Space needed by high-bypass fans

[chornedsnorkack]

How much are high-bypass turbofans bigger that 1950-s, 1960-s pure turbojets or low-bypass turbofans, comparing engines of similar thrust?

Also, are they heavier or lighter?

I have heard quite some planes have actually had engines removed and different engines installed - this is sometimes possible. And pure turbojets have more noise than high-bypass turbofans, as well as having more fuel burn per thrust.

Turbofans work by sucking in a lot of air and accelerating it to moderate speeds, compared to a pure turbojet. Does it mean that a high-bypass turbofan will not work well near another turbofan, as they compete for air to suck in? Pure turbojets and low bypass turbofans are sometimes mounted in pairs, inlets practically next to one another, as on B-52, VC-10 and Concorde. Could high-bypass turbofans be mounted near each other?

[Tim Zukas]

Wonder why nobody's speaking up?

I guess you can get today's weights off the manufacturer websites. The JT4A-11 with 17500 lb sea-level static thrust weighed about 5000 lb, and the Conway (about same thrust, as used on DC-8-40 and 707-420) was around 4500 lb. JT4A diameter 43 inches, about the same as a JT8D.

[barit1]

I don't have the weights handy, but a DC-8-60 w/JT3D's could be converted to a -70 series w/CFM56's. The new engine is larger diameter (less ground clearance) and more thrust but lighter. Over 100 planes were converted 20 years ago.

[The African Dude]

Can't be bothered to find the data but purely for discussion purposes as there was only one other reply:

"How much are high-bypass turbofans bigger that 1950-s, 1960-s pure turbojets or low-bypass turbofans, comparing engines of similar thrust?
Also, are they heavier or lighter?"

Reduction in weight is important for aero engines. The heavier the aircraft, the greater lift required, requiring greater airspeed, thus greater thrust and therefore more fuel is consumed. The heavier the engines, the more fuel used to get from A to B. Therefore you'll find engine manufacturers will only increase the weight of their shiny new engine design in comparison to an old one of similar thrust if there's a much more compensatory increase in efficiency as a result. Bluntly speaking that's never going to happen. So what I'm trying to say is that RR, IAE etc. will work to reduce the engine weight in design and so you SHOULD find them to be LIGHTER.


"I have heard quite some planes have actually had engines removed and different engines installed"

Yes - for example, check out pictures on a*rliners.net of BA's 747's in the 80's and then do a reg search for the same aircraft today if still flying.


"And pure turbojets have more noise than high-bypass turbofans,"

This is due to the pressure fluctuations of a single hot jet stream. In a turbofan, the hot streat is surrounded by a 'cold blanket' from the bypass flow. This reduces pressure fluctuations which we hear as sound, i.e. noise.


".. as well as having more fuel burn per thrust."

Again, yes but turbofans can't reach supersonic speeds, and in these situations a turbojet must be used regardless of the lower efficiency.


"Does it mean that a high-bypass turbofan will not work well near another turbofan, as they compete for air to suck in?"

There is plenty of air to go around :) And if there was a slight, very slight decrease in the local air pressure at the inlet nozzle entry, it would have no calculable effects on the efficiency of the engine (which takes into account the air pressure on entry to the compressor).


"Could high-bypass turbofans be mounted near each other?"

Yes, but there are no additional benefits to doing so. However, maintenance practices would be restricted, and in addition high-bypass turbofans such as the RR-Trent or GEnx are way too big / noisy for mounting on teh side of the fuselage, or stuck to eachother... etc.

:}

[barit1]

...
Reduction in weight is important for aero engines. The heavier the aircraft, the greater lift required, requiring greater airspeed, thus greater thrust and therefore more fuel is consumed. The heavier the engines, the more fuel used to get from A to B. Therefore you'll find engine manufacturers will only increase the weight of their shiny new engine design in comparison to an old one of similar thrust if there's a much more compensatory increase in efficiency as a result. Bluntly speaking that's never going to happen...


Well --- not quite.

