The Boeing 737NG Fin: Why does it look like that?

[@MentourPilot]

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[@yahia.benali]

thanks for the awesome explanation but some thing came to my mind sir when you said with the dorsal fin the fin stalls at higher angle of attack now when we fit a dorsal fin to the vertical stab it would actually increase the aspect ratio and we know that low aspect ratio aerofoil stalls at higher angle of attack so i’m a bit confused here

[@mwjones71]

At Hobby Airport in Houston Texas, USA (KHOU), Continental Airlines (now a part of United Airlines - UAL) had a maintenance facility that did heavy checks on 737 Classics and NG's. The hangar was not designed for the taller stabilizer of the NG's, and it could not clear the doors (the hangar did not have the small "cutout" door like the one shown in the video). They built a custom jack and dolly rig that would lift the nose gear up, dropping the tail towards the ground (one small bump would have meant a tail strike it was that far to the ground), and then the plane could be pulled into the hangar. Once inside, the nose would be lowered back to the ground and the stabilizer would neatly nest in the rafters. While I don't have video of this operation, I saw it a few times when I supported that airport, and it was fascinating to see a plane entering the hangar like it was at V1

[@AndrewSkow1]

How does it make more sense to tilt up an aircraft that costs tens of millions instead of trimming a notch out of the hanger?

[@mwjones71]

This hangar was rather old (one of the adjacent hangars reportedly had ties to Howard Hughes) and the door tracks would interfere with cutting a notch for the tail. Short of rebuilding the whole side of the hangar the jackstand method was more cost effective, since the facility was always on the short list to be closed (there had been repeated talks to move the maintenance operations to Bush Intercontinental (KIAH), since Continental/United had not had regular passenger service from KHOU in years), but I can't say for certain (I moved away from Houston several years ago)

[@agentanderson3976]

Vr, perhaps?

[@MrAwyork]

Hopefully an aircraft doesn't have a problem tilting up a bit.

[@JulieAV]

Too bad the tail doesn't fold down. The 717 (KC135) tail could (but rarely did). I saw one folded over. I think someone on base saw the same boeing article I did and had to try it.

[@higherordersoliton]

When you said "OK?" I almost replied lol.

[@skytrainii8933]

You are right, the dorsal does add more stabilizing yaw moment for countering the yaw moment caused by engine failure (or more correctly, during any high yaw attitude condition). But it is more often necessary to prevent a condition called Rudder Lock. As with a swept wing, the center of pressure moves aft as the angle of attack increases for the swept vertical stabilizer. This aft movement causes the natural position (called the float angle) of the rudder to change. When the float angle and the required deflection match, you get a condition called Rudder Lock. The dorsal is an extremely effective and efficient way to delay rudder lock to higher yaw angles than you would see during an engine failure. While I was not involved in the B373's design, I was directly involved in other swept wing business jets which have dorsals. In all these cases, we added a dorsal to prevent rudder lock.

[@ErickC]

The 737 is a specific case because the engines are not where they were originally designed to be, and much more powerful than on the original airplane. The fin on the 731/732/73S, you will notice, has no extension. The extension added area without modifying the structure of the stabilizer (it's just a fairing), which meant that they could simply adapt the existing process.

[@dougball328]

@@ErickC My comment is meant in general and not aimed at you specifically, Erick. As one of the aerodynamicists who was involved in the 737-300 development I feel the need to clarify a few things. The airplane had a low speed, high alpha directional control problem. The wing separated flow was blanketing the empennage. This low dynamic pressure flow meant that the surfaces could not generate the load to counteract yawing moments from say, an engine out. Having spent my early career on the F-16 I suggested the dorsal. So yes, it was a stolen idea. It is not the area that matters but the vortices that get shed. The explanation that is given about this is correct. As for the engine location. It is hung at exactly the same spanwise location as the JT9Ds, that would be wing buttock line 204.4 (sometimes you use some numbers so many times you never forget them!) Obviously the vertical location is different. As a young engineer the integration of the CFM-56 presented a variety of challenges - not the least of which was the belief that the ovalized nacelles would not work. On the contrary, they worked perfectly.

[@nix4644]

I'm not a pilot, but love the technology and physics involved. Very interesting and informative. Thanks.

