LeSving wrote:
If that factor is 10, 20 or 50%, who cares, it’s just an arbitrary number found in the design code to be used in the design . It’s not something that shall ever be met.
?!? The factors of safety required for certification are specifically stated in the certification standards, and are demonstrated by test (I witness some of those test) That factor of safety has saved a few careless or unfortunate pilots over the years.
For example, during my flight testing:
Holy mackerel!
@Pilot_DAE, what kind of safety measures do you take when carrying out this kind of tests?
Parachute? If so, how do you get out of the plane?
Did you do any testing of the DA42 in icing conditions?
No parachute, I’m not interested in trying to exit at that speed! Safety measure is a very careful touch, and very smooth air! For the last 30 years, I’ve been a fair weather pilot only, I avoid ice like the plague! (From lessons learned during my first 10 years of flying!).
I would guess that the structural implications of flying overloaded would vary according to where the weight was carried e.g. some tip tank conversions give a gross weight increase. All about bending loads. Otherwise the risk of structural failure will increase by flying below MTOM. The arcs in the ASI are set for max weight, if you are light the green arc should be reduced. The stalling speed will increase with weight so the wing will stall before it fails.
I agree that operating occasionally a bit overweight is far from being the worse way of abusing an aircraft. Its obviously most significant during take off and early climb, also landing overweight is more damaging for the structure than take off. However operating an MEP overweight needs much more care, single engine performance is often marginal anyway. On take you would be much worse off than in a SEP, you would have more than twice the chance of engine failure combined with a much higher stalling speed.
Also much depends on why the weight was fixed. My plane was certified in the UTILITY category, you could also buy one at that time certified in the NORMAL category (giving a 70kg gross weight increase). The only difference between the planes was the POH. So clearly I’m not going to worry too much about being a bit overweight!
Anyone who advocates slavish observance to the POH has obviously never seen a 1970s Robin handbook. More a pamphlet than a book! I once read through a Beech Duchess POH and was amazed at the copious detail offered.
Back on the topic of weight…
A load test (to 3.8G X 1.5 factor) prior to my flight test:
Ballast for flight test, and preflight load testing:
Amazing and interesting pics, Pilot_DAR! Thanks for posting them.
It does “kind of” show that you know what you are writing about 
Pilot_DAR wrote:
The factors of safety required for certification are specifically stated in the certification standards, and are demonstrated by test (I witness some of those test) That factor of safety has saved a few careless or unfortunate pilots over the years.
The point is that there is a load going up to a limit. There always has to be a factor in these circumstances, no matter if it is an aircraft or a car seat, a bridge or whatever. For load factor it is chosen to be 50%, and that is an arbitrary number. It could have been 20% or 70%, it doesn’t matter, it’s just a choice. But – it always has to be larger than zero even for a zero safety factor so to say. Again, this has nothing to do with a good design or not in a real practical situation.
Cool pictures
LeSving wrote:
Again, this has nothing to do with a good design or not in a real practical situation.
I don’t agree. We spend effort to assure that some secondary structures will fail between 1.5 and 2 times the required maximum limit load, or else, we have built it too heavy/strong/expensive. The ultimate capacity of the materials is known very exactly, and so a balanced design will consider the relationship of structure to fasteners to assure that there is not a disproportionate amount of one vs the other = waste. For more complex “whole” structures, this requires engineering skill, which I do not profess to have.
Similarly, there is no point making a plane with oodles of power, yet a structure which cannot withstand the loads which that power would enable, or the other way, a rugged plane, with not enough power to climb away with the intended load.
I will present an example: An older Canadian designed plane (they only made 5) with 290 HP, six seats, fuel capacity of 245 liters (388 pounds), and a gross weight of 3500 pounds. Its empty weight was 2782.0 pounds. Oops… 3500 – 2782 = 718 pounds useful load. Six 170 pound occupants weigh 1020 pounds, you can’t even fill the seats and carry any fuel! I have a half dozen hours flight testing one of these on wheels and floats (which are more heavy) with full fuel, and two occupants: 2782 + 370 + 388 = 3540. I was 40 pounds overgross with only two of the six possible occupants. This design, rugged as can be, was much too heavy – design mismatch. And 290 HP lifting 3500 pounds is poor… (Small wonder they never sold any!).
Fuji_Abound wrote:
Three very interesting examples. Thank you.
There’s another one by the way: The Cirrus SR20 has a MTOW of 3000 lbs and MLW of 2900 lbs.
Pilot_DAR wrote:
We spend effort to assure that some secondary structures will fail between 1.5 and 2 times the required maximum limit load, or else, we have built it too heavy/strong/expensive
In that case it is not a design factor, but a pure safety factor to be tested. What exactly does CS say? It will also be a difference what is considered if for instance a structure can carry 100% with no adverse effects and up to 150% with some adverse affects, yet no total failure.
