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Nick Figaretto
10th Feb 2002, 15:40
Does anyone have a simple explanation on why Fuel Flow is lower on a turbo-prop at low altitudes than on a jet engine?

As far as I know, on short route sectors (low cruising altitudes) the burn of a turbo-prop regional aircraft may be down to half of a comparable Regional Jet.

I think I've got a general idea on why FF is high at low altitudes on a jet. (It is generally due to high density of the air, which requires more fuel. (...?))

I've also got a general idea on why a turbo-prop is relatively ineffective (=high FF) at high altitude. (It is due to low propeller efficiency in low density air.)

But why is the FF of a turbo-prop so (relatively) much less at low altitudes? It is still a jet engine driving a propeller?

Nick.

John Farley
10th Feb 2002, 16:40
In order to drive the prop, the turbine stages of a turbo prop are specifically designed to extract as much energy as possible from the “jet” before it goes to atmosphere. Thus there is considerably less wasted energy in its exhaust compared to a jet. Hence less fuel flow for a given amount of power.

As you say, the turbo prop blows some of this advantage if you go too high as the prop efficiency degrades up there.

Keith.Williams.
10th Feb 2002, 18:40
Nick,

To understand this phenomenon we need to consider what the propulsion systems actually do. In the case of both turbojets and turbo-props, the first step is the conversion of chemical energy in the fuel into thermal energy. The efficiency of this process varies with inlet air temperature and RPM. The colder the incoming air, the more efficient the thermal process will be. Although it might be tempting to imaging that thermal efficiency also increases with increasing RPM, this is not entirely true. Engines are designed to be most efficient at cruising RPM. For a turbojet this is typically in the 85% to 95% RPM range. Thermal efficiency declines as RPM moves above or below this band.

The thermal energy must then be converted into thrust power. This process is most efficient when the aircraft TAS is very close to the speed of the jet exhaust or propeller wake.

In a turbojet, this second conversion is carried out by the engine and propelling nozzle. In a turboprop it is carried out by a power turbine and the propeller. So the efficiency of this process in a turboprop depends to a great extent upon the efficiency of the propeller. These are designed to be most efficient at relatively low altitudes and speeds.

The overall efficiency of an aircraft is the product of the efficiency with which fuel is turned into thrust power (thermal/propulsive efficiency) and the efficiency with which that power is used to push the aircraft forward (TAS :) rag ratio). So to achieve the maximum overall efficiency, aircraft must operate at the altitude producing the best combination of thermal/propulsive efficiency and TAS :) rag ratio.

Engines are designed to be most efficient when in their cruise condition, which for a turbojet is typically in the 85% to 95% RPM range. So turbojet aircraft are designed so that the thrust in this RPM range matches the drag when cruising in the cold air at high altitude. At lower altitudes the denser air makes the engines capable of producing too much thrust. So RPMs must be reduced below the most efficient range. So jet aircraft flying at low altitude have high fuel consumption rates because their engines are operating below their optimum RPM range.

In turboprops, although the colder air at high altitude improves thermal efficiency, the higher TAS at any given CAS, reduces propeller efficiency. At high altitude the losses caused by propeller inefficiency outweigh the higher thermal efficiency provided by the colder air. This problem is further compounded by the fact that these engines are designed to run in their optimum RPM range when cruising at relatively low altitudes. So at higher altitudes the lower air density requires them to run at RPMs above their optimum range.

Or more simply, turbojets are designed to be most efficient at high altitudes and turboprops at low altitudes. So turbojets at low altitude and turboprops at high altitude, both burn more fuel because they are outside of their optimum conditions.

Mago
10th Feb 2002, 19:31
A simple explanation would be that the combustion chamber in the TP´s is smaller than the ones on the TJ´s.

Let`s compare similar size engines and see the difference in FF; ou could not compare a PW120 with a CFM56.

Keith.Williams.
11th Feb 2002, 02:30
I must confess that being considered too complicated is something of a novelty to me. I am usually accused of over simplifying things!. .

CANVEN

You are of course correct, but your approach adds little insight to the subject. A glider has no combustion chamber and so must achieve the best possible fuel efficiency. But there is more to the original question than this.

BIK 116.80

You also are of course correct, but are missing the point of the original question. It related to altitude not speed. It is true that increasing altitude also increases TAS at any given CAS, but there is more than this to the question in hand.

Consider first a pure turbojet with its small mass of air and big acceleration. Fly at 400 kts at 1000 ft and observe the fuel flow. Now climb to 40000 ft while maintaining constant TAS, and the fuel flow will decrease. One (inadequate) explanation for this is that as altitude increases, the reducing air density reduces the drag at constant TAS, so we are doing less work and therefore require less fuel.

