The Magical, Mystical, Full-Rotor-Disk Shot

July 01, 2021  •  Leave a Comment

One of the toughest shots to make in air-to-air photography, when the subject is an airplane with propellers, or a helicopter, is capturing the aircraft's propulsive components such that they visually form an unbroken disk. Prop-driven airplanes and helicopters share the same issues and challenges in this mystical quest, but my specialty is helicopters, so I'll use them to illustrate the why and how of full-disk photography.

(If your eyes start fluttering closed in the middle of the forthcoming, there is a TL;DR near the end — feel free to jump ahead.)

Helicopters are magic, as evidenced by this MD Explorer cruising through Monument Valley, in northern Arizona, on a thin disk of spinning composites. This particular aspect of the magic was revealed with a shutter speed of 1/30 second.

Why Full-Disk Photos?

One reason for creating full-disk photos of helicopters or airplanes is: it looks pretty. Define "pretty?" I'd say it's pretty because a circle is a complete shape, a simple one that doesn't require our brains to figure things out. They are easy to look at and we understand instinctively what the circle represents.

It can imbue a photo of a machine, one that is in actuality madly beating the air into submission, with a sense of calm. In our minds, the "sound" of a rotor system that is forming a smooth disk is a smooth sound. If individual blades are seen, sweeping through just a part of their circular path around the hub, we feel those blades clawing through the air. There's a whomp or growl to seeing individual blades, and that's fine. With a complete disk, there's a whoosh.

Images of aircraft with full-disk rotors, thus, convey a different character to the action, a different emotion. Other elements of an image, that are brought along with the techniques required to achieve these effects, might also be desired, and I'll touch on those later.

Another reason might be: such images are less common because they are more difficult to achieve. Being less common, viewers are less accustomed to seeing them, thus are more interested in seeing them and, as always, keeping a viewer's attention is part of good communications.

Feel Free to Skip the Math

Several factors figure into capturing an image in which the rotor blades are rendered as an unbroken disk. In terms of the aircraft, those factors are the number of blades and how quickly the blades are spinning. Fortunately, an image showing a full disk doesn't require each blade to make an entire revolution — from the moment the camera shutter is opened to the moment it closes, each blade must merely spin far enough to reach the position where the next blade was when the shutter opened.

In general, that means an aircraft with more blades (prop or rotary) is typically easier to capture, full-disk-ish, than one with fewer blades. I've prepared a totally uninspired illustration of the requisite rotational angles for your viewing pleasure. (Note: I am not aware of any helicopter with nine blades, but MT-Propeller is working on a nine-bladed prop for possible electric propulsion aircraft, so you never know…)

This handy illustration would have to be combined with additional data to calculate a shutter speed that would yield full-disk photos. That additional data is: how fast is the rotor system turning? Should you perform those calculations? Um, not really, unless you've got a hankering.

The MD Explorer has a five-blade main rotor, while this Sikorsky S-92 has but four. Notice the slight gaps in the disk occasioned by the blades not having moved quite far enough during the exposure to visually blend into a single circular shape. My camera was set to expose for 50% longer here than for the Explorer photo, 1/20 second, but even that wasn't quite slow enough.

Different helicopter models spin their rotor heads at different speeds, different revolutions per minute (RPM), but no matter which helicopter model you look at, they each spin so the tips of their blades are moving at a speed that is similar to other helicopters. It's an aerodynamics thing that is affected, in large part, by the diameter of the rotor system — the larger the diameter, the slower it must rotate in order to keep the tips at their optimum speed.

Therefore (don't you just hate hearing that word? It's so stuffy), while having more blades in a system would, all other factors being equal, allow higher shutter speeds in rendering a full disk, the helicopters with a larger number of blades also tend to be the ones with larger rotor diameters, which means they turn more slowly. It's like aeronautical engineers are conspiring against us.

To figure out the shutter speed needed to render a full disk for any helicopter, all you have to do is take the rotational speed of the main rotor system, calculate how many degrees of rotation the blades are sweeping per second, then account for the blade-to-blade angle, based on the number of blades (see the uninspired illustration), and … sheesh … I've run out of energy just typing that!

Fortunately, while this number stuff might actually be straightforward, it's not required if you're shooting digitally and can review images on the fly. (Get it? On the fly?) I've not photographed every model of helicopter out there (yet), but if you're zipping along with a helo or prop-driven airplane in your sights, dial your shutter speed down pretty low and pop off a few shots. Review, and adjust your speed accordingly.

