I was just wondering what direction should you turn the high speed
needle on two strokes when the barrometric pressure goes up or down?
If I understand correctly,
high pressure- richen
low pressure- lean
just a theorectical question.
Yep. Thankfully when flying full scale, we don't consider humidity in
our density altitude calculations. There *IS* a formula for doing so,
but it's a pain in the butt to use and the difference you see isn't
"felt" by the aircraft..
| temperature increase: lean
| temperature decrease: richen
| moisture increase: lean
| moisture decrease: richen
| Mix them together and you can get really confused at times!
... or you can just do what most people do, and that's adjust the
needle until it's at the maximum RPM, then richen it until it slows
down a few hundred RPM, not figuring out any of the factors that
required you to change it :)
Air temperature changes affect the mixture more than
barometric changes. The barometric pressure drops roughly 1" Hg for
every 1000 feet of altitude change, but barometric pressure changes on
the ground aren't usually all that great. When we fly full scale, we
need to adjust the mixture once we've reached altitude, and if we're
climbing a long way we might have to lean it somewhat in the climb.
Air density is heavily affected by its temperature. This is seen
in the difference in aircraft performance from the same runway, at the
same barometric pressure, between hot and cold days. On a hot day we
might have to lean before takeoff to get enough power to safely climb
away. On a cold day it can't wait to get off the ground and climb like
If you live near an airport there is a good chance they have a recorded
weather status advisory. The message will have all kinds of info such a
wind direction and speed, temperature, pressure and density altitude. I'm
always amazed as to the variation we see in density altitude with
atmospheric conditions. Our airport sits at 5,500' elevation and it is not
uncommon that under certain conditions the density altitude soars to over
9,500 feet! Not only does this hurt the engine performance but the
aerodynamics suffer too. Much different that when I lived and flew on the
Texas Gulf coast at 15' elevation above MSL.
Here's a question, and I do not know
the proper answer. Is the reported wind
speed in INDICATED airspeed or
Second part to the question: If the probe
that measures the wind speed is a pitot
tube then there is no doubt that it reads
indicated airspeed. What if the probe is
an anemometer or windmill type device?
Which is it responding to?
It always seems that the higher I go to fly, the
stronger a 15 mph wind feels.
As far as the aircraft goes, there is no
argument. The aircraft responds to Q.
1/2 rho Vsquared
Marlowe wrote in message ...
WIND speed is what is reported by ATS. AIRSPEED is the velocity of an object
moving through an airmass. In the case of wind speed reported by ATS it is
measured with an anemometer. Also IIRC it is indicated in degrees magnetic.
AFAIK there is no compensation for density altitude in the wind speed
reported by ATS.
True airspeed is indicated airspeed with compensation for instrument and
position errors, compressibility and density altitude.
There is a good discussion on the subject at
Actually it should be the reverse although I doubt anyone could feel much
difference except with extreme altitude changes.
As to the original posters question; as density altitude increases, your
mixture will richen and require you to lean the needle setting.
I think you're thinking about ATIS. They report the
airport condition, including wind.
I agree with the rest of your response.
I still wonder if they are reporting the same as
a calibrated pitot/static system or if they are
reporting what a calibrated pitot/static with
corrections for non-standard temperature
and pressure (sea level standard day)
would be. i.e. does an anemometer
need such corrections?
Compressibility isn't much at mach .01
A second part of the question would be -
just what corrections ARE applied when
calibrating an anemometer. I have been
involved in more than a few airspeed
system calibrations. There is a lot more
to those calibrations than the majority
of pilots know about. The advent of
INS, GPS, and other electronics has
changed the nature of these processes
and I have little experience with current methods.
Rob Dover wrote in message ...
Wish I could do that but in an r/c car it's difficult to tell at times
because of the noise of the other cars around you. We usually just richen it
a bit at the start of the day and then rely on a combination of what the
driver 'feels' the throttle is like, what we can hear and see of the car
exhaust and how well the car idles as it comes back into the pits after a
fast couple of laps. We rareley if ever touch the bottom needle (only when
initially setting up the engine and carb) just the top end and barrel stop
(idle) screw (no air bleed screws on these carbs - would mean unfiltered air
getting into the engine).
Airport windspeeds are measured with accurate anemometers.
Pitot systems aren't accurate at low windspeeds, since dynamic pressure
increases by the square of the increase in speed. It would take
extremely sensitive pitot-type indicators to register a light breeze.
They report ground winds in degrees magnetic and in knots
(nautical miles per hour). One knot is about 1.15 MPH.
The wind speed indeed increases with altitude above the ground.
The friction of the ground slows the air near it, and also causes a
directional change. The wind will come more from the right as you
climb, more from the left as you descend. It's a result of Coriolis
Force, caused by the Earth's rotational velocity imparted to moving air
masses. As well, the air near the ground cools more at night, gets
heavier, and tends to disconnect from the upper air layers, so that
there's usually little wind at night and into the morning. We've seen
no wind on the ground at 8 or 9 AM, yet at 500 feet there might be 25
or 30 knots. As the air warms up, it begins to mix with the upper air,
and the ground winds pick up. So the best model flying times will be
late evening or early morning.
Of course, a temperature inversion can change all that.
We fly a couple of miles south of the Great Salt Lake. Early mornings
Lake is cool, dessert surface is cool. Very little wind or no wind a
all. Then the dessert surface heats up (especially in the summer
about tennish or so. Wind starts to blow coming from the cool lake.
Time to pack up