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How are gross weights derived?
I've been looking at various models on the W&B page and I find different Gross Weight limits for the same models. Some seem to follow engine HP but others don't. How are the Gross Weights derived???? This is of significant interest to me since I'm Bubba sized.
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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.
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Thanks John,
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. |
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Ramp check
They would use the same info you use to calculate your Wt & Balance :cool: You are required to have the empty weight of the aircraft as part of your on-board documentation (remember ARROW ?) and then they would simply weigh whatever you have on board :eek:
Dennis Glaeser 7A Fuselage (just rolled) |
Experimental = Liberty
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I'm With Jack
I'm with Jack on this one. 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. I set the GW on my -8 at 1800 based upon Van's recommendation but quite honestly, I wish it were higher. Did Van's determined the RV-8 1800# GW met the Utility category load limits of +4.4 and -1.76 and then established the Acrobatic GW of 1350 (or something like that) based upon the Acrobatic category load limit of +6 and -3. Given that these asssumptions are true, if we arbitrarily set a higher gross weight then in reality we don't meet (even thought we don't necessarily have to) the established standard for Utility category. Like all assumptions, mine are based upon a lack of knowledge. And for me comparing these load limits to certified category aircraft is just for my comprehension. I'm really curious to know what impact setting a higher gross weight will have on the load limit and consequently the operational limits of my airplane.
Rick McBride well equiped but rather portly RV-8 |
MTOW analysis
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Gross Weight
In the certified world, gross weight is set based on reaching a mandated limit on one of the following:
- max stall speeed - structural limits (G's - depending on category - aerobatic, utility, normal) - single engine climb performance (multi-engine) Experimental aircraft don't have mandated requirements for any of these, so that's why you're allowed to establish whatever gross weight meets your own needs. I suspect that Van's' gross weights are based on what they've advertised as structural limits - they've tested their main structures (i.e. wing spar) to the advertised limit load G's based on their advertised gross weight. Dennis Glaeser |
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. |
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.
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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. |
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. |
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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. |
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. |
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. |
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. |
confusion reigns
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RVBYSDI Steve |
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? |
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. |
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? :confused:
Best, |
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If you operate off rough strips at more than 1800 lb, you may run into problems with the landing gear or its attachment structure. If you stick to smooth runways and smooth touchdowns, limit the g to some reasonable value when over 1800 lb (3 g is probably more than safe), and reduce speed to VA when in turbulence, then I believe there is little increase in risk with operations at 2000 lb. |
Yes, as a homebuilder you can set your gross weight to whatever you like. However, you will have to show your DAR/Inspector how you arrived at anything over what the designer recommends. Unless you can show me engineering data to back up your new gross weight, I will not approve anything over utility category for an RV. You can find this weight by extrapolating from the aerobatic gross weight set by Van's Aircraft from 6 gs to 4.4 gs.
Mel...DAR |
What about flying RV's?
Here's another question along the same lines.
I purchased a flying RV-8A with gross weight at the usual 1800 lbs. Obviously I am not the builder. I would like to be able to up the gross weight limitation to 1900 lbs. I recently spoke to two builders at a fly-in who told me that the process is relatively easy. One of them got the procedure from a DAR. I spoke to another individual in Michigan via e-mail who confirmed the process -- he in fact followed the process to sell his RV. Supposedly you make an entry in the aircraft log removing the airplane from operational airworthy status and placing it back into Phase I testing status. Then you retest the aircraft weight and balance for the newly desired gross weight. When testing is complete you must revise the weight and balance section of the aircraft operating limitations. Finally, return the aircraft to operational airworthy status with the new gross weight and a descriptive writeup in the aircraft log -- no inspection required. The two builders I spoke to were sure that the original builder could do the above, but not sure if a non-builder buyer such as myself could do it legally. Any opinions? |
Yes, a non-builder can do this to HIS/HER aircraft as well. However, be careful. If anything happens, the insurance company may ask you to provide the same justification data that a DAR would want to see. Also, when putting the aircraft back into phase I, you must get concurrance from the local FSDO for your test area.
Another important point..your operating limitations MUST allow this procedure. The currently issued op limits do allow this, but, you are bound by YOUR operating limitations. The older op limits require a recurrent certification for any major changes. Mel...DAR |
Here is a dumb question. If a builder sets a higher GW for their RV, to they have to fly it at that weight while in Phase 1 testing?
I've always wondered if a builder can set his GW at 2500 lbs but has never actually flown it at that weight. |
MEL,
Are you going to be at the Texas RV Fly-In on May 20th? If so, could I chat with you about this? |
Yes, the aircraft needs to be flown at the new gross weight to the forward and aft cg limits during phase I. The purpose of phase I testing is to prove that the aircraft is safe to the limits of the operating envelope. If you change that envelope, you need to prove the new limits are safe.
Yes, I will be at the Texas Fly-in. Well.....I don't really like to talk about airplanes, but if you twist my arm just right......... Mel...DAR |
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Be careful. Fly with the big head, not the little head. :) |
Why Va decreases at light weights
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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 |
Mel,
Thanks for your reply. I will be sure to contact my local FSDO prior to revising the gross weight. I had not thought about that aspect. |
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.
