Glider Instrumentation and Plumbing

1
Glider Instrumentation and Plumbing

• NCSA Safety Seminar
• 11 Feb 2006

2
Poor vs. Good Instrumentation

• Ultimately to get the most on of your glider you
  need to know and understand the instrumentation
  and what it is actually telling you, not doing
  so, could lead you to make wrong decisions
• Understand of TAS vs. IAS
• Executing Best Speed to Fly
• Identifying real lift

3
Pressure Ports

• There are 3 main types of pressure ports on
  gliders
• Static
• Altimeter, airspeed, vario
• Dynamic
• Ram or Pitot (pressure increases with speed)
• Bernoulli venturi (pressure decreases with speed)
• Total Energy Static Dynamic
• Hybrid Bernoulli Static
• Dynamic Static

4
Static

• True static is a measure of the atmospheric
  pressure surrounding the glider as if it was not
  moving
• Usually a simple hole on the fuselage or the side
  of a tube normal to the air flow
• Slipping and skidding can effect the reliability
  of this measurement
• Multiple static ports can reduce this problem

5
Dynamic

• Always greater or lesser than the static when the
  glider is moving, measure of motion
• Most common is Ram and is usually a large or
  small hole directly pointed into the relative
  wind
• Comparing the static to the dynamic is a direct
  measure of airspeed
• As the air density changes with altitude,
  indicated airspeed (IAS) is slower than true
  speed (TAS). How does this relate to VNE,
  maneuvering, stall speed?
• "To convert IAS to TAS multiply the your
  altitude, in 1000s of feet, by a factor of about
  1.5 to get the percentage increase to apply." 
    e.g. TAS _at_ 88 knots at 6500 feet 88 8.8
  97 knots. The factor increases with altitude
  reaching about 2 at 30 000 feet.

6
Bernoulli pressure     P  ½ r V²  constant

• This pressure is always lower than the static
  when the aircraft is in motion
• Very rarely used to measure airspeed these days
• This effect is the most common used in
  compensating static pressure to give a Total
  Energy . Pure geometry between
• ports can achieve this.

7
Grob 102, SS plumbing

• 1 Dynamic Ram
• 8 Static Ports ( 3 independent groups )
• 1 Total Energy
• The point here is that it can be complicated and
  good intentions can lead to bigger problems

TE and Pitot
4 static ports
4 static ports
8
SS details

• Airspeed
• Dynamic from tail and separate static
• Altimeter
• Separate static
• Sage Vario
• TE from tail ½ liter flask
• Cambridge Electric Vario
• Separate static dynamic
• Does not use TE from the plane and does its own
  TE using the pitot and true static, tunable

9
Checking for Normal

• Airspeed behaves normally with angle of attack,
  side slip, skidding, etc.??
• Air speed, altimeter, and vario does not change
  with vent open or closed
• Total energy compensated vario shows only glider
  polar response in still air

10
What to Look for

• Pure static (in still air)
• Vario swings with pitch and speed changes
• Pull ups show positive rates of climb (stick
  thermal)
• Very difficult to identify thermals, center lift,
  and adjust speed to fly
• TE compensated (in still air)
• Push over shows only glider polar sink rates
  (MaCready Zero)
• During pull ups, vario never goes above minimum
  sink rate, always stays negative
• Under and over compensation, tuning is possible
• All of our gliders should have this, the hardware
  is there and if it is set up properly it should
  work. In general leaks are killers. Response
  time is also tunable with gust filters combined
  with the right volumes.
• Netto
• Adds additional compensation to TE that removes
  the polar sink rates
• Only shows the air mass motion at any speed
• To my knowledge, our club does not have a glider
  with this type of compensation
• Modern instrumentation can switch between TE and
  Netto automatically while transitioning between
  thermaling and cruise.
• Very , very nice for identifying thermal on a
  high speed run

11
In Summary

• When you and your glider are working in
• harmony with the outside conditions, great
• things can happen
• The experience can be both predictable and fun
  beyond belief
• Please take the time to familiarize yourself with
  the instrumentation on board and learn how to
  make the best possible use of it
• Also, understand when it is giving correct
  information