does a passenger plane require less power at higher altitudes?

Answer 1

The "simple" answer is this:

I'm assuming you are talking about turbine engines (jets or turboprops.) In that case, the higher you go, the less power you have available to you due to a decrease in density of one of the ingredients needed for combustion, namely oxygen.

But, also because of the decrease in air density, your true airspeed increases with altitude due to a decrease in parasite drag. This is roughly a 2% increase for every 1,000 feet of altitude. So, if you were going 200 knots at sea level with a certain amount of power, at 30,000 feet you are going ~320 knots. (30 * 2 = 60, 60% of 200 is 120, 200 +120 = 320)
That's an approximation, and it gets farther off the higher you go, so I'm sure someone is going to jump in and correct me on that number. I bet it's higher than 320.

So, at higher altitudes, you get less power, and therefore a lower rate of fuel consumption. The good news is, you also go faster. That's why airliners will typically fly as high as they can everywhere they go. It's not unusual for a jet airplane to spend the first half of its flight climbing, and the second half descending, with no real level flight in between, especially on shorter flights. They're trying to hurry up to the fast and efficient altitudes.

So, if you're talking "power" in terms of fuel, then yes, jet airplanes require less "power" to fly at higher altitudes.

If you are talking about thrust, then no, they require the same amount of thrust, and they have less of that available to them the higher they go. This is what ultimately determines the highest that an airplane can go - when it doesn't have enough available thrust to climb anymore.

Answer 2

Part of it is that they are made to work with the less air at the higher altitude. The biggest reason is there is less drag due to less air pressure. With less pressure against the body of the plane, it takes less energy to move it faster. A good way to do this is to take a tennis ball and throw it to someone. It is an easy throw. Then get into a pool. Now try to throw the ball while under water. It is very difficult. This is an extreme example of the difference, but it shows the difference in densities the item is travelling in.

Answer 3

I'm trying to understand the exact intent of your question in order to give you a decent answer. In one way, of course the pilot reduces power when he reaches cruise altitude, so yes, less power is required for cruise than for climb.

If you mean less power due to atmospheric drag on the plane, again the answer is yes, simply because the atmosphere is thinner in the flight levels.

If you are referring to engine efficiency, the jet engine is definitely more efficient at altitude than on the deck, for a couple of reasons. Again, the drag factor, and the fact that the air is so much colder in the flight levels, and the colder the air, the better the jet operates.

So I guess the answer to your question is yes, they require less power at high altitudes than low, for all the reasons stated above.

Answer 4

It is a tricky question. When you read this, the answer may be as clear as mud for a while.

First it must be understood that turbine airplanes measure thrust or power as a percentage. 85 to 95% of an engines power is applied on the ground during takeoff. And you now the feeling of being thrown back in your seat as the plane accelerates. That same 'kick-in-the pants' experience at high altitudes is not possible with the same power setting.

Why? A turbine engine is less efficient at higher altitude. So it is like have a smaller engine when you are flying high. When you apply, lets say 90% of power to a big engine as compared to a small engine, you get more power of course.

So now to your question,
does a passenger plane require less power at higher altitudes? If you are speaking in terms of percentages, than yes. Flying at the same airspeed down low as compared to up high would require using more of the engines total power. Where it gets tricky is when you think about how much power or thrust is actually being produced by the engine in both situations. Remember the engine is theoretically smaller at high altitudes? So the amount of power being produced by an airplane flying high at a given true airspeed is less.

Told you, clear as mud right?

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After reading the below statement, I have to say that you would have to press the gas harder the higher you go. Thin are does mean less drag... but also less thrust you get from the engine for a given power setting.

Answer 5

No not really. I don't think so. maximum speed does decrease but not power, per say. Power generally decreases with altitude until an altitude is reached where 75% power at full throttle is maximum available power...At which point, power is stabilized based what cruise level: ie takeoff (100%); fast cruise (65-75%); normal cruise( 65%); economy (55-65%)...

Answer 6

Think of it a little more simple than the above answer.
To accelerate your car to 60 mph requires you to press the accelerator down to get the momentum...Once you reach 60 mph you are pressing less on the gas.
There is less drag on the aircraft at altitude therefore less throttle is needed.

Answer 7

yes