Re: Vortex generators on engine nacells?

From:         Jop Vlaskamp <vlaskamp@cuci.nl>
Date:         27 Jun 96 12:56:55 
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At 12:24 24-06-96, you wrote:
>  I just took my first ride in a 737-400 (I'm typically flying longer
>flights on larger planes) and noticed what appears to be a two-hand sized
>fin on the fuselage side of the engine..
>What is the purpose of these?

Vortex generators etc. usually have to do with stall speed/characteristics.
The fins you mentioned are called "strakes".
The notes I took during my Aerodynamics ATPL-course say the following on the
subject of strakes:

" Sorts of strakes:
1) Strakes placed on the fuselage: to maintain the directional control
during large slip-angles.
2) Strakes mounted on the tail engine (DC-10): to create a nose-down
pitching moment.
3) Strakes on the wing-engines (B737): Reduce the local stall speed in this
area at large angles of attack."


I know that smaller planes, with which I am more familiar, have similar
devices. For example the Piper PA-28-161 Warrior and the Piper Arrow have
"stall strips". Those are strips placed on the leading edge of the wing.
The airflow becomes a turbulent one when it passes the strip. A turbulent
airflow has more energy than a laminar one (I know, it sounds like a
contradiction), and therefore "sticks" to the wing better. It prevents the
airflow from separating from the wing and thus the part where they are
installed from stalling.

Often there are stall strips, stall fences, vortex generators etc. installed
to prevent the wingtip from stalling before the wing root does.
A wing tip stall will make your ailerons ineffective.
The wing root stalling first will make sure the aircraft buffets before it
stalls: the turbulent airflow hits the elevator. It also helps create the
desired nose down movement when the aircraft actually stalls: the negative
taillift will disappear when the air, which would normally proceed via the
wing to the elevator, changes into a turbulent "chaos".


Greetings,

Daan Vlaskamp (Dutch CPL)
The Netherlands