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Over gross ?

Considering an accident/incident

Considering an accident/incident where damages were suffered, and may have been caused by an overweight condition (beyond Vans listed weights), all an opposing counsel would need to fry the offender can be found at:
http://www.vansaircraft.com/public/flyrvs.htm

Vans refers to "maximum design gross weight":

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).

Who can argue that any experimental builder certifying his experimental aircraft for a maximum gross weight exceeding the designer's "maximum design gross weight" is not "letting it all hang out?" .....legally.

Barney, in Memphis
RV-3 & RV-4 flying
 
L.Adamson said:
As I have said before, and years before that.............these same questions come up every few years. Then we get the same repetitive answers from those who are only guessing with their "moral" high ground thinking. It's even got to the point of criticizing builders for upping the weight, and then daring to sell their aircraft to some unsuspecting buyer. And at the same time, you get potential buyers who are criticizing the builder for not upping the gross to start with.
Click to expand...
And at the same time we get the same repetitive answers from armchair non-engineers who don't understand that the strength of the wing isn't the only consideration you need to think about when setting a gross weight.

As I said before, get someone from Van's who will actually condemn the 1850 gross weight on an RV6A, then we'll talk about it.
Click to expand...
Why limit yourself? Since no RV-6's have crashed while being operated at 2200lb gross, why not set it there? Van hasn't condemned a 2200 lb gross either, so it must be fine, right? :rolleyes:
 
Not condemned?

Snowflake said:
Why limit yourself? Since no RV-6's have crashed while being operated at 2200lb gross, why not set it there? Van hasn't condemned a 2200 lb gross either, so it must be fine, right? :rolleyes:
Click to expand...

See here is the problem suddenly a 1650 gross has gone to a 1850 gross, now a 2200 gross. next it will be 2500..........

Van hasn't condemned it? Take a look at the page LAvion quotes above.

The following line is taken straight off that page.

No RV should ever be operated above its design gross weight limit.

What do you expect Van to do contact every member who steps beyond this limit........ thats not his job. What van has done is set in stone a limit, thereby limiting his liability. Once you exceed those limits you are putting yourself in the hands of the civil liability lawyers if the worst should happen.

Van will just point at the limitations he sets and that line, you then have to explain to the court why you went outside these limits.

BTW don't 'assume that people like me are not engineers because we may have served with the police. I served an engineering apprenticeship (5years), have a degree in engineering and continued to practice throughout my police service. I also have a law degree so know a little bit about that too.

Bottom line here for the original poster of this question is given all that has been said, Am I happy to operate my aeroplane above Vans specified gross on the basis that I will carry the liability if an accident occurs. Am I happy that my airframe can take the loads, am I happy with my ability to cope with any situation, like gust loads which may develop.

Have a good day I look forward to meeting some of you in the next few weeks when I am in California!!
 
Steve, I agree with you. I was just pointing out the fallacy in the suggestion that simply because a lot of people had set their gross weights higher than recommended, and that they did so "because everyone else before them did it", that it was somehow safe. And if you're going to pull a number out of the air that isn't supported by the data, well, an inch is as good as a mile as they say.
 
Problem

Snowflake,

We are on the same page..... the problem with sites like this is that you never know who is writing on the other end. I would like to think that we are all well motivated aviators and builders who are trying to help and look after each other, but of course I am very familiar with the other side of human nature.

It just seems strange to me that people keep coming up with the phrase Van has not condemned it when that line exists in Vans Literature.


No RV should ever be operated above its design gross weight limit.
 
WAM120RV said:
No RV should ever be operated above its design gross weight limit.
Click to expand...

On the other hand, Van gave Jon Johanson a one time approval for a gross weight increase of 136% on his stock built RV4 with tank modifications. This was required because it was built in Australia. At a normal gross of 1500 lbs, that would be 2040 lbs. Seems he's flown it around the world in different directions, at least three times.....perhaps more.

http://www.vansaircraft.com/public/jj-plane.htm
 
It just so happens I snagged a copy of Dan’s W&B table just before he took his site down.

Looking over the numbers he had collected at that time, here is what I found:
These numbers are only for those aircraft with an inflated GW.

