# More "What's that thing on that plane?"... **URL:** \ **Category:** Factual Questions\ **Created:** [July 10, 2007, 11:35pm UTC](https://boards.straightdope.com/t/more-whats-that-thing-on-that-plane/411281 "2007-07-10T23:35:53Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![John\_DiFool](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_difool/32/19543_2.png) [@John\_DiFool](https://boards.straightdope.com/u/John_DiFool)\ **Post date:** [July 10, 2007, 11:35pm UTC](https://boards.straightdope.com/t/more-whats-that-thing-on-that-plane/411281/1 "2007-07-10T23:35:53Z")
Inspired by the AWACS thread, I’ve always wondered about two features of airliners which have puzzled me: For those models with engines on the rear of the fuselage (727 and DC-9/MD-80), they have a lot of what for the lack of a better term I call “pods” on the wings (visible on [this](http://www.vosen.de/images/ng_ng2_a_321_05.jpg) model). I assume they are for stability or somesuch. On the models with engines on the wings only (e.g. [747](http://infoavia.ru/images/b-747.jpg)) there’s what looks like an exhaust outlet at the rear of the fuselage. What exactly is vented through this outlet? ---
**Author:** ![Capt\_B.Phart](https://avatars.discourse-cdn.com/v4/letter/c/9fc29f/32.png) [@Capt\_B.Phart](https://boards.straightdope.com/u/Capt_B.Phart)\ **Post date:** [July 10, 2007, 11:44pm UTC](https://boards.straightdope.com/t/more-whats-that-thing-on-that-plane/411281/2 "2007-07-10T23:44:54Z")
[QUOTE=John DiFool] On the models with engines on the wings only (e.g. [747](http://infoavia.ru/images/b-747.jpg)) there’s what looks like an exhaust outlet at the rear of the fuselage. What exactly is vented through this outlet? [/QUOTE] [Its an APU](http://en.wikipedia.org/wiki/Auxiliary_power_unit) ---
**Author:** ![racer72](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/racer72/32/3075_2.png) [@racer72](https://boards.straightdope.com/u/racer72)\ **Post date:** [July 11, 2007, 12:19am UTC](https://boards.straightdope.com/t/more-whats-that-thing-on-that-plane/411281/3 "2007-07-11T00:19:13Z")
The pods you speak of are called canoe fairings. They provide an aerodynamic cover for the flap tracks and actuators that move the flaps up and down. They have nothing to do with stability. The model in your picture is a poor example, it does not show the flaps as a separate part of the wing assembly. ---
**Author:** ![RealityChuck](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/realitychuck/32/195_2.png) [@RealityChuck](https://boards.straightdope.com/u/RealityChuck)\ **Post date:** [July 11, 2007, 1:12am UTC](https://boards.straightdope.com/t/more-whats-that-thing-on-that-plane/411281/4 "2007-07-11T01:12:20Z")
Snake repellers. ---
**Author:** ![JRDelirious](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jrdelirious/32/9531_2.png) [@JRDelirious](https://boards.straightdope.com/u/JRDelirious)\ **Post date:** [July 11, 2007, 2:47am UTC](https://boards.straightdope.com/t/more-whats-that-thing-on-that-plane/411281/5 "2007-07-11T02:47:28Z")
[QUOTE=racer72] The pods you speak of are called canoe fairings. They provide an aerodynamic cover for the flap tracks and actuators that move the flaps up and down. They have nothing to do with stability. The model in your picture is a poor example, it does not show the flaps as a separate part of the wing assembly. [/QUOTE] [Picture of flaps in full