OK matt_hooks, I take your point about upgoing and downgoing blades. Will think about your contention that it has "nothing to do with slipstream on the rudder".

Being a bit of a purist myself, I can appreciate your objection to the use of the term "critical", if indeed it has such a specific meaning in the propeller context.

In relation to jet engines, you say "there's no assymetric blade effect, it really matters not a jot which way they spin".
Are you sure this continues to apply with today's massive fans, not always ducted the full length of the nacelle? If you are, perhaps you could explain why.

Phew, it's a bit late to be banging on like this! [Can't type fast enough.]

Chris

OK Chris, I'm going to assume that you understand the mechanics of the downgoing blade effect wrt AOA of the blade etc.

So now take the example of your fan. As each blade is very small in comparison to the size of the disc, and they spin relatively fast, the difference in AOA between the upgoing and downgoing blade is very small, in fact to all intents and purposes negligible!

Also the intake is designed to present the air to the fan blade evenly and perpendicular to the rotation. Although it will not be perfectly perpendicular, again it goes towards reducing to negligibility the difference of AOA between upgoing and downgoing blades.

As for the fan not being ducted along the full length of the engine, this is irrelevant to any discussion of downgoing blade effect. If you were referring to it as presenting the possibility of a rotating airstream from the engine, then I take your point. However, remember that behind every rotor there is a stator, which effectively straightens out the airflow.

A good cutaway diagram HERE of a high bypass turbofan engine.

Thanks Matt,

I like the cutaway, and remember the CF6 well (in its Dash-50 and Dash-80 form). Must admit that I'd wondered about the effect of the stator blades in the duct, as well as the VSVs (variable-pitch anti-compressor-stall stator-vanes).

I hear what you say re fan speed. But I also recall that one of the main limitations on propeller rpm is the tips going supersonic. How does that make the fan always "faster"? As for the fan blades being "very small in comparison with the size of the disc", have you looked at some of the more recent designs? The old CFM-56 ones are quite big (only about 32 to fill the circumference, as I recall), the V2500 bigger, and so on.

OK, I'm a bit out of my depth in this argument of semi-ducted fans with stators; versus unducted propellers. But, in addition to wake turbulence issues caused by wing-tip vortices, I remain to be convinced that jet-engine slipstream, for want of a better word, is completely non-vortex.

As for fan slipstream, and its possible effect on the fin and rudder, I will buy dinner for any (one!) forumite who can disprove my contention that it is noticeable as you spool up the CFM-56 engines of an A320 on a rolling crosswind take-off. [Yes, I'm aware it's got rudder-fine steering.]

Chris

Matt

Please explain why my flight manual refers to a critical engine?

Hawker 700, twin jet!


Cheers

RK

The critical engine in a jet or piston or turbine-propeller is the one that was misidentified and shut down improperly!!!

Weathercocking in the air ?
 

Quote from AndyGiov [Feb26/14:50]:
In the aeronautical engineering slang... weathercocking is possible once airborne as well, not only while on the ground!
Indeed, "weathercock stability" is the nickname given to tendency of the aircraft to "naturally" align its nose into the wind...
[Unquote]

Sorry for the late reply, and it's going to be lengthy, and sometimes didactic (for the new guys). It seems your knowledge of dynamics theory is greater than mine. The rustiness of the comparatively little aerodynamics I was taught has already been demonstrated on this forum. But I do have a career of practical flying experience to draw on.

WEATHERCOCKING IN FLIGHT ?

As I understand it, you are saying that – if, for example, an aircraft with aerodynamic directional stability is airborne and on a westerly heading, flying within an air mass that is moving from north to south – a series of horizontal gusts from its right will each induce a sideslip, leading to a slight movement of heading towards the wind. A series of these events would have the effect of gradually bringing the heading of the aircraft round towards the north, i.e., into the wind. As the wind rises and falls, the gusts striking the tail could be regarded as sometimes positive, sometimes negative. So your proposition is counter-intuitive, but in the dynamics of fluid it has the ring of truth.

