5 ms·

Is that true even if it has a large mass from cargo?

lazide 5y ago
Yes. It always has to have a much larger surface area in proportion to the weight it is carrying than any heavier than air craft, by definition. That means it has to have more volume to lift more weight. More volume means more surface area (though not linearly!). More surface area means more impact from strong winds, updrafts, downdrafts. And we haven’t figured out any plausible sort of propulsion that can even momentarily provide enough force to counteract something like a strong sudden downdraft without being too bulky to be practical.
- hansvm 5y ago
  I think they still have a point though: Let X be any linear measurement of the airship (like length). The forces on the airship are proportional to X^2, and the max mass is proportional to X^3. Consequently, acceleration from wind and whatnot tends to 0 as the airship size increases (when fully loaded). Moreover, the necessary propulsion similarly scales with X^2 (which is convenient, because that's the amount of space you have to place the propulsion), while requiring increasingly negligible fractions of the ship's carrying capacity as the ship size increases.
  - lazide 5y ago
    Nice! Any chance you’d be interested in doing the math to figure out power to surface area ratios and what it would have to be to have the maneuvering capabilities of say a Cessna 172 in something like a blimp?
    - hansvm 5y ago
      If I'm not mistaken that's impossible as the size of the blimp increases (similar idea -- X^2 max power, X^3 mass, consequently acceleration and maneuverability are poor). The point was more that heavy winds aren't an issue for a sufficiently large blimp, even without maneuverability, because the impact of the storm on a blimp is negligible.
      - FabHK 5y ago
        I'd think the forces would still be formidable. But yes, assuming the airship withstands the forces, it would not be thrown around as much (specifically: the accelerations and displacements would be lower) as it gets bigger. That seems plausible to me, by your x^2/x^3 argument.
      - lazide 5y ago
        That doesn't make sense though - if the entire air mass is moving, and there is insufficient propulsion to go faster than the airmass is moving - then that airmass will carry the blimp into whatever that airmass hits? There is too much surface area for much else to happen right? Microburst wind speeds can hit 270km/h or more. Updrafts can hit 10,000 ft/minute or more [https://en.wikipedia.org/wiki/Downburst][https://www.britannica.com/science/updraft] https://en.wikipedia.org/wiki/Downburst][https://www.britann...
        hansvm 5y ago
        A few points: - The O(1/X) acceleration property prevents a 300km/h wind from getting the blimp to speed quickly. The "entire air mass moving" doesn't change that; you'll see wind flowing around the blimp, wind becoming turbulent and reversing directions, wind losing velocity and converting to heat and sound, local portions of the blimp temporarily deforming, potential blimp damage, and all kinds of other effects from a microburst, but you won't see a high mass-to-surface-area-ratio object have its center of mass accelerate quickly from wind drag. - The blimp _would_ need to have sufficient propulsion to counteract average wind forces over some time period. If you had a sustained downdraft with squared velocity averaging 270^2km/h over the surface of the blimp for any substantial length of time then the blimp would need equivalent upward propulsion to avoid _eventually_ crashing into the ground. For a sufficiently large blimp though, "eventually" can be extended as far as you'd like by reducing the acceleration induced by such forces and allowing you to average external forces over a longer time period before experiencing any negative repercussions.
