A Personal Approach to Blimps











By Dan Nachbar

The best thing about flying in a small airplane is the view. The worst thing is the noise. What I want is to be able to enjoy the view without having to put up with the noise.

In particular, I want an aircraft well suited to 'air therapy.' Air therapy is the practice of taking an aircraft into the sky, usually on a short flight, for the restorative value that comes from just being in the sky. Air therapy flights are often made in the evenings, after work. Usually such flights don't go anywhere. The 'mission' here is not transportation. The mission is joy. A "joyride" as it were; but in the less flip sense of that term. Some airplane pilots call it "warmimg up the oil."

If you can't understand what I'm talking about, my guess is that you are not a pilot. So go find one and ask them about it. My guess is they'll have a pretty good idea of what I'm describing.

Hot Air Balloons Lack Control
One reasonable candidate for the sort of aircraft I'm seeking is a balloon. I very much like being in a balloon -- as a passenger. But I find piloting a balloon anything but soothing.

For the pilot, hot air balloons present a very significant challenge: the pilot does not have the final say in which way the balloon goes -- the wind does. The pilot can usually gain some limited directional control by changing altitude. But if the pilot wants to go West and the winds at all altitudes are blowing East, the balloon is going to go East. While some folks may find that sort of enforced spontaneity an exhilarating delight, I find it bloody unnerving.

To my way of thinking, the lack of directional control makes every balloon landing a de facto emergency. Not a life-threatening sort of emergency to be sure. Thousands of hot air balloons fly and land safely all the time. But on every flight, the balloon pilot must use a combination of wits, skill, and experience to locate and reach a safe place to land. Sometimes it's easy, sometimes not. But for other types of aircraft, that sort of decision-making is only required in emergencies. It's simply a matter of taste whether the making of such decisions is considered fun. For me, it's not.

Further, in many cases one can't tell in advance how much work and/or hassle is going to be involved in packing up the balloon and hauling it back to civilization. That sort of uncertainly takes yet more fun out of the whole experience for me.

I guess I have control issues. I want to have a darn good idea how each flight is going to end as well as where it is going to end.

Gliders Require Constant Focus
Another choice is gliders. I like gliders, both as a pilot and as a passenger. But flight in gliders is divided into two parts -- gliding (going down) and soaring (going up). If all one does is the gliding part, the ride is too short to be much fun. As a practical matter, one must engage in some soaring.

Soaring is fun, but it is definitely not a leisurely undertaking. In fact, I find great joy in the deeply consuming nature of the task of finding and utilizing the updrafts of air necessary to sustain flight. All the world's problems drop away as I focus on the task. It's wonderful. Successful soaring is deeply gratifying. But the great bane of soaring is that one must focus on the work at hand whether one wants to or not. Otherwise, a very short flight. Every glider should have a sign posted: No "joyriding" allowed (per order of the laws of physics).

Electric Motors Provide Only Very Short Flights
When I started musing on the goal of silent flight, the first thought that came to mind was to look for an airplane with an electric motor. There is already one electricly launched glider called the Antares (See the Antares site for further details). It has grand total of 11 minutes of motor running time on a 50 horsepower motor/prop that puts out only 45 decibels when running. Getting longer flights is pretty tough, given the power-to-weight ratios of current technology. Maybe things will be different in 5 or 10 years. But there is no way to take a reasonably long joyride today.

But I think the idea of electric propulsion is promising. The trick is to find an aircraft that will work with the high weight and low power available today. The answer: an airship.

Airships Offer Promise But Have Their Own Problems
Airships don't go very fast. But, for my pleasure trip needs, I really don't need to go very fast or very far. And the amazing thing about going slow is that it requires very little power. For instance, a 4-horsepower motor can drive a reasonably sized airship at 10 mph, so one can fly for quite a while on an Antares-glider-sized battery pack.

In addition to being slow, airships have other problems. The biggest is cost. In addition to the cost of the craft itself, there is the cost of a place to put the darn thing when it's on the ground. It isn't practical to leave it outside when a storm comes along. There is also the cost (in dollars or beer) of a whole gang of people to grab onto ropes and drag the ship around when it is on the ground.

