load limit?

[sleddriver71]

Just sitting around thinking again. Let's say your aircraft is classified in the normal category and is rated for up to 3.8 positive G's. Let's assume that the gross weight of this aircraft is 2500 lbs. So, at 3.8 G's (correct me if I'm mistaken), the wings are supporting 9500 lbs. Now, by regulation, this same airplane is limited to 3.8 G's, even at a lesser weight. So, at 2000 lbs, if we pulled 3.8 G's, the wings would only be supporting 7600 lbs. My question is, wouldn't it be just as safe to pull 4.75 G's at an aircraft weight of 2000 lbs as it is to pull 3.8 G's at 2500 lbs? In both cases, the wings would be supporting 9500 lbs. Just wondering what you all thought about that?

'You never know how fast you can go until you fall down.'

 

[Catbert]

It's more than just what the wings will carry. We also have to consider fixed weight items and their effects on the structure at certain g-loads.

For example. An engine that weighs 300 pounds at 3.8 g's will have an effective weight of 1140 pounds on its supporting structure. That engine mount will only handle so much weight. Its load bearing capacity won't change with the weight of the aircraft.

Now repeat this for lots of other fixed-in-place structures. The floor supporting the battery. The landing gear bay. Maybe the max load for the floor in the baggage compartment is 75 lbs/sq ft under normal circumstances, and it's rated for normal category 3.8 g's. You load that floor to its max allowable weight on the ground, then pull more than 3.8 g's, you have exceeded the load it was designed for. None of this will change with the weight of the aircraft either.

In real life, aircraft have something of a fudge factor built in as well. But that's a whole other ball of wax.

 

[sleddriver71]

That's a good point, I didn't think about the other fixed items in the aircraft. I was just thinking about the weight on the wings.

 

[aucfi]

It's more than just what the wings will carry. We also have to consider fixed weight items and their effects on the structure at certain g-loads.

For example. An engine that weighs 300 pounds at 3.8 g's will have an effective weight of 1140 pounds on its supporting structure. That engine mount will only handle so much weight. Its load bearing capacity won't change with the weight of the aircraft.

Now repeat this for lots of other fixed-in-place structures. The floor supporting the battery. The landing gear bay. Maybe the max load for the floor in the baggage compartment is 75 lbs/sq ft under normal circumstances, and it's rated for normal category 3.8 g's. You load that floor to its max allowable weight on the ground, then pull more than 3.8 g's, you have exceeded the load it was designed for. None of this will change with the weight of the aircraft either.

In real life, aircraft have something of a fudge factor built in as well. But that's a whole other ball of wax.

Well said with great points. Not bad for "Instructor Trash"

 

[Catbert]

Thanks.

 

[Fugawe]

Just sitting around thinking again. Let's say your aircraft is classified in the normal category and is rated for up to 3.8 positive G's. Let's assume that the gross weight of this aircraft is 2500 lbs. So, at 3.8 G's (correct me if I'm mistaken), the wings are supporting 9500 lbs. Now, by regulation, this same airplane is limited to 3.8 G's, even at a lesser weight. So, at 2000 lbs, if we pulled 3.8 G's, the wings would only be supporting 7600 lbs. My question is, wouldn't it be just as safe to pull 4.75 G's at an aircraft weight of 2000 lbs as it is to pull 3.8 G's at 2500 lbs? In both cases, the wings would be supporting 9500 lbs. Just wondering what you all thought about that?

'You never know how fast you can go until you fall down.'

Sled,

Wow, that's a lot of thinkin' But, you are correct in that available G changes with GW.

I flew a jet years ago that we memorized a sliding scale for G available based on weight. We watched the fuel and knew how much G was available before we entered a maneuver. It was all fuselage fuel, no wing fuel.

New jets have computers that constantly calculate available G,even rolling G's (outside wing is generating a little more lift than inside). You get audible warnings as the Gs increase, as well as over-G warnings if you exceed available G. You need the audible warnings because you're way too busy to watch a G-meter.

I don't know if civilian aerobatic planes have audible systems in them - any folks know? I know most of those competition planes are built so tough they probably seldom get near the load limits.

