Jet aircraft question
Discussion
I've had a couple of books on the development of jets over Christmas, and something often mentioned is that early designs which used engines on the wings (as per the Meteor) fell out of favour as they were difficult to control if one engine was lost.
Makes sense - but all modern airliners have the engines on the wings, and even 2 engine types seem barely affected by the loss of an engine, even on take-off. So what has changed? Is it all down to modern control systems, or better aerodynamic design, or...?
Makes sense - but all modern airliners have the engines on the wings, and even 2 engine types seem barely affected by the loss of an engine, even on take-off. So what has changed? Is it all down to modern control systems, or better aerodynamic design, or...?
ndtman said:
Single spool engines resulted in high gyroscopic precession. Later multi spool engines with contra rotating assemblies are not as bad. Could be utterly wrong though.
Trent 900 circa 2006 was the first RR civil large engine to counter rotate its spools and that isn't on a two engined plane. I'd go with modern aircraft having much better thrust to weight ratios at low altitude and low speeds combined with better control responses from fly by wire systems and FADEC's on the engines.
Nowadays, the loss of an engine isn't so crucial in terms of thrust. Certification standards dictate that engine out performance (namely take-off climb performance, having lost an engine at the worst possible time) meets certain criteria. They aren't particularly comfortable criteria, in so much as they have you touring the countryside at 400ft while you accelerate; but modern airliners far exceed these requirements.
The issue with two engined aircraft is controllability. As you can imagine, of one wing-mounted engine fails, you'll have a big yawing force due to asymmetric thrust. This leads to a loss of controllability at low speed - until you have sufficient airflow over your control surfaces to control this yaw. Again, performance parameters are in place to ensue that you won't be committed to taking off with an uncontrollable aircraft. If you're interested, these speed are known as min control speeds ; Vmca (in the air) and Vmcg (on the ground).
Single engine flight can still be a handful, if you consider whopping great engines like GE-90s trying to swing the aircraft round, but as mentioned, control surfaces are larger and more authoritative on modern aircraft. Consequently, given the aircraft certification, you don't need to worry about it being uncontrollable, if you fly it properly.
Other factors help, like reduced thrust take-offs, and on some aircraft, other aids such as 'thrust asymmetry control', which senses asymmetric thrust and helps out by automatically applying a bit of rudder, and systems that link the rudder to the control column at low speeds.
All in all though, we know that the aircraft is perfectly capable of being suitably controlled after losing an engine, so long as it's flown in an appropriately coordinated way.
The issue with two engined aircraft is controllability. As you can imagine, of one wing-mounted engine fails, you'll have a big yawing force due to asymmetric thrust. This leads to a loss of controllability at low speed - until you have sufficient airflow over your control surfaces to control this yaw. Again, performance parameters are in place to ensue that you won't be committed to taking off with an uncontrollable aircraft. If you're interested, these speed are known as min control speeds ; Vmca (in the air) and Vmcg (on the ground).
Single engine flight can still be a handful, if you consider whopping great engines like GE-90s trying to swing the aircraft round, but as mentioned, control surfaces are larger and more authoritative on modern aircraft. Consequently, given the aircraft certification, you don't need to worry about it being uncontrollable, if you fly it properly.
Other factors help, like reduced thrust take-offs, and on some aircraft, other aids such as 'thrust asymmetry control', which senses asymmetric thrust and helps out by automatically applying a bit of rudder, and systems that link the rudder to the control column at low speeds.
All in all though, we know that the aircraft is perfectly capable of being suitably controlled after losing an engine, so long as it's flown in an appropriately coordinated way.
So it sounds like a bit of everything - better aerodynamic design to make sure that there is enough control surface available, coupled with better control systems to reduce the workload, and staying within a flight envelope that makes sure you don't end up in a position where control would be lost. And I guess that the design parameters are so different, what works for a civil airliner is probably completely innapropriate for a smaller transonic aircraft.
Interesting stuff, thanks all.
Interesting stuff, thanks all.
CrutyRammers said:
So it sounds like a bit of everything - better aerodynamic design to make sure that there is enough control surface available, coupled with better control systems to reduce the workload, and staying within a flight envelope that makes sure you don't end up in a position where control would be lost. And I guess that the design parameters are so different, what works for a civil airliner is probably completely innapropriate for a smaller transonic aircraft.
Interesting stuff, thanks all.
I guess a lot of it is driven by the relevant airworthiness regs. There are still lots of twin engined aircraft where an engine out during take off would be much more of an issue than with a modern Boeing or Airbus simply because the regulations aren't so arduous.Interesting stuff, thanks all.
I'd also argue that early aircraft had what would, these days, be classed as a "low lift, high drag" wing. I.e. they take quite a lot of power to remain flying! Modern wings, designed with countless iterations of CFD and millions of hrs in the wind tunnel, have an enviable Lift to drag ratio. Hence, loosing an engine doesn't necessarily jeopardise the airframes flying qualities in the same fashion.
Max_Torque said:
I'd also argue that early aircraft had what would, these days, be classed as a "low lift, high drag" wing. I.e. they take quite a lot of power to remain flying! Modern wings, designed with countless iterations of CFD and millions of hrs in the wind tunnel, have an enviable Lift to drag ratio. Hence, loosing an engine doesn't necessarily jeopardise the airframes flying qualities in the same fashion.
I wouldn't. Aerofoil design has progressed but not as much as you'd think. Especially not when you're heading for max C_l. Many modernish aircraft are happily flying around with aerfoil sections designd in the 30's and 40's. What's changed perhaps is the standards designers are prepared to accept and CFD can certainly be used to get that last 3%. The reality is though that good designers can deliver 90+% of optmum capability with a pencil and paper, or perhaps also MS ExcelThe answer to the OP's question is tail volume which has increased dramatically since the beginning of the jet era. Tail volume is the area moment integral of the vertical (or horizontal) stabilizer w.r.t the mass centroid, hence the units of volume.
Edited by AER on Saturday 28th December 10:22
Something I hadn't considered until watching one of those lovely BBC4 documentaries about the jet age is that on propeller driven planes, the prop wash gives considerable control authority even at low speeds. On jet planes the thrust is usually diverted away from the controls since it would melt them, so they don't become useful aerodynamically until the airflow over them is sufficient.
davepoth said:
Something I hadn't considered until watching one of those lovely BBC4 documentaries about the jet age is that on propeller driven planes, the prop wash gives considerable control authority even at low speeds. On jet planes the thrust is usually diverted away from the controls since it would melt them, so they don't become useful aerodynamically until the airflow over them is sufficient.
Also what one has to remember is what aircraft those pilots struggling to control a Mosquito were used to. A high performance twin engine prop plane would have had considerable more thrust at low speed (hence why early jets had a very long take off roll) and a much quicker throttle response. So a Meteor would have been considered a much greater handfull.
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