An aircraft's maximum altitude?

Have been wondering for a while now, what exactly determines a planes maximum altitude?

Is it the power of the engines?

The shape of the wings?

Materials used etc?

Or just plain aerodynamics?

Done some research on the subject, but know real conclusive answer's!

Anyone help?

I think maybe the pressure determines the max altitude, like how high they can go before the veins in their eyes crack and their head busts.

Because the air gets thinner the higher you go your maximum altitude depends on at what height the air is too thin for your engine to produce enough thrust for you to climb comined with when the air is too thin for your wings to generate more lift than is needed simply to stay straight and level. Example; the big winged and Big engined versions of the B-57 (with additional underwing engines too) can fly higher than all other Canberra's.

Depends on the type of aircraft (rotorwing, fixed wing, prop, jet, etc.).
Rototwings beat the air into submission. When the air gets too thin to beat, it no longer submits.

Some engines will not continue to operate at higher altitudes, once they get so far up, they don't work at all.

I'm sure there are other factors, and I know we have some aerospace engineers around because I've approved a few applicants of A.E.'s recently. Maybe they'd like to chime in?

So Waynos, what you're saying is, that the power and thrust of the engine's, are a major factor then?

The engines. How much oxygen in the air. Engines do not currently work in high atmosphere without oxygen. Speed.

NASA tested an engine that could take aircraft higher and faster awhile back.

Scram Jet Engine Test

Also the speed of the Earths rotation and gravity require that aircraft go very fast in order to break out of earths gravity hold.



Brainiac stuff on getting into Orbit

Quite right, waynos.
Other factors:
The ability of a pressurized cabin to withstand such pressure determines the maximum altitude at which an aircraft can fly.
Propulsion systems in relation to thrust-to-weight ratios, etc.
The aircrafts airframe design and structure.
Performance of the aircraft in relation to its intended purpose.
Aircraft controls and avionics.
The aircrafts aerodynamics.





seekerof

[edit on 1-11-2005 by Seekerof]

You must first understand how the "Bernoulli Effect" works. If you understand what creates lift then we can start to understand what limitations increased altitude has in comparison to wing shape etc...

Study the Bernoulli Effect here.

Once you understand that lift is created by having less pressure above the wing than below it, you can see how the reduced pressure at altitude can reduce, or eliminate lift all together.

So airplanes that have more pronounced wing shapes that create more pressure below the wing can naturally handle higher altitude. But it gets more complex after that.

A plane like the U2 had super altitude but went real slow. It achieved this because of its pronounced curvature of the wing. Jet fighters on the other hand, have more subtle curved (and angled) wings and need to make up for the reduced pressure with speed.

So...For the U2 the solution was more radical wing curvatures to force more pressure under the wing and fighters use speed to increase the flow over the wings and increase pressure that way. Same effect, different method.

Now think about what happens to any plane at very slow speeds and altitudes. Every plane has a "stall speed", the speed where the air flowing over the wings doesn’t provide enough lift to support the weight of the plane any more. This is fancy talk for "crashing" (or landing for that matter).

Now think about the high altitudes less atmospheric pressure. The higher you go the less pressure. Theoretically there will come an altitude with reduced enough pressure to stop providing enough lift to support your planes weight, no matter how radical your wing is or how fast it can go.

Now to answer your question: The main factor has more to do with the nature of your plane than a rule that affects every single plane. It is for all planes however a mixture of wing shape and power, just a different mix depending on plane style.

So how about this then!

If you had a plane that could go to 60,000 ft, because of the height it was designed for, and with the engine's that it had as standard, could you then change engine's, or increase thrust, to make the aircraft go higher? Obviously if the aircraft's frame/structure could withstand it!

Is it possible?

Originally posted by veritas 7
So how about this then!

If you had a plane that could go to 60,000 ft, because of the height it was designed for, and with the engine's that it had as standard, could you then change engine's, or increase thrust, to make the aircraft go higher? Obviously if the aircraft's frame/structure could withstand it!

Is it possible?

Sort of... with enough thrust behind it, even a brick will fly.

I think the lift provided by the wings would be a major factor with what you describe above. Notice that the U2 has very long wings to provide as much lift as possible for high altitude flight. Sure it can fly at low altitudes too. But, for example, an F-16 can't go as high as a U2 because its wings don't provide enough lift at higher altitudes.

The short answer to veritas's question is that everyhting he (or she...let's hope there are some more female aero buffs out there!) mentioned applies.

The longer answer is that the best way of saying 'this determines your flight ceiling' is to look at something called specific excess power.

Ps=(V/W)*(T-D) V=Velocity W=Weight T=Thrust D=Drag

Plotting an aircraft's Ps contours against Mach# and Altitude create the classic 'flight envelope' that is so often mentioned. On this plot are line of constant Ps. Following these lines allows one to trade speed for altitude and vice versa with no input from the engine. Moving up a Ps level requires additional work from the engine...or perhapds a payload drop. Pushing the envelope refers to being at the maximal Ps value for an a/c-engine combo. Thus the pilot is flying as high as they should theoretically be able to for a given speed or as fast as they should be theoretically able to for a given altitude.

The real fun begins when you realize that engine thrust is by no means linear. It is a highly non-linear function of dozens of variable which all change as flight conditions change.

Here's your basic Aerospace Engineer, with a basic answer.

The four forces of an object in flight, are Thrust, Lift, Weight, and Drag.

The Force from the engines, moves the aircraft forward, creating aerodynamic Lift over the wings, tails, and body of the aircraft. When the lift is greater than the Weight of the aircraft, it will lift off the ground and fly. The aircraft's efficiency in flight is limited by several types of aerodynamic Drag.

Every textbook on aerodynamics has a diagram like this one:

www.grc.nasa.gov...


