Gbradleyiii
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Do DC10 Aerial Firefighting planes have to pressurize the cabin?

Thu Oct 31, 2019 2:38 am

I love to see the big DC10 Aerial bombers drop their phoscheck to surprise wildfires.
I wonder if they need to pressurize the cabin for these sorties.
They don’t seem to go very high.
 
GalaxyFlyer
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Re: Do DC10 Aerial Firefighting planes have to pressurize the cabin?

Thu Oct 31, 2019 1:31 pm

They probably do, unpressurized flight for some planes causes fuselage stresses. C-5 unpressurized flight required a Lockheed waiver.
 
jetblueguy22
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Re: Do DC10 Aerial Firefighting planes have to pressurize the cabin?

Thu Oct 31, 2019 5:22 pm

GalaxyFlyer wrote:
They probably do, unpressurized flight for some planes causes fuselage stresses. C-5 unpressurized flight required a Lockheed waiver.

Curious if you'd know the answer, but what about for paratrooper drop aircraft like the C-130/C-17? I doubt they're getting a waiver every time from the manufacturer, so is there an airframe limit to the drops? Or are they just stressed for that kind of role?
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GalaxyFlyer
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Re: Do DC10 Aerial Firefighting planes have to pressurize the cabin?

Thu Oct 31, 2019 8:06 pm

In the C-5, pressurization puts the fuse in tension reducing flex due to air loads. Think of an inflated balloon. Airdrop was not an essential design element and wasn’t done much. Besides the waiver was not for airdrop as much as long ferrying flights to maintenance. The 130 and 17 were both airdrop designs and no waivers needed. Most military airdrop is low altitude, HALO being the exception, high altitude (up to .F250).

GF
 
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Re: Do DC10 Aerial Firefighting planes have to pressurize the cabin?

Thu Oct 31, 2019 8:34 pm

I was told something long ago about why it's good to have a plane pressurized down low, specifically a little bit of pressure on takeoff/landing is what it was about. Think of squeezing a coke can that hasn't been opened and then squeeze an empty one. Which one would rather be bouncing around in the air in.
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Gbradleyiii
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Re: Do DC10 Aerial Firefighting planes have to pressurize the cabin?

Fri Nov 01, 2019 3:01 am

The reasons I was wondering, was because:
(1) I have heard that the number of pressurization cycles is what takes the life out of the air-frame.
(2) I don't know how long it takes to pressurize, or if pressurization would slow them down when they have to get airborne quickly.
 
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fr8mech
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Re: Do DC10 Aerial Firefighting planes have to pressurize the cabin?

Fri Nov 01, 2019 4:29 am

Gbradleyiii wrote:
The reasons I was wondering, was because:
(1) I have heard that the number of pressurization cycles is what takes the life out of the air-frame.
(2) I don't know how long it takes to pressurize, or if pressurization would slow them down when they have to get airborne quickly.


(1) Takeoff/Landing Cycles, not pressurization cycles, though those numbers would be pretty close, but I don't think anyone counts pressurization cycles.

(2) Pressurization is automatic (when everything is working normally) and occurs as the aircraft climbs (and descends).
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DocLightning
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Re: Do DC10 Aerial Firefighting planes have to pressurize the cabin?

Fri Nov 01, 2019 5:03 am

Good points made here and additionally: most airliners that are converted away from pax use, be they freighters, tankers, or something else, do not have pressure bulkheads aft of the cockpit. A flat bulkhead like that is extremely heavy and again, pressurization stiffens the fuselage. Only a few very special variants (Guppy, DreamLifter) have pressure bulkheads aft of the cockpit and unpressurized cargo areas.
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Max Q
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Re: Do DC10 Aerial Firefighting planes have to pressurize the cabin?

Fri Nov 01, 2019 7:36 am

DocLightning wrote:
Good points made here and additionally: most airliners that are converted away from pax use, be they freighters, tankers, or something else, do not have pressure bulkheads aft of the cockpit. A flat bulkhead like that is extremely heavy and again, pressurization stiffens the fuselage. Only a few very special variants (Guppy, DreamLifter) have pressure bulkheads aft of the cockpit and unpressurized cargo areas.



Your post is confusing, freighter conversions rarely relocate the pressure bulkhead, all aircraft have these aft of the cockpit however
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fr8mech
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Re: Do DC10 Aerial Firefighting planes have to pressurize the cabin?

Fri Nov 01, 2019 8:03 am

Max Q wrote:
Your post is confusing, freighter conversions rarely relocate the pressure bulkhead, all aircraft have these aft of the cockpit however


They have a 9G bulkhead/restraint protecting the flight deck, not a pressure bulkhead. Doc is trying to relate that the pressure vessel remains the same after conversion.
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Re: Do DC10 Aerial Firefighting planes have to pressurize the cabin?