[@Josh_rga32]

I always learn something new with your videos. I appreciate that you do this for all of us!

[@andyb.1200]

Who dislikes these videos? It's free information that's also well explained. What's the problem? Keep up the awesome work Petter. Your videos are excellent.

[@khalidabdulghani]

This is what I love about Mentor pilot - he answers all of these weird things about the 737 which I've ALWAYS been wondering about. For example, the the bottom of the engine is flat and the the flap fairings are. Mentor, I hope you see this, and thanks man you've answered so many of my question - I love you man! 💙

[@Dragon-Slay3r]

Lol

[@DaWolf805]

It's also interesting to note that the Embraer E-Jet family had dorsal fins on the original family, but now in the upgrade to the E2 family, the dorsal fin is removed, despite the engine upgrades the plane received. This was reportedly done to increase the aerodynamic efficiency, and thus decrease the fuel burn of the aircraft, something that was a major focus of the redesign. A source I found does say that the vertical stabilizer is overall smaller, as well, so presumably with modern computer simulations they were able to shave some of the margin off the yaw performance and know that it would still be safe.

[@AmbientMorality]

Andy Modern stability augmentation helps with that. Manufacturers can reduce required stability a lot and use computers to make the plane feel natural, so horizontal and vertical stabilizers are getting smaller. If the computers failed it would be harder to fly, but not a safety risk.

[@davidchristensen6908]

It is an elegant way of solving the height problem. 737 is just an amazing workhorse. Thanks for explaining this.

[@MichiganWildcat]

Man I love your channel, great information delivered in a format for us non pilot folks can understand. Thank you!

[@MentourPilot]

I’m so happy that you like it!

[@davethewave2197]

To be honest I never really noticed it. But now I can't unsee it

[@majortom4543]

Thats because you werent looking.

[@9HighFlyer9]

I can't say that it ever caught my eye. Even if it did it looks like the C172 im used to so I wouldn't have thought it weird.

[@francodibartolo2138]

@@majortom4543 n

[@sunbulah9779]

Are you serious?????!!!!!!!

[@Dragoncannon4]

Just learned about it as a crew chief for F-16

[@southpark5555]

That was just awesome - this video. When I was guessing the purpose, I mistakenly thought that it provided more reinforcing strength to the tail plane so that it doesn't break off when the rudder is used to the extreme, or the aircraft is being used at some limit in extreme turning. The video mentioning the vortex forming functionality to allow the same size rudder to perform satisfactorily during an engine failure ------- very clever indeed!

[@DumbledoreMcCracken]

Good to hear you speak knowledgeably. So many people spout nonsense.

[@rohan-ghosh]

All Aviation enthusiasts and geeks always needed a guy JUST LIKE YOU. THANK FOR EVERYTHING AND LOOKING FORWARD TO MORE....

[@DenSanAZ]

Thanks for the explanation. I was a FA for AWA, hired in '89 when we were mainly an all 737 fleet (757 joined the fleet 2 years before I started - Republic Airlines owned 6 757s with RR engines when purchased by NWA who used 757 with PW. Those 6 757s were offered to AWA, starting their usage of the type). Anyway, while in training we learned how to tell our 737-200 (and a few 737-100) apart from our growing 737-300 fleet at a distance by both the engines and the tail - but we were never told why the tail was different. Now, 29 years later, I finally know why!

[@henktl3580]

"The Classic and the Jurassic...." :-) Thanks for a good video. Maybe do one on the informal funny 'pilot jargo'n' like this. Would be fun.

[@jayski9410]

I love these lectures on aerodynamics. How about one on "creative" uses of reverse thrust? I've seen it used in flight (to lose altitude fast?) and I've even seen a 737 use it to leave a gate when they couldn't get a push-back. And why does it look like the A380 only has the reversers on the inboard engines? And can the rudder control the yaw if an engine fails while in reverse?

