# pressure changes

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.
Correct
Ray Shearer
Don't forget:
temperature increase: lean temperature decrease: richen
moisture increase: lean moisture decrease: richen
Mix them together and you can get really confused at times!
Chris
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 crazy.
Dan
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 TRUE airspeed?
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. But. 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).
Chris
ATS = Air Traffic Services includes Air Traffic Control (ATC), Flight Services (FSS), Flight Planning and probably others.
Good question, I'm afraid I don't have an answer though.
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. Dan
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
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