Flare Physics

Originally published in Hang Gliding and Paragliding Magazine, August 2014

As pilots we tend to focus a lot of our thoughts on flying. The sad fact is, no matter how epic the flight, all flights end. We’ve all seen that some pilots end their flights with a little more flair than others. Landings are an unavoidable necessity, so we should give them much respect. We talk of various landing techniques and applying them in different ways much like an artist creating a masterpiece. What I’d like to delve into is not the art of landing, but the science. I like science because it’s concrete and repeatable. Science follows rules and knowing these rules helps us land well consistently.

Since we have no choice but to work within the laws of physics, let’s define a few real quickly. I’ll try to keep it short and sweet since I’m no scientist.

  • Inertia is an object’s tendency to resist changes in motion, and it is measured in “mass”. Objects with more mass are more resistant to changes in their velocity- if you’ve ever run out of gas and tried to push you’re your car you know what I mean!
  • Momentum is almost the same thing, but incorporates velocity into things. Momentum is defined as the product of mass times velocity, which mathematically tells us what we already know: it takes a lot of force to stop a heavy object that is moving quickly. It’s harder to stop a bowling ball than a Ping-Pong ball rolling at the same speed.
  • An object in motion will remain in motion until acted on by an outside force.
  • Our gliders and bodies disturb the air we fly through, and we end up pulling some of that air along behind us. Air that is being pulled behind an object is called “entrained” air. The force required to continually pull this entrained air is part of what we refer to as “drag”.
  • The amount of drag created is equal to the square of airspeed. Going twice as fast means four-times as much drag. Three-times as fast is nine-times as much drag, and so it goes.
  • Gravity sucks, everything and always. But it’s not fair and sucks things with more mass harder than things with less mass… Think of “weight” as a measure of how hard gravity pulls on something. Although there are some technical differences between weight and mass, they are directly related and for our simplistic purposes we can consider them the same.
  • A stall is the separation of airflow over the top surface of the wing, which is the result of too high an angle of attack. This can occur at any airspeed.
  • Energy comes in two states. There’s potential energy, or stored energy- in hang gliding this is altitude. And then there’s kinetic energy, or energy in motion- in hang gliding this is airspeed.
  • Energy can be transferred from one kind to another, but energy cannot be created or removed entirely. We’ll come back to why this is so important…

The goal of every landing is to get both our altitude above the ground and our groundspeed to zero. Our landing approach determines where this occurs, but it is our FLARE that actually accomplishes this- so that is where we will focus.

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Landing on a hot day in Utah, Tom Galvin flares. Flaring hard would rotate the glider but not fully stop his forward momentum. Flaring smoothly allows the glider to climb and transfer the energy of that momentum away.

We all know we should be flying our whole approach with additional airspeed, carrying that airspeed right to ground level. In physics terms, what we are doing is taking potential energy- altitude- and transferring it into kinetic energy- airspeed. As we round out at ground level we have a “ground skim” period, where we are using that kinetic energy to offset the force of gravity. Without this ground skim phase, our flare is trying to offset the force of our forward momentum AND gravity’s pull, and that’s a lot to ask of even the most perfect flare. Since gravity doesn’t stop sucking, it’s best to eliminate all descending momentum well before we try to stop our forward momentum. While in our ground skim we still have “drag”, due in part from the entrained air we pull behind us, so kinetic energy gradually decreases. As we’re bleeding energy, we’re approaching the point where we have to flare before the root of the wing stalls and the nose drops and we whack.

In physics terms, there are actually two very different types of flares. First is the idea of using the wing as an air brake. This is how many people envision our flare working, and in most cases it’s effective- but this flare will never produce a no-step landing without a headwind. For this flare (I call it the “air brake flare), we push out very quickly, and the glider rotates nose-up with very little direction change taking place. Now we have a nose-up wing traveling horizontally through the air and creating a lot of drag, so kinetic energy (airspeed) drops off very quickly. The main issue with this, if we want our ground speed to be zero, is that the drag created by the wing is equal to the square of our airspeed. By the time we’re going half-as-fast, the wing is making four-times less drag. Once we’re going one-third as fast, the wing is making nine-times less drag. Because the amount of drag drops off much faster than our airspeed, we’ll never reach zero airspeed without having to run. If we have a headwind we might reach zero groundspeed, which is what we really care about when landing.

This style of flare works best on lower performing gliders, which weigh less, and therefore carry less momentum (less force is needed to stop them). Lower performing gliders also have a greater sail area and therefore create more drag when the wing is used as an air brake. The “air brake flare” is NOT particularly effective in no wind or at high altitude, and it’s especially ineffective on high performance gliders that weigh more- which means more momentum so they’re harder to stop- and less surface area (doesn’t create as much drag to slow you down). Another issue with this flare is that when the wing stalls the mass of entrained air behind us, which was traveling with us, is now detached from our wing and tries to continue traveling past us. If you’ve ever landed in no wind and felt like you got pushed from behind just after you flared, that was your entrained air slamming into you!

