Fins On Engine Cowlings

AirframeAS · Wed Aug 04, 2004 12:53 am

Take a look at these pictures:

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Photo © Aaron Mandolesi - Mile High Images

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Photo © Daniel & Robert Fall

Take a look at the engines and the fins on both a/c...

Here is my question:

On the 737s, there is one fin on the inboard engine cowling. I can understand why there is one and how it behaves if the engine falls off inflight. I know the fin helps force the engine to 'fly' away from the aircraft if it falls off. I worked at AS and I know how the engine would perform if it were to fall off.

Now what about the D10? I notice that each engine on both sides of the engine cowlings has two. Why two? Whats the reason for it and how does it 'behave' of the engine falls off inflight? Also why doesn't the tail engine have any fins?

Thanks in advance!

AUAE · Wed Aug 04, 2004 1:10 am

I don't think the fin has anything to to with the engine falling off. The fins just help improve airflow over the wing. If you happen to land on a humid day you can see the distrubance the engine creates in relation to the wing. Those fins just help mitigate the distrubance.

Shawn

broke · Wed Aug 04, 2004 1:15 am

The fins are actually called either strakes or vortex generators. They have nothing to do with directing the engine away from the airplane if the nacelle would separate from the wing. They are there to generate a vortex or whirling tube of air. At low speeds and higher angles of attack the airflow over the airplane may separate especially at the inboard wings and/or horizontal stabilizers. While a smooth flow of air is ideal (laminar), any airflow turbulent or not, is better than flow separation.
So the strakes generate a rotating tube of air (a vortex) which will more closely follow the wing contour or the horizontal stabilizer contour at flight conditions where flow separation may occur. This is usually at lower speeds and higher angles of attack (the angle between the wing and the airflow). At cruise speed the airframe is optimized for low drag and reasonably good airflow, so strakes and vortex generators are not useful during that flight condition.

vzlet · Wed Aug 04, 2004 1:18 am

Other refs here:
https://www.airliners.net/discussions/tech_ops/read.main/88683/

avioniker · Wed Aug 04, 2004 1:31 am

The fin has nothing to do with the "flight" characteristics of an engine departing the airframe.

It's there to smooth the airflow across the vertical and horizontal control surfaces (depending on the aircraft) and prevent boundary layer separation which increases stall speed of the aircraft at higher Angles of Attack.

In the case of the 737NG family it's called a "Vortex Control Device" and it's there to smooth the airflow around the wing.

If an engine is going to depart the wing it will be as a result of a failure of the fuse pins or bolts and the design of the struts which will, hopefully, ensure positive separation with minimum additional damage.

SlamClick · Wed Aug 04, 2004 2:10 am

AirframeAS you have just learned a very valuable lesson. That is that many of the people you work with, even study under, are full of crap. As has been explained, the devices are for flow control over the wing, around the disruption caused by the big fat engine cowl.

Most of the misinformation that is spread is not the result of someone intentionally lying about it, but maybe someone's guess taken by another as a fact. I can't recall all the wrong things like that I've heard over the years.

By the way, the full-of-crap quotient is about the same, maybe higher here in these forums. It will take you a while to decide who to trust.

avioniker · Wed Aug 04, 2004 2:18 am

WELL PUT Mr. Click...
Way too many people out there making uninformed guesses. The biggest problem now days is that they're doing the instructing.
Time to get back to teaching theory and requiring demonstrated proficiency before you're allowed to teach or design... Smile/happy/getting dizzy

Comments???

manzoori · Wed Aug 04, 2004 2:21 am

Well said Slamclick.

AirframeAS, thanks for the laugh mate... ! Wink/being sarcastic

Cheers!

Rez
Big thumbs up

MITaero · Wed Aug 04, 2004 3:10 am

By the way, the fins aren't traditional vortex generators and are not strakes; they're called "chines". They do generate a vortex which protects the wing over the engine where there's no leading edge device (i.e. slat).

Starlionblue · Wed Aug 04, 2004 7:04 am

After a couple of incidents where the departing engine(s) did not help (the Israeli 747 in the Netherlands comes to mind), the airframers figured out that it was a better idea for engines to stay with the plane no matter what. It's easier to, say, extinguish a fire than deal with the structural, stability and aerodynamic consequences of an engine falling off.

videns · Wed Aug 04, 2004 12:37 pm

I heard that the reason for DC-10s and MD-11s having two fins instead of the more commonly seen single one is because of a patent that McDonnell owned. Does anybody know if this is true? And if so, wouldn't Boeing (Who bought McDonnell/Douglas) be able to use these patented devices?

MITaero · Wed Aug 04, 2004 2:54 pm

>And if so, wouldn't Boeing (Who bought McDonnell/Douglas) be able to use these patented devices?

Yes, but we won't use two unless they're necessary. I haven't heard about the patent (or the logic behind it), but someone else might have.

N766UA · Wed Aug 04, 2004 3:00 pm

Isn't that a strake? Just like on the nose of Mad Dogs.

QantasA332 · Wed Aug 04, 2004 3:22 pm

Isn't that a strake?

Yes - strake is just another name for that same airflow-kinetic-energy-increasing device. 'Chine' is the term used more often when engine nacelles are involved, but 'strake' is fine to use as well...

Cheers,
QantasA332

AirframeAS · Thu Aug 05, 2004 3:05 am

AirframeAS you have just learned a very valuable lesson. That is that many of the people you work with, even study under, are full of crap.

