Ideal RC Blimp Materials

July 5, 2017

Remote Control Blimp

In order to understand the cost of a high quality blimp, the common expression “you get what you pay for” should be considered and understood before jumping to the lowest cost provider. At a cursory glance, all RC blimps across multiple producers appear more or less the same while costs vary significantly by company. In truth, the underlying materials used to create a blimp predominantly influence the overall price passed on to the consumer, and therefore it is worth understanding those different materials and their trade-offs in order to make an educated decision about a non-trivial purchase.

High Quality Before Low Cost

The first consideration that should be made as a consumer is the source of the production material used. Chinese parts are ubiquitous throughout the world, and it’s worth mentioning that in the RC Blimp market at least, those parts are generally low quality. Moreover, for those that purchase blimps made from Chinese parts, in the likely scenario that the manufacturer has over-promised and underdelivered, the consumer has no recourse. A low-quality blimp that still requires costly helium to fly at all serves as nothing more than a deflated pile of sunk cost and shattered dreams.

Unfortunately both for our own business at EBlimp and to the people we interact with, it’s all too familiar to see a potential client abandon a purchase in favor of a cheaper vendor, only to return a few weeks later with an identical request and a dramatically smaller budget.

At EBlimp, not only is our supply chain limited to high-quality U.S. vendors, but all of our work is backed by a one year warranty. Our business relies heavily on its reputation in order to thrive: the vast majority of incoming clients were referred from existing ones, and repeat business constitutes a significant portion of our revenue. Moreover, much of our work is done in the form of piloting a blimp from our own inventory, and low quality materials that risk a loss of reliability or durability would not be tolerable for our own reputation.

Too Low Envelope Rigidity

An envelope for an RC Blimp is ideally composed of polyurethane in order to maximize helium retention. This material is also more rigid than typical polyvinyl chloride (PVC) or thermoplastic polyurethane (TPU) that Chinese manufacturers generally utilize. In general, Chinese RC blimp products will say they’re made with urethane, but technically speaking, they’re actually referring to TPU.

One of TPU’s properties is that it can stretch, much in the same way that latex does. Therefore, the balloon can stretch, so when any force is applied to the material, the envelope distorts into a different shape. For example, wind resistance during forward movement can flatten out a blimp’s nose, thereby reducing its aerodynamic efficiency, and the balloon will also not be able to hold the gondola steady as the airship’s motors apply force against its own airframe.

One could compare the above concept to driving a car with a flat tire. At low speeds, the flat tire is tolerable, but faster speeds will result in increasing instability. Rubber is soft and the air it contains is fluid, but under pressure the tire becomes rigid, and a well-designed blimp will mirror the positive properties of a tire. It’s no coincidence that Goodyear, a major producer of tires, started producing blimps because it was able to leverage its intellectual property for other purposes.

Both PVC and TPU simply don’t have the properties to maintain rigidity to form an ideal lighter-than-air vehicle.

A way to identify a blimp made from less rigid and therefore non-ideal materials is that the corresponding aircraft needs supporting ropes originating from the gondola to keep it mounted and to maintain structural integrity. The ropes help, but they don’t address the root problem these bad materials introduce. In flight, the aircraft will bend and distort, and unfortunately for a consumer, those negative characteristics will not be displayed in full from cherry-picked photo advertisements. Additionally, as a remote pilot for a blimp, it’s easy to recognize the feedback from the aircraft as it’s controlled, and those distortions will not only be visible, but they can be felt in the form of increasingly worse responsiveness. Airships are already by definition lighter than air, and poor flight characteristics will only amplify the forces due to wind. This includes reduced speed, sudden shifts in momentum, slower response times to control input, and uncomfortable oscillations in the blimp’s shape.

Too High Envelope Rigidity

If rigidity alone determined a blimp’s quality, then the opposite end of the spectrum would provide an ideal blimp material. But, with increased rigidity come additional trade-offs that quickly outstrip the benefits with burdensome costs. In particular, nylon can act as the primary material for a rigid airship.

The main benefits of rigid airships is that they can travel at dramatically higher speeds than their non-rigid counterparts. So in some circumstances for very specific needs, nylon works as a reasonable solution to develop a high-speed blimp. But for the overwhelming majority of our customers, high speeds on an airship only increase liability with no real benefit for the actual purpose of the aircraft.

But rigid, nylon airships incur serious drawbacks. There’s an actual danger resulting from the airframe’s inability to stretch at all: the envelope will easily pop as a result of dramatic pressure changes inside. Those pressure changes can occur from simple weather changes outside, and as the balloon tries to expand along with the gases inside, the nylon won’t give and instead results in the envelope popping.

Nylon also tears easily. While intuitively this wouldn’t be a property of nylon because of its high tensile strength, even the slightest puncture, whether that be from a tree branch, a barbed wire fence, a cactus, or any other structure in a particular environment, will trigger an immediate rip. Ripstop nylon exists to mitigate the problem just described, but this particular material isn’t airtight, making it unsuitable by itself for an airship envelope. Moving down the line of mitigation techniques, coated ripstop nylon could be used to make the blimp airtight again. But once nylon is coated in any material such as PVC, the ripstop characteristics are nullified.

To understand why coating increases ripstop nylon’s brittleness, ripstop materials must first be understood. These work by a composition of fibers that slide when pressure is applied. So the resulting bend in what otherwise would have caused a tear is instead dispersed from the first fiber to the second fiber it slides to followed by the third fiber and so forth until the impact has been absorbed. But once an airtight coating is introduced, the sliding behavior is eliminated. Pressure against one of the fibers is again isolated to a single one. This behavior is analagous to bed sheets or any other piece of fabric. These are difficult to tear and bendable, but if an experiment were conducted to apply glue to the fabric, the glued area once dry will have become brittle and breakable when it was previously foldable. At a much lower level, the fibers that were once bound together to form a single thread can no longer bend and slide with one another.

Rigid Nylon Over a Polyurethane Bladder

It would be possible to combine the benefits offered by nylon in terms of rigidity and offset its negative air retention characteristics with an internal polyurethane bladder. The resultant envelope would be both airtight and rigid, and the nylon would not be subject to tears. The rigid airframe would keep the internal pressure high, and indeed, the performance of the aircraft would be superior to alternative designs.

But the cost of such a design is its high maintenance and expensive inputs. In a blimp with a standard bladder, wear and tear will result in small punctures that are easily manageable with the application of simple tape. In a two-layered envelope, when a leak eventually occurs, finding that leak is far more difficult because the leak is covered up by a nylon bag. And if and when the leak is discovered, fixing it still requires a substantial effort because of the nylon exoskeleton. So the only way to resolve the problem is to pump all of the air out of the envelope into another bag because it’s unlikely that one would so easily part with expensive lifting gas. The internal bladder needs to be removed then re-inflated with some other air, and the leak then needs to be patched. Now the entire process needs to be reversed to get the bladder back into its nylon exoskeleton. All in all, what would have taken a few minutes with a conventional design now takes hours.

Mylar

Mylar is typically associated with helium balloons and is an excellent material for that case because of its extremely high air retention. For a purely hobby application where low cost and performance wins the day, it’s a perfect choice. But for any application requiring a reliable and durable solution, mylar won’t sustain typical wear and tear. Mylar tears easily with little to no stretch, and those same tears will rip across the entire balloon in the blink of an eye. For a large airship, this can bring the entire aircraft crashing to the ground immediately. Even something as small of a grain of sand ejected from a machine can puncture the material.

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