Van's Air Force
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How are gross weights derived?
- Thread starter JackT
- Start date
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.
DGlaeser
Well Known Member
Ramp check
They would use the same info you use to calculate your Wt & Balance
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
Dennis Glaeser
7A Fuselage (just rolled)
They would use the same info you use to calculate your Wt & Balance
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 Dennis Glaeser
7A Fuselage (just rolled)
rickrv8
Well Known Member
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
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
DGlaeser
Well Known Member
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
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
keen9a
Well Known Member
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.
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
Well Known Member
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.
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.
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:
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.
Kevin Horton
Well Known Member
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:
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.
Brockster
Well Known Member
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.
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.
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.
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RVbySDI
Well Known Member
confusion reigns
RVBYSDI
Steve
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:
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%.
Is there a reasonable way to evaluate this addition in terms of risk?
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%.
Is there a reasonable way to evaluate this addition in terms of risk?
keen9a
Well Known Member
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.
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.
Kevin Horton
Well Known Member
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 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
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?
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
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
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
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
Kevin Horton
Well Known Member
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?
Be careful. Fly with the big head, not the little head.
DGlaeser
Well Known Member
Why Va decreases at light weights
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
Here is the way I like to explain Va:Brockster said:
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
osxuser
Well Known Member
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.
Captain Avgas
Well Known Member
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.
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.
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.
vlittle
Well Known Member
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
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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gmcjetpilot
Well Known Member
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.
CATEGORY --------------------------------------- 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
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 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/umodule2.html#flight_envelope
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 certificated
weights for certain airplanes operated in Alaska.
(b) The maximum certificated weight approved under this section may not
exceed --
(1) 12,500 pounds;
(2) 115 percent of the maximum weight listed in the FAA aircraft
specifications;
(3) The weight at which the airplane meets the positive maneuvering load
factor requirement for the normal category specified in ?23.337 of this
chapter; or
(4) The weight at which the airplane meets the climb performance
requirements under which it was type certificated.
(c) In determining the maximum certificated weight, the Administrator
considers the structural soundness of the airplane and the terrain to be
traversed.
(d) The maximum certificated weight determined under this section is added
to the airplane's operation limitations and is identified as the maximum weight
authorized for operations within the State of Alaska.
weights for certain airplanes operated in Alaska.
(b) The maximum certificated weight approved under this section may not
exceed --
(1) 12,500 pounds;
(2) 115 percent of the maximum weight listed in the FAA aircraft
specifications;
(3) The weight at which the airplane meets the positive maneuvering load
factor requirement for the normal category specified in ?23.337 of this
chapter; or
(4) The weight at which the airplane meets the climb performance
requirements under which it was type certificated.
(c) In determining the maximum certificated weight, the Administrator
considers the structural soundness of the airplane and the terrain to be
traversed.
(d) The maximum certificated weight determined under this section is added
to the airplane's operation limitations and is identified as the maximum weight
authorized for operations within the State of Alaska.
CATEGORY --------------------------------------- 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
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
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/umodule2.html#flight_envelope
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John C
Well Known Member
So how much margin do you really have?
Weights and Maneuver Speeds - there is design and the real world - lets mix them up - I agree with the posts that you are much safet sticking to the design.
((I make a lot of general statements and do not include all the footnotes and qualifiers. If I have let out an important qualifier, please jump in. This is meant to be a general discussion about maneuver speeds and RVs, and how extra weight may affect you))
Gs, lift, forces, stresses, and back. Let?s leave stresses out of this. We design to Gs as the FARs dictate. Force is what breaks wings. The design weight and design G define the design lift or force. The G you see in the manual is the design or limit load. I then apply a 1.5 factor to limit load to get ultimate load. That mean you are not supposed to permanently bend the wing below limit load and not break them below ultimate load (whole bunch of footnotes here). This is important -- you may have no margin above maneuver speed before you can seriously damage your airplane, that is bend it. Is there much difference between bending it and breaking it in static test? In the real world? More on that in a moment.
Wings may bend permanently if I exceed the design weight and design G (lets ignore the 1.5 factor for a minute). Thus, if I am heavier than design, my wings will bend at the same force, but at a lesser G. If I am at a lighter weight, my wings will still break at the same force, but at a higher G. All of this is possible above the maneuver speed. So, regardless of the weight, at maneuver speed I can reach limit load and bend the wing. If I bent my wing, I may feel like killing myself.
