Cherokee_Cruiser
Bronteroc
Is the click, like, and subscribe generation entering the NTSB? Of course they want to record you.
Lifeguard Fatal Five in Nevada Feb 24, 2023
- Thread starter fholbert
- Start date
Is the click, like, and subscribe generation entering the NTSB? Of course they want to record you.
BEEF SUPREME
Well-Known Member
OK so am I the only one that thinks it is odd we have a member here who is saying multiple PC-12's have fallen apart in mid air?
Itchy
Well-Known Member
To your point on hitting flat, it sure looks like this one did too.
Roger Roger
Bottom of the list
Right. This isn’t a twin Cessna that is 40 years old and had no structural inspection requirements for the first 30 of those. When a PC comes apart it’s because it hit something, whether something solid or a max load factor
BEEF SUPREME
Well-Known Member
But they haven't fallen apart and don't have a history of falling apart. There has been one confirmed due to an inexperienced pilot flying directly into a massive S Florida Thunderstorm and completely dorking up the recovery. That's it.
Roger Roger
Bottom of the list
Yeah that’s what I was getting at
I can't find where anyone said that in this thread.
Closest I found was someone saying "wouldn't be the first" or something like that. Followed by lots of people talking about that one instance.
BEEF SUPREME
Well-Known Member
I guess technically it "wouldn't be the first" but the one time it has ever happened was when it was flown directly into a massive thunderstorm. It's kind of a stretch to call it a precedence.
This isn't a common occurrence. It happened once with a guy who had more money than brains at the controls. Not a professional pilot.
///AMG
Well-Known Member
It ain’t too bad, anything more than 4 takes some physical effort, 7+ normally doesn’t last too long due to aerodynamic/drag reasons, unless in a slicked off lightly loaded jet at low MSL alt, and then you are in for a real ride as it accelerates on or near the G limiter........
inigo88
Composite-lover
This took a little bit of research and phoning some friends, because this is a load case I’ve generally taken for granted (in my defense some engineers generate loads and some consume them for new designs, I’m a consumer
).One of the first things you make when generating structural loads is the V-n diagram (V for velocity, n for vertical acceleration aka Gs). You can actually have accelerations in all three Cartesian x,y,z directions so we call vertical acceleration Nz, but it’s shortened to n here. (Edit: the graph calls it “load factor”, but it’s the vertical acceleration Nz, aka “n”, in units of “G’s” which is actually a unit less scaling factor.) You can actually correlate the diagram below with color ranges on the airspeed indicator:
(Source:Flight Envelope | Support)
Point A is Vs.
Point B is Va - The speed at which you can make full control deflections without negative consequences. For example max elevator deflection will result in an accelerated stall before you can develop sufficient G to over-stress the airframe.
The horizontal lines to the right of point B and point C are your positive and negative G limits respectively.
Vno - the line between the green and yellow arc has to do with gust envelope protection which is on a separate chart, but suffice to say if you hit unexpected turbulence and encounter a large vertical gust in the yellow arc you may not be protected from a structural failure, hence Vno must only be exceeded in smooth air.
And finally Vne is the never exceed speed, the point after which flutter may be encountered, parts may depart the aircraft, etc.
I bring up the V-n diagram because the FARs and Mil-Specs for airworthiness certification apply a maximum symmetric positive and negative acceleration (the lines to the right of points B and C on the V-n diagram) and NOT a rolling G.
The next thing we need to touch on is what spanwise lift distribution actually looks like in symmetric vs rolling applications.
Say this glider is making a wings level pull into a loop and the pilot pulls aft stick right to the positive max G limit in the airworthiness certification. That distribution of lift FORCE (note G is acceleration, now we are talking about the force of lift acting on the wing in lbs) will look something like this:
These wings are just fancy looking cantilever beams with an applied elliptical lift force distribution, so the designer/stress analyst is going to size the wing spar(s) to a couple of key load cases described in the airworthiness regulations but generally positive max up bending (at symmetric max positive G) is going to be the sizing load case.
Now telling you guys to not throw full aileron deflection in at max positive G isn’t going to surprise 90% of you because you know not to do that over maneuvering speed anyway, but the fighter pilot community can’t accept that because specific dogfighting maneuvers actually require it. “Flat Scissors” comes to mind: Scissors (aeronautics) - Wikipedia
So let’s say you’re in a fighter trying to get someone off your tail and you throw in full aileron deflection to reverse your turn, what happens to the spanwise lift distribution shown in the glider picture above?
Recall that an aileron works by changing the camber of the airfoil on the wing. Down aileron = more cambered airfoil = more lift force. Up aileron = less cambered airfoil = less lift force. Look in the opposite direction of your aileron input and that’s the down aileron side, because that’s the side that’s making more lift and “picking that wing up” to roll, like this:
(source:Ailerons)
Note that this is a simplified view of the spanwise lift distribution, it’s actually still elliptically distributed like in the glider picture, except now at the inboard edge of the aileron there’s a big step input on the down aileron side with even more lift than before. Think of these big arrows as resultant vectors showing the sum of the distributed forces in the glider picture. And if you integrate the area under the curve of the lift distribution, your total lift force on the down aileron wing is going to be greater in the rolling case than in the symmetric pull case.
