Dear pilots,

After a research process that started immediately after Bill Lemon's tumbling we are presenting the conclusion about the stability qualities from the Ghostbuster. Sorry if this was a long time, but we have stretched this research until we were absolutely sure of the results.

The terminology adopted in Germany makes a distinction between tucking and tumbling.

• Tucking is the quick rotation forward like a forward loop.
• Tumbling is the recurrence of the quick rotation forward until recovery, inverted flight, parachute opening or glider brake.

Our final conclusion was issued based with data obtained from many different sources:

1. · aero dynamical data from more then twenty runs in the DHV certification test truck with the Ghostbuster in all possible modifications
2. · aero dynamical data from the Exxtacy and Ghostbuster certification
3. · reports from the accidents by the pilots
4. · physical examination from the broken gliders sent back to us
5. · test flights by our pilots
6. · reports from many Ghostbuster pilots all over the world
7. · an independent analysis performed by Dr. Martin Jursa who was the technical director of the DHV and performed a deep theoretical and practical research about tumbling characteristics in hang gliders.

We would like to stress that the research was made exceeding all the possible configurations that a Ghostbuster could possible have after leaving our quality control. This was done to examine if a particular combination of extreme sweep and sail tension could induce a loss of stability with the twist distribution inside the factory tolerances. None of this tests have showed any critical result according to today's certification tests.

The aerodynamically analysis performed by Dr. Martin Jursa consisted in comparing the stability curves from the different configurations from the Ghostbuster and the certified Exxtacy.

His analysis showed that the stability curves from the Ghostbuster are better then from the Exxtacy. This somewhat surprising, due to the difference in safety records, agrees with the safety aim set when the Ghostbuster was designed. The Ghostbuster was carefully designed with a large tip area to be more spin resistant and improve the load distribution. There is also turbulator sewing along the span of the upper sail that prevents a laminar stall.

Where does the difference in safety records then come from?

Normally a pilot entering a turbulence zone reacts by adopting an ergonomic "alert" position, where the bar lies somewhere between his chin and breast. The amount of weight shifting afterwards depends on how dangerous the pilot judge the situation after a while.

The Ghostbuster has 7-10 cm more backwards bar position compared to an Exxtacy. For an average size pilot this difference in the alert position represents 3-4° higher angle of attack, which will approximately result in 5 km/h less speed. The speed and the angle of attack differences cannot explain why a Ghostbuster would tuck and an Exxtacy not, because in extreme turbulence the gust variation can be much bigger then that, therefore statistically it should have already happened that some Exxtacy would have tucked.

Hanggliders are not only tailess but also apply weight shift in the longitudinal axis (speed axis). When a pilot move his body forward he is not only increasing speed because of reducing the angle of attack but more important he is also shifting his weight and with that the CG (centre of gravity) forward. The CG position is very important for all hang gliders, gliders and planes in turbulent conditions.

In the mechanics of flight the biggest safety factor for longitudinal stability is how much the centre of gravity of the aircraft is in front of the neutral point. This distance is called the static margin.

If the same pilot keeps his specific "alert position" in a Ghostbuster and an Exxtacy, there will be a very big difference of stability. As you can see in the table below for our example we have more then 125% in difference in the static margin. It is important to understand that it doesn't matters which speed the pilot is flying, if he keeps the same bar position relative to his body this difference in stability will always be present in all speed range and even more dramatic in lower speeds.

If we change now the pilot position by 7 cm with new cables, we double the static margin of the Ghostbuster. A second very important positive effect of having bigger static margin is the resistance to tip stall and spin.

Conclusion:

We have had two tasks for our research:

1. To find the proofed reason for the difference in safety statistics between the Ghostbuster and the Exxtacy
2. To find a solution to modify the glider in a way that the pilots can fly safe without any special advise or tools and with no reduction of in-flight fun or performance.

The difference in static margin is very reasonable explanation for the incidents. The Ghostbuster by its pitch curve is more stable than the Exxtacy, today's certification tests do not evaluate the static margin and we also have not done it during our design period, but you can be sure that in all future developments we will take care of this point.

With the change of the front cables, we can provide to the pilots a modification, which will not in anyway decrease in-flight fun or performance, but will increase safety margin to the well proven Exxtacy standard, by having better spin and stall resistance and recovery.

New front cables and rear shackles for all Ghostbuster pilots can be ordered free of charge from your importers and dealers.

Flight Design, Stuttgart 26th September 2000