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G limitations - what do they mean?

P

prkaye

Well Known Member
When a designer posts "G" limitations for an aircraft, what does that mean precisely, and how is it arrived at? For e.g. I think Vans posts +4G limit for the -9A. Does this mean they've put the -9A spar in a fixture and watched it snap at +4G, and they've done analysis to know this is the first thing that will break? Or is it the point at which the structure will BEGIN to deform? Does it include a safety margin, meaning that the design is supposed to be OK for up to and including +4G? Where do these numbers come from?

Please don't turn this into another "aerobatics in a -9/9A" thread - I'm honestly just interested in finding out what published G limitations really mean.
 
Calculated

Hi Phil,
Van builds a 50 % safety margin into the RV's as do certified manufacturers. If the 9 has a 4G limit, that is at max gross weight. Just say it flies at 1750 lbs, then times 4= 7000 lbs on the wings. They can take that without any permanent deformation. If you pull it to 5.5 Gs at the same weight, then there is 9625 lbs and there can be permanent deformation but not failure...or so they hope.

The -4, 6, 7 and 8 all have a 6G design (at gross weight) and should stay together to 9 G's but then the airplane may not be re-usable because of permanent skin deformation. This all only really applies if the airplanes are built well, to Van's standards.

Several years ago, a couple of guys pulled the wing/wings off an -8 and died. Van had the remains of the airframe examined at a lab and the conclusion was that they had subjected the airplane to more than 9G's.

I'm not sure if Van has sandbagged the wings to their respective G limits but I recall seeing wings bowed from sandbags...a pretty precise loading with the bags placed carefully to simulate real loads. Many manufacturers will go to destruction on one or more sets of wings to find the failure points (where they break) and how many G equivalent.



I'm sure you'll get some more detailed info from the engineering guys.

Regards,
 
As I have understood it, for experimental aircraft they do not neccesarily have to mean anything. The normal thing is that +4g means it can withstand at least +4g with a safety factor of 1.5, that is +6g before structural damage.
 
the conclusion was that they had subjected the airplane to more than 9G's
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Wow... without a G-suit, I would think not many people could withstand that for more than a couple of seconds (if that) before blacking-out! Canadian fighter pilots are put in the centrefuge at +6G and have to remain concious for 15 seconds, and apparently it's not easy. 9 G???????
 
Take a look at the latest update for the 12. There was some stuff in there about testing the wing. They did test it to failure. They were happy to report that it went past the calculations and then when additional weight was applied, it broke.

There was allot about this posted during the investigation about the 8 demonstrator.
 
prkaye said:
Wow... without a G-suit, I would think not many people could withstand that for more than a couple of seconds (if that) before blacking-out! Canadian fighter pilots are put in the centrefuge at +6G and have to remain concious for 15 seconds, and apparently it's not easy. 9 G???????
Click to expand...

The 9 G's was most likely from a quick pull on the stick, while straight and level; and not from aerobatic type manuvering. The whole episode was only a few seconds.

L.Adamson
 
All Correct

pierre smith said:
Hi Phil,
Van builds a 50 % safety margin into the RV's as do certified manufacturers. If the 9 has a 4G limit, that is at max gross weight. Just say it flies at 1750 lbs, then times 4= 7000 lbs on the wings. They can take that without any permanent deformation. If you pull it to 5.5 Gs at the same weight, then there is 9625 lbs and there can be permanent deformation but not failure...or so they hope.
Click to expand...

If Van is using a 1.5 safety factor then the design is good up to 4Gs with no structural permanent deformation and 6Gs (ultimate load) with no failures. If you pull more than 4Gs you better do a thoural inspection because something inside may have deformed. Unless you look at Van's analysis you do not know what the weakest point is and it varies for different G loadings (maneuvers) and manufacturing variation (builder short cuts, mods, etc)

L.Adamson said:
The 9 G's was most likely from a quick pull on the stick, while straight and level; and not from aerobatic type manuvering. The whole episode was only a few seconds.
L.Adamson
Click to expand...

Most people can easily stay conscious for a 9G instantaneous manuever. It only happens for a second so the blood does not have a chance to be pulled from your upper body like in a sustained manuever.
 
pierre smith said:
The -4, 6, 7 and 8 all have a 6G design (at gross weight) and should stay together to 9 G's but then the airplane may not be re-usable because of permanent skin deformation. This all only really applies if the airplanes are built well, to Van's standards.
Click to expand...
The 6g limit on the -4, -6, -7, -8 are NOT at gross weight. The aerobatic load limits are at a reduced gross weight.
 
