Sunday, February 12, 2012

Time for Essence

Finesse is without dimension, a timeless ratio.

Essence is a condensation of many quantities. We use essence, departing full, finding little enroute, and arriving depleted. Essence, converted, changes with time.

The glider pilot finds the day full of lift, rising air adding essence with ease. At sunset, or perhaps before, the lift ends. Playing far from home, the pilot's essence is spent deciding that it is time to return lest this essence evaporate.

The pilot's sense of time is not regular. A crisis passes in a second by the clock, in a week by the mind. What does this mean for essence?

When the crisis takes a long time, the craft seems to slow, perhaps hover. Height stays the same but speed goes to zero, and so it seems less essence is burned. Slowing time requires
energy from the pilot, so his essence is burned, not the craft's.

When events rush by, the pilot burns more and more personal essence but may never be able to think as fast as the craft is moving. The pilot's tanks may empty soon.

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Friday, October 28, 2011

Leveling Off

[Another few words from my work on using essence and finesse to think about flying]

Levelland, Texas. The Llano Estacado. Cattle, oil, essence for pilot and craft. Cruising above the Caprock. Easy.

Start on the ground. Fuel, noise, sublimation.

Climb. Speed, check. Vertical speed, check. Chemical essence turns to height. The textbooks say nothing is changing, but the ground falls away, the air thins. The needles slip from truth
to lies.

Now make it all stop. But how? Push the nose down, lose finesse? Pull the throttle back, save essence? Both must change.

As height increase, speed is low. As speed is low, height increases.

But height stops growing. So speed must grow. Change of finesse. Gently push the nose to level.

Now speed is growing. Drag grows, not lift. Continue and finesse will vanish.

Level, using more essence robs the craft of finesse. A double dilemma. Everything we have worked for, scrimped for, sacrificed for, thrown away.

My candle burns at both ends.
It will not last the night.

It will not last to shed a lovely light.

Pull the throttle back.

[poem from Edna St.-Vincent Millay]

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Saturday, October 8, 2011

Finessing Failure

Engines fail, and pilots since the Vickers Vimy have worked hard to handle this safely.

A standard part of multiengine training is the drag demo. You pick a reasonable airspeed and set the power to fly level. Then, put the landing gear down; after a few transients, the airplane settles into a steady descent rate. "Write that down: gear down is 500 feet per minute," or whatever.

After you do the same thing with flaps, you go on to see what happens with a windmilling propeller, and finally with a windmilling propeller in the zero-sideslip configuration. (In a multiengine airplane with asymmetric thrust, flying with the wings level amounts to a mild slip, as a yaw string will clearly demonstrate.)

The student gets a lot of good knowledge out of this demonstration: windmilling propellers produce a lot of drag (in most multis it is about the same as the landing gear), and zero-sideslip really improves performance.

Too bad we're teaching the wrong thing.

For the past little while I have been rethinking all of the problems of aerodynamics from the point of view of two parameters, essence and finesse. (I've written about this here and here, among other places.) Briefly, essence is composed of all of the energy (potential, kinetic, chemical, intellectual, emotional) available to the airplane. Finesse is simpler; it's simply the glide ratio at the current speed, something a glider pilot learns from the polar.

So what does a multiengine pilot faced with a failed engine need to know? The descent rate is in feet per minute, which might be useful if the pilot wants to be able to stay aloft until, say, the earthquake ends. But that's not usually the concern; the concern is Can I make it to the airport?, and the proper number to know is measured in feet per foot; it's finesse.

The thing is that finesse is quite easy to calculate, even among those who don't like to calculate. The key fact involved is that 1 knot is very close to being 100 feet per minute, so the vertical speed indicator pointing to 600fpm is also pointing to 6 knots. So, to find the finesse, divide the airspeed by the VSI reading. Done. You can refine this by considering true airspeed, but this way your estimated glide ratio is lower, ie, more conservative.

Ground speed is another matter, so if you want the best effective finesse divide GPS groundspeed by the VIS reading.

It is worth remembering that a 3 degree descent angle is a 20:1 glide ration; that's the same fact as the fact that the target descent rate for an ILS approach is 5 times the groundspeed.

The essence of an engine failure is more complex, no matter how many engines there are. One thing is sure: panic makes no essence available, which is why regular practice is essential.

One other thing: with an engine failure the chemical energy -- fuel -- that the airplane carries become ballast, ballast that burns. Be careful.

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Sunday, September 11, 2011

Finding Finesse

[This is another short essay exploring the roles of essence and finesse in aeronautics. Remember that "essence" is largely energy, while "finesse" is lift over drag.]

