Help! Need a Bit of Technical Input - DC-3 Landing Gear vs. A380
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Smart Arse Assumptions
CC, if your comment was aimed at me you may like to have another look at the heading of my post. I asked if it was a Wind up, no more or less.
DC-3/C-47 L/G
Quote from MarkerInbound (my emphasis):
"The gear is held down in the down position by trapped hydraulic pressure and a mechanical safety latch which holds the gear linkage to the aft end of the wheel well when the gear is extended and the safety latch is engaged. The safety latch is controlled by a short lever on the floor just inboard of the Captain's seat. So retracting and extending the landing gear is a multi-step process. To retract the gear you first rotate a clip at the end of the safety latch lever 90 degrees forward so it is no longer holding the safety latch lever to the floor. You then raise the safety latch lever to a 45 degree position from the floor. The safety latch lever also controls a dog and shoe on the landing gear selector valve so now you can raise the landing gear lever to retract the landing gear. When climb power is set you move the landing gear lever to the "OFF" position. When the landing gear lever is moved to OFF the safety latch lever drops to a ~20 degree position off the floor."
That's a very neat description, if I may say so! Rather better than I was given in my type conversion. For young aviators: very important to raise the safety latch fully, as it's fairly stiff and quite a long reach from the right-hand seat, positioned as it is on the floor under the skipper's right elbow. The landing-gear lever (behind and below the co-pilot's left elbow, as MI writes) consists of an aluminium tube over a foot long and about an inch in diameter, usually fitted with a rubber hand-grip on the free end. My second P2 flight of a C-47 Dakota on the line was a heavily-laden night take-off (0430 hrs). The combination of my failing to raise the safety latch enough, followed by a degree of desperation on my part, and a L/G lever that had a circumferential fatigue crack near its pivot point led to a very interesting result...
"The gear is held down in the down position by trapped hydraulic pressure and a mechanical safety latch which holds the gear linkage to the aft end of the wheel well when the gear is extended and the safety latch is engaged. The safety latch is controlled by a short lever on the floor just inboard of the Captain's seat. So retracting and extending the landing gear is a multi-step process. To retract the gear you first rotate a clip at the end of the safety latch lever 90 degrees forward so it is no longer holding the safety latch lever to the floor. You then raise the safety latch lever to a 45 degree position from the floor. The safety latch lever also controls a dog and shoe on the landing gear selector valve so now you can raise the landing gear lever to retract the landing gear. When climb power is set you move the landing gear lever to the "OFF" position. When the landing gear lever is moved to OFF the safety latch lever drops to a ~20 degree position off the floor."
That's a very neat description, if I may say so! Rather better than I was given in my type conversion. For young aviators: very important to raise the safety latch fully, as it's fairly stiff and quite a long reach from the right-hand seat, positioned as it is on the floor under the skipper's right elbow. The landing-gear lever (behind and below the co-pilot's left elbow, as MI writes) consists of an aluminium tube over a foot long and about an inch in diameter, usually fitted with a rubber hand-grip on the free end. My second P2 flight of a C-47 Dakota on the line was a heavily-laden night take-off (0430 hrs). The combination of my failing to raise the safety latch enough, followed by a degree of desperation on my part, and a L/G lever that had a circumferential fatigue crack near its pivot point led to a very interesting result...
A short question on the side, if I may.
When a fresh tire is taken from the store and installed on a rim, it is obviously initially filled with ambient air at atmospheric pressure. How is the air flushed out and replaced with pure nitrogen? Is it inflated, deflated and again inflated until mathematically only minuscule traces of oxygen are left; will simply pressurizing it to the required value with pure nitrogen do, or are there any other methods used?
When a fresh tire is taken from the store and installed on a rim, it is obviously initially filled with ambient air at atmospheric pressure. How is the air flushed out and replaced with pure nitrogen? Is it inflated, deflated and again inflated until mathematically only minuscule traces of oxygen are left; will simply pressurizing it to the required value with pure nitrogen do, or are there any other methods used?
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The main reason for the Nitrogen is that Nitrogen molecules are bigger then all the molecules that make up regular air. (Yes including O2 and H2O)
Fill a tire with Nitrogen and it will stay pressurized longer then when you fill it with regular air.