Fri Nov 01, 2019 3:03 pm

fr8mech wrote:
Max Q wrote:
Your post is confusing, freighter conversions rarely relocate the pressure bulkhead, all aircraft have these aft of the cockpit however


They have a 9G bulkhead/restraint protecting the flight deck, not a pressure bulkhead. Doc is trying to relate that the pressure vessel remains the same after conversion.


Exactly.
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Max Q
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Re: Do DC10 Aerial Firefighting planes have to pressurize the cabin?

Sat Nov 02, 2019 3:59 am

fr8mech wrote:
Max Q wrote:
Your post is confusing, freighter conversions rarely relocate the pressure bulkhead, all aircraft have these aft of the cockpit however


They have a 9G bulkhead/restraint protecting the flight deck, not a pressure bulkhead. Doc is trying to relate that the pressure vessel remains the same after conversion.



As was I, re-read his post, he states ‘most aircraft converted to freighters do not have pressure bulkheads aft of the cockpit’


Well that’s just incorrect, of course they do, as do all pressurized aircraft


What I think he meant however was certain, highly modified and / or specialized aircraft only pressurize the cockpit area and / or immediate vicinity and have an ‘ aft’ pressure bulkhead located very close behind these compartments


In lieu of the usual aft pressure bulkhead at the rear of the former passenger cabin
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JayinKitsap
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Re: Do DC10 Aerial Firefighting planes have to pressurize the cabin?

Sat Nov 02, 2019 10:20 pm

Aircraft pressurization occurs as the plane rises in elevation. At cruise, most airlines prior to the 787 maintained the interior at 8,000 ft elevation, even if at fl41. So the pressurization is the difference from 41 and 8. I am unaware of any plane that intentionally pressurizes the plane below 8,000 ft elevation.

Thin shells have far higher buckling strength in compression if the hoop stresses are in tension, the weaker buckling mode is into the vessel, all of the buckling modes outwards from the vessel take far higher stresses. With Al, the outward modes take 3x to 4x the axial compression of the inward. A pressure of 1 psi on shells the diameter of an airliner inward modes above the outward.
 
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Re: Do DC10 Aerial Firefighting planes have to pressurize the cabin?

Sat Nov 02, 2019 10:47 pm

JayinKitsap wrote:
Aircraft pressurization occurs as the plane rises in elevation. At cruise, most airlines prior to the 787 maintained the interior at 8,000 ft elevation, even if at fl41. So the pressurization is the difference from 41 and 8. I am unaware of any plane that intentionally pressurizes the plane below 8,000 ft elevation.

Thin shells have far higher buckling strength in compression if the hoop stresses are in tension, the weaker buckling mode is into the vessel, all of the buckling modes outwards from the vessel take far higher stresses. With Al, the outward modes take 3x to 4x the axial compression of the inward. A pressure of 1 psi on shells the diameter of an airliner inward modes above the outward.


Loads of planes pressurize below 8,000’ cabin altitude most airliners that are cruising below maximum altitude are below 8,000’ cabin, perhaps not much. The newer bizjets operate 10.3 psid, 4,500’ cabin at F450.

GF
 
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Re: Do DC10 Aerial Firefighting planes have to pressurize the cabin?

Sun Nov 03, 2019 4:02 am

GalaxyFlyer wrote:
JayinKitsap wrote:
Aircraft pressurization occurs as the plane rises in elevation. At cruise, most airlines prior to the 787 maintained the interior at 8,000 ft elevation, even if at fl41. So the pressurization is the difference from 41 and 8. I am unaware of any plane that intentionally pressurizes the plane below 8,000 ft elevation.

Thin shells have far higher buckling strength in compression if the hoop stresses are in tension, the weaker buckling mode is into the vessel, all of the buckling modes outwards from the vessel take far higher stresses. With Al, the outward modes take 3x to 4x the axial compression of the inward. A pressure of 1 psi on shells the diameter of an airliner inward modes above the outward.


Loads of planes pressurize below 8,000’ cabin altitude most airliners that are cruising below maximum altitude are below 8,000’ cabin, perhaps not much. The newer bizjets operate 10.3 psid, 4,500’ cabin at F450.

GF


I'm sure you are right, cabin pressure is controlled by a back pressure valve. I do not know how they dial in the pressure, it would be easy to set for the differential between 80 and F450 anything below F450 would have a nice low cabin pressure, say 4,000 at F400. A single setting mechanical back pressure valve is similar to a relief other than it is designed to have continuous flow not emergency release.
 
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Re: Do DC10 Aerial Firefighting planes have to pressurize the cabin?

Mon Nov 04, 2019 1:58 am

JayinKitsap wrote:
GalaxyFlyer wrote:
JayinKitsap wrote:
Aircraft pressurization occurs as the plane rises in elevation. At cruise, most airlines prior to the 787 maintained the interior at 8,000 ft elevation, even if at fl41. So the pressurization is the difference from 41 and 8. I am unaware of any plane that intentionally pressurizes the plane below 8,000 ft elevation.

