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-   -   How are gross weights derived? (https://vansairforce.net/community/showthread.php?t=7368)

keen9a 04-28-2006 03:20 PM

Rick, FAR part 23 has all the detail you'd ever want for how certified aircraft set the max gross weight (they just call it maximum weight). Given the performance of RV's, I think its a pretty safe assumption that loads is the driving factor.

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.

Now, since we're experimentals, we get to decide what our requirement should be.

Kevin Horton 04-28-2006 06:27 PM

I understand that Van used FAR 23 structural criteria when designing the various RVs. His analysis and static load testing showed that the aircraft met the FAR 23 aerobatic category requirements at the recommended aerobatic gross weight, and utility category requirements (or better) at the recommended gross weight.

Quote:

Originally Posted by keen9a
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.

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.

But, when it comes to Van's recommended limiting load factors, we should not assume that the structure is any stronger than it needs to be to withstand the load factor limits that Van recommends.

We have no regulatory requirement to abide by Van's recommendations. But we can't cheat on the laws of physics. The aircraft should fly just fine at higher weights. But we would need to make a corresponding reduction in the g we pull, stay on smooth runways and slow below VA if we anticipate significant turbulence.

fodrv7 04-28-2006 06:38 PM

Coffin Corner
 
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.

Kevin Horton 04-28-2006 06:51 PM

Quote:

Originally Posted by fodrv7
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.

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.

We would need to know how much the weight of the fuselage and its contents have increased, before we can calculate how much g would be safe at the higher weight.

fodrv7 04-28-2006 06:54 PM

Zero Fuel Weight
 
Ah! Zero Fuel Weight limited.
Of cousre when Jon took-off slightly over weight for the Antarctic the 1150 litres of fuel was in the wings.
Thanks Kevin.

Brockster 05-03-2006 11:12 AM

Va deceases as max weight decreases
 
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?

1. As the builder of an experimental aircraft I can set my own gross weight. Some have chosen to use a higher gross than Van specifies so that they would be in compliance if ramp checked.

2. I will make the assumption that Van set max gross weight based on structural limits.

3. The weights Van specifies for acrobatic flight or Utility Category would stay the same. As the builder I assume we could change these also but this would not be prudent without some type of engineering documentation.

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.

jcoloccia 05-03-2006 12:55 PM

If it's something like the firewall busting off (too many G's and the engine falls off) the stress on the wings is irrelavant. Plane gets lighter, you can pull more G's at a slower speed. Manuervering speed goes down as weight goes down (i.e. the acceleration of the airframe as a whole is the limiting factor).

If the limit is the wings falling off (or some other lift generating thingy), weight doesn't have an effect, on VA I don't think. All changing the weight would do is affect how many G's you register before something breaks. So an overweight plane may very well break at 2 G's.

I trust Van's analysis and his numbers but unless we run the analysis ourselves or Van's comes out and gives us guidance we don't really have anyway of knowing how the numbers were set, although it's a good bet that since a few wings have flown off due to over stress that the wings are probably the first things to go. Personally, though, I don't want anything going without my consent so I'll follow Van's recommendations.

RVbySDI 05-03-2006 01:11 PM

confusion reigns
 
Quote:

Originally Posted by Brockster
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.

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.

RVBYSDI
Steve

JackT 05-03-2006 02:08 PM

Reality Question
 
Let's say I want to build a RV-8 with a pretty sporty engine, OE fuel capacity, and basic IFR equipment. Let's say I do a pretty good job of controling weight and get about 1150# BOW. Full fuel adds 273# for BOW + fuel at 1423. Now since I'm "bubba" sized, I want 500# useful load which means I need a 1923 Gross (Maximum) Weight.

That's 123# over the Vans recommended GW which is jsut under a 7% increase.

If I round it to 2000# GW, it's a 200# increase for plus 11%. :confused:

Is there a reasonable way to evaluate this addition in terms of risk?

keen9a 05-03-2006 03:17 PM

11% is a lot! That's the limit of my judgement that I'mm willing to toss out to a meassage board.

I'm personally not capable of explaining all the in and outs of aircraft loads with a few paragraphs. I'd need charts and graphs and several hours just to get through the basics. In light of that, I would suggest reading, er studying this. The title is "Aerodynamics", but it covers loads.


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