Asking how the pilot sees finesse evolving starts with asking how the pilot sees finesse.

Seeing finesse is easiest when the ship has no power. Keep your speed steady. Pick a bug on the windshield, and make a mental note of what it hid on the ground. Say it's a haystack; focus on the haystack. If the haystack falls below the bug, your glide will go beyond, and if the haystack moves above the bug, your glide will fall short.

Keep your speed steady. If you will be short, pick another dead bug further up the windshield and note the shrub that it obscures. Watch the shrub. See if it goes up or down. Pick an bug on the opposite side, and play the game again, hoping for a smooth field that moves neither up nor down.

Finesse takes you to the spot that does not move.

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Wednesday, August 10, 2011

More on Essence

[I have been working with the concept of essence, which encompasses all of the forms of energy that go into flying.]

Try not to take a side in this debate: does an airplane’s elevator control speed, or does it control height? Both sides have strong arguments, and neither side will ever be convinced by the other. The end of the argument is the passion of everyone involved.

Now sweep the question aside. Elevator has no effect on essence, at least for a while. By thinking – and thinking is part of essence – the question no longer matters, although the passion should still remain. Elevator can only change our finesse.

Sweep the question aside again. Why do we care about speed and height? We do not; these are surrogates for our true goal. We really aim to follow a certain path. Focus on your goal.

Forethought adds essence to our craft, both in the form of anticipation and in the form of fuel. We believe that we have the essence to complete the mission, and must manage its conversion from chemical or atmospheric energy to the kind of energy – height or speed – that we desire.
Too little dynamic energy – too little height, or too little speed, or too little of both – means taking some chemical or atmospheric energy from our stores. So open the throttle.

Some cars store energy from braking, but we have not figured out how to do this in airplanes. Both slowing down and going down destroy our essence. The rumble when the spoilers or landing gear extend carries essence from the airplane into the passenger’s brain, or even the pilot’s brain, and we have yet to think of a way to use that energy. “Welcome to Santa Monica,” the large sign reads, “Please fly quietly.” Preserve your essence: approach a little high. Now you have too much essence. So close the throttle.

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Wednesday, August 3, 2011


Essence is a funny word. It is existence itself - esse means "to be" in Latin - and
touches the core of its object. Over time we have refined its usage, adding perfumes and foods - the essential oils. France goes further; there, airplanes (and cars, and trucks) are powered by essence.

Thus fuel becomes the essence of flying. Sailplane pilots roar in protest, but theirs is a narrow view. Fuel stores energy, but only of a particular kind, and when lacking we find a different source.

Energy - whether from fuel or the brain - keeps us aloft, moving forward, but not yet feeling good. The finishing touch - the finesse - completes the cycle.

Finesse is the end of flying, not the perfect landing but the very purpose. Finesse describes the wing, in a particular way. The usage has not taken off in English, but in French it describes the performance of the wing.

Energy and the wing: that's flying.

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Thursday, March 17, 2011

Flying with Finesse

One of my students suggested a book I'd never read, Henk Tennekes The Simple Science of Flight (MIT Press, 1996). My heart sank a little when he lent me his copy. "This is a book I'd like to write," I sighed.

Tennekes has the audacity to apply the basics of Aeronautical Engineering to all flying creatures, "from insects to jumbo jets." The discussion is clear and gentle. He puts all of the material together -- and I mean all of the material. For example, study The Great Gliding Diagram, which shows the polar curves for 10 flying objects: the cabbage white, the Gossamer Albatross, a typical sailplane, a swift (the bird, not the Globe-Temco beauty), a real albatross, a budgerigar, an ultralight, the Fokker Friendship, a pheasant, and the Boeing 747. You can compare the performance requirements directly, and there are many interesting consequences about wing loading, wing shape, power requirements, and the like.

Apparently the French call "L/D," the ratio of lift to drag, the finesse. I will now use this term forever. By the way, we owe the concept of the polar graph that shows vertical speed versus horizontal speed to another French engineer, Gustave Eiffel, of "Tour d'Eiffel" fame.

My current aeronautical thinking is that everything depends on two parameters: energy, which I wrote about in this post, and L/D, which I wrote about here. Those posts are just the beginning, of course, hence my idea to write a book explaining every maneuver and performance calculation in terms of these two.

But it would be a nerdy book, full of equations and tables. Tennekes does not shy away from equations (Stephen Hawking claims that every equation reduces the sales of a book by a factor of two), but his gentle prose and simple ink drawings really evoke the beauty of these simple ideas. Perhaps with Tennekes's inspiration I can write something with a little more finesse, too.

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