What drives a gross weight changing with an engine size? I guess I expected the recommended gross weight to be the same for an airframe regardless of the engine in it, but the 9A has a different gross weight recommendation depending on the engine.
Huh. I hadn't given it a ton of thought. I just assumed the gross weight was what the airframe could physically "handle," as opposed to also making considerations for performance.
Please educate me.
If i want to conduct phase one on my plane at a higher gross weight than what Vans recommends, what are the rules?
If I test it at a higher gross weight, can I now use that number for my gross weight after phase 1?
If analysis is needed, what analysis do i need to show?
With the inherent variability of experimental airplanes, how does one set the gross weight for a entirely scratch built airplane? Does one have to show the detailed mechanical analysis to the DAR for a one -off design to get an airworthiness certificate? Or is the flight testing sufficient?
Thanks in advance,
Paul,
thanks for your insight.
I wish there was a more concrete process for establishing a higher gross weight; this loosey-goosey stuff and shopping for a DAR bothers me.
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.
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)...
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 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"?
Back in my 135 air taxi days in Alaska, we were legally allowed to increase the gross weight of our aircraft.
FAR 91.0323 (b) The maximum certificated weight approved under this section may not exceed - (2) 115 percent of the maximum weight listed in the FAA aircraft specifications.
EDIT: On a 3300lb. Cessna 185, Thats a 495lb. increase
Except it is now and never has been legal to do with a Cessna 185 (If I understand FAR 91.323 correctly).
It is only allowed on specific older aircraft that were originally certificated under the Department of Commerce, and later the CAA, and even then it is under very specific requirements that are not relevant for privately operated aircraft.
In my opinion not at all relevant to modern aircraft, RV's included.
If interested, people should do a bit of research on the origins of why this was allowed. My take is that it was a willingness to trade some levels of safety in order to increase others.
For most of my life I have been fascinated by long distance flights in small airplanes, starting with Max Conrad in the early 60s. Currently Bill Harrelson's record breaking flights have been of interest. Guam to Jacksonville FL, 7038nm nonstop, a new world record.
I think all those airplanes may have been a bit over gross. Maybe two times the normal gross.
Without going into the intricacies of the FAR...when doing the oral on a 135 check ride and asked to do a weight and balance, the 115% rule indeed was allowed for C-185s, C-206s, Cub's and whenever else we flew. At least in the '90s, this rule was applied across all bush planes regardless of the vintage.
have compassion for the folks that have to worry very day about density altitude…..
We hope at least, that the designer....or more likely the design team... has considered everything that is realistically possible to consider and made the best compromise decisions along the way. Without having the full big picture someone doing a modification is making some blind assumptions for sure....but it might not necessarily be wrong/dangerous/stupid/or whatever....