h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

OK, just been flying and this thought piqued my curiosity whils looking down at the clouds that forever cover the North Sea.

Why do commercial planes fly at 35,000 ft? It's an almost uniform (although I recall that Concorde flew even higher) height amongst passenger airliners and I wondered where the decision to fly at this particular height came from.

A few pros and cons occur to me over great height when flying.

Pros

1) Low air pressure means less air resitance/turbulunce I guess. Hence a smoother flight and lower fuel cost (less drag)
2) If anything happens mechanically or otherwise there is more time to recover the plane. Plenty of airspace to play with.
3) Low air pressure/low drag means greater speeds can be achieved.
4) Some corking views are possible.
5) It's less noisy for those on Terra Firma.


Cons
1) It's damn cold up there. -60 C we were told, ice must be a problem. Low temperatures make materials brittle.
2) Low air pressure means less lift and possible higher fuel costs to get this lift (cf. point 1 above - is their a trade off going on here?)
3) Clouds can get in the way of the nice views.
4) If the cabin is punctured everyone needs oxygen.

So can anyone enlighten me?

Speed surley. The faster you go the more lift so as the planes are going fast they fly high. This doesn't seem like the full story though I'm sure I'm missing something.

Well I can't explain it for you, but I think I can speculate wildly on the answer to "is there a trade off going on here?)". As with most things in life, probably yes. I would think that the angle of attack on the wing, and any other aerodynamic measures which produce lift, would also generate horizontal drag. Whether its an equivalent amount I don't know. Perhaps there is a sweetspot for speed/fuel consumption which is met at that height because of this?

That's what I reckon too Bouncy.

Sigsfried, fighter jets can fly only a few hundred metres from the ground at great speed. I don't think height is proportionaly to speed although I'll no doubt get proved wrong

Yes I suppose they can. It may be more a case of most efficient hieght is proptional to speed. Certainly there will be more lift lower so maybe the Fighter jets compensate for this in some way.

I assume it's because of greater air density at lower levels but planes have to fly slower or have stronger air frames at lower altitudes. Two pieces of evidence in support. I have been on a couple of flights that have had to reduce altitude and flight times increased. Also the RAF, I think in the first Gulf war, used Bucaneers for fast, low level attacks because they had strong air frames for their Aircraft Carier roll.

It is a trade off. At higher altitude the air is much thinner, so more power is needed to create the lift. The engines run out of puff for the same reason humans do at altitude, so you need to pour more fuel in. There's less drag though, so relative speed is higher. Somewhere between 30,000 and 39,000 feet is optimum depending on aircraft and engine design.

Concorde flew much higher (around 50,000 feet) and at far greater speed, but fuel costs made this an expensive bird to operate.

However, another real reason is pressurisation. Higher altitude means a significantly stronger airframe structure is required to cope with the loads. Stronger structures add weight, which uses more fuel. Everything in modern aircraft design is based upon efficiency. The more you can carry and the faster you can carry it per gallon of fuel is the goal.

Let's take 'em in order:

Pros

1) Low air pressure means less air resitance/turbulunce I guess. Hence a smoother flight and lower fuel cost (less drag)

On the money. Less air = less drag = greater mpg. That's pretty much it.

2) If anything happens mechanically or otherwise there is more time to recover the plane. Plenty of airspace to play with.

Only a minor consideration, since most commercial air transport works on the principle that the airplane keeps working for the whole of the flight.

3) Low air pressure/low drag means greater speeds can be achieved.

Again, on the money, but is part and parcel of (1), in a way. 500 miles at 500 mph is preferable to 500 miles at 250 mph.

4) Some corking views are possible.

No airline in the world would operate commercially on anything so trivial unless that was the WHOLE point of the journey and they were charging a premium - as in the "fly over your house" type flights local aerodromes sometimes do.

5) It's less noisy for those on Terra Firma.

I don't think that would be a consideration on its own. We're just lucky it's more economical to fly high.

Cons
1) It's damn cold up there. -60 C we were told, ice must be a problem. Low temperatures make materials brittle.

Now, this is a tricky one. Ice is certainly a problem on the ground, and is a cause of many crashes on take off. On the other hand, if you're rushing through even quite thin air at 500mph, you're going to get some pretty good friction. A lot of the clever technology in Concorde was there to deal with how HOT the airframe got at 60,000 ft, not how cold. The SR-71A Blackbird, which could fly at more like 100,000 ft, actually leaked fuel all over the place at takeoff, BY DESIGN, because at cruising speed the metal body panels expanded due to the heat of air friction.

But they were both exotic supersonics. I have no idea what the surface temperature of a 747 would be at 500mph and 35,000 ft. I'd love to know though.

"2) Low air pressure means less lift and possible higher fuel costs to get this lift (cf. point 1 above - is their a trade off going on here?)"

On the money again. The height of 35,000 ft must be a "sweet spot"...

"3) Clouds can get in the way of the nice views."

Again, I can't imagine that would be a consideration. After all, where I live clouds can get in the way of nice views I would see from certain places on the ground...

"4) If the cabin is punctured everyone needs oxygen."

Again, commercial aircraft do tend to assume that they'll arrive with an intact fuselage more often than not.

So, basically there's a graph you could draw (probably quite a complicated one...) where you trade off fuel efficiency against cruising height. Or speed against cruising height. There'll be a "sweet spot". Above the sweet spot, fuel efficiency falls because you need more power to generate lift because of thin air. Below that spot, fuel efficiency falls because you need more power to overcome drag.

