jcoloccia said:Vans gives a suggested gross weight (on the -7, for example, it's 1800 lbs). We as the builders, however, can set the gross weight to whatever we want. For example, Dan has set his to 1950 lbs. I'll probably do something similar on mine (not 1950 but something north of 1800). I'll set mine to 1820 or so, shoot for 1800 and if I'm a couple of lbs over at a ramp check so be it.
Jack, with an experimental aircraft you can set the MTOW to whatever you like. It's not a request. A lot of guys set it pretty high just so that they are sure to never fail a ramp check.JackT said:I see most -8s at 1800# but there is one over 2200#. What is the certification or engineering basis for requesting a GW this high? I haven't dealt with experimental before and I'm very hazy on this issue.
Now you're getting into serious Kevin Horton territory - I'm way out of my league!rickrv8 said:What criteria is used to establish these weights? I understand I can set the GW at whatever I want simply because it's experimental but I really would like to understand the analysis that went into the numbers.
Here's what it comes down to: CYArickrv8 said:What criteria is used to establish these weights?
This above description is relevant to the way that VC and VD are defined for type certificated aircraft (see FAR 23.333). The structure must be able to withstand vertical gusts of a certain intensity at those speeds. If we are lucky, we will never actually experience the 3000 ft/mn vertical gust that the structure must be able to handle at VC. But, there is a pretty good chance we will hit that 1500 ft/mn gust that is assumed at VD.keen9a said:The FARs are ridiculously conservative in my opinion. You basically take a theoretical instantaneous vertical gust that cannot occur in nature to define the load, and then you apply your 50% safety margin to that. Of course, you're also assuming that you magically got to an altitude where the magic gust might occur (gusts become not so vertical close to the ground) while burning no gas.
Pete, the basic idea is good, but the exact sums won't work. 2G at 3600 lb would provide the same loads as 4g at 1800 lb, if the extra 1800 lb were evenly distributed throughout the whole airframe, including the wings. But, in real world cases, most of the extra weight is in the fuselage somewhere. The aircraft gross weight has doubled, but the weight in the fuselage has more than doubled. That means the wing bending moments have more than doubled.fodrv7 said:Kevin,
If you were to fly your RV way over gross, say 3600lb, and you never exceeded the speed at which a 2G stall occured, would this prevent the design loads from being exceeded.
Pete.
I am there with you on the weight and load factor confusion. I have also read in numerous articles and books on G loading that the light airplane experiencing turbulance will be more likely to be damaged than if the same airplane at gross weight experienced that same amount of turbulance. Yet, here we have all of our RV "engineers" discussing the issue of max gross weight and its affect on stressing the airframe. I would like some "expert" explanations of this issue from which I could glean some knowledge.Brockster said:4. An aircraft weighing less than the specified weight for a category, doesn't mean we can exceed specified G limits or maneuvering speeds for that category and in fact maneuvering speed decreases as weight decreases due to higher G on set before the stall, per the article in AOPA.
When I first read this thread and then the AOPA article I got a little confused because people talked about stressing the aircraft more with higher weight and the article talks about stressing the aircraft more with less weight and my brain started to lock up. Any clarification would be appreciated.
It all depends on how you operate the airplane. If you want to pull 6g at 2000 lb, you would be putting significantly more load on the airframe than it was designed for, and the risk of structural failure would be increased. Bad idea.JackT said:If I round it to 2000# GW, it's a 200# increase for plus 11%.
Is there a reasonable way to evaluate this addition in terms of risk?
If that yahoo understands that laws of physics still apply to him, and he flys the aircraft appropriately, he may do OK. But ignoring the laws of physics does not obligate the laws of physics to ignore you.Kahuna said:How about that yahoo that put his 8 from 1800 to 2270? Thats 26% increase. What a freak! Wonder where he gets off putting up numbers like that. Its a wonder he is still alive. Or is he?
Here is the way I like to explain Va:Brockster said:Trying to understand what everyone has explained in this thread and reading the article in this months AOPA magazine "Va-weight and see the G" do I have this straight?
<snip>
When I first read this thread and then the AOPA article I got a little confused because people talked about stressing the aircraft more with higher weight and the article talks about stressing the aircraft more with less weight and my brain started to lock up. Any clarification would be appreciated.
osxuser said:Remember that wherever you set the limit when you manufactor the airplane, it will stay there for the life of the airplane... do you really want someone else to be able to fly your airplane (if and when you sell it) at X amount over the suggested gross weight because you wanted to be sure you would never get busted in a ramp check? I'd be careful not to set it too high. I think Dan's 1950LB number is probably a really good compromise.
Dennis,Brockster said:Dennis,
Thanks for your explaination on Va and weight. I understand now.
I'm afraid I have to disagree with your 'multiple load' statement. Once the wing stalls, from any combination of gusts and/or control input, any further increase in AOA (from any source) will actuall reduce the load, not increase it You ARE protected from aerodynamic loads breaking the wing when flying at the proper manuevering speed for your weight.John C said:<snip>
What do I mean by multiple loads. You are not necessarily protected against all gusts and flight control inputs below maneuver speed. In theory, below maneuver speed, you can have the monster gust and the wing will stall before it breaks. Or you can put in a full-control input and, again, the wing will stall before it breaks. However, you may not be protected from both at the same time. That is, a big gust and a hard flight control input to fix the upset may take a wing off even though you are below maneuver speed. You also may not be protected from multiple large control inputs. Consider that you are protected from one single event, not two simultaneous events.