Type___# Samples___Avg OG #__Avg OG %____Range over GW
RV-3________2_______150_______14%______100 to 200 lbs
RV-4_______24_______116________8%_______50 to 300 lbs
RV-6_______58_______172_______11%_______50 to 500 lbs
RV-7_______34_______108________6%_______50 to 200 lbs
RV-8________5_______174_______10%_______50 to 470 lbs
RV-9_______10________85________5%_______50 to 100 lbs
RV-10_______4_______138________5%______100 to 200 lbs
 
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Jon Johanson

Hi Larry

Yes, I met Jon at the LAA rally in England during I think his first world trip. His canopy had cracked while flying over Greenland if I remember correctly. I gave him what assistance I could to get on his way.

He is a remarkable man and quite a charactor. If I remember correctly the main issue was whether the landing gear would stand up to the load.

Again what you should do is look at the context of the permission. The theory is that the aeroplane will take off at this weight............ burn off a load of fuel and land at a much lighter weight. Quite different to taking your 300lb plus buddy up burn off an hours fuel and land on a repetative basis. A one off permission is exactly that, it does not imply permission for everyone else to follow suit.

You are lucky in the states to have the freedoms you have and OK you can set your own limits........... but without doubt if you go outside what VAN says and there is an accident someone will come after you with a law suit.

There is another way of looking at this....... say you were in the market for an RV and you found two identical RV's. The only difference was that one had been operated over Vans gross throughout its life, the other had not. which would you buy.... the one with a hard life or the other one.

Actually, there is a really simple answer to this, if you have a six and want to lift more weight buy a seven. If thats not enough buy a 10!!

I have been considering building a 6 when I finish the 4. I flew a 7 recently and it was great....... so because of the extra load and aerobatic load that perhaps where I should go.

Anyway, I think you know why I have a problem with this from the posts above........ an aeroplane with a gross of 1600 has gone to 2200 in a few pages. As someone has said in a different thread thread when is it going to stop....... when there is a fatality?

Anyway best wishes to you and the rest of VAF I will be in your fine country in a few hours, and am looking forward to a flight in fellow Brit Mark Alberry's 8 in LA. Then we will visit the Grand Canyon and you might even get me up in one of them Helium Copters.

Someone once told me that helicopters dont fly, they are so ugly the earth just repels them.......:eek:
 
WAM120RV said:
There is another way of looking at this....... say you were in the market for an RV and you found two identical RV's. The only difference was that one had been operated over Vans gross throughout its life, the other had not. which would you buy.... the one with a hard life or the other one.
Click to expand...

I've been around the RV market......for a whole lot of years...

Therefor, the aircraft with different specified gross weights will most likely be far from identical. Option one (1650#)is going to be bare bones with a wooden prop. And it will be lucky if it's received a paint job in it's 15 years of operation.

Option 2 (1850 GW) will most likely have a constant speed prop, leather seats, some insulation on the floor, and a sidewall upholstery job. Probably has a 2 axis auto-pilot too! And best of all, it will be painted...........instead of saying "I get to pick my own color scheme".

This is usually how the RV6(A) scenario plays out. I know which one............I'd want...

L.Adamson -- 6A, 1850 GW, Hartzell CS
 
One thing to consider in this discussion. Virtually every aircraft over times sees increases in the allowed gross weight. Often even on certified aircraft this is done without any other changes. As the aircraft flies over time and data is gained the weight is increased. This even applies to airliners and military aircraft. As I mentioned before there is a tremendous amount of data over time to support the higher weights on some of Vans aircraft. Delta recently upped the gross weights on all its 767ER's without any airframe changes. Just a note from Boeing saying its ok. The increased weight is used to takeoff for cross country flights and burned off before landing. I don't know anyone going out and loading a RVXX beyond the vans weights an launching directly into acro ect..

George
 
sailvi767 said:
I don't know anyone going out and loading a RVXX beyond the vans weights an launching directly into acro ect..
Click to expand...

Even though my RV6A GW was increased from 1650 to 1850, the aerobatic weights have remained the same as Van's listings. It seems that most others do the same.
 
L.Adamson said:
On the other hand, Van gave Jon Johanson a one time approval for a gross weight increase of 136% on his stock built RV4 with tank modifications. This was required because it was built in Australia. At a normal gross of 1500 lbs, that would be 2040 lbs. Seems he's flown it around the world in different directions, at least three times.....perhaps more.
Click to expand...
This is true. However, i'm sure it came with restrictions. Jon had to be very careful what conditions he was flying in right after takeoff. If it was really gusty, I doubt it would have been all that safe. And while his gross may be "on the books" at 2000lb, I doubt he makes a habit of flying it anywhere near that weight.

