This is how I explain it to CFIs and fellow students and they look at me like I have no idea what I'm talking about... I guess I'll send them this video as well. lol.
I probably would have started with explaining the stalling AOA. Then the various AOA’s required for certain speeds explaining each at 1G. Then design load factor limits and load factor margin. I think not including the Va diagram made the discussion not correlate to the handbooks. But nice job. Nice graphics.
For the commercial emergency descent skill. Bank the plane 30-45° to maintain a positive load factor. Why is 0° bank nosedive lifting in your seat (negative G) and a bank creating a positive load factor?
This is how I explain it to my students. Devil's advocate (in case a student asks me), why does quadrupling the angle of attack quadruple the lift? Why is it a one to one ratio like that?
I’m not so happy with the explanation, because you’re neglecting the ailerons completely. “Control inputs” is not only referring to the elevators. The video basically explains, why you could stall an aircraft at any speed and attitude. The aileron input is more dangerous because it is neither depicted in the flight envelope, nor is it expressed by load factor. Why is the manoevering speed inside the green arc and far below the yellow arc? It’s the wing root bending moment which gives an opposite force from both wings on the wing spar. It cannot be expressed by g-forces but be calculated by the surface area of the ailerons, wind forces (speed!) and lever (wingspan). Please keep that in mind.
It's a shame instructors are still perpetuating this half-truth explanation of Va, it isn't exactly wrong but incomplete enough to setup a poor grasp of the true underlying hazards and fundamental physics and engineering. Yes, the conservative answer is that Va is reduced with reduced mass, and this will pass the test and keep you alive. But the reason is not really about reducing total lift, this practice of reducing Va is simply a catch all to cover the worst case load distribution so you don't need to account for the details of the actual load plan beyond simple compliance with the basic station mass limitations. The aircraft wing structure is not going to be over stressed (let alone approach breakup) by maintaining Va at a constant CAS, although some local sub-structures like seats or baggage floor may be over stressed. The engineering side of the matter is that the load factor is an engineering design goal used in combination with desired aircraft mass to select the required minimum structural working strength of components (or conversely with aftermarket revisions to determine the allowable mass at an existing strength). Design load-factor taken in isolation [without mass] is not a structural limit. The load force(pounds) product is structurally relevant, the individual factors [mass and acceleration] going into the product are not. Va also covers large inputs beyond just pitch, the forces of roll and yaw are both reduced with reduced mass as they are primarily inertia based. "VA-the calibrated design maneuvering airspeed. This is the maximum speed at which the limit load can be imposed (either by gusts or full deflection of the control surfaces) without causing structural damage. Operating at or below maneuvering speed does not provide structural protection against multiple full control inputs in one axis or full control inputs in more than one axis at the same time." (faa-h-8083-25c pg. 11-18, pg 5-42 also has a decent explanation of the loading on the airframe.) If the aircraft mass is changed proportionally and uniformly across all stations, then structural loads at a given total lift will always be the same even though the load factor may have increased or decreased drastically. In fact this is covered in "Aerodynamics for Naval Aviators" and examples are given for adjusting load factor for mass and keeping Va constant, rather than adjusting Va to hit a constant load factor. The reason the private pilot civilian realm adjusts Va with gross weight is that it is convenient and accommodates lazy load planning and preflight calculations, specifically the case where the overall gross weight may have been reduced but one station remains at maximum capacity and the assumption that the maximum working load for the local structure of the station is based on the same load factor of the operational category (3.8 for normal). This is why you will see many C172 also have a 4.4G utility envelope in addition to the normal category envelope with reductions on allowed mass in individual stations. There is no magic that allows a sudden step function between 3.8 and 4.4 at 2000 pounds mass, physically there are smooth curves connecting the two categories and loading limits. Notice that (for a C172M) 3.8×2300=4.4×2000=8800 pounds with some minor rounding, the same total structural load in pound force. So the fixed-mass stations (such as engine mounts) are all designed for a load factor of at least 4.4 with an overall structural working load of 8800 pounds of lift, a lift which will always occur at max AoA at the same CAS (112mph in the example C172M) And why some io-360 conversion poh supplements do not allow for utility category it is a slightly more massive engine on existing mounts so the maximum vertical acceleration is less than 4.4G.
@@Dub636 Maybe. A youtube comment can only spoon feed so much, and not everyone has the elementary engineering foundation that they like to tell themselves.