The other type of flare embraces the physics law that states energy can be transferred but not eliminated. If it’s physically impossible to eliminate our forward kinetic energy, we need to get creative and transfer it somewhere else. If the push-out in our flare is slower so that the glider transfers kinetic energy (airspeed) into potential energy (altitude) as efficiently as possible, we take our forward momentum and turn it into vertical/climbing momentum. Since gravity sucks, we don’t climb for long before coming back down. I’ve taken to naming this type of flare the “¼ loop” flare; it’s an excellent way of visualizing it, albeit not exactly what we are performing. It’s not entirely accurate because, as we climb, our airspeed decreases and as the wing begins to stall it’s losing it’s ability to create lift and continue changing our direction of travel. Unless you flared really early, and climbed really high, we’re not really going to be climbing vertically- so it’s not really ¼ of a loop (again that’s just a great way to visualize it). Since we’re not climbing perfectly vertically, we haven’t eliminated ALL of our forward momentum yet. But since we climbed a little, we’re not done and on the ground yet, either! As gravity sucks the last of our upward momentum and we begin to descend down toward the ground again, our glider is forced into a tail slide. Up high this would be terrifying and we’d surely throw our chute, but it’s ideal in this situation because the tail sliding glider creates a force pulling against our forward direction of travel. One other thing that makes this type of flare so effective is that, when the wing stalls and separates from the entrained air we’re pulling behind us, that air has an upward component. I’m not really sure if the upward component of that air helps us land softly, or if it just passes over us rather than slam into us… but either way climbing a little before separating from that mass of entrained air is very helpful.

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Flaring a little early is no big deal, just slow your flare motion even more so that you end your climb with the nose up, and hold it. Unless you’re ridiculously high, physics guarantees you’ll come down softly. This landing was super-soft!

This “¼ loop flare” is particularly effective on high performance gliders, because they efficiently transfer that kinetic energy back into potential energy as we climb a little in our flare. High performance gliders also weigh more, which means gravity sucks them harder, which makes it hard to climb to an uncomfortable height during our flare (although it can be done). If there is any drawback to the ¼ loop flare, it is that it requires the glider to be balanced prior to your flare. If the wings aren’t level, or there is any amount of yaw oscillation going on, you’re probably better off combining the air brake flare with running it out. My favorite thing about the ¼ loop flare is that, because the push-out motion is done pretty slowly, it gives my brain time to process how the glider is reacting to my push-out. If I start my flare and climb quickly, I know I had a little too much energy, and I can hold that bar position until I stop climbing before finishing my flare (commonly called the “two-stage flare”, which works great). I feel this flare technique eases the pressure of perfectly timing the flare, because if we’re a little early on the flare the physics just work that much better, and we have time to adjust the pitch-rate of our flare so we don’t zoom way up high. I will point out that waiting too long to flare means we do not have enough energy to climb a little, so flaring a touch early is much better than being even a tiny bit late. The same can be said for the “air brake flare”- since that is a very fast nose-up rotation of the glider, and we barely change direction at all, if you’re a little early it’s not so bad because the wing is making a lot more drag at that higher airspeed. The air brake flare can really save your bacon if you misjudged your approach and NEED to flare early because you’ve run out of LZ or there is an obstacle in your way. And just like the ¼ loop flare, if you are too late with the air brake flare the wing isn’t making much drag to slow you down, so you’d better plan on running (running more and faster than usual, since the air brake flare almost always requires running already).

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We can see the ground is rushing by horizontally as master pilot Paul Voight flies through his “ground skim” waiting to flare.

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As he starts his flare, we can see he still had significant ground speed. He times his flare so he has enough kinetic energy to climb just a little, and flares smoothly so that the glider transfers that energy efficiently.

 

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At the top of his “¼ loop” flare, we can see that the glider did climb a little, but more importantly we can see that he now has little or no forward momentum.

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Voila! Paul Voight has used the laws of physics to perform a no-step landing in no-wind!

Looking at the bigger picture, all of the landing techniques we talk about are just different ways of accomplishing a safe return to Earth. It makes little difference to our wing or to gravity which techniques we select, or even how well we execute them. Our fate is in how well we apply and abide by the laws of physics, and it is these laws that determine the outcome of our landings. By understanding the big picture of what we must do, we can truly see what we need to accomplish. We need to approach the ground with extra airspeed so that we can round out and have a “ground skim” phase so that we only have horizontal momentum. Unless we have a strong headwind, we want to flare smoothly and with enough kinetic energy (airspeed) to climb just a little. In the ¼ loop flare it is the slower flare motion and the little bit of climb that really makes the physics work toward a complete stop and no-step landing. And because this is science we’re talking about, it works every time! We might need to alter the inputs or techniques we use from one landing to the next, but following the laws of physics will result in soft landings every time!

Now that we better understand how to flare, we can land with more flair. Go out, get high, fly far, and stick those landings!

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A late flare is a tough situation either way: not enough energy to climb for a “¼ loop”, and not enough airspeed to make enough drag for the “air brake” method. If you wait too long it’s physically impossible to achieve a full-stop landing.

5 thoughts on “Flare Physics

    1. RyanVoight

      Induced drag is from a wing that’s creating lift. A fully stalled (air-brake flared) wing is just parasitic drag

       
      Reply
  1. Daniel Moser

    Not true.. Even a fully stalled wing has a positive lift coefficient, and is therefore producing a lot of induced drag.

     
    Reply
    1. RyanVoight

      Does any of this debate change how we should land our gliders? I’m all for further learning/understanding, and I’m certainly no aerodynamicist… But in the big picture what are you saying?

       
      Reply
      1. Daniel Moser

        No, not at all!.. I think your advice is quite good.. and your landing skills are the best !! Better than mine, I’m not ashamed to admit! 😉 It’s just your explanation of the physics involved that is lacking, and I’m sorry if it comes across as nit-picking .. that’s not my intention, I assure you. Having studied physics & aerodynamics nearly all my adult life (and that’s a loooong time), I have learned a few things that I hope are worthwhile sharing with others.. the “back side of the power curve,” as airplane pilots know it, is the low airspeed regime where induced drag is dominant. The early standard Rogallo wings illustrated this beautifully.. you could “parachute” the wing to a small landing spot very reliably by using high induced drag at low speed… sometimes called “stalled” but it is still producing a lot of lift and induced drag.

         
        Reply

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