[@notjackschannel5380]

This is a correct explanation of why a dorsal fin was added corresponding to the lengthened fuselage and larger thrust engines. But in addition to making the vertical tail more effective (essentially by adding lifting area), it also makes the rudder more effective. At high AOAs, the boundary layer gets very thick on the upper surface of the fuselage in the vicinity of the rudder. Which can make the rudder hinge moment non-linear and can lead to rudder lock - a condition where the rudder is aerodynamically locked in a hard-over position. Adding the dorsal sheds a high-energy vortex (as this video explains) which reduces the effect of a large separated boundary layer - thereby making the rudder more effective. A more effective rudder is more efficient than just making a larger rudder. This is also a preferred fix as it allows the design to reuse a large amount of already designed structure. Source: Am an aerospace engineer

[@deandanielson8074]

Hi Petter, I just caught up with your fine video on the reason for the dorsal fin on the 737. A great explanation and very interesting. Thank you for what you're doing for pilots and aviation in general. - Dean from Minnesota

[@Warriorking.1963]

Fantastic video! I'm binge watching all your videos, and have to say this channel is a genuine wealth of information.

[@yabbadabbadoo8225]

Here logic and physics is explained with great clarity. 10/10 here.

[@saadn.3348]

Love the topic! Thank you for the info Captain. Always look at the B737 and wondering why it has this unique fin--which I adore!!

[@amirz1321]

I love watching your videos. So precise and clear for everyone to be able to understand. And obviously a very knowledgeable pilot!

[@abangwout232]

I have absolutely No clue, what kind of people put a dislike on these interesting tutorial videos that you and cpt Joe make. Thank you guys so much for your time to make videos

[@dandel351]

If you look at WW2 fighter planes, the P47 Thunderbolt had a similar tail modification after they cut down the rear fuselage to suit the bubble canopies. A new longer dorsal fin was added to stabilize the aircraft as it had a huge prop torque effect at full power.

[@coriscotupi]

The dorsal fin is also a convenient way of enlarging the vertical stabilizer surface area without the need to completely redesign the stabilizer itself. Being able to increase the area while at the same time keeping existing tooling for manufacturing is a huge saving in manufacturing costs.

[@gavindoyle1016]

I learn something new with every video. Love it Petter, thank you.

[@ledzepgirlnmful]

Petter, thanks so much for your always informative and intriguing videos surrounding aviation... I love your expertise, how you break the information down as well ! I look forward to your upcoming posts! Many blessings to you, and your lovely family as well, for allowing you to teach the rest of us, outside your usual, professional, and hectic life ! It's appreciated by so many !! Genevieve (aviation fan) from NM, in the States ; )

[@ShreyasJani]

Thanks for this. 737 has always been a favorite, right from the engine covers to the cockpit glass design.

[@OlivierGabin]

Never though about the explanation you give here for the shape of the 737's rudder. Seems obvious when properly explained, like you do here. Thanks a lot !

[@simongreenidge6454]

Forgive me if someone has already offered this tip, but in English 'prolongs' is used mainly with reference to time; "He had a prolonged wait in the departure lounge." 'Elongates' would have been more appropriate; "To avoid making the rudder taller, it was elongated with a dorsal fin extension."

[@TheWeatherbuff]

I was unaware that anyone else (besides me) even noticed the different fin configuration. Lesson learned. Others actually do pay attention to such things! :-) I greatly enjoy your channel. Peace.

[@isaacjamestea9652]

one of the best channels on youtube.

[@ArrowTrajectory]

Don't forget the length of the fuselage (or lack of length in the 737's case), plays a part in the effectiveness -- and therefore size -- of the rudder. Great videos!

[@promerops]

I was thinking just that, in respect of the very elegant fin on the 777, which has no dorsal fin or even (Airbus style) radiused fillet.

[@JA-pb7oz]

Thank you Petter, as ever you've taken a technical subject and given an easy to understand answer.

[@samerhaddad280]

it's a pleasure to see you in command mentour, albeit in the sim...

[@FPVREVIEWS]

The VS fillet also helps reduce drag (by acting like an aerodynamic fairing) in cruise flight, saving fuel. For aircraft that do not have CR fans, a slight offset can be an efficient way to counter torque from the propulsion system.

[@ChrisWilliams-cc5ee]

great info captain! as usual :) take care!

