Air Speed Indicator

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planet4ever said:
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.

[youtube]http://www.youtube.com/watch?v=X9SRAzxVBnE[/youtube]

[youtube]http://www.youtube.com/watch?v=bPSElw8qEsI[/youtube]
 
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.
 
BrendanDolan said:
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.
 
AndyH said:
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:
 
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 said:
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.
 
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.
 
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).
 
BrendanDolan said:
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,.... :)
 
Nubo said:
BrendanDolan said:
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.
 
Nubo said:
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.
Now, back in the days when I drove my VW bus, and later my VW camper, I was always on the lookout for anything that might suggest a crosswind. These were both Gen 2, not the new-fangled Vanagon or Transporter, and boy, did they like to jump around!
 
A variometer is a differential altimeter, usually very sensitive. It measures how fast one is rising (climbing, or gaining altitude) or falling (losing altitude).

It would generally show you if the road you were driving was going uphill or down.

With an audio output, it would go "... weeeeeeeee e e e e e ..." as you drove across (up, over, and back down) an overpass.

(commercial pilot rating)
 
Driving in a car, especially at low or moderate speeds, the relative wind might come from any direction. The "string" (tattletail) indicator might be "streaming out" in any direction.

This "any direction" is easiest to imagine when the car is stopped.

If the real (over-the-ground) "wind" is 40 mph, it will "push" the car back, sideways, or forward depending upon the car's direction relative to the wind.

In this case, a car going 50 mph would experience a low of 10 mph "wind-drag" when driving downwind, and 90 mph "drag" when driving directly onto the wind, ... a very significant difference in evergy usage for an EV.

Uphill, into a wind on a rainy day ... will eat a lot more e-fuel.

Also, there might be a (rare) steep street that the EV, without low gears to use, just cannot go up.

Consider a 3500 pound car with 500 pounds of passengers and cargo (4000 total) ...
With 400 ft-lbs of torque on the axle, on a 16" rim (approx 1 foot tire radius), that produces 400 pounds of "push" on the road/car, only enough to go up about a 12% grade.

The occasional "really steep" street could be in the 20% (to perhaps 30%) range.

How much torque does the LEAF have at the wheels?
 
garygid said:
...
Also, there might be a (rare) steep street that the EV, without low gears to use, just cannot go up.

Consider a 3500 pound car with 500 pounds of passengers and cargo (4000 total) ...
With 400 ft-lbs of torque on the axle, on a 16" rim (approx 1 foot tire radius), that produces 400 pounds of "push" on the road/car, only enough to go up about a 12% grade.

The occasional "really steep" street could be in the 20% (to perhaps 30%) range.

How much torque does the LEAF have at the wheels?

Nissan isn't saying yet, at least on the LEAF site. But here's a wild guess:

They are saying 107 Horsepower and top speed of 88 MPH.
With a 24" wheel diameter, this would be about 1200 wheel rpm
Motor rpm is the wild guess -- 12,000 sounds reasonable
Horsepower = (torque * EngineRpm) /5252
doing the math, I get engine torque of 47 lb-ft
Effective gear reduction is 12,000/1,200 = 10:1
torque at the wheels = 470 lb-ft at maxRPM

At this point, it doesn't matter what the actual engine rpm is, since it's used to factor both torque and the effective gear reduction; any change just cancels out.

But, one can't assume that torque is constant over the rpm range, even with an electric motor. See the power and torque curve for the Tesla

torquegraph_v2.gif


You can see the torque at maxRPM is only about 1/5 that of the low range.

So if we use that value to extrapolate and go even further out on the limb, the low-speed torque at the wheels of the LEAF would be 2350 lb-ft

If my crazy math is right, you'd be ok here:

http://en.wikipedia.org/wiki/Baldwin_Street,_Dunedin

Edit: -- okay max power on the Tesla example peaks earlier too, so it looks like another factor is necessary for my wild guess. Divide by 2, to get 1175 lb-ft at the wheel for low-speed torque.
 
garygid said:
A variometer is a differential altimeter, usually very sensitive. It measures how fast one is rising (climbing, or gaining altitude) or falling (losing altitude).

It would generally show you if the road you were driving was going uphill or down.

With an audio output, it would go "... weeeeeeeee e e e e e ..." as you drove across (up, over, and back down) an overpass.

(commercial pilot rating)

Thanks for the clarification. Never had a checkride in an airplane where I had to know that one, and thinking about it, it would be more than useless.

Flying a DG505 after flying the Schweizer 232 was actually rather annoying, as my brain hadn't learned to pay less attention to the "wuuu wuuu wuuuuu wuuUUUUUU" coming out of that thing. Mildly annoying, but at least it keeps your eyes out of the cockpit. :cool:
 
Sailplanes (fixed-wing gliders) use the instrument as an important, primary indicator.

Large aircraft usually include a rate-of-climb indicator.

In some light aircraft the variation of weight on your seat is one (coarse) indicator, an small changes in eardrum pressure can detect fairly subtle altitude changes.
 
Nubo said:
Nissan isn't saying yet, at least on the LEAF site. But here's a wild guess...


They give some specs here:

http://www.nissan-newsroom.com/JPN/en/PRODUCTS/ZEROEMISSION/images/01/NNR/100611/leaf_presen_100611-05.pdf

280Nm engine torque, or about 207 ft-lb. Well, that's no too terribly far from my SWAG, which would give about 235 ft-lb peak engine torque (5*47). Of course the rest of my guess is still just that. I think I saw elsewhere today the acceleration numbers, which I guess you could also use to figure the wheel torque. I'll leave that as an exercise for the reader. i.e., I'm too lazy :)
 
Nubo said:
Since the biggest determinant of power consumption is wind resistance, a bit of thinking about Air Speed vs. Ground Speed is in order. With a headwind of 20mph, a ground speed of 60 would be consuming power at the 80mph rate. Conversely, with a tailwind of 20mph, a 70mph cruise speed would only be consuming the equivalent of 50mph rate.

LOL, great idea!
Very true.

I will counter that a simple algorithm of "power used / speed achieved" will give a good approximation of wind, that is if on level ground.
 
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