This is especially handy in the low pressure situations that aircraft are in when flying. You will still have pressure in you tires before landing.
Fill a tire with Nitrogen and it will stay pressurized longer then when you fill it with regular air.
This is especially handy in the low pressure situations that aircraft are in when flying. You will still have pressure in you tires before landing.
Hi Tu.114,
"When a fresh tire is taken from the store and installed on a rim, it is obviously initially filled with ambient air at atmospheric pressure. How is the air flushed out and replaced with pure nitrogen? Is it inflated, deflated and again inflated until mathematically only minuscule traces of oxygen are left; will simply pressurizing it to the required value with pure nitrogen do, or are there any other methods used?"
Now why didn't I ever think of that? In expectation of being corrected by someone who does it, however, I think your second idea might suffice, because of the high tyre pressures used in the kind of a/c we are talking about. To take a fairly modest example, 180 psi, that's roughly 12 times ambient if the inflation is being done at sea-level. After inflation, once the temperature has returned to ambient and the desired 180 psi (differential above ambient) is achieved, I reckon the mass of nitrogen inside would be about 13 times the 0 psi mass. As ambient air is only about 20% oxygen, that would suggest that the oxygen content would now only represent about 1.5% of the gas content of the tyre.
In the case of a car tyre at, say, 30 psi, I reckon the mass of nitrogen would be only about 3 times the 0 psi figure. But the higher percentage of oxygen remaining is probably not a problem in that application (see below).
Quote from D-OCHO:
"The main reason for the Nitrogen is that Nitrogen molecules are bigger then all the molecules that make up regular air. (Yes including O2 and H2O)
Fill a tire with Nitrogen and it will stay pressurized longer then when you fill it with regular air."
I'm sure you're right about the lower leak rate, but my understanding is that the main reason is to prevent the kind of oxygen-fuelled tyre explosion that brought down a Swissair Caravelle shortly after take-off from Geneva in about 1966. IIRC, before take-off in fog the a/c had taxied at a high power setting along the length of the runway, using continuous braking, in an attempt to raise the RVR to the minimum requirement for take-off. But there are many other causes of overheating tyres.
"When a fresh tire is taken from the store and installed on a rim, it is obviously initially filled with ambient air at atmospheric pressure. How is the air flushed out and replaced with pure nitrogen? Is it inflated, deflated and again inflated until mathematically only minuscule traces of oxygen are left; will simply pressurizing it to the required value with pure nitrogen do, or are there any other methods used?"
Now why didn't I ever think of that? In expectation of being corrected by someone who does it, however, I think your second idea might suffice, because of the high tyre pressures used in the kind of a/c we are talking about. To take a fairly modest example, 180 psi, that's roughly 12 times ambient if the inflation is being done at sea-level. After inflation, once the temperature has returned to ambient and the desired 180 psi (differential above ambient) is achieved, I reckon the mass of nitrogen inside would be about 13 times the 0 psi mass. As ambient air is only about 20% oxygen, that would suggest that the oxygen content would now only represent about 1.5% of the gas content of the tyre.
In the case of a car tyre at, say, 30 psi, I reckon the mass of nitrogen would be only about 3 times the 0 psi figure. But the higher percentage of oxygen remaining is probably not a problem in that application (see below).
Quote from D-OCHO:
"The main reason for the Nitrogen is that Nitrogen molecules are bigger then all the molecules that make up regular air. (Yes including O2 and H2O)
Fill a tire with Nitrogen and it will stay pressurized longer then when you fill it with regular air."
I'm sure you're right about the lower leak rate, but my understanding is that the main reason is to prevent the kind of oxygen-fuelled tyre explosion that brought down a Swissair Caravelle shortly after take-off from Geneva in about 1966. IIRC, before take-off in fog the a/c had taxied at a high power setting along the length of the runway, using continuous braking, in an attempt to raise the RVR to the minimum requirement for take-off. But there are many other causes of overheating tyres.
The main reason for the Nitrogen is that Nitrogen molecules are bigger then all the molecules that make up regular air. (Yes including O2 and H2O)
Fill a tire with Nitrogen and it will stay pressurized longer then when you fill it with regular air.
This is especially handy in the low pressure situations that aircraft are in when flying. You will still have pressure in you tires before landing.
