Question - Decreasing TAS at Height, benefit?

[wangus]

A colleague of mine and I were discussing some aspects of principles of flight (pre-type rating course)and we both got stumped on something. We are both frozen atpls so please be gentle on our knowledge.
As you complete a long distance flight and the aircraft lightens due to fuel burn, you gradually increase flight levels. If you are cruising at a constant M0.79, assuming ever decreasing temperature with increased height, your TAS will be decreasing, at least up to the tropopause. If this is the case, as you get higher you are in fact flying slower due to the temperature decrease, while maintaining .79. Now the STUPID question. Is the advantage the increased efficiency of the engine?? Why doesn't the increased TAS slightly lower outweigh the gain of the higher FL? I completed my theory 4 years ago, and cannot remember. Sat here scratching my bald head.
Why fly higher and accept reduces TAS?????

[LeftHeadingNorth]

the simple answer is that the reduced fuel burn outweighs the decrement in TAS....

[nitpicker330]

Damn, he beat me to it. :ok:

In any Aircraft you want the best ( lowest ) kg per ground nautical mile. Kg/gnm.

Pick the best FL to give the answer with regards to FF ( A/C weight, Mach Etc ) and G/S

[mustafagander]

There is a lot of design work goes into this, but simply put the aircraft has an "optimum altitude" for each combination of weight and temperature. It will be tabulated in the AFM. Off optimum you lose, it's that simple. If you're low you can claw back a bit by flying slower.

The gas turbine engine has a design point of compressor speeds which is most efficient and it (they) hang off an airframe which has a most efficient AoA. At optimum altitude these two points are in harmony. As weight reduces, less thrust is required as well as less lift meaning that both the wing and the engine tend to move away from design points for sensible cruise speeds. Climbing tends to restore this balance but generally the engine has the most influence so we climb more to ensure the engine is operating at designed RPM and hence efficiency. Wings tend to be more tolerant of off design conditions than engines.

In general terms as altitude increases, air density and (importantly) temperature both decrease. Lower density air means less drag but of course less lift. Lower temperatures mean better engine efficiency. As has been previously posted, there is a reduction in TAS but fuel flow falls faster while flight times increase. See the trades now needing to be balanced?

A quick and dirty reply from me but there are some on these fora who have devoted a good part of their working lives to such number juggling and I'm sure that one at least will correct any errors I am posting here.

[wangus]

Thanks for your initial responses. My brain has just managed to further confuse itself. The wing doesn't know or care if it is at sea level or FL390. The IAS/CAS determines the performance of the wing. Why doesn't the same apply to the engine? If you are blasting through the less dense air at FL451 in a 747-400, for argument's sake at (MADE UP NUMBERS) Mach 0.85 and CAS of 230 Kts, isn't the engine ingesting / compressing air at the equivalent of 230 kts at sealevel? Isn't the ram effect creating the same conditions as sea level? If the wing doesn't care about TAS, only ACTUAL air flowing over it, ie IAS/CAS, why is the engine different? Why is is improving while wing remains subject to CAS?:confused: If the same amount of air is passing through the engine, why does the fuel burn lower from 230kts at sea level?

[Jonty]

The jet engine is at its most efficient at about 90% RPM. At 230KTS at sea level the engine will be nowhere near its most efficient operating range. And if you try to get the engine up to 90% RPM then you will, in all probability over speed the aircraft.

So we have to take the aircraft to a level where the engine can operate at its most efficient, and where it is supplying enough thrust so as to keep the aircraft in the sky and not over speed it. This is done by reducing the mass airflow into the engine (climbing to high level) and thus the amount of fuel required to burn that mass air flow is subsequently reduced.

So the engine is not improving, the performance of it is getting worse as you climb. However it has far too much performance at sea level, so we need to reduce that performance to a level that means that we can operate it at its most efficient RPM without damaging the aircraft.

If that makes sense?

[stubby1]

If you see the drag vs TAS curve, we want the best TAS/DRAG.

