[KaiMartin]

The gap works in the same way that sailboats do with jib sails. There is an interaction between the two "wings" which enables the airflow to stay attached at very high angles of attack so they do not stall. Airliners use slats on the leading edge on take off and landing to reduce the stall speed. It is like having a deep high lift aerofoil with two single surfaces defining the total shape of that deep aerofoil.

I am sure that an aerodynamicist will have a more scientific explanation for it and the sailing fraternity talk about the venturi effect through the slot, but whoever is correct the effect is real and enabled me to fly slower than any other hang gliders I have flown of similar wing area.

The sailing lore is flawed. The venturi effect requires a significant static pressure differential between both sides of the "nozzle". Air speed is much too low and the slit is way too large for such pressure to build.

A hand waving explanation based on mainstream aerodynamics would go like this:
An airfoil stalls if the air in the boundary layer near the upper surface looses too much energy. This allows the air to flow backward against the general flow and create the large vortex that is notorious to induce high friction and very little lift. In aerodynamic speak: the flow separates.
The closely spaced airfoils of jib and mainsail interact favourably to increase the energy in each others boundary layer. The main sail generates a low pressure volume just behind the jib. This low pressure accelerates the air on the top side of the jib. Its energy increases, flow separation at the jib is prevented at angles that would normally induce a stall. The jib generates a volume of high pressure just in front of the main sail. This pressure adds to the acceleration on the top side of the mainsail. Again, an imminent stall is avoided.

Sounds too good to be true? Well, there is a draw back: The combined wing area would be more efficient if it were configured as a single wing with longer span. So you essentially trade glide ratio for better maximum lift. Airliners manage to get the best of both worlds. They retract slats and slotted flaps on cruise and deploy them only on start and landing. I noticed that the famous dragonfly uses slotted flaps to achieve high lift at low minimum speed.

---<)kaimartin(>---

[ARP]

Kai,

As I said the aerodynamicists are at odds with the sailing fraternity but the reality is, it works. A longer span increases the weight of the glider and the controllability if only weight shift is being used. One thing yet to be tried is elastic ties linking the battens of the jib and main holding the sails together at low A of A but can open with increased angles when flying slow. Effectively the same idea of retracting slats on airliners.

Tony

[Rick Masters]

I suspect you both are right but both are wrong as well.
The reason is the keel.
An effective water keel prevents sideslip and allows efficiencies of more than twice the wind speed on a horizontal vector.
Compared to the keel of a water ship, the keel of an air glider does almost nothing.
Try tacking a hang glider.
I've raced 5-meter cats past the point of flipping them over at high speed. They haul balls!
You are getting a lot of air over a catamaran's sail on a strong tack with one hull flying.
Nothing is comparable on a hang glider at moderate speed.
Just like the difference between a hang glider and a paraglider, the faster vehicle utilizes the wind to greater advantage.
You are getting a lot more useful work out of a cat sail than half a hang glider could ever deliver.
Apples and oranges.

[ARP]

Rick,

The wind speed is not relevant to the efficiency of the glider as it is moving within the air mass and its airspeed is relative to that mass. The energy to fly is derived from gravity with the glider constantly "falling" in respect to the air mass. Having the ability to fly slowly allows easy take off and landing and to core small thermals to gain the height which then converts to energy to fly, derived from gravity.

With the ability to close the slot the glider should be able to fly more efficiently between thermals and then open them to loiter, at slow speed, to gain height in the rising air.

As you say apples and oranges as the glider has nothing to lever against, as the boat does on the water. However the sail efficiency depends on how it effects the airflow that it is operating in. Interestingly there is a paraglider system that uses a water foil on the end of a line which allows the paraglider to kite itself up and tack across the wind. So it acts as a hybrid aircraft/sailboat.:- http://youtu.be/sXpPyWx7JoQ

Tony

[Frank Colver]

That paraglider sailing system reminds me of the old sport off using an off-shore wind (Like the Santa Ana's here in SoCal) to send a kite free flying over the horizon. One attaches a small bucket as a drag and if the kite and drag are in correct proportion, and the kite design is for stability, away goes the kite out of sight across the sea.

Of course in this case the kite is flying straight down wind, not tacking.

Frank C.

[ARP]

" Try tacking a hang glider. " No reason it could not be done using the sea anchor as shown in the video. Others have suggested a similar system could work completely in the air at the interface of different air masses where a differential in velocity exists.

[JoeF]

The realm is quite advanced. Kiteboarding is an air-wing--water-wing combo. Ben Franklin seemed to be part of the start of this realm. And the air-only tacking using two couple wings has been being explored for decades: FFAWE is a keyword to be used in Google for traces of several formats of coupling two wings by tether to move, fly, and soar. http://energykitesystems.net/0/FFAWE/

Notice that each wing set may be controlled or set at tacking angles relative to the opposing wing set.

[KaiMartin]

The reason is the keel.
An effective water keel prevents sideslip and allows efficiencies of more than twice the wind speed on a horizontal vector.
Compared to the keel of a water ship, the keel of an air glider does almost nothing.
Try tacking a hang glider.

For yachts the component of the total air force which is directed orthogonal to the direction of movement is a nuisance they accept in order to obtain the parallel component which provides propulsion. For gliders it is quite the opposite. To them the orthogonal component is the lift that keeps them aloft. The parallel component is a nuisance which slows the glider down.

In a way, the weight of the pilot does to the wing what the keel does for the yacht. It prevents the wing to accelerate upwards with ridiculous rates. The weight of course also provides gliders with thrust. So the thought experiment to remove the pilot weight by suddenly switching off gravity does not quite work. But imagine an ultralight with its powerful engine still running while gravity is off. It will jump into the sky. You get the idea.

In summery, gliders do not fight the force which makes a sail slip. They embrace it. There is no need to tack a glider because the weight of wing and pilot already provides the tack. And if we do additional tack, we usually call it a "tow"

[KaiMartin]

One thing yet to be tried is elastic ties linking the battens of the jib and main holding the sails together at low A of A but can open with increased angles when flying slow.

The idea is neat.
But I have a feeling, that aerodynamic forces do not vary enough to make such a system work all by itself. In particular, the forces would have to act against the tendency of narrow slits to close when blown through.

---<)kaimartin(>---

[ARP]

Kai,

That remains to be seen but I have had not had the gap trying to close on my glider and sailboat sails remain open under load. The lower the angle of attack becomes the less force will be trying to open the slot. The tension of the elastic would need some trial and error to get right. I would prefer not to have to resort to a separate mechanical closing option. Automatic L/E slats work without additional mechanics.

If it works then the variable geometry will be automatic, adjusting itself according to the pilots pitch input. The amount of gap opening will be variable according to the aerodynamic flight loads. At high speed the apparent (combined genoa/main) aerofoil section will be flattened and at slow speed the section increases to provide the required lift. I have put genoa instead of jib as the wings/sails are overlapping.

Tony