So I think this is the crux of the problem. The engineers are in compliance with the load requirements as long as the wing is designed not to break at the max positive G with symmetrically applied lift force. But throw aileron in and now your lift becomes asymmetric, and the lift on the down aileron wing exceeds the max design value and could result in structural failure.
From a design standpoint there’s a few ways you can tackle this. If your airplane truly needs to do this maneuver then you dream this up as an extra design load case and it becomes your new sizing load case (above and beyond the max requirement in the airworthiness regs). All your structure is going to weigh more than what’s technically required though, and you have to go back to your customer and say “your max payload weight has decreased by X amount, can you live with that?” Inevitably they’ll probably decide they don’t want to live with that, at which point your other option is to:
a. Design your wing for the max positive symmetric G value. The wing attach point must react all the load distribution at the attach point with the fuselage, so now you have a max reaction shear and moment value.
b. Add full aileron deflection and see how much your lift force increases on the down aileron side, and see how much that increases your reaction shear and moment value.
c. Dial back the vertical G force (Nz) applied via aft stick while rolling until the total lift force and reaction shear and moment on the down aileron wing fall back to the original values from part a.
Essentially you end up with two limitations, your max symmetric G value and a reduced max rolling G value and the pilots must know and memorize both. When getting ready to roll in a maneuver like a flat scissors the pilot has to remember to back off the back pressure from the max pull value to the rolling value, reverse the turn with full aileron deflection and then load back to max G once the stick is centered. I think this is what @MikeD and @///AMG are talking about, but you guys correct me if I’m wrong. Fundamentally by doing that you guys are ensuring that the force on the wings never exceed the design limit load case, even when a deflected aileron is applying additional lift on one side.
Hope this helps, I had a fun time learning about it myself!
///AMG
Well-Known Member
Nice thanks @inigo88
I have nothing at all to add to that nice description (you know far more than I do), but I will say that a true "flat" scissors is flown at absolute minimum airspeed. If someone is behind you, you aren't in the flats.....you're just about to be a guns kill. At such slow speed, G available is well below whatever rolling G limit might exist. I imagine you may be thinking more of what we would call "1 circle" maneuvering, or a specific type of 1C maneuvering called a "weave". 1C flow at higher airspeeds, or perhaps a loopy rolling scissors with a lot of energy involved can get you into the realm where a rudder or nose hi to nose low reversal could get you into unhappy rolling G. We actually see it a lot more commonly while doing a safe escape maneuver following a roll-in type delivery of GP bombs or strafing. Low alt, high speed, lots of G available, sometimes with a lateral avoidance component to the maneuver.
I have nothing at all to add to that nice description (you know far more than I do), but I will say that a true "flat" scissors is flown at absolute minimum airspeed. If someone is behind you, you aren't in the flats.....you're just about to be a guns kill. At such slow speed, G available is well below whatever rolling G limit might exist. I imagine you may be thinking more of what we would call "1 circle" maneuvering, or a specific type of 1C maneuvering called a "weave". 1C flow at higher airspeeds, or perhaps a loopy rolling scissors with a lot of energy involved can get you into the realm where a rudder or nose hi to nose low reversal could get you into unhappy rolling G. We actually see it a lot more commonly while doing a safe escape maneuver following a roll-in type delivery of GP bombs or strafing. Low alt, high speed, lots of G available, sometimes with a lateral avoidance component to the maneuver.
inigo88
Composite-lover
Thanks man, that’s super interesting. I was envisioning something at much higher speed, like a 2 circle above 400 knots with a roll reversal. But your real world examples make more sense, especially the egress from GP or strafing where you’re trying to climb and turn at the same time at high speed.
ppragman
FLIPY FLAPS!
You’re my hero.This took a little bit of research and phoning some friends, because this is a load case I’ve generally taken for granted (in my defense some engineers generate loads and some consume them for new designs, I’m a consumer
One of the first things you make when generating structural loads is the V-n diagram (V for velocity, n for vertical acceleration aka Gs). You can actually have accelerations in all three Cartesian x,y,z directions so we call vertical acceleration Nz, but it’s shortened to n here. (Edit: the graph calls it “load factor”, but it’s the vertical acceleration Nz, aka “n”, in units of “G’s” which is actually a unit less scaling factor.) You can actually correlate the diagram below with color ranges on the airspeed indicator:
View attachment 70116
(Source:Flight Envelope | Support)
Point A is Vs.
Point B is Va - The speed at which you can make full control deflections without negative consequences. For example max elevator deflection will result in an accelerated stall before you can develop sufficient G to over-stress the airframe.
The horizontal lines to the right of point B and point C are your positive and negative G limits respectively.
Vno - the line between the green and yellow arc has to do with gust envelope protection which is on a separate chart, but suffice to say if you hit unexpected turbulence and encounter a large vertical gust in the yellow arc you may not be protected from a structural failure, hence Vno must only be exceeded in smooth air.
And finally Vne is the never exceed speed, the point after which flutter may be encountered, parts may depart the aircraft, etc.