Presumably, the above is a theoretical long-term effect. My flying experience is admittedly limited to conventional aeroplanes from 30 - 380 kts IAS, 30 - 500 kts TAS, and in wind speeds up to about 180 kts; but I've never noticed this effect in practice. This may be because the pilot will always correct sideslip with rudder, but the corrections required from horizontal gusts are minimal on large aeroplanes. This applies particularly on a swept-wing airliner with yaw dampers, where pilots are discouraged from making rudder inputs in symmetric flight, especially at high speed.

For modern tail-less aeroplanes, which may have inadequate aerodynamic directional stability, my presumption is that artificial stability is achieved by asymmetric deployment of computer-controlled, wing-mounted air brakes. Such an aircraft, like the B-2, might be less subject to the phenomenon you describe?

DRIFT IN FLIGHT

What is evident to the aircrew in flight, of course, is the phenomenon of drift. An aeroplane on the approach to land, with no sideslip, at 120 kts TAS in a beam crosswind of 40 kts, tracks over the ground at nearly 20 degrees different from its heading. That difference of track (TRK), compared with heading (HDG), is known as the drift angle. "Left" drift is the result of a crosswind from somewhere on the R/H side of the aircraft; bow, beam, or quarter.

An aircraft flying at 480 kts TAS, pointing directly across a 160-knot jetstream, experiences the same angle of drift as the example above. [For the uninitiated, both examples are routine on jet aeroplanes.]

WEATHERCOCKING ON THE GROUND

To define the expression "weathercocking", we need to remind ourselves what a weathercock is, and how it works.

A weathercock is essentially a weather vane (wind-direction indicator), normally mounted on the top of slim buildings, such as church steeples. Its shape represents the profile of a male chicken, wings folded. [The male chicken is known as a "cock" in England; a "rooster" in America.] The male displays large tail feathers, particularly when courting females. On the ground, standing, the large tail means that it has to face ("head") into any stiff wind; to avoid being turned and blown over.
Like the rooster on the ground, a weathercock "heads" into wind. This is because, in place of two legs, it has one central leg in the form of a rod, which pivots on a bearing that is fixed on the building.

In the case of a conventional aircraft on the ground, the rod of the weathercock is replaced by the main gear legs. [Gliders often have just one.] If it is stationary on a windy day, the brakes are released, and any wheels on other legs allowed to castor, it will be free to rotate on its main leg(s). If it is provided with vertical tail surfaces to provide directional stability in flight, and if the wind is strong enough, the aircraft will turn to face into the wind. This is known as "weathercocking".

The tendency for this conventional aeroplane to turn into the perceived wind persists as long as its weight is fully or partially supported by the main gear legs, whatever its speed along the ground. At higher IAS, the rudder is more effective in controlling this tendency, whereas a steerable nosewheel (or fixed-straight tailwheel) becomes less effective as the ground speed rises.

PS: Reviewing the above, my ears burn – imagining the chorus of comment that I am labouring the bleedin’ obvious. But even matt_hooks – for all his knowledge of aerodynamics – was confusing weathercocking and drift early in this thread (posts #6 – #11). So it may not be so obvious if you are an aspiring aviator, whose current experience is only virtual.

Chris, I understand how you feel---let me put it all technical for 'em:}


once free of the ground the AIRPLANE DOES NOT FEEL THE WIND!

before achieving wings level at lift off YOU SHOULD BE SIDE SLIPPING how would you otherwise TRACK TRUE along the run way!!!

the airplane responds only to GUST...Affecting TOTAL MOMENTUM---:ugh:


The Critical engine [for a pilot] is the one that you don't properly deal with trying to act like captain Octopus!

RK, this is indeed a little puzzling, especially when you consider the location of the engines. The assymetric thrust produced by an engine failure with tail mounted engines is much less significant than a similar situation with wing mounted engines (much greater moment of action about the C of G in the horizontal plane).

Would you be able to type the exact wording of the flight manual with regards to the mention of "critical engine" as the context might give us a clue?

My understanding has always been that a critical engine will under all circumstances have a greater effect, whereas in the case of the downwind engine effect it could be either engine.

As always, I stand to be corrected of course. This is just my understanding of the phraseology used in the certification.

Chris. I have no direct knowledge of the effect you describe on the A320. However, I would contend that with the rudder mounted above the fuselage and the engines a good distance below, it is unlikely that the airflow from the engines would impinge on the rudder/vertical stabiliser surfaces.