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Dennis,
Thanks for your explaination on Va and weight. I understand now. |
Landing gross
This thread has primarily dealt with Maximum Take-off Weight (MTOW) but nothing much has been said about Maximum Landing Weight (MLW). The two can be entirely different numbers. For instance I fly a Cessna 182R and the MLW at 1338 kg is considerably less than the MTOW at 1406 kg. In other words if you're at MTOW you have to fly off approx 100 litres of fuel before you can legally land.
So RV builders may be able to increase their MTOW for a number of legitimate structural reasons....such as carrying additional fuel in auxiliary wing tanks. However it may well be unwise to land above Vans specified gross (RV7A = 1800 lbs). In particular I suspect the nose gear on the A models has questionable redundancy at Vans gross of 1800 lb. Therefore for those considering upping the gross, they might find that there is greater structural justification in increasing the MTOW but leaving the MLW at Vans gross. |
Excellent point in my opinion. Question: Would it be prudent to qualify any increase in MTOW with a limitation over 1800 pounds as Normal category only (i.e. 3.3g)? Limit the landing weight to 1800#?
Peter Schwarzenbach RV7 Fuse Quote:
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Here is my take, flame away if you must. Unless you design aircraft for a living, stick to the major design and CG limitations. Van has been doing this for a while and knows what he is doing. Sure, there is probably a pretty large safety margin figured in, but use it for safety, not to fatten up your RV. If the RV doesn't meet you mission profile, find a design that does.
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I was hoping this would stay a fairly technical thread with some science and experience.
Mean spirited opinions have nothing to offer. |
Gross weight
Van's specs his aircraft with a gross weight (1750 lbs in the case of my RV-9A).
I calculated a loading envelope that would allow up to 1800 lbs without increasing the wing loading above spec. In order to do this, you need to calculate the maximum HS/elevator down force with nominal gross weight (1750) at most forward CofG. From this, you can calculate a loading envelope that increases the gross weight as the CofG moves aft, without exceeding the maximum wing load (1750 + forward CofG elevator down force). This explanation may not be clear, but the result is that I can carry extra weight in the baggage compartment without increasing the wing loading because it reduces the down force requirement on the HS. By specifying this envelope, you can ensure that the maximum wing loading never exceeds what it would be with Van's spec at forward CofG. For more information (Excel Spreadsheet), go to my website and click the link near the top. Vern Little website |
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I second that thanks. Your explanation was very straight forward. I appreciate it. RVBYSDI Steve |
The FAA lets commecial operators do it
FAA says allow some operators to increase gross weight in Alaska for some
ops. Max shall not exceed 15% of FAA specs, not that its a given. Ref: Sec. 91.323 - Increased maximum certificatedCATEGORY --------------------------------------- LIMIT LOAD ...Normal ---------------------------------------- 3.8 -1.52 ...Utility (mild acrobatics, including spins) ---------- 4.4 -1.76 ...Acrobatic -------------------------------------- 6.0 -3.0 Basic math ratio of limit Normal to Utility load factors gives approx 15%, no doubt related to where they got 15% in FAR 23.323. Hummm :rolleyes: I agree keep it light in the first place. There is no great reason to be way over Van's recommend est of empty wt. I am anti-dual everything that is so popular or late. Last RVator (1st of 2006) "Van the man" talks to this subject. I just got a call from a guy who is looking at buying 1215lbs RV-7. If you do the math you are at or over 1,800 lbs, full fuel, crew=2. no bags. It is a single place acro plane. The 1,600 lbs is a hard limit to me. There is no justification in my mind to exceed acro gross limit, but I expect someone might rationalize, I promise not to pull more than 3 g's. My OPINION: As arbitrary as it sounds, plan A is keep GW with in VAC limits. Bar that, I think a 100 lb - 125 lb or 7.5% bump on gross, what ever is less, is reasonable, if you promise to fly in normal category load limits. Plan A is best. From my Rocket Science math :rolleyes: Max GW at +15% works out to 2070 lbs for the RV-7/9, and 1840 lbs on the RV-6. That's too sporty for me. Of course there is no relief on CG, which must be in limits, unless you do stability and control flight test. Please no argument, this is what I would do and is as arbitrary as anything, this is what gives me warm fuzzies. George PS: Keep in mind performance and landing weight. Probably will not be a problem but consider it. You may recall your great solo sea level weight performance, and than get a surprise operating high, hot, and heavy with extend gross wt. I would set landing weight at Vans gross, so you need burn off fuel when flying w/ "extend gross". That is what we do on large aircraft. If you think weight and balance is a pain, try it on a large aircraft, Max Zero Fuel wt., Max t/o wt., Improved climb t/o wt., Assumed temp t/o wt., Max Landing wt., while still able to take fuel and payload needed. Here is a link to the V-N diagram: http://www.auf.asn.au/groundschool/u...light_envelope |
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