Erm... don't really know for sure but my best guess is that they would perform a function similar to wing fences - ie. prevent separation of the airflow from the nacelle surface as it flows around and up the inside of the engine. Separation would create unwanted drag, and possibly unstable airflow over the slats/intake
Passengering with a window seat on cold, humid days I have noticed that condensation trails form above and behind the fin, indicating a pressure drop - I suspect this is a vortex forming behind the fin to 're-energise' the airflow and preventing laminar separation.
...proceeding below Decision Height with CAUTION...
Have been thinking about his one myself following a recent trip on a BMI A321. I initially though that AB might have hit the nail on the head. However, I don't think that the nacelle reaches a great enough incidence for the airflow to separate significantly. However, at incidence, cylinders shed vortices from their upper surfaces, and a guess of mine was that the fins may be present to modify the form of these vortices.
Otherwise, they may be present to control some flow characteristic due to the interaction of the fusalage/nacelle/wing in that area. Either way, i'm going to quiz one on my departments academics to see if they can enlighten us!
O.K. Here goes.... The 'fins' are refered to as chines, they are there to redirect the airflow where it interacts between the nacelle and the wing. Have you noticed they're always on the inside if the nacelle? where the airflow will be most turbulent. From memory, on the B767 they provide something in the order of a 5 knot improvement in the stall margin. Therefore, if missing, No dispatch!
I think its to reduce the isolated nacelle/pylon drag coefficient by reducing the boundary layer drag, keeping the airflow attached over the pylon, and to weaken any shock interference caused by the nacelle.
Pod mounted engines also produce lift when they are placed at an angle of attack, this is normally express as a function of the wing angle of attach, nacelle incidence angle, and the nacelle upwash or downwash angle.
The design of engine nacelles, pods, and mounts is a tricky aerodynamic challenge to make sure the engine get clean undisturbed air in all normal attitudes, and to reduce as much as possible the interference drag of the installation.
Pods mounted in front of the wing have an upwash angle (like A320), behind the wing a downwash (like B717), you can use the upwash to generate additional lift on the nacelle.
Also pods are normally designed so that the have a zero local sideslip, sometimes it is necessary to toe-in the nacelles like on a B747 to achieve this.
The devices to which you refer are vortex generators. The extended nacelles, at high alpha, cause early separation in the wing/nacelle region. This results in stall values which are not music to the accountants' ears - stall speed dictates V2 speed which dictates TODR and Vref for LDR - and the accountants have a wonderful desire to flog aeroplanes - hence the aerodynamicists install VGs to delay separation and lower the stall speed, etc. They do this by trailing a quite intense vortex which comes over the leading edge and down along the upper surface of the wing.
Something in the same sort of way that military fighters such as the F/A 18 have a leading edge extension to permit very high alpha manoeuvring by having intense vortex flow over the fins.
The main concern is despatch with ONE VG missing. I recall a significant PacRim operator some years ago which did just that on a 733 as I recall - presuming that this was an MEL option. They ran around the network for a number of days before the regulator got wind of it - and the airworthiness guys got quite aggro.
[This message has been edited by john_tullamarine (edited 17 May 2001).]
The 733 has a set three vortex generators (about 3cm high) on the inboard and outboard nacelle near the wing leading edge, and a large fin (chine) on the inboard nacelle about a foot forward of the vortex generators.
The MEL allows one vortex generator to be missing at each set (ie four total on the aircraft), but the chines must be there for dispatch.
CB .. My comments related only to the big VG out front ... that one is a big no-no for despatch with it missing. It has a significant effect on the flight test stall speed. It was one of these which was missing on the aircraft to which I referred previously.
Whether people want to call the big VGs chines, fins, or whatever is immaterial. Their purpose is as vortex generators.. the shed vortex can usually be seen in humid ambient conditions at higher alpha and curls over the leading edge and back along the wing upper surface quite effectively. The smaller VGs are, presumably, for local leading edge/nacelle/pylon flow control.
