REFLECTOR: Limitations to the limitations

[Al Gietzen]

Truth be told a Vne pointer should move

with shifts in density altitude, and not be a painted red mark on a dial

face.  Published airspeed limits are really TAS limits which are

modified by the manuafacturer with a compensation factor for indicated

airspeed application by the pilot.      

 

The TAS formula must account for density (or Rho) in the equation:   Q

(dynamic pressure) equals 1/2 Rho(density altitude) times velocity

squared.  TAS is what your airplane feels and responds to.  200 IAS at

5000 and 200 IAS at 200 are not the same real speeds.  Vne is a TAS

limit not an IAS limit, most days, the higher you go the bigger airspeed

error due to deviation error in RHO from actual back to standard day.

 

Fortunately modern panel equipment does the calculation for you, and gives a
continuous readout of TAS.  Both my EFIS system and engine monitor display
TAS.

 

  ALSO PLEASE

REMEMBER the formula calls for velocity squared, which means that

unexpected 10 knot vertical shear is really 100 units of difference.

Not 10

 

True, it goes as the square, but that 100 units of difference has to be
compared to the forward velocity squared as well; which at 200 kt is 40,000.


 

So it seems that all we know is that there is a potentially destructive
limit out there somewhere, but we don't know where it is.  And that it best
be determined by analysis or in a wind tunnel.  However, current experience
suggests that it is somewhere beyond 200 kts IAS; and possibly beyond 230 -
250 KTAS if everything is properly balanced and rigged. (Seem about right?)

 

Catto's analysis says my plane could go 228 kts WOT, level flight, at sea
level on a standard day (IAS = TAS).  With a normally aspirated engine, I
should likely be OK at any altitude in level flight.  I'll just have to
watch the speed (TAS) in a descent.

 

Al

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