I think what you may experience is the engine exhaust impinging on the (much larger in surface area) side portions of the fuselage. Also you speak of a rolling take off. Hence I would suggest that a speed increase, coupled with the jetwash acting on the fuselage could account for the effect you describe. Especially if you are referring to an increased effectivenes of the rudder in the up-wind direction, requiring a reduction in applied rudder pressure as you apply T/O power.

It's also possible, just throwing ideas up in the air here, that the upwind engine gets cleaner air than the downwind engine, allowing it to produce marginally more power and hence acting to correct the tendency to weathercock. This could easily manifest itself as an apparent increase in rudder effectiveness due to jetwash on the tail fin.

Can anyone say categorically that on the A320 the jet exhaust doesn't cause increased rudder effectiveness? Well, I'd say that dinner is pretty safe Chris, it's not easy to prove a negative. However I think I've come up with a couple of viable alternative answers.

Matt

Thanks for taking the time, I quote:

The aircraft has a negative weathercock stability on the ground, that is, in a crosswind take off it tends to yaw further out of wind. The critical engine is therefore, on the downwind or lee side.

I'm with you, it seems not to fit the classic critical engine definitions I remember from my four-prop days!

Regards

RK

Yeah, it's a strange one. Just speaking to one of my friends who flies the 700 and his version of the flight manual doesn't mention critical engines. Maybe there is an updated version where they have changed the phraseology.

I would suggest that the main gear is set a long way back so the yawing moment caused by the keel area ahead of the gear is greater than that behind but still the use of the term "critical engine" is a little odd. I'm going to check my CS25 for the exact definition of the term again.

Just a thought.
As CAA used to use a crosswind for certification and now JAA/EASA don't then it depends on when the ac was certified and by whom.

I've flown identical L1011s with big differences in VMCG just through certification.

The HS 125/Hawker has a small rudder, small fin and short fuselage and a surprisingly high Vmcg and Vmca (Vmca 114 kt for the 125-800 if my memory is reliable). I no longer have my copy of the 125-800 aircraft manual, but it DOES refer to the critical engine (as being the into wind side, if memory serves).

Sobering fact -
 

Fifty-odd threads, and still trying to define/understand what is a "critical engine"...?
I am seriously concerned about this XXIst Century generation of pilots.
Retiring in November...
Am I safe to buy a ticket, and soon qualify as SLF...?
My old Peugeot 205 seems safer and safer as years go by.
Even driving 1000 km in a day or two...
xxx
:)
Happy contrails

Moggiee

Your memory may well be reliable, but I would be surprised if their was a significant difference between the 700 and 800, especially given the major control/flight surfaces were largely untouched until the later models.

I have the Vol2 in front of me and, cross-my-heart-and-hope-to-die-promise, it talks about the downwind engine.

This does make sense given that the aircraft is prone to negative weathercock on the ground. Generally, she's a unstable little beast in any gusty crosswind:{ but given the choice I'd rather not have any engine failure!

Old hats
 

Hello Buenos Aires,

Do I detect a certain frustration in your post last night?
[ Fifty-odd threads, and still trying to define/understand what is a "critical engine"...? ]

Your post Of Feb21 was not in vain: many of us got a timely reminder of the dangers of using tiller-steering during the take-off (and landing) run. Also, you pointed out the crosswind effect that D. P. Davies, presumably, had incorporated into the UK ARB (CAA) figures for VMCG (and VMCA ?). Why not the FAA? Sounds like politics.

So your post was not lost on all of us. For the benefit of those who missed it, and may not be inclined to turn back the pages, I hope you will not mind me reposting some extracts from what you wrote...

Quote from BelArgUSA [Feb21/22:12]:

We have concluded that the procedure to "guard the tiller" during the takeoff roll to 80 knots increased the likelihood for overcontrol of the nosewheel, and that we should eliminate the procedure. From then on, our procedure changed to instruct the captain not to guard the tiller, and place his hand on the wheel, after alignment for takeoff on the runway. I was instructed by the chief pilot to effect the new procedure to all types (747, 737, MD-80 and A-310s), effective immediately. There were no further problems since the procedure changed.

