Van's Air Force
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Gross weight increase.
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Don’t think you can increase the gross weight on an existing aircraft.
I know of several -10’s that were originally certified for over 2700 lbs. I believe the builders simply claimed they would be ‘in a condition for safe operation’, and supposedly would test them at the new higher weight during phase one. For an already flying airplane, you need to speak with a DAR. I *think* a new gross weight can be assigned, if you can convince the dar that it’s safe, and then go back to phase one and test it somehow. Clearly, just measuring the climb rate is not sufficient. 3000 lbs is >10% increase. I wouldn’t be casual about this.
rocketman1988
Well Known Member
Yep, the performance testing is one tiny facet of the whole picture. The more important part is the structure. Going over the recommended weight is reducing the engineering margin, the magnitude of which is information the average builder doesn't have.
If you are 10% over, the plane will likely not fall out of the sky in normal conditions, HOWEVER, you have no idea how close you are to the structural limits. Point is, if you find those limits, you may not get a chance to report them...
Kyle Boatright
Well Known Member
Walt, where is the gross weight recorded for administrative purposes? I remember giving my DAR a sample W/B, but am unsure if/where that number was recorded for the FAA's records.
RV8JD
Well Known Member
Here is something else to consider: At the Van's recommended Max Gross Weight for the RV-3B, RV-4, RV-7/7A, RV-8/8A and RV-14/14A, (and I think the RV-6/6A also) the airplane's Design Load Factor Limits are the the Utility Category Limits (+4.4/-1.75 G).
However, at the Van's recommended Max Gross Weight for the RV-9/9A and RV-10, the airplane's Design Load Factor Limits are the Normal Category Limits (+3.8, -1.5 G).
So increasing the Max Gross Weight slightly above the Van's recommended numbers on the RV-3B, RV-4, RV-7/7A, RV-8/8A and RV-14/14A, and the RV-6/6A, gets you somewhere between the Utility Category Limits (+4.4/-1.75 G) and the Normal Category Load Factor Limits (+3.8, -1.5 G).
But increasing the Max Gross Weight above the Van's recommended numbers on the RV-9/9A and RV-10, gets you less than the Normal Category Load Factor Limits of +3.8, -1.5 G, which is definitely not recommended for normal operations.
Ref: https://www.vansaircraft.com/flying-an-rv/
Having said all that, I would stick with Van's recommended Max Gross Weight numbers, as they have done the design analysis, ground testing, and flight testing to validate their airplanes for all of the different design load conditions (static, dynamic, maneuver, gust, and landing).
However, at the Van's recommended Max Gross Weight for the RV-9/9A and RV-10, the airplane's Design Load Factor Limits are the Normal Category Limits (+3.8, -1.5 G).
So increasing the Max Gross Weight slightly above the Van's recommended numbers on the RV-3B, RV-4, RV-7/7A, RV-8/8A and RV-14/14A, and the RV-6/6A, gets you somewhere between the Utility Category Limits (+4.4/-1.75 G) and the Normal Category Load Factor Limits (+3.8, -1.5 G).
But increasing the Max Gross Weight above the Van's recommended numbers on the RV-9/9A and RV-10, gets you less than the Normal Category Load Factor Limits of +3.8, -1.5 G, which is definitely not recommended for normal operations.
Ref: https://www.vansaircraft.com/flying-an-rv/
"The RV-3B, RV-4, RV-7/7A, RV-8/8A and RV-14/14A 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 RV-9/9A, RV-10 and RV-12 are not designed for aerobatic flight.
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).
No RV should ever be operated above its design gross weight limit."
Having said all that, I would stick with Van's recommended Max Gross Weight numbers, as they have done the design analysis, ground testing, and flight testing to validate their airplanes for all of the different design load conditions (static, dynamic, maneuver, gust, and landing).
Walt
Well Known Member
I don't believe the FAA has any record of WB/GW on an EAB, and they don't need to be involved if you decide to change it.