Fill a tire with Nitrogen and it will stay pressurized longer then when you fill it with regular air.
This is especially handy in the low pressure situations that aircraft are in when flying. You will still have pressure in you tires before landing.
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Tu.114 & Chris: I believe you are right, new aircraft tires are simply inflated using nitrogen initially.
tdracer: You're conflating molecular weight and molecular size (kinetic diameter). N2 weighs less than O2, but N2 is (very slightly) larger in size than O2.
However, I agree that the effect on leak loss is likely negligible for all intents and purposes.
For racing, using nitrogen can indeed reduce pressure fluctuations.
If you fill tires with regular air then you're subject to whatever humidity level is in the ambient air at the time. On humid days this can result in a lot of water vapor in the tires.
At race speeds, the tire's internal temperature can rise to more than 100 C, the boiling point of water. This causes the water vapors to expand rapidly and the tire pressure can increase significantly. Hence the use of dry nitrogen instead of regular air.
tdracer: You're conflating molecular weight and molecular size (kinetic diameter). N2 weighs less than O2, but N2 is (very slightly) larger in size than O2.
However, I agree that the effect on leak loss is likely negligible for all intents and purposes.
For racing, using nitrogen can indeed reduce pressure fluctuations.
If you fill tires with regular air then you're subject to whatever humidity level is in the ambient air at the time. On humid days this can result in a lot of water vapor in the tires.
At race speeds, the tire's internal temperature can rise to more than 100 C, the boiling point of water. This causes the water vapors to expand rapidly and the tire pressure can increase significantly. Hence the use of dry nitrogen instead of regular air.
Not sure why water vapor would expand greatly at 100 Celsius, but any liquid water that had condensed in the tire surely would vaporize and expand hundreds of times. Also, water tends to condense in air compressors and get into air lines, so the fill air could have a lot more water than the ambient air. (It could also have less.)
Another advantage of nitrogen is that it reduces oxidation. Not just the explosive kind, but the gradual kind that causes rubber to deteriorate over time. How big a factor that is in overall tire life, I don't know.
Another advantage of nitrogen is that it reduces oxidation. Not just the explosive kind, but the gradual kind that causes rubber to deteriorate over time. How big a factor that is in overall tire life, I don't know.
but any liquid water that had condensed in the tire surely would vaporize and expand hundreds of times
Oh, and unlike aircraft tires which are inflated to around 200 psi, we typically ran around one bar pressure - in which case whatever was inside the tire prior to beading was a significant percentage of the total. Peekay, I talked to a chemist buddy, and at least according to him, unless the Oxygen was "ionized" (missing electrons), the O2 molecule would 'act' larger than the N2 molecule when it came to passing through other materials. N2 and O2 are basically symmetrical molecules so their molecular weight does closely relate to their size (H2O, being highly asymmetric, does act much differently than what it's molecular weight would suggest).
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And even when you throw H20 into the mix, it only comes into play if there is enough moisture that it changes state
Per Dalton's law, the total pressure for moist air is the sum of (dry air partial pressure) + (water vapor partial pressure)
The individual partial pressures can be calculated from the gas law in molar form:
pV = (m/M) R T
where m is the mass and M is the molecular weight and R is the ideal gas constant.
In other words, the rise or fall of the partial pressure relates to the inverse of the gas molecular weight. Or, equivalently, proportional to R/M, which is the specific gas constant.
Water vapor has a molecular weight of 18.02 g/mol. Dry air = 28.96 g/mol.
Water vapor's specific gas constant is 461.5 J/(Kg K), dry air = 287 J/(Kg K).
So we can see, given the same rise or fall in temperature, the water vapor partial pressure will change (461.5/287) = 1.6 times faster than the partial pressure of air.
Typical "dry air" sources available track side still has a lot of moisture in it.
Oh, and unlike aircraft tires which are inflated to around 200 psi, we typically ran around one bar pressure - in which case whatever was inside the tire prior to beading was a significant percentage of the total.
Typical "dry air" sources available track side still has a lot of moisture in it.
For racing applications, the air in the tires are typically removed (towards but above vacuum) before nitrogen is added, to achieve better than 95% Nitrogen, same as in aviation.
For racing applications, the air in the tires are typically removed (towards but above vacuum) before nitrogen is added, to achieve better than 95% Nitrogen, same as in aviation.