Thin shells have far higher buckling strength in compression if the hoop stresses are in tension, the weaker buckling mode is into the vessel, all of the buckling modes outwards from the vessel take far higher stresses. With Al, the outward modes take 3x to 4x the axial compression of the inward. A pressure of 1 psi on shells the diameter of an airliner inward modes above the outward.


Loads of planes pressurize below 8,000’ cabin altitude most airliners that are cruising below maximum altitude are below 8,000’ cabin, perhaps not much. The newer bizjets operate 10.3 psid, 4,500’ cabin at F450.

GF


I'm sure you are right, cabin pressure is controlled by a back pressure valve. I do not know how they dial in the pressure, it would be easy to set for the differential between 80 and F450 anything below F450 would have a nice low cabin pressure, say 4,000 at F400. A single setting mechanical back pressure valve is similar to a relief other than it is designed to have continuous flow not emergency release.


It’s an outflow valve, actually they’re installed in pairs-2 per plane. They are controlled by electronic controllers with manual back-up and cannot be set above about 14,500’ in the high elevation airport mode. Most have a set schedule of pressurization done automatically.

GF
 
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fr8mech
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Re: Do DC10 Aerial Firefighting planes have to pressurize the cabin?

Mon Nov 04, 2019 2:23 am

GalaxyFlyer wrote:
It’s an outflow valve, actually they’re installed in pairs-2 per plane. They are controlled by electronic controllers with manual back-up and cannot be set above about 14,500’ in the high elevation airport mode. Most have a set schedule of pressurization done automatically.

GF


The B757/776 each have 1 valve, with 2 separate control channels, plus a manual control. I think the DC10/MD11 also have 1, though something is nagging me about the wheel well, maybe that’s the A300.
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JayinKitsap
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Re: Do DC10 Aerial Firefighting planes have to pressurize the cabin?

Mon Nov 04, 2019 3:28 am

fr8mech wrote:
GalaxyFlyer wrote:
It’s an outflow valve, actually they’re installed in pairs-2 per plane. They are controlled by electronic controllers with manual back-up and cannot be set above about 14,500’ in the high elevation airport mode. Most have a set schedule of pressurization done automatically.

GF


The B757/776 each have 1 valve, with 2 separate control channels, plus a manual control. I think the DC10/MD11 also have 1, though something is nagging me about the wheel well, maybe that’s the A300.


Most pressure vessels have both a pressure control and a safety relief. If just one valve there would probably be a relief panel or rupture disk as well. Blowing the rear bulkhead could be considered 'bad'. :stirthepot:
 
Max Q
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Re: Do DC10 Aerial Firefighting planes have to pressurize the cabin?

Mon Nov 04, 2019 7:44 am

Years ago, I watched a 727 pull up to the gate and noticed something odd, it looked like a piece of the fuselage on the forward nose section was deflected outward


Indeed it was, that was the overpressure relief valve that had opened


I don’t remember how it happened, as a brand new FE it was interesting though
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fr8mech
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Re: Do DC10 Aerial Firefighting planes have to pressurize the cabin?

Mon Nov 04, 2019 12:04 pm

JayinKitsap wrote:

Most pressure vessels have both a pressure control and a safety relief. If just one valve there would probably be a relief panel or rupture disk as well. Blowing the rear bulkhead could be considered 'bad'. :stirthepot:


But only the outflow valve is controllable, the pressure relief valves act automatically, and are independent of the cabin pressure controllers and the manual control.
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JayinKitsap
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Re: Do DC10 Aerial Firefighting planes have to pressurize the cabin?

Mon Nov 04, 2019 7:39 pm

fr8mech wrote:
JayinKitsap wrote:

Most pressure vessels have both a pressure control and a safety relief. If just one valve there would probably be a relief panel or rupture disk as well. Blowing the rear bulkhead could be considered 'bad'. :stirthepot:


But only the outflow valve is controllable, the pressure relief valves act automatically, and are independent of the cabin pressure controllers and the manual control.


I agree, the outflow valve is controllable and can be set for the desired pressure from the cockpit, the relief is set for the maximum allowed differential pressure, it will relieve the pressure above the maximum design pressure, usually the set point in industrial piping and tanks at 5% over the design pressure.
 
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Re: Do DC10 Aerial Firefighting planes have to pressurize the cabin?

Fri Nov 08, 2019 7:53 pm

DocLightning wrote:
Good points made here and additionally: most airliners that are converted away from pax use, be they freighters, tankers, or something else, do not have pressure bulkheads aft of the cockpit. A flat bulkhead like that is extremely heavy and again, pressurization stiffens the fuselage. Only a few very special variants (Guppy, DreamLifter) have pressure bulkheads aft of the cockpit and unpressurized cargo areas.

The Dreamlifter video in viewtopic.php?f=5&t=1434647 shows a video of the installation of this one-off case of using a pressure bulkhead immediately behind the cockpit to create the large unpressurized cargo area.
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