How to achieve a dramatic increase in a glider’s performance with a simple measure.
Where does the air go that comes into the cockpit through the vents?
This is by no means a new question.
Wolf Lemke specifically designed the fins of LS gliders with an opening which was supposed to allow the air to exit without causing major damage to the airflow. However, he never measured the effect.
Schleicher gliders have a similar opening in the fin.
Our CEO Holger Back remembers that Klaus Holighaus once said to one of his fellow team members in a competition that “if it gets tough, shut the vent!”
I also asked Wilhelm Dirks this question years ago and was told that the fuselage of the DG-808 is virtually open at the tail, allowing an unhindered exit for the air.
Nobody however investigated any further and tried to find out what really happens and how one might be able to influence it.
Only when Holger Back and Stefan Dörnemann talked to our engineer Clemens Mandl about this issue and asked him what his thoughts were, Clemens thought of various ways of testing the effect of the airflow and came up with astonishing results.
Air flow:
First of all Clemens Mandl (who is responsible for the detailed design of the LS10) worked out whether the pressure in the LS10’s cockpit actually changes when you open or close the vent. The result was that opening the vent caused an increase in pressure of e.g. 0.5mb at 110kph (54kts) and 2mb at 200kph (108kts). This proves that the incoming air cannot exit the aircraft without resistance.
As described, LS gliders have a 40mm diameter opening in the back of the tail fin. The idea was that the air flows out of this opening and around the rudder. This however can’t really work as there is a mylar seal on the rudder. The air will therefore be deflected to the top and bottom. What Clemens noticed next was that when he moved the stick back, i.e. when he generated a lower pressure on the underside of the elevator, there was a gurgling-whistling noise in flight. This was the air causing the top mylar seal to vibrate. Aha!
It was also fairly clear that some of the air will find its way into the wings. On the DG-808 with its torsion rods for the ailerons the wings are sealed “airtight”. (Wilhelm Dirks knew about this phenomenon!) On the LS10 and other gliders with the automatic “Hänle” control connections such an airtight seal is not possible. Some of the cockpit air therefore has to flow out at the aileron connections and cause vortices.
Everybody knows that flies disturb the laminar profile. But what about the canopy frame? The canopy gap is much bigger than a fly and can never be fitted so accurately that it would not have an impact on the laminar airflow. Could it even be that the cockpit air escapes through the canopy gap and causes the – at that point still laminar – airflow to become turbulent over a large area? This question had also been asked in the past, with the consequence that at one Worlds some pilots had their canopy gaps taped after getting in. Luckily this dangerous activity was soon banned.
Anyway, we see that the cockpit air is definitely responsible for “aerodynamic disaster”
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