Now as wt reduces in cruise due fuel burn, we have two choices

1. Reduce IAS by reducing thrust (hence fuel flow). The TAS reduces BUT Drag is propor tional to IAS2 , hence better TAS/DRAG. (constant altitude method)

2. As wt reduces start climb such that TAS is constant or reducing to maintain constant MNo. whilst in this the TAS is reducing but the relationship between IAS & TAS is such that IAS reduces to effectively reduce DRAG much more hence giving better TAS/DRAG. (climb cruise method)
;)

So better TAS/DRAG in climb cruise method

You can check & confirm with the RAF AP3456.. good book.

[wangus]

Thanks for all your inputs. I still cannot get my head around one point. If the WING is thinking it is flying 230kts at great height, ie IAS/CAS, despite a TAS of dramatically more, why is engine different. Isn't it experiencing the same INDICATED airflow into the intake as the wing is flying through? If straight and level at 100ft at 230kts requires 60% N1 (made up) then based on the confusion, why doesn't 230 kts at FL390 require 60% N1???? I understand that the air is less dense, but by flying a TAS of, I don't know, 454 kts, and therefore getting 230 kts indicated, if the wing doesn't know the difference between 100 ft and FL 390, why is engine any different? Why doesn't the high EAS/IAS of 230 kts mean engine is getting enough air? If engine is approaching starvation, how is wing happily thinking "I'm flying along at a lovely 230kts.."
Thanks in advance for your inputs and patience!

[kenparry]

and therefore getting 230 kts indicated, if the wing doesn't know the difference between 100 ft and FL 390, why is engine any different? Why doesn't the high EAS/IAS of 230 kts mean engine is getting enough air? If engine is approaching starvation, how is wing happily thinking "I'm flying along at a lovely 230kts

You are right about the wing if you ignore Mach effects, but the engine is very different. It is a thermodynamic machine turning fuel into thrust. It is definitely getting "enough air" at high level. The big difference is the thermal cycle conditions. At ISA sea level, input static temp is +15 deg C, at FL 350 it is around -56 deg C. This means the compressor has very different inlet conditions, and if you examine the maths of the thermodynamic cycle you can show that the efficiency is improved by that lower inlet temperature.

[Da-20 monkey]

If you look in the books you will find a TAS versus power graph for a jet engine. I was also a bit stumped at the fact that jet power increases with TAS instead of IAS.
The way I made sense out of it: A climbing jet:

1. At a higher TAS there is a greater column of air flowing through the engine per time.

2. This may allow the fan/ compressor to rotate faster, improving it's compression ratio. This should increase the N1 in theory.

3. I'm not sure about this; but I think that as long as the thrust lever is not moved while climbing, the engine wants to maintain a constant N1 RPM, so it will simply decrease fuel flow to compensate for the improved compression ratio.

Also, I believe the highest level flight TAS for a jet can be achieved at the changeover altitude where, in a climb, one would go from a constant IAS to a constant Mach climb speed. this is at about 25.000ft.

But, jets climb higher. One of the reasons for this must be for compressibility effects, which will also improve fuel flow.

Also, at the tropopause you have a reasonably good TAS at a cold temperature, which also means a denser column of air through the engine per time.

It may make more sense if you think of IAS as a certain pressure working on the airplane per time, and TAS as a column of air flowing over the airplane per time.

Please correct me if i'm wrong, this certainly is an interesting topic, but not always easely explained in the books! :ugh:

[wangus]

Thanks guys. I think my brain is getting around this. Are the following statements all correct???:

1) Using the 777, GE90 as an example, the (vague memory), the 112000 lbs of thrust it claims would be produced, at ISA condition, and ISA only. By the time you reach the cruise, the thrust ACTUALLy being produced had decreased GREATLY.

2) Just because you have a high EPR and N1 setting at FL350, it doesn't mean you are producing the same thrust at great height as you are at FL110 with the same EPR and N1 setting. You will in fact be producing LESS thrust with same setting.

3) Due to the lower ambient air pressure at height, there is less resistance to the engine exhaust which allows greater exhaust velocity and therefore energy and efficiency. (Action and reaction - Greater action?)

4) A turbine will automatically, at height,:confused confused turn at higher RPM at a given fuel input level and attain the same compression pressure.

