To fly a given IAS, the thrust required at altitude??

[electriclightfoot]

Can anyone help me with this question:

1) To fly a given IAS, the thrust required at altitude:

a) will be less than sea level
b) will be the same at sea level and will given by the same throttle
position
c) will be the same at sea level but will require advanced throttle
position
d) will be greater than see level

Answers with an explanation please.

Thanking you in advance.

[Tom Power]

a................................

[Pitch+Power]

I'll go for C

:rolleyes:

[AVIATOR1982]

Due to the reduced density of air at any altitude above sea level, to maintain the same IAS you will need to fly at a faster TAS, thus you will require more thrust, this will manifest itself by the need to increase power and push the throttle forward to maintain the same IAS. This can clearly be seen by refering to the basic thrust required graph, i.e as TAS increases more thrust is required. On this basis personally i would be selecting D.......:ok:

[Andy_20]

Hmm, im slightly 50/50 on this because as Aviator said less density should mean more thrust, but doesnt a decrease in air density give a reduction in drag? My first thought was A...

Happy to be proved wrong though.:8

[tmpffisch]

Reasonably basic question. Answer should be D.

IAS is a measure of the amount of air molecules flowing past you. Because of decreased air density (read air molecules), to maintain the same IAS at altitude you'll need to fly faster. Along with this, increased thrust is also required at altitude because the engine is less efficient (less power), because you need to travel faster to create the same amount of lift, and because the prop is creating less thrust too.

Answer is D

[AVIATOR1982]

On another note if you think about it if this was not the case any powered aircraft would in theory be able to climb infanitely high as excess thrust would always be available.

The only reason something like a C172 cannot climb above about 10,000ft is because all its excess thrust is being used to maintain enough TAS to enable the wings to continue to produce lift and keep it in the air. Therefore it cannot overcome the forces acting against the climb and no climb will be possible.

Of course this is very basic stuff there are obviously many other variable's as tmpffisch quite rightly stated volumetric efficiency has a part to play as well, however thats an entirely different topic.

@Andy 20 regarding reduced drag, yes less friction results in less drag however we are talking about maintaining a CONSTANT IAS therefore the same amount of molecules are passing in any given time frame, which will mean there is effectively no reduction in drag.

Just my 2 cents worth, I by no means claim to be any kind of expert....

[mad_jock]

Well i reckon that its C

For a given IAS and none accelerated flight the amount of drag you will getting will be the same as the only variable you are changing is density. And as it cancels out of both sides of the steady state equation it doesn't change a thing for a fixed IAS. This is also backed up by the fact there are 2 answers with "the sames as at sea level"

Now the power position bit of a difficult one to visualise unless you fly things that get up high.

As you get higher the air density decreases so the amount of bite it has on the air get less. But because there are that many things are changing its a bit of a crap question as some intelligent engine control systems fiddle with the fuel flow so that you don't see it.

The easest way to visualise this is that climb is a factor of excess thrust. As you would expect at a given climb power and airspeed you would expect that your rate of climb would decrease the higher you get until the point it reaches zero. So intially you have and excess of power which relates to a lower power lever setting for flying S&L and at zero climb you have the larger power setting but you are still at S&L.

Well thats my theory anyway how to explain it. In practise its the same as well as the torque rolls back the higher you get as the blades fine out. And you keep nudging the levers up as the EGT decreases so S&L (sub FL60) at 180 knts is maintain with about 40% of the range @ 65% torque (35% to spare) and cruise at 250knts TAS but still 180knts IAS is done with 90% of the levers range and still @ 65% torque which is all you can get out of the old girl without exceeding EGT limits

[mad_jock]

Actually as the question uses throttle its easy to explain you need to increase.

The same theory holds that the thrust must be the same.

The throttle needs to be opened up because the power output of the engine is related to the amount of fuel burned so as you get higher the density drops off which means for the same setting of the throttle fewer oxygen molecules get in so less fuel is burn so you are not maintaining thrust.

So the only way to get the same mass of fuel burn't is to get back to the same amount of oxygen molecules as you had before. Because the density has dropped you have a larger volume through and the only way to do that is to advance the throttle.

[AVIATOR1982]

@ Mad Jock I agree with what you are saying, however read what the question is asking for....For a given IAS the THRUST required at altitude will ?............. The only possible answer is D. The examiner clearly wants to know if you are aware of the BASIC principle that as you go higher you will need to fly faster to maintain IAS, and as TAS is directly proportional to the amount of thrust required it can only be D.

For what its worth the answers with throttle included are only in there to try to make you think its more complicated than it seems by trying to get you all confused between thrust and power, and thats the exact trick with these exams, the moment you start getting too complicated is the moment you start getting all the wrong answers bouncing around in you're head YOU HAVE TO KEEP TO THE BASICS and work the answer from there...

[Capt Pit Bull]

trying to get you all confused between thrust and power

Aviator1982,

sorry to have to say it, but in your case the examiner has succeeded.

Thrust required = Drag.

Drag = 1/2 rho V^2 S Cd

Fixed IAS = Fixed 1/2 rho V^2

Therefore thrust is unchanged.

