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  #11  
Old 09-27-2022, 12:12 PM
Mikeyb Mikeyb is offline
 
Join Date: Nov 2018
Location: Pasadena ca
Posts: 206
Default

To me building a well proven Vans kit meant that the only thing that was experimental about it was that I built it. I never imagined someone would go outside the W/B envelope. I don’t think an airplane built to operate outside those limits should be registered as an RV.
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Last edited by Mikeyb : 09-27-2022 at 12:34 PM.
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  #12  
Old 09-27-2022, 12:20 PM
rvbuilder2002 rvbuilder2002 is offline
 
Join Date: Jul 2005
Location: Hubbard Oregon
Posts: 9,975
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Larry is right on.

Arbitrarily choosing to increase specific performance limits, whether it be speed, weight, etc., is a kin to you doing all the work to design your own airplane and then once it is built simply make the conscious choice to up some of the limits beyond those that you used when doing all of the engineering and design work for said airplane.
Wouldn’t make much sense would it?

The design limits specified by vans are the design limits. Anyone making a choice to go beyond them just because they don’t meet their personal needs, is degrading their and their passengers safety margins.

As for the difference in the RV-6 and RV-7 design gross weight… The RV-7 is higher because it was specifically designed to be higher. The purpose of the RV-7 design was to be an improved and updated version of the RV-6. Wanting a larger (heavier) engine and higher payload carrying ability were two of the things that customers mentioned most regarding the RV-6, so those were taken into consideration when the engineering of the RV-7 was done.
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Scott McDaniels
Hubbard, Oregon
Formerly of Van's Aircraft Engineering Prototype Shop
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RV-6A (aka "Junkyard Special ")
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  #13  
Old 09-27-2022, 12:34 PM
skelrad skelrad is offline
 
Join Date: Mar 2005
Location: Redmond, WA
Posts: 357
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I was mostly curious why gross weight recommendations would change with different engines on the same airframe, but I didn't realize people played around with the numbers beyond those recommendations. Searching on the topic now, it looks like something that has been beat around a bit. Huh.
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  #14  
Old 09-27-2022, 12:42 PM
rvbuilder2002 rvbuilder2002 is offline
 
Join Date: Jul 2005
Location: Hubbard Oregon
Posts: 9,975
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Quote:
Originally Posted by skelrad View Post
I was mostly curious why gross weight recommendations would change with different engines on the same airframe, but I didn't realize people played around with the numbers beyond those recommendations. Searching on the topic now, it looks like something that has been beat around a bit. Huh.
Yes it has.
Some have a true understanding of why the limits are what they are, and some choose to consider it a suggestion (even though it is far from that).

The different gross weights on the RV-9 are indeed related to climb performance.
Even though as a kit manufacturer we are not required to design to FAR requirements, they are used as guidelines for many design parameters on RV kits because most of those requirements exist for very good reasons.
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  #15  
Old 09-27-2022, 01:15 PM
terrye terrye is offline
 
Join Date: Feb 2010
Location: Vancouver, BC, Canada
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Default Explain Gross Weight for Engine Choice

When discussing “design margins”, “safety factors”, or more properly “factors of ignorance”, most builders do not understand that they do not own these. The only one that owns these is the design engineer. He (or she) is the only one that understands the assumptions, simplifications, and design standards used to create the structure. In general, he is the one that designs the physical test regime, either ground or flight based, and signs off that the structure meets or exceeds the design standards.

In airplane design, the load factors are specified based on the category of airplane. In the GA certified world, these are normal, utility and aerobatic. Applying these load factors to the weight of the airplane results in the limit load. For example, the positive limit load of an aircraft in the aerobatic category with an aircraft weight of 1600 lbs is (1600lb x 6 g load factor) = 9600 lbs. On top of this limit load, a “safety factor” of usually 1.5 is applied to give the design ultimate load. So, in the example case, (9600 lbs x 1.5) = 14,400 lbs. This is the point at which the structure is about to fail catastrophically.