The OP asked if GW can be increased legally and the answer is yes, is it prudent, that's up to the builder.
Keep in mind one of the first things on an NTSB report is if the AC exceeded GW, by any amount. If you did, you're immediately at fault.
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Canadian_JOY
Well Known Member
I own an amateur-built aircraft built by another person. That person was well aware that most loading scenarios put the aircraft dangerously close to its commonly-listed gross weight. When going through initial airworthiness certification he claimed an extra hundred pounds on the gross weight. When I asked him why he did this he explained the fear of being found overweight. After all, it only takes one pound over to be over...
I fly the aircraft based on its more common gross weight, but am very glad I have that "legal padding" in the event something happens and I made a mistake in my weight calculations.
As far as I know, there is no regulatory binding gross weight limit for amateur built experimental aircraft. The airworthiness requirements are contained in the operating limitations, which are requried to be in the aircraft for flight. No gross weight limit is in that document, at least for my aircraft.
It is wise to have weight and balance information in a POH or similar document for ready reference in the aircraft, but there is no regulatory requirement to have such a document. If you have such at document, it's contents are at your discretion. In the W&B info in my checklist I note the limit as "Van's recommended maximum gross weight is 2700 lbs."
As others have noted, if you decide to fly over the designer's recommended gross weight, you are eating in to the design safety margin.
Bill Boyd
Well Known Member
A small point, but still to be noted: you are reducing margin until you have burned fuel, expended ordnance, thrown baggage overboard or ejected unruly passengers sufficient to bring the aircraft back into the 2700 pound MTOW/gross weight limit. After that, you're good. For most RV-10's, that happens at about 60-80 pounds per hour in cruise, much faster in climb.
jnorris
Well Known Member
Walt,
Not sure about other FAA offices, or inspectors, but every aircraft we certificate through the Minneapolis office has a weight and balance report submitted to the FAA via the AWC system, which goes into the aircraft record in OKC. So there may be paper trail with FAA regarding the originally reported gross weight of the aircraft.
As for the FAA being involved, there is a paragraph in the operating limitations stating that any operation conducted in Phase 2 (Normal operations) must be tested in a flight test period (AKA Phase 1). This includes testing to the desired gross weight. If you claim a gross weight of a certain number, the FAA will assume that the aircraft was tested to that weight. (Whether or not this testing actually takes place is up for debate, but that will be the assumption by the FAA.) So, if you are operating the aircraft at a weight higher than that to which it was (theoretically) tested, you are in violation of your operating limitations. Now, that being said.....
Or it could be up to a subsequent owner. The operating limitations contain a provision for making "major changes" to the aircraft. Raising the gross weight would be a major change, and the required testing could be done by anyone, not just the builder. So in answer to the original poster's question, yes, the gross weight could be changed subsequent to original certification.
Also keep in mind that, if you are operating the aircraft in excess of the designer's recommended gross weight, you are definitely a test pilot, so be aware and be prepared.
The critical issue is CG. There are many examples of type certificated airplanes that had gross weight increases with no structural changes. One example is Piper Aztec which went from 4800# to 5200# with no structural changes.
Tip tanks on Piper Comanche and Twin Comanche allow a gross weight increase of 180# which is the weight of the tip tank fuel. IIRC the Max Conrad world record flight in the TC was at nearly two times the certificated weight of the airplane at takeoff. The tip tank increase only applies when there is fuel in the tip tanks.
Tip tanks on Piper Comanche and Twin Comanche allow a gross weight increase of 180# which is the weight of the tip tank fuel. IIRC the Max Conrad world record flight in the TC was at nearly two times the certificated weight of the airplane at takeoff. The tip tank increase only applies when there is fuel in the tip tanks.
Many airplanes will fly when over gross weight. Now how they fly when forward or aft of the recommended CG limit could make them unstable or unrecoverable from a stall.