[@Sacto1654]

This was also the reason why when the British Civil Aviation Authority was certifying the 747-420 for BOAC, they noticed a problem with yaw stability with the original shorter vertical tail surfaces. As such, Boeing built a taller vertical tail design and a small ventral fin to correct this issue--a modification that was retrofitted to other 707's and to new-build 707's.

[@techmantra4521]

Nice to see you have your high-vis jacket on Sir. Great video as usual.

[@devonnewest7990]

I wondered that for a long time!!! Thank you, another great vid & happy subscriber :-)

[@RevElaineNjerveZack7]

Great informative video.

[@LaunchPadAstronomy]

I’ve always wondered what was going on with the 737 fin. Thanks for the excellent explanation!

[@Stoney3K]

My first guess was that the APU was actually hidden underneath it, but then I learned that the dorsal fin wasn't there on the Jurassic models.

[@bobbypaluga4346]

As much as I’ve read about the industry and Boeing in particular, I have always wondered about the”fin” forward of the horizontal stabilizer.

[@beeleo]

Don't forget about everyone's favorite, the P-51 Mustang. When it transitioned from the 'C' model razorback version to the 'D' model with the bubble canopy, it lost the side area behind the cockpit. North American Aviation gave it a dorsal fin to make up the difference.

[@nnamdichibuzor6032]

Very cool! Never thought vortex lift would play a part in the dorsal fin!

[@blumac9801]

Aerodynamics is the explanation to everything on an airplane.

[@skytrainii8933]

Even the coffee maker in the galley??

[@spicypeppers2019]

SkyTrain II definitely

[@cnordegren]

Very informative video. Always best explained by a German! Very concise yet systematic.

[@antimonni]

You mean a Swede, right? ;)

[@airfoxtrot2006]

Very nice video Mentour! Hope your weekend goes well.

[@Greggory1987]

I very much enjoyed learning about this topic....thank you for a great video!

[@Hi11is]

The B-17 Flying Fortress started out without strakes as the model 299, the accepted model had a strake from the trailing edge of the wing to the tail.

[@gilbertleeds8112]

Love your channel. I would be very confident with you as my Captain. Lear jets have ventral fins on all their aircraft which is why their stall characteristics make them so stable.

[@professorbitbang]

Your videos are always interesting. 4:21 yaw pronounced perfectly :)

[@TaunusTV]

That's another question answered and perfectly explained ! Thanx & thumbs-up !!

[@all-yw2yr]

Thank you Petter great info, good to see you on the field.

[@fredorman2429]

In my recollection the Boeing B-17 and B-29 had dorsal fins in addition to the standard vertical stabilizer.

[@WillFastie]

Very interesting. I always noticed the dorsal fin but never realized it had a deeper purpose.

[@jadefalcon001]

An intro from OUTSIDE? Like, not in buildings? MADNESS! ANARCHY!

[@runarandersen878]

I have never noticed it or wondering about it, still it was interesting and makes sense. So nicely explained. I will probably look for it next time flying.

[@eDriver]

nice explanation video. Well done!

[@timsanders9111]

Great explanation. I always wondered why the 737 had a dorsal fin. Thanks!

[@johnhorency3791]

During development the B-17 bomber went through a similar process. The earlier models had a much smaller vertical stabilizer. The final design is similar to the 737.

[@RichieRouge206]

Great video as always. The 737 is such an icon

[@mytech6779]

Most of what you described applies more to the [movable] rudder control surface rather than the fixed stabilizer area specifically. The [fixed] stabilizer area must be balanced against dihedral effects with regard to the significant impact on dutch roll oscillation and spiral dive tendency. Dorsal and ventral fins are more often added for this stability balance while rudder design is more of a modification target for when adjusting yaw control effectiveness. A dorsal component may also be added for structural reasons, achieving the needed surface area while keeping weight low and avoiding excess leverage.

[@rthdavid]

I really enjoyed this video. I found it very informative and learned something new. I've also downloaded your App. to my smart phone. Keep up the good work.

[@fdenisiuc]

Great video as always, Captain!

[@sumatrasumatra9336]

Congratulations for your 16 years at your airliner!

[@richardshiggins704]

Very interesting . Also shortened models such as B747SP and A318 have taller tail fins to compensate for the shortened body to provide better longitudinal stability .