If you have a loading scale that explains limitations vs. G-loading, like normal category 1800, utility 1600, aerobatic 1400, I could see that. But you haven't suggested that any such thing is necessary. Your -6 is registered at 1800, right? What G-load would you take your -6 to at that weight?
 
Snowflake said:
Your -6 is registered at 1800, right? What G-load would you take your -6 to at that weight?
Click to expand...

You'd have to know what an actual 6 wing tested to..............to really answer that question, wouldn't you?

Those 6 wings, the ones that attach in the middle. Amazingly strong! :)

L.Adamson --- 1850#
 
L.Adamson said:
You'd have to know what an actual 6 wing tested to..............to really answer that question, wouldn't you?
Click to expand...

No, I'm just asking what *your* operational limit is at *your* gross weight in *your* airplane. Do you know? Will you share?

Beyond the wing, there are other structures on the aircraft that become limiting factors at extreme load limits. The forces on your horizontal stab and elevator, for example, are directly proportional to the gross weight and max G loading. Van did change the skin thicknesses in the tail at some point in the RV-6... Not sure when though.
 
I know this subject has been done to death and has drifted a little from the original discussion. However, I find it interesting that there seems more support for NOT increasing gross over recommended weights than is normal.

There seems to be 3 issues:

PERFORMANCE - We all know that VANS aircraft perform remarkably. Unless you are very hot and high, the performance at higher weights is unlikely to be a problem. Unfortunately, this adequate performance at higher gross may persuade some people that it is OK without considering other factors.

STRUCTURE - VANS tests the aircraft at the appropriate weight and G using a 50% safety factor. The wing must support this without permanent deformation. (What I don't know is if they go further to see when it ultimately fails, but no-one designs an aircraft heavier or stronger than it needs to be or the performance/payload suffers.) So, if you increase your gross by 10-15%, it is unlikely that pulling the G limit at the higher weight is going to cause failure which is why we don't hear of over-gross aircraft falling out of the sky every day. Again, this fact is going to persuade people that being over-gross is OK given the "evidence". What you are doing, of course, is eating into your 50% safety factor. So what happens on the day that you get airborne, meet someone joining the circuit the wrong way and over-G the aeroplane avoiding him? Now the safety factor has gone. Probably will never happen.......... The other issue is long-term fatigue and wear & tear at the higher weights. Rivets working loose, cracks forming quicker than they might otherwise do. This is why the one-off approvals are just that. The pilot and authorities accept that, statistically, for the round-the-world trip or whatever, the aircraft is unlikely to fail or be subjected to long-term fatigue in the short term. Do it consistently, though, and the odds start mounting up against you.

BALANCE - No testing has been done by VANS on the validity of the CG limits at higher weights. Certainly for larger aircraft, the CG limits normally narrow at higher weights and you simply can't assume that the quoted limits apply and provide acceptable handling at over-gross weights. Again, anyone planning a one-off approval is probably going to test the aircraft incrementally and get some data.

Bottom line - my opinion - simply because an aircraft has adequate performance, seems to handle OK and doesn't fall out of the sky doesn't mean it's OK to arbitrarily increase the gross weight. The better option is to stop eating meat and potatoes 3 times a day, have some salad and make some visits to the gym - that's what I'm doing to improve the payload! And I am "only" around 200lb - I'm not over-weight, I am under-height ;)
 
over gross

I believe Jon Johanson's RV4 has been in a museum for several years. Did he not have some fuel near the wingtips?? Fuel at or near the wingtips reduces bending moment on the wings. One certified example of this is the Piper Twin Commanche with optional wingtip tanks of 30 gallons total. Gross weight with tip fuel is 125# greater than standard, so the tip fuel ALMOST gets a free ride.
 
Yep.

Jon had 18 gallons in each wingtip, IIRC. My Cessna 310 had 55 gallons in each tip for the reasons you mention.