Most metal airplanes are designed with a 1-1/2 safety factor. This means that a 6 G airplane was designed for a 9 G load. Having done a TEST flight at the recommended aft CG limit, I will add that my RV-6 will fly and recover from a stall but it is very pitch sensitive. Not fun to fly when at the recommended aft CG limit.
The landing gear is another factor. The gear should withstand a hard landing at the designer's gross weight but if over gross and a hard landing is made, what is going to happen? Most likely there could be damage to either the gear or structure the gear is attached to.
Going above the designer's recommended gross weight cuts into the safety margin of the airplane.
Others have addressed the legal issues of a gross weight change.
Pilots and aircraft owners need to manage their risk to a level that they are comfortable with.
rocketman1988
Well Known Member
"...Most metal airplanes are designed with a 1-1/2 safety factor..."
How do you know?
Where is the data?
Have you talked to the design engineers?
Your statement is making a huge assumption, as there isn't any requirement for that safety factor, especially in experimental aircraft. That is part of the problem; you don't KNOW what margins the engineers have designed to. They may be more than 1.5 but they also may be less.
The points about the CG and other structures are excellent points, which illustrate that changing one parameter in an aircraft design (gross weight, in this case) will most definitely affect many other parameters (and structures) of the aircraft.
As most everyone has said, exceeding the design parameters is much more involved than just changing a number on a data plate.
How do you know?
Where is the data?
Have you talked to the design engineers?
Your statement is making a huge assumption, as there isn't any requirement for that safety factor, especially in experimental aircraft. That is part of the problem; you don't KNOW what margins the engineers have designed to. They may be more than 1.5 but they also may be less.
The points about the CG and other structures are excellent points, which illustrate that changing one parameter in an aircraft design (gross weight, in this case) will most definitely affect many other parameters (and structures) of the aircraft.
As most everyone has said, exceeding the design parameters is much more involved than just changing a number on a data plate.
I wish it was not called a safety factor because it really is just a sudo aviation industry standard for two design points, when the structure could deform and when the structure could fail. metal will permanently deform before it fails. The 6 G limit load means the structure is designed to not deform before 6 Gs. The safety factor of 1.5 means the structure will not fail/crack/break before 9 Gs ultimate load but you could have permanently bent the structure any time above 6 Gs. For an RV-6A, which has a designed to max gross weight of 1650 lbs, if you increase max gross weight to 1800 lb you may get some structure to be permanently deformed at 5.5 Gs.
The Pitts S2A was the first airplane type certificated in the aerobatic category for a long time. The S!S and S1T were later type certificated. The plus 6 minus 3 was simply a number required for certification. I personally saw a S1T at the nationals that showed 10 G. Curtis Pitts looked at that and had a big smile.
It is reasonable to believe that every S1S flown in airshows or competition was flown at 7 G positive and way over 3 negative.
Only one factory S1S ever has had a major structural failure. That airplane had a long history of abuse.
The original RV3 was presented as a 6G aerobatic airplane with certain maneuvers such as tailslides prohibited. Service experience proved the airplane was way less than a 6 G airplane. It took two major redesigns to fix the problem. On the other hand there has never been an inflight breakup of an RV6 and few if any with the RV4. Several with the RV7 and the one with the factoryRV8 which was probably a 10G or greater event.
The things working against the aerobatic RV's are the relatively light controls, the relatively low stall speed and the rate of acceleration following a loss of control. Vans "Aerobatic Epistle" explains how easy it is to put a 20 G load on an aerobatic RV. At the other end of the spectrum is the Sukhoi SU29 with that huge bubble canopy causing a lot of drag. From a hammerhead to Vne in the 29 takes 3500 ft vertical down with full throttle. In an RV6 I would guess you would go well beyond 300 m/h.
I do not advocate overloading any airplane. But as a practical matter in the aerobatic RV's going 100# over the published max gross is insignificant. The many long distance RV flyers have proven that. The racers have proven that speed is not the big deal that some make it. Personally in my non RV I am very comfortably flying over the design weight but not over Vne.
