The world of aerospace moves fast. It uses the highest of high technology to propel people at dizzying velocities. But the roots of aerospace are slower and come from the tranquil, almost sedate drift of a hot-air balloon.
However, even here, technology moves on. Back in 1783, the Montgolfier Brothers’ waxed-paper construction was held aloft by burning straw; today’s balloons are made from technical fabrics and inflated with propane. And the role of the engineer is as crucial here as anywhere else in the aviation industry.
Simon Forse, chief engineer and managing director of Lindstrand Hot Air Balloons, said: ‘We’re one of the very few aircraft manufacturers left in this country to hold full manufacture and design approval from the European Aviation Safety Agency. We design and build standard balloons and special shapes for promotional purposes, and those are essentially one-off aircraft with all the type approval and data that entails.’
Lindstrand, as the name suggests, was founded in 1992 by legendary ballooner and adventurer Per Lindstrand. Initially making all types of lighter-than-air and inflatable structures, the company split into two in 2003. Lindstrand himself runs Lindstrand Technologies, which specialises in using heavy fabrics, weighing upwards of 250g/m2; it makes helium-filled structures such as airship envelopes and products such as inflatable flood barriers and twin-skinned temporary buildings. More recently, it has begun to investigate the space industry - it produced the landing balloons for the ill-fated Beagle 2 Mars probe.
Lindstrand Hot Air Balloons uses lighter fabrics, generally polyurethane-coated 60g/m2 ripstop nylon and a heavier silicone polymer-coated fabric - known as Hyperlife - at 90g/m2. Forse explained that this puts it into the realm of piloted lighter-than-air vehicles.
He said: ‘We demerged the two companies because it became obvious that they were addressing completely different areas, and most especially because there is a fundamental difference between the technologies associated with them. All of Lindstrand Technologies’ fabrics are welded together, whereas my fabrics are all sewn. As soon as you start working with those heavier fabrics, the way you deal with them has to change. Below 100g/m2, the sewing machine stays still and the article moves along it; with heavier fabrics the article is too heavy to move around, so you have to move the machinery instead of the product. That drives the rest of the production process and how it’s organised.’
Forse is in his element at Lindstrand; he’s been piloting hot-air balloons since the age of 14. ‘I got my pilot’s licence a few years after that,’ he said, ‘but when I started my degree I didn’t have ballooning in mind. I thought I was going to be trained to mend aircraft and was surprised when they told me I would be learning to design them too. The vacancy with a balloon-manufacturing company came up by chance, but it was too good an opportunity to miss.’
Forse now runs the biggest balloon-making company in the UK and it is a surprisingly large concern; its 40 employees produce around 60 balloons per year, 95 per cent of them for export, generating £2m of turnover per year. He said: ‘The strong technical background from my training and from working as an engineer has definitely helped me position our products in the professional market - although they do say that the eventual destination of all engineers is to end up in management.’
Many of the products are standard, teardrop-shaped balloons of varying sizes, but one of the distinctive parts of the business is ‘special’ shapes. Generally for promotional purposes, these can take a variety of forms; recent examples have included the shape of the helmet and mask of Star Wars character Darth Vader, the ubiquitous cartoon English bulldog from the Churchill insurance adverts and the airship that was used in a Top Gear stunt to lift a caravan as its gondola. It’s here that engineering know-how comes into play.
‘We divide special shapes into three categories,’ explained Forse. ‘One is just a modification of the standard teardrop shape and there are inflatable appendages added on. The next most complex is rotationally symmetric shapes, such as bottles; that’s a swept volume, once you’ve established your outer co-ordinate geometry. After that, you are into non-rotationally symmetric and those are bespoke designs to client request.’
There is an art to making shapes such as these. Generally, a loose brief from the client will go to the graphics department, which will come up with several concepts. Forse said: ‘Our design engineers’ skill is to look at these shapes and determine where the true lifting volume - the part of the shape that supports the gondola and pulls it upwards - is. Within that lifting body, we’ll have the stressed formers to allow it to do its job, carrying the vertical load. The rest of the shape we refer to as 3D artwork - it’s not part of the structural aircraft and is regarded as an add-on when it comes to the approval of the balloon. If it is ripped off, it doesn’t compromise the safety of the aircraft.’
