Why do airplanes have to fly at 30,000+ft.

Altitude has saved huge jetliners from destruction even after losing all engine power. Believe it or not, they’ll glide, and the higher you are, the more time and distance you have to find a place to land.

Google the “Gimli Glider” - a Boeing 767 that lost both engines (ran out of fuel) at 40,000 feet. The altitude gave them time and distance to get to a landing strip.

The Concordes were certified up to 60,000 ft iirc, although they mostly flew somewhere in the mid 50,000ft range, depending on air temperature and whatnot. Four (gigantic, fuck-off) engines, but not really typical for a civilian aircraft.

See post #11.

FWIW, the novelization said it was a ground-effect craft like the 'ekranplan" concept.

Or even #8.

Yes, the jet stream is a west to east event for the most part. And a headwind affects a plane more than a tail wind of the same speed.

There was another caseof a plane that hit ash in the atmosphere from a volcanic eruption which shut all four engines down. The pilots had no idea what the cause was at the time. The would glide and lose altitude, the engines would clear and start, then when they regained altitude they’d go right back in the ash and the engines would shut down again. Kind of maddening when you don’t know the reason.


If you look at some airline schedules from say LA to London, the trip east is about 30-45 minutes shorter than the trip west.

Another reason why jet airliners fly at high altitudes - they were designed to.

At low altitudes, the greater deinsity of air creates more drag from just forcing the airplane through all those molecules. As you go higher, there’s less air, so less parasitic and form drag.

But as you go higher, the wings have to work harder, and this causes another form of drag - induced drag. This is the drag produced as a result of the creation of lift. When the curves for parasitic drag cross, you’ve found the lowest point of overall drag for that airplane. That’s your best cruising altitude.

One of the ways to reduce induced drag is to use a wing with a larger aspect ratio - long, thin wings vs short, fat wings.

Look at this overhead picture of a Boeing 747-400. Look at how long and thin the wings are. Now compare it to a picture of something meant to go low and slow, like my old Grumman AA1B.

You don’t want long, skinny wings on all airplanes because it makes them harder to manoever, harder to park and taxi, and heavier. So if you do your flying down low, you trade off high altitude efficiency for other advantages. But if you main interest is going a long way as fast as possible on as little fuel as possible, you want to design your airplane to fly as high as you can, subject to engines being available that work efficiently at that altitude.

Sorry, but this sounds just a mite unlikely. Isn’t the orbit of anything - suns, planets, space-junk - based more on reaching the perfect arc despite gravity; some sort of compromise between orbit and collision? Also, how could you not have an atmosphere unless you were living on a weird gasless planet in a bad sci-fi novel or something(?)

He talks to much.

He needs his mama.

How apparent is this? I note from personal experience, and anecdotes from Huey drivers that they don’t fly very high. And even then, we’re not talking more than 125 KIAS. Obviously it depends on the design/envelope of the helo, but just how noticeable is it?

Yup. I know Huey drivers from Minot.

Orbits are ellipses. I’m not sure what you mean by a “perfect arc.”

And I’d say that for small rocky planets (as opposed to gas giants), the lack of atmosphere is not all that uncommon. Mercury has no atmosphere of significance, and neither does the Earth’s Moon, which is larger than Pluto (of course, Pluto has recently been demoted from planet status).

I was stoned. Sorry.

My ignorant disbelief was more of an invitation for proper scientists to explain why a satellite can’t orbit an earth-sized (gasless) planet at 50ft, more than anything.

As for your point about relatively gasless bodies; is the mass of a solid object the *only * determinant of it’s atmosphere’s mass? Probly not…

I don’t have either POH handy, but with a Robinson R22 Beta V[sub]NE[/sub] at sea level is 102 knots. At some (unremembered) altitude V[sub]NE[/sub] will drop to zero.

The issue is retreating blade stall. As you already know, air pressure is less the higher you go. In a fixed-wing aircraft, this means that the aircraft must fly faster to get the same amount of lift. (i.e., get the same amount of air over the wings per unit of time.) In a helicopter in forward flight (or in a hover in a headwind) the advancing blade has a higher airspeed than the retreating blade. That’s why there’s all that ‘flapping’ and ‘feathering’ going on.

