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ANZUS340
Topic Author
Posts: 62
Joined: Sat Nov 17, 2007 2:30 am

Little "fins" located FWD on F/A-18C and D.

Mon Apr 19, 2021 2:51 am

I have a question regards the F/A-18 C and D models. On the top of the LERX directly above the air intake inlets are what appears to me to be small fins. I on each side of the aircraft. They are rectangular or more trapezoid shaped in appearance. Does anybody know their purpose by chance?

I have not seen pictures of them on any variants other than the C and D. Were they on earlier models at any time?

I have wasted many hours trying to find out what they are for without any success. Thanks for your time all.


Image
 
ThePointblank
Posts: 3854
Joined: Sat Jan 17, 2009 11:39 pm

Re: Little "fins" located FWD on F/A-18C and D.

Mon Apr 19, 2021 4:56 am

These are also present on A and B models as well and are called LEX fences:



They were installed in response to an issue that was causing premature cracks and fatigue in the vertical stabilizers. This structural damage limited the first batch of planes to a few hundred flight hours, as opposed to the several thousand flight hours the Navy required for the service life of its aircraft. The cause of the structural cracks was eventually traced back to the LEX vortices impacting on the vertical tails and creating loads the tails weren't designed to handle.

Specifically, the issue was due to a phenomenon called vortex bursting. As a vortex travels downstream, it enlarges and becomes weaker. If the rotational velocity of the vortex drops low enough, the increasing pressure within the vortex causes it to lose its tornado-like structure and break apart, and it was found that this was occurring just forward of the vertical stabilizers. This was causing extreme buffeting on the vertical stabilizers, and it just happened that frequency of loads induced by vortex bursting coincided with the natural frequency of bending in the vertical tail.

It took a team from NASA using the F/A-18 High Alpha Research Vehicle (HARV) to figure out what was going on; NASA conducted a series of test flights using their HARV and injecting smoke into LEX vortices to observe the vortices in flight, which identified the issue.

To deal with the the vortices, McDonnell Douglas implemented a number of design changes to the F/A-18. The first is most visible; a pair of LEX fences were installed on the LERX, which creates a second unsteady vortex that interacts with the vortex created by the leading edge extension. This interaction strengthens the rotation of the main vortex so that vortex bursting is eliminated in the vicinity of the vertical stabilizers and actually improved the Hornet's performance under high angles of attack.

The second big change were a series of L brackets installed on the vertical stabilizers where they meet the fuselage to provide increased structural strength.
 
ANZUS340
Topic Author
Posts: 62
Joined: Sat Nov 17, 2007 2:30 am

Re: Little "fins" located FWD on F/A-18C and D.

Mon Apr 19, 2021 6:00 pm

ThePointblank wrote:
These are also present on A and B models as well and are called LEX fences:



They were installed in response to an issue that was causing premature cracks and fatigue in the vertical stabilizers. This structural damage limited the first batch of planes to a few hundred flight hours, as opposed to the several thousand flight hours the Navy required for the service life of its aircraft. The cause of the structural cracks was eventually traced back to the LEX vortices impacting on the vertical tails and creating loads the tails weren't designed to handle.

Specifically, the issue was due to a phenomenon called vortex bursting. As a vortex travels downstream, it enlarges and becomes weaker. If the rotational velocity of the vortex drops low enough, the increasing pressure within the vortex causes it to lose its tornado-like structure and break apart, and it was found that this was occurring just forward of the vertical stabilizers. This was causing extreme buffeting on the vertical stabilizers, and it just happened that frequency of loads induced by vortex bursting coincided with the natural frequency of bending in the vertical tail.

It took a team from NASA using the F/A-18 High Alpha Research Vehicle (HARV) to figure out what was going on; NASA conducted a series of test flights using their HARV and injecting smoke into LEX vortices to observe the vortices in flight, which identified the issue.

To deal with the the vortices, McDonnell Douglas implemented a number of design changes to the F/A-18. The first is most visible; a pair of LEX fences were installed on the LERX, which creates a second unsteady vortex that interacts with the vortex created by the leading edge extension. This interaction strengthens the rotation of the main vortex so that vortex bursting is eliminated in the vicinity of the vertical stabilizers and actually improved the Hornet's performance under high angles of attack.

The second big change were a series of L brackets installed on the vertical stabilizers where they meet the fuselage to provide increased structural strength.


Thanks so much for the in depth response. I found it very interesting and informative. Never would I have guessed that such a "small" addition could have such an impact on the durability of the aircraft. Very cool.
 
reltney
Posts: 713
Joined: Fri Jun 25, 2004 1:34 am

Re: Little "fins" located FWD on F/A-18C and D.

Wed May 12, 2021 6:21 pm

Look at and compare airplanes. Little fins as explained by pointblank are also called “wiffle dust”. When a plane enters testing, and pre production flying, little thing crop up. Wiffle dust is added to correct airflow issues and it’s a cheap quick effective fix. Many planes in all category have it. Vortex generators are the most common . Little fins here, slots there, slight rigging of a flight control can correct major issues. Airflow not going as predicted can cause havoc as it did on the 18. Look at the dog tooth in the 15 stab....wiffle dust..the prototype did not have it. the beech 1900 looks like an angry porcupine because of all the wiffle dust.. the cowl on a 767 engine or DC10 engine has a vortalon . The bottom of a Dc-9 thru MD90 wing has a vortalon. My spelling might be bad on that.

FYI, wiffle dust is a magical dust that fixes things... it also corrects air flow.

Cheers
 
acecrackshot
Posts: 215
Joined: Sun Nov 15, 2020 4:22 am

Re: Little "fins" located FWD on F/A-18C and D.

Thu May 13, 2021 1:59 pm

Regarding the 1900, many of the extra airfoils weren’t for stability per se (as the King Air series is quite aerodynamically stable) but to avoid the requirement for a stick pusher/stick shaker/stall avoidance system other than the existing King Air series leading edge indicator.

Essentially, the 1900 is basically impossible to spin.

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