Gross Weight
Moderators: GAHorn, Karl Towle, Bruce Fenstermacher
Re: Gross Weight
I've been told for the last 23 years by a number of people that a gross weight increase was coming really soon for big engine 170s.
I have zero confidence that anyone from the fuzz is gonna stick their neck out to approve anything of the kind.
Prove me wrong.
Please.
I have zero confidence that anyone from the fuzz is gonna stick their neck out to approve anything of the kind.
Prove me wrong.
Please.
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ghostflyer
- Posts: 1395
- Joined: Mon Apr 21, 2008 3:06 am
Re: Gross Weight
Well we learn some thing new every day . I “think”I know why the 170 hasn’t had an all up weight increase with an increase in power. . It appears the wing doesn’t have the same structure as a later model 172. For example When a wing extension is done a number of strength ing stainless plates are fitted to the main spar . There is also a right angle extrusion is fitted to the rear of the fuel tank rear spar. So the research goes on. Please advise if I am not on the correct track of enlightenment .
Re: Gross Weight
It’s just not that simple. Wing strength is not the issue. The 170B/172/L19 have similar if not identical wings. Tail-feathers are also shared between 170B /L19. Anyone/Everyone knows the structure of the 170 can handle increased weight…and balked-landing-climb requirement with flaps deployed can be addressed with horsepower…and that is what makes for the common wish for increased gross weight.
Cessna faced this with small increases in subsequent 172 models. The 172C got 50 lbs increase with streamlining/fairing changes and lager models got the H.P. with the Lycomings…but the rear doorposts could not handle the balked-landing-climb stresses the flaps placed on the doorposts…so the solution for a weight increase…was a max-flap reduction. (No more 40-degrees.)
While the street believes H.P. should equate to greater weight…there’s just too many other details necessary to meet the certification rules…. of which most of us are unaware. To carry significantly more weight skin-gauges must be increased and that leads to greater E.W. and that conflicts with U.L. and G.W.
There’s one thing we all know, however. The way to get a G.W. increase is with Money…an intimidating amount of money. Thompsons’ book discussed the G.W. increases in a round-about-manner when the design-engineers were observing a 210 on the hydraulic stress-frame…and they kept increasing the loads to the design limit and everyone had big smiles because no popping-rivets and no wrinkling skins were observed. The smiles disappeared when a couple hundred pounds MORE stress was added and still no popped rivets…no wrinkled skins. Frowns appeared when still MORE stress was applied and no indications of structural failure appeared.
Why were they unhappy when they saw their design had so much reserve strength..?? Because that meant they had FAILED in their computions…they had OVER-built the airplane….which was now heavier than necessary…which steals from Useful Load. They had to go back to the dwg-boards and reduce structural weight to meet the intended design. It’s a balancing-act.
Adding H.P. alone isn’t the answer to increased G.W. …. but I doubt many O-360 or IO-360 operators worry excessively about an extra hundred or two pounds.
Cessna faced this with small increases in subsequent 172 models. The 172C got 50 lbs increase with streamlining/fairing changes and lager models got the H.P. with the Lycomings…but the rear doorposts could not handle the balked-landing-climb stresses the flaps placed on the doorposts…so the solution for a weight increase…was a max-flap reduction. (No more 40-degrees.)
While the street believes H.P. should equate to greater weight…there’s just too many other details necessary to meet the certification rules…. of which most of us are unaware. To carry significantly more weight skin-gauges must be increased and that leads to greater E.W. and that conflicts with U.L. and G.W.
There’s one thing we all know, however. The way to get a G.W. increase is with Money…an intimidating amount of money. Thompsons’ book discussed the G.W. increases in a round-about-manner when the design-engineers were observing a 210 on the hydraulic stress-frame…and they kept increasing the loads to the design limit and everyone had big smiles because no popping-rivets and no wrinkling skins were observed. The smiles disappeared when a couple hundred pounds MORE stress was added and still no popped rivets…no wrinkled skins. Frowns appeared when still MORE stress was applied and no indications of structural failure appeared.
Why were they unhappy when they saw their design had so much reserve strength..?? Because that meant they had FAILED in their computions…they had OVER-built the airplane….which was now heavier than necessary…which steals from Useful Load. They had to go back to the dwg-boards and reduce structural weight to meet the intended design. It’s a balancing-act.
Adding H.P. alone isn’t the answer to increased G.W. …. but I doubt many O-360 or IO-360 operators worry excessively about an extra hundred or two pounds.
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons.
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons.
Re: Gross Weight
Increasing the gross weight of a certificated aircraft can be accomplished by applying for an STC and providing the FAA with engineering data that demonstrates compliance with safety regulations. It’s pretty straightforward. The engineering process might prove it is not possible either due to excessive cost or limitations of the original design. If it’s ever going to happen, someone has to commit to the serious work and investment required.
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ghostflyer
- Posts: 1395
- Joined: Mon Apr 21, 2008 3:06 am
Re: Gross Weight
One point that we all agree on it’s takes money to have that small increase in weight uplift. As previously stated to me it would be “cheaper to buy a new Cessna 172” than try and go through the process of a STC. What little knowledge I have on the structural aspects of the 170 airframe I will be endeavouring fly well below the max weight uplift. I have cleaned out all the junk carried under the back seat and the cargo area. I have been on a weight reduction Itinerary for a couple of years now . [Aircraft that is ] .
However with the Wings X STC to increase the span of the wing it’s interesting to note all the stainless steel strips added to the main spar. Their claim to this was its what the later Cessnas have already fitted .
However with the Wings X STC to increase the span of the wing it’s interesting to note all the stainless steel strips added to the main spar. Their claim to this was its what the later Cessnas have already fitted .