- Dylan16807 5y ago
  > Yes. It always has to have a much larger surface area in proportion to the weight it is carrying than any heavier than air craft, by definition. What do you mean 'by definition'? A heavier than air craft lifts with its surface. It has a relatively fixed surface area to weight ratio, no matter how big it gets. A lighter than air craft lifts with its volume. The bigger you make it, the less surface area you have per weight.
  - lazide 5y ago
    Heavier than air craft can be denser than air, by definition, and therefore have a lower ratio of surface area to weight - and hence less drag. This allows them to go fast (potentially), and power through problematic air turbulence with minimal impact. Lighter than air craft must (by definition), have an overall density less than the surrounding gas. This means their surface area and volume for any notable weight must always be quite large, and the corresponding influence from the surrounding air mass is always much, much higher, and their ability to fight any change in direction is always much less. Imagine what it would take to get a blimp to go the cruising speed of an airliner, and it might make more intuitive sense. And even those airliners avoid storms when they can. It might not be impossible - but it would require a degree of engineering not even considered here.
    - Dylan16807 5y ago
      The mistake you're making is to assume the two types of craft have similar-enough shapes that you can mix up surface area and volume. But even though a heavier than air craft has a lot less volume, it doesn't have to have less surface area. Planes are pretty flat, and have a much higher ratio of surface area to volume. A small delicate plane can weigh less than 50 grams per square meter. Or we could look at planes designed for human-powered flight. Those are ruthlessly optimized so you know they have no more surface area than necessary, and they weigh well under a kilogram per square meter, even if you added a motor on top. The balloons google was using to lift mini cell towers, at 50 feet wide, had about 2 cubic meters of helium per square meter of surface area. So about two kilograms of payload per square meter. And if you made it bigger you could turn that into five or ten kilograms per square meter without even trying. Is it extremely hard for a blimp to beat an airliner, which even for a plane has a small surface area? Yes. But lots and lots of other plane designs lose to a big blimp. Some of them even lose to a small blimp. Especially slow planes. And this is only talking about reasonable plane designs.
dredmorbius 5y ago
Cargo mass adds to structural stresses, and subtracts from mass available for increased strength and resilience.
romwell 5y ago
>Is that true even if it has a large mass from cargo? Nope. It's just plain false. To wit: During its career, Graf Zeppelin had flown almost 1.7 million km (1,053,391 miles), the first aircraft to fly over a million miles. It made 144 oceanic crossings (143 across the Atlantic, and one of the Pacific), carried 13,110 passengers and 106,700 kg (235,300 lb) of mail and freight. It flew for 17,177 hours (717 days, or nearly two years), without injuring a passenger or crewman. It never crashed, and was retired at the dawn of WWII. The huge airships get a bad rep because the only country which could successfully build and operate them was Nazi Germany. As for dirigibles operated by the US, the UK, and the USSR... yup, none of them actually worked. Doesn't say anything about the vehicle type.