action.](http://www.airliners.net/open.file?id=1229933&WxsIERv=Obrvat%20767-375%2FRE&Wm=0&WdsYXMg=Nve%20Pnanqn&QtODMg=Pnytnel%20-%20Vagreangvbany%20%28LLP%20%2F%20PLLP%29&ERDLTkt=Pnanqn%20-%20Nyoregn&ktODMp=Whar%2019%2C%202007&BP=1&WNEb25u=Crgre%20Avpxrefba&xsIERvdWdsY=P-SCPN&MgTUQtODMgKE=NP%20853%20sebz%20YUE%20svany%20nccebnpu%20gb%20ehajnl%2016%20PLLP%20%5BAvxba%20Q2Kf%2C%2080-400IE%5D&YXMgTUQtODMgKERD=390&NEb25uZWxs=2007-06-30%2017%3A08%3A25&ODJ9dvCE=&O89Dcjdg=24306%2F258&static=yes&width=1024&height=695&sok=JURER%20%20%28ZNGPU%20%28nvepensg%2Cnveyvar%2Ccynpr%2Ccubgb_qngr%2Cpbhagel%2Cerznex%2Ccubgbtencure%2Crznvy%2Clrne%2Cert%2Cnvepensg_trarevp%2Cpa%2Cpbqr%29%20NTNVAFG%20%28%27%2B%22svany%22%20%2B%22nccebnpu%22%27%20VA%20OBBYRNA%20ZBQR%29%29%20%20BEQRE%20OL%20cubgb_vq%20QRFP&photo_nr=11&prev_id=1229964&next_id=1229868) Since the flaps do not only move up and down but also project backwards behind the wing, while also lowered, they require extension and retraction mechanisms that may not fit entirely inside the structure. ---
**Author:** ![JRDelirious](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jrdelirious/32/9531_2.png) [@JRDelirious](https://boards.straightdope.com/u/JRDelirious)\ **Post date:** [July 11, 2007, 2:55am UTC](https://boards.straightdope.com/t/more-whats-that-thing-on-that-plane/411281/6 "2007-07-11T02:55:17Z")
Missed the edit window… in rear-engine AC they may be more visible due to there not being also the very conspicuous engine-mount pylons, but it’s not so much engine position as wing design that dictates how many or how large they’ll be. BTW the rear-engined planes do also have APUs, just that the exhaust/intake in some may be concealed behind the lateral engine cowlings. Embraer RJs very clearly have them in the tail. ---
**Author:** ![Like\_Fry](https://avatars.discourse-cdn.com/v4/letter/l/3da27b/32.png) [@Like\_Fry](https://boards.straightdope.com/u/Like_Fry)\ **Post date:** [July 11, 2007, 3:04am UTC](https://boards.straightdope.com/t/more-whats-that-thing-on-that-plane/411281/7 "2007-07-11T03:04:49Z")
Those pods also help distribute the stagnant flow along the bottom of the wing, increasing efficiency. Fry ---
**Author:** ![DSYoungEsq](https://avatars.discourse-cdn.com/v4/letter/d/c6cbf5/32.png) [@DSYoungEsq](https://boards.straightdope.com/u/DSYoungEsq)\ **Post date:** [July 11, 2007, 12:56pm UTC](https://boards.straightdope.com/t/more-whats-that-thing-on-that-plane/411281/8 "2007-07-11T12:56:29Z")
[QUOTE=Like Fry] Those pods also help distribute the stagnant flow along the bottom of the wing, increasing efficiency. Fry [/QUOTE] Even on a treadmill? :dubious: ---
**Author:** ![Philster](https://avatars.discourse-cdn.com/v4/letter/p/13edae/32.png) [@Philster](https://boards.straightdope.com/u/Philster)\ **Post date:** [July 11, 2007, 1:29pm UTC](https://boards.straightdope.com/t/more-whats-that-thing-on-that-plane/411281/9 "2007-07-11T13:29:45Z")
Yes, even on a treadmill. 🙂 ---
**Author:** ![NBYodo](https://avatars.discourse-cdn.com/v4/letter/n/a9a28c/32.png) [@NBYodo](https://boards.straightdope.com/u/NBYodo)\ **Post date:** [July 11, 2007, 1:58pm UTC](https://boards.straightdope.com/t/more-whats-that-thing-on-that-plane/411281/10 "2007-07-11T13:58:31Z")
_Especially_ on a treadmill. ---
**Author:** ![Cervaise](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cervaise/32/16693_2.png) [@Cervaise](https://boards.straightdope.com/u/Cervaise)\ **Post date:** [July 11, 2007, 3:09pm UTC](https://boards.straightdope.com/t/more-whats-that-thing-on-that-plane/411281/11 "2007-07-11T15:09:03Z")
[QUOTE=RealityChuck] Snake repellers. [/QUOTE] Not true. Snakes are free climbers. ---
