On 12/03/2009 10:18 AM, Stuart Buchanan wrote:
> I took the opportunity to check the PoH against the simulator
> experience. While I didn't go as far as getting the OAT exactly
> right, the errors I came across were fairly signficant (using a HUD
> to get accurate altitude/TAS etc.)
>
> As I think you've noted before, the climb rate is too high - I
> consistently see 1000ft/min up to 8000ft ASL, instead of approx
> 800ft/min at sea level, and 400ft/m at 8000ft ASL.
>
> In contrast, the cruise speed is a bit too low - I don't recall what
> I saw at sea-level, but at 8000ft ASL, I saw 107 KTAS rather than 120
> KTAS (though as that was very close to the IAS, it may be that the
> environment was not quite right).
I'm really glad you're looking into this. I will happily
help as much as I can, but I've spent about 200 hours in
hospitals in the last three weeks, which cuts into my
flying and virtual flying time....
> I'm not sure what to make of this.
Being a test pilot is very demanding work. Nothing easy
about it.
> Perhaps the drag and power should
> be reduced, or possibly the alpha drag needs to increase?
A distinct possibility.
Suggestion: Try plotting the power curve. Start by
measuring the power-off vertical speed as a function
of airspeed.
The top of the curve should correspond to the POH
power-off best-glide speed. If the induced drag
("alpha drag") is off, the curve will be wildly
off. Finding the exact top of the curve is not
easy, but a little curve-fitting helps a lot.
The next step is to measure the power-on power curve.
In the real airplane, this is a pain in the neck
because engine power depends so strongly on altitude.
In the sim the pain is slightly less, because you
can easily record _thrust_ horsepower (thrust times
TAS) as you go along, and take that into account in
the analysis.....
I suggest turning on auto-coordination. The yaw
axis behavior in the C172p sim is quite unrealistic,
and we don't want that to pollute the power and drag
measurements. We can deal with the yaw axis some
other time.
> I suspect I need to use JSBSim directly to tune these parameters
> better.
There's nothing easy about that, either.
At one point I started writing a wind tunnel based
on JSBSim, i.e. something that would systematically
map out the aerodynamic coefficients. But I got
a negative amount of cooperation, so I moved on
to other things. If there is sufficient interest
maybe that effort could be revived.
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