Drone or blimp: what the difference is really made of

Before 2013 a camera at two hundred feet meant a crane, a helicopter, a very long mast or a bag of gas. The first mass market camera multirotors arrived that year, the regulatory framework for commercial flight in the United States followed in August 2016, and within two seasons the tethered platform had lost the general case.¹ Anyone comparing the two today is not asking which is better. They are asking whether their job is one of the exceptions.

The honest way to run the comparison is criterion by criterion, with the regulatory ceilings stated rather than assumed, because those ceilings differ and they are not the ones most people expect.

Moored balloon ceiling

500 feet above the surface, under 14 CFR Part 101

Small unmanned aircraft ceiling

400 feet above ground, under 14 CFR Part 107

Aerostat endurance

Hours, limited by weather and gas loss

Multirotor endurance

Typically 20 to 45 minutes per battery

The comparison, criterion by criterion

Two rows in that table are worth expanding, because they are where most of the practical difference lives.

Placed against flown

A multirotor is flown. It holds a coordinate, returns to it after a gust, and can repeat the same position on a later visit closely enough that two images taken months apart overlay. That repeatability is the reason it took over progress documentation and survey work almost entirely. A tethered aerostat is placed: its height comes from the winch, its bearing from where the anchor is, and its actual position from the wind. Move the picture and you move the ground station.

One machine goes where you tell it. The other goes where the wind agrees to leave it, and then stays there all afternoon.

The trade in a sentence

Endurance, and what it is worth

Battery endurance is not usually the constraint people imagine. A twenty five minute flight is ample for a set of stills, and swapping batteries costs a minute. Endurance matters when the subject, not the photographer, sets the timing: a ceremony that starts late, a concrete pour that runs long, a weather window you are waiting for, a wildlife observation that has to be uninterrupted. In those cases a platform that can sit at two hundred feet for four hours without a single interruption is doing something a multirotor genuinely cannot.

That is also the reason large tethered aerostats remain in service for surveillance and communications relay, where dwell time is the entire point and no one cares that the platform cannot move.² Camera work simply inherits a small version of the same argument.

The rules are not symmetrical

It surprises people that the balloon has the higher ceiling. Part 101 caps a moored balloon or kite at five hundred feet above the surface, while Part 107 caps a small unmanned aircraft at four hundred feet above ground level, with the well known exception that it may go four hundred feet above a structure when it stays within four hundred feet of that structure.³

The asymmetry runs the other way on everything else. A drone operator holds a remote pilot certificate and can obtain airspace authorisation for controlled airspace, often in minutes, through the automated system. A tethered crew has no equivalent: Part 101 keeps them well clear of airports, requires advance notice to air traffic control above a certain height, and asks for pennants on the line and a device that deflates the balloon if it escapes. The paperwork is smaller and the geographic restrictions are larger. Both regimes are set out on the airspace page.

When the tethered platform is still the right answer

• the camera has to stay in one place for hours rather than minutes;
• the payload is heavier than a small airframe will lift, or needs to hang absolutely still;
• the exposure is long, at dusk or at night, and propeller vibration is unacceptable;
• the venue is indoors, or the subject is noise sensitive;
• the position wanted is a fixed one that the crew can reach on foot, and precision matters less than dwell.

And when it is not: anything that needs to be repeated on a schedule from an exact coordinate, anything on a site with no ground room, anything on a windy day, anything with a two hour window including travel. Which, honestly, is most commissions. That is what the market decided, and the decision was correct.⁴