Jon
11-23-2009, 11:39 AM
Airship Design Considerations
Summary
This Post sets out a theoretical semi-rigid thermal Airship design. I suggest improvements to current performance limitations using known technology to reduce development risks and costs.
Summary 1
1) The Impractical Dream 1
2) Existing Airships 2
3) Modest Design Goals 3
4) Design Suggestions - How do we achieve these modest aims? 4
5) The Market 7
6) Challenges 8
1) The Impractical Dream
Purple Prose
“Imagine a creature, vaster than any aeroplane, crawling into stately view over the horizon, small at first and hard to see, its great bulk swims inexorably towards you, periodically illuminated by internal pillars of flame, making fleetingly visible the most splendid Chinese lantern anywhere on earth. Closer it comes, its true size manifest at last, it dominates the sky, the low rumble of its engines pushing it fast against the wind. And there, at the base of the envelope, passengers seeing you waving, a small dot of enthusiasm beneath them, break off from their conversations, turn and wave back. Only the pilot, his eyes flicking from his instruments to the far horizon, does not. He has the future of aviation in his hands. Then it is gone, plunging onward to its destination, hundreds of miles away. Someday you think, someday, I will live this dream. “
The Dream
To build airships.
The dream is alive in popular imagination – films are littered with them, as are books, and the genres of steam punk and role playing games. In reality however it is quite dead. The gulf between modelling and the great beasts of the skies is unreachably great. The rise of the machines – aeroplanes, has seen to that. Dreamers, having no money, and those with money having no plausible prospect of a market, have no place to see their hopes realised. How then, shall a dreamer focus his ambition on a more practical prospect? He must lower his gaze from the mighty whales of heavy lift freight, commercial tourism and governmental largess to the minnows of lighter than air sport ballooning.
2) Existing Airships
There are two main types still flying: The Helium types that exist in limited numbers serving niche markets, and excite any number of speculative news items, and the thermal airships produced primarily by Geta-flug and Cameron Balloons. The latter are low-to-nil wind capable, portable leisure hot air balloons with engines.
Current State
Helium and Hydrogen have proven impractical for the sporting amateur. The costs and technical genius are beyond his everyday wallet and humdrum mind. Helium airships are beyond anyone except major corporations and billionaires, require extensive infrastructure and crew. There is no prospect of a billionaire turning his attention towards the construction of a new Zeppelin.
Hot air Ballooning has settled into a steady state backwater of special shapes and dawn and dust tourist experiences. It is a grand spectacle, but it does not develop.
Thermal airships purchase costs are in the low 100,000's, but they are toys.
Current thermal airships
Once there was a stronger passion to produce thermal airships.
Colt and Geta-flug and Cameron have between them a small range of strikingly similar airships with the performance characteristics of snails. With a top speed of 20 mph and flight durations of between 1 and 3 hours, these are effectively still hot air balloons on anything but a still wind day. Why then have they not progressed technically?
Primarily because the concept is from a hot-air ballooning background of non-rigidity and ground portability. They can be packed into a trailer and transported, and in fact due to the performance window, have to be. The lack of rigidity in the envelope makes it impossible to be aerodynamic, the gigantic airbrake cannot be overcome with internal pressurisation. Given that typical winds in the UK range from 10 to 25 mph, an airship needs a top speed of 50 mph to be functionally viable - i.e. to travel unaided from one airstrip to another.
3) Modest Design Goals
What we want to achieve is set out below, our guiding principles are useful performance and cost minimisation.
3.1) Costs per flight as a low as possible
Helium is therefore out of the question and we are restricted to hot air.
3.2) Lightweight and cheap construction costs
An inflatable envelope is by far the cheapest to develop. It has its own drawbacks, particularly relating to performance.
3.3) Speed of approx 50 mph
This is a somewhat arbitrary goal, but we must have a speed sufficient to cope with normal wind conditions in a head wind and still make reasonable progress. Achieving this while retaining a non rigid structure is difficult and is where innovative design is key to success.
3.4) Range 3 hours
We must have the capacity to reach airstrips at reasonable distances to refuel.
3.5) Lift – 1 or 2 people at most
Initial development must be for as small a vehicle as possible because development will inevitable involve many costly and time consuming iterations and adjustments, notwithstanding the performance benefits from increased lifting volume, these cannot be a first step.