5)Are there any turbine engines where you MANUALLY decrease the mixture at height? If so, what are you monitoring to get the correct mixture?

6) Does the lower air temperature result in greater expansion from the compressor through to the turbines, and onward, allowing for greater energy to be converted into efflux AND fan rotation via the spool?

[wangus]

Hi DA, just noticed your post.

1. At a higher TAS there is a greater column of air flowing through the engine per time.

This is the statement where I am struggling. My understanding is that despite the higher TAS, there ISN'T actually the equivalent of any more air flowing through the engine. In 10 seconds of flight, there is the equivalent of 10 seconds of the CAS/IAS air passing through the engine. In my understanding, (which I'm sure IS wrong!) TAS irrelevant.

You are probably right. I just cannot see it. Asked a couple of friends who fly A320s and 737s, and they scratched their heads. "It's all auto throttle.... Not sure..."!!!!!!!!!!!!!!

[Checkboard]

http://www.pprune.org/tech-log/10655-why-jet-engines-more-fuel-efficient-high-altitude.html

The efficiency of a heat engine is largely governed by the compression ratio. A heat engine is one that converts heat in a gas (added by a fuel) to do work. All combustion engines, like the Otto cycle (four stroke) engine, Diesel engine and Brayton Cycle (gas turbine) engine are heat engines.

How does the amount the air/fuel mix is compressed effect the efficiency? Imagine a piston travelling up a cylinder and compressing the gas in that cylinder (like pushing in the handle of a bike pump with your thumb over the exit hole). If released, the piston would now travel back down the cylinder until the pressure inside equalled the atmospheric pressure. The work done in compressing the gas is recovered as the piston moves back down the cylinder under the pressure of the compressed gas (less friction losses etc.)

If the piston compresses the gas to half its original volume, then it has a compression ratio of 2:1, as the piston recovers it can do work based on the compression it started with. The more the air is compressed, the more work can be done as it recovers. Otto cycle piston engines have a compression ratio of around 8:1.

Now lets introduce some combustion after the piston has compressed the air, as happens in an Otto cycle engine (a "suck, squeeze, bang, blow four stroke). If the charge was only compressed at a 2:1 ratio, then the effect of the combustion can only do work until the piston recovers to ambient pressure. The more the piston compresses the charge, the further away from ambient pressure it is and thus more work can be done by the piston as it recovers. So the higher the fuel/air mix is compressed, the more efficiently that fuel can expend its energy.

Why not compress the charge more in a four stroke, and get even better efficiency? Because as the charge is compressed it heats up, until at higher compressions it heats enough to explode at the wrong time ("known as "pinging", "knocking" or detonation) which may create enough pressure to exceed the strength of the engine components. Typically you put holes in pistons or cylinder heads - not considered a good thing.

Diesel engines actually take advantage of this effect and use "compression ignition" instead of a spark plug, so they can achieve double the compression of Otto engines and are consequentially more efficient. An aviation diesel engine, burning kerosene (Jet fuel) is being developed and should be available on the Socata Trinidad in a year or so. It is lighter, has less moving parts and burns less fuel with more power than your current Lycoming or Continental.

All this can be proved mathematically from first principles through the laws of thermodynamics, a branch of Physics usually studied at second year degree level in Mechanical Engineering. I won't bore you with the proof, but if you are really interested you can click here.

The same principle applies in gas turbine engines, although technically you don't talk about compression ratios, but rather cycle pressure ratios. Known as the thermal efficiency, or internal efficiency of the engine the higher the pressure ratios, the higher the efficiency of combustion. As the compressor is not a cylinder travelling a fixed distance, but a "fan blowing air into a small space" the pressure ratio is governed by the engine RPM.
So gas turbine engines like to run at high RPM.

Next you need to look at the propulsive efficiency, or external efficiency of the engine (the study in Physics known as mechanics). The amount of thrust provided by an engine (propeller or jet) is related to the amount of air they throw out the back, and the speed at which they throw that air. In symbols, if m kilograms is the mass of air affected per second, and if it is given an extra velocity of v meters per second by the propulsion device, then the momentum given to the air per second is mv, so Thrust = mv (per second).