<snip>
Rod Macado?s article in this months AOPA magazine raises some concepts that are correct. He says that lighter weights could mean the need for a lower maneuver speed. That could be true but is probably not applicable to this discussion or for RVs. Basically, if you are at lighter weights, you can pull more Gs before the wings break off. Things attached to the airplane could break off at those higher Gs. So, to protect for those other things breaking off, you would need to use a lower maneuver speed. I would not worry about that for our RVs.
Regards, John.
At sea level my IO-360-A1B6 burns nearly 20 gph at full rich, full throttle. Average takeoff fuel flow out of my home base at 652' is about 17.5 gph (I don't often use full rich mixture).JackT said:What is the fuel burn on a 180/200 hp RV-8 at TO/Climb power? (How quckly will the actual weight come down?)
I agree! Way NOT intuitive!DGlaeser said:Here is the way I like to explain Va:
In any flight condition (other than a stall), your wing is operating at something less than the stall angle of attack. Lets call the difference between where you are and stall the 'reserve AOA'. This 'reserve AOA' can be turned into an instantaneous load by either abrupt control movement, or a vertical gust.
At any given airspeed, if you are flying heavy, you have less 'reserve AOA' because the wing is operating at a higher AOA to handle the weight. So the amount of load that the reserve AOA can generate is less.
Conversely, when flying light, you have more reserve AOA, which can be turned into a higher instantaneous load.
The amount of load generated is a function of airspeed - higher speed, higher load generated by the reserve AOA. The maximum load the airframe can handle is fixed.
Va is the speed which, at gross weight, the reserve AOA will generate the maximum load the airframe can handle. If you are below gross weight, you have more reserve AOA and that same airspeed can now generate more load than the airframe can handle! So, when flying light, you have to slow down in turbulence (or when doing abrupt maneuvers) - more reserve AOA requires less speed to generate the same load.
Hope that helps. It's not an intuitive concept!
Dennis Glaeser
DGlaeser said:I'm afraid I have to disagree with your 'multiple load' statement. Once the wing stalls, from any combination of gusts and/or control input, any further increase in AOA (from any source) will actuall reduce the load, not increase it You ARE protected from aerodynamic loads breaking the wing when flying at the proper manuevering speed for your weight.
As for Rod's comments: lighter weights DO require lower maneuver speeds - that is true for any aircraft including RVs - see my explanation earlier in this thread.
Dennis Glaeser
I wasn't intending to be mean spirited, I truly believe the great majority of builders should build and fly these aircraft as designed, and that before you start a project, you do need to make sure the design meets your requirements. If it doesn't, then I believe it makes more sense to find a design that will do what you want, than to modify a design to make it do what you want. Designing an airplane is a relatively well understood science, and in general, the Vans aircraft are well designed. In a well designed plane you really cannot get something for nothing. If you want to add to one aspect of the plane, you take away from another. If you want more weight, you will take away from other areas including performance, handling qualities, and safety in my opinion. You as the builder have the option of doing what you want, just don't do it thinking that Vans numbers are arbitrary or have a huge unneeded safety margin built in that you can just take away without any untoward effects.JackT said:I was hoping this would stay a fairly technical thread with some science and experience.
Mean spirited opinions have nothing to offer.
Engineers are like wives. Neither wants to accept generalities. They both want precise factual information that they can use to manipulate and control every nuance of behavior. The engineer to control the behavior of the machine. The wife to control the behavior of the husband.jonbakerok said:I know we all would like to think that Van's has carefully calculated and tested the engineering load factors on everything from the spar to the glovebox lid, but it's probably a fantasy.
The 1600 pounds that Vans recommended for my 6A didn't leave much room for luggage with two normal-sized people. I had done all the math and it was obvious my airframe was easily capable of well over 2000 lbs gross weight in the air. I figured the 1600 lb limitation must be due to those spindly gear legs.
About that time, the 7 came out, with its 1800 gross weight.
So I called the factory to see if I could order a pair of those new gear legs for my 6. The response? "They're the same legs". I explained my calculations and how I came to the conclusion that the gear was the limiting factor. He (I think it was Tom) confirmed that, yes, the gear is the limiting factor. "So howcome the 7 can handle 1800 lbs while the 6 is limited to 1600?", I asked.
The answer -- "Because we noticed that so many 6 drivers were using an 1800 lb gross without any problems".
John C said:Dennis, what I was describing is different than your scenario. There are a number of design conditions and each is evaluated separately. For example, one is for symmetrical loading of the wings at the maneuver load and maneuver speed. Another is for full aileron at maneuver speed and 2/3 maneuver load. One does not have to design for both symmetrical maneuver load and full aileron input at the same time. The combination would create a higher load in wing (including twist) at the moment of stall but the higher load is not a design requirment. Thus, my comment that you are not necessarily protected with a full aileron input at maneuver load. The same is true for various conditions of rudder input and sideslip angles at maneuver speed. You have to meet each condition, but not several at a time.
Regards, John.