Falling From the Sky: The Instability of Stealth Aircraft

1
Falling From the SkyThe Instability of Stealth
Aircraft

• Cooper Hammarlund
• 5/28/07
• Fluid Mechanics II

2
Radar and Stealth Aircraft

• Radar
• Sends out radio waves which are reflected to a
  receiver.
• Main Advantage of Stealth Aircraft
• Low radar cross section (RCS) makes it harder to
  detect.
• Main Disadvantage of Stealth Aircraft
• Creating a plane with a low RCS conflicts with
  optimal aerodynamic design.

3
B2 Spirit Stealth Bomber
Flying wing design results in a low radar cross
section. Active Since 1997
4
F-117A Nighthawk
Active Since 1983 Set to be Retired in 2008
Flying A-Frame House Design The Hopeless
Diamond
5
Flight Control and Aircraft Stability
Aircrafts rotate around 3 Axes. Optimal aircraft
design incorporates devices to stabilize and
control the movement about each axis.
Traditional aircraft use three independent
controls
Ailerons Controls Roll Rudder Controls
Yaw Elevator Controls Pitch
6
Ailerons
Aileron in neutral position
Lift Decreases, Drag Decreases due to greater
effective camber
Lift Increases, Drag Increases
Side Effect Adverse Yaw Because the drag
increases when the lift is increased. The wing
that tilts up is slowed down. Therefore, when the
airplane banks to the right, it also turns
left. Corrected by Rudder
7
Rudder and Vertical Stabilizer
A vertical stabilizer set far behind the
aircrafts center of gravity keeps the nose of
the plane from swinging left or right. This
eliminates the aircrafts instability about the
vertical axis.
A rudder increases drag in the direction it is
tilted, thus slowing that side of the aircraft
causing it to turn in that direction.
8
Elevator and Horizontal Stabilizer
A horizontal stabilizer set behind the aircrafts
center of gravity keeps the nose of the plane
from swinging up or down. This eliminates the
aircrafts instability about the lateral axis.
Elevators increase or decrease the lift of the
back of the plane. By decreasing the lift in the
back, it forces the nose of the aircraft up and
vice versa.
9
Stealth Technology
Eliminate Corner Reflectors (No right angles or
orthogonal panels on aircraft). Reflect radar
waves away from source. The most effective way of
doing this is through the use of flat, angled
panels which return the waves in specific
directions away from the source. This means the
elimination of round body panels.
Corner Reflector
Engines must be concealed within the body or the
wing of the aircraft. Coated with radar
absorbent material. Fuel tank and weapons must
also be stored internally.
Faceted Airframe
10
Faceted Airframes

• Triangular and Trapezoidal Surfaces Replace Round
  Surfaces.
• Flow separation occurs at lower angles of attack.
• Flow cannot change direction instantaneously.
  Acceleration would have to be infinite.

Traditional rounded leading airfoil edge.
11
Faceted Surfaces
The body and the leading edges of the wings on
the F117 are faceted.
While the body is more rounded, the leading edges
on the B2 wings are not.
12
Suppressed Vertical Tails F117

• Tails are non orthogonal (V-tail)
• Combining rudder and elevator into ruddervator
  requires a more complex control system.
• Horizontal and Vertical stabilizers are combined
  into one tail that results in less stability.
• Tails are set completely above the body of the
  aircraft
• More susceptible to deep stall

13
Suppressed Vertical Tails B2

• B2 completely eliminates vertical tails
• No vertical or horizontal stabilizers. Results in
  the aircraft being unstable about all three axes
• Need split ailerons to control yaw.
• Split ailerons do not produce linear yaw moment.
• Creates destabilizing yaw moment.

14
Parallel Line Platforms

• Features on the aircraft are designed to align
  with predetermined radar reflection lines.
• Causes all radar waves to bounce back in set
  directions, away from the receiving station.
• The aircraft cannot be designed for optimal
  aerodynamics.

15
Fly-By-Wire Control

• The solution to stealth aircraft instability.
• Aircraft sensors constantly measure positions,
  pressures etc.
• Every pilot action is sent through a computer,
  and the computer executes the command based on
  what the pilot wants the aircraft to do.
• Quadruple redundancy.
• Computers continuously fly the aircraft and make
  minor adjustments to correct the instabilities
  inherent to the stealth design.
• Computers compensate for weight of aircraft and
  whether bomb doors are open or not.
• If the computer system fails, the aircraft
  literally becomes uncontrollable.

16
References

• Abzug, Malcome Larrabee, Eugene. Airplane
  Stability and Control a history of the
  technologies that made aviation possible.
  Cambridge Aerospace Series. 2002.
• Aircraft Design Synthesis and Analysis. Desktop
  Aeronautics. 2006
• Photos
• Richard Seaman
• http//www.richard-seaman.com/Aircraft/AirShows/Ed
  wards2005/B2/index.html
• wikipedia
• http//www.air-attack.com/page/20/Stealth-Technolo
  gy.html
• http//virtualskies.arc.nasa.gov/aeronautics/tutor
  ial/wings.html
• NASA Glenn Research Center
• http//www.grc.nasa.gov/WWW/K-12/airplane/
• http//www.atfx.org/photos/b2a.jpg
• http//www.rcgroups.com/forums/showthread.php?t22
  9775
• http//www.bugimus.com/stealth/b2_sil.jpg