A propeller engine uses a large m and a small v, a gas turbine engine uses a small m and a large v. 10 kg of air given a velocity of 1 m/s has the same thrust as 1 kg/s of air given a velocity of 10 m/s. Which is most efficient? Well, the rate at which kinetic energy is given to the air (the work done) is ½mv² watts. So the first case requires 5 watts of work and the latter requires 50 watts of work. Clearly the piston is more efficient. Problems occur as the speed increases, and the propeller efficiency breaks down.

An easier way to think of it is by considering the waste energy in the flow. Stand behind a propeller engine at takeoff, and it will knock your hat off. Stand behind a jet giving the same thrust and it will knock you off your feet! Waste energy dissipated in the jet wake, which represents a loss, can be expressed as [W(vj-V)²]÷2g (W is the mass flow, vj is the jet velocity, V is the aircraft velocity, so (vj-V) is the waste velocity). As the jet exhaust leaves the gas turbine at roughly the same speed whether it is standing still or moving, the faster the engine is moving the less waste energy lost. Assuming an aircraft speed of 375 mph and a jet velocity of 1,230 mph the efficiency of a turbo-jet is approx. 47%. At 600 mph the efficiency is approx. 66%. Propeller efficiency at these speeds is approx. 82% and 55% respectively.
So gas turbine engines like to fly at high TAS.

The problem is that at high RPM at sea level turbo-jets are sucking in very thick air, so when they add a heap of fuel and burn it they make tremendous amounts of thrust - good for take-off but way too much for level flight at low level (with the associated high IAS).
Aircraft generate too much drag at high Indicated Air Speed and keeping the engine turning at an efficiently high RPM at sea level would quickly exceed the aircraft speed limitations.

As you increase altitude, however, the amount of thrust the engine can produce reduces (as it is sucking in "thinner" air) even though it is still operating at high compression ratios (for good thermal efficiency). Also you can have a high TAS (for good propulsive efficiency) at a low IAS (for lower drag on the airframe). There are a few other advantages, like the cold temperature, which keeps the turbine temperatures down as well.

So jet engines like to fly at high power and at high TAS, while aeroplanes like a moderate IAS, and the regime were all this can be achieved together is at high altitude.

[keith williams]

My understanding is that despite the higher TAS, there ISN'T actually the equivalent of any more air flowing through the engine. In 10 seconds of flight, there is the equivalent of 10 seconds of the CAS/IAS air passing through the engine.


Let's test that argument.

If we ignore the slight difference between EAS and CAS then for a constant CAS we must have a constant dynamic pressure.

Pdyn = 1/2 Rho TAS squared.

Lets call msl density Rho1
And let's call msl TAS TAS1
And let's call Pdyn Pdyn1

At msl Pdyn 1 = 1/2 x (Rho1) x (TAS1) squared

Now let's go to an altitude at which the density is 1/4 of the msl value

For the same CAS we must have the same Pdyn and for the same Pdyn we must have

Pdyn 2 = 1/2 x (1/4 Rho1) x (TAS 2) squared.

For the CAS to remain unchanged Pdyn 1 must be equal to Pdyn 2.

So to compensate for the fact that Rh0 2 is only 1/4 of Rho 1, (TAS 2)squared must be four times (TAS 1) squared.

This means that TAS 2 must be 2 x TAS 1

So in climbing to our altitude at which density is 1/4 of the msl value our TAS has doubled to maintain constant CAS.

Now let's look at what this means for the air mass flow rate

Mass flow rate of air going though the air linlet isl

Air Mass Flow = Inlet Area x Air Density x TAS

At msl we have Inlet Area x Rho1 x TAS 1

At our higher altitude we have Inlet area x 1/4 Rho1 x (2 x TAS1)

This means that the mass flow rate at the higher altitude is only 1/2 of the msl value.

This reduction in air mass flow rate causes the thrust at any given combination of CAS and RPM to decrease as we climb.

[wangus]

Thanks Checkboard and Keith Williams, and everyone else who has contributed. Going to read, re-read, and re-read these responses again. I think I'm with you. I think.....:eek:

[Escape Path]

For me, post #2 on the thread linked by checkboard sums it all up quite nicely.