It is reasonable to believe that every S1S flown in airshows or competition was flown at 7 G positive and way over 3 negative.
Only one factory S1S ever has had a major structural failure. That airplane had a long history of abuse.
The original RV3 was presented as a 6G aerobatic airplane with certain maneuvers such as tailslides prohibited. Service experience proved the airplane was way less than a 6 G airplane. It took two major redesigns to fix the problem. On the other hand there has never been an inflight breakup of an RV6 and few if any with the RV4. Several with the RV7 and the one with the factoryRV8 which was probably a 10G or greater event.
The things working against the aerobatic RV's are the relatively light controls, the relatively low stall speed and the rate of acceleration following a loss of control. Vans "Aerobatic Epistle" explains how easy it is to put a 20 G load on an aerobatic RV. At the other end of the spectrum is the Sukhoi SU29 with that huge bubble canopy causing a lot of drag. From a hammerhead to Vne in the 29 takes 3500 ft vertical down with full throttle. In an RV6 I would guess you would go well beyond 300 m/h.
I do not advocate overloading any airplane. But as a practical matter in the aerobatic RV's going 100# over the published max gross is insignificant. The many long distance RV flyers have proven that. The racers have proven that speed is not the big deal that some make it. Personally in my non RV I am very comfortably flying over the design weight but not over Vne.
The spelling is pseudo and is used incorrectly. A US type certificated aerobatic airplane with no maneuver restrictions will handle 7g every day for years. They do not start to bend at 6G. The Extras and similar airplanes with composite or partial composite structures are designed for plus/minus 10 G or more. The composite structure portions are designed for 20G, a safety factor of 100%.
If a pilot who has been flying a Pitts at 7G transitions to a monoplane he/she will likely be flying the monoplane between 8G and 10G within a few flights in the monoplane.
If a pilot who has been flying a Pitts at 7G transitions to a monoplane he/she will likely be flying the monoplane between 8G and 10G within a few flights in the monoplane.
CaptPausert
Active Member
Keep in mind that the effects of increased weight doesn't just affect the maximum structural load of the aircraft. It will also affect the repeat stress loading and decrease the effective fatigue life of the aircraft. We had one at work where an increase in gross weight of 20% decreased the maximum service life of the airframe by half and significantly increased the frequency of required inspections. Even if the airframe is well under yield loads you are still putting fatigue damage on the airframe every time you fly. there generally isn't an issue with most GA aircraft because most people will never get near the design life. however, if you increase the gross weight, it will change that number potentially significantly. It also tends not to be a linear relationship and depends on how the airframe is designed so it is hard to predict without doing a full analysis of the airframe structural loads.
rocketman1988
Well Known Member
Data and source or just anecdotal?
rocketman1988
Well Known Member
I guess we can disregard the design limits as you have shown them to be in error.
Multiple sources, published and personal experience. I flew my S1s at 6 1/2 to 7 for ten years. Sukhoi plus ten and minus seven for practice and a contest flight always seems to be 1 G greater. One of the first things I did in the Sukhoi was to high speed stall it pulling to vertical. Moderate speeds, probably half way between stall and Vne. It took 9 G to stall the airplane.
I built four Pitts S1S aircraft and rebuilt a couple others. The damage that I found on one rebuild was from taxiing into a solid object with the left lower wing.
I built four Pitts S1S aircraft and rebuilt a couple others. The damage that I found on one rebuild was from taxiing into a solid object with the left lower wing.
The numbers are not design limits but are the numbers presented to the FAA for certification.
rocketman1988
Well Known Member
Great but those are NOT RVs.
Personal experiences do not substitute for data; published sources?
rocketman1988
Well Known Member
Really. What engineering credentials do you possess to support this statement?
The “School of Hard Knocks” doesn’t count.
CaptPausert
Active Member
If you want to increase beyond the manufacturer recommended structural limits I recommend reading the following texts. When you can fully understand these you will have a good basis to do the calculations required to determine safe limits.
