Long Day

By lex, On Tue – May 18, 2004

It’s a strange feature of life in the Navy these days – while the Army and Marines have their hands full on the ground in Iraq, the purely naval mission is complete. For twelve years every deploying carrier battle group knew right where they’d be going, and every air wing knew exactly what threat to train to. Nearly every TACAIR pilot now in the fleet has some measure of combat experience, between Desert Storm, Iraqi Freedom and of course all the brush fires in the Balkans back during the 90’s.

When I first joined the fleet, only the commanders and those more senior had combat experience, dating from Vietnam. They had racks of campaign ribbons (chest candy), air medals, etc. There was a sharp difference in the chest candy between those old enough to have served in Southeast Asia, and those who just missed it. From 1972, cold warriors were always ready, but hardly ever committed. Now even lieutenants junior grade have seen the wolf.

Still, it feels strange to be focusing on the next threat when people are still fighting and dying overseas.

 

Lileks hits it out of the park today. If you haven’t given him a read, please do so. Now . Money graf? K:

This smothering gloom, this suppurating corrosion – this isn’t us. This isn’t who we are. If it is, well, we’re lost, because it contains such potent self-hatred that we’ll shrink from defending ourselves, because what we have built isn’t worth defending. Thanks for the push, al Qaeda! We’ll take it from here.

But it’s not us. It’s some, but they don’t set the national temperament. They can set the mood, but not the character. Yet. This war is ours to lose if we want to.

You want to lose it? Me neither.

Meatball, lineup, AOA.

Had a question in today’s comments:

Thanks for another look into a Naval Aviator’s life. AOA, in the groove & glide path have something to do with the “Ball Call?”

It’s amazing what you take for granted. The things that mean life or death to you are so much jargon to someone else. I sometimes feel silly spelling out our acronyms, explaining fundamental concepts.

Then I remember that most of my readers aren’t naval aviators.

Here’s a pic of an FA-18 rolling “into the groove,” i.e. final approach –

Long Day 1

You can clearly see the line up: centerline in the middle (duh!), ladder lines on the outside. The angle deck, which permits the simultaneous launch and recover of aircraft is clearly pointed to port. So you can see that as the ship travels forward through the ocean, the angle deck moves somewhat to starboard – you have to chase line up a bit. And since the deck is so much shorter than an airfield runway, you have to hawk line up very carefully. And each time you dip your wing to chase line up, you spill a bit of lift, requiring compensating power adjustments on the throttles.

Anyway.

On the port side, amidships, you can see a thin green line. If your eyes are sharp you can also see a yellow blur between the green line. That’s the meatball, or ball, on the Fresnel lens.

Here’s a better look at the lens:

Long Day 2

Although blue in this picture, the vertical column of lights appears yellow in flight – the lens, or “source,” is prismatic: You only see a certain array of lights, depending on your altitude, or glide slope. The horizontal bar of green lights is the reference line, or “datum.” If you’re above glide slope, the yellow ball appears above the datum. If you’re on glide slope, it appears directly between the green horizontal lines. And if you go very low, the ball turns red.

I’ve never seen a red ball in flight. When it gets that low, I add power and close my eyes.

The ball is about 15 feet wide at a half a mile, where the tolerances are greater. At touchdown, it’s 18 inches in width. To touch down on a three degree glide slope with your eyeballs inside a wedge of 18 inches breadth requires a considerable degree of precision.

And sometimes, the deck is moving. Up and down, up and down.

The red lights that ring the source are the wave off lights. If the ball gets red and you don’t add power, these lights start to flash, and you’ll have another opportunity to prove your skills. On the next attempt. In that way, flying the ball is kind of like golf: The worse you are, the more practice you will get.

Finally there is angle of attack, or AOA. Strictly speaking, AOA is a measure of the difference between the relative wind crossing the wing and the wing’s mean chord line.

Isn’t that clear?

But in practice, AOA, when combined with your (continuously adjusted) throttle setting, equals vertical performance, or rate of descent. The FA-18 lands at a rate of descent of about -750 feet per minute. Navy landing gear are stressed to take this shock, which would certainly fold the undercarriage of lesser aircraft. When you’re on speed (on AOA), on glideslope, and on centerline, you’ll catch the number three arresting cable. And as long as you don’t look like a total idiot getting there, you’re half way to an OK pass, a 4.0.

Swept wing aircraft (suitable for supersonic flight – the wing sweep delays the onset of transonic air mass compressibility) have a relatively flat lift over drag curve. The example below is from a straight wing aircraft, with a relatively steep L/D curve:

Long Day 3

You can see that as AOA (the alpha symbol) increases on the left graph’s horizontal axis, the coefficient of lift increases on the vertical axis – up until stall speed.

Next we’ll discuss airspeed versus drag:

Long Day 4

Parasitic drag is a measure of the airframe efficiency – a needle has very low parasitic drag, while a flat plate driven through the air would have a very high parasitic drag. Parasitic drag (blue line) increases more or less linearly as a function of airspeed – it’s the amount of energy required to push your dad’s Nash Rambler, complete with shark grill, at 95 miles an hour.

Induced drag (red line) is drag created in the act of creating lift – the more lift (the higher AOA), the greater the drag. If you’ve ever been driving down the freeway with your hand outside the window in the shape of an air foil, and then gradually raise your fingers (increase the AOA) eventually you’ll reach the point where the wind (drag) forces your hand back rather than up. It rises more sharply as the air foil exceeds its optimally efficient AOA.

The two (parasitic and induced) combine to create total drag (black line). Where the combined curve is the lowest, we find the L/D max point, which is to say, that point at which lift is greatest as compared to total drag. This is where aircraft generally fly their approach speeds, which directly correspond (it should be too obvious by now to state) to optimum AOA. Flying faster than that speed means longer landings, the tendency to float in ground effect and on an aircraft carrier, greater stress on the tail hook and arresting gear. Flying slower than L/D max means that the stall margin is reduced, requiring greater power to maintain the same attitude and rate of descent. This is called being “on the back side of the power curve.”

Which is a bad thing.

So. Anyway.

Back to the lens.

The lens is rolled to provide a different “look” to each of the airwing’s several type of aircraft. The roll angle depends upon the aircraft’s hook-to-eye value – the FA-18, which comes in relatively fast (i.e., at a lower AOA) has a small hook-to-eye-value. The F-14, a much larger (rather more ungainly looking) aircraft, has a much higher hook-to-eye value. If an F-14 pilot is to catch the same three wire that the FA-18 pilot is striving for, his eyeballs must be higher in absolute terms (relative to the hook point) than the FA-18 pilot’s eyes.

And when you “call the ball,” you’re basically declaring that you’ve finished your landing checklist, are within gross weight limits to land, and see the glide slope reference. You’re more or less asking permission to land. “Roger, ball” from the LSO means, “Continue.”

—————————–

And now I am (if possible) more tired of writing about aerodynamics and the aircraft carrier than you are of reading about it. Drop a note in the comments box if you really want more.

You know I’ve got it.

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1 Comment

Filed under Best of Neptunus Lex, by lex, Carroll "Lex" LeFon, Carroll LeFon, Lex, Naval Aviation, Neptunus Lex

One response to “Long Day

  1. Pingback: Index – The Rest of Neptunus Lex | The Lexicans

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