But now try to do the same with a turboprop, but at a more realistic 250 kts. This time we will find that fuel flow decreases up to about 25000 ft then starts to increase again. We will never get to 40000 ft because decreasing overall efficiency (thermal x propulsive) makes this impossible.

Both aircarft experienced decreasing drag, but the effect on fuel flow was quite different.

In the case of the jet, the decreasing air temeparture improved thermal efficiency, so fuel flow decreased. The turboprop also benefitted from decreasing temperature, but this was more than balanced by the decreasing propeller eficiency.

Checkboard
11th Feb 2002, 11:56
I wrote a treatise on this a little while ago, which is no in the archives. You might find it interesting.

<a href="http://www.pprune.org/ubb/NonCGI/ultimatebb.php?ubb=get_topic&f=71&t=000009" target="_blank">Why are jet engines more fuel efficient at high altitude?</a>

It's about halfway down the first page.

Nick Figaretto
15th Feb 2002, 23:58
Thanks alot for good anwers!

Would it be true, then, that if a turbo jet engine and a turbo prop engine, which produces the same amount of thrust, and were both designed be most efficient at a relatively low altitude (let's say FL150) the FF would be practically the same? Or would a turbo prop still be more fuel efficient?

..I guess it probably still would vary with TAS, as turbo jets would be more efficient at high TAS than a turbo prop?

. .Nick.

Keith.Williams.
16th Feb 2002, 21:07
Nick,

The basis of your question appears to be that "At low altitude the engines of a jet produce too much thrust, so have to run them at an inefficiently low RPM. By using fewer or less powerful engines, will we be able to run them at their optimum RPM at low altitude, thereby achieving the same level of fuel efficiency as we do at high altitude?". . . .The short answer to this question is no!

The speed at which an aircraft moves over the ground is proportional to TAS, whereas the drag it generates is proportional to CAS. At 4000 ft for example, . .the TAS is about twice the CAS. This means that by operating at 40000 ft we cover the ground at twice the speed, but pay no extra penalty in terms of drag. So the airframe is more aerodynamically efficient at high altitude. This in turn means that whatever we might achieve in terms of the fuel efficiency of the engines, a jet will always get more miles per gallon at high altitude that at low.

This does not however mean that your idea has no merit. It is commonly used when a high speed jet requires to loiter for long periods. In the case of maritime patrols for example, the NIMROD has 4 engines and is able to cruise at reasonably high speed at high altitude. But to loiter for 5 or 6 hours over a small area of ocean, 1 or 2 of its engines are shut down, thereby enabling the others to be operated closer to their optimum RPM. But this does not mean that the remaining engines are achieving the same fuel efficiency as they would at high altitude. The problem here is one of propulsive efficiency.

Propulsive efficiency is a measure of how efficiently the mechanical energy in the hot gas is transferred to the aircraft. 100% efficiency means that all of this energy is transferred to the aircraft and none of it is wasted in the exhaust. As explained by Checkboard in a previous post, propulsive efficiency is determined by the ratio of the TAS of the aircraft to the TAS of its jet plume (or prop wash). The closer these two speeds become, the greater will be the propulsive efficiency.

When the two speeds are equal, the exhaust gas will be laid down behind the aircraft as if it had never been disturbed. The air will have been given no mechanical energy, so propulsive efficiency will (according to the equation quoted by Checkboard) be 100% . Unfortunately this also means that the gas will not have been accelerated, so no thrust will be produced, so no energy will be given to the aircraft. This in turn means that propulsive efficiency will be zero. So 100% propulsive efficiency can never be achieved. But propulsive efficiency can still be maximised by keeping the aircraft TAS and exhaust TAS as close as possible. Jet engines produce thrust by taking a small mass of air and giving it a very large acceleration. So for maximum propulsive efficiency jet aircraft must fly at high speed.

So if we reduce the size or number of our jet engines to get high RPM and good thermal efficiency at low altitude, we will need to fly at very high TAS to achieve good propulsive efficiency. But at low altitudes the TAS is much closer to the CAS, and drag is proportional to CAS. So at low altitude, the high TAS required for high jet engine propulsive efficiency will incur very high drag forces. Power required is equal to drag x TAS, so this low altitude high TAS flight will require a great deal of power. So although the higher RPM will improve specific fuel consumption, this benefit will be lost due to the higher power requirement.

The most effective solution to this problem is to take our jet engines and fit propellers to them. This will increase the mass flow of air, enabling the same thrust to be generated using much lower acceleration rate. This in turn will give good propulsive efficiency at reasonably low airspeeds which will incur low drag and low power required. But then we will have turned them into turbo-props?

brain fade
20th Feb 2002, 05:01
TILT!