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Why does the DC-9-80 have this cusp in its fuselage?

Does the MD-80 seat layout affect the balance of the aircraft?Did the cusp in the Boeing 377/C-97/KC-97’s fuselage structure cause fatigue problems?Why is the fuselage on an airliner circular-shaped?MD81: Why are there so many lights on the fuselage?Is the Boeing 737 unusually susceptible to stress fatigue?Why do airliners have “pressure bulkheads”?Why do most commercial aircraft have their fuselage over the wing, instead of under?Is bleed air for fuselage pressurization at altitude purified or sterile/ If so how is this doneHow does the fuselage handle the different loads?Why does this 757 have a propeller engine attached to the fuselage?Why does the Aurora D8 have vents on the bottom of its fuselage?Did the cusp in the Boeing 377/C-97/KC-97’s fuselage structure cause fatigue problems?

9

$begingroup$

The fuselage of the McDonnell Douglas DC-9-80 (marketed as the "MD-80") has a pronounced cusp at floor level, as can be seen in this diagram (courtesy of Boeing, eater of McDonnell Douglas - at least, according to @JWalters):

Although the DC-9-80's fuselage cusp is nowhere near as extreme as the Boeing 377 Stratocruiser's, it still seems like an unnecessary stress concentrator with no apparent reason for being there; why did McDonnell Douglas put in the cusp, despite its potential for accelerating fatigue damage to the fuselage?

aircraft-design fuselage dc-9-family fatigue

share|improve this question

asked 22 hours ago

SeanSean

4,92122461

$endgroup$

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  $begingroup$
  Not a unique feature of the DC-9. It's easier to name mainline jet the aircraft that aren't double lobe so they don't have a cusp: A300/310/330/340 (same circular fuselage), A380 (elliptical), 777 (circular).
  $endgroup$
  – user71659
  19 hours ago
  

  
  
  
  


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  It was inherited from the DC-8. The DC-8 was initially planned with a smaller fuselage width but the upper half was enlarged when the Boeing 707 was made wider before it. The DC-9 cross section was a smaller version of the DC-8 cross section.
  $endgroup$
  – Peter Kämpf
  13 hours ago
  

  
  
  
  

add a comment | 

9

$begingroup$

The fuselage of the McDonnell Douglas DC-9-80 (marketed as the "MD-80") has a pronounced cusp at floor level, as can be seen in this diagram (courtesy of Boeing, eater of McDonnell Douglas - at least, according to @JWalters):

Although the DC-9-80's fuselage cusp is nowhere near as extreme as the Boeing 377 Stratocruiser's, it still seems like an unnecessary stress concentrator with no apparent reason for being there; why did McDonnell Douglas put in the cusp, despite its potential for accelerating fatigue damage to the fuselage?

aircraft-design fuselage dc-9-family fatigue

share|improve this question

asked 22 hours ago

SeanSean

4,92122461

$endgroup$

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  $begingroup$
  Not a unique feature of the DC-9. It's easier to name mainline jet the aircraft that aren't double lobe so they don't have a cusp: A300/310/330/340 (same circular fuselage), A380 (elliptical), 777 (circular).
  $endgroup$
  – user71659
  19 hours ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  $begingroup$
  It was inherited from the DC-8. The DC-8 was initially planned with a smaller fuselage width but the upper half was enlarged when the Boeing 707 was made wider before it. The DC-9 cross section was a smaller version of the DC-8 cross section.
  $endgroup$
  – Peter Kämpf
  13 hours ago
  

  
  
  
  

add a comment | 

$begingroup$

The fuselage of the McDonnell Douglas DC-9-80 (marketed as the "MD-80") has a pronounced cusp at floor level, as can be seen in this diagram (courtesy of Boeing, eater of McDonnell Douglas - at least, according to @JWalters):

Although the DC-9-80's fuselage cusp is nowhere near as extreme as the Boeing 377 Stratocruiser's, it still seems like an unnecessary stress concentrator with no apparent reason for being there; why did McDonnell Douglas put in the cusp, despite its potential for accelerating fatigue damage to the fuselage?

aircraft-design fuselage dc-9-family fatigue

share|improve this question

asked 22 hours ago

SeanSean

4,92122461

$endgroup$

share|improve this question

asked 22 hours ago

SeanSean

4,92122461

share|improve this question

asked 22 hours ago

SeanSean

4,92122461

asked 22 hours ago

SeanSean

4,92122461

asked 22 hours ago

SeanSean

4,92122461

3 Answers
3

active

oldest

votes

9

$begingroup$

It's not a stress concentrator; it's just the opposite. What you're missing is that the floor itself at the pinched part forms a tension bridge that allows a more or less 'ovalized' circle while still maintaining tension loading on the skins and frames as if it was a pure circle.

If I had a rubber balloon filled with air and was able to run a string internally from one side to the other, attached to the walls of the balloon, and then drew the string in to pinch the sides of the balloon into a figure eightish profile, I'd have the same thing. All the loads on the skins are still in tension, as well as the bridging floor beams (the string).

share|improve this answer

answered 20 hours ago

John KJohn K

20k12458

$endgroup$

add a comment | 

4

$begingroup$

To add to John's answer, why did they do it? To make more room in the passenger area.