There are a bunch of other oddities that I learned about when I started to look into airship operations. For instance, once moored to a mast, the ship needs to be kept fairly buoyant or else it will tip over. But if you make it too buoyant it will bounce around too much and hurt itself, even in fairly mild winds. Also, the bloody helium slowly leaks all the time and needs to be "topped off" . . . an expensive and annoying task.

Most surprisingly I learned that the "balloonettes" (balloons within the balloon) on helium airships that I thought were used to adjust the ship's buoyancy do nothing of the kind. They are used to compensate for the expansion of the helium due to changes in altitude and temperature.

So helium ships don't fly based on buoyancy alone. Instead, the ships are kept slightly "heavy" and flown by driving along under power with the nose pitched up and having the resulting aerodynamic forces lift the ship. The lack of buoyancy control creates a whole bunch of other distinctly unpleasant complications I won't bother to enumerate. The end result is that a helium airship seems like a lot of bother and expense.

The Good News: Hot Air Avoids the Problems of Helium
The other popular choice of lifting gas for aircraft is hot air. Hot air has a lot of advantages. It's cheap to make (just burn some fuel, such as propane), so it can be disposed of at the end of each flight. Since the gas bag can be deflated, a hot air airship does not need a big storage building.

It's also easy to control the buoyancy of hot air. Just change its temperature. So a hot air airship can be made to stay put when its on the ground. As a result, it doesn't need a big ground crew to wrestle it around. Once it is down, it will stay down.

The Bad News: Hot Air Requires a Bigger Airship Than Helium
However, a given volume of hot air can lift only about one third as much as the same volume of helium. Consequently, in order to carry the same load, a hot air airship needs to be bigger than a helium airship. Such an increase in size will necessarily hurt performance (i.e., speed). Also, bigger ships tend to be more expensive. But, due to the wonders of a mathematical concept called surface-to-mass ratios, a hot air airship does not have to be all that much larger than a helium one.

Doing the Math on Size and Cost
In fact, one does not need to make the ship three times longer, and three times wider, and three times taller. Rather, the hot air airship needs to be only 3 times bigger in volume. And delightfully, the volume of any object is related to the third power ("cube") of the individual dimensions. So one need only increase each dimension by the cube root of 3. The cube root of 3 is a bit less than 1.5 (i.e., -- 1.5 times 1.5 times 1.5 is approximately 3.) In other words, a hot air airship only needs to be 1.5 times bigger (on each dimension) than a helium ship in order to have the same lifting capacity.

That is not to say that a hot air airship will cost only 1.5 times more than a helium ship. Cost and performance are much more closely related to the surface area of a ship than to its individual dimensions. Surface area is related to the individual dimensions as a second power (or "square"). So a first guess would put the cost penalty of hot air at the square of 1.5, or 2.25 (i.e., 1.5 times 1.5 is about 2.25.) So in terms of horsepower, fabric, etc., a hot air airship will need about 2.25 times more than a helium ship with the same lifting capacity.

Actually, it's not quite that bad. In particular, remember those ballonets that helium ships need to have in order to control the pressure inside the envelope? Well, those "balloons within a balloon" provide no lift but take up a fair bit of space. In fact, the space inside a typical helium ship is something like 25% (by volume) ballonet. So, when you knock off the space taken up by the ballonet, the cost difference between hot air and helium comes down to a factor of 1.9.

One can refine this sort of cost guesswork much further. One things to consider is the weight compounding phenomenon that can drive costs up (if the motor is bigger, then it weighs more, which means a yet bigger volume for lifting gas, which means yet more horsepower, etc.). On the other hand, the fabrics used to contain the lifting gas can be much lighter and less expensive for hot air than for helium. (The reason is that letting a little hot air leak is no big deal, you can just make more. But even a "small" Helium leak can be a very expensive over time.) So, as a conservative first guess, a slightly higher number, say 2.0, is a reasonable "back-of-the-envelope" approximation for the additional cost factor of hot air.

Now, a cost factor of 2 is nothing to sneeze at, to be sure. But, for my money, it seems well worth it, given the significant cost and operational advantages of using hot air (remember that there is no hangar or ground crew needed). So, why not consider a hot air airship?

Existing Hot Air Airships Have Trouble Keeping Their Shape
And in fact, there are companies that currently make hot air airships. But the existing craft have some serious limitations. Specifically, when using hot air for an airship, it is quite difficult to maintain the structural stiffness of the gas bag.