Further, "elevator down" to "increase nosewheel effectiveness" is worthless. It is obviously worthless at low speeds, worse, it does increase aerodynamic drag at higher speeds. Directional control should rely only on aerodynamic forces, once the aircraft starts rolling for takeoff. Elevators are kept in a "neutral/faired" position.

...the British CAA accounts for crosswind effect on VMCG. In the case of 747-200/300s as an example, in the minimum V1 speeds (restricted by VmcG), speeds in the FOM/QRH are different in airplanes certificated as per FAA and CAA.
VmcG/V1 speeds are higher for CAA certification.

[Unquote]

I know what you mean about back-seat syndrome...
Chris

Goes to prove a ) that my memory is not reliable and b ) that there IS a critical engine on some jets (despite the assertions of some posters here).

It is 4 years since I last saw the 125-800 manuals, so I can be excused, I think.

Chris Scott:

BelArgUSA's comments about NOT guarding the NWS tiller up to 80kt are all very well on an aeroplane with rudder fine steering. However, if he tried that on an aeroplane without such a system (HS125, VC10 and, I believe, BAE146) then a strong crosswind would make for VERY interesting takeoff as he woulkd have no directional control below 60-80kt.

I would put money on him having zero flight time on the VC10, and probably the same for the HS125 series.

Horses for courses - when it comes to SOPs for particular aeroplanes.

Moggiee

My memory doesn't last four days, never mind four years!

Excused!;)

Regards

RK

Hola Moggiee -
xxx
Please be aware that I always indicate clearly what airplane type(s) in any discussion regarding, shall we say, procedures, or performance. Specifying the type permits then to apply/compare to the type(s) aircraft you fly.
xxx
I mentioned that the PanAm 747s were devoid of rudder pedal NWS, and yes, my pilot experience includes many PIC hours flying the 707s, which never had any nosewheel steering on rudder pedals. Despite that, while requiring some amount of limited steering on tiller, good effectiveness of the rudder was obtained as early as 40-50 kts. Weather conditions of 30 kts direct crosswinds are not an everyday situation.
xxx
I see you deal with Hawker HS-125... In my F/O and F/E days with PanAm, with limited salaries, I was moonlighting as a part time Lear Jet type rating instructor, and practiced numerous "V1 engine failure" power cuts... and this not in a simulator, but in the actual aircraft - a nasty little airplane, the LR-24, where an engine slammed to "idle" at V1 plus a few knots required full rudder application, and a prayer of hope... I estimate that I survived some 500+ "engine cuts" and... I am still alive.
xxx
I had a trainee slamming on the wrong rudder one time, and a buddy instructor had a trainee wearing "high heel cowboy boots", and by pushing the rudder, the heel got jammed under the rudder pedal. From then on, we required trainees to wear Nikes or Adidas sport shoes, no Guccis, please.
xxx
I am just your average line pilot, nothing spectacular, but I have excellent reputation as classroom, simulator and line training instructor, and I take pride in a 99.9% pass rate of "my guys", if they survive my bad humour and corny jokes in the classroom. Being a clown is a teaching technique.
xxx
When I write something about flying an airplane, believe me, I try to do it for the benefit of all pilots, should they be trainees, or experienced. I have been paid to teach, study and learn for my entire career, and I learned a lot from my trainees as well.
xxx
:)
Happy contrails

Bottom line...
 

In summary, at the risk of sounding un-aviation like, the Critical engine is the one, that should it fail, it generaly creates the most asymetrical yawing tendencies, creates the most drag, puts the most damper on performance...and requires the pilot to put the most opposite or correcting imputs to deal with a critical engine failure.

If you use this analogy, every aircraft has a crtical engine...even jets due to the rotation of the engines, slipstream affects..

IMHO...the critical engine is really the one that the PILOT has the most hard to time with...like some people deal with right crosswinds better then others, or if the left engine is out, they just turn better to the left, then they do to the right..

This might be splitting hairs, but planes don't fly themselves, pilot's do.

It's purely a semantic point as to how the term "critical engine" is defined for the purposes of certification.

Yes, in certain circumstances on a jet aircraft it is worse to lose one engine than the other. Xwind takeoff is just one example.

The most critical engine on any twin-engined aircraft is the one thats still working.