati]

What do negative G ratings for aircraft represent ?

For example a stunt plane like the popular ZI 540, says it is good for 20 positive Gs and something like ten negative G’s

I only know about Negative G’s from a skydiving context, like if the clouds cover up the exit-point on the climb to altitude, making it illegal by the FAA standards to jump – the plane will descend faster than free-fall to save on gas/time/money.



So why would an aircraft have a negative G rating? I can understand how positive G’s STRESS the strength and flexibility of a wing, ultimately to the point of distortion and breaking.

How do Negative G’s stress a(n) wing/aircraft? Does the plane, ‘get too light’? :magnify_1

I can see how positive = more weight= more stress= limits ….but I don’t yet understand how negative = less weight = breaking?

Or if it more to do with glide and the ably to fly? Is the negative G rating, more about the aerodynamic efficiency of a wing?

Would a plane over its negative G rating break-apart or stall/fall from sky?

Thank you for your time and help.

Joe

 

[Brett327]

Negative Gs don't mean less weight, they mean more weight, but in the opposite direction. Without geeking out on the physics minutiae, if you think of +2Gs pulling down on the structures of an airframe, you can think of -2Gs as pushing up in the same way. The physiological effects on aircrew are usually of greater concern for negative Gs. Again, think of it as the reverse of what happens when you pull positive Gs. Instead of all the blood pooling in your lower body/extremities, it rushes into your head and can cause "red-out" in your vision, pain, and bursting of blood vessels. An airframe is subject to those same forces, so design considerations and limitations will dictate just how much negative force it can tolerate.

Brett

 

[916Ducati]

Ah, I get it - the direction of force, cools thanks.

Is a red out more dangerous than a black-out? is there a negative 'g-lock' ?

can the body handle more positive G's than negative ones?

Thanks
Joe

 

[Brett327]

Ah, I get it - the direction of force, cools thanks.

Is a red out more dangerous than a black-out? is there a negative 'g-lock' ?

can the body handle more positive G's than negative ones?

Thanks
Joe

The body can definitely do more + than -. It's got more to do with pain, vision and vascular trauma than LOC. There's a physiological phenomenon called "push-pull" where your + tolerance can be significantly reduced by going from - to + Gs. I'm not a fighter type, so there may be some caveats (Guns D?), but there's generally not a great deal of tactical use in flying sustained or high negative Gs.

Brett

 

[scoober78]

Negative Gs don't mean less weight, they mean more weight, but in the opposite direction.

Kinda a minor point....but it actually means acceleration, not weight.

1G= the normal acceleration of gravity, in the normal direction...
-1G= the normal acceleration of gravity, in the opposite direction...
2G= twice the normal acceleration of gravity in the same direction...

Not trying to nerd anyone out but...when thinking of G's in terms of aircraft it helps since the acceleration of the aircraft turning or pitching or...is what is actually causing the phenomena.

<edit>
Brett...I in no way meant to imply that you didn't know this...

 

[BurghGuy]

Also, not trying to "geek out" on anyone, but your both right.

F=ma anyone? The amount of force (weight felt) is directly proportional to the acceleration. The Gs are indeed, a measure of acceleration in relation to the normal pull of gravity, but the end result is a "force" that felt in a certain direction.

 

[scoober78]

Also, not trying to "geek out" on anyone, but your both right.

Actually...no....

G's are a measure of acceleration...

You are correct...F is proportional to acceleration, but also to mass...so, if you and I are accelerated in a 2G acceleration, we will feel different forces because we have different masses.

You sound like you know this but its not what you said.

 

[Brett327]

Jesus Christ. See what happens when the engineers get a hold of things. It's like monkey's fvcking a football. Keep all that math to yourselves - it unnecessarily complicates what amounts to a very simple explanation. Acceleration, weight, whatever. The perceived force from the aircrew's/airframe's perspective is weight. The next person who posts an equation gets a virtual People's Elbow, followed by a kick to the larynx.

Brett

 

[skidz]

Isn't this a bit late for phryday fysics? or is it early?

 

[Fmr1833]

Jesus Christ. See what happens when the engineers get a hold of things. It's like monkey's fvcking a football. Keep all that math to yourselves - it unnecessarily complicates what amounts to a very simple explanation. Acceleration, weight, whatever. The perceived force from the aircrew's/airframe's perspective is weight. The next person who posts an equation gets a virtual People's Elbow, followed by a kick to the larynx.

Brett

I don't know, Brett, I think an equation is just what we need here. How about:

"StupidFvcking equations + UnneccesarilyComplicated posts = a reduction in my give a sh!t factor."

or simply,

SFeq + UCpst = -(Gs).

Also, I'm a tad bit disappointed in the AW crew here. 8 posts in and I'm the first one to call out 916Ducati for using the phrase, "my tummy feels light and I feel floaty." I haven't heard that kind of talk since my first hook up when I was in pre-k. Oh, those were the days, two root beers and you were golden...seriously...tummy?...floaty?...are we fivey?

 

[Brett327]

I don't know, Brett, I think an equation is just what we need here. How about:

"StupidFvcking equations + UnneccesarilyComplicated posts = a reduction in my give a sh!t factor."

or simply,

SFeq + UCpst = -(Gs).

Also, I'm a tad bit disappointed in the AW crew here. 8 posts in and I'm the first one to call out 916Ducati for using the phrase, "my tummy feels light and I feel floaty." I haven't heard that kind of talk since my first hook up when I was in pre-k. Oh, those were the days, two root beers and you were golden...seriously...tummy?...floaty?...are we fivey?

That was great. You get a waiver for above average humor.

Brett

 

[scoober78]

haven't heard that kind of talk since my first hook up when I was in pre-k. Oh, those were the days, two root beers and you were golden...seriously...tummy?...floaty?...are we fivey?

My keyboard is not your friend...funny $%^*

virtual rep in fact...

 

[skidz]

rep to FMR, nicely done. How many jumps do you have Ducati?

 

[Catmando]

... I'm not a fighter type, so there may be some caveats (Guns D?), but there's generally not a great deal of tactical use in flying sustained or high negative Gs.

Very true…………except only when suddenly finding yourself descending rapidly, inverted, and being too close to the dirt!