Power = Force x Speed.

Power required = Drag x TAS

Less density, same IAS means increased TAS.

Therefore power required is increased

(And that not even taking into account reduced power for a given RPM by density reduction.)

C FTW!

pb

pb

[Andy_20]

SO what you are saying there is that the same amount of thrust is required top keep a constant IAS but due to the air density power required is increased? :confused:

[Capt Pit Bull]

Andy,

As the air is less dense you're physically moving faster for a given ammount of drag. This means more power is required.

However, since the air is less dense, you don't actually need any more thrust.
When IAS is fixed, you are talking about flying with a fixed value for the 1/2 rho V^2 bit of the lift and drag formulae. As you climb, as rho (air density) is decreasing, then your aircraft must be increasing in V, (which is TAS).

Remember, IAS isn't really a speed, in the proper 'Physics' sense of the word. The only one out of IAS/RAS/CAS/EAS/TAS etc that is truly a speed is TAS, and thats what the V is in the lift formula.

This is why aircraft handle depending on IAS, but Navigate based on TAS.

Topics such as stability or Prop efficiency depend on both the 'IAS/RAS/EAS' bit (for aerodynamic forces) and also the 'TAS' bit with regards to the geometry of the physical motion of the aircraft.

pb

[Snowiey]

IAS is a measurement of air pressure created by the moving air flow (Total - Static = dynamic). If a certain IAS is the same at altitude as it was at sea level then the TAS will be higher. The IAS being constant indicates that the same amount of air molecules is passing over the airframe as it was at sea level. Yes, the air density is reduced at altitude, but this is compensated by travelling at an increased TAS to maintain the constant IAS (and therefore - constant amount of air molecules).
However, for a given throttle setting/position the engine will not be giving the same amount of power at altitude as it was at sea level. (due to the decreased air density). Therefore the throttle would have to be advanced to overcome this, but the thrust required remains constant as the air resistance (drag) is also constant (same amount of air molecules) due to the increased TAS needed to maintain the constant IAS. I'd go with 'C'.

[Andy_20]

How do you guys know this information, so readily available to you? :confused:

Thanks Capt. i shall look forward to these chapters when i start Perf next week. Advice is much appreciated! :ok:

[mad_jock]

Power, work and the other definitions at school aged 13.

Force equilbrium and newtons laws school aged 15.

The fact that any physical system can be explained by just looking at the energy contained in it. Aged 21 at my graduation, speach by some old hairy shipbuilder, a look of dawning came across half the engineers present and ment nothing to any of the rest. Was a bit erked that a 4 year Uni degree can be sumarised in one sentance but hey ho.

How these basic concepts fitted in with aircraft aged 29 when I did the ATPL's.

Every sim check we do we maybe get 100-200 questions on this sort of stuff and other subjects in the ATPL. So you do get used to answering these types of questions. ;)

[Capt Pit Bull]

How do you guys know this information, so readily available to you?

Years of flying and teaching, and spotting the same basic misconceptions cropping up in many students.

It is true, you don't have to understand this stuff, you can pass the ATPLs by rote learning, but a solid understanding of the basic physical quantities and laws of motion makes it hugely easier.

Too many people (and too many poorly written aviation books) muddle up important (but relatively simple) concepts. Here's 10 off the top of my head:

IAS is not a speed.
Momentum is not the same as Kinetic Energy.
Momentum is not inertia.
Kinetic Energy is not a vector.
Force is not the same as Power.
Lift is not defined upwards.
Weight is not Mass.
Weight doesn't change when you pull G.
Moving objects don't need a force to keep them moving.
Accelerations are caused by unbalanced forces, not vice versa.

pb

[Andy_20]

Thanks for the replies, im relatively new in comparison. Im finding the ATPL's ok, no dramas there at all! it just confuses me when i see people giving such detailed explanations, wondering where the hell theyve got that information from!

On another note, how much ATPL information do you think you need to restore for a career in aviation? How much do you think is relevant? and how much of it will you actually use?

For example, GNAV, i understood all the basics, but in reality in some sense the ATPL exam isnt really practical navigation.

Am i making any sense? (answers on a postcard please)..:ok:

[AVIATOR1982]

Well you may well be right, I did very clearly state that I am no expert it has been a while since i studied this subject although I did get 93% in the exam at the time, having done some proper hunting via google i can find several sources that actually say thrust required decreases as altitude increases how acurate this is i don't know but I think I will do some looking up and try and enhance my understanding of these principles, thats the beauty of aviation you learn something new every day.........

[Andy_20]

That is what confused me at first and made me think A was correct, as i read it the other day in the Bristol notes!

[Merritt]

several sources that actually say thrust required decreases as altitude increases

Im only a lowly ppl'er but my understanding is that this is correct if you maintain a constant TRUE air speed. If you maintain a constant IAS, you are actually increasing your TAS.

From the engines point of view, it looses thrust as altitute increases (all other things being equal) because the air is less dense. In order to maintain the same thrust, the throttle needs to be advanced.


Steve