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I'm reading a chapter about flight performance during the cruise phase and the influence of altitude.

My book (Aerodynamica, prestatieleer en vliegtuigtechniek by Bas Vrijhof on page 155, written in Dutch) describes the following:

Naarmate de hoogte toeneemt, neemt de luchtdichheid af. De afnemende luchtdichtheid heeft een effect op het benodigde vermogen, en dus op de prestaties. Om bij een lagere luchtdichtheid te vliegen is meer vermogen nodig. Om bij een lagere luchtdichtheid voldoende lift te houden moet de TAS toenemen, dat kost vermogen.

Translated to English:

Airdensity decreases with altitude. A decrement in airdensity influences flight performance. To fly at a lower airdensity more engine power is needed. To maintain enough lift at lower airdensity the TAS needs to increase, this comes with the cost of engine power.

This is totally clear to me. Then the book says.

Endurance... Meer vermogen betekent een hogere brandstofverbruik. De endurance neemt daarom af.

Translated:

Endurance (amount of fuel per hour), more engine power means a higher fuel consumption. The endurance decreases with an increase in engine power.

Again, this is also clear to me.

Range... De range verandert niet bij toenemende hoogte. De range is niet afhankelijk van de vlieghoogte. Wel neemt de Vmax range toe als de hoogte toeneemt.

Translated:

Range (maximum flight distance, amount of fuel per km) doesn't change with an increase in altitude. However, the Vmax range increases with an increase in altitude.

The last sentence is confusing me. Because my instincts say that a higher fuel consumption comes with a shorter flight distance. So my question is:

Is it really true that the altitude doesn't influence the flight range? Does the book has an error?

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    $\begingroup$ Piston or jet? It is academically true for piston, see: How does the efficiency of a piston aircraft change with altitude? $\endgroup$
    – user14897
    Commented Mar 5, 2019 at 16:08
  • $\begingroup$ @ymb1 Correct me if I'm wrong. Flying higher means a shorter air time, and an increase in Vmax range. So, if I would fly with the Vmax range at a high altitude, then the max. air-time would be shorter compared to lower altitudes. But, flying with the Vmax range at relatively high altitudes means that the aircraft is able to cover the range / distance in a shorter time. So, the range doesn't change with altitude. So, the statement the book made is true. $\endgroup$
    – Julian
    Commented Mar 6, 2019 at 9:54

3 Answers 3

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Your book may not be wrong, but it is misleading, because the answers depend on the the aircraft type, and the type of engine. (piston, turboprop, turbojet, turbofan)

In general:

All aircraft have the best range at higher altitudes.

Piston powered aircraft have the best endurance at low atitude.

Jet powered aircraft have the best endurance at their optimum altitude. (not too low and not too high)

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  • $\begingroup$ May I know why a piston powered aircraft has the best endurance at low altitude? $\endgroup$
    – Julian
    Commented Mar 5, 2019 at 16:26
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    $\begingroup$ Maximum endurance is attained at minimum power setting and minimum speed for level flight. (low speed means less parasite drag) Any increase in altitude means a higher true airspeed for flight, and more parasitic drag. Piston engines are efficient at low altitudes even while operating at low power and RPM. Jet engines are optimized for high altitude by design and are not efficient at low altitudes. $\endgroup$ Commented Mar 5, 2019 at 16:38
  • $\begingroup$ "All aircraft have the best range at higher altitudes." What about balloons? $\endgroup$ Commented Mar 6, 2019 at 10:05
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    $\begingroup$ @Julian Balloons are aircraft. faa.gov/aircraft/air_cert/design_approvals/balloons $\endgroup$ Commented Mar 6, 2019 at 22:12
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    $\begingroup$ @AnonymousPhysicist Ok a balloon is an aircraft, my bad. I edited the question title. $\endgroup$
    – Julian
    Commented Mar 7, 2019 at 9:44
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Jet engines can make the same horsepower/thrust at basically all altitudes due to their compressors, thus the same fuel burn - this lets them take advantage of the less dense air/less drag at higher altitudes, and perhaps take advantage of jet stream tail winds. Well, maybe not the same, see here for more How does turbojet thrust change with altitude?

Smaller planes can be equipped with turbochargers, or turbonormalizers, to make sealevel horsepower at higher altitudes and also take advantage of the thinner air.

Non-turbo engines like mine and most small planes are limited to leaning the mixture to keep the engine from running rich/wasting fuel as (generally) climb above 3000 feet. We can't get fully as high to the thinnest air, but we're also burning less fuel once cruise altitude is reached.

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    $\begingroup$ "Jet engines can make the same horsepower/thrust at basically all altitudes" is wrong, see, aviation.stackexchange.com/q/16727/14897, to expand on / correct that part perhaps. $\endgroup$
    – user14897
    Commented Mar 5, 2019 at 16:14
  • $\begingroup$ @ymb1 Is the lower thrust value at higher altitudes compensated for by the less dense air? I get the feeling that you don't need as much force to move an airplane at 30,000 versus sea level. $\endgroup$
    – zymhan
    Commented Mar 6, 2019 at 14:45
  • $\begingroup$ I think you can say "the lower thrust value at higher altitudes is caused by the less dense air". $\endgroup$
    – CrossRoads
    Commented Mar 6, 2019 at 14:48
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any aircraft has an optimal altitude for cruising. account for burn at takeoff and landing, then the cruising altitude is the one minimizing the costs. since combustible is the main cost, then is easy to determine which altitude is the economical one, even customized for the trip length and wind direction. however, for practical reasons and separation of air corridors, the economical altitude also depends on imposed constraints related to air traffic.

for distances of less than 1000km, the 6000m 675km/h turboprops of small capacity are the most economical. embraer and bombardier are flying those routes. for longer distances, the 11000m 980km/h large airliners are the most economical. for planetary distances, the largest airships are the most economical. the altitude is limited to 11000m.

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    $\begingroup$ Can you please add some sources? This looks like a good answer in the making. $\endgroup$
    – dalearn
    Commented Dec 27, 2019 at 2:43
  • $\begingroup$ I’m sorry, I don’t have sources. Is what I’ve learned in pilot ground school $\endgroup$
    – WindSoul
    Commented Dec 27, 2019 at 18:36

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