On another note, Zeke makes comment regarding nacelle lift. Possibly he/she is thinking of the normal (ie perpendicular) force seen at either the propeller disc in props or the front of the nacelle in jets.
This is due to the change in direction of airflow as the disc or inlet is traversed. The flow, viewed from the side, appears somewhat similar to the upwash/downwash pattern past a wing, and has the same result - a significant vertical force. This force has material implications in respect of static stability - for instance, on piston-turboprop conversions, the prop disc usually is relocated out near the nose of the aircraft to allow the CG to be kept under control with the lighter engine. However, this then causes a large destabilising vertical force out forward during, eg, missed approach as the power/thrust is increased. As a result, it is common on piston/turboprop conversions to require installation of a variable downspring arrangement (usually called something in the nature of a stability augmentation system) to overcome a potentially very nasty unstable pitch up tendency due to the normal force referred to above.
[This message has been edited by john_tullamarine (edited 19 May 2001).]
Just in case you are interested.... On the Boeing 717 there are nacell fins on the outer and lower section of the nacell that are about 6 foot long extending nearly the entire length of both engines, up to the reverser camshell. On an aircraft I flew the other day there was one missing. The dispatch deviation required neather of them to be fitted but reduce the ZFW CoG limit therefore making it 7.5% MAC inside the aft limit (about 35% Mac) I can only assume that this was required for added protection against deep stall charactoristics of a T tail aircraft and as the engines are fitted to the aft of the fuselage you would think that there is no relation between the fins and the wing. Food for thought.... Cheers.
How interesting - have never taken a look at the back end of the machine - I shall have a look at an MD95 (B717 ? .. .. passing fad) tail end in a week or so when the opportunity arises.
As you observe, a VG on the 717's engine nacelle clearly is not very relevant to the wing airflow .. but will, nonetheless, be a flow control device.
One needs to keep in mind that such devices are a means of getting around a flow problem - usually an undesired separation - and can appear, in appropropriate shapes and sizes - in many and varied parts of an aircraft.
I would expect that the intent, in this instance, is to delay nacelle flow separation's blanking of the lower rudder -which is relevant to aft cg stall (and spin) recovery considerations.
I think we might be at cross purposes somehow here,
(a) While I have no problem with your stated lemma that a nacelle VG would not be intended for actively effecting a stall/spin recovery during any particular event, such devices may be very relevant to determining the aft cg limit for which satisfactory stall and recovery characteristics can be demonstrated. It follows that an MEL-sanctioned absence of such a device would, necessarily, involve consideration of appropriate cg restrictions to the aft limit to retain handling qualities not inferior to the normal installation. Two matters are of direct relevance -
(i) The aft cg limit is determined by a number of factors, including stall characteristics. As the cg moves aft, recovery progressively becomes more critical and eventually may be ineffective, as has been demonstrated in a number of fatal accidents over the years. Such a consideration is, for example, one factor in the typical light aircraft dual normal/utility category certifications having different cg envelopes for each of the two categories.
(ii) For the same reason that an aerobatic empennage design geometry minimises tailplane wake blanketing of the fin and rudder, I suggest (without yet having had an opportunity to look at the devices referred to) that the structure observed by Spinash (presumably an Impulse man/woman - and no doubt quite relieved by the ACCC's decision to permit the QF deal) are VGs - intended to delay, or otherwise control, flow separation - which would blanket, at the very least, the lower fin and rudder region.
Hence, one would expect the sort of MEL restriction to aft CG which Spinash mentions. And a 7-8 pc restriction is not a trifling amount.
(b) I must confess to absolute and total confusion by your second comment. Apart from stating a lemma - that the nacelle is in the wing downwash flow field - you appear only to be restating my earlier observation.
On the other hand ... if I have demonstrated gross technical ignorance in these matters, I would appreciate further enlightenment....