That's all there is to it!

Sort of.

A Guimbal Cabri G2 hovering above the ramp at Middle Georgia State University's Eastman Campus allows a simple example of how shutter speed affects the visible sweep of a rotor system.

Why Can't We All Just Get Along … Slowly?

First up, in the challenges, is: slow shutter speeds require a steady camera during the length of time the shutter is open. A venerated rule of thumb for non-blurry photos, when hand-holding a camera, is to shoot no slower than the reciprocal of the focal length of the lens. To wit, in the photos of the Cabri, above, since I am definitely hand-holding the camera, the shutter speeds indicated would pair nicely with a 40 millimeter lens for the image on the left, and with a 13 millimeter lens on the right. However, my zoom lens was not set at those focal lengths. On the left, the lens was at 135 millimeters, on the right it was 165. That venerated rule suggests I should have left the shutter open for less than a tenth of the time I did.

(My article "Shootin' Steady" details how best to keep your camera steady.)

Fortunately, of course, many camera/lens systems have built-in stabilization features. Unfortunately, those systems are optimized for hand-holding a camera on otherwise solid ground, not for countering the ups and downs and buzzes and jiggles of a helicopter in flight. That's why I attach my camera/lens to a hefty, high-power gyroscopic stabilizer, and even that is no guarantee of sharp images at these lackadaisical shutter speeds with long lenses. Why not? Because sometimes the failure is not at my end of the image-making.

Here's a close-up of the full-disk Cabri photo. Notice how the front of the helicopter is reasonably sharp. Now, look at that tail — horrid. Why? Because the aircraft was yawing during the 1/13 of a second that I was making this image. Not my fault, man!

For this segment of the air-to-air mission with the Cabri, I was aided by having both the target aircraft and the chase merely hovering. When both aircraft are pushing along at cruising speeds, there's more airflow over and into the chase aircraft, which makes for more vibrations and perturbations, and that makes capturing full-disk shots much less likely. The solution? Shoot a lot.

How much "a lot?" A lot, a lot!

28 frames, shown as shot, with a single frame (highlighted in blue) yielding a definitely sharp image of an MD Helicopters MD 530F with its high-visibility rotor blades in full disk mode.

Air-to-air images of Virginia Beach, Virginia, Police Department helicopter, in and around Mesa, Arizona.

And here's that winning image. Why so few good ones for all that shooting? If you'll look at that full disk, you can actually see the blades are not just meeting, but are overlapping themselves. My shutter speed, I see in the image metadata, was 1/15 second, yet this aircraft can display a full disk at 1/40. By going even slower, shutter-speed-wise, the aircraft is not just the only sharp thing in the image, the background is not just a little blurry, but very blurry with motion, adding energy and interest to the shot.

Also, in defense of one-out-of-twenty-eight, we were moving along at, probably, 60 knots, and speed always lends itself to more speed bumps, if you catch my meaning. More burbles and jiggles, which cuts down on the success rate. Thus — shoot a lot, a lot.

This Airbus EC135 doesn't have high-viz rotor blades, but they are white with yellow tips, so we can see the disk. Again, shot at 1/15 second, so a little overlap of the blades and a very motion-blurred background.

What Could Go Wrong?

One oddity I noticed in a recent shoot, with another MD 530F, is visible in the following image.

Rather, perhaps I should characterize the oddity as what is not visible: the rotor disk! Some combination of blade color (black?), lighting, and the background have conspired to render all but their roots invisible.

See? I knew it wasn't zipping along on just rotorblade roots — we can see the yellow tips arcing against the water (though only barely against that bit of land on the right). I saw the (in)visible problem in that first photo when I checked what I was capturing while we were flying along, so I didn't bother further pursuing the full disk against that background.

What that lesson teaches is: sometimes it doesn't work out.

Another issue that can arise, in the confluence of camera and background, is dirt. What dirt, you ask? Camera dirt, I reply.

On the left is a screen shot of the image as I was converting it from its RAW state to a TIFF. Each arrowhead is pointing to a speck of dirt that had to be cleaned off the image. On the right is the cleaned version.

I was shooting at 1/50 second so, even though this Airbus AS350 doesn't exhibit a full rotor disk, to achieve the proper exposure my camera constricted the iris in the lens to f/22. Also called the aperture, a setting of f/22 is great for putting as much of a scene as possible, from near to far, in focus. Cool.

