Parabolic Arc (or the Incredible Floating Kowalski)

One of the cool things about the Blackhawk (and all its variants) is that it can pull “negative G’s”.  A “G Force” is one times the force of gravity.  So, just sitting or standing, we are all experiencing one G.  If a person experiences two G’s (from  acceleration or deceleration, typically), then he or she feels twice the force of gravity.  Zero G’s is when something is weightless or in free-fall.  A negative G is when the force of gravity is coming from a direction opposite from where it typically comes.  So, a stunt pilot flying level upside down is experiencing one negative G.  If I remember correctly (and it was a long time ago), the Blackhawk was capable of 3.7 positive G’s and 1.5 negative G’s.

The real significance of this was the advantage it provided in maneuverability when flying low level while following terrain.  This was especially true when compared to the Huey, which the Blackhawk replaced.  The Huey had a teetering rotor system, which meant that it had two rotor blades which were connected to each other and sort of hung, teeter-totter style, on the mast.  This allowed the rotor disc to tip as needed.  However, if the rotor ever became unloaded (i.e., experienced zero Gs), then that teeter-totter could just flap around at will.  This posed the very serious danger of mast bumping—the root of the blades banging against the mast and snapping it off. Then the helicopter would experience what was called “Blue Blades”.  (One blew this way and the other blew that way).  In other words, the rotors would depart the aircraft (which typically isn’t a good thing.)  So, in a Huey, you always kept positive Gs on the aircraft.

The Huey and its teetering rotor system.

Then along comes the Blackhawk with a completely different rotor system and a limit of negative 1.5 G.  This allowed, for example, a crew to fly fast, hugging the ground, up one side of a ridge and down the other.  When cresting the top, to keep low along the terrain, the nose would have to be pushed over to start the descent on the other side.  This would unload the rotor (the helicopter’s inertia is wanting it to continue upward while the rotor is directing it downward, so the rotors are temporarily not having to support the weight of the helicopter.)  This is the same thing that happens when you’re driving a car quickly over a short, steep hill.  It feels like you’ve gotten airborne for a second and your stomach sort of falls out…you were experiencing less than one G.  (If you hit your head on the roof, you were experiencing a negative G.)

The Blackhawk and its fully-articulated rotor system.

Now, actual negative Gs in and of themselves aren’t really all that fun. All the blood rushes to your head, and there is a danger of “whiting out” or fainting.  

Zero Gs, on the other hand, can be very fun because you’re weightless, literally free-falling for a short period of time.

This can be experienced in a helicopter by quickly (and sort of violently) reducing the collective lever all the way down so there is no lift in the system.  But, autorotative aerodynamics quickly catch up and the helicopters slows down its vertical descent, so this technique, in addition to being violent, doesn’t provide zero g’s for very long.

A much more gentle, effective, and longer way of experiencing zero Gs in an aircraft is by flying a parabolic arc.  This is the same arc that a bullet or cannon ball follows after it leaves the gun barrel.  In fact, (leaving friction with the air aside), as soon as a bullet or cannon ball leaves a barrel, it is no longer accelerating from the gunpowder explosion but is affected only by gravity and follows a parabolic arc up, up, up, until it tips over then goes down, down, down, accelerating toward the earth at gravity’s 9.8 meters per second squared.  The entire time, the ball or bullet is “feeling” completely weightless. (Again, disregarding the effects of friction with the air.)

This same parabolic arc, if done properly, can be flown by an aircraft.  Of course, an aircraft isn’t shot out of a gun and can’t be flown all the way into the ground like a bullet (well, it can–but just once), so the time that it can experience zero Gs is much less.  Nonetheless, astronaut training aircraft like the KC-135 Reduced-Gravity Aircraft (AKA the “Vomit Comet”)  can sustain zero Gs for up to 25 seconds at a time by using a parabolic arc-type maneuver.  (For more info about that, click here.)

Gemini astronauts in the original vomit comet.
Steven Hawking in the modern vomit comet.

It didn’t take long, after transitioning from Hueys to Blackhawks to discover that truly the main benefit of its negative G capabilities wasn’t that it could hug terrain so as to much more effectively evade the enemy, or that in an emergency maneuvering situation there was no danger of mast bumping.  No, the most important thing was that we crews could now have fun experiencing zero Gs when we wanted to!  Yippee!

I’m not sure how I first discovered this.  I can’t remember if it was shown to me by another pilot, or I read about it.  In fact, I think it might even have been demonstrated to us at Ft. Rucker during the transition class to show vividly (especially to those Vietnam vets who had been flying Hueys for years and in whom it was ingrained to never, ever unload the rotor system) that the Blackhawk’s rotors were not going to depart the aircraft when flying negative Gs.  The instructors wanted Blackhawk pilots to understand its increased maneuverability in case it was needed in combat.