3.6) Self-portable avoiding the need for road transportation
We want to avoid additional support vehicles for daily operation; a self contained unit with the speed, range and survivability to move itself is preferable by far. We wish to do away with the concept of a portable hot-air airship.
3.7) Avoid requirement for a hanger in daily operation
Development and repair will require a hanger, there is no avoiding this, but reducing the costs of a hanger at each overnight stop is crucial. This means we must have the ability to survive moderately adverse weather conditions, which in turn means significantly reducing the profile of the envelope, by collapsing it and securing it into wind, with or without a ground anchor. Large rigids were notoriously prone to wind damage.
4) Design Suggestions - How do we achieve these modest aims?
The primary limitation on operations is caused by the shape of the envelope and its rigidity or otherwise in the face of external pressures.
Performance losses may happen in a number of ways:
4.1) By deformation of the envelope shape as it encounters resistance – typically seen in the nose cone flattening.
4.2) In movement throughout the envelope – flapping and general fabric movement causes severe additional air resistance
4.3) Temperature transfer from the nose, due to the volume of air at a lower temperature impacting the envelope, leading to reduced lift.
4.4) The shape of the envelope. This will not be perfectly aerodynamic to start with, given that there are additional compromises which must be taken into account such as the presence of the gondola, the need for roll stability and difficulties in control and maintaining structural integrity with a particularly long cylindrical shape, and the challenge of creating a wing-like profile.
Rigid airships have been proven, however the additional weight is too much given that the lifting strength of heated air is so weak. Such a craft must also have a hanger, or robust ground anchoring to prevent destruction in strong winds.
A completely fabric envelope has likewise been tested and found wanting. It acts as a gigantic air brake, but it does have important benefits over a rigid structure, apart from cost. It can be deflated and packed away, it can be detached and mended or amended separately from the gondola and crucially it is lightweight.
We therefore wish to keep the fabric envelope, but to stiffen it, especially at the nose. We are not interested in improvements in materials - realistically, there is no budget for envelope fabric development, only for cutting a different shape. Therefore, we prefer a Semi-rigid airship design, with a keel along the base of the envelope.
The Keel
A long keel has two advantages. It enables a longer and more streamlined shape - a completely non-rigid can be 'bent' if it is long, with unpleasant consequences. A keel enables a rigid structure to be constructed across the front of the envelope - maintaining the shape and hence greatly reducing the power required to drive it forwards. The envelope can also be collapsed onto the keel and secured to it.
In addition, the rigid keel means that burners can be located in more than one place - something that becomes important as the envelope becomes longer and more cylindrical. This should also mean that the envelope can be re-inflated without the absolute need for a cold air inflator. Given enough space held open above each burner it can inflate the envelope alone, should it have to do so – i.e. at a foreign airstrip, the only requirements will be for fuel, and not equipment.
Rigidity of the Envelope
My preference is for a Ram-air design, though this may need to be used in conjunction with a traditional structure of metal struts supporting the centre of the nose, with radiating battens.
A Ram-air design is where a series of internal tubes are used. These are sewn along the inside of the main envelope, and are open at the leading edge, so that forward movement as slow as 5 mph will hold them rigid with air and force the envelope to stay in place. This concept is how modern parachutes and paragliders maintain their structure, without any rigidity of their own. The faster they move through the air, the firmer their structure becomes. The internal envelope will be separate from the ram-air tubes in that no lifting air can transfer into the ram-air section. The ram-air openings can be fitted with valves to ensure pressurisation remains high, but this is additional weight and should be unnecessary.
Ram-air advantages:
It too is entirely collapsible as part of the main envelope.
A buffer of stationary air is a perfect thermal insulator at the nose of the airship.
The faster the airship is driven through the air, the better it maintains its shape.
Air itself, scooped up by an unmoving cup shape will form its own invisible leading edge to the air it passes through, where a flapping shape constantly creates shocks, resistance and wear.
Disadvantages
Unheated air is heavy, as is fabric (more than struts in any large use), so use must be minimised. This probably means limiting it to the nose, in a hemispherical pattern.
The main inlet tube will probably still require a physical spar to hold it up and enable it to inflate easily.
5) The Market
Why did airships die? Because of speed and safety and above all Cost. The financial case is really not too promising, so we must limit our ambitions accordingly.