It is important to remember that, opposite a submarine, pressure is higher on the inside of the aircraft in flight, rather than lower. So structural design favors a cross brace to hold it together. A submarine would be strongest if it were perfectly spherical.

Also, notice the "pinched" fuselage has lower frontal area, resulting in better fuel economy from less drag.

The MD-80 design, with its clean wings (engines mounted on rear fuselage), lived on as the
Boeing 717.

share|improve this answer

answered 14 hours ago

Robert DiGiovanniRobert DiGiovanni

2,3591316

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  The passenger area would be larger if they removed the concave area, so I don't understand the point of your first paragraph.
  $endgroup$
  – David Richerby
  12 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  Could be statisticly passenger + seat has more volume on average than luggage.🤔 But, apparently, enlarging the upper half was the best engineering choice. Also seen on the Super Guppy.
  $endgroup$
  – Robert DiGiovanni
  9 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  I mean using an egg-shaped cross-section. You see the point where it pinches in? Push that out to give a smooth curve: the passenger area gets larger and the floor still acts as a tension beam.
  $endgroup$
  – David Richerby
  8 hours ago
  

  
  
  
  


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  @David Richerby: Shoulders are generally wider than feet, and that's where the increased room is.
  $endgroup$
  – jamesqf
  8 hours ago
  

  
  
  
  


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  @DavidRicherby An oval shape like that disrupts the circular arc for each part circle and introduces bending forces in the frames above and below the floor beams as pressurization forces push out. You can do it that way, but you will add a bit of weight stiffening the frames. The double bubble profile is the lightest way to achieve an oval-ish cross section.
  $endgroup$
  – John K
  4 hours ago
  

  
  
  
  

 | 
show 1 more comment

1

$begingroup$

The answer is soap bubbles:

They are filled with slightly higher pressure air, and when they're attached to each other, they have a planar "reinforcement-like" part between them. (see the picture)
This is similar to your "attached circles with reinforcement between them"-style structure. (DC fuselage)

share|improve this answer

answered 7 hours ago

NyosNyos

111

New contributor

Nyos is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

$endgroup$

add a comment | 

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9

$begingroup$

It's not a stress concentrator; it's just the opposite. What you're missing is that the floor itself at the pinched part forms a tension bridge that allows a more or less 'ovalized' circle while still maintaining tension loading on the skins and frames as if it was a pure circle.

If I had a rubber balloon filled with air and was able to run a string internally from one side to the other, attached to the walls of the balloon, and then drew the string in to pinch the sides of the balloon into a figure eightish profile, I'd have the same thing. All the loads on the skins are still in tension, as well as the bridging floor beams (the string).

share|improve this answer

answered 20 hours ago

John KJohn K

20k12458

$endgroup$

add a comment | 

9

$begingroup$

It's not a stress concentrator; it's just the opposite. What you're missing is that the floor itself at the pinched part forms a tension bridge that allows a more or less 'ovalized' circle while still maintaining tension loading on the skins and frames as if it was a pure circle.

If I had a rubber balloon filled with air and was able to run a string internally from one side to the other, attached to the walls of the balloon, and then drew the string in to pinch the sides of the balloon into a figure eightish profile, I'd have the same thing. All the loads on the skins are still in tension, as well as the bridging floor beams (the string).

share|improve this answer

answered 20 hours ago

John KJohn K

20k12458

$endgroup$

add a comment | 

$begingroup$

It's not a stress concentrator; it's just the opposite. What you're missing is that the floor itself at the pinched part forms a tension bridge that allows a more or less 'ovalized' circle while still maintaining tension loading on the skins and frames as if it was a pure circle.

If I had a rubber balloon filled with air and was able to run a string internally from one side to the other, attached to the walls of the balloon, and then drew the string in to pinch the sides of the balloon into a figure eightish profile, I'd have the same thing. All the loads on the skins are still in tension, as well as the bridging floor beams (the string).

share|improve this answer

answered 20 hours ago

John KJohn K

20k12458

$endgroup$

share|improve this answer

answered 20 hours ago

John KJohn K

20k12458

answered 20 hours ago

John KJohn K

20k12458

answered 20 hours ago

John KJohn K

20k12458

4

$begingroup$

To add to John's answer, why did they do it? To make more room in the passenger area.

It is important to remember that, opposite a submarine, pressure is higher on the inside of the aircraft in flight, rather than lower. So structural design favors a cross brace to hold it together. A submarine would be strongest if it were perfectly spherical.

Also, notice the "pinched" fuselage has lower frontal area, resulting in better fuel economy from less drag.