LOL!!!! Yeah, I guess I just learned something I didnt know. I guess my counterparts at AS were a full of *bleep*. I guess I'll take everybody's anwser as correct. Thanks, y'all! Appreciated it. After all these years, I thought that the vortex generator on the engine cowling was for safety! How embarassing!!!

AirframeAS, thanks for the laugh mate... !

You're most welcome, Manzoori!! LOL!!

Thanks for the educational forum, gentlemen! LOL! Big thumbs up

N243NW · Thu Aug 05, 2004 4:49 am

One thing I've noticed is that quite often these nacelle-mounted strakes/vortex generators produce a thin line of condensation during takeoff. It's a neat little phenomenon and intrigues me as to how this condensation trail is created. Any insights?

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-N243NW

eksath · Thu Aug 05, 2004 5:25 am

...
So the strakes generate a rotating tube of air (a vortex) which will more closely follow the wing contour or the horizontal stabilizer contour at flight conditions where flow separation may occur.

Great explanation and hear is an inflight view of the afore mentioned "tube of air" caused by the vortex generator on a 734 on takeoff.

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Klaus · Thu Aug 05, 2004 5:40 am

N243NW: One thing I've noticed is that quite often these nacelle-mounted strakes/vortex generators produce a thin line of condensation during takeoff. It's a neat little phenomenon and intrigues me as to how this condensation trail is created. Any insights?

As far as I understand it, this would indicate a relatively strong drop of local pressure and temperatue which causes humidity in the air to condensate.

The strakes are designed to keep the airflow "ordered" rather than chaotic in this particular situation and they seem to interact quite strongly with a relatively small volume of air to achieve that, while the larger surfaces interact with a larger volume of air but at lower pressure changes. That would be the reason why under certain circumstances the strake vortices can reach the point of condensation while the low pressure above the wing may remain just below the condensation point (there are other examples where both exhibit condensation).

(Warning: This information is non-authoritative and should be perceived as preliminary until further confirmation... Wink/being sarcastic

)

Starlionblue · Thu Aug 05, 2004 8:07 am

As far as I understand it, this would indicate a relatively strong drop of local pressure and temperatue which causes humidity in the air to condensate.

Correct. The lower the pressure and pressure, the less moisture the air can retain. The lower pressure makes the water precipitate out, creating the contrail.

This is exactly the same phenomenon as creates more traditional contrails. Warm air from the engine meets very cold outside air.

MITaero · Thu Aug 05, 2004 8:31 am

>This is exactly the same phenomenon as creates more traditional contrails. Warm air from the engine meets very cold outside air.

No it's not. This is the same phenomenon as is seen in tip vortices, but neither have anything to do with the engine's effect on air temperature.

And can we call them chines, please? Smile

Klaus · Thu Aug 05, 2004 8:39 am

As far as I know, in engine-generated contrails the additional water vapour created in the combustion process (from hydrogen contained in the fuel combined with oxygen from the air) pushes the relative humidity level over the edge of condensation when the temperature drops down to the ambient level again.

It´s oversaturation rather than underpressure relative to the ambient air in that case.

JeffDCA · Thu Aug 05, 2004 10:06 am

Correct Klaus, the water vapor from the engine exhausts increase the relative humidity to 100%. This water vapor, on being expelled from the exhaust freezes due to the cold temperatures, making a line of ice crystals, also known as a contrail.

Cheers,

Jeff

Starlionblue · Thu Aug 05, 2004 10:40 am

Maybe I'm not getting this but they sound like the same phenomenon to me.

The characteristics of the environment change in such a way that the air has to release water.

the water vapor from the engine exhausts increase the relative humidity to 100%. This water vapor, on being expelled from the exhaust freezes due to the cold temperatures, making a line of ice crystals, also known as a contrail

AND

The lower the pressure and temperature (mistyped before), the less moisture the air can retain. The lower pressure makes the water precipitate out, creating the contrail.

Klaus · Thu Aug 05, 2004 11:16 am

Starlionblue: Maybe I'm not getting this but they sound like the same phenomenon to me.

Yes and no.

Fundamentally, it´s the same process. But at least ideally(!), engine contrails will tend to persist due to oversaturation (increase of humidity), while decompression condensation will tend to resublimate/evaporate again when the air returns to the ambient pressure and temperature level (temporary separation of air and humidity).

[Edited 2004-08-05 04:18:03]

Starlionblue · Thu Aug 05, 2004 11:42 am

Aaaaaaaaaaaaaaaaaaah! I get it now. Thx.

c172heavy · Sat Aug 07, 2004 10:03 am

Not to confuse this topic any further, but in reference to the picture posted by Eksath, there is a row of "bumps" just aft of the leading edge: would these be VORTEX GENERATORS?

P.S. - As this is my first post allow me to quote Homer -- "Woo Hoo!"

Starlionblue · Sat Aug 07, 2004 10:19 am

I think those are strakes, but I'm not sure.

Welcome to A.nut. Cool username.

QantasA332 · Sat Aug 07, 2004 11:03 am

Not to confuse this topic any further, but in reference to the picture posted by Eksath, there is a row of "bumps" just aft of the leading edge: would these be VORTEX GENERATORS?

Those are indeed vortex generators. As long as you know where each different device is located and how it does its job, don't worry too much about the terminology. VGs, strakes, chines...they're all really doing similar sorts of things in the end!

Cheers,
QantasA332

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