REFLECTOR: Limitations to the limitations

[Terry Miles]

The January '06 issue of Kit Planes carries an article about indicated
airspeed and the mechanics of what makes the pointer move.  Chuck Jensen
pointed to an article on Van's Website admonishing people about big
horsepower and controled flight well above usual GA altitudes.  Both
articles are worth your time.   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.      

Airspeed indicators look at the pitot sensor, compare it to static
sensor and then affectively work the mechanical equivalent of an
algebraic equation to give you a needle deflection with one of the
variables in that equation set to a constant value.  Your IAS needle
thinks RHO as a constant, but RHO is not constant.  

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.


Vertical gust (turbulence) is another element impacting Q.  It is the
equivalent of having Mother Nature push you up another 10 knots and
maybe into flutter regime just when you only wanted to go two knots and
sneak back before the ailerons ripped off --and (as the Van's article
confirms) this could happen to you before you could even sense it in the
cockpit.  ...and further it could be at the same INDICATED airspeed you
used in last week's high altitude testing without a hitch.  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.       

My two cents
Terry Miles


-----Original Message-----
From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On
Behalf Of Scott Derrick
Sent: Sunday, December 11, 2005 5:19 PM
To: Velocity Aircraft Owners and Builders list
Subject: Re: REFLECTOR: Vne (was 'interesting read')


I think it was about 250 KTAS. 

At the time at was not aware that flutter testing was done at TAS not 
IAS.   I figured with a Vne of 200, a test to 210 should be done.

I would imagine others(Velocity's) have gone at least as fast as I
have..

Scott

Al Gietzen wrote:

>  
>
>  
>
> I have flight tested my plane to 210 KIAS at 10,000 feet. Giving the
>
> stick and peddles multiple whacks. Seemed rock solid.
>
>  
>
> I guess I will have to test up to 24,000 when I get my turbo charged
>
> engine installed, as thats the turbo's limit for 100% output.
>
>  
>
> Scott
>
>  
>
> Scott;
>
>  
>
> Care to estimate the TAS that goes with the 210 IAS?
>
>  
>
> It would be interesting to know to what speeds folks have flown their
> Velocities; either deliberately or otherwise.  It would also be 
> interesting to know about the flight data gathered (and configuration)

> in the professional flight testing, and what the estimated margin is 
> on the suggested Vne.
>
>  
>
> Al
>
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