G Loads

After the wing failure on the RV-8, Van did a sandbag load test of an actual amateur built wing. As reported in an older issue of the "Aviator", the wings withstood 9 G's. They were deformed but did not fail. He went on to state in another article on aerobatics that an RV flying at redline speed will be subjected to over 16 G's with rapid full deflection of the stick. That is why it's called "The Wing Removal Lever:!!!!!!!!!!:eek:
 
Manuever Speed

mannanj said:
He went on to state in another article on aerobatics that an RV flying at redline speed will be subjected to over 16 G's with rapid full deflection of the stick. That is why it's called "The Wing Removal Lever:!!!!!!!!!!:eek:
Click to expand...

Manuever speed is max speed that you can be flying, have maximum control stick input, and not over G (pull more than 6Gs) the structure. Van lists that at only 116 kts for the RV-6. Not very fast. Be careful out there!
 
Not to muddy the waters here, but I understand this is TAS for all V-speeds. So, you may be indicating 205 mph at 8500 feet but you're actually above redline by a few mph (in a -7). Same thing goes for Va with that full-deflection of the controls. Or am I mistaken?

Update: after thinking about this for a few seconds, I may be thinking of only Vne as the one measured with TAS. The others may be indicated (seeing as how you need to color-code your IAS guage)...can anyone straighten me out?
 
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lostpilot28 said:
Not to muddy the waters here, but I understand this is TAS for all V-speeds. So, you may be indicating 205 mph at 8500 feet but you're actually above redline by a few mph (in a -7). Same thing goes for Va with that full-deflection of the controls. Or am I mistaken?

Update: after thinking about this for a few seconds, I may be thinking of only Vne as the one measured with TAS. The others may be indicated (seeing as how you need to color-code your IAS guage)...can anyone straighten me out?
Click to expand...
Va is the speed at which the wing can generate enough lift at the stall to reach the max allowable g. Lift is defined by CAS, not TAS (and yes, for the pedants out there, I am ignoring the tiny difference between EAS and CAS here). So, forget about TAS when thinking of Va.
 
L.Adamson said:
The 9 G's was most likely from a quick pull on the stick, while straight and level; and not from aerobatic type manuvering. The whole episode was only a few seconds.

L.Adamson
Click to expand...

An Air Force friend told me that if you pulse the stick you can remove the wings almost instantly, even in the USAF's best.

Whenever this topic comes up everyone always talks about the wings. What about he HS? What are odds of snapping the HS before the wings?
 
N941WR said:
...Whenever this topic comes up everyone always talks about the wings. What about he HS? What are odds of snapping the HS before the wings?
Click to expand...
Just about everyone used to think you couldn't break anything on the airplane below Va. AA 587 proved that it might be possible to break something, with pulsing, below Va.
 
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Mel said:
The 6g limit on the -4, -6, -7, -8 are NOT at gross weight. The aerobatic load limits are at a reduced gross weight.
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I second Mel's post.

All RV's that are approved by Van's for aerobatics have a specified Aerobatic gross weight. This is less than the full (non aerobatic flight) gross weight.

For my RV-6A the normal gross weight is 1650 lbs but the aerobatic gross weight is 1375 lbs.
 
lostpilot28 said:
Not to muddy the waters here, but I understand this is TAS for all V-speeds. So, you may be indicating 205 mph at 8500 feet but you're actually above redline by a few mph (in a -7). Same thing goes for Va with that full-deflection of the controls. Or am I mistaken?

Update: after thinking about this for a few seconds, I may be thinking of only Vne as the one measured with TAS. The others may be indicated (seeing as how you need to color-code your IAS guage)...can anyone straighten me out?
Click to expand...


Sonny - I think that what you are remembering are the discussions on flutter - those speed limits are TAS, and are real, but are not directly related to the g-limits being discussed here.

Paul
 
Whatever the G limits are finally determined to be, keep them to a minimum. Going under the knife for faulty hemorrhoids is no fun - been there, done that. The surgeon commented during the pre briefing, pilots are his #1 source of business so be forewarned....:) I say again, the entire business was NO FUN.
 
Ironflight said:
Sonny - I think that what you are remembering are the discussions on flutter - those speed limits are TAS, and are real, but are not directly related to the g-limits being discussed here.

Paul
Click to expand...