I bring up the V-n diagram because the FARs and Mil-Specs for airworthiness certification apply a maximum symmetric positive and negative acceleration (the lines to the right of points B and C on the V-n diagram) and NOT a rolling G.
The next thing we need to touch on is what spanwise lift distribution actually looks like in symmetric vs rolling applications.
Say this glider is making a wings level pull into a loop and the pilot pulls aft stick right to the positive max G limit in the airworthiness certification. That distribution of lift FORCE (note G is acceleration, now we are talking about the force of lift acting on the wing in lbs) will look something like this:
View attachment 70119
These wings are just fancy looking cantilever beams with an applied elliptical lift force distribution, so the designer/stress analyst is going to size the wing spar(s) to a couple of key load cases described in the airworthiness regulations but generally positive max up bending (at symmetric max positive G) is going to be the sizing load case.
Now telling you guys to not throw full aileron deflection in at max positive G isn’t going to surprise 90% of you because you know not to do that over maneuvering speed anyway, but the fighter pilot community can’t accept that because specific dogfighting maneuvers actually require it. “Flat Scissors” comes to mind: Scissors (aeronautics) - Wikipedia
So let’s say you’re in a fighter trying to get someone off your tail and you throw in full aileron deflection to reverse your turn, what happens to the spanwise lift distribution shown in the glider picture above?
Recall that an aileron works by changing the camber of the airfoil on the wing. Down aileron = more cambered airfoil = more lift force. Up aileron = less cambered airfoil = less lift force. Look in the opposite direction of your aileron input and that’s the down aileron side, because that’s the side that’s making more lift and “picking that wing up” to roll, like this:
View attachment 70120
(source:Ailerons)
Note that this is a simplified view of the spanwise lift distribution, it’s actually still elliptically distributed like in the glider picture, except now at the inboard edge of the aileron there’s a big step input on the down aileron side with even more lift than before. Think of these big arrows as resultant vectors showing the sum of the distributed forces in the glider picture. And if you integrate the area under the curve of the lift distribution, your total lift force on the down aileron wing is going to be greater in the rolling case than in the symmetric pull case.
So I think this is the crux of the problem. The engineers are in compliance with the load requirements as long as the wing is designed not to break at the max positive G with symmetrically applied lift force. But throw aileron in and now your lift becomes asymmetric, and the lift on the down aileron wing exceeds the max design value and could result in structural failure.
From a design standpoint there’s a few ways you can tackle this. If your airplane truly needs to do this maneuver then you dream this up as an extra design load case and it becomes your new sizing load case (above and beyond the max requirement in the airworthiness regs). All your structure is going to weigh more than what’s technically required though, and you have to go back to your customer and say “your max payload weight has decreased by X amount, can you live with that?” Inevitably they’ll probably decide they don’t want to live with that, at which point your other option is to:
a. Design your wing for the max positive symmetric G value. The wing attach point must react all the load distribution at the attach point with the fuselage, so now you have a max reaction shear and moment value.
b. Add full aileron deflection and see how much your lift force increases on the down aileron side, and see how much that increases your reaction shear and moment value.
c. Dial back the vertical G force (Nz) applied via aft stick while rolling until the total lift force and reaction shear and moment on the down aileron wing fall back to the original values from part a.
Essentially you end up with two limitations, your max symmetric G value and a reduced max rolling G value and the pilots must know and memorize both. When getting ready to roll in a maneuver like a flat scissors the pilot has to remember to back off the back pressure from the max pull value to the rolling value, reverse the turn with full aileron deflection and then load back to max G once the stick is centered. I think this is what @MikeD and @///AMG are talking about, but you guys correct me if I’m wrong. Fundamentally by doing that you guys are ensuring that the force on the wings never exceed the design limit load case, even when a deflected aileron is applying additional lift on one side.
Hope this helps, I had a fun time learning about it myself!
Bob Ridpath
Pit Bull love
Even as one with no skin in the game, this discussion is tremendous and enlightening. Thank you all for the technical and personal expertise you've chosen to share!
🦈💜
Well-Known Member
Hope this helps, I had a fun time learning about it myself!
Thank you so much for this! That's how I understood it, but I loved the way you stepped through it, and some of those diagrams brought back memories! ^_^ (I also may borrow the glider lift image for teaching aerodynamics)
In fighters, especially in dogfighting, we try to avoid loaded rolls, as they tend to lag in roll rate….trying to fight the G load in the turn. To get some serious roll rate while reversing a turn, it’s best to unload first (forward elevator/stab), then throw the roll in with aileron/roll spoilers…..placing your lift vector (that exists straight up out the top of your canopy) where you are wanting the jet to go, then loading on the G’s again with aft aft stick/stab in order to get the jet going in that direction. That’s the fastest way to reverse turns and get the jet moving in a different direction quickly.
🦈💜
Well-Known Member
As weird as this sounds, this is actually a fundamental component of modern upset training as well.
mightynimbus
Well-Known Member
Urban dictionary describes the scissor maneuver quite similarly
For severe unusual attitudes, when you have to get the jet back upright quickly, it does make aerodynamic sense. So I can easily see that.
