Re: [Jsbsim-devel] C-172 Performance

[David M. <dav...@gm...>]

On Wed, 13 Oct 2004 08:13:24 -0700, Gregory Pierson
<gpi...@so...> wrote:

> Also remember, a lot of POH's have passed through a marketing department.
> And during this translation the numbers can become embellished some. When a
> customer is looking to buy an aircraft, they compare POH's, so marketing may
> fudge things to make the aircraft look better.

I don't know that they embellish so much as choose the best
circumstances.  For example, the cruise speeds in my Warrior's POH are
for *mid* weight, not maximum gross.  I can reach them when I'm flying
alone in the plane (even lighter than the test weight), but I'm 4 or 5
knots slower with the plane fully loaded.

Another trick, deliberate or not, is airspeed calibration -- indicated
airspeed is almost always lower than calibrated airspeed below Vy and
higher than calibrated airspeed above Vy, so it looks like the plane
flies faster and stalls slower than it really does.

Many pilots believe that their planes are slower than the POH because
of tachometer errors or mistakes calculating power settings for
density altitude (or even an inability to do so).  I often hear people
saying things like "I've never been in a Warrior that flew over 115
knots true", but upon asking them about RPM and altitude, it turns out
that they never fly over 65% power.  Other people use average
groundspeed instead of true airspeed, but average groundspeed (even
over hundreds of trips) will *always* be lower than true airspeed
because you spend more time in headwinds than tailwinds.


All the best,


David

-- 
http://www.megginson.com/