A couple of downsides to wingtip fuel is a dutch roll tendency, until you get used to handling it..and some airplanes are placarded against spins because of the high moment of inertia with that much fuel at the tips...spins may be unrecoverable:eek:
 
paul330 said:
I know this subject has been done to death and has drifted a little from the original discussion. However, I find it interesting that there seems more support for NOT increasing gross over recommended weights than is normal.
Click to expand...

Most likely, because the same subject pops up every few years. Between this forum, and the Matronics, which I used to use...............it's been discussed a lot. It looks like many who use to care, to respond, are not going to bother anymore.

L.Adamson
 
paul330 said:
STRUCTURE - VANS tests the aircraft at the appropriate weight and G using a 50% safety factor. The wing must support this without permanent deformation.
Click to expand...

Incorrect. 50% Safety factor is to failure. Permanent deformation is allowed to occur anytime after design limit load (6 gs for the RV-6) and failure is allowed after ultimate load (1.5X design limit load or 9 gs). If you surpass 6 gs then you ought to inspect the aircraft for permanent deformation.
 
paul330 said:
I stand corrected. Even more reason NOT to increase GW.........
Click to expand...

Not exactly. First you have to determine that the wing actually started
deformation just above 6 G's. Suppose that it went much higher than that, when actually tested.

L.Adamson
 
L.Adamson said:
Not exactly. First you have to determine that the wing actually started
deformation just above 6 G's. Suppose that it went much higher than that, when actually tested.
Click to expand...
I understand that was done at one point at Vans. On a factory-built wing, uniformly loaded under what one might deem "ideal conditions". I don't know, but I doubt they considered aileron deflection during that test. Still, it would be interesting to know how far the wing went before deformation occurred. And how far it went before destruction occurred.

Still, it's relevant to ask what *other* parts of the structure were tested to 6G and beyond... Engine mount? Tail surfaces? Anything? After all, the wing isn't always the first thing that fails under high G.

You didn't answer my earlier question either... You said that you still use the 1375 Aerobatic limit, so 6G, but what G-limit do you impose at your 1850 gross?
 
It's very simple. VANS tests the airframe to the quoted weight and g +50%. If you make no structural improvements and up the gross weight, you MUST be eating into the 50% margin. If your regulatory regime allows that and you are happy to do so, then go ahead - fill your boots. As a life-long professional pilot, it makes my skin crawl. Limits are limits and the 50% belongs to the designer, not the pilot........
 
The "wing", and the scientific ;) testing assessment 'Those 6 wings, the ones that attach in the middle. Amazingly strong!' is not where I would direct my concerns... As stated much earlier, this apsect is relatively easy to mitigate:
RV-6A W&B
Aerobatic Gross Weight............................1375 lbs [6g]
leads to an equivalent at 4.4g (Utility) of 1875lbs.

It is other areas that are less certain, brakes / landing gear. Of course, RV-6 engine mount cracks have never been heard of ;)
 
Another thought

I had another thought which is something big jets have. It is obvious builders increasing the gross weight over what Van's has published is a fact of life. So make the increase for takeoff only, with the limitation that landing weight will be the gross weight Van's has established. There might be an emergency which would require landing above this weight, in this case a mandatory inspection of the appropriate parts of the airplane would be required (gear, spars skin etc). This seems like a way to mitigate some of the safety concerns.

You can make the same requirement for exceeding certain "G" limits, lets call it 3 Gs. If one encounters this above the Van's gross weight then down the plane until inspection in completed. In the fighter world over Gs would happen, depending on how severe it would require different levels of inspection. I will say that in the military world, G limits have more to do with the fatigue life of the airframe than what the wing can handle.

More fuel for the discussion, but I had not seen this proposed, seems to me it might settle some concerns. This is kind of along the lines of what George mentioned.

Cheers
 
RVG8tor said:
I had another thought which is something big jets have. It is obvious builders increasing the gross weight over what Van's has published is a fact of life. So make the increase for takeoff only, with the limitation that landing weight will be the gross weight Van's has established. There might be an emergency which would require landing above this weight, in this case a mandatory inspection of the appropriate parts of the airplane would be required (gear, spars skin etc). This seems like a way to mitigate some of the safety concerns.

You can make the same requirement for exceeding certain "G" limits, lets call it 3 Gs. If one encounters this above the Van's gross weight then down the plane until inspection in completed. In the fighter world over Gs would happen, depending on how severe it would require different levels of inspection. I will say that in the military world, G limits have more to do with the fatigue life of the airframe than what the wing can handle.