[@ahmadtheaviationlover1937]

Very well explained! Thank you very much. Ur explanations are very concise and clear

[@MentourPilot]

Thank you! I am so happy to hear that you like them!

[@Shadowfax-1980]

The P-51D model also featured a dorsal fin that wasn’t on earlier P-51 “Razorback” models.

[@Lozzie74]

Yes, when the area behind the canopy was removed (turtle deck) they discovered a loss in yaw stability

[@edp2260]

The Lockheed C-130 also uses the dorsal extension on the vertical stabilizer.

[@vaughanellis7866]

This is the same solution was used on the P51 Mustang when it had stability issues after it was up engined to the Merlin.

[@ShamirMuhammad]

You got me at "classic and jurassic". HAHA. Is that how you refer to old aircraft models??? Or did i mis-hear? Still hilarious and superbly appropriate! Nice one.

[@umm_rm]

Fab video again, is that a puppy i spy in the lower left corner lol.

[@pronoe]

Very interesting, I used to use this feature to help me differentiate an A320 from a B737, I had no idea this had an actual purpose.

[@orionwesley]

Fascinating. I noticed it but I never questioned it. Also, subscribed & also followed on Instagram.

[@yourfuneraldirector6432]

Always noticed it but thought it was a transition for the air to smoothly pass the rudder. Boeing changing it so its fit Southwest hangers is a great example of good customer relations.

[@chicagoman58]

great explanation! I didn't know that the size and shape of the vertical stabilizer was used to counteract forces during engine failure.

[@ryansmithza]

Very interesting! I hadn't even noticed this before but makes perfect sense! Thanks for the video!

[@ThomasGrillo]

Interesting to note, that same fin shape is present in the Cessna, and a few other light planes. Thanks for the video. :)

[@johnkelley9877]

Thanks for the great explanation of the tail.

[@gabby5ify]

Great video as always. btw, I really enjoyed your instagram live today!

[@echaurijpl]

Eye opening analogie of the dorsal fin to a double delta wing

[@nicksGLI]

It's called the vertical stabilizer. It looks like that because it is vertical, and stabilizes the plane. ;)

[@fourstripes3322]

Nice informative vid Petter! WOW! Ive never seen the whole side of a pobeda 737 before

[@monibstar]

Thank you for answering.. I always had this question in my mind :) Now i know why 737 has tht dorsal fin ☺👍

[@dimitriskataivatis5846]

Very informative! I've always wondered about the specially shaped fin. Thank you captain!

[@usnwildcat]

I always wonder where he gets all this information from. Especially when it comes to very detailed question I hardly believe those are discussed duting an atpl or a type rating. Keep the good work up!

[@johnp139]

usnwildcat Google?

[@gonzchi]

Excellent video, as always! Thanks!

[@Guust_Flater]

My first thought was that it makes the fin construction stronger, due to lower angle and can make the actual fin part lighter.

[@davidbartlett7344]

I always enjoy your topics

[@opman711]

Thanks for the Southwest shoutout!

[@Borkopf]

Liked also for "Pobeda"-B-737 at the beginning

[@FlyingInlines]

Another Amazing video :) The tail of the 737-800 is one of my favourite parts of the aircraft. (behind the cockpit and the engines :D)

[@HEDGE1011]

Aviator Veronika If your favorite part of the 737 is the cockpit you haven't spent much time there.

[@luvr381]

Reminds me of the dorsal fillet they added to P-51s. Also, I can't recall what aircraft I saw it on, but some had a folding rudder to allow hangar clearance.

[@DipanGhosh]

Video in collaboration with yourself! Nice one :)

[@HEDGE1011]

Great explanation but I would hasten to add it was a lower cost change to basically bolt on the dorsal extension than to redesign the entire vertical stabilizer. Smart, low-risk, and cost-effective from Boeing's perspective.

[@juanmiguelcortarello6823]

Excellent video and very interesting!

[@Azivegu]

I love that you have an app, it is really cool. I wont download it, but that is because I'm a minamilist with my phone, but I have checked it out and love it. But one piece of critique I have, dont post a picture of your app with 27% battery left xD I'm not being serious, but its just the kind of things I pay attention too xD Keep up the good work my friend!