Let’s say your rotor tips are going around at 400 knots, and you’re flying at 100 kts. The advancing blade tip has an airspeed of 500 kts and the retreating blade tip has an airspeed of 300 kts. This could be a problem, as one may expect the helicopter to roll. But the blades ‘flap’. IIRC (you know how it is, you learn something for a test…) the advancing blade, having a higher airspeed, generates more lift and flaps up. This reduces the relative angle of attack and reduces the lift. Similarly, the retreating blade flaps down, which increases the AOA and increases lift. From the textbook: ‘The blades flap to equilibrium.’

If you fly a helicopter fast enough, the retreating blade will not have enough air flowing over it, or be able to flap enough, to sustain lift. The airfoil stalls. That’s when you roll out of control. Since the air is ‘thinner’ up high, the speed at which a helicopter can fly must be lower so as to keep enough air flowing over the retreating blade to sustain lift.

Johnny, that was a great, detailed response–makes sense to an engin-nerd like myself.

I have to assume, however, that you can feel in the sticks when the helo just doesn’t want to cooperate with you though, most likely because your retreating blade is just at the cusp of failing. . .

Is it something you can feel before you exceed it?

It’s them sensitive fingers that I’m sure make the difference. . .

Yes, of course it does. What I said was that a headwind affects an airplane more than a tailwind, given the same speed.

Example: a plane that flies 100 miles per hour and has a headwind of 90 mph will take 10 times as long to get to destination (effective speed is 10 mph). If the same plane has a 90 mph tailwind it will arrive in roughly half the time (effective speed is 190 mph).

I understand that. I was agreeing with you, and providing a real world cite.

At the onset of retreating blade stall the helicopter will shudder and the nose will pitch up. At this point you still have a chance. Slow it down. Speed up, and the retreating blade will stall and you will roll left (in an American helicopter; right in Russian and French helicopters, and some homebuilts).

I’ve never gotten that close. Even in training, an unexpected gust could put you over the edge. So I had to be able to describe what would happen without having experienced it. (We did do ‘settling with power’ or ‘vortex ring state’ though – at a safe altitude.)

It is probably less scary flying 600 MPH at high altitude than it would be at 1000 ft. If you have ever driven a go-cart you know that 30mph can feel much faster than 80mph in a car does.

Engine effiency increases a temperature and density decreases. Temperature is pretty constant from around 30,000’ to about 70,000. Commercial aircraft tend to operate at the lower edge of this, as it takes extra fuel (and time) to climb higher. Because of the absence of temperature lapse rate above 30,000’ air density drops faster with altitude than it does below 30,000’. As air density decreases, the speed needed to generate enough lift to support the airplane (stall speed) increases.

The upper limit of altiude for passenger aircraft is due to the stall speed reaching essentially mach 1… A little under as there are points on the airframe where the local airspeed exceeds the average. It is not uncommon for buisness jets to be only a few knots above stall when flying at maximum altitudes. The convergence of mach limit with stall speed is known as one of the “coffin corners” of the operating envelope. Operating altitude beyond 45,000’ or so starts getting very difficult to acheive…sort of like the effective 200mph “barrier” for motorcycles. Supersonic aircraft do not have this issue, but effiency of such is a joke.

Above around 70,000’ air temperature starts increasing rapidly…a much stronger slope than the troposperic lapse rate. As a result, air breathing engines essentially quit working at 75,000’ or so. The sudden increase in temperature is pretty much what establishes the border between troposphere and stratosphere.

The stratosphere starts at around 30,000 ft, and airliners routinely fly in the stratosphere. And the tropopause (the border you mention) isn’t the lower boundary of a sharp increase in temperature. It’s the transition point from a gradual positive lapse rate (decrease in temperature with increasing altitude) to a gradual negative lapse rate (increasing temperature).

No scientist is going to explain why that can’t happen, because it can happen. There is no reason a satellite can’t orbit a smooth, Earth-sized planet with no atmosphere at only 50 ft above the ground.