@@whoanelly737-8 Basic arithmetic and some middling effort to go beyond the first result returned by reddit doesn't take an engineering degree, but is a prerequisite to one.
I've never understood this. Say 4Gs and 2500 lbs are our limits to make the math easier. that means the wing has to produce 10000lbs of lift to get to the 4G limit. If you weigh 2000lbs, pull the yoke back at the same speed, the lift is the same 10000lbs. you will experience 5gs. For the wings it's the same.
I think you're saying the force on the wings was 10000lbs before removing mass, so should be able to withstand the same 10000lbs (5G) after losing weight. Ie, the reduced Va at lower weight doesn't make sense? I used to think that too. The confusion is that there is more than one way to overstress an airframe. The wings won't rip off at 5g as you surmise, (same 10000lbs) but the engine mounts may snap trying to muscle the heavy engine around to accelerate at 5g etc. Imagine an imaginary extreme case, where the wings still exert 10000lbs force but cause 30G acceleration - the pilot dies. The load limit isn't just cantilever wing mounts.
You are correct that the main wing structure experiences the same load (actually less stress if the remaining mass is just fuel in wing tanks). And if you proportionally reduced the mass of all substructures, Va would not change. This topic is almost universally taught terribly in the pilot community. The practice of reducing Va is to accommodate fixed mass stations and a worst-case distribution of the variable-mass (aka payload) without adding extra load planning calculations. So using your numbers you may reduce total gross mass from 2500 to 2000, but if the baggage area remains at its maximum normal-category limit of 50 pounds of luggage then that station remains limited to 4G. 50×4g=200 pound force on the local structure, but 50×5g=250 pound force on that local structure. My main comment has some more detailed explanations including aircraft with 2 or more categories (normal utility acrobatic).
This is how I explain it to CFIs and fellow students and they look at me like I have no idea what I'm talking about... I guess I'll send them this video as well. lol.
This is so true😂 one of the toughest things to teach personally
Rough that CFIs don't already understand this. And aren't teaching to their students "here's why Va means it'll stall before it breaks"...
I am a AGI/IGI/CFI/CFII...I never could explain it as well as you just illustrated...your graphics are the difference maker
Bravo!! Never had this explained so well.
by far the best explanation of Va ever. boldmethod did a really good job too to be fair. but this is goated
I’ve never understood this well before you made this video. Thank you!
This is actually quite difficult to understand but I manage to grasp it while studying. Nice to have a video on it now 😊
Thanks for fixing so quickly
Excellent video. Thanks
I probably would have started with explaining the stalling AOA. Then the various AOA’s required for certain speeds explaining each at 1G. Then design load factor limits and load factor margin. I think not including the Va diagram made the discussion not correlate to the handbooks. But nice job. Nice graphics.
Great video!
Epic! Keep it up!
For the commercial emergency descent skill. Bank the plane 30-45° to maintain a positive load factor. Why is 0° bank nosedive lifting in your seat (negative G) and a bank creating a positive load factor?
Good vid.
Best explanation I've ever seen.
I'll be using this for all future students.
This is how I explain it to my students. Devil's advocate (in case a student asks me), why does quadrupling the angle of attack quadruple the lift? Why is it a one to one ratio like that?
EPIC OMG
I’m not so happy with the explanation, because you’re neglecting the ailerons completely. “Control inputs” is not only referring to the elevators. The video basically explains, why you could stall an aircraft at any speed and attitude. The aileron input is more dangerous because it is neither depicted in the flight envelope, nor is it expressed by load factor. Why is the manoevering speed inside the green arc and far below the yellow arc? It’s the wing root bending moment which gives an opposite force from both wings on the wing spar. It cannot be expressed by g-forces but be calculated by the surface area of the ailerons, wind forces (speed!) and lever (wingspan). Please keep that in mind.
Holy cow! Thank you
2000lbs of lift does not "quadruple" to 6000lbs of lift haha but great video otherwise.
It's a shame instructors are still perpetuating this half-truth explanation of Va, it isn't exactly wrong but incomplete enough to setup a poor grasp of the true underlying hazards and fundamental physics and engineering. Yes, the conservative answer is that Va is reduced with reduced mass, and this will pass the test and keep you alive. But the reason is not really about reducing total lift, this practice of reducing Va is simply a catch all to cover the worst case load distribution so you don't need to account for the details of the actual load plan beyond simple compliance with the basic station mass limitations.
The aircraft wing structure is not going to be over stressed (let alone approach breakup) by maintaining Va at a constant CAS, although some local sub-structures like seats or baggage floor may be over stressed.