Not cool is how a tiny aperture (f/22 is tiny) also brings into visibility the tiny flecks of dirt that can end up on the sensor inside your digital camera. And if the image contains areas of little detail, like a blank sky or snow or a smooth lake, those areas are prone to show the need for a little housecleaning.

To forestall collecting such flecks, you would avoid swapping lenses — say, from a wide-angle to a long zoom, then to some intermediate lens, perhaps to an extra-long one, then back to the wide-angle — especially in environments that might have swirling dust. Or smoke. Or ash. This photo, however, was made near the end of a week of traveling the western United States, capturing images of aircraft involved in firefighting. I shot thousands of frames using a variety of lenses, swapped on and off 3 different cameras, which offered numerous opportunities for dust/smoke/ash to wander into the sensor area.

Even if you start with a clean sensor, swapping lenses in a helicopter can expose the sensor to new flecks, and it's not unusual to be swapping lenses during air-to-air missions, in the quest for a variety of compositions. High shutter speeds are matched with large apertures, where sensor dirt might be unnoticeable. But when you slow the shutter speed on a camera, for proper exposure the aperture must constrict, and when it constricts … well, fleck happens.

In and around Rotors of the Rockies of Broomfield, just outside Denver.

Another consequence of a small aperture enabling things near and far to be in focus, is seeing things in focus, near and far.

Wait! Is that a bad thing? It can, at least, be a less-than-great thing.

In this shot of a Schweizer 300C, I got the full disk (thanks to the white tips), but the background is also a bit distractingly in focus, especially along the right half of the image. It's not terrible, but whereas the photo of the blue MD 530F zipping over the green field has motion blur helping visually separate the aircraft from the scenery, this 300C is much farther from its scenery, and we are viewing the aircraft more from its nose, so that scenery isn't so much zipping by as it is, kinda, just sitting there in reasonable focus.

That doesn't preclude this from being a useful image, but just know that not all full-disk photos are the same. In this image, I can almost imagine the Schweizer is resting its left skid on the rocky ridge.

Enough with the lecture. Let's wrap this thing up!

What We Learned (AKA TL;DR)

Images of aircraft with full disks, whether of an airplane's or a helicopter's, are possible when the shutter speed of the camera is slow enough such that the propeller/rotor blades move far enough during the exposure time that each blade visually merges with the next.

The maximum shutter speed at which that occurs depends on the number of blades and the speed at which the blade system is turning.

Rotor systems with more blades tend to allow faster shutter speeds while still forming a single, visual, disk. Rotor systems that are larger in diameter tend to turn more slowly, requiring slower shutter speeds. Simple math can be used to calculate the required shutter speed for each model of helicopter or airplane.

My experience with helicopters, so far, shows shutter speeds of between 1/10 second and 1/40 second are required to achieve a full disk. Skip the math and start in that range, check the results, adjust as necessary, and shoot a lot, a lot.

Practice good shootmanship regarding keeping a stable camera, as the slow shutter speeds easily produce images blurred by camera motion (and sometimes even subject motion, though there's basically nothing you can do about the latter).

Bonus Images

C-2A(R) at El CentroC-2A(R) at El CentroPhoto call at NAF El Centro

I figured I'd throw in one image that shows both an airplane and a helicopter, so I can at least send any prop-driven-airplane photographers on the right path. The shutter speed for this image is 1/80 second. As would be expected from all of the writing up above, the main rotor blades of the Bell AH-1Z in the background are blurred, but still distinctively, individually, visible. The Grumman C-2A(R) Greyhound in the foreground shows blades that are easily overlapping other blades.

My point is, shooting airplanes with full-disk can be accomplished with much higher shutter speeds. Just look at this next shot:

T-45C on or above NAF El CentroT-45C on or above NAF El CentroPhoto call at NAF El Centro

That same Greyhound, landing at Naval Air Facility El Centro (you can see the burned-rubber smoke where his main gear touched down), and my shutter speed was 1/160 second. That is my go-to shutter speed for photographing helicopters such that their main rotor blades are obviously in motion; for this C-2, the blades have made a delightful disk o' gray.

An Airbus H145 at 1/160 second. Plenty of blade blur, but nowhere near filling out a full disk.

'Nuf said.


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