Truly inexplicable

First off, this is even more of a self-indulgent post than normal, because it largely stems from being a helicopter/rotorcraft enthusiast, so if you’re here looking for bugs or rabid atheism, this one isn’t going to deliver. On the other hand, there are absolutely no pictures of food, cats, or myself attached. And while I am going to talk about the vacation out to North Topsail Beach, it will be very hard to classify this as gloating in any way. Credit that as you will.

Like last year’s trip, the military aircraft could be seen from time to time while we were out there, just not as many this year. North Topsail is bordered to the northeast by New River Inlet, and directly across from that sits a military exercise airstrip; at night, flares can often be seen in that vicinity, and by day one can view the helicopters occasionally doing low passes and ‘insertion’ exercises (for want of a better word – don’t go thinking that I have the terminology correct.) One of the days we were out there, we watched a pair of USMC V-22 Osprey tiltrotor aircraft doing passes on the field.

a pair of V-22 Ospreys banking towards closer approach
Now, I sidetrack a little here, because this photo shows a marker buoy out in the water off New River Inlet, more or less centered in the opening of the inlet but well off in the distance – this was shot at 300mm focal length and cropped tighter from there. Remember when I pointed out the speck of light on the horizon under that crescent moon? I decided to do a little research, and pulled up a boating chart of the inlet. The marker buoy is clearly defined, and I was able to do a few measurements, very rough ones. Essentially, the buoy would have been somewhere around 3.5 kilometers from my position on the beach when I took that moon shot before sunrise. But was it the correct direction?

plot of buoy in New River Inlet, North Topsail Beach NCFor that, I pulled up Stellarium, set my viewing position for the lat/lon coordinates of the spot on the beach, rolled the date back to the timestamp from the photo, and checked the moon position. Everything looked kosher, and put that little light in the photo at almost precisely due east (true, not magnetic.) Comparing that to the chart lines up pretty damn closely – close enough that I would have seen any other lights had this not been the source. Using the section of chart I’ve included here, the buoy is the purple circle, while my viewing position for the crescent moon photo is about where the orange arrow is on the shoreline, lower left; my position for the Osprey photo above was on the southwest side of the inlet right where that line designating the road ends, while the airstrip that I mentioned is outlined in bright green, upper right. For now, I’m going to consider the identification of the light in the crescent moon photo to be confirmed. Don’t ask me why I bothered; this is the kind of thing that I do out of curiosity.

You may have noticed the difference between the teeny little speck in the crescent photo and the distinctive buoy in the Osprey photo – this was because I was shooting at entirely different focal lengths for different purposes between the two. But wait! As I was about to type this explanation out, I started recalling that perhaps I did go in as tight on that light while out on the beach – not that day, but a later one. Started poking through the folders at some of the remarkably plain frames, and sure enough: I have a distinct photo of the buoy taken from the same vantage as the crescent. Could have saved myself a lot of trouble if I’d thought of that earlier.

All right, enough of that, back to the choppers. It’s not immediately apparent from that first photo up there, but the nacelles are rolled forward. Tiltrotors get their moniker from, imagine this, tilting their entire turbines on the end of the wings forward for high-speed, fixed-wing style flight, but they have to tilt back to upright again for hovering and landing (they can also perform a short rolling takeoff with the nacelles tilted slightly forward, producing mostly vertical lift but also some forward thrust.) As the pair of aircraft circled around our vantage and approached the airstrip, the nacelles could be seen to roll back, visible here; the two aircraft have transitioned to different degrees.

pair of V-22 Ospreys approaching landing zone
It goes without saying that the day was hardly clear, and in fact it was occasionally spitting a bit in that halfhearted non-rain that occurs sometimes.

Curious, to me, was that these aircraft were still on the ‘crosswind’ leg of their approach, having to turn another 90° to line up with the airstrip, yet the nacelles were already rolling back. Thinking about it, however, I realize that their stall speed with rotors forward would be about that of a mid-size aircraft, probably better than 80 knots, so as they reduce towards that speed the rotors better be more upright as the lift goes away. Since the Osprey can maneuver in all directions like a standard helicopter with full vertical rotors, they could still maintain a halfway decent approach speed – just not as fast as they could with the turbines rolled forward.

pair of V-22 Osprey on final approach over hidden airstrip
The airstrip was hidden from our view behind the trees, but as you might surmise from this image with the turbines and rotors full vertical, they certainly weren’t pulling a fast approach; as we watched, there was some question that they were even performing a full landing at all, instead just passing over the field. It was pretty sedate.