**Author:** ![Morgenstern](https://avatars.discourse-cdn.com/v4/letter/m/f9ae1b/32.png) [@Morgenstern](https://boards.straightdope.com/u/Morgenstern)\ **Post date:** [July 11, 2007, 3:19pm UTC](https://boards.straightdope.com/t/more-whats-that-thing-on-that-plane/411281/12 "2007-07-11T15:19:38Z")
Those things on the rear of the fuselage are only on the female 747. 😉 ---
**Author:** ![Dorjan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dorjan/32/12440_2.png) [@Dorjan](https://boards.straightdope.com/u/Dorjan)\ **Post date:** [July 11, 2007, 3:25pm UTC](https://boards.straightdope.com/t/more-whats-that-thing-on-that-plane/411281/13 "2007-07-11T15:25:03Z")
[QUOTE=JRDelirious] [Picture of flaps in full action.](http://www.airliners.net/open.file?id=1229933&WxsIERv=Obrvat%20767-375%2FRE&Wm=0&WdsYXMg=Nve%20Pnanqn&QtODMg=Pnytnel%20-%20Vagreangvbany%20%28LLP%20%2F%20PLLP%29&ERDLTkt=Pnanqn%20-%20Nyoregn&ktODMp=Whar%2019%2C%202007&BP=1&WNEb25u=Crgre%20Avpxrefba&xsIERvdWdsY=P-SCPN&MgTUQtODMgKE=NP%20853%20sebz%20YUE%20svany%20nccebnpu%20gb%20ehajnl%2016%20PLLP%20%5BAvxba%20Q2Kf%2C%2080-400IE%5D&YXMgTUQtODMgKERD=390&NEb25uZWxs=2007-06-30%2017%3A08%3A25&ODJ9dvCE=&O89Dcjdg=24306%2F258&static=yes&width=1024&height=695&sok=JURER%20%20%28ZNGPU%20%28nvepensg%2Cnveyvar%2Ccynpr%2Ccubgb_qngr%2Cpbhagel%2Cerznex%2Ccubgbtencure%2Crznvy%2Clrne%2Cert%2Cnvepensg_trarevp%2Cpa%2Cpbqr%29%20NTNVAFG%20%28%27%2B%22svany%22%20%2B%22nccebnpu%22%27%20VA%20OBBYRNA%20ZBQR%29%29%20%20BEQRE%20OL%20cubgb_vq%20QRFP&photo_nr=11&prev_id=1229964&next_id=1229868) Since the flaps do not only move up and down but also project backwards behind the wing, while also lowered, they require extension and retraction mechanisms that may not fit entirely inside the structure. [/QUOTE] A.K.A Flap track Fairings. Here’s a better [pic](http://content.answers.com/main/content/wp/en/thumb/d/db/250px-Wing.bmi.a320.labelled.arp.750pix.jpg) ---
**Author:** ![Rocketeer](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rocketeer/32/4437_2.png) [@Rocketeer](https://boards.straightdope.com/u/Rocketeer)\ **Post date:** [July 11, 2007, 5:53pm UTC](https://boards.straightdope.com/t/more-whats-that-thing-on-that-plane/411281/14 "2007-07-11T17:53:01Z")
One of the big aerodynamic improvements on the 737 Next Generation airplane was when they redesigned the trailing edge flaps so that one of the flap fairings could be combined with the engine strut fairing. Eliminated a bunch of drag. The redesign also eliminated a lot of parts, saving a load of weight, too. ---
**Author:** ![racer72](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/racer72/32/3075_2.png) [@racer72](https://boards.straightdope.com/u/racer72)\ **Post date:** [July 11, 2007, 6:18pm UTC](https://boards.straightdope.com/t/more-whats-that-thing-on-that-plane/411281/15 "2007-07-11T18:18:56Z")
[QUOTE=Like Fry] Those pods also help distribute the stagnant flow along the bottom of the wing, increasing efficiency. Fry [/QUOTE] I just showed your post to a 737 engineer. His reply was “I don’t think so”. He would also like to know what “stagnant flow” is, he has never heard that term in 33 years at Boeing. ---
