Un aliante, cinque polari.

What the lever changes, and what it cannot.

Flap changes the shape of the drag curve and the stall speed. It does not change the wing area, the span or the mass, so it cannot rewrite the induced drag that dominates slow flight, and it cannot buy a better best glide ratio than the aerofoil has in it. What it buys is the right to be efficient over a much wider speed range than a fixed wing manages: a fixed-flap glider is optimised at one speed and pays everywhere else, while a flapped one moves its optimum to meet you.

Landing flap is a different job. The large positive settings are there to steepen the approach and lower the touchdown speed, and they are all drag — the polar does not so much shift as collapse. Nothing on the schedule below applies to it.

The schedule: flap follows speed, not mood.

Flown properly, the lever moves on a number, not a feeling. Thermalling slowly wants the bucket high, so positive flap; the cruise between thermals wants it low, so neutral and then negative as you accelerate. The crossings land at similar speeds for similar gliders, which is why club advice like +1 for thermals, 0 for cruise, −1 above 140 works even though it is somebody else's glider.

The habit that separates a tidy pilot from a busy one is leading with the lever. Select negative flap before you push over, not after the speed has built, and select positive before you slow into the climb, not once the wing has already started to complain. A flap change at the wrong moment costs energy exactly when you were trying to save it.

Placards, and the way ballast moves them.

Every positive notch carries its own maximum speed, and those numbers get lower as the notch gets larger — a highly cambered wing at speed is a hinge moment the flap system was not built for. They are indicated speeds and they are limits, not guidance. The red line in the tool is that placard: push past it and the model turns the marker red, which is the polite version of what happens to the aircraft.

Water ballast complicates the schedule in a way worth stating plainly. Adding mass raises every speed in the family by the same square-root factor, so the crossing speeds move up with it — the schedule you memorised dry is too slow when full. The placards, however, do not move at all. A ballasted glider therefore has less room between the speed the schedule wants and the speed the lever allows, and the gap closes fastest in the large positive notches.

Why the computer wants to know where the lever is.

Speed to fly, netto, the final-glide arithmetic and the range ring at the top of the screen all start from one polar. In a flapped glider that polar is a moving target, and the computer has two ways to cope. It can assume you are following the schedule, which is what happens when you enter one set of coefficients and nothing else — an assumption that stays true for as long as you are disciplined. Or it can read the lever directly.

A flap-position sensor is a small thing that removes a permanent guess. With one fitted the instrument knows the notch. On NAVIA that selects the polar for the notch you are actually in, so the speed-to-fly ring, the netto reading and the arrival height follow the wing rather than the plan; on the Era and LX 10K the sensor reports the lever position to the instrument over CAN. The difference is not dramatic minute to minute. Over a fifty-kilometre cruise flown one notch away from the schedule it is forty to sixty metres of arrival height, and more than a hundred if the notch is wrong at high speed — which is to say, it is the margin you were arguing about at the last thermal.

Nel tuo cockpit.

The family of curves is the reason two of the other articles here matter more in a flapped glider than in a fixed one. Ballast scales all five polars at once — the ballast article has that arithmetic — and bugs and rain degrade all five together, which the degradation article turns into a number you can watch on the glide. The one thing the lever cannot do is make up for either.