V-22 Osprey disappearing behind the trees
Eventually, they did drop down and vanish behind the trees – you can just make out the second one peeking out here right before dipping from view. And yes, we would have had a much better view from that little marina, but had no way of getting there. No, it was way too far to take the kayaks, especially given the wicked current of the inlet. Not to mention that, of the six of us on the trip, only two had the faintest interest in observing this closer, and one of those (e.g., not me) had actually traveled by tiltrotor before and so wasn’t that enthusiastic about a closer look.

pair of V-22 Osprey departing airstrip
I had predicted a fairly fast reappearance of the aircraft, thinking they were doing a troop dropping exercise, but I was proved wrong, since it took several minutes for the Ospreys to take off again, and still without any apparent haste. They can transition to forward flight profile pretty quickly, which is a hell of a ride according to several sources, but not this time around. It took better than 90 seconds from their reappearance to even start to roll the nacelles forward, and the pilots were also pretty slow in retracting the landing gear. Certainly not what I expected.

Bored yet? Okay, good! One more.

AH-1W Super Cobra passing overhead
While walking on the beach the previous day, we were treated (or whatever) to a close pass overhead from an AH-1W Super Cobra assault helicopter, more-or-less a predecessor of the well-known AH-64 Apache. The Cobra started operations in Vietnam, a dedicated attack aircraft with a lot of components from the UH-1 Huey; that they’re still in operation, albeit much updated now, says something about their versatility and performance.

There’s a trait here that’s visible if you look very closely, courtesy of the humid conditions of the day. The effects of turbulence and air pressure caused by the rotor path created a very short-lived condensation trail extending from the tips and lasting only for fractions of a second, not even a full revolution of the blades but captured by the shutter speed. Let’s have an enhanced look at it:

Ah-1W Super Cobra with contrast altered to show tip trails sharper
Around the back and underneath the helo, but also around the front, stretch two faint curved lines of condensation, and here we delve into the engineering of helicopters a little. As the rotor spins, the tips are naturally passing through the air faster than the base of the blades, while the entire aircraft is traveling forward (usually) through the air mass. The impact of air resistance and turbulence is strongest at the tips and this is where the greatest amount of noise comes from. But as the aircraft moves forward at greater velocities, the rotor tip that is advancing around in front may start coming close to breaking the sound barrier, which has profound effects on aerodynamics, while the retreating tip comes close to matching the speed of the aircraft in reverse, essentially stopping dead in midair and thus providing no lift at all; this produces a very real limit on the airspeed that any standard helicopter can achieve (and why tiltrotors can greatly exceed this since they are not supported by the rotors in high-speed flight.) Make the rotors longer and the effect becomes worse, but make them shorter and lift is compromised, which requires broader rotors to compensate for, which increases drag and noise, and so on and so on. Helicopters with two main rotors blades like this one produce a significant amount of noise and a distinctive beating sound, but, the type of rotor head that’s necessary for them is lower maintenance and produces far less vibration – there’s a lot of stuff that I’m skipping right now for brevity (not enough, I know, but I’m saying it could be worse.) The design of the rotors, and the articulation thereof, and the turbine power needed to drive them, is a very elaborate and interactive discipline, and it amazes me that it was first successfully hashed out in the 1930s.

But let’s go back to that photo, because it shows something interesting. The condensation path ahead of the aircraft seems very close, because the helicopter is moving forward through the (more-or-less) stationary air, advancing through the trail. And if you look close, the trail behind the helo can be seen against the tail rotor and boom – which is well below the main rotor blade; in fact, because of our view angle here, that trail has to be quite well below the rotor ‘disk.’ Of course, this is a function of the lift that the rotors provide, the downward force against the air that keeps it airborne, so the trails are actually spiraling down from the rotor disk, which we might see better if we viewed the aircraft directly from the side. Bear in mind that typical rotor speeds are in the realm of 300 rpm; you can even see the faint blurring towards the tips of the rotors despite the 1/4000 second shutter speed of this image. So you can take a stab at how little time has passed between the rotor arcing over the tail and when the photo was taken, and know the condensation trail was forced down that far in that period of time. Pretty cool really.

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