Balloons experience no relative wind. They literally go with the flow. But as soon as any aircraft starts to move through the air, that motion creates a "relative wind." In an ordinary hot air balloon, the pressure caused by the relative wind bumping into the gas bag will cause it to deflate. Deflating a gas bag in flight leads to a very short flight indeed.

In order to resist deflation due to the relative wind of motion, there has to be some stiffness added to the bag. On helium airships, the gas bags are sealed and pressurized (like a toy balloon is pressurized). The higher internal air pressure keeps the bag from deflating.

Sealing Up the Gas Bag Has Practical Limitations
Most hot air ships don't have the option of having sealed gas bags, because they need a constant supply of oxygen to feed the burners. There is one brand of hot air airship with sealed bags. The bags are sealed before takeoff, and no fresh oxygen is brought into the bag during flight. These ships use the expanding gas from the propane burners to pressurize the bag. After an hour or so, the oxygen in the bag is used up by the burners and the burners go out, "starved" for Oxygen. In my view, any approach with such a severe limit on flight duration is not worth serious consideration.

Some Designs Use Blowers to Pressurize the Gas Bag
The more sensible, and thankfully more common, method of pressurizing a hot air airship is through the use of a fan/blower to constantly pump in fresh air. The big problem with fans is that they are noisy. Also, the pressures that can be safely obtained by the lightweight fabrics used in hot air ships are relatively low. This leads to a fairly floppy envelope, lots of drag, and poor performance.

The other irksome problem with using gas pressure to maintain the shape of the craft (either for helium or hot air) is that a ruptured seam or other damage to the envelope causes not only a loss of lift but also a loss of structural integrity and consequently an increase in drag. So just when the pilots need to get somewhere in a hurry, it's hard to move very fast. Not a soothing thought.

Another issue is that during the development/construction of my craft, I want to be able to set the envelope up and mess around with it while it's on the ground. And I'd like it to just sit there while I poke around or contemplate what needs to be done next. It's hard to do that if you need to have a fan blowing away whenever the envelope is inflated.

A Rigid Frame Avoids the Problems Created by Pressurization
The alternative to using gas pressure to keep the envelope in shape is to add some sort of rigid support to the bag. This is what the Zeppelins did in the days of yore. But a rigid design, even a hot air one, has the disadvantage of needing a large (and presumably expensive) place to store the ship on the ground. Needing storage also limits where the ship can spend the night. So the choices are to either figure out a way to take the storage building along with the airship, or find a way to fold up and store the rigid structure.

I have chosen to try the second approach -- to design a folding rigid structure.

Current Burners Are Noisy
There is one other source of noise on hot air craft, namely the noise of the burners. Hot air balloon burners are typically very loud. What is the point of using quiet, electric motors if the burners are going to be blasting away?

However, burners don't inherently need to make noise in order to work. For instance, the burner on my gas stove at home is essentially silent. I've had extended consultations with two of the world's premier experimental balloon building experts (Don Piccard and Brian Boland -- see our People page for details) on the topic of burner noise. Both of these fellows agree that nearly noiseless burners can be built if one wishes to do so.

Why do balloon burners remain so noisy? Well, it seems that for most balloon pilots it's great fun to have the ability (and excuse) to make big blasting noises whenever they wish, nobody has bothered to build and market a quiet burner. Go figure.

In Summary
That's the thinking behind the design of the craft called the Skyacht Personal Blimp. I started out looking for an aircraft that does 3 things:

1) doesn't make a lot of noise
2) lets me control where I'm going
3) stays in the air without requiring continuous intense effort

No existing aircraft fits that bill. The closest thing available is a hot air balloon. In order to make a hot air balloon completely meet my needs, it must have three novel features:

  • quiet, electric propulsion
  • rigid support structure for the gas bag that is foldable on the ground
  • hot air for lift created by quiet burners
It's by far the most complex and difficult building project I've ever taken on. It would have been impossible to make the progress to date without the world-class co-workers and technical advisors with whom I've had the good fortune to collaborate.

You can click here to trace through the process of bringing this dream to reality.

Copyright 2002-2010 Skyacht Aircraft, Inc.