The MD-80 design, with its clean wings (engines mounted on rear fuselage), lived on as the
Boeing 717.

share|improve this answer

answered 14 hours ago

Robert DiGiovanniRobert DiGiovanni

2,3591316

$endgroup$

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  $begingroup$
  The passenger area would be larger if they removed the concave area, so I don't understand the point of your first paragraph.
  $endgroup$
  – David Richerby
  12 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  Could be statisticly passenger + seat has more volume on average than luggage.🤔 But, apparently, enlarging the upper half was the best engineering choice. Also seen on the Super Guppy.
  $endgroup$
  – Robert DiGiovanni
  9 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  I mean using an egg-shaped cross-section. You see the point where it pinches in? Push that out to give a smooth curve: the passenger area gets larger and the floor still acts as a tension beam.
  $endgroup$
  – David Richerby
  8 hours ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  $begingroup$
  @David Richerby: Shoulders are generally wider than feet, and that's where the increased room is.
  $endgroup$
  – jamesqf
  8 hours ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  $begingroup$
  @DavidRicherby An oval shape like that disrupts the circular arc for each part circle and introduces bending forces in the frames above and below the floor beams as pressurization forces push out. You can do it that way, but you will add a bit of weight stiffening the frames. The double bubble profile is the lightest way to achieve an oval-ish cross section.
  $endgroup$
  – John K
  4 hours ago
  

  
  
  
  

 | 
show 1 more comment

4

$begingroup$

To add to John's answer, why did they do it? To make more room in the passenger area.

It is important to remember that, opposite a submarine, pressure is higher on the inside of the aircraft in flight, rather than lower. So structural design favors a cross brace to hold it together. A submarine would be strongest if it were perfectly spherical.

Also, notice the "pinched" fuselage has lower frontal area, resulting in better fuel economy from less drag.

The MD-80 design, with its clean wings (engines mounted on rear fuselage), lived on as the
Boeing 717.

share|improve this answer

answered 14 hours ago

Robert DiGiovanniRobert DiGiovanni

2,3591316

$endgroup$

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  $begingroup$
  The passenger area would be larger if they removed the concave area, so I don't understand the point of your first paragraph.
  $endgroup$
  – David Richerby
  12 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  Could be statisticly passenger + seat has more volume on average than luggage.🤔 But, apparently, enlarging the upper half was the best engineering choice. Also seen on the Super Guppy.
  $endgroup$
  – Robert DiGiovanni
  9 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  I mean using an egg-shaped cross-section. You see the point where it pinches in? Push that out to give a smooth curve: the passenger area gets larger and the floor still acts as a tension beam.
  $endgroup$
  – David Richerby
  8 hours ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  $begingroup$
  @David Richerby: Shoulders are generally wider than feet, and that's where the increased room is.
  $endgroup$
  – jamesqf
  8 hours ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  $begingroup$
  @DavidRicherby An oval shape like that disrupts the circular arc for each part circle and introduces bending forces in the frames above and below the floor beams as pressurization forces push out. You can do it that way, but you will add a bit of weight stiffening the frames. The double bubble profile is the lightest way to achieve an oval-ish cross section.
  $endgroup$
  – John K
  4 hours ago
  

  
  
  
  

 | 
show 1 more comment

$begingroup$

To add to John's answer, why did they do it? To make more room in the passenger area.

It is important to remember that, opposite a submarine, pressure is higher on the inside of the aircraft in flight, rather than lower. So structural design favors a cross brace to hold it together. A submarine would be strongest if it were perfectly spherical.

Also, notice the "pinched" fuselage has lower frontal area, resulting in better fuel economy from less drag.

The MD-80 design, with its clean wings (engines mounted on rear fuselage), lived on as the
Boeing 717.

share|improve this answer

answered 14 hours ago

Robert DiGiovanniRobert DiGiovanni

2,3591316

$endgroup$

share|improve this answer

answered 14 hours ago

Robert DiGiovanniRobert DiGiovanni

2,3591316

answered 14 hours ago

Robert DiGiovanniRobert DiGiovanni

2,3591316

answered 14 hours ago

Robert DiGiovanniRobert DiGiovanni

2,3591316

1

$begingroup$

The answer is soap bubbles:

They are filled with slightly higher pressure air, and when they're attached to each other, they have a planar "reinforcement-like" part between them. (see the picture)
This is similar to your "attached circles with reinforcement between them"-style structure. (DC fuselage)

share|improve this answer

answered 7 hours ago

NyosNyos

111

New contributor

Nyos is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

$endgroup$

add a comment | 

1

$begingroup$

The answer is soap bubbles:

They are filled with slightly higher pressure air, and when they're attached to each other, they have a planar "reinforcement-like" part between them. (see the picture)
This is similar to your "attached circles with reinforcement between them"-style structure. (DC fuselage)

share|improve this answer

answered 7 hours ago

NyosNyos

111

New contributor

Nyos is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

$endgroup$

add a comment | 

$begingroup$

The answer is soap bubbles:

They are filled with slightly higher pressure air, and when they're attached to each other, they have a planar "reinforcement-like" part between them. (see the picture)
This is similar to your "attached circles with reinforcement between them"-style structure. (DC fuselage)

share|improve this answer

answered 7 hours ago

NyosNyos

111

New contributor

Nyos is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

$endgroup$

share|improve this answer

answered 7 hours ago

NyosNyos

111

New contributor

Nyos is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

answered 7 hours ago

NyosNyos

111

New contributor

Nyos is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

answered 7 hours ago

NyosNyos

111