**Author:** ![Zambini57](https://avatars.discourse-cdn.com/v4/letter/z/4491bb/32.png) [@Zambini57](https://boards.straightdope.com/u/Zambini57)\ **Post date:** [July 11, 2007, 7:55pm UTC](https://boards.straightdope.com/t/more-whats-that-thing-on-that-plane/411281/16 "2007-07-11T19:55:26Z")
[QUOTE=racer72] I just showed your post to a 737 engineer. His reply was “I don’t think so”. He would also like to know what “stagnant flow” is, he has never heard that term in 33 years at Boeing. [/QUOTE] He may be referring to the “boundary layer” of air that adheres to the surfaces of airfoils. When air flows around a wing shaped surface, (and we all know what that is so let’s not get into Bernoulli and all that), there is a thin layer of air that does not flow smoothly over the airfoil surface, but instead clings to it. This results in excess drag. To eliminate the stagnant boundary layer, many wings employ “vortex generators”. These small “winglets” perpendicular to the surface of the airfoil, create an airflow which helps to eliminate the boundary layer, and therefore reduce drag. I’m surprised that your engineer of 33 years has never heard of this. Or that he couldn’t make the connection between “stagnant air” and the boundary layer. I think it was just his “professional” stuck-up-idness that went into play because an “amateur” didn’t use the correct terminology. ---
**Author:** ![Rocketeer](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rocketeer/32/4437_2.png) [@Rocketeer](https://boards.straightdope.com/u/Rocketeer)\ **Post date:** [July 11, 2007, 8:01pm UTC](https://boards.straightdope.com/t/more-whats-that-thing-on-that-plane/411281/17 "2007-07-11T20:01:27Z")
Boundary layer control or not (and despite **zambini57** ’s assertion, they’re not intended for that), if Boeing could get rid of flap fairings altogether we’d do it in a heartbeat. ---
**Author:** ![Zambini57](https://avatars.discourse-cdn.com/v4/letter/z/4491bb/32.png) [@Zambini57](https://boards.straightdope.com/u/Zambini57)\ **Post date:** [July 11, 2007, 8:12pm UTC](https://boards.straightdope.com/t/more-whats-that-thing-on-that-plane/411281/18 "2007-07-11T20:12:03Z")
[QUOTE=Rocketeer] Boundary layer control or not (and despite **zambini57** ’s assertion, they’re not intended for that), if Boeing could get rid of flap fairings altogether we’d do it in a heartbeat. [/QUOTE] Doesn’t matter if they are intended for it or not. If they perform the function, then your point is irrelevant. You’re not getting rid of flap fairings until you get rid of flaps. Back to your drawing board. ---
**Author:** ![Like\_Fry](https://avatars.discourse-cdn.com/v4/letter/l/3da27b/32.png) [@Like\_Fry](https://boards.straightdope.com/u/Like_Fry)\ **Post date:** [July 11, 2007, 9:30pm UTC](https://boards.straightdope.com/t/more-whats-that-thing-on-that-plane/411281/19 "2007-07-11T21:30:43Z")
Ok, I had some of my terminology wrong, but the idea is the same. The pods help control [shock induced flow seperation](http://www.aerospaceweb.org/question/aerodynamics/q0240.shtml). Fry ---
**Author:** ![Zambini57](https://avatars.discourse-cdn.com/v4/letter/z/4491bb/32.png) [@Zambini57](https://boards.straightdope.com/u/Zambini57)\ **Post date:** [July 12, 2007, 1:06am UTC](https://boards.straightdope.com/t/more-whats-that-thing-on-that-plane/411281/20 "2007-07-12T01:06:29Z")
[QUOTE=Rocketeer] Boundary layer control or not (and despite **zambini57** ’s assertion, they’re not intended for that),. [/QUOTE] Just to clarify, I never asserted that flap fairings had anything to do with the boundary layer. My issue was that an engineer could not deduce that when someone used the term “stagnant” air he may have been referring to the boundary layer. [Next page](https://boards.straightdope.com/t/more-whats-that-thing-on-that-plane/411281.md?page=2)