Our market is wealthy individuals who already can afford balloons or thermal airships, the cost should not be excessively more than is already charged for these. A new development can and should charge a premium, though until the technology is proven the market is very limited.
Sales could also be made on the following grounds:
Advertising – as with existing balloons there is scope to generate some income from advertising, but this is unlikely to be much. There is in addition some scope for film work. These sources of income are not to be relied upon, though they may provide useful supplementary funding. A thermal airship is better than an unpowered balloon in terms of focusing advertising and considerably cheaper than a helium blimp. There may be some middle ground in the market when the economy picks up.
Transportation – as an ordinary commercial service this is impossible, given the regulatory and reliability issues.
Tourist Flights – This could be a limited business, though again there are regulatory and certification issues. Costs per passenger would be very high. Given safety concerns it is unlikely that low level flights over cities and towns would be permitted. Once airships can be established as reliable in the hands of hobbyists, it may be something to consider, as may the scale (and hence passenger carrying capacity) of airships, but this is for the future.
Leisure – Private pilots are likely to make up the majority of the purchasers.
6) Challenges
Frame: lightweight and thin, maintaining a stable structure for the engines to push through the considerable air resistance, surviving heavy landings. Enabling the envelope to be collapsed neatly on to it. Stable on the ground. Enabling structural battens or struts at the front of the envelope to keep the envelope still.
Engines: lightweight and powerful. Manufacturers such as Bailey, who construct 4 stroke paramotor engines have the engineering skills to improve upon the current generation, but this is a secondary area of potential improvement.
Envelope – Keeping it as lightweight as possible.
Envelope – internal construction preventing upending (heat all at one end), rolling and porpoising.
Envelope – Avoiding heat loss at speed – especially from the front, insulating
Envelope – Employing wind resistance and air pockets to stiffen the envelope without too much weight
Envelope – heating at multiple positions along the length of the envelope, multiple burners and gas lines/control lines to the pilot
Agility – through the use of additional stern mounted engine, or prop separate from engine (bow thruster)
Size – Managing a large envelope without a large ground crew
Size – a large enough envelope to carry several crew may be unmanageably large. Because there are existing hot air balloons with vast lift capacity, I hope this is not the case.
Noise – engines capable of driving the vast bulk of such a beast are likely to be noisy, which may well restrict operation.
Wheels – for manoeuvring on the ground without inflation – i.e. turning into wind or towing, lightweight enough to be bourn aloft, strong enough to survive hard landing.
Crew – even in light winds, inflating and deflating the envelope will be difficult tasks, the sheer surface area being a vast sail. If the airship is directly into wind and the semi-rigid nose can shield this process somewhat, this will help enormously. It is to be hoped that the airship can transport its own ground crew, or that only a limited number of additional people will be required. It is also to be hoped that the deflated envelope can be held ‘neatly’ atop the frame, avoiding too much contact with the ground – the structure of the frame may need to be too heavy and unstable to achieve this. Compression straps may be needed all around to pull the material into non-sail closeness. Frame will be needed over each burner nozzle, to enable inflation to begin without searing the envelope.
Gondola - existing ones are made from steel tubing - it must be possible to make them from lighter material.
Burner technology is not worth spending additional money on.
Disadvantages of a longer shape
Despite the obvious airborne advantages, there are a number of issues arising, primarily to do with ground handling.
A long shape will tip and impact the ground at the front and rear. A floppy envelope can bear this with ease, but a keel requires shock absorbing.
A long shape will not turn easily. Requiring a bow or stern thruster, which again will need to bear a few heavy contacts with the ground.
Facilities – construction and maintenance will require a hanger, there is not any way around it. It is to be hoped however, that at other airstrips the airship will require only fuel and can survive in the open with reasonable care, facing into wind, possibly with a nose anchor. Rain will inevitably increase the weight of the envelope considerably and may make inflation impossible until a breeze has dried it sufficiently.
Progressing these ideas
Best to start small! Despite the great advantages of scale, such ideas can only be tested and proven using models to establish designs that will work before scaling them up. And consider the possible sporting market - machines with a lifting capacity of 1 to 4 people is as much as should realistically be planned for. If. And it is a very BIG IF, these get anywhere, then larger and more sophisticated developments will follow. But these will be for another generation, and no-one should think otherwise.