More fuel for the discussion, but I had not seen this proposed, seems to me it might settle some concerns. This is kind of along the lines of what George mentioned.
Click to expand...

Certainly, for a 200 lb. increase, that would be overkill, considering so many airplanes over so many years have not had a problem. As I was googleing some info yesterday, this thread popped up. Actually, it wasn't this thread. It was one from two years ago.............that looks and reads almost identical. Like I said...........about every two years... :(
 
That is OK

L.Adamson said:
Certainly, for a 200 lb. increase, that would be overkill, considering so many airplanes over so many years have not had a problem. As I was googleing some info yesterday, this thread popped up. Actually, it wasn't this thread. It was one from two years ago.............that looks and reads almost identical. Like I said...........about every two years... :(
Click to expand...

Every two years sounds good to me to address any good subject on this forum, after all there are new builders who don't know what they don't know and might not think to search on a subject. As it cycles through more get involved in the debate and new builders will have something to ponder or avoid, and more information might come to light.

Looking forward to the debate in two years :)

Cheers
 
Its been over two years...

Time to stir the pot. I submitted my paperwork to the FAA for the certification process. In it, I set the gross weight at 1,700 pounds(a 200# increase from manual). The FAA actually called Vans to talk to them about it. Vans essentially said that "a lot of people have done it" with no problems at all, and that the airplane will easily handle it. Mention was also made that the reason they have not increased the gross weight is because they would have to reopen the testing process on a plane they don't sell many of.
 
In case you can't find any accidents

Here is a link to an "experimenter" who just didn't know when to stop increasing his gross weight. 450 Gallons of fuel to make the trip nonstop from Basel Switzerland to Oshkosh. Poor fellow crashed into a multiplex apartment building, set it on fire and perished in the accident. No special maneuvers here just made it off the ground and was first to arrive at the site of the accident.

http://www.nbcnews.com/id/19914766/...uilding-playground-pilot-killed/#.VRcA72b88nI

On the other hand:

Bob Buck,
noted aviation author writes in his book, "NORTH STAR OVER MY SHOULDER" that after carefully calculating every load on their civilian DC3s so as not to ever exceed the stated maximum gross weight of 25,200 pounds, they were
prescribed a new maximum gross weight by an army air corps bureaucrat
of 32,000 pounds. The same airplane also got a new name and was called a C-47. It was 1942 and the war effort required maximum efficiency and the stated gross weight was simply not going to accomplish what needed to be done.

Some food for thought.
 
I think it is prudent to consider the fact that I have hear that Van's has made approvals for folks to flynwith extra tanks installed to make long legs such as in the Pacific.
Personally I agree with Pierre, and being in the business he is in it is crutial to keep things for maximum manerverability..

Folks operating in Alaska have a special deal if they are operating Part 121/135 or flying fish and game folks around under FAR 91.323 (a)(2). 115 percent of max gross if you meet the other conditions...i have heard some folks say in Indiana and Ohio where I started to fly say,"good enough for them good enough for me."

I designed and made my RV to Van's guidelines and kept the Gross weight at 1800. It could be changed on my 7 but i don't see a need to go back to Phase one testing. My bladder is a 2 1/2 hour deal so stopping to get fuel works fine for me... My wife likes the stops too. We meet a lot of new folks at all the Various airports...

As I know Pierre and 42 years ago we use to fly an Aero Commander Shrike into his airport to pick up a good customer. Been a long time. Since I have seen Him but that being said it is always good to see an old friend and not read about them in the Obits.

Old saying from Honda motorcycles..."STUPID HURTS"

Jack
 
N427EF said:
Bob Buck,
noted aviation author writes in his book, "NORTH STAR OVER MY SHOULDER" that after carefully calculating every load on their civilian DC3s so as not to ever exceed the stated maximum gross weight of 25,200 pounds, they were
prescribed a new maximum gross weight by an army air corps bureaucrat
of 32,000 pounds. The same airplane also got a new name and was called a C-47. It was 1942 and the war effort required maximum efficiency and the stated gross weight was simply not going to accomplish what needed to be done.

Some food for thought.
Click to expand...

Risk vs reward is completely different in wartime. Also, long term fatigue related failures aren't much of a concern when the airframe isn't needed to last very long.
 
