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More on Speed is Life
Date:2013-2-26    Publisher:本站原创

More on Speed is Life

Hang Gliding & Paragliding Magazine
August 2006

By Brad Spencer


When I submitted Speed Is Life for the June issue of Hang Gliding & Paragliding magazine, I realized that some one (probably with a degree in physics) would take me to task on the equation I included. Fair enough and I?ll take the heat for that, but Mike?s probably doing the same thing (in his article in the August issue). I don?t know the specific data he used, but if the coefficients of lift and drag are not included (along with other factors), his graph is equally misleading. It is not a simple physics equation to plug into a computer and spit out a graph, it?s an aerodynamics problem. I will grant you that the differences in altitude lost during recovery will not be very much either way (diving for speed first versus going immediately to maximum lift angle of attack), but it?s my suggested recovery technique that will more likely save your hide, and Mike concurs with that. So my question is: Why the long-winded discourse to prove me wrong, only to say I?m right?

I agree that control is more important than speed. And yes, I agree that flying a ?modern flex-wing hang glider? with too much speed in some other situations could be a problem ? it?s that way with any airplane. But trying to gain too much speed in my low-altitude scenario will not be a factor because the impact will slow you right back down. To say control is life is just pabulum. Control is like the body?s brain, and speed the blood. The body can live without the brain ? my Yorkshire Terrier proves that every stinking day. Without blood, you die. Therefore, blood is life, speed is life.

Mike?s statement that ?you can?t reduce the radius by speeding up? is true based on my poorly chosen Newtonian equation (and I said it was incomplete), but Newton never flew a hang glider nor a Phantom. I never said you can reduce your turn radius by speeding up?okay, maybe in a round-about way I did because ? yes, you can! ? and I?ll prove it in a moment. My statement was that you increased your "g" available by speeding up. Remember, it is the "g," or as Mike correctly calls it, "acceleration," that turns the glider, not the speed. Speed gives the wing the ability to create the lift used to create the g used to create the turn.

For example, a glider at 10 mph with an average-sized pilot will not produce even 1 g for unaccelerated (or normal) flight, much less for a turn, and will therefore not only have a huge turn radius (if one at all) but must also be in a big descent to maintain any resemblance of flight. If you speed the glider up to around 20 mph, you should have around three or more g?s available and will be able to outturn the slower glider. Increase to 25 mph, and you might have five g?s. How much altitude would you have to trade off in order to gain the speed to make a four-g 180-degree turn? Probably a lot less than you would lose in an attempt to fly around at 10 mph.

Your best turn is where your airspeed allows for the maximum g your wing can produce (usually around four to six g?s) at the maximum lift AOA, without causing structural fatigue or failure; this is called your "cornering speed" (see the graph). If you accelerate more than that, you risk increasing your turn radius at best and structural failure at worst.

The quickest way to increase your speed to the cornering speed is to reduce your wing?s angle of attack to zero or nearly so. Why? Because: Any creation of lift creates drag. Drag will slow your progress towards your cornering speed. Pushing to achieve ?maximum lift angle of attack? creates maximum (induced) drag.

 

In this graph, the curved line represents the g available at ?maximum lift angle of attack." You can "fly" at any speed to the right or below the g-available line, just be aware that with any speed greater than the cornering speed (in the circle) you have the ability to overstress the glider. Any speed to the left or above the g-available line represents a stall conditions and is non-flyable.

The difference between ?maximum lift angle of attack? and stall on a hang glider is fractionally small. Any attempt to ?immediately go to maximum lift angle of attack? by all but the most experienced pilot will probably result in a stall, and you would not recover. I?ve seen it happen. Where exactly is this max lift AOA anyway? How do you get to it? Does any instructor in the world teach it, demonstrate it, or even mention it? It would really be insane to try that in a low altitude/airspeed situation.

Mike states: ?The recovery to level flight consumes the least altitude when it is started as soon as possible.?

I have to draw attention to this. There is a problem with Mike?s well-intentioned and conditionally true statement: What you start as soon as possible may not be the right thing to start. A captain in my squadron (later to become a Marine Corps general) told me that almost every emergency checklist should start with ?wind the clock.? (Yes, the Phantoms had wind-up clocks.) Essentially what he was saying was to think about what you are about to do. You almost always have time. Too many emergencies have been greatly compounded by fear and rushing. Instinctually I wanted to push the control bar and fly out of my situation, but that would have been the wrong thing to do and I knew it because of what I had been taught. In the end, you should trust your training, not what you ?might intuitively suspect.?

My method of recovery is, in the given situation, the most idiot-proof. Regain flying speed as quickly as possible by accelerating at near-zero angle of attack (pulling in or at least easing pressure on the control bar), then pushing out to achieve maximum lift angle of attack. Slightly fast or slow then is less of a problem than stall recovery in our scenario.

Mike?s statement that ?it may have limited applicability to hang glider flight? baffles me. I?m not comparing hang gliders to blimps or space capsules. I?m comparing delta-shaped wings to delta-shaped wings. The scale may be quite different, but the formula stays the same. Everything I?ve stated above applies to every airplane I have flown, from Ravens to Phantoms to Cessnas to Stratocruisers.

Thanks for allowing me to ramble on. I really shortened down my stories and comments as much as possible. Remember your training, and be safe.

An afterthought: The "speed is life" phrase came from my big brother, Robert (Bob) Spencer, retired Navy Lt Cmdr. His call sign was Roper. Bob passed away from heart failure shortly after the magazine article came out. He was only 58. I told him about the article, but he was just too weak to read it. I regret not crediting him with the catchphrase. He left me with many other gems that perhaps I can write about later.

 

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