SoRB

The points about the views were somewhat frivolous I'll admit. After all, 400,000 ft + would make for an even better view

Thanks for that, I'd forgotten to take into consideration that the thing is moving at several hundred miles an hour. Yes, that would make up somewhat for the cold temperature.

Concorde grew 6inches when it was at cruising speed. I think the surface temperature was about 200DegC.

Grew 6inches. *sigh*...show off.

If they flew low, they'd be coaches.

B

<>

Given the lack of air at 35,000 feet, we can also assume there is less airborne water. I'd be surprised if there is sufficient water in the atmosphere at that elevation to produce ice, even if a 500mph wind (or faster if heading into the jet stream) wasn't blowing over every external surface.

A jet's engines achieve their power by basically compressing air, mixing it with fuel, igniting the combination, and spraying the result out a narrow nozzle. Some of the energy produced is necessary to drive the compressor, which in a commercial jet's case is a large fan. As you go up in altitude you'll get less drag, but eventually you'll get to a point where you have to use more of the engine's energy to drive the fan faster in order to get enough air intake. The more energy you're using on the fan, the less is left over for propulsion.

Jet fighters are a different animal altogether. They have no fans. They compress air based on the shape of the air inlets. At low altitude there is more air, and thus they work more efficiently at low altitude and produce more energy. A high altitude craft like the SR-71 has to take some extreme measures for air compression in order to run at best speed up there... measures which make it operate poorly at low speed and altitude. There are some serous tradeoffs made there.

There's another important factor. Most of the weather happens below 30,000 feet, and flying above the weather is preferable to flying through it, as you'll know if you've flown through any on the way up or the way down, or been subject to turbulence en route. And then there's the jet stream., which mostly does its thing between 35,000 and 40,000 feet if I'm remembering my A level geog. correctly. If an aircraft can catch the jet stream as a tail wind it can improve journey times and fuel costs.

But the corrolary is that any jet flying into the jet stream has to fight against it. Yet even east -> west flights maintain altitudes between 30,000 - 40,000 feet, and anyone who flies round trip across significant longitudes notices the big difference between flight times in each direction. Apparently the jet stream isn't enough of a concern to cause the plane to change its elevation.

Is there anything regarding takeoff/landing that comes into the equation?
Ideally, we want an aircraft that can reach its destination as fast as possible - a good fraction of the speed of sound whilst avoiding the problems that can happen if we get *too* close to it.

Also, for safety we want an aircraft that can take off (with a full fuel load) and land at manageably slow speeds, which even with the assistance of various panels extending out of the rear of the wing does require fairly hefty wings.

Given the drag of air, to cruise at the kinds of speeds we'd like to cruise at in an aircraft that can also take off and land at sensibly low speeds, is going high really the only option?

The jet stream doesn't happen everywhere - there are only certain flights that can take advantage of it as a tail wind and they do - the same flight travelling in the opposite direction will take a route that avoids it. As for all the other flights that travel between 30,000 and 40,000 feet, well that's where all the other factors come into play.

I don't think takeoff speeds are much of a concern. The takeoff theory seems to be to get the plane up to best speed as quickly as possible. If there were some need to take off at slower speeds, I would imagine they'd be deploying the various wing panels that add lift. I don't recall ever seeing them deployed except at landing, and I've flown far more times than I've cared to.

Landing speeds are important, because you need to be able to brake at the end. Landing speeds are really only controlled by two factors:

1) How much lift can I generate at low speed?
2) How much do I weigh?

Aircraft descents are actually controlled falls, in which you cruise at a speed which generates slightly less lift than you require to remain aloft. That's why you continue to go down even with the pointy end facing up.

Long-distance flight actually makes things easier at landing, because near the end of a fuel load you weigh a whole lot less than you did at takeoff. Less weight means you need less lift, which means you can remain aloft at a lower speed.

You can still fly a commercial jet at 500mph at 500 feet above sea level. At that speed and elevation the plane will generate more lift than you actually need, because the air is so much thicker there, and you could design the wing to retract further than current designs do. Your need to compress air would be reduced, and therefore more of the energy would go straight to propulsion, so the engine would be more efficient.

Of course, you'd also be flying under the weather, which would make food service an adventure. And fuel efficiency would be decreased due to drag.

"Jet fighters are a different animal altogether. They have no fans. They compress air based on the shape of the air inlets."

Is that similar to ramjets?

Even early jet fighters used turbojets, where all the air entering the engine was compressed by a rotating compressor, passed through the combustion chamber and the through the rear turbines.

Many modern jet fighters use [low bypass] turbofans, with not all the air entering the front of the engine and being compressed by the initial turbine stage actually passing through the combustion chamber and rear turbine - in this sense they are somewhat like a commercial jet engine, only with relatively less air bypassing the combustion/turbine stages.

A ramjet, where the movement of the aircraft and air inlet geometry provide all the necessary compression won't work at all at low airspeeds (much below Mach 1), and is only really efficient at speeds higher than most fighter aircraft fly at.

SR-71s did use an engine which was part turbojet and part ramjet, but that was for a very specialised aircraft. Other than that, it's turbines of one configuration or another for all jet fighters.

The Harrier uses a high-bypass turbofan - the thrust from the fan, directed to the front nozzles, balances the jet thrust going to the rear nozzles. Modern commercial engines actually produce most of their thrust from the fan, which makes them quieter - a larger volume of air moves at slower speeds -> less noise.

All aircraft have a performance envelope that relates altitude and speed. Fighter aircraft are exactly the same as airliners in that respect: they will go fastest at 30-40,000 feet. They can go pretty fast at ground level too only because they have such a huge power-to-weight ratio.