The engineering side of the matter is that the load factor is an engineering design goal used in combination with desired aircraft mass to select the required minimum structural working strength of components (or conversely with aftermarket revisions to determine the allowable mass at an existing strength). Design load-factor taken in isolation [without mass] is not a structural limit. The load force(pounds) product is structurally relevant, the individual factors [mass and acceleration] going into the product are not.
Va also covers large inputs beyond just pitch, the forces of roll and yaw are both reduced with reduced mass as they are primarily inertia based. "VA-the calibrated design maneuvering airspeed. This is the maximum speed at which the limit load can be imposed (either by gusts or full deflection of the control surfaces) without causing structural damage. Operating at or below maneuvering speed does not provide structural protection against multiple full control inputs in one axis or full control inputs in more than one axis at the same time." (faa-h-8083-25c pg. 11-18, pg 5-42 also has a decent explanation of the loading on the airframe.)
If the aircraft mass is changed proportionally and uniformly across all stations, then structural loads at a given total lift will always be the same even though the load factor may have increased or decreased drastically. In fact this is covered in "Aerodynamics for Naval Aviators" and examples are given for adjusting load factor for mass and keeping Va constant, rather than adjusting Va to hit a constant load factor.
The reason the private pilot civilian realm adjusts Va with gross weight is that it is convenient and accommodates lazy load planning and preflight calculations, specifically the case where the overall gross weight may have been reduced but one station remains at maximum capacity and the assumption that the maximum working load for the local structure of the station is based on the same load factor of the operational category (3.8 for normal).
This is why you will see many C172 also have a 4.4G utility envelope in addition to the normal category envelope with reductions on allowed mass in individual stations. There is no magic that allows a sudden step function between 3.8 and 4.4 at 2000 pounds mass, physically there are smooth curves connecting the two categories and loading limits.
Notice that (for a C172M) 3.8×2300=4.4×2000=8800 pounds with some minor rounding, the same total structural load in pound force. So the fixed-mass stations (such as engine mounts) are all designed for a load factor of at least 4.4 with an overall structural working load of 8800 pounds of lift, a lift which will always occur at max AoA at the same CAS (112mph in the example C172M) And why some io-360 conversion poh supplements do not allow for utility category it is a slightly more massive engine on existing mounts so the maximum vertical acceleration is less than 4.4G.
Come on!
you just said a lot without saying basically anything. A nothing burger. A word salad
@@Dub636 Maybe. A youtube comment can only spoon feed so much, and not everyone has the elementary engineering foundation that they like to tell themselves.
Half truths that cover the margin is better than something most people cannot grasp without an engineering degree.
@@whoanelly737-8 Basic arithmetic and some middling effort to go beyond the first result returned by reddit doesn't take an engineering degree, but is a prerequisite to one.
I've never understood this.
Say 4Gs and 2500 lbs are our limits to make the math easier.
that means the wing has to produce 10000lbs of lift to get to the 4G limit.
If you weigh 2000lbs, pull the yoke back at the same speed, the lift is the same 10000lbs. you will experience 5gs. For the wings it's the same.
I think you're saying the force on the wings was 10000lbs before removing mass, so should be able to withstand the same 10000lbs (5G) after losing weight. Ie, the reduced Va at lower weight doesn't make sense?
I used to think that too. The confusion is that there is more than one way to overstress an airframe.
The wings won't rip off at 5g as you surmise, (same 10000lbs) but the engine mounts may snap trying to muscle the heavy engine around to accelerate at 5g etc.
Imagine an imaginary extreme case, where the wings still exert 10000lbs force but cause 30G acceleration - the pilot dies. The load limit isn't just cantilever wing mounts.
You are correct that the main wing structure experiences the same load (actually less stress if the remaining mass is just fuel in wing tanks). And if you proportionally reduced the mass of all substructures, Va would not change. This topic is almost universally taught terribly in the pilot community.
The practice of reducing Va is to accommodate fixed mass stations and a worst-case distribution of the variable-mass (aka payload) without adding extra load planning calculations.
So using your numbers you may reduce total gross mass from 2500 to 2000, but if the baggage area remains at its maximum normal-category limit of 50 pounds of luggage then that station remains limited to 4G. 50×4g=200 pound force on the local structure, but 50×5g=250 pound force on that local structure. My main comment has some more detailed explanations including aircraft with 2 or more categories (normal utility acrobatic).