AndyH
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Re: Air Speed Indicator

Mon Jun 07, 2010 8:15 pm

planet4ever wrote:Gee, Brendon, what Nubo said sounded right to me. I don't know much about wing aerodynamics, but I had the impression a plane would be in danger of crashing if there wasn't a net airflow from front to rear. In fact if your ground speed is 100 MPH and there is a 90 degree crosswind of 50 MPH, wouldn't you be flying only 30 degrees off from directly into the effective wind? I'd call that "more or less directly into the relative wind."
An airplane has to have airflow over the wing relative to the air it's in. Thrust from the engine(s) (or putting the nose down) makes that happen. It has nothing to do with ground speed. I've hovered an older Cessna 172 over Srn Illinois by pointing the nose into a 40mph wind, dropping 40° of flaps, and adjusting power. One can take-off or land with zero ground roll if the wind is right.




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Re: Air Speed Indicator

Tue Jun 08, 2010 3:51 am

I understand. But the discussion began by Nubo pointing out that when flying an airplane the air is always approaching the plane "more or less" from the front. We're talking air speed, not ground speed. While in a car, he reminded us, the air could in theory be approaching the car from any direction. This causes complications for an air speed indicator. Brendan disagreed, and I took Nubo's side.

Apparently Nubo's use of the term "relative wind" and mine of "effective wind" were either not clear enough or were ignored.
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Re: Air Speed Indicator

Tue Jun 08, 2010 8:08 am

BrendanDolan wrote: To be fair, an airplane on has a 1 in 360 chance of flying "directly" into the wind.
From the airplane's perspective, once airborne it is always flying directly into the "relative wind" during level flight. This is why the conventional "pitot tube" airspeed indicator can work effectively. The wind relative to the ground is a separate but important matter.
Depending on your route of flight, it's common to never have a tailwind or a headwind. Only time you are generally going to have a headwind is on take off and landing. Everything else is fair game.
[/quote]

The opposite is true. The airplanes ground track is the sum of its propulsion through the "relative wind", and the force and direction of the wind as measured against the ground. In this sense, the pilot is ALWAYS under the influence of wind, regardless of its direction. That is, the airplane has a 360 in 360 chance of being affected by the wind, unless the wind velocity is zero -- the same as a hot air balloon.
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Re: Air Speed Indicator

Tue Jun 08, 2010 8:26 am

AndyH wrote: An airplane has to have airflow over the wing relative to the air it's in. Thrust from the engine(s) (or putting the nose down) makes that happen. It has nothing to do with ground speed. I've hovered an older Cessna 172 over Srn Illinois by pointing the nose into a 40mph wind, dropping 40° of flaps, and adjusting power. One can take-off or land with zero ground roll if the wind is right
Haha, exactly! I did this with an instructor on an approach. Not quite at a standstill, but close. After about 5 minutes, the controller said "Very funny. You gonna land anytime soon?" :lol:
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Re: Air Speed Indicator

Tue Jun 08, 2010 10:29 am

I like it!

And since some seem to be missing the point, there is a significant difference between car and airplane on this due to two factors: speed relative to the wind, and directional changes. If the wind speed is 30 MPH a plane traveling 150 MPH is producing most of the airspeed by its own velocity, regardless of wind direction, thus the apparent wind direction is virtually always nearly a headwind. The wind speed makes only a 20% change even if directly ahead or behind. Also, airplanes, once aloft tend to travel more or less directly toward their destination, although of course some turns are necessary. So as long as they are going in a long straight line and the wind direction remains constant, that airspeed indicator is useful. A car, on the other hand may be traveling no more 65 or 70 MPH, usually much slower, so a 30 MPH head or tailwind can change the airspeed as much as 50%. But the auto rarely travels a long distance in a straight line, and even when they do, winds at ground level shift or diminish or increase a lot due to terrain features such as buildings and hills. Thus even if you had one, it would not be often that you could use the information to adjust driving style and those few times you can, the change would be steady and evident from the the state of charge indicator. It's sort of like the old joke: "Is my turn signal working?" "...Yes. No. Yes. No. Yes. No...."

P.S. There is of course one other huge difference that makes it much more important for a pilot to know airspeed (rather than just ground speed) than for a car driver: if the airspeed falls too low, the plane can fall out of the sky. I don't anticipate that problem with the Leaf.
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Re: Air Speed Indicator

Tue Jun 08, 2010 2:46 pm

Rat wrote:I like it!

And since some seem to be missing the point, there is a significant difference between car and airplane on this due to two factors: speed relative to the wind, and directional changes. If the wind speed is 30 MPH a plane traveling 150 MPH is producing most of the airspeed by its own velocity, regardless of wind direction, thus the apparent wind direction is virtually always nearly a headwind. The wind speed makes only a 20% change even if directly ahead or behind. Also, airplanes, once aloft tend to travel more or less directly toward their destination, although of course some turns are necessary. So as long as they are going in a long straight line and the wind direction remains constant, that airspeed indicator is useful. A car, on the other hand may be traveling no more 65 or 70 MPH, usually much slower, so a 30 MPH head or tailwind can change the airspeed as much as 50%. But the auto rarely travels a long distance in a straight line, and even when they do, winds at ground level shift or diminish or increase a lot due to terrain features such as buildings and hills. Thus even if you had one, it would not be often that you could use the information to adjust driving style and those few times you can, the change would be steady and evident from the the state of charge indicator. It's sort of like the old joke: "Is my turn signal working?" "...Yes. No. Yes. No. Yes. No...."