"Imagine this example: you are on a mid-size airliner with a gross weight of 270,000 lbs. Just before leaving the gate, the captain comes on the PA system and says: “we’ve overbooked more than usual today, so we’re going to assume that the factory engineers over-designed this airplane and allowed an abundant safety margin. We’re going to take off at 280,000 lbs. instead. So move over, there are 50 more passengers coming on board."

This is pretty much exactly what happened to me on a recent airline flight. An announcement came on the PA that because of weight restriction problems passengers were encouraged to carry on everything possible as carry on luggage did not count against the weight restriction. So either some very strange rules of physics applied in this case, or indeed, the airline is assuming that the airplane is over designed and safe to operate at above maximum gross weight.

We did get off the ground okay and didn't break up in flight in a full CRJ 900.
 
Passanger weights

n5lp said:
"Imagine this example: you are on a mid-size airliner with a gross weight of 270,000 lbs. Just before leaving the gate, the captain comes on the PA system and says: ?we?ve overbooked more than usual today, so we?re going to assume that the factory engineers over-designed this airplane and allowed an abundant safety margin. We?re going to take off at 280,000 lbs. instead. So move over, there are 50 more passengers coming on board."

This is pretty much exactly what happened to me on a recent airline flight. An announcement came on the PA that because of weight restriction problems passengers were encouraged to carry on everything possible as carry on luggage did not count against the weight restriction. So either some very strange rules of physics applied in this case, or indeed, the airline is assuming that the airplane is over designed and safe to operate at above maximum gross weight.

We did get off the ground okay and didn't break up in flight in a full CRJ 900.
Click to expand...

Just to let you know, these big airlines use average fixed weights for passangers, there are winter and summer weights. That is why they ask you to carry on your stuff when the W/B becomes and issue. Now one could argue the weight used might not fit the average passanger these days with those of us well above that. I think the winter weight used is somethin like 188 lbs, but realize every kid onboard is only half that.

Because this type of system is used there is a large safety factor built in, just my opinion.

We take off in our fully loaded 767 at Max Takeoff Weight and still used reduced power takeoff. This power setting is predicated on losing an engine and still being able climb out on one, in fact procedurally we don't automatically push up the power on the good engine, the takeoff data assumes you lose the engine so you can make the climb on one. Of course you still have the option to push up the power on the good engine. Obstacles and terrain on the departure are what will drive the need for a full power takeoff. In really high terrain we would have a special path to fly that keep us away from the terrain at out single engine climb capability.

So RVs just like airliners have some wiggle room. Nice day long runway you can takeoff with more than design gross weight. Based on the performance of my RV-8A I did make my max gross 1900 lbs, 100 over design, so far that has not been and issue, I flew to OSH at 1900 lb limit and had no issues. Doing this is a choice, the fact others do it successfully is a data point you can use to make your own decision.

Cheers
 
Weight

Using the RV8 example of 100# increase in weight. Round numbers to make it simple: original gross weight 1800#, G limit at that weight 4G, 4 x 1800=7200 divided by 1900(new gross weight), new G limit is 3.7-- lets call it 3.8.
Then there are the long distance flights. The gentleman in the Lancair 4 that recently broke a world record was probably 50% over the normal gross weight.
 
When figuring an up-rated gross weight, a common approach seems to be to use the acro gross weight (in the case of the 4/6/7/8 anyway), multiply by six, then divide by whatever max G you think you need to yield the gross weight.

This ignores the possibility that other parts of the airframe may be limiting. For example, using the RV-6A numbers (since that is what I am familiar with without looking other numbers up), an acro gross of 1375 multiplied by 6g gives 8250lbs-g, which then divided by 3.8g yields about 2171 lbs. All good, right?

What if it's not the wing spar which is limiting? Using the other end of the weight range, Van's gross of 1650lbs multiplied by 4.4 (the utility max load factor) yields 7260lbs-g, which then divided by 3.8g results in 1910lbs. Hmmm. Not quite 2171.

Yet another approach would be to graph the weight vs. load factor and use that to find the weight for a specific load factor. Going that route shows that a maximum load factor of 3.8g would limit gross weight to about 1750lbs. Even further away from what the simplistic calculation based on the wing spar would suggest.

