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planet4ever
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Re: Air Speed Indicator

Wed Jun 09, 2010 5:08 pm

Nubo wrote: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!
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IndyLeaf
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Re: Air Speed Indicator

Thu Jun 10, 2010 5:08 am

Rat wrote:I like it!

And since some seem to be missing the point, there is a significant difference between car and airplane...
Nuff said...

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garygid
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Re: Air Speed Indicator

Thu Jun 10, 2010 7:37 am

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)
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garygid
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Re: Air Speed Indicator

Thu Jun 10, 2010 8:11 am

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?
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Nubo
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Re: Air Speed Indicator

Thu Jun 10, 2010 10:50 am

garygid wrote:...
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

Image

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.
I noticed you're still working with polymers.

BrendanDolan
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Re: Air Speed Indicator

Thu Jun 10, 2010 6:02 pm

garygid wrote: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. 8-)
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garygid
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Re: Air Speed Indicator

Thu Jun 10, 2010 11:15 pm

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.
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Nubo
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Re: Air Speed Indicator

Fri Jun 11, 2010 3:17 pm

Nubo wrote: 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/P ... 611-05.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 :)
I noticed you're still working with polymers.

darthracing
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Re: Air Speed Indicator

Tue Jul 05, 2011 10:50 pm

Here's a little airspeed indicator kit for you Arduino hackers out there :P

http://store.diydrones.com/Kit_MPXV7002 ... 2dp-01.htm

HoustonFlier
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Re: Air Speed Indicator

Thu Jun 28, 2012 2:00 pm

Nubo wrote: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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