I can have a ?legal? airplane and break, rather than just bend it at the 1.0 factor or limit load (more a little later).
Now, back to the 1.5 factor, what does the 1.5 factor buy you? Not much if you need it. There are monster gusts out there. Such as penetrating a thunderstorm or mountain induced turbulence. Most of us will not be flying in those conditions but if you get caught and you are above Van?s design weight, you don?t have much margin when you hit that lesser monster gust. NTSB and news reports suggest that Scott Crossfield?s airplane came apart in flight during a thunderstorm penetration. It could be from severe gusts or a loss of control and rapidly accelerating past maneuver speed with subsequent maneuvers, or additional gusts. I would bet that in many cases of upset, it takes less than five seconds to use up that entire so-called margin as I streak past maneuver speed. The 1.5 factor would raise the ?9A loads from limit (at maneuver speed of 118 mph) to ultimate at 144 knots, only a 26 mph increase. The RVs accelerate very fast. So, how much margin do you really have?
Now, back to how you can ?legally? break an airplane at limit load. I have always been told that the 1.5 factor makes up for a lot of sins. Such as?bad assumptions to set the FAR design requirements, bigger gusts, material deficiencies, manufacturing deficiencies (yours), aging, wear, damage, multiple loads (such as gust and full aileron), ?.? So, if you had a recorder, and found that your wings did break at design load (limit load) your airplane has met the intent of the design requirements. What that means is the 1.5 factor may not be there and does not have to be there.
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.
Higher empty weights normally mean more static load on the nose gear. Higher gross weights can certainly mean higher loads on the nose gear. Higher loads on the nose gear can make it much easier to get the fork into the sod. It will flatten the tire more, is your tire pressure ok? It will push the tire further into the sod, how soft is the field today?
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.
Just remember, we engineers get to assume anything except responsibility. You assume the responsibility, and the risks, if you start playing with our wonderful designs.
Regards, John.
Weights and Maneuver Speeds - there is design and the real world - lets mix them up - I agree with the posts that you are much safet sticking to the design.
((I make a lot of general statements and do not include all the footnotes and qualifiers. If I have let out an important qualifier, please jump in. This is meant to be a general discussion about maneuver speeds and RVs, and how extra weight may affect you))
Gs, lift, forces, stresses, and back. Let?s leave stresses out of this. We design to Gs as the FARs dictate. Force is what breaks wings. The design weight and design G define the design lift or force. The G you see in the manual is the design or limit load. I then apply a 1.5 factor to limit load to get ultimate load. That mean you are not supposed to permanently bend the wing below limit load and not break them below ultimate load (whole bunch of footnotes here). This is important -- you may have no margin above maneuver speed before you can seriously damage your airplane, that is bend it. Is there much difference between bending it and breaking it in static test? In the real world? More on that in a moment.
Wings may bend permanently if I exceed the design weight and design G (lets ignore the 1.5 factor for a minute). Thus, if I am heavier than design, my wings will bend at the same force, but at a lesser G. If I am at a lighter weight, my wings will still break at the same force, but at a higher G. All of this is possible above the maneuver speed. So, regardless of the weight, at maneuver speed I can reach limit load and bend the wing. If I bent my wing, I may feel like killing myself.
I can have a ?legal? airplane and break, rather than just bend it at the 1.0 factor or limit load (more a little later).
Now, back to the 1.5 factor, what does the 1.5 factor buy you? Not much if you need it. There are monster gusts out there. Such as penetrating a thunderstorm or mountain induced turbulence. Most of us will not be flying in those conditions but if you get caught and you are above Van?s design weight, you don?t have much margin when you hit that lesser monster gust. NTSB and news reports suggest that Scott Crossfield?s airplane came apart in flight during a thunderstorm penetration. It could be from severe gusts or a loss of control and rapidly accelerating past maneuver speed with subsequent maneuvers, or additional gusts. I would bet that in many cases of upset, it takes less than five seconds to use up that entire so-called margin as I streak past maneuver speed. The 1.5 factor would raise the ?9A loads from limit (at maneuver speed of 118 mph) to ultimate at 144 knots, only a 26 mph increase. The RVs accelerate very fast. So, how much margin do you really have?