Granted, my racing was amateur, not professional, but I never, ever saw "air removed" from tires after beading prior to inflation (and I'm talking hundreds, if not thousands, of tire mountings). I did however, on a regular basis, see soapy water used as a lube to seat the bead. Some of us used a water-free compound to lube the bead to allow it to seat (the stuff I used was called "Tire Snot") but we were not in the majority (although we did tend to run up front). Most of the time, the pressure required to set the bead was higher than the running pressure so they simply bleed the tire down to something close to the desired running pressure. On occasion, the tire valve was removed and all pressure allowed to escape, then the valve installed and the tire inflated to the desired pressure.
Not only did I never see a vacuum used to evacuate the tire, I never even saw the needed equipment to do it. And again, my point has always been there is no meaningful difference between dry air, and nitrogen when it comes to how tire pressure behaves. Racers use nitrogen because it's cheap and readily available, not because it's better than dry air.
Peekay,
I have to say that I don't get it. pV = (m/M) R T gives you the pressure at a given temperature, not the change in pressure. If you had two identical flasks, and filled one with water vapor and one with dry air, the pressure in the first would be higher. No surprise there -- water is less dense, and therefore a given mass has to be compressed more to fit a given volume.
But (m/M) in your equation doesn't change with temperature. R, of course, is also constant, and the volume of a tire (V) we can also assume is constant. So if you double the absolute temperature (T) in your equation, pressure (p) has to double as well -- none of the other numbers change. And that's true whatever the value of (m/M) is.
It is true that neither water vapor nor air is an ideal gas, and they probably do expand at somewhat different rates. But it isn't as simple as a ratio of molecular weights, and I'm virtually positive the difference is much less profound.
I have to say that I don't get it. pV = (m/M) R T gives you the pressure at a given temperature, not the change in pressure. If you had two identical flasks, and filled one with water vapor and one with dry air, the pressure in the first would be higher. No surprise there -- water is less dense, and therefore a given mass has to be compressed more to fit a given volume.
But (m/M) in your equation doesn't change with temperature. R, of course, is also constant, and the volume of a tire (V) we can also assume is constant. So if you double the absolute temperature (T) in your equation, pressure (p) has to double as well -- none of the other numbers change. And that's true whatever the value of (m/M) is.
It is true that neither water vapor nor air is an ideal gas, and they probably do expand at somewhat different rates. But it isn't as simple as a ratio of molecular weights, and I'm virtually positive the difference is much less profound.
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The ratio in pressure between the two flasks will be the same. But the absolute amount of rise or fall in pressure within each flask will not be the same.
It's basically a simple linear equation, y = mx + b. If you have two lines at two different slopes (assuming same intercept), then at any given x the ratio of y between the two lines will remain the same, but obviously one would rise/fall faster than the other. Here (m/M) is the slope aka derivative or rate of change.
But in general I would agree with you guys that if there's liquid water -- due to condensation or sloppiness -- then that would be the dominant factor, barring extreme temperature & humidity conditions.
It's basically a simple linear equation, y = mx + b. If you have two lines at two different slopes (assuming same intercept), then at any given x the ratio of y between the two lines will remain the same, but obviously one would rise/fall faster than the other. Here (m/M) is the slope aka derivative or rate of change.
But in general I would agree with you guys that if there's liquid water -- due to condensation or sloppiness -- then that would be the dominant factor, barring extreme temperature & humidity conditions.
ChrisScott, thank You, that makes sense. Filling the tire from its air-filled initial state and thereby diluting the 1/5 of oxygen with massive amounts of nitrogen will reduce its amount below what is required to sustain a combustion.
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Hey! I studied this thread, made some notes, wrote my piece, it was published, and what do you know - I ran into an Emirates A380 Captain who had read it and complimented me for "nailing the A380 landing gear."
I stood there smiling and thinking.....wait til the Ppruners hear about this! Thank-you! Thank-you! Thank-you!
p.s. I'm am not paid for these literary jaunts, but if I were, I'd buy you all a beer.
I stood there smiling and thinking.....wait til the Ppruners hear about this! Thank-you! Thank-you! Thank-you!
p.s. I'm am not paid for these literary jaunts, but if I were, I'd buy you all a beer.