P.S. There is of course one other huge difference that makes it much more important for a pilot to know airspeed (rather than just ground speed) than for a car driver: if the airspeed falls too low, the plane can fall out of the sky. I don't anticipate that problem with the Leaf.
Rat, I don't think you quite have the concept of relative wind.

Consider you are floating downstream in a canoe. Your speed relative to the water is zero, yet your speed relative to the riverbank may be 10 mph. Say you point directly at the riverbank and start paddling. Your speed relative to the water may now be 2 mph, yet you are still going downstream at 10mph. The canoe doesn't "know" about the riverbank. It's world is the water. A "water-speed indicator" mounted on the front would register 2mph. And it would still indicate 2 mph if you paddled directly downstream or directly upstream. In the later case your speed relative to the riverbank would be 8 mph, in the former, 12 mph. As far as the canoe is concerned, it's travelling through the water at 2mph -- straight ahead.
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Re: Air Speed Indicator

Tue Jun 08, 2010 3:06 pm

Its all a very interesting conversation. But, in the end, if you want maximum distance from your EV... drive slower.

Under 45 wind speed is considered of negligible impact on vehicles. It think it is 17MPH or so for bicycles, so drive at 15 MPH and you can go further than anybody. Without any instruments at all.

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Re: Air Speed Indicator

Tue Jun 08, 2010 5:45 pm

Looks like I dumbed my post down a little too much. :lol: I'm a commercially rated pilot, so I try not to over do it on messageboards.

Per "relative wind," yes, you are always flying within a small angle of it in most normal situations. Unless you're doing Sean Tucker style aerobatics, the angle between your flight path and your direction of travel is within a few degrees.
The opposite is true. The airplanes ground track is the sum of its propulsion through the "relative wind", and the force and direction of the wind as measured against the ground. In this sense, the pilot is ALWAYS under the influence of wind, regardless of its direction. That is, the airplane has a 360 in 360 chance of being affected by the wind, unless the wind velocity is zero -- the same as a hot air balloon.
Again, dumbed that statement down a little too much. I didn't say that it wasn't affected by the wind, rather that the only time you're generally safe the assume the wind is coming mostly as a headwind, is on take off and landing. I've been flying for 15 years, and I've only done a few quartering tailwind landings. :shock:

Don't get me wrong, I'm not saying that an airplane either A. doesn't have to have airspeed to fly, B. it's not at a small angle to the relative wind, or C. it isn't affected by the wind.

Also, the 1/360 comment means if winds are 210@40, the only heading you can fly to point the nose directly into the wind is 210 (and if you slow flight dirty, you'll hang out in the same spot...I love doing that in the late afternoon when the winds near the coast are up).

I also would have an issue with a yaw string on my windscreen in a car. I'd always want to fight the urge to coordinate the airpla...I mean car. :lol: Wait...thinking that one through, if you're on the freeway, and not sliding the Leaf, you'd be pointed into the relative wind coordinated, so you'd just have a piece of yarn on your windscreen telling you that you're coordinated, unless you had a massive crosswind (which is possible, but not terribly common).
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Re: Air Speed Indicator

Wed Jun 09, 2010 11:21 am

BrendanDolan wrote:I also would have an issue with a yaw string on my windscreen in a car. I'd always want to fight the urge to coordinate the airpla...I mean car. :lol: Wait...thinking that one through, if you're on the freeway, and not sliding the Leaf, you'd be pointed into the relative wind coordinated, so you'd just have a piece of yarn on your windscreen telling you that you're coordinated, unless you had a massive crosswind (which is possible, but not terribly common).
Haha, step on the ball! :lol:

Yeah, I really didn't think things through. I'm always interested in the wind when I'm on the bike; it just makes so much of a difference and I want to know if my perceived exertion is explained by wind, or if it's just me. So I'm always on the lookout for flags, smokestacks, tree leaves, grass, anything to clue me in on windspeed and direction. I suppose a system could be integrated into the car, even though it is in contact with the ground, to give true air speed, but it would be inordinately complicated. So I have to agree with the folks that say to just monitor your power useage and adapt. And I'll still be looking for flags, smokestacks,.... :)
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Re: Air Speed Indicator

Wed Jun 09, 2010 4:26 pm

Nubo wrote:
BrendanDolan wrote:I also would have an issue with a yaw string on my windscreen in a car. I'd always want to fight the urge to coordinate the airpla...I mean car. :lol: Wait...thinking that one through, if you're on the freeway, and not sliding the Leaf, you'd be pointed into the relative wind coordinated, so you'd just have a piece of yarn on your windscreen telling you that you're coordinated, unless you had a massive crosswind (which is possible, but not terribly common).
Haha, step on the ball! :lol:

Yeah, I really didn't think things through. I'm always interested in the wind when I'm on the bike; it just makes so much of a difference and I want to know if my perceived exertion is explained by wind, or if it's just me. So I'm always on the lookout for flags, smokestacks, tree leaves, grass, anything to clue me in on windspeed and direction. I suppose a system could be integrated into the car, even though it is in contact with the ground, to give true air speed, but it would be inordinately complicated. So I have to agree with the folks that say to just monitor your power useage and adapt. And I'll still be looking for flags, smokestacks,.... :)
I pay attention to the wind all the time too, but usually to see if it's calm enough to sneak out of the office and fly my RC planes... :lol: I burn through the LiPo's too quickly when it's windy.

For the Leaf, a variometer might do it (I've flown plenty of gliders, but I'm not rated, so this is subject to errors/omissions), since they figure out what you are doing in a column of air.
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