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.

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?

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.

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.britann...

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.

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.

> 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.

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.

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.

Is it possible to make a heavier than air aircraft with terrible enough surface area to weight and power ratios that it will make a blimp seem easy to control in bad weather? Sure, I guess. I wasn't saying you couldn't if you read my comment - I was saying you can make a heavier than air aircraft with a LOWER surface area to weight/power, unlike lighter than air aircraft, so you can avoid being knocked around as much, have less drag, etc.

Heavier than air craft are far more versatile in general.

You can never get a lighter than air craft to an overall density higher than air by definition, and that is hugely limiting.

A 50 foot wide balloon (r=25ft), would have a surface area of 7853 square feet (729 m^2) if an ideal sphere. If you add up all of the wing and control surfaces on a 757 [https://www.b757.info/boeing-757-200-specifications/], you get 3992 square feet. Add another several thousand for the fuselage, and you're probably in the same ballpark.

The balloon you're talking about has a volume (assuming a perfect sphere, r=25ft) of 65,449 cubic feet (1853 m^3). Per [https://www.airships.net/helium-hydrogen-airships/], that seems to pencil out at around 4000 lbs of lift for helium, and 4500 lbs of lift for hydrogen (in 'real world' situations), add 20% to be closer to ideal. Or 2.5m^3 of gas per square meter of surface area. But the literal maximum amount of lifting force you can get is 1.01kg/m^3 with helium and 1.2kg/m^3 with hydrogen.

That really isn't much lift for something that big. You could scale it up, but then you're talking more surface area no? a LOT more surface area? We'll figure that out later.

Said 757 weight will vary from 130,000 lbs-255,000 lbs (empty to max takeoff weight), or 59k-72k lbs of payload if configured as a freighter. Each engine produces 36,000-43,000 lbs of thrust depending on model.

So for the 757, it is lifting (payload alone, on top of it's own weight, fuel, etc.) 3.4kg/m^2, and empty, is lifting 7.5kg/m^2. If you look at max takeoff weight, it's hitting 32kg/m^2. Way more if you care about just the airfoils of course. And to hit that takeoff, it is likely going over 200+km/h.

For a balloon to lift the same weight as the 757 at max takeoff weight, you need one with a volume of at least 114520 m^3 (for helium, ideal) or 96388 m^3 (for hydrogen, ideal), which is a minimum of r=30m for helium and r=28m for hydrogen (ideal). That is a sphere approximately 95-100ft in diameter.

That comes out to a surface area of 11309m^2 for helium and 9852m^2 for hydrogen (assuming perfect spheres, which don't happen).

That is 6.1x the surface area for helium, and 5.3x for hydrogen, assuming everything is perfect - and there is zero way you could drag that through the atmosphere or control it in any way like you can a 757 (or a Cessna, even), even if you used the same engines.

And even if you use a balloon big enough to literally lift a 757 at max takeoff weight, you're weight to surface area ratio is just hitting 10kg/m^2. 1/3 of the 757, and that means you have 3x more 'surface' to drag through the air for the same available weight (aka power/airframe) budget.

so you need to be talking multiple max-takeoff-weight-of-a-757 worth of ballon lifting capacity before you start getting in the same ballpark from a raw 'surface exposed vs weight' perspective. If we use weight as a raw proxy for power (roughly probably correct), you get the same setup.

And from a air resistance/drag perspective (what we care about here), it still isn't even all that close due to airfoil shape vs giant spheres. If you're using a blimp/zepplin shape, you're trading off airframe weight for aerodynamics, but it doesn't help as much - you end up having to spend a lot of your weight budget structuring it more like an airfoil, because the density still has to be low, so the shape has to be much bigger, and you have less budget for engines - so the heavier than air craft actually have an even bigger advantage. But even doing these very basic comparisons show it pretty clearly enough.

If you need to move through air faster than the air itself is moving, density helps - by reducing the surface area (and hence impact of these winds) and allowing you to have more engines, or a fancier airframe, or whatever. If you need to resist weather and similar forms of strong, high speed wind currents and changes, you need to be able to move through the air fast, and preferably have a strong frame.

Lighter than air craft are hindered in this by having a cap on their density, and for buildable/practical sized craft, high surface areas to weight ratios (which is a proxy for strength of airframes and available power).

> I was saying you can make a heavier than air aircraft with a LOWER surface area to weight/power, unlike lighter than air aircraft, so you can avoid being knocked around as much, have less drag, etc.

You didn't say that you "can" make a heavier than air craft with a better ratio. You said that "by definition" lighter than air craft will have a worse ratio than any heavier than air craft. That's a very different statement! (And I'm being fair, I'm interpreting "any" as "any reasonable".)

> You can never get a lighter than air craft to an overall density higher than air by definition, and that is hugely limiting.

That's true, but the statement I objected to was that weight:surface-area is worse by definition, not any statement about volume.

> 757 stuff

The problem with that chain of logic is that you're starting with some of the best planes around for surface area vs. weight, and then trying to make a blimp that beats them.

Of course that's super hard to do!

But if you take a slow ultralight plane instead, you'll see that it's not very hard to beat with a blimp. The kind of plane that cruises at 35mph and not 500mph.

The truth isn't that [reasonable] planes automatically beat [reasonable] blimps. It's that planes similar to a 757 beat reasonable blimps. That's a much weaker statement.

There are lots of reasonable plane designs that might only hit 5kg/m^2, and it's easy to make a blimp that beats that. Or the 10kg/m^2 in your math, that's not something that takes unreasonable materials to reach in a blimp.

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tl;dr: If you demand a blimp beat 30kg/m^2, it probably won't happen. But in the 2-10kg/m^2 range, sometimes planes beat blimps and sometimes blimps beat planes, using reasonable designs for both. "[an airship] 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." is a false statement.

A factor I forgot to mention - heavier than air craft, because of their better surface area to weight ratio (ability to be more dense) can pack in more engines for a given amount of surface area - and go faster, and produce more lift per unit of surface area, due to the reduced drag.

So heavier than air craft do NOT have a fixed surface area to weight ratio, they have a surface area/airfoil to power ratio, which can vary widely depending on the effectiveness of the engine.