horizontal wind velocity at different altitudes. The term sailing is used
because the operation of a sailboat is a good analogy of how the guidance
system works. The balloon is analogous to the sailboat keel and the wing is
analogous to the boat’s sail. A sailboat uses the difference in densities
between the air and water. A balloon guidance system takes advantage of the
different densities and wind speeds of air between two altitudes.
A winged balloon guidance system exploits the natural wind field variation
with altitude to generate passive lateral control forces on a balloon by an aerodynamic surface deployed beneath the balloon. There is generally a vector
wind difference between altitudes separated by several kilometers. This results
in a relative wind at the wing which allows a vertical rudder that changes the
main wing’s angle of attack to generate a lift force. This lift force can be
directed horizontally across the natural flight path of the balloon. This force is
transmitted along the tether to the balloon, causing the balloon to drift across
the winds at that altitude. Over a period of time, a small force on the order of
100 newtons (22.5 lbf) can deflect a balloon hundreds (if not thousands) of
kilometers away from where it would’ve gone if it had simply drifted with the
prevailing winds.
A high altitude stratospheric balloon conducting scientific work could deploy a
StratoSail BGS wing on a 15 km (49,000 ft) tether and the wing would still be well
above commercial airspace and above most military aircraft. Studies have been
undertaken of smaller, lower altitude balloons using substantially shorter tethers
but coordination with air traffic control authorities would be needed in order to
achieve safe flight.
5.10.3 Validation Tests
There has been successful wind tunnel testing in the Lucas Adaptive Wall Wind
Tunnel of the California Institute of Technology to validate the performance of a
1/8th scale winged guidance system. This covered main wing and wing assembly
operation over a typical range of angles of attack. The results confirmed that the
wing design performed as modeled in the range of relevant Reynolds numbers in
stratospheric flight operations. (Note: Reynolds numbers are used to predict the
transition from laminar to turbulent flow.)
In addition, there have been successful test flight experiments with a 1/4th scale
model balloon guidance system suspended below a blimp tethered to the ground
and conducted in windy conditions. The results were very successful in showing
in actual flight conditions that the full-scale system will perform as predicted in
the Reynolds number regime in which it must operate. Furthermore, a full-scale
mechanical prototype was built under NASA funding.
118 Commercial Corporations and Applications
because the operation of a sailboat is a good analogy of how the guidance
system works. The balloon is analogous to the sailboat keel and the wing is
analogous to the boat’s sail. A sailboat uses the difference in densities
between the air and water. A balloon guidance system takes advantage of the
different densities and wind speeds of air between two altitudes.
A winged balloon guidance system exploits the natural wind field variation
with altitude to generate passive lateral control forces on a balloon by an aerodynamic surface deployed beneath the balloon. There is generally a vector
wind difference between altitudes separated by several kilometers. This results
in a relative wind at the wing which allows a vertical rudder that changes the
main wing’s angle of attack to generate a lift force. This lift force can be
directed horizontally across the natural flight path of the balloon. This force is
transmitted along the tether to the balloon, causing the balloon to drift across
the winds at that altitude. Over a period of time, a small force on the order of
100 newtons (22.5 lbf) can deflect a balloon hundreds (if not thousands) of
kilometers away from where it would’ve gone if it had simply drifted with the
prevailing winds.
A high altitude stratospheric balloon conducting scientific work could deploy a
StratoSail BGS wing on a 15 km (49,000 ft) tether and the wing would still be well
above commercial airspace and above most military aircraft. Studies have been
undertaken of smaller, lower altitude balloons using substantially shorter tethers
but coordination with air traffic control authorities would be needed in order to
achieve safe flight.
5.10.3 Validation Tests
There has been successful wind tunnel testing in the Lucas Adaptive Wall Wind
Tunnel of the California Institute of Technology to validate the performance of a
1/8th scale winged guidance system. This covered main wing and wing assembly
operation over a typical range of angles of attack. The results confirmed that the
wing design performed as modeled in the range of relevant Reynolds numbers in
stratospheric flight operations. (Note: Reynolds numbers are used to predict the
transition from laminar to turbulent flow.)
In addition, there have been successful test flight experiments with a 1/4th scale
model balloon guidance system suspended below a blimp tethered to the ground
and conducted in windy conditions. The results were very successful in showing
in actual flight conditions that the full-scale system will perform as predicted in
the Reynolds number regime in which it must operate. Furthermore, a full-scale
mechanical prototype was built under NASA funding.
118 Commercial Corporations and Applications