It's total propulsion SYSTEM weight that's important. Don't forget to count the weight of the mission fuel.

So on a short-range aircraft, specific fuel consumption (ie engine efficiency) might be less important than paring engine weight to the bone, because the fuel carried is a smaller weight compared to the payload.

On a long-range bird it's the other way around. The fuel load at TO is a LOT heavier than the payload, so the emphasis is all-out on efficiency - sipping fuel like tea. To do this might require a slightly heavier engine carcass and more system optimizing - a reasonable tradeoff IF it saves fuel.

So in the real world - the answer is "It all depends..." :8

[chornedsnorkack]

Can't be bothered to find the data but purely for discussion purposes as there was only one other reply:
"How much are high-bypass turbofans bigger that 1950-s, 1960-s pure turbojets or low-bypass turbofans, comparing engines of similar thrust?
Also, are they heavier or lighter?"
Reduction in weight is important for aero engines. The heavier the aircraft, the greater lift required, requiring greater airspeed, thus greater thrust and therefore more fuel is consumed. The heavier the engines, the more fuel used to get from A to B. Therefore you'll find engine manufacturers will only increase the weight of their shiny new engine design in comparison to an old one of similar thrust if there's a much more compensatory increase in efficiency as a result. Bluntly speaking that's never going to happen. So what I'm trying to say is that RR, IAE etc. will work to reduce the engine weight in design and so you SHOULD find them to be LIGHTER.
http://www.concordesst.com/concordeb.html
It seems that replacing the pure turbojets with low-bypass turbofans on Concorde B was expected to add over 5 tons engine weight - yet somehow stretch the range.

"And pure turbojets have more noise than high-bypass turbofans,"
This is due to the pressure fluctuations of a single hot jet stream. In a turbofan, the hot streat is surrounded by a 'cold blanket' from the bypass flow. This reduces pressure fluctuations which we hear as sound, i.e. noise.
".. as well as having more fuel burn per thrust."
Again, yes but turbofans can't reach supersonic speeds, and in these situations a turbojet must be used regardless of the lower efficiency.
Or low-bypass turbofan. I think a plenty of military planes flying supersonic are said to have low-bypass turbofans rather than pure turbojets.

"Does it mean that a high-bypass turbofan will not work well near another turbofan, as they compete for air to suck in?"
There is plenty of air to go around :) And if there was a slight, very slight decrease in the local air pressure at the inlet nozzle entry, it would have no calculable effects on the efficiency of the engine (which takes into account the air pressure on entry to the compressor).
Wasn´t it a major problem with the inlet design of Concorde that the engines disturbed airflow into each other?

"Could high-bypass turbofans be mounted near each other?"
Yes, but there are no additional benefits to doing so. However, maintenance practices would be restricted, and in addition high-bypass turbofans such as the RR-Trent or GEnx are way too big / noisy for mounting on teh side of the fuselage, or stuck to eachother... etc.
:}

How noisy are the high-bypass engines of MD-90 compared to the 1960-s engines of DC-9 first versions?

[The African Dude]

Thanks barit1, learned something there! :)

Chornedsnork...
"Wasn´t it a major problem with the inlet design of Concorde that the engines disturbed airflow into each other?"
How did they solve it then?

[shuttlebus]

I seem to remember that Brian Trubshaw's book covered the intake issues with Concorde in great detail.

As there are ex-Concorde pilots on line here, you may get a better reply, but from memory, it came down to scheduling of the intake ramps to prevent surge.

A 1960's computer was responsible for all the computations and I believe even today the infomration is deemed "commercially sensitive". There was a rumour that these computers were removed from the aircraft adter their final flights by the respective airlines - of course it might have been Airbus ensuring that no-one had the opportunity for a joyride :ok:

Incidently, the SR-71 also suffered from surges (called an unstart) when airflow disturbance caused the engine thrust to decay. It accounted for the loss of at least on eairframe due to the high yaw rate induced by such an event at Mach 2.5+.

Regards,

Shuttlebus