Summary
This Post sets out a theoretical semi-rigid thermal Airship design. I suggest improvements to current performance limitations using known technology to reduce development risks and costs.
Summary 1
1) The Impractical Dream 1
2) Existing Airships 2
3) Modest Design Goals 3
4) Design Suggestions - How do we achieve these modest aims? 4
5) The Market 7
6) Challenges 8
1) The Impractical Dream
Purple Prose
“Imagine a creature, vaster than any aeroplane, crawling into stately view over the horizon, small at first and hard to see, its great bulk swims inexorably towards you, periodically illuminated by internal pillars of flame, making fleetingly visible the most splendid Chinese lantern anywhere on earth. Closer it comes, its true size manifest at last, it dominates the sky, the low rumble of its engines pushing it fast against the wind. And there, at the base of the envelope, passengers seeing you waving, a small dot of enthusiasm beneath them, break off from their conversations, turn and wave back. Only the pilot, his eyes flicking from his instruments to the far horizon, does not. He has the future of aviation in his hands. Then it is gone, plunging onward to its destination, hundreds of miles away. Someday you think, someday, I will live this dream. “
The Dream
To build airships.
The dream is alive in popular imagination – films are littered with them, as are books, and the genres of steam punk and role playing games. In reality however it is quite dead. The gulf between modelling and the great beasts of the skies is unreachably great. The rise of the machines – aeroplanes, has seen to that. Dreamers, having no money, and those with money having no plausible prospect of a market, have no place to see their hopes realised. How then, shall a dreamer focus his ambition on a more practical prospect? He must lower his gaze from the mighty whales of heavy lift freight, commercial tourism and governmental largess to the minnows of lighter than air sport ballooning.
2) Existing Airships
There are two main types still flying: The Helium types that exist in limited numbers serving niche markets, and excite any number of speculative news items, and the thermal airships produced primarily by Geta-flug and Cameron Balloons. The latter are low-to-nil wind capable, portable leisure hot air balloons with engines.
Current State
Helium and Hydrogen have proven impractical for the sporting amateur. The costs and technical genius are beyond his everyday wallet and humdrum mind. Helium airships are beyond anyone except major corporations and billionaires, require extensive infrastructure and crew. There is no prospect of a billionaire turning his attention towards the construction of a new Zeppelin.
Hot air Ballooning has settled into a steady state backwater of special shapes and dawn and dust tourist experiences. It is a grand spectacle, but it does not develop.
Thermal airships purchase costs are in the low 100,000's, but they are toys.
Current thermal airships
Once there was a stronger passion to produce thermal airships.
Colt and Geta-flug and Cameron have between them a small range of strikingly similar airships with the performance characteristics of snails. With a top speed of 20 mph and flight durations of between 1 and 3 hours, these are effectively still hot air balloons on anything but a still wind day. Why then have they not progressed technically?
Primarily because the concept is from a hot-air ballooning background of non-rigidity and ground portability. They can be packed into a trailer and transported, and in fact due to the performance window, have to be. The lack of rigidity in the envelope makes it impossible to be aerodynamic, the gigantic airbrake cannot be overcome with internal pressurisation. Given that typical winds in the UK range from 10 to 25 mph, an airship needs a top speed of 50 mph to be functionally viable - i.e. to travel unaided from one airstrip to another.
3) Modest Design Goals
What we want to achieve is set out below, our guiding principles are useful performance and cost minimisation.
3.1) Costs per flight as a low as possible
Helium is therefore out of the question and we are restricted to hot air.
3.2) Lightweight and cheap construction costs
An inflatable envelope is by far the cheapest to develop. It has its own drawbacks, particularly relating to performance.
3.3) Speed of approx 50 mph
This is a somewhat arbitrary goal, but we must have a speed sufficient to cope with normal wind conditions in a head wind and still make reasonable progress. Achieving this while retaining a non rigid structure is difficult and is where innovative design is key to success.
3.4) Range 3 hours
We must have the capacity to reach airstrips at reasonable distances to refuel.
3.5) Lift – 1 or 2 people at most
Initial development must be for as small a vehicle as possible because development will inevitable involve many costly and time consuming iterations and adjustments, notwithstanding the performance benefits from increased lifting volume, these cannot be a first step.