Airframe Structural Design & Airframe Stress Analysis and Sizing by Michael Niu $67-75 on amazon
Analysis and design of flight vehicle Structures by E.F Bruhn this one isn't in print anymore, but you can get used copies. They tend to be a bit pricy around $350
Airframe Structural Design & Airframe Stress Analysis and Sizing by Michael Niu $67-75 on amazon
Analysis and design of flight vehicle Structures by E.F Bruhn this one isn't in print anymore, but you can get used copies. They tend to be a bit pricy around $350
CaptPausert
Active Member
I did find a copy of the Analysis and design of flight vehicle Structures by E.F Bruhn online as an ebook. Just a fair warning it is pretty dense engineering.
You are right it is not a requirement for EXPERIMENTAL aircraft.
I am making an ASSUMPTION that any engineer worth his weight would attempt to design to FAR 23.2230 the same way I would.
Kyle Boatright
Well Known Member
I will add my $0.02 which is "Why is 3.8G's a magic number for the normal category?" I realize it is the standard, and you need those for certification. But why 3.8? Why not 3.5 or 4 or something else? 3.8 seems fairly arbitrary.
I think the normal limit load for aircraft is 2.5 Gs. Multiply the safety factor which is 1.5, you get 3.75Gs. I am not sure 3.8 is round off but I think even the big boys use 3.75Gs to test the ultimate load, like the various popular wing break videos from the 777 or other large airplanes.
Kyle Boatright
Well Known Member
For light aircraft, normal is 3.8. Utility is 4.2, and Aerobatic is 6. All those numbers have a basis, but picking a specific number in any of those categories has a fair amount of "arbitrary" in it.
Utility category load factor for light aircraft is 4.4 G’s (not that that matters in the context of this discussion since most people that fly over design gross weight don’t analyze that decision based on engineering and technical details).
Previous post by me on the higher gross weight debate
Post in thread 'Wt and Balance question'
https://vansairforce.net/threads/wt-and-balance-question.222745/post-1732528
The 150% safety factor (ultimate load) is never considered by a designer to be used in the normal operation of an aircraft.
One simple reason is that it is acceptable for any g loading above limit load, to cause permanent deformation.
Ultimate load is the value that must be exceeded before catastrophic structural failure occurs.
rocketman1988
Well Known Member
You know what they say about assumptions.
You didn’t answer the question.
Rvbuilder2002 provided a link to a previous thread and some insight to the 1.5 margin.
I am sorry you do not understand the answer. I am not going to try and explain it to you.
What's the difference between personal experiences and testing? It is data. Engineering calculations don't translate 100% accurately in the real world, that's why even engineers test after design. Sandbagging a wing, dropping an airframe to test the gear, or hydrotesting a pressure vessel, as examples.
rocketman1988
Well Known Member
No, you do not understand the question, so I will ask again:
What credentials do you have to support you statement of using the engineering safety factor to exceed the design limits?
rocketman1988
Well Known Member
So where is the data to support you statements concerning exceeding design limits?
rocketman1988
Well Known Member
I apologize. My comments were directed to someone who did. I replied to the wrong post, sorry.
Jpm757
Well Known Member
I have inspected 2 RV-4's with180 Lyc's, and C.S. props. Both aircraft had higher gross weights, 1700 and 1800 lbs respectively. One of the aircraft had cracks on both gear legs, the other had 2 cracked dynafocal rings and wrinkled firewall.
Don't try to make your RV into something it is not.
Don't try to make your RV into something it is not.
The EAB Category became law in late 1952, a few months before the founding of EAA. Initially there was no provision for carrying passengers. The subject of EAB passengers came up in a very early EAA meeting. Early on it was just talk. This can all be found in the Experimenter(Now Sport Aviation) archieves. The Wittman Tailwind was originally designed by Steve as a personal traveling. There was no intent to sell plans. The prototype first flew in 1953. Steve worked with the CAA(FAA) concerning approval to carry passengers. In Dec 1953 Steve performed flight test with the CAA Inspector observing. The tailwind was loaded to gross weight with sandbags in the right seat and baggage area. Recording G meter and polaroid camera. The airplane was dived to 110% of Vne and a pullup to 3.8G. The airplane was then approved to carry passengers-not for hire. In 1954 Steve started selling plans and by summer of 1955 there were two more examples flying. The first five Tailwinds were flight tested as described above and observed by CAA.