There is a big difference between 2171lbs and 1750lbs. I did up my gross weight, but used the third approach to determine it. Anyone else do something similar?
 
Brad,

I have pondered similar extrapolations from time to time. Could you explain the difference between your calculation method 2 and 3? Seems to me they would be the same. I must be missing something.

Bevan
 
Bevan said:
I have pondered similar extrapolations from time to time. Could you explain the difference between your calculation method 2 and 3? Seems to me they would be the same. I must be missing something.
Click to expand...

The basic issue with extrapolating an up-rated gross at 3.8g (or another arbitrary value) from a single data point such as the max gross weight for acro or maximum gross weight for utility operations is that doing so presumes that other factors change similarly which we can see is not necessarily the case.

For example, at 1650lbs gross, both the second and third methods I outlined above will predict a max load factor of 4.4g. Great. But working it backwards to 1375, the second method results in a max load factor of 5.3g - which we know is incorrect (but conservative, which is good). Method three shows the correct result - 6.0g.

Going the other way, the second method predicts that at 1850lbs, the max load factor is 3.9g - above our 3.8g target. But is it? If we follow the slope of the line described in method three in my previous post, the max load factor here is only 3.2g - significantly below our desired target of 3.8g.

The following chart shows methods 1 (extrapolation from max load factor at acro gross), 2 (extrapolation from max load factor at utility gross), and 3 (extrapolation from load factors at both acro and utility gross weights):
16992894055_c16fa34421_b.jpg

​
Of the three methods I outlined in my previous post, only method #3 gives load factors that match Van's limits for both acro gross and recommended utility gross weights. Because of this reason, I believe it reasonable to accept the 1750lb max gross for a 3.8g load factor rather than the 1850+lb max gross as suggested by method #2.

I think I should repeat the obvious that these only impact air loads and are ignorant of loads placed on the landing gear etc. but I hope that explains the differences between the two approaches.
 
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jrs14855 said:
Using the RV8 example of 100# increase in weight. Round numbers to make it simple: original gross weight 1800#, G limit at that weight 4G, 4 x 1800=7200 divided by 1900(new gross weight), new G limit is 3.7-- lets call it 3.8.
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Not to pick on you specifically, your summary was just the most compact.

These extrapolations (and the implied "that's all you have to do") are a good example why non-engineers shouldn't play with their gross weight.

Extrapolating the gross weight via reduced G limits is only valid for the flying surfaces that carry the loads, and doesn't take into consideration the distribution of that added weight you want to carry.

Here's a simple example that might help people understand why you can't just increase your gross willy-nilly: Let's say you have an RV that was designed for 6G loading with a 250lb firewall-forward package (say an O-320 and fixed-pitch prop). Not wanting to be the slowest RV in your group, you opt instead to put a 375-lb firewall forward package... IO-360 with a constant speed metal prop.

Note that you've increased your FWF load by 50%. In a 6-G pull in the heavier configuration, you're loading the engine mount 50% more than with the design engine package... Which means it's like pulling 9G with the smaller engine. So even though you've extrapolated down your G-limit by 5-7% for the added 125lb on the gross weight, if you ever pull to your extrapolated limit you'll still be well beyond the design limit of the firewall.

The same consideration goes for increasing gross just so you can carry a heavier passenger, or more baggage... The baggage floor and seat pan were only designed for certain loads at certain G.
 
gross weight

Hey Rob when are you going to run the numbers for the RV 6 on that super computer so we can lay this to rest . I thought in a post 2 or 3 years ago that was the plan.
Bob
 
Weight increase

Snowflake said:
Not to pick on you specifically, your summary was just the most compact.

These extrapolations (and the implied "that's all you have to do") are a good example why non-engineers shouldn't play with their gross weight.

Extrapolating the gross weight via reduced G limits is only valid for the flying surfaces that carry the loads, and doesn't take into consideration the distribution of that added weight you want to carry.

Here's a simple example that might help people understand why you can't just increase your gross willy-nilly: Let's say you have an RV that was designed for 6G loading with a 250lb firewall-forward package (say an O-320 and fixed-pitch prop). Not wanting to be the slowest RV in your group, you opt instead to put a 375-lb firewall forward package... IO-360 with a constant speed metal prop.