3.6) Self-portable avoiding the need for road transportation
We want to avoid additional support vehicles for daily operation; a self contained unit with the speed, range and survivability to move itself is preferable by far. We wish to do away with the concept of a portable hot-air airship.
3.7) Avoid requirement for a hanger in daily operation
Development and repair will require a hanger, there is no avoiding this, but reducing the costs of a hanger at each overnight stop is crucial. This means we must have the ability to survive moderately adverse weather conditions, which in turn means significantly reducing the profile of the envelope, by collapsing it and securing it into wind, with or without a ground anchor. Large rigids were notoriously prone to wind damage.
4) Design Suggestions - How do we achieve these modest aims?
The primary limitation on operations is caused by the shape of the envelope and its rigidity or otherwise in the face of external pressures.
Performance losses may happen in a number of ways:
4.1) By deformation of the envelope shape as it encounters resistance – typically seen in the nose cone flattening.
4.2) In movement throughout the envelope – flapping and general fabric movement causes severe additional air resistance
4.3) Temperature transfer from the nose, due to the volume of air at a lower temperature impacting the envelope, leading to reduced lift.
4.4) The shape of the envelope. This will not be perfectly aerodynamic to start with, given that there are additional compromises which must be taken into account such as the presence of the gondola, the need for roll stability and difficulties in control and maintaining structural integrity with a particularly long cylindrical shape, and the challenge of creating a wing-like profile.
Rigid airships have been proven, however the additional weight is too much given that the lifting strength of heated air is so weak. Such a craft must also have a hanger, or robust ground anchoring to prevent destruction in strong winds.
A completely fabric envelope has likewise been tested and found wanting. It acts as a gigantic air brake, but it does have important benefits over a rigid structure, apart from cost. It can be deflated and packed away, it can be detached and mended or amended separately from the gondola and crucially it is lightweight.
We therefore wish to keep the fabric envelope, but to stiffen it, especially at the nose. We are not interested in improvements in materials - realistically, there is no budget for envelope fabric development, only for cutting a different shape. Therefore, we prefer a Semi-rigid airship design, with a keel along the base of the envelope.
The Keel
A long keel has two advantages. It enables a longer and more streamlined shape - a completely non-rigid can be 'bent' if it is long, with unpleasant consequences. A keel enables a rigid structure to be constructed across the front of the envelope - maintaining the shape and hence greatly reducing the power required to drive it forwards. The envelope can also be collapsed onto the keel and secured to it.
In addition, the rigid keel means that burners can be located in more than one place - something that becomes important as the envelope becomes longer and more cylindrical. This should also mean that the envelope can be re-inflated without the absolute need for a cold air inflator. Given enough space held open above each burner it can inflate the envelope alone, should it have to do so – i.e. at a foreign airstrip, the only requirements will be for fuel, and not equipment.
Rigidity of the Envelope
My preference is for a Ram-air design, though this may need to be used in conjunction with a traditional structure of metal struts supporting the centre of the nose, with radiating battens.
A Ram-air design is where a series of internal tubes are used. These are sewn along the inside of the main envelope, and are open at the leading edge, so that forward movement as slow as 5 mph will hold them rigid with air and force the envelope to stay in place. This concept is how modern parachutes and paragliders maintain their structure, without any rigidity of their own. The faster they move through the air, the firmer their structure becomes. The internal envelope will be separate from the ram-air tubes in that no lifting air can transfer into the ram-air section. The ram-air openings can be fitted with valves to ensure pressurisation remains high, but this is additional weight and should be unnecessary.
Ram-air advantages:
It too is entirely collapsible as part of the main envelope.
A buffer of stationary air is a perfect thermal insulator at the nose of the airship.
The faster the airship is driven through the air, the better it maintains its shape.
Air itself, scooped up by an unmoving cup shape will form its own invisible leading edge to the air it passes through, where a flapping shape constantly creates shocks, resistance and wear.
Disadvantages
Unheated air is heavy, as is fabric (more than struts in any large use), so use must be minimised. This probably means limiting it to the nose, in a hemispherical pattern.
The main inlet tube will probably still require a physical spar to hold it up and enable it to inflate easily.
5) The Market
Why did airships die? Because of speed and safety and above all Cost. The financial case is really not too promising, so we must limit our ambitions accordingly.