The following is based on a phone interview i did with the co builder of one of the five. I had been hearing the story for years and wanted to confirm it. The two builders had agreed that one would do the initial flight and the other would do the CAA test. Both were present for the CAA test. At a high speed the sandbag in the right seat slipped forward against the stick. Speed reached close to 300 m/h(Vne is 185). 8.3 positive G and 3.8 negative. The airplane landed safely. The CAA man saw the slightly pushed in windshield and a bulge in the lower fabric from the sandbag. The CAA man said "I can't certify that airplane). When the test pilot told him what had happened he signed off the airplane.
The first five Tailwind's were all tested in the same manner.
I later heard Steve Wittman tell that story. He cautioned the just because one airplane survived 8.3 G it did not mean the next one would.
The following is based on a phone interview i did with the co builder of one of the five. I had been hearing the story for years and wanted to confirm it. The two builders had agreed that one would do the initial flight and the other would do the CAA test. Both were present for the CAA test. At a high speed the sandbag in the right seat slipped forward against the stick. Speed reached close to 300 m/h(Vne is 185). 8.3 positive G and 3.8 negative. The airplane landed safely. The CAA man saw the slightly pushed in windshield and a bulge in the lower fabric from the sandbag. The CAA man said "I can't certify that airplane). When the test pilot told him what had happened he signed off the airplane.
The first five Tailwind's were all tested in the same manner.
I later heard Steve Wittman tell that story. He cautioned the just because one airplane survived 8.3 G it did not mean the next one would.
Regarding post 44, I don't believe either of those issue's were CAUSED by the higher gross weight. The issues with the landing gear portion of the engine mount are well known and are a design issue. The engine mount and gear leg damage are indicative of extremely hard landings. Additional possibility of improper heat treating of the gear legs.
I never said that one should exceed the design limits.
It seems short sighted to say that a higher aircraft weight would have zero influence.
Not all RV-4's have had cracking issues with firewall brackets (even legacy ones with the lighter brackets).
Are some of the pilots just more skilled than others?
Maybe, but it is doubtful that that is the only variable.
I believe there are a bunch of different factors.
One of those being that the original design wasn't up to the type of loads it would be subjected to in some instances, with variables to that being everything from surface condition of the runways being used, etc.
So I think everyone would agree that higher weight was not the root cause, but to say that higher weight can have no influence on failed brackets is just plain wrong.
Part 135 and 121 operators in Alaska are allowed to operate 15% over certificated gross weight. (apparently used to be 10%) This is apparently permanent for the operator.
Jerrie Mock, who was the first woman to fly solo around the world, was at 900# overweight in a early model Cessna 180.
Max Conrad was 2000# over gross in the Comanche 250 for the nonstop flight Casablanca to Los Angeles.
Current approvals for long distance ferry apparently do not have and specific limit.
AC 21-4B discusses this in detail.
Articles about Dick Rutan and others would be a good reference for someone who wants to do the earthrounder adventure.
The V tail Bonanza that is often on display near the Vintage Red Barn is another long distance flyer.
Jerrie Mock, who was the first woman to fly solo around the world, was at 900# overweight in a early model Cessna 180.
Max Conrad was 2000# over gross in the Comanche 250 for the nonstop flight Casablanca to Los Angeles.
Current approvals for long distance ferry apparently do not have and specific limit.
AC 21-4B discusses this in detail.
Articles about Dick Rutan and others would be a good reference for someone who wants to do the earthrounder adventure.
The V tail Bonanza that is often on display near the Vintage Red Barn is another long distance flyer.
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