Don't let a "K-12" website disappoint, all aircraft pilots, mechanics, and engineers, start on a same similar lesson page.

So to answer Veritas 7's question:

Have been wondering for a while now, what exactly determines a planes maximum altitude?

In level flight, the aircraft will climb until the thrust of the engines and the lift of the wings, is diminished by the air flowing into or over them. A combination of lack of oxygen for the engines and a lack of aerodynamic pressure for the wing's lift.

When the diminished thrust and lift is equaled to the combination of weight and drag, it has reached it's limit.

At Max Thrust, this would be the aircraft's Maximum Altitude.

At an altitude a little lower than that, where the engines work efficiently to travel over distance (fuel mileage), the aircraft is said to be at it's Cruise Altitude.

These maximum values for the aircraft, will vary accordingly, with the temperature and density of the air around it. So in the cold Winter, the aircraft can fly higher, and in the hot Summer, it can fly lower.

That's the basics, and it gets more complicated after that.

There are many websites on Aerodynamics:

NASA Beginner's Guide to Aerodynamics
Petester's Basic Aerodynamics
Basic Aerodynamics at dynamicflight.com
DMOZ list of links on Basic Aerodynamics

Originally posted by ZPE StarPilotHere's your basic Aerospace Engineer, with a basic answer.

Thanks ZPE StarPilot. That was educational. I didn't learn much that was new to me, but your presentation of the factors involved were worthwhile. I hadn't thought about the difference between maximum altitude and crusing altitude in relation to engine efficiency.

Thanks

- McGrude

As was stated earlier wing shape makes a huge difference in the performance of the plane. The wings of a 747 are big and fat in the front for low speed, high lift performance, where the wings of a F-15 are as thin as they can be for high speed performance. The more curve you put in a wing, to a certain extent, the more lift you get and the higher theoretical altitude you can reach, with the right engines. However, the more curve you put in the more drag you are creating, which will start to lower your altitude because your engines have more to overcome. You have to find just the right balance between the two for the optimum performance you are looking for. The U-2 wings are actually quite narrow, but have a wicked curve on them, but there is so little drag from the rest of the plane it can reach the altitude it flies at quite easily.

Originally posted by ZPE StarPilot

In level flight, the aircraft will climb until the thrust of the engines and the lift of the wings, is diminished by the air flowing into or over them. A combination of lack of oxygen for the engines and a lack of aerodynamic pressure for the wing's lift.

When the diminished thrust and lift is equaled to the combination of weight and drag, it has reached it's limit.

At Max Thrust, this would be the aircraft's Maximum Altitude.

At an altitude a little lower than that, where the engines work efficiently to travel over distance (fuel mileage), the aircraft is said to be at it's Cruise Altitude.

These maximum values for the aircraft, will vary accordingly, with the temperature and density of the air around it. So in the cold Winter, the aircraft can fly higher, and in the hot Summer, it can fly lower.

That's the basics, and it gets more complicated after that.

temperature also changes the climb characteristics of the aircraft, which greatly affects my job. they climb fast on cooler days, and like dogs on hotter days (except the regional jets....they climb like crap regardless...lol).

dont forget that the amount of fuel on board changes enroute, changing the best cruise altitude and speed over the course of the flight.

another interesting fact is that airliners, at their cruise altitudes, have only a five or ten knot window between stall speed and max safe speed, which is why even a little turbulence at altitude will cause an aircraft to need an immediate descent.

not really important, but an interesting tidbit.

[edit on 1-11-2005 by snafu7700]

ZPE, strange, I'm learning to fly and I still haven't come across that similiar page, we use the alot of stuff to learn though. But yea you're right on the basic forces of flight and stuff, I already know all of that.

Anyways, Bernoulli is basically saying, the higher your velocity, the lower your pressure. It's an inverse effect, Velocity goes up, pressure goes down.

I believe Ramjets and Scramjets are best performing AIR BREATHING engines at higher altitudes, Ramjets for subsonic-supersonic speeds and Scramjets for above Supersonic speeds. However, what I don't know is how a ramjet would take off the ground, since it can't compress it's own air or create enough of an airflow to put out thrust.

Does anyone know how an SR-71 BlackBird takes off? Because it uses a Ramjet and I don't know what else it uses to get off the ground, because I know a ramjet doesn't get the Blackbird from the runway to the air.

Shattered OUT...

Shattered:

You are right that a pure sc/ramjet is worthless below mid supersonic speeds (Mach 2-3). The blackbird uses what is called a turboramjet. It's a turbojet that has a bypass that functions as a ramjet at higher altitudes. Thus it functions as a normal aircraft engine at slower speeds and allows for greater efficiency at higher Mach numbers.

Originally posted by McGrude
...with enough thrust behind it, even a brick will fly...

Anyone else thinking of the space shuttle here?


Anyway, does the strength fo the structure not come into play at all? I mean as in as you go higher in order to maintain a pressurised cabin, the strucutre has to be strong enough to stop an explosion of gas escaping the cabin (I refer to the cockpit as cabin as well). We can get people in to space is pressurised capsules however surely that gets quite heavy so the other factors mentioned come into play?

Or am I just being dense here!?!

Originally posted by Infidellic

Originally posted by McGrude
...with enough thrust behind it, even a brick will fly...

Anyone else thinking of the space shuttle here?

Not really, considering that the Space Shuttle is an aerodynamic designed known as, a Wing Body aircraft.

Sure the Shuttle is made up of Ceramic bricks, and the rockets and main fuel tank might as well be a giant rectangular shape with alot of gas in it, but the shuttle is still aerodynamic.

Shattered OUT...

Remember, that the SR71, uses a ramjet, and can fly extremely high, say a maximum of 125,000ft, but look at the wingspan of the aircraft, does'nt look like it could deliver much lift at all, could someone explain?