Fugawe
--no more Gs, if I'm sloshing coffee I'm maneuvering too aggressively

 

[A Squared]

Just sitting around thinking again. Let's say your aircraft is classified in the normal category and is rated for up to 3.8 positive G's. Let's assume that the gross weight of this aircraft is 2500 lbs. So, at 3.8 G's (correct me if I'm mistaken), the wings are supporting 9500 lbs. Now, by regulation, this same airplane is limited to 3.8 G's, even at a lesser weight. So, at 2000 lbs, if we pulled 3.8 G's, the wings would only be supporting 7600 lbs. My question is, wouldn't it be just as safe to pull 4.75 G's at an aircraft weight of 2000 lbs as it is to pull 3.8 G's at 2500 lbs? In both cases, the wings would be supporting 9500 lbs. Just wondering what you all thought about that?

'You never know how fast you can go until you fall down.'

It's more than just what the wings will carry. We also have to consider fixed weight items and their effects on the structure at certain g-loads.

For example. An engine that weighs 300 pounds at 3.8 g's will have an effective weight of 1140 pounds on its supporting structure. That engine mount will only handle so much weight. Its load bearing capacity won't change with the weight of the aircraft.

Now repeat this for lots of other fixed-in-place structures. The floor supporting the battery. The landing gear bay. Maybe the max load for the floor in the baggage compartment is 75 lbs/sq ft under normal circumstances, and it's rated for normal category 3.8 g's. You load that floor to its max allowable weight on the ground, then pull more than 3.8 g's, you have exceeded the load it was designed for. None of this will change with the weight of the aircraft either.

In real life, aircraft have something of a fudge factor built in as well. But that's a whole other ball of wax.

Good question and good answer. Just to take this one more step, this is *exactly* why manuvering weight goes *down* when gross weight decreases. It's not to keep from ripping the wings off, it's to protect the secondary structure. It seems that very few pilots understand this. When you ask why manuvering speed decreases with weight, you get all sorts of silly answers with nonsense like "big ships crashing through waves better" and "reserve angle of attack"

I flew a jet years ago that we memorized a sliding scale for G available based on weight. We watched the fuel and knew how much G was available before we entered a maneuver. It was all fuselage fuel, no wing fuel.

New jets have computers that constantly calculate available G,even rolling G's (outside wing is generating a little more lift than inside). You get audible warnings as the Gs increase, as well as over-G warnings if you exceed available G. You need the audible warnings because you're way too busy to watch a G-meter.

If I'm not mistaken, this was the cause of the loss of an F-15 at Galena, AK a while back. The pilot (whose wife was on base that day) did a low pass with a hard pullup. As I understand it, he didn't have the correct weight of the external stores entered into his computer, the computer thought the plane was lighter, and he pulled to the max gomputed G in the pullup and pulled a wing off the plane.

 

[Fugawe]

If I'm not mistaken, this was the cause of the loss of an F-15 at Galena, AK a while back. The pilot (whose wife was on base that day) did a low pass with a hard pullup. As I understand it, he didn't have the correct weight of the external stores entered into his computer, the computer thought the plane was lighter, and he pulled to the max gomputed G in the pullup and pulled a wing off the plane.

Don't remember the accident, and I haven't flown those jets for years, but I thought they recognized the stores and programmed automatically. Could've been a malfunction, or pilot screwup -- either way, not good.

Years ago an F-15 at Tyndall AFB lost a wing and the pilot died when he (F-15 student) pulled an estimated 15 Gs in a high speed pitch back. It's not that hard to get a quick G spike at speeds near the mach when you're looking back over your shoulder which is what happened to him apparently -- the instructor was watching from the other jet.


Fugawe

 

[FL420]

this is *exactly* why manuvering weight goes *down* when gross weight decreases.

Just to clarify, we all know you meant to type speed instead of weight.

"G overshoot" is(or was, back in the dark ages) a common source of overstress as the speed bled off from supersonic through transonic into subsonic. The Center of Pressure(CP) would rapidly shift forward closer to the CG as airflow over the wing became subsonic resulting in "stick force lightening." A pilot holding a constant large rearward stick force would almost instantaneously find the same stick force pulled the stick even further back and added up to(IIRC) a G and a half or even more as the CP moved rapidly forward.

 

[A Squared]

Just to clarify, we all know you meant to type speed instead of weight.

Yeah, that is what I meant, thanks. Hope nobody was thinking "WTF is he talking about?"