This article was originally published in Fly RC’s January 2016 issue.
By Mike Lee
The one constant question that is asked about sailplane flying is how do you know when you have found a thermal or lift? I’m going to explain that process for you, but first, what is a thermal?
A thermal is quite simply a rising body of air that, for our purposes, is showing you the graphic proof that heat rises. The atmosphere we live within as a guest of Mother Earth is not a uniform mass of mixed gases of predominantly nitrogen, oxygen, carbon dioxide and other gases. It is a constantly moving, swirling, rising and dropping mixture of air bodies reacting to changes in temperature. If it were not for this constant movement, we could not survive as the wind would not be able to blow the air around to allow living organisms to intake the necessary oxygen needed to live. You know … the plants take in carbon dioxide and release oxygen, while the animals intake oxygen and release carbon dioxide, but, let’s stop at the part about the constantly moving air. When a body of air is warmer than the other bodies of air surrounding it, the warmer body will rise above the cooler air. This rising body of air is a thermal.
How does the warm air body form itself? A basic example might be right outside your window as you read this, in the form of a parking area. People like to congregate on the grass because it is soft, comfy and cool. Remember the cool part. Now we go to the parking lot where we find the temperature is typically a bit warmer. The sun heats up the parking lot and helps to form a warm body of air. That body of air is being held down to the ground by the atmosphere above it, until it gets warm enough to push itself past the cooler air, thus forming a thermal.
In the desert flatlands of the Southwestern U.S., it is common to see “dust devils”; miniature versions of a tornado. These occur when two bodies of air end up moving towards each other and collide. The body with the warmer air will try to rise above the other body and the leading edge of warmer mass tries to curl around the cooler mass, creating the rotating column of air we call the dust devil.
On a larger scale, you can have a very large mass of air moving across the ground in a weather or storm front. Think of these as the same action produced by waves hitting the beach. If there is moisture in the large mass, it may be heated as well as physically heavier due to the moisture content. Such a front can rise up to form massive clouds and windy conditions, but not so much as to deter experienced pilots.
These guys know that smaller fronts can come in after clearing a line of mountains and the moving air is deflected by the ground causing the air push upward to give the pilot some very workable thermal weather.
If you ever wish to see what thermals look like, just check out an old lamp fixture called the “Lava Lamp”. This throwback to the 1970’s is back and the rising and falling globs of lava react very similarly to thermals in their movement and formation. Sometimes you see a lone glob move around while at other times you see a column of lava lifting up. Airborne thermals look the same!
So now that we know a little bit about how thermals are formed, how do we get the plane to detect a thermal and how will we know when it does? I’ll start with some ground work. Let me say that any plane can fly in lift given an adequate amount of lift present versus the plane being thermalled. I have held pattern planes in a thermal, as well as many sport, trainers, scale and even a racing model. The first requirement is that the plane is set up correctly. The “correctly” part is different on each plane you fly, but it is the combination of the plane being perfectly balanced and trimmed for as much “hands-off” flight as possible. You want the plane to fly on its own without you correcting it. This allows the plane to show you what is moving in the air around it. Let’s say you have your plane just gliding along, not needing much control movement to maneuver and able to hold a straight line. This is good, as the plane must be able to fly pretty much hands off at low speed. Now just watch the plane fly and look for it to make an un-commanded turn or nose pitch change. It could be a thermal you are flying through.
Most of the time you are flying for thermals, you are looking for the plane to be disturbed by the rising air. In this case, maybe the right wing picked up and caused the plane to bank to the left. The thermal is present on the right and you need to turn right to engage it. Now you are in the sizing phase. You have found the thermal, but don’t know how large it may be in order to stay with it. Try to fly through it again and if you think you have found it, fly a tidy sized figure-eight maneuver. By watching the plane to see if it falls or rises as you pass through, the plane will show you where the edges of the lift may exist and defines the area of lift that you need to work with. Don’t get discouraged during this learning process. It takes a sharp eye and good reflexes to find lift and react to the movement the plane makes when it engages the lift. It takes patience to get tuned into watching the plane closely for that un-commanded movement, but once you catch that first thermal and ride it out, you’re going to get hooked! You are in tune with nature and taking advantage of her power for your flying. Gotta love it!
For those of you who live close to rolling hills and small mountains, slope lift may be your ticket as the air comes up the hillside to literally push your plane upward. A brisk breeze coming up the slope can mean you have some sizzling speed coming your direction on the slope. How much speed can be attained depends on the wind speed and the aircraft. Indeed, a dedicated slope speed model can attain 100 mph for sporting pilots and using a technique known as dynamic soaring, the slope speed becomes ballistic! The current world record is a bit over 500 mph! We will take a closer look at that style of flying in a later issue. It is a mind blowing experience!
In the arena of Discus Launched Gliders (DLG), the object of the sport is to be able to thermal the plane up and out from a ground level hand launch. A typical DLG model weighs in from nine to 12 ounces using three or four servos for dual ailerons, elevator and rudder. The ailerons are normally mixed to provide flaperons for glide path control. These are excellent models to learn efficient thermal flight techniques. I fly mine to train myself how to handle low level lift with a larger, competition model. When flying DLG, you become more sensitive and aware of the air surrounding you. Sure, you can get tired of throwing if you’re not finding thermals, but that’s what drives you to know more about how they form and how to detect them. Watch for signs of lift by observing the trees, tall grass, dust blowing and small birds chasing bugs. If one tree is being blown by the wind and the other trees around it are not moving, you have a thermal. If a field of grass is waving in the wind save for a large spot standing still, you could be looking at lift. If the breeze coming through feels warmer than the ambient air, you have a thermal. Launch into it! Even if you only gain a few seconds longer flight, you’re learning and that’s what is important.
So, now you know a little more about thermals and lift and how to engage the lift to get your model to stay aloft. For those pilots who want to achieve proficiency in non-powered flight, there is an organization known as the League of Silent Flight (LSF) and they have a self-improvement program designed to take you from beginner up to expert skill levels by way of accomplishing certain goals and tasks. As a Level 1 pilot, you might have to perform several flights lasting 10 minutes, while in Level 4 that task grows up to be two hours, non-stop. You also perform slope flight, landing tasks, cross country flying (one of my favorites) and logging contest competition scores. The supreme test is the Level 5 slope task of flying for eight hours! Less than 150 pilots have accomplished this goal and become Level 5 pilots, but when you get there, you will be an expert. Check it out on the web. When you think you have this thermal stuff down, check out the LSF and try the tasks. Stay Aloft!