You're right, Paul...now that I think about it. Thanks for clearing that up. :D
 
I was in Van's shop and saw the remains of the -8 wing that was tested to failure. It was amazing to see the results of the loading. The inboard skin had a huge rip in it. I don't know if the skin ripped which allowed the spar to fail or if the spar failed which caused the skin to rip... Whatever the cause, it makes you appreciate how the skins and skeleton depend on each other for structural integrity. Absolutely fascinating art/science!!!
Don
 
Pardon my ignorance, but I can't remember what "EAS" is (not sure if I ever learned this term in my PPL training actually)...
 
pierre smith said:
Hi Phil,

Several years ago, a couple of guys pulled the wing/wings off an -8 and died. Van had the remains of the airframe examined at a lab and the conclusion was that they had subjected the airplane to more than 9G's.
Click to expand...

I think they made a spar change after this as well. The spar doubler now extends past the outer edge of the fuel tank.

I also think the F-16 is FBW. From what I understand on that aircraft is there is no chance to fly by hand. If the computers fail, the aircraft will not even fly straight and level.
 
Yes...

Sid Lambert said:
I think they made a spar change after this as well. The spar doubler now extends past the outer edge of the fuel tank.

.
Click to expand...

Yep, I understood that the wing broke at the outboard end of the fuel tank, where the computer said it would, not at the spar attach near the fuselage. Dunno about the spar mod but hey, it took 9 G's the way it was originally.

Regards,
 
The RV-8 "B" wing

I talked to Van's guys at Oshkosh, and they told me that the change in the RV-8 wing was not for the RV-8, but was a change when designing the RV-7. They wanted to increase the GW of the RV-7 and didn't want to make two different wings. The RV-8's limits did not change, as the original wing met design standards, and the extra strength was just extra weight for the 8.

I ask him if that was the truth, or just the company line. He assured me that he was telling the truth. The RV-8 "B" wing is stronger, and heavier. The load limits are the same for both.
 
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Van' website

From Van's website: The RV-3B, RV-4, RV-6/6A, RV-7/7A, and RV-8/8A have been designed for the operational stress limits of the aerobatic category (+6.0/-3.0 G) at and below their aerobatic gross weights. The operational stress limits for these aircraft between their aerobatic gross weights and their maximum design gross weights are utility category (+4.4/-1.75 G).

The design operational stress limit for the RV-9/9A is utility category (+4.4/-1.75 G) at less than 1600 pound gross weight and is standard category (+3.8/-1.5 G) between 1600 pounds and the aircraft?s design gross weight.

The design operational stress limit for the RV-10 is standard category (+3.8/-1.5 G).
 
prkaye said:
Pardon my ignorance, but I can't remember what "EAS" is (not sure if I ever learned this term in my PPL training actually)...
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EAS = Equivalent Airspeed. If you read the standard private pilot ground school stuff you'll get nice (but useless) words on the difference between TAS and EAS, and EAS and CAS, like "EAS is CAS corrected for compressibility" - that sure clears things up :)

Lets look at this from an aerodynamics point of view. If we look at the standard equations for lift and drag, we find that they start with 1/2 times the density times the speed (1/2 rho V^2, where rho is how you pronounce that Greek symbol for density). This term, 1/2 rho V^2, is the dynamic pressure - this is the pressure over and above the ambient pressure that is created when the moving air "hits" the aircraft. It is a very important parametre that determines lift, drag, aerodynamic forces, etc. If the air that we flew in was an incompressible fluid, the dynamic pressure would depend on CAS. In this case, our stall speeds, lift, drag, aerodynamic loadings on surfaces, etc would all depend exactly on CAS. But, the wonderful air that lifts us is compressible. In the world of compressible fluids, the dynamic pressure varies exactly with EAS instead of CAS. The pilot ground school stuff hint at this when they talk about a compressibility correction to get from CAS to EAS.

If EAS is so important, why do our airspeed indicators show CAS, and not EAS? That is because there is no simple mechanical design that can take pitot and static pressure and output EAS. A CAS indicator just needs a diaphragm with pitot pressure on one side, and static pressure on the other side, and a gearing mechanism to convert movements of the diaphragm to movement of a needle.

