Re: Unsafe At Any Speed?


David J. Gall
 

Hi, Al,

There's an area of the art of aviating that no one ever seems to mention any
more, and that's the gulf separating experiment from application. As pilots,
we are so used to reading a number from an instrument then applying some
conversions to it -- by charts and tables, or by computers -- to get to the
information we want (in this case, TAS) that we find it difficult to
conceive of the process in reverse. As TEST pilots, however, that is exactly
what we must do. As pilots, we read the conversion charts; as TEST pilots,
we CREATE the conversion chart.

Each of us who has built an airplane and done the test flying ought to be
intimately familiar with this topic, because so much of the rest of our
airplanes' performance is built around an assumption that our airspeed
indicator is accurate. Your statement about flying on the wing of another Q
illustrates this point. Although you may not have gone to the trouble of
preparing a formal airspeed calibration chart, you have noted the
discrepancy from one airplane to the next. You've compared your airspeed to
a reference, just not the reference of the ground. If we all make our
comparisons to the groundspeed and we all follow the same procedure to do
so, then we have a reasonable standard for comparison between airplanes
without having to fly formation on one-another. This is how "rules" get
written. The FAA has a whole passel of rules for us to use, based in part on
bureaucratic BS and in part on 100 years of test flying.

The term "True Indicated Airspeed" is directly from page 43 of AC 90-89A,
the FAA's "Amateur-Built Aircraft and Ultralight Flight Testing Handbook,"
Chapter 4, Section 3, paragraph 5 "Airspeed In-Flight Accuracy Check,"
subparagraph b. If you haven't heard or seen it before, then I must presume
that you have never gone through the process of calibrating an airspeed
indicator on a newly-manufactured airplane. Anyone who has done so must have
encountered the value, even if they didn't know the "official" FAA name for
it.

If you have built and test flown an airplane without encountering the
concept embodied in the "True Indicated Airspeed," regardless of the name
you may have used for it, then you have missed an opportunity to standardize
your airplane and know what it is really doing. I've seen lots of guys who
have saved every single receipt for the cost of building their pride-and-joy
and can account for it by categories like "tools" and "paint;" it's such a
shame that so many of us haven't taken the time to perform the most
rudimentary airspeed checks and record them. What was the point of building
your own plane if you literally DON'T KNOW what you have after all that
sweat?

I'm not picking on you, Al, or singling out anyone else. We all have the
fudge-factor "it's good enough" going on some times, and if you fly one
plane from its birth, you "know" these things without having to formally
document them. And as an engineering PhD I know routinely says, if you can
figure things to within 20% and be right, major corporations will buy you
whatever you want. But as a co-worker is fond of saying, it IS important to
know "the book" well enough to know what lie to tell, and when. Some of us
just aren't "lying" very well.

I think I'll spend the rest of the evening rounding up the last year or so's
worth of Sport Aviation back issues and reading the series of articles on
test flying.


David J. Gall, amateur
I think I got the right subject line this time :-)

----- Original Message -----
From: <kittleson1@...>
To: <Q-LIST@...>
Sent: Friday, October 20, 2000 12:31 AM
Subject: [Q-LIST] what speed?


David,

For people who weren't confused before they read you response, they may
be after.

I have never before, heard or seen anything called "True Indicated
Airspeed".

It's one or the other, not both. (Except ,of course, if there is no
instrument error and that you are flying at sea level on a standard
day.....all very unlikely)

Rarely is the indicated airspeed (IAS) accurate enough to use without
factoring in the installation error to get your calibrated airspeed
(CAS).

My pitot/static tube makes things pretty close, but my experience flying
the wing on one other Q showed at least 8 mph difference in IAS with
other bird...the other one being highly optimistic.

Just don't go fast enough to worry about equivalent airspeed,(EAS).

Al Franken




On Wed, 18 Oct 2000 23:32:59 -0400 "David J. Gall" <David@...>
writes:
Pat,
snip>

Run the true groundspeed through your E6B computer for altitude and
temperature and it becomes True Indicated Airspeed. Be careful,
though,
because you need to make that calculation backwards from the TAS
calculation
you're used to. Whereas you would normally take your indicated
airspeed in
cruise and use your E6B to find your TAS, here you have the TAS
(true
groundspeed) and you're trying to find out what your airspeed
indicator
should read. Once you've done that, compare this against what your
airspeed
indicator actually indicated during the flight (you did fly a steady
airspeed at constant altitude and RPM, didn't you?) and you'll have
one data
point on your airspeed calibration card for your airplane flight
manual. See
AC 90-89A Chapter 4, Section 3, paragraph 5.

With the advent of GPS, we no longer have to fly a measured course
over the
ground; just take your GPS groundspeed readings while traveling in
two
opposite directions, then average them. That's your true airspeed as
you
would find it in no wind conditions. If you wish, plug that into
your E6B
for temperature and altitude (backward calculating, again), and
you'll know
what your airspeed indicator >should< have been reading at the time.


David J. Gall

----- Original Message -----
From: "Pat Panzera" <panzera@...>
To: <Q-LIST@...>
Sent: Wednesday, October 18, 2000 4:24 PM
Subject: Re: [Q-LIST] Wingtip Lenses




James Postma wrote:

This is why record attempts are two way over a closed course.
YOu can
do
the same with your GPS by doing a 180 and averaging the two. Do
it into
the
wind if you can and then downwind to minimize crosswind
components.

That sounds all fine and dandy, but can you guarantee the wind
speed,
direction and temperature will remain constant for each run? I
can't.

And here's an extreme example. Say you want to find the
PERFORMANCE
capability of tiny single seater, 1/2 veedub, 80mph vne
aircraft.

You plot a course between 2 points. Your down wind leg has a
100mph
tailwind. You arrive at the finish line and turn around. Start
heading
toward the new finish line and never make it because you are
flying
backwards across the ground. What dose this tell you about the
performance of this aircraft?

Ground references for performance verification is ridiculous.
If you want to know how fast your airplane is capable of
traveling,
look at your airspeed and adjust what you see for alittude and
temperature.

If you want to know how long it's going to take to go someplace,
use
this new PERFORMANCE information against the predicted winds to
get an
anticipated ground speed.

Record attempts are done both ways because it's the only possible
way
to attempt to factor out any tail wind. In all reality, anyone
hoping
to set a new record, would hope for no wind at all, especially
if
it's over a timed course.

Hope this helps.

Pat



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