Last month we discussed how to recognize when your sailplane blundered or knowingly
was flown into what you were looking for, a thermal. This month we will discuss
some of the techniques used to fly your sailplane in a thermal. Generally, what
one wants to do when flying in a thermal is to maximize the altitude gain for
the time spent in the thermal.
The first and most obvious factor that affects the altitude gained is to fly
smoothly. It is very hard for the sailplane to gain any altitude when the aircraft
is either stalling or flopping around almost out of control. So, guys the first
thing to learn is how to make flat, controlled, tight turns. This just requires
practice or time on the sticks. You also may, if your radio has the function,
set up your controls so that low-rate on the dual-rate switch gives you less
control surface movement, which makes it easier to fly smoothly. Warning, remember
when you are in low-rate, it can lead to a serious situation when on landing
approach your sailplane rolls violently the wrong way and you do not have enough
control to counter act the roll. Dual-rates on the control surfaces can also
be used to help keep your sailplane in one of those really turbulent thermals
that is always trying to spit out your aircraft. I have put my sailplane in
this type of thermal before and found it necessary to use high-rates to keep
control of the aircraft and stay in the thermal.
You may have noticed a large group of the flyers at the field are flying the
“full house” or multi-function type of sailplane. One reason for
flying this type of sailplane is the ability to change the camber of the wing.
I will not discuss the finer points of camber of a wing here, which is a subject
all to its self. Suffice it to say that increasing the camber of a wing also
increases the lift of the wing and increased lift means more altitude gain.
This is accomplished on the full house sailplanes by dropping the flaps slightly
or possibly even the ailerons as well when in a thermal. I personally have my
Vision radio programmed to give me full span trailing edge drop with up elevator
and also on a sliding side lever. The drop with the elevator allows the sailplane
to turn tighter without losing altitude, while the side lever drop seems to
help slow the flying speed and sink rate of the aircraft. This has two effects
when used properly, it increases the altitude gained in the thermal by decreasing
the sink rate of the aircraft and allows one to keep the aircraft in those smaller
thermals as they drift down wind.
“Drifting down wind," is another subject we need to discuss. Far
too many times I have watched a neophyte flyer find a thermal and continue to
circle in the same spot over the field. If you have heard of “The Old
Buzzard” he uses the analogy of the atmosphere being a river of air. This
means that thermals drift with the wind, which also means that for your sailplane
to stay in the thermal it must drift down wind at the same speed as the thermal.
Since you are standing at a fixed point, what is a circle to the sailplane,
when the wind is blowing at all appears to be an ellipse to you. At times it
may be necessary to actually turn the aircraft down wind and fly down wind in
order to catch-up to the thermal. You may ask how elongated the ellipse needs
to be or how far down wind you need to fly. I cannot say, again it just takes
practice and experience to get a feel for how rapidly the thermal is drifting
that day. To complicate it even more sometimes it seems as though different
thermals drift at different speeds on the same day when the wind seems to be
the same.
Have you ever heard one of your fellow flyers say, “I really cored that
one,” or something similar? Well, what they mean is that they were able
to find what was probably the center of that thermal. The center of a thermal
is considered the most rapidly rising section. I usually try to find it by watching
my sailplane for how quickly it gains altitude as it circles. If on one side
of the turn the aircraft appears to gain more altitude than the other, I elongate
the next circle in that direction. By doing this I shift the circle of the aircraft
towards what may be the true center of the thermal. Quite often you can locate
a much stronger section of the thermal this way and gain more altitude than
if you had just circled and drifted downwind with what you originally found.
Also, sometimes if you do not over control your sailplane and force it in a
direction away from the thermal, the air rushing into the thermal will cause
your sailplane to center-up in the thermal naturally. For this to happen though
your sailplane must be properly trimmed so that it responds to the air currents
around it, this usually means that the center of gravity of the sailplane needs
to be as far aft as you can safely afford it to be.
Well, that wraps-up the last planned segment of the articles I had planned to
write. I hope that they were helpful to those of you that wanted to read them.
Also, I hope that they gave you some ideas of what to look for and some things
to do when trying to catch that elusive thermal. So go out and try to find one
and let me know if you find something that might help me find one as well that
I have failed to mention.
M.W.
