On Monday, August 16, 2010 08:59:38 pm Ron Jensen wrote:
> On Monday 16 August 2010 21:19:35 Ron Jensen wrote:
> > On Monday 16 August 2010 20:58:44 Jon S. Berndt wrote:
> > > What's your second thought?
>
> O.k. third thought. An FCS function (so it will work in
> current flightgear) applied by external forces at the correct area... I'm
> working up an example of this idea now..
>
> The following code is built around a 1700 hp engine pushing 67 inHg as an
> example. It seems to work in FlightGear but hasn't been extensively
> validated. Keep the direction vector normalized if you adjust that...
>
> Ron
>
> Paste the channel below into your <flight_control> section
>
> <channel name="exhaust thrust">
> <fcs_function name="aero/function/engine-exhaust-thrust">
> <function>
> <product>
> <property>propulsion/engine/power-hp</property>
> <value>0.000588</value> <!-- 1/1700 hp to nomalize engine/power-hp
> --> <property> propulsion/engine/map-inhg </property>
> <value>0.0149</value> <!-- 1/67 inhg to nomalize engine/map-inhg
> --> <value> 70 </value> <!-- 70 lbs of thrust at full power and map -->
> </product>
> </function>
> <output> external_reactions/exhaust-thrust/magnitude </output>
> </fcs_function>
> </channel>
>
> And then add an external_reactions section between <flight_control> and
> <aerodynamics>:
>
> <external_reactions>
> <force name="exhaust-thrust" frame="BODY">
> <location unit="IN">
> <x> 96.5 </x> <!-- this is the CG for the example aircraft -->
> <y> 0.0 </y>
> <z> -9.0 </z>
> </location>
> <direction>
> <x> 1.0 </x> <!-- straight down the aircraft longitudinal axis
> --> <y> 0.0 </y>
> <z> 0.0 </z>
> </direction>
> </force>
> </external_reactions>
>
This seems to be working. A few comments.
The 70 lb thrust figure was for an early merlin spitfire (IE 1938) with about
1200 HP war emergency power and with the pre NACA exhaust pipes. The spitfire
used in the NACA tests in 1942 had a Merlin XLV engine which would have made
it a mark V with 1470HP. So for a 150 octane V-1650-7 (with NACA exhaust
pipes) that a late model P-51D would have been using the exhaust thrust at max
power (1940 HP at 81 inHg) should be closer to 200 lbs.
I am a little confused about why you include engine HP in the thrust function.
For super charged engines there is a considerable amount of power used for
driving the super charger (probably about 150HP on low blower and about 350HP
on high blower at 67 inHg for this engine) so this would result in lower
exhaust thrust on high blower. But the exhaust thrust (at the same RPM)
should be the same if not higher at higher altitudes for a given manifold
pressure.
If my memory of physics is correct the thrust should be proportional to:
V^2 x M
Where V is the velocity of the ejecta and M is the mass of the ejecta. The
amount (or mass) of ejecta should be the same per engine revolution at a given
manifold pressure. This makes me think the function should be using RPM
instead of HP.
In addition I think altitude comes into play as well since it seems logical to
me that the velocity of the ejecta would be higher at higher altitudes. That
is lower exhaust back pressure should result in higher exhaust gas velocity
but I don't know if the velocity increase is enough to worry about. But it is
squared so even a small change in the ejecta velocity could be a significant
factor.
I think the location of the force should be about where the center of the
engine is located since this is approx. where the exhaust stacks are located.
I changed my local version to use RPM instead of HP and adjusted things to
better reflect the V-1650-7. I also moved the location of the force.
This is what I am now using:
<channel name="exhaust thrust">
<fcs_function name="aero/function/engine-exhaust-thrust">
<function>
<product>
<property>propulsion/engine/propeller-rpm</property>
<value>0.00079365079</value> <!-- 1/1260 to nomalize
engine/propeller rpm -->
<property> propulsion/engine/map-inhg </property>
<value>0.01234567</value> <!-- 1/81 inhg to nomalize
engine/map-inhg -->
<value> 200 </value> <!-- 200 lbs of thrust at full RPM and
map -->
</product>
</function>
<output> external_reactions/exhaust-thrust/magnitude </output>
</fcs_function>
</channel>
<external_reactions>
<force name="exhaust-thrust" frame="BODY">
<location unit="IN">
<x> 36 </x> <!-- center of engine -->
<y> 0.0 </y>
<z> 0.0 </z>
</location>
<direction>
<x> 1.0 </x>
<y> 0.0 </y>
<z> 0.0 </z>
</direction>
</force>
</external_reactions>
Hal
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