Yes. It always has to have a much larger surface area in proportion to the weight it is carrying than any heavier than air craft, by definition. That means it has to have more volume to lift more weight. More volume means more surface area (though not linearly!). More surface area means more impact from strong winds, updrafts, downdrafts. And we haven’t figured out any plausible sort of propulsion that can even momentarily provide enough force to counteract something like a strong sudden downdraft without being too bulky to be practical.

hansvm 5y ago
I think they still have a point though: Let X be any linear measurement of the airship (like length). The forces on the airship are proportional to X^2, and the max mass is proportional to X^3. Consequently, acceleration from wind and whatnot tends to 0 as the airship size increases (when fully loaded). Moreover, the necessary propulsion similarly scales with X^2 (which is convenient, because that's the amount of space you have to place the propulsion), while requiring increasingly negligible fractions of the ship's carrying capacity as the ship size increases.
- lazide 5y ago
  Nice! Any chance you’d be interested in doing the math to figure out power to surface area ratios and what it would have to be to have the maneuvering capabilities of say a Cessna 172 in something like a blimp?
  - hansvm 5y ago
    If I'm not mistaken that's impossible as the size of the blimp increases (similar idea -- X^2 max power, X^3 mass, consequently acceleration and maneuverability are poor). The point was more that heavy winds aren't an issue for a sufficiently large blimp, even without maneuverability, because the impact of the storm on a blimp is negligible.
    - FabHK 5y ago
      I'd think the forces would still be formidable. But yes, assuming the airship withstands the forces, it would not be thrown around as much (specifically: the accelerations and displacements would be lower) as it gets bigger. That seems plausible to me, by your x^2/x^3 argument.
    - lazide 5y ago
      That doesn't make sense though - if the entire air mass is moving, and there is insufficient propulsion to go faster than the airmass is moving - then that airmass will carry the blimp into whatever that airmass hits? There is too much surface area for much else to happen right? Microburst wind speeds can hit 270km/h or more. Updrafts can hit 10,000 ft/minute or more [https://en.wikipedia.org/wiki/Downburst][https://www.britannica.com/science/updraft] https://en.wikipedia.org/wiki/Downburst][https://www.britann...
      - hansvm 5y ago
        A few points: - The O(1/X) acceleration property prevents a 300km/h wind from getting the blimp to speed quickly. The "entire air mass moving" doesn't change that; you'll see wind flowing around the blimp, wind becoming turbulent and reversing directions, wind losing velocity and converting to heat and sound, local portions of the blimp temporarily deforming, potential blimp damage, and all kinds of other effects from a microburst, but you won't see a high mass-to-surface-area-ratio object have its center of mass accelerate quickly from wind drag. - The blimp _would_ need to have sufficient propulsion to counteract average wind forces over some time period. If you had a sustained downdraft with squared velocity averaging 270^2km/h over the surface of the blimp for any substantial length of time then the blimp would need equivalent upward propulsion to avoid _eventually_ crashing into the ground. For a sufficiently large blimp though, "eventually" can be extended as far as you'd like by reducing the acceleration induced by such forces and allowing you to average external forces over a longer time period before experiencing any negative repercussions.
Dylan16807 5y ago
> Yes. It always has to have a much larger surface area in proportion to the weight it is carrying than any heavier than air craft, by definition. What do you mean 'by definition'? A heavier than air craft lifts with its surface. It has a relatively fixed surface area to weight ratio, no matter how big it gets. A lighter than air craft lifts with its volume. The bigger you make it, the less surface area you have per weight.
- lazide 5y ago
  Heavier than air craft can be denser than air, by definition, and therefore have a lower ratio of surface area to weight - and hence less drag. This allows them to go fast (potentially), and power through problematic air turbulence with minimal impact. Lighter than air craft must (by definition), have an overall density less than the surrounding gas. This means their surface area and volume for any notable weight must always be quite large, and the corresponding influence from the surrounding air mass is always much, much higher, and their ability to fight any change in direction is always much less. Imagine what it would take to get a blimp to go the cruising speed of an airliner, and it might make more intuitive sense. And even those airliners avoid storms when they can. It might not be impossible - but it would require a degree of engineering not even considered here.
  - Dylan16807 5y ago
    The mistake you're making is to assume the two types of craft have similar-enough shapes that you can mix up surface area and volume. But even though a heavier than air craft has a lot less volume, it doesn't have to have less surface area. Planes are pretty flat, and have a much higher ratio of surface area to volume. A small delicate plane can weigh less than 50 grams per square meter. Or we could look at planes designed for human-powered flight. Those are ruthlessly optimized so you know they have no more surface area than necessary, and they weigh well under a kilogram per square meter, even if you added a motor on top. The balloons google was using to lift mini cell towers, at 50 feet wide, had about 2 cubic meters of helium per square meter of surface area. So about two kilograms of payload per square meter. And if you made it bigger you could turn that into five or ten kilograms per square meter without even trying. Is it extremely hard for a blimp to beat an airliner, which even for a plane has a small surface area? Yes. But lots and lots of other plane designs lose to a big blimp. Some of them even lose to a small blimp. Especially slow planes. And this is only talking about reasonable plane designs.

dredmorbius 5y ago

Cargo mass adds to structural stresses, and subtracts from mass available for increased strength and resilience.

romwell 5y ago

>Is that true even if it has a large mass from cargo? Nope. It's just plain false. To wit: During its career, Graf Zeppelin had flown almost 1.7 million km (1,053,391 miles), the first aircraft to fly over a million miles. It made 144 oceanic crossings (143 across the Atlantic, and one of the Pacific), carried 13,110 passengers and 106,700 kg (235,300 lb) of mail and freight. It flew for 17,177 hours (717 days, or nearly two years), without injuring a passenger or crewman. It never crashed, and was retired at the dawn of WWII. The huge airships get a bad rep because the only country which could successfully build and operate them was Nazi Germany. As for dirigibles operated by the US, the UK, and the USSR... yup, none of them actually worked. Doesn't say anything about the vehicle type.