It seems to me that if planes flew only a couple thousand feet above the surface instead of 30,000 or 40,000, it would reduce the circumference of the path they fly, and thus reduce air travel time. Also, the air down lower is more dense, so lift is easier to create and maintain, thus reducing fuel consumption. This may not be practical across Nepal or Colorado or something, but on international trans-Atlantic/Pacific flights where there’s nothing but (basically) flat water, why not?
actually, it takes more fuel for a plane to fly at lower altitutes than at higher ones. Drag plays a major consideration, and there is more drag at lower altitutes.
The atmosphere is less dense at altitude. Your indicated airspeed may be the same as at a lower altitude, but your ground speed will be greater (assuming winds are the same).
Although headwinds are common, aircraft can often find favourable winds aloft. From http://www.smm.org/general_info/bhop/jetstreams.html :
Nothing like a 150mph tailwind to shorten a trip! (Of course, that can cause problems at the destination airport if you get there too early.)
Also, engines operate most efficiently at a given fuel/air ratio. If the air pressure is reduced, the amount of fuel required is also reduced. Engines lose power for that reason as they go higher, but the most efficient altitudes (that is, the lower power vs. lower fuel flow ponit) are higher.
Finally, flying higher gets the aircraft out of the weather. If you have 25,000 foot cloud tops, the jet can be flying happily along in the sunshine 10,000 feet above them.
>>>>>>>>>
The atmosphere is less dense at altitude. Your indicated airspeed may be the same as at a lower altitude, but your ground speed will be greater (assuming winds are the same).
>>>>>>>>>
If you were flying higher at 500 MPH, wouldn’t your ground speed be less than if you were flying lower at 500 MPH? Kind of like how a record moves fastest at the rim…even though towards the center it is moving slowly. So, if you put a red dot on the rim of the record at recorded its speed at 500 MPH, a red dot an inch from the center might read 100 MPH (or something like that). So, if you made that one an inch from the center do 500 MPH, it would be sailing around (the earth) like nobody’s business, even without the tailwind.
>>>>>>>>>
Also, engines operate most efficiently at a given fuel/air ratio. If the air pressure is reduced, the amount of fuel required is also reduced. Engines lose power for that reason as they go higher, but the most efficient altitudes (that is, the lower power vs. lower fuel flow ponit) are higher.
>>>>>>>>>
Are the most efficient altitudes higher because that’s how the engine is designed? I thought that was a problem with car engines, that in the mountains fuel consumption was miserable.
Good point about the weather, though.
Think of the equivalent arcs that you looking at.
Assume circumference of Earth at surface is 24,000 miles.
Assume diameter of Earth is 24,000 / PI = 7640.9 miles.
Assume distance traveled is 1000 miles.
Assume spherical Earth.
That is, if the distance of the arc between two points on the Earth’s surface is 1000 miles, then you have approx 1000/24,000 of a circle, or 15 degrees of the 360 degree arc about the Earth. To cover the same 15 degrees 35,000 feet above the Earth, you must go:
Add 2*(35,000 ft/5280 ft/mile) miles, and you get D at 35,000 ft of 7654.1 miles, or a C at 35,000 ft of 24,046 miles. That same 15 degrees at this D is 1001.92 miles.
So yes, if you did both at 500 mph, your ground speed would be slightly less. But you’re also assuming that you could economically travel 500 mph at a lower altitude, which given the drag involved most commercial planes probably couldn’t do it effciently.
You can’t compare IC engines with aeroderivative gas turbines in almost any context. The engines are designed to be most efficient at high altitudes, becuase that’s where the ground speed of the plane can be the highest due to drag.
I did the above without calculator or references - hope I didn’t make any mistakes.
the increase in distance flown because of altitude is less than 1 part per 1000, and so is negligible (the radius of the earth being about 6366 km).
All the other factors have a much greater incidence.
The relative distance between the red dot at the edge of a record and the red dot an inch from the centre is greater than the distance between the Earth’s surface and 35,000 feet. The atmosphere, relative to the planet, is “paper thin”. The “advantage” of being up high is negligable. Also, an aircraft in flight is not attached to the centre of rotation, as the edge of a record is.
Also, your indicated airspeed (IAS) may be 500 knots, but your true airspeed (TAS) will be greater because of the reduced air density.
[quote[Are the most efficient altitudes higher because that’s how the engine is designed? I thought that was a problem with car engines, that in the mountains fuel consumption was miserable. [/quote]
A piston engine needs about a 15:1 air/fuel ratio to run efficiently. Since the air pressure at altitude is less than at sea level, the amount of fuel required to achieve that 15:1 ratio is less. Aircraft engines (at least piston engines, which you don’t really see much above 15,000 or so) have a “mixture” control. As you fly higher, the ratio can be maintained at peak by leaning the mixture (i.e., reducing the amount of fuel).