The difference between CAS and EAS is not practically important to us RV folks, as EAS is almost exactly the same as CAS in the airspeed and altitude range that RVs operate at. For example, at 200 kt CAS at 10000 ft, the difference between CAS and EAS is one kt. The difference between CAS and EAS becomes greater at higher speeds and altitudes. At typical airliner speeds and altitudes the difference between CAS and EAS is about 10 kt. At very high speeds and altitudes, the difference becomes quite important, which is why SR-71s have airspeed indicators that read in EAS rather than CAS.

I've cut a few corners here - the full story, with all relevant tangential discussions, would take many pages.
 
Knowing Van's I would say this is more plausible.

Any company that can combine parts in inventory will increase profit. If it means having safer parts then all the better.

Danny King said:
I talked to Van's guys at Oshkosh, and they told me that the change in the RV-8 wing was not for the RV-8, but was a change when designing the RV-7. They wanted to increase the GW of the RV-7 and didn't want to make two different wings. The RV-8's limits did not change, as the original wing met design standards, and the extra strength was just extra weight for the 8.

I ask him if that was the truth, or just the company line. He assured me that he was telling the truth. The RV-8 "B" wing is stronger, and heavier. The load limits are the same for both.
Click to expand...
 
G's

Did anyone mention that there is a time factor (duration of G load) involved? Van noted that the airframe must withstand the 150% (1.5 times normal max G) load only for a few seconds. Presumably, longer duration loads would increase deformation and risk of failure. I've often wondered about extremely short duration "bumps" that occur either IFR or in the clear. These can be very sharp and strong. I don't think my G meter responds quickly enough to register these accurately and many can occur as a single event at a speed well above maneuvering. Hopefully these are not damaging. Any engineers out there who know the answers? A lot of high time airframes are still together and flying, so maybe there's no cause for concern. Bill
 
New wings

Hi Bill/guys,
The jolts are cumulative as many older airplanes have had AD's issued concerning wing replacement, including my Air Tractor at 8000 hours. I seriously doubt if it has had more than a momentary 3G's but I do 1.5-2 G pulls very often in my work.

The Beech T-34, the Beech -18 others have either had restrictions or modifications performed on the spars.

I just don't think that there are any RV's with 5000 or more hours on them yet and I may not see one in my lifetime with that many hours, to see how well they hold up with age.

Bear in mind that my Air Tractor is built to normal category 3.8 G specs and most RV's are built to 6 G giving us a much better margin for accumulative cyclical wing flex.

Regards,
 
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The certification requirements specify ultimate load for 3 seconds without failure. Permanent deformation is permissible.

Pete
 
max G or accumulation ?

Hi Pierre,

I am wondering if cumulative cycling would be more relevant in terms of fatigue life than the actual 'G' load is in terms of wing strength ?

In other words, if I pull 6 G once every time I fly, is that any safer than 2000 hours of flying in rough air that would accumulate a lot more cycles but of lesser intensity ? I also notice that some military trainers, such as the PC-9, have around 6 or so G-meters, measuring various loads across the airframe, and all accumulative that is used for maintenance and airframe life calculations.

Cheers

Martin

(Pierre wrote : The jolts are cumulative as many older airplanes have had AD's issued concerning wing replacement, including my Air Tractor at 8000 hours.
Bear in mind that my Air Tractor is built to normal category 3.8 G specs and most RV's are built to 6 G giving us a much better margin for accumulative cyclical wing flex.)
 
Fatigue Life

Airframe cycles are fatigue and typically defined by spectrums. These spectrums define number of cycles and frequencies that the structure needs to meet. Typically fatigue referes to much higher frequency then that associated by normal turbulence. More like vibrations. That being said, Gear and engine are designed to cycles like number of landings and number of engine starts or number of over temps.
I do not know what FAR Part 23 calls for general aviation to be designed to. In military aircraft they are designed to things like 20 year life, 6,000 hours, and a very detailed spectrum of fatigue frequencies.
 
Thanks...

......Philip. Nice to have our own resident Aeronautical engineer contribute. I complained and moaned when I had to spend north of $11,000 in 2001 for a new, stronger spar splice plate on the wings' center section. This, after an Air Tractor lost a wing in Arizona, a fatal end.

The factory had tested the original splice by bending the wings up and down literally thousands of times with hydraulic rams without failure but the real life experience revealed what many considered a design flaw.

The result was an 8000 hour lower steel spar cap life. At that point, the wings come off, go back to the factory where they remove the leading edges and ribs and lower spar caps and replace them with new, for another 8000 hour life. Cost...$32,500 thankyou. Talk about drilling out a bunch of rivets:eek:

Regards,
 
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