The design process makes heavy use of CAD packages and stretches them to the limit. Forse said: ‘We were briefly a test site for AutoCAD, because we were pushing the surface-modelling techniques quite hard, and we still do, even in the advanced releases. Recently, we’ve been playing with 3D laser scanning; if a client comes to us with a product or object they want turned into a balloon, laser scanning is a quick way to get a surface model into the CAD system as quickly as possible.’
The design process can be a delicate balancing act between the client’s wishes and the practicalities, Forse added. ‘We try to tailor what we put in front of a client so we end up with a rational aircraft. You can do an awful lot with fabrics, but we have to look at it from a pilot’s perspective. It’s no good building something that no one in their right mind would want to fly.’
Biography
Simon Forse
Managing director
Lindstrand Hot Air Balloons
Education
- BTEC/HND Mechanical and Aeronautical Engineering from Hatfield Polytechnic
Career
- 1985 Mechanical engineer at BAJ Underwater Systems Division, working on design and manufacture of underwater weapons systems
- 1988 Design and airworthiness engineer at Thunder and Colt, ending up as head of R&D
- 1992 Chief engineer at Lindstrand Hot Air Balloons
- 2000 General manager of Lindstrand Hot Air Balloons
- 2003 Present managing director of Lindstrand Hot Air Balloons
- Interests Forse is rated to fly hot-air balloons, airships and gas balloons on a Private Pilot’s Licence, and is a British Balloon and Airship Club Category 2-rated senior balloon inspector
Q&A - More than just hot air
What technological innovations have there been in hot-air ballooning in recent years?
The performance of the balloons has come on in leaps and bounds. In sports ballooning, the focus is to achieve very high rates of ascent and descent - the faster the pilots can get into the air, the better they can hit the targets in the sport. We changed the profile of the classic teardrop shape and doubled the rate of ascent to 2,000ft per minute.
Apart from the balloons, what other engineering input is there?
The burners are very important; we produce those ourselves, and they are specifically design- ed for ballooning. There are a lot of parameters we have to think about. Power output is high on the list of desirables; there are issues with increasing the power and also decreasing the noise they produce.
What are the other priorities?
We focus a lot on simplicity and keeping the moving-part count low. That has implications in keeping the burners easy to operate, while also making them easy to maintain. It also keeps the cost down, although to be honest cost is quite a way down the priority list. We also have to think about the radiant heat, which we want to suppress - as much of the heat as possible has to go up into the balloon, rather than spreading outwards from the burner. There are also more esoteric design criteria, such as shut-off time, which is the delay between releasing the valve handle and the flame disappearing. Long shut-off times make the flame less controllable, which causes difficulties when you are inflating in windy conditions.
Is the combustion process itself significant?
Efficiency of combustion makes a big difference, and the shape of the flame is an important criterion - if it’s too wide, it can burn the mouth of the envelope in windy inflations. In simple terms, what we are trying to do is extract the largest amount of energy possible from the fuel and direct it upwards rather than outwards, while producing as little noise as we can.
Has computer design been useful here as well?
Not as much as it has in envelope design. We do use CAD, but only in the late stages; the burners are batch produced, so that means we can afford to spend more time developing them. You tend to have an idea and then go and test it, and that works faster than using computational fluid dynamics (CFD). The boundary conditions have never really been established for CFD, so you don’t get a firm recommendation from it for how you should shape the burner. That’s primarily because we are dealing with an unusual form of combustion. You have a tank of liquid propane that passes over a vapourising coil, which expels a mixture of liquid and vapour, whereas most combustion is based around straight vapour. The conditions we work with are rarely stable, because the pressure of the propane cylinder varies with temperature and performance varies in a major way.
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