It appears that I may have lit your wick, somewhat. Although this discussion, while still related to VG considerations, might now be the subject of another thread, to comment briefly -
(a) In real life aeroplanes, CG has a measurable bearing on stall speed (which, rather than alpha, is the normal pilot's practical concern). Certification (ie AFM) figures usually are determined for maximum forward CG. Recovery character is critical for aft CG. Removal of a control device has a material affect on configuration - hence my inference based on the reported MEL CG restriction.
A simplistic reading of FARs can be rather misleading. One also needs to review FAA regulatory and flight test advisory data (ie the FAA's thoughts on what the rules might mean). In addition, one always needs to be cautious in that the current rule may be materially different to the rule applying at the time at which the Type design standards were frozen.
I was not suggesting that flow control on the nacelles would have anything much to do with direct stall protection - only that such considerations may be very relevant to setting CG limits which, in turn, have an affect on stall character.
(b) OEI considerations may well be quite relevant - unless a Douglas contributor perhaps wishes to comment, we probably cannot resolve that one.
(c) Re CG envelopes, load factor, agreed, is one consideration - but your comment ignores the very significant aft CG limit restriction in utility category, which has not a great deal to do with load factors.
(d) Does not your comment re nacelles influencing upstream flow contradict your previous position ?
(e) I have no difficulty with your comments regarding where upwash and downwash live, breed and have social gatherings - aerodynamics was my first engineering discipline and I have done my share of analysing tunnel data.
(f) Nacelle pressure/force distribution is a complex matter and the forces so obtained, agreed, are not insignificant. However, I suggest that the main contributor to vertical lift at alpha in the realworld aircraft is the normal lift force at the front of the nacelle, rather than simple considerations of upwash flow fields.
(g) I merely presumed from his/her details, that Spinash flys for Impulse.
For my own interest in interpreting your comments, do you have an extensive PE background ?
John and Zeke, I am truly enlightened (and entertained) on both of your comments regarding the VG's on the 717. Is it possible that the nacell fins on the engine of the 717 are there (perhaps in addition to the reasons you have explained) to act in a similar way to which winglets or wing fences work. In that they reduce the amount of induced drag by restricting the span wise flow and therefore creating smaller vortices at the wing tip (in this case aft of the nacell). If this was possible, could that change the characteristics of the airflow over the lower portion of the fin/rudder, perhaps leading to improved controllability during single engine ops, especially at high AoA's?? I am guessing that there would be little influence from the nacell over the fin normally due to the jet efflux??? Another thought (driven by ignorance) The Beech 1900 and the Lear 60 (I think) have large ventral fins. Assuming they act in a similar aerodynamic way to each other, is it possible the 717 nacell fins are there for a similar reason? I would appreciate your comments as I am merely a pilot with an interest for things I do not understand. i.e. Aerodynamics!!! John, Yes I am pleased at the ACCC's decision. Your presumption is correct.
The 1900 tailets and finlets are there mainly to increase the CofG range from 4% MAC to 40%MAC making it real easy to load the aircraft.
The lears have a lot of problems with stability, espically up high. The dorsal fins are there to improve the dutch roll behaviour of the aircraft.
Don't worry about not understanding aerodynamics, people designing aircraft make a lot of assumtions becuase its just too hard to work out exactly whats happening. Even today no one is ever sure that a new aircraft type will come back successfully from its test flights.
Never seen a table for stall speed vs CofG in any AFM or any dial/ecam/PFD symbol for CofG position in real life aeroplanes...
The amendment status of the FAR etc on the TCDS is relevent as you pointed out, but the current amendment status reflects the changes which may prevent future accidents.
Regulatory and flight test advisory data (whatever they are called I thought something else) are a good read when you trying to work out how to comply with the regs, and the reasons behind them.
I would have to blow the dust off the books to read up on the utility cat stuff (not real interested in doing that), from memory it was just load factors changing the V-n diagram, maybe the aft CofG limit has to do with the design loads on the empenage...
[This message has been edited by Zeke (edited 23 May 2001).]