I do remember using it very effectively when on a field exercise in Morocco when I was flying Blackhawks for the Army.  This was back in 1986.  We’d often take grunts with us when doing training flights to give them some fun.  A common characteristic of the area in which we were flying was the prevalence of huge mesas.  The desert would be flat for miles and miles until there would be a very steep and deep (often 500-feet deep) valley that would get washed out only during the very infrequent rains.  This cliff-like boundary wasn’t “smooth” but consisted of multiple bumps of land that would project out before falling almost vertically down into the valley. These projections were the mesas.

From the cockpit, we could see these valleys coming up, but the grunts in the back could only see out the side windows.  So what we would typically do (after having all of them remove their helmets and hold them in their laps upside down) is get down right on the deck (our wheels about 10’ from the very flat desert floor) going as fast as we could, which would have been about 170 mph.  Then, we’d head out to the end of one of these mesas, and as soon as we crossed over the cliff’s edge, we would dump power while pushing the hose over until we were experiencing zero Gs (basically flying the backside of the parabolic arc).  All the passengers saw was the ground disappear as the helicopter “fell” over the cliff.  It only lasted perhaps five seconds, but it was invariably enough of an startling and scary experience for many of them to puke in their helmets.  Ah, the good old days!

But by far the best memory I have of pulling zero Gs was when flying Jayhawks (the Coast Guard variant of the Blackhawk) when stationed in Sitka, Alaska.   I can’t for the life of me remember who my copilot was, but the crew in the back consisted of Troy Brevik and Barry Kowalski.   Barry was very squared away (as was Troy), but Barry was always just a bit of a smart ass to me—in a good way—I think because we had the same first name.  Obviously, “Barry” isn’t a very common first name, so there was this sort of connection, mostly unspoken because of the Officer/Enlisted relationship.  Anyway, we occasionally teased each other a bit.

From what I can remember (with some help from Troy, who also vividly remembers this incident), we were returning from an ATON (Aids to Navigation) mission up north at Cape Spencer lighthouse.  

Jayhawk on lighthouse helipad in Alaska

It was a long leg back home, so Barry had broken out his Coast Guard-provided “box lunch” to eat on the way back.  Nothing unusual in that.  Except that I happened to glance back in the cabin and saw that he had decided to sit on the floor, unsecured by any seatbelt or gunner’s belt.1  He was just sitting in the middle of the cabin floor with all his box lunch ingredients spread out around him: a sandwich, a piece of fruit, an (open) bag of chips, a open soda, and a few other items and condiments.

I suddenly saw an amazing opportunity I couldn’t pass up.

However, I needed to let the other crew members know what I was planning so they wouldn’t freak out. Barry was busy playing with his food so hadn’t noticed me looking back, but Troy did, so I gave him a knowing smile and started pulling the checklists down from where they were hanging on their hooks near the ceiling circuit breaker panel.  (When pulling negative Gs, anything that isn’t secured will float for awhile and can end up in peculiar places, so those of us with experience in such things had learned what we needed to secure, and the checklist were always first.)  So, Troy quickly figured out what was coming and tightened his seatbelt.  I then mometarily reconfigured the intercom so that only the copilot could hear me, told him what I was planning, and that I would be taking the controls in a second.  He acknowledged with a nod and a grin.

One quick final look around the cabin to make sure everything was as it should be (especially that the cabin door was secured!!!), then a scan outside to make sure there were no other aircraft or clouds around, then I said over the intercom to the copilot for all to hear, “Do you see that bird?”  He said no, so I said, “I have the controls”. I took the controls then pulled the nose of the helicopter up about 30 degrees (ostensibly to avoid a bird) while pulling an armful of power (the gunpowder part of the parabolic arc).  Then, when our nose had gotten high enough, I gently nosed the aircraft over while reducing power until we were riding that wonderful parabolic arc with its zero G’s.  I looked aft and was presented with one of the funniest sights in my life:  Barry and all his food were floating in mid-air in the middle of the cabin.  He looked completely astonished while desperately trying to grab his soda and chips so they wouldn’t spill everywhere.  I was laughing so hard I could barely recover the helicopter, but as we approached 30 or so degrees nose down, I very gently brought the nose and power back up to set him as softly as possible back down on the cabin floor.  

Luckily, Barry could take it as well as he could give it, and although he was initially a bit peeved about having to clean up the food, he eventually was able to laugh about it. 

  1. Worn by flight mechanics when the door was open for hoisting ↩︎

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