While normally called a “safety factor”, this is really a catch all for “factors of ignorance”. It includes such things as: defects in the material (aluminum sheet and plate in the case of RVs), defects in manufacture, defects in service, etc. All those things that the designer cannot control.

So whenever I read in these forums about builders installing larger engines than recommended, selecting an arbitrary gross weight above that recommended, flying at a speed that exceeds Vne, or pulling more Gs than the airplane was designed for, I ask myself what have they done to ensure the safety of the airplane? Have they done any calculations? Have they done any testing? In most cases, I suspect the answer is no.

The designer is the only one that owns the “design margins”. Not the builder, not the pilot, not the mechanic. Any builder that encroaches on the “design margins” is showing their own ignorance.
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  #16  
Old 09-27-2022, 01:34 PM
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Dan 57 Dan 57 is offline
 
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Quote:
The designer is the only one that owns the “design margins”.
Absolutely.
And obscure by itself... assume we'd take 2 team of Engineers, and have them recalculate, both of them and separetely , e.g. the RV-6 and the RV-7, using non-empirical methods as used in modern analysis... my bet is that the results would be quite interesting.

As to the -9(A) having different Gross Weights for different engines... bizarre, as most other models also have different engine/power options, but only one Gross Weight (Mass really, but that is another subject in itself)...
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  #17  
Old 09-27-2022, 04:27 PM
rvbuilder2002 rvbuilder2002 is offline
 
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Quote:
Originally Posted by Dan 57 View Post
Absolutely.
And obscure by itself... assume we'd take 2 team of Engineers, and have them recalculate, both of them and separetely , e.g. the RV-6 and the RV-7, using non-empirical methods as used in modern analysis... my bet is that the results would be quite interesting.

As to the -9(A) having different Gross Weights for different engines... bizarre, as most other models also have different engine/power options, but only one Gross Weight (Mass really, but that is another subject in itself)...
There are multiple factors that set a specific gross weight limit.

For most of the RV models that is mass because if even the smallest recommended engine is used the airplane will still meet all of the other performance criteria (rate of climb at gross, etc.)
For the RV-9 the different gross weight limit with the smaller engines isn't because the airframe structure can't handle the higher load (because it is exactly the same structurally), In that case as already mentioned, it is because if flown at the normal max. gross weight with the smaller engine, the airplane would not be capable of a minimum rate of climb (particularly with the high pitch fixed pitch prop that likely would be used to achieve the high cruise speed that the airplane is capable of).
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Opinions, information, and comments, are my own unless stated otherwise.
You are personally responsible for determining the suitability of any tips,
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Scott McDaniels
Hubbard, Oregon
Formerly of Van's Aircraft Engineering Prototype Shop
FAA/DAR
RV-6A (aka "Junkyard Special ")
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  #18  
Old 09-28-2022, 07:59 AM
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airguy airguy is offline
 
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Location: Garden City, Tx
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Scott - can you tell us what that target climb rate is?
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  #19  
Old 09-28-2022, 09:24 AM
rmarshall234 rmarshall234 is offline
 
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Default The Margin

These words from Van had such a profound impact on me at the time, that I clipped it and pasted into my log book.
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  #20  
Old 09-28-2022, 10:29 AM
PilotjohnS PilotjohnS is offline
 
Join Date: Jan 2016
Location: Southwest, USA
Posts: 2,528
Default Same thing, different feelings

So i raised the question and started this thread drift, sorry , but happy it went there.

So a builder can artificially increase the gross weight in two ways:
1) The builder can apply for airworthiness and do the phase 1 testing with a higher than recommended gross weight.
2) The builder can do all the airworthiness stuff as recommended by the manufacturer. Then, when his mission demands, he can concisely and deliberately overload the airplane.

Either way is the same bad outcome, except the first way the pilot may feel better because he "Certified it" at the erroneous overweight gross weight.

So what I hear you all saying is that certifying the plane at a higher gross weight is just as bad as intentionally taking off "over gross"?
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