How does altitude affect the aircraft performance?

[hin0727]

Hi
Sorry for this stupid question... but this really bothers me a lot
So to start with, as altitude increases(let say pressure altitude),the air density decreases, and so as the drag(parasite drag), so the aircraft will have its maximum aerodynamic performance at its service attitude....
But I have just read a book, which says high attitude decreases the aircraft performance...I guess its because of the lower air density that deceases the amount of airflow pushing rearward, resulting in the decease of thrust and hence aircraft performance?!?!

So after all, do high altitude decrease or increase in the aircraft performance?!
thanks!

[Wizofoz]

Hino,

Your book is more likely to give you the answers, as it is a long and complex subject.

What do you mean by Performance? Maximum speed? Maximum range? Jet? Turbo prop? Piston? Turbo Normalised?

In general terms, you have the basic idea- Drag is (simplistically) proportionate to Calibrated Airspeed- as altitude increases, True Airspeed increases as a function of CAS, so all other things being equal speed and range will increase with altitude.

That, however, assumes thrust is constant. As altitude increases, available power reduces, so the point at which maximum performance is available is the highest altitude at which the power required for a particular CAS is available.

[BOAC]

So to start with, as altitude increases(let say pressure altitude),the air density decreases, and so as the drag(parasite drag), so the aircraft will have its maximum aerodynamic performance at its service attitude.... - that is where you went wrong. A/C generally fly at an IAS. At the same IAS, drag is the same (it actually increases with altitude, but that 'is not important right now') regardless of altitude.

[hin0727]

So u guys are saying drag will only proportional to IAS/EAS, but has nothing to do with the air density??

However, based on the fact that true airspeed increases with altitude for constant IAS ,isn't it because of the lower air density at high altitude that a smaller amount of drag presents so that the TAS becomes higher??

[Wizofoz]

No both are true.

For the same TAS drag will be lower if air density is lower- but so is IAS.

For a given IAS, TAS increases as air density reduces, but drag remains,constant. ( This is the simple answer, there are other factors)

[hin0727]

So for a constant IAS, TAS increases in climbing is due to the reduction of air density, but nothing to do with drag, because drag is only proportional to IAS, is that right?:ok:

[dubbleyew eight]

within the limits of normal comprehension altitude does not effect the aeroplane it merely effects the engine performance.

in the less dense air at altitude the aircraft flies faster over the ground but experiences the same dynamic pressures and indicates the same airspeed.

takeoff and landing at altitude are different from sea level because of the need for speeds that achieve the dynamic pressures that the aircraft works with.

[BOAC]

takeoff and landing at altitude are different from sea level because of the need for speeds that achieve the dynamic pressures that the aircraft works with. - possible confusion warning? I don't think hin is ready for that! Let's leave him/her with the knowledge that the take-off and landing IAS is still the same?

[Wizofoz]

So for a constant IAS, TAS increases in climbing is due to the reduction of air density, but nothing to do with drag, because drag is only proportional to IAS, is that right?

Yes, that's it. IAS is really a measure of Dynamic Pressure, which is ALSO what drag is proportional to, so drag remains constant for a given IAS.

So, if you assume constant power, you achieve a higher TAS as you climb at a constant IAS.

HOWEVER (With the possible exception of a turbo-normalized Piston Engine) available thrust REDUCES with altitude, so at some point you won't be ABLE to maintain a constant IAS, and your performance will begin to degrade.

Now, some high performance aircraft will reach their service ceiling (perhaps mandated because of Pressurization as an example) while still having enough power to achieve VMO- and thus will ALWAYS be able to achieve a higher performance by climbing.

Most, however, reach there "Best" altitude lower than their "Highest" altitude.

[Natstrackalpha]

that is where you went wrong. A/C generally fly at an IAS. At the same IAS, drag is the same (it actually increases with altitude, but that 'is not important right now') regardless of altitude.

Some other countries teach CAS where we would teach RAS, different, `tis true.

[BOAC]

In 51 years of lifting my backside into the air in a machine I have never seen an ASI calibrated in either CAS or RAS - have you?

[Natstrackalpha]

No, it is IAS.