Note that you've increased your FWF load by 50%. In a 6-G pull in the heavier configuration, you're loading the engine mount 50% more than with the design engine package... Which means it's like pulling 9G with the smaller engine. So even though you've extrapolated down your G-limit by 5-7% for the added 125lb on the gross weight, if you ever pull to your extrapolated limit you'll still be well beyond the design limit of the firewall.

The same consideration goes for increasing gross just so you can carry a heavier passenger, or more baggage... The baggage floor and seat pan were only designed for certain loads at certain G.
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Experience has proven that your engine/prop example is not realistic, except for the 12 and maybe the -3. Examples: some well known racers that are likely being operated far in excess of published Vne. This is potentially a far greater danger than operating a small amount over the original gross weight.
I feel I was very specific that my example was non aerobatic. My example is a very legitimate method for modest gross weight increases in the non aerobatic envelope. A good example would be an airplane that is seriously overweight for whatever reason, heavy paint, lots of equipment, plush interior etc. There is no legitimate reason not to increase the gross weight using the formula that I posted. In normal category it is reasonable to believe that one would never encounter turbulence or other conditions that would come close to 3.8 G's.
I do agree that there should be a reasonable limit for occupant weights and baggage weight.
There is one other factor that has not been mentioned. Wingtip tanks on small production aircraft often get a "free ride", ie the gross weight in increased by the weight of fuel in the tip tanks, with the stipulation that all weight over the original gross weight must be fuel in the tip tanks.
 
Weight

Should have mentioned that several of the "racers" have the IO 360 with metal blade constant speed.
 
jrs14855 said:
In normal category it is reasonable to believe that one would never encounter turbulence or other conditions that would come close to 3.8 G's.
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This is not correct.
In fact it is exactly why we have a specified maneuvering speed, and a yellow arc range on the airspeed indicator.
Is it common? No, but it is possible. That is one of the reasons for the FAA's requirement to publish a Vno speed (top of green arc).
 
jrs14855 said:
Examples: some well known racers that are likely being operated far in excess of published Vne. This is potentially a far greater danger than operating a small amount over the original gross weight.
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Yes, but not far over the published Vne *and* at max G at the same time.

A good example would be an airplane that is seriously overweight for whatever reason, heavy paint, lots of equipment, plush interior etc.
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That right there shows either a lack of understanding of the concept, or you just worded your point poorly. Additional load that was equally distributed through the structure of the airplane, say as paint, I agree would have a small effect that could be approximated by extrapolating (it wouldn't be perfectly accurate because the wings carry their own weight and the fuselage doesn't, like your mention of the tip tanks). But additional load that is concentrated in any one area, is no different in the cockpit with heavy upholstery, or on the firewall with a heavy engine.

With no access to the engineering analysis done by Van on the aircraft so you could know what's been over-designed and what is near its limit, you can't say that adding 100 lb to any one area is either safe or prudent. Add it to your cabin, and it's "only" a 5% increase in your gross weight, but it's a 25% increase in loading on your seat structure (assuming 50lb per seat, and 200lb standard passenger) when you hit turbulence. If you want to extrapolate, your G-limit is now down to 3G.
 
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6 Gun said:
Hey Rob when are you going to run the numbers for the RV 6 on that super computer so we can lay this to rest . I thought in a post 2 or 3 years ago that was the plan.
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I did run the numbers on the wing structure and fuselage interface alone, to answer a specific question I had regarding one part of the structure. I was not looking to increase the gross weight. The best ways to increase your baggage capacity are to build the airplane light to start with, and to get your own structure off the couch and stay fit.

An acquaintance who is my height but has 20 pounds on me was showing me his new all-carbon-fibre bicycle, that weighs 4 pounds less than my aluminium-frame bicycle that cost 1/10th as much. He wasn't very impressed when I pointed out that I was still 16 pounds lighter overall, *and* I had $9000 in the bank. Gotta keep your eye on the big picture. :)
 
Snowflake said:
An acquaintance who is my height but has 20 pounds on me was showing me his new all-carbon-fibre bicycle, that weighs 4 pounds less than my aluminium-frame bicycle that cost 1/10th as much. He wasn't very impressed when I pointed out that I was still 16 pounds lighter overall, *and* I had $9000 in the bank. Gotta keep your eye on the big picture. :)
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Yeah, but I don't need tie-downs in high winds. :)
 
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