Our market is wealthy individuals who already can afford balloons or thermal airships, the cost should not be excessively more than is already charged for these. A new development can and should charge a premium, though until the technology is proven the market is very limited.
Sales could also be made on the following grounds:
Advertising – as with existing balloons there is scope to generate some income from advertising, but this is unlikely to be much. There is in addition some scope for film work. These sources of income are not to be relied upon, though they may provide useful supplementary funding. A thermal airship is better than an unpowered balloon in terms of focusing advertising and considerably cheaper than a helium blimp. There may be some middle ground in the market when the economy picks up.
Transportation – as an ordinary commercial service this is impossible, given the regulatory and reliability issues.
Tourist Flights – This could be a limited business, though again there are regulatory and certification issues. Costs per passenger would be very high. Given safety concerns it is unlikely that low level flights over cities and towns would be permitted. Once airships can be established as reliable in the hands of hobbyists, it may be something to consider, as may the scale (and hence passenger carrying capacity) of airships, but this is for the future.
Leisure – Private pilots are likely to make up the majority of the purchasers.
6) Challenges
Frame: lightweight and thin, maintaining a stable structure for the engines to push through the considerable air resistance, surviving heavy landings. Enabling the envelope to be collapsed neatly on to it. Stable on the ground. Enabling structural battens or struts at the front of the envelope to keep the envelope still.
Engines: lightweight and powerful. Manufacturers such as Bailey, who construct 4 stroke paramotor engines have the engineering skills to improve upon the current generation, but this is a secondary area of potential improvement.
Envelope – Keeping it as lightweight as possible.
Envelope – internal construction preventing upending (heat all at one end), rolling and porpoising.
Envelope – Avoiding heat loss at speed – especially from the front, insulating
Envelope – Employing wind resistance and air pockets to stiffen the envelope without too much weight
Envelope – heating at multiple positions along the length of the envelope, multiple burners and gas lines/control lines to the pilot
Agility – through the use of additional stern mounted engine, or prop separate from engine (bow thruster)
Size – Managing a large envelope without a large ground crew
Size – a large enough envelope to carry several crew may be unmanageably large. Because there are existing hot air balloons with vast lift capacity, I hope this is not the case.
Noise – engines capable of driving the vast bulk of such a beast are likely to be noisy, which may well restrict operation.
Wheels – for manoeuvring on the ground without inflation – i.e. turning into wind or towing, lightweight enough to be bourn aloft, strong enough to survive hard landing.
Crew – even in light winds, inflating and deflating the envelope will be difficult tasks, the sheer surface area being a vast sail. If the airship is directly into wind and the semi-rigid nose can shield this process somewhat, this will help enormously. It is to be hoped that the airship can transport its own ground crew, or that only a limited number of additional people will be required. It is also to be hoped that the deflated envelope can be held ‘neatly’ atop the frame, avoiding too much contact with the ground – the structure of the frame may need to be too heavy and unstable to achieve this. Compression straps may be needed all around to pull the material into non-sail closeness. Frame will be needed over each burner nozzle, to enable inflation to begin without searing the envelope.
Gondola - existing ones are made from steel tubing - it must be possible to make them from lighter material.
Burner technology is not worth spending additional money on.
Disadvantages of a longer shape
Despite the obvious airborne advantages, there are a number of issues arising, primarily to do with ground handling.
A long shape will tip and impact the ground at the front and rear. A floppy envelope can bear this with ease, but a keel requires shock absorbing.
A long shape will not turn easily. Requiring a bow or stern thruster, which again will need to bear a few heavy contacts with the ground.
Facilities – construction and maintenance will require a hanger, there is not any way around it. It is to be hoped however, that at other airstrips the airship will require only fuel and can survive in the open with reasonable care, facing into wind, possibly with a nose anchor. Rain will inevitably increase the weight of the envelope considerably and may make inflation impossible until a breeze has dried it sufficiently.
Progressing these ideas
Best to start small! Despite the great advantages of scale, such ideas can only be tested and proven using models to establish designs that will work before scaling them up. And consider the possible sporting market - machines with a lifting capacity of 1 to 4 people is as much as should realistically be planned for. If. And it is a very BIG IF, these get anywhere, then larger and more sophisticated developments will follow. But these will be for another generation, and no-one should think otherwise.