Now, back to how you can ?legally? break an airplane at limit load. I have always been told that the 1.5 factor makes up for a lot of sins. Such as?bad assumptions to set the FAR design requirements, bigger gusts, material deficiencies, manufacturing deficiencies (yours), aging, wear, damage, multiple loads (such as gust and full aileron), ?.? So, if you had a recorder, and found that your wings did break at design load (limit load) your airplane has met the intent of the design requirements. What that means is the 1.5 factor may not be there and does not have to be there.
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.
Higher empty weights normally mean more static load on the nose gear. Higher gross weights can certainly mean higher loads on the nose gear. Higher loads on the nose gear can make it much easier to get the fork into the sod. It will flatten the tire more, is your tire pressure ok? It will push the tire further into the sod, how soft is the field today?
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.
Just remember, we engineers get to assume anything except responsibility. You assume the responsibility, and the risks, if you start playing with our wonderful designs.
Regards, John.
DGlaeser
Well Known Member
Weights and Maneuver speeds
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'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 itJohn C said:
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
RV-8 speeds & weights
First I would like to thank everyone for the very educational glimpse into the physics of the limitations.
More questions:
Is there a design maximum zero fuel weight for a RV-8?
What is the fuel burn on a 180/200 hp RV-8 at TO/Climb power? (How quckly will the actual weight come down?)
First I would like to thank everyone for the very educational glimpse into the physics of the limitations.
More questions:
Is there a design maximum zero fuel weight for a RV-8?
What is the fuel burn on a 180/200 hp RV-8 at TO/Climb power? (How quckly will the actual weight come down?)
dan
Well Known Member
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:
Climb is so quick with the powerplants you mentioned (assuming you have a C/S prop) that even if you're going up to 12,000' MSL from sea level, even if you're burning an average of 15 gph getting there, you'll only burn about 3 gallons.
The amount of fuel you burn off in an RV climb is almost negligible. I did a study of LOP versus ROP climbs to see how effectively it could extend my range. The difference worked out to about 0.2 gallons on my average cross country leg...not worth bothering.
Anyway, the point is that fuel burned in the climb in a well-climbing RV will not have a significant impact on the gross weight or CG.
mlw450802
Well Known Member
I agree! Way NOT intuitive!DGlaeser said:
I apologize if this is too tedious but I had to actually work through some of the accepted relationships to prove to myself the sliding Va speeds according to weight.
1. Stall speed= sqrt(flying weight/gross weight)*gross weight stall speed
2. (flying speed/ stall speed)^2 = max possible G loading.
(following are assumptions on weight and stall speeds)
Let's assume an RV 7 with 1800pound gross weight and an established stall speed at gross of 60 kt.
using relationship 1. for a 1500 lb flying weight:
Stall speed = sqrt(1500/1800)*60 = 54.77kt at 1500lb.
lets assume the maximum g loading you want to incur is 4g.
Relationship 2. says (flying speed/stall speed) must be 2, [(flyspeed/stallspeed)^2=4] so the maneuvering speed would be 120kt at gross weight but only 109.54 kts at 1500lb.
(109.54/54.77)^2 = 4 = (120/60)^2
Again, I apologize for this if it was in the AOPA article listed earlier. It just helped me to work through the non-intuitive nature of the problem.
-mike
John C
Well Known Member
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
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
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.
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:
jonbakerok
Well Known Member
It ain't all science
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".
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".
RVbySDI
Well Known Member
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:
So many of us builders are engineers or wish we were engineers. We expect to be able to calculate for every variable with precise measurements and reasons for those measurements and thus have total control over our projects. Does everything have to have .001 tolerances or have some precise number associated with it in order for it to work well? If 1800 lbs is the limit will the airplane crash and burn at 1801 lbs? Why the numbers are set the way they are is not always based on metalurgic qualities, physics, aerodynamics or any other "scientific" calculations. Sometimes they may be based on best estimation generalizations.
RVBYSDI
Steve
John C said:
An example of this is the AA airbus out of New York that crashed in November of 2001 . The vertical fin seperated from the aircraft shortly after takeoff.
"But the NTSB has determined that the pilot flying the Airbus caused the vertical fin to break off by rapidly moving the rudder from side to side. The Airbus was flying at a speed below Va and so, according to pilot dogma, should have been immune from a structural failure. But we pilots were wrong."
Full article on the Flying website here: http://tinyurl.com/e3kqc
