IAS and altitude and thrust

[mickypitch]

Hi I think I must be thick but I cannot grasp why at altitude more thrust is not required to keep the same IAS as at sea level?
As density gets less at altitude then everything tells me that you need to travel faster to get the same pressure through the Pitot tube?? Hence more thrust?
Can someone explain this to me as I cant seem to grasp it.
Cheers :(:

[garywoodrow]

i might be completely wrong, sorry if i am, i still have not even started my PPL but wouldn't the plane travel faster with the same thrust because there is less air resistance stopping the plane from traveling in a forward? :confused:

Or maybe not.. you have got me confused now i probably wont be able to sleep now! lol :ugh:

[moggiee]

I'm no aerodynamicist but here goes with my understanding of it.

Weight remains the same, regardless of altitude, therefore the lift requirement remains the same regardless of altitude. In level flight weight=lift.

For a given configuration, lift is proportional to Indicated Air Speed (IAS) so the IAS required to generate that lift remains the same at all altitudes. Whilst True Air Speed (TAS) will increase with altitude, the amount of lift generated at (say) 150kt IAS will remain the same at any altitude.

For a given configuration, drag is proportional to IAS. So at the above mentioned 150kt, the total drag will remain unchanged at all altitudes - because it's related to IAS (and therefore effectively sidesteps the issue of air density). For a steady speed drag=thrust.

A set amount of thrust is required to overcome that drag and because the drag remains unchanged at all altitudes, so does the thrust requirement.

Of course, because of reduced air density at higher altitudes, an engine requires a higher RPM (jet or fixed pitch prop) or blade angle (variable pitch prop) to produce a given amount of thrust. Whilst a jet may need 70% N1 for 210kt at 10,000' and 90% N1 for 210kt at 30,000' the actual amount of thrust being produced will be the same.

I think that makes sense.

[skyhighbird]

I did do Advanced Aerodynamics during my Aero degree and I won't even bother explaining the full answer cos I'll sound like a geek and and I probably wouldn't understand it myself but I think you have posed the question in an incorrect way (from a science point of view- current pilots please correct me).

"why at altitude more thrust is not required to keep the same IAS as at sea level?". I believe this question is posed incorrectly and is actually a wrong statement.

Firstly, you don't keep and don't want to keep the same IAS at altitude as you get at sea level. Remember, TAS (true Air speed) is the speed of a body through relatively undisturbed air mass. You are right in one way that in order to keep the plane flying at high-density altitudes, the aircraft must travel through the air mass at a high rate of speed. Hence, what’s actually increasing is the true airspeed.

So what you are really asking or should be asking is:

"How come the fact that the plane is traveling faster doesn’t show up on the airspeed indicator"?.

It’s because the airspeed indicator displays indicated airspeed (IAS), which is derived from the impact pressure—the number of molecules jammed into the pitot tube at a given moment. Since there are fewer molecules available at higher-density altitudes, the pitot tube must pass through the atmosphere faster to jam as many air molecules down its throat as it would passing through thicker, more densely populated air.

So yes, you do need to go faster but you don't want your IAS to read 490kts at FL390!!! It is usually around 290knts (whatever - depends on aircraft, height, as we all know etc) but it doesn't mean you are only flying at 300-odd miles per hour.

Please forgive me if I got a bit complicated here. I haven't done any Flying Theory so maybe I have given the answer in a convoluted way in line with what I've studied. Perhaps an ATPL pilot can give the answer in 2 sentences! PLus, it be good to know if I am incorrect or waffling on as this question could come up in an interview!


XXX

[sick_bag]

you dont need more thrust at altitude - you need more power, which is not the same thing.

thrust is to overcome drag, so if you have the same IAS at altitude and at low level, the thrust required will be the same, because the drag is the same. This is a slight simplification, because theres an effect caused by the higher Mach number at altitude, but in essence, same IAS = same thrust.

However, at altitude the TRUE airspeed is higher, so the power must have increased, because power = thrust * TAS. And power is energy per unit time. And energy is fuel. Which is why the fuel flow is higher, even though the thrust is the same. (but you still get more miles per Kg of fuel, which is the name of the game)

engine RPM is not directly related to thrust, it depends on the ambient conditions too, so the RPM will be higher at altitude, to produce the same thrust.

[mickypitch]

Hi - thanks for the replies.
I guess the way to remember it is that thrust must equal weight which does not change with altitude. I kept thinking of power as thrust which is not right.
Cheers all :ok:

[matt_hooks]

Brian.

As Mach increases, TAS decreases?

Don't think so old fella.

Without being personal, I don't think I agree with anything you said in your post. :bored:

It's the old "three lines on a graph"

You have three lines. One representing TAS, one representing IAS and the other Mach number.

You can do all sorts of interesting things by making one or other of the lines straight. One thing you CAN'T do is make TAS inversely proportional to mach number.

Anyway...

Without putting up loads of graphs of thrust/drag and power/drag curves it's difficult to answer this question.

Suffice to say, at higher altitudes you will probably find that the aircraft become MACH limited rather than IAS (or TAS) limited.

So the chances are you won't be trying to maintain the same IAS.

Anyway, the crux of the matter is that your IAS for a given amount of lift at a given pitch attitude will be the same, no matter what altitude you're at. So yes, you WILL be going faster, and you WILL require more POWER.

POWER and THRUST are two different things!

It's far too complicated for simple minds like mine to explain, just get hold of examples of the power/thrust against drag curves for IAS and TAS, and it will all become blindingly obvious!

[BEagle]

mickypitch, remember that thrust opposes DRAG.

Drag = 1/2 rho x V**2 x Cd

Keeping it simple and ignoring compressibility, your IAS is the 1/2 rho x V**2 part, so drag = k x IAS x Cd

So if IAS is the same, then as rho decreases with altitude, TAS must increase. Which, ignoring compressibility, has no effect on drag if IAS is kept constant and Cd is constant. Which means that Thrust = k x IAS x Cd as well.

As for 'more' thrust, well, it is rarely possible to set thrust as a specific value. Normally you set a parameter such as rpm or MAP in a prop ac, rpm, EPR, P7 or many other things in a jet ac. In the horrible RAF Jetstream, we even used to set 'blade angle'.... The characteristics of each engine mean that, for the same THRUST value, you would need to set a different parameter at altitude compared with sea level.

There are some real PofF howlers on this thread - hopefully not from any licence holders.....

Neither Thrust nor Power equal Weight! Unless you're in a zero IAS Harrier in the hover, that is, when all the weight is opposed by 'jet lift' thrust.