While your car may be using the same fuel/air ratio at 8,000 feet as it does at sea level (actually, I’m sure modern cars compensate), you are not maintaining a constant altitude. In the mountains you’re constantly driving up and down hills. When you go uphill, you step on the throttle to maintain your speed. You burn more gas.
Okay, I didn’t know drag was such a factor…and that the distance travelled was so negligible. Not that I’m surprised that 95 years of aeronautical engineers knew what they were doing. Thanks for clearing that up…
Which reminds me: I recently flew to/from Germany (and came up with that question in flight) on Lufthansa Airlines. For some reason, they don’t allow CD players. However, every other airline I’ve flown on (never transcontinental) has allowed them. Why?
You know? One thing about flying helicopters 500 feet above the ground at 70 knots, is that I almost never use my E-6B. Haven’t flown a fixed-wing in years. So I’m very rusty on my calculations. Anyway, let’s say the OAT is 25°C. My altitude is 5,000 feet. According to my little window, density altitude would be about 7,100 feet. If my CAS is 100 kts, then my TAS shows 115 knots. Is that right?
Now let’s say the OAT is -30°C and my pressure altitude is 35,000 feet. So I set -30 over the 35 in the pressure altitude window. My density altitude window shows 37,200 feet.
Right. When I look at 500 kts on the “MINUTES” (CAS) scale, it shows 950 kts on the “MILES” (TAS) scale! That doesn’t seem right! WTF, over?
This has been discussed a few times (at least once in GQ, and once in the BBQ Pit). Here’s one link where I posted the appropriate regulation:
http://boards.straightdope.com/sdmb/showthread.php?threadid=12807
How’d you get from 115Kt TAS to 500? I’d say that (1) you are reading a ouija board by mistake ;(2) you have a HELL of a tailwind that the E6 knows about somehow, or (3) you are not setting the pressure altitude opposite OAT correctly. What color is the helo you fly?
Two seperate calculations. The first was OAT = 25°C, pressure altitude = 5,000 feet, and CAS = 100kts.
The second was OAT = -30°C, pressure altitude = 35,000 feet, CAS = 500kts.
The first try was using typical “Bugsmasher” numbers, and the second was for something higher and faster.
As I remember it,
Blue=LAPD
Green/tan=LASD
Red=LACFD
other=traffic reporters/air ambulances
I just fly for fun. I fly a white one. Here’s me landing in Lancaster:
http://pw1.netcom.com/~heliboy/n61413crop.jpg
BTW: I actually READ THE INSTRUCTIONS! for the E-6B. It looks like I was figuring correctly (i.e., given the conditions, 500 kts CAS = 950 kts TAS). Since the speed of sound is less than 950 kts, I’m guessing that the CAS on the hypothetical airliner would be less than 500 kts. Serves me right for not being an airline pilot.
Old geezer checking in. Have you ever taken a DC-3 from point to point? They were unpressurized and had to keep it to 12,000 feet or less.
There’s a lot of WEATHER in the lower altitudes. It’s one thing to fly over turbulence at 40,000 feet.
It’s another thing to flythrough it at 10,000.
So when you’re figuring an arc of travel, include an arc AROUND bad weather at low altitudes.
What about when an engine shits itself, particularly a single-engined aircraft. When you have sufficient height your glide path is extended, and your then have more options to getting yourself out of trouble.
There’s a saying that a few of the pilots I see regularly use to describe any plane crash: the pilot ran out of height, speed and ideas simultaneously.
Kinda sums my argument up nicely.
The report is out on JFK Jr’s crash. It reminds me of one of the (admittedly uncharitable) phrases I use: “He was dead before he ever left the ground.” Interesting thing about pilots. We look at a crash and always see human error. “That would never have happened to me!” If a plane is flying along and a wing falls off, we say, “That stupid pilot! He should have done a more thorough preflight!”
Doug, the only time I flew in a DC-3 was when I was a little kid in the 70s. My mom worked at MYF (in San Diego) and a DC-3 was there giving rides. Short flight, but I remember it was neat!
There has even been talk about suborbital flights that would take you half way around the world in a couple of hours.
Folks seem to be talking past each other on the issue of airspeed, which is generating some confusion. An airspeed meter works by counting the number of molecules of air entering an opening (usually on the end of a little tube) in a given amount of time. Airspeed, as measured by such a device, is also the relevant quantity for computing drag. There’s two ways to reduce your effective airspeed: You can reduce your true speed, or you can reduce the density of the air-- either one will cause less molecules to enter your tube per second. This is why airspeed is different at altitude, it has nothing to do with the distance from the center of the Earth.
DOH! I keep forgetting that people don’t know what a pitot-static system is!