[hin0727]

Thank you everyone!!
Well explained!:ok:

[Onceapilot]

Despite BOAC's experience, many of us fly KCAS.:)

[BOAC]

OAP - jolly good - I am aware of the functions of ADCs. I was flying CAS 40 years ago. This thread began with a simple question from someone who needed a bit of help in understanding the effects of altitude on performance. As so often on PPRune, the thread becomes the bandstand on which some folk can display their 'knowledge' about aviation matters that are not actually relevant to the topic and before long the thread will be full of erudite mathematical equations and the like and hin will probably have fainted.:)

In the interest of wiz and I trying to keep things reasonably simple for hin0727, we tried to 'keep away' from corrections to IAS - the OQ indicated absolutely no need to enter into that domain. Hin appears to have read a book which is guiding him/her in the right direction. Introducing CAS/EAS will merely confuse at this stage. Now then, tell me - is the speed you see on the ASI the speed indicated (which makes it the 'Indicated Airspeed'?) or not? As an 'oap', do you rotate when the given speed is indicated on your ASI? What is your definition of 'indicated'? If you were teaching an 'ab initio' about flying, would you launch straight into CAS/EAS?

[gernie]

Hey guys. Not sure of what drag are you talking about, but I´ve always been taught that both parasite and airframe drag reduces with an increase of altitude due to the reduction in air density. That is why jets like to fly high to achieve best SFC (using less fuel) and less airframe drag.

Maybe you could elaborate a bit more?

Thanks

[Qwerti]

Aerodynamics at high altitude are slightly more interesting due to some factors that Im sure the book will explain later.

I will only back up what WizOfOz said.. Stick to the book because the answer to the question if Performance decreases with altitude could be yes & no, depending on what we are talking about.

[pattern_is_full]

The key issue is defining "performance."

Thanks to reduced drag at higher altitudes (up to a point) the aircraft can travel faster (true airspeed). Even with reduced power and thrust (less air to burn with fuel, and less air to push for thrust).

And has to travel faster, to generate enough lift in the thinner air.

TAS is not the only measure of performance, however.

A takeoff at a high altitude will take longer - less thrust for acceleration, and more TAS needed, to reach rotation speed measured in IAS.

If you have a "drag race" between two identical planes taking off - one at sea level and one from 6,000 feet (2,000 meters) - the plane at sea level will have much better performance - a shorter takeoff roll in both time and distance, and better climb performance (obstacle/terrain clearance capability) after liftoff. Accelerating at the low end of the speed range (from zero to Vr/V2), the lower drag of thinner air doesn't help performance much.

[FE Hoppy]

@BOAC
The speed displayed on many PFDs is in fact CAS not IAS despite the fact that it is indicated.
Of course you then have the good old Avro RJ that displays something called computed air speed which is a crap attempt at CAS.

[AtomKraft]

Isn't it handy that an aircraft will stall at (about) the same ias, no matter what the altitude or TAS?

Nothing short of convenient, so it is.

[hawk37]

Anyone with a ball park figure on how much the stalling CAS will increase as one goes from seal level to 40,000 feet?? percent or number of knots?

Or a guess on how much the stalling aoa in degrees will decrease??

[john_tullamarine]

Isn't it handy that an aircraft will stall at (about) the same ias, no matter what the altitude or TAS?

Are you quite sure of that ?

Anyone with a ball park figure

Plenty of useful references on the net to review .. this Boeing article might be a good starting point -

Aero 12 - Angle of Attack (http://www.boeing.com/commercial/aeromagazine/aero_12/attack_story.html)

[Nock187]

To echo some other comments, we may be sidetracking off the OP's question, which is really quite a high level one.

Also agree it depends on what you mean by "performance". Perhaps a starting point maybe to have a look at the theory behind "Flight Envelopes". This was my first introduction to performance theory back in my teens when started reading some discussion from Chuck Yeager I got a hold of on why aircraft "perform" differently at different altitudes
en.wikipedia.org/wiki/Flight_envelope (http://en.wikipedia.org/wiki/Flight_envelope)

In terms of ability to accelerate, climb or maneuver, generally this is better at lower altitude (and worse at higher altitude) due to higher lift and higher excess power (Generally, although that can depend on the powerplant and airframe combination in question). These are probably the key "performance" characteristics to keep in mind when looking to fly the plane safely. Certainly, this has been topical in the spate of high-altitude stalls/incidents.

Now, in terms of speed, I tend to feel true "performance" is true airspeed. Yes IAS can be different, but on a discussion on aircraft performance...somewhat irrelevant unless we want to talk about a question separate from that the OP asked.

Maximum speed depends on the airframe and powerplant in question. Some planes are designed they will achieve this at near ground level, others will achieve it at higher altitudes.

[stator vane]

Being in the dog house! Funny how we get ourselves into those situations!