REFLECTOR: Limitations to the limitations

[Terry Miles]

Scott,

I know you are well intentioned and so am I.  If I get it correctly,
what Al might pursue is a TAS based Vne for his use on his airplane
above FL180.  If it's an IAS figure, then it must adjusted for altitude
and temp.  He will also perhaps be able to provide an IAS-based revised
Vne good for all of us and published as a revision by the factory, but
limited to altitudes below say 14,000 to pick an arbitrary cut off
point.  That would be wonderful...where do I contribute!!

I am not a whiz at this stuff.  Like you have, on several occasions now,
I too want to urge caution when I read of things beyond where I would go
myself.  I disagree with some of your statements, but the methodolgy is
a bit more complicated and includes my understanding of applied
aerodynnamics as taught to me over the years with airplanes traveling a
lot higher and a lot faster than a Velocity can go.  I am no expert in
this area, and should never be taken as such.  Using your Grant airport
example Vr and Vlo are unchanged.   I am not familiar w/ Grants but I
assume you are talking high density altitude.  TAS required to get to Vr
would change on the same basis that Vne changes from the IAS based
cockpit sense of safe speed.  I miss your point, and I am trying to fit
seat cushions and not write emails that likely only a half dozen people
will skim.  

In any event I yield the point to keep the discussion simple that
airplanes do not rotate on a TAS...they take off at IAS.  IAS is our
conventional gauge of speed.  In case my poor phasing led you to assume
otherwise, I was not advocating for TAS as a cockpit replacement for
IAS.  All your IP manuals have it right.  Use your airspeed indicator.


As I mentioned, and will repeat, the difference between indicated and
true diverges with changes in RHO, and RHO changes with altitude,
generally speaking.  Contrary to your notion of stall, indicated Vs will
increase with decreases in density altitude, even tho Vs is
conventionally defined in IAS terms.  In fact if you have the power
available, you can take any airplane to an altitude where one
knot(IAS)faster and you go into high speed upset, and one knot (IAS)
slower and you go into low speed stall.  

Bottom line is this, whether you use IAS or TAS, when you get into the
biz of pushing the edges of any published flight envelope, proceed with
caution.  That's all I wanted to say. Whatever your IAS needle or your
air data compute tells you, read that number with care and with full
understanding.  Above FL180 use IAS if you like, just write a
performance page for every 5 degrees off of standard day and every 2000
feet of pressure altitude.   I am more than OK with that.   Oh, and make
sure your OAT probe is adjusted for ram rise.     
   
Terry 

-----Original Message-----
From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On
Behalf Of Scott Derrick
Sent: Monday, December 12, 2005 5:57 PM
To: Velocity Aircraft Owners and Builders list
Subject: Re: REFLECTOR: Limitations to the limitations



> 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.
>
Thats not completely true. The airplane will stall at the same IAS no 
matter what the altitude, thus it responds to indicated airspeed.  Stall

speed has absolutely nothing to do with TAS, doesn't "feel" it at all.

Your Vr is set by IAS, not TAS, they are not even close at my airport. 
Just try taking off at Grants in the summer at 75 TAS, practice your 
abortive takeoff procedure first, I'll buy the popcorn.

So as any elementary flight instruction manual will tell you,  the plane

flys by IAS.  Of course we both know there are exceptions, but they are 
the exception, not the rule.

>   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.
>
Thats not exactly true either.

Vne is normally established for 3 things.

1.) The amount of Wind load the structure can handle. That is 100% IAS, 
TAS doesn't matter.

2.) Gust load, or G loading.  This is also determined by IAS, because 
the plane feels IAS, not TAS.  TAS don't matter.

3.) Flutter.  This can be effected by a combination of IAS, TAS, 
structural materials, rigging etc...  TAS should be taken into 
consideration but it certainly isn't the final word on it.

Scott


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