Forum: F-16 Design & Construction

I'm not sure about the role of the small fins at the back of most drop tanks. I can think of two reasons:

To avoid unwanted vortices by smoothing the airflow
To make sure that when the tank is dropped it goes down nose first because of the small amount of lift at their back provided by the fins.

Then why some drop tanks don't have fins (on the Eagle, Hornet, the centerline ones in the Viper and Fulcrum).

Hope you can shed some light on this for me...

Thanks in advance for your answers.

F-16.net Sponsor

Very good question. The centerline doesn't have them either. It can't be to make the nose drop first, that is done by the aft pivot fitting which insures the nose goes first.

Maybe it's just there to look cool!!

I believe they are there to force the nose down and make sure once it detaches from the aft pivot that it goes down to avoid hitting the horizontal stabs on it's way back or something like that.

That's maybe why the centerline tank doesnt havem because there is nothing behind it to hit.

Look at the tank, and consider this: it hangs nose down, naturally- even in flight, to a degree. So, in flight, the air pressure on the top of the tank will force the front end down, helping it fall that way away from the airplane. The aft pivot fitting is likely there to keep the tank from going aft the instant it is jettisoned, so it goes more "down" than "back" in the first few milliseconds. The fins in the back may add a little roll stability to keep the tank from twisting too much while the aft pivot fitting is still in contact, and help to stabilize the tank somewhat while it seperates from the disturbed air around the jet. Or, as mentioned below, it straightens any airflow that may be 'rotating' down the sides of the tank, so there aren't any vortices hitting the stabs a mere three feet back. Or, LM just couldn't think of any other sharp objects to put there for us crew cheifs to run in to.

Last edited by Roscoe on Mar 10, 2006 - 05:05 AM; edited 1 time in total

VPRGUY wrote:

Or, LM just couldn't think of any other sharp objects to put there for us crew chiefs to run in to.

Yea, that's the ticket Very Happy

Actually, it all depends on the local airflow. If fins are required to keep the tank from attempting a rejoin, then they will be added. Fins add drag, they won't be added unless they are required.

I worked at Eglin where all the SEEK EAGLE (stores compatibility and certification) work was done. Saw some amazing video and film where tanks, bombs, and other stores performed unexpected whiferdils and rejoins after release. Saw a GBU-12 rip off the outer stab on a Strike Eagle in a fairly benign, straight and level release. Raise the hair on your neck.

Elite 1K Joined: May 26, 2005 - 08:39 PM Posts: 1496

Ok, "rejoin" has got to be one of the most benign sounding euphemisms for an event that is actually spectacularly bad!

Of course we all know an F-15 can fly perfectly fine as long as there is a control stick and working engines. Who needs wings or stabs when you can just add more thrust? Smile

To make Amateurs think they're some sort of missiles or bombs! Very Happy

I think the fins are there just to keep the aft end of the tank in the aft pivot fitting while it drops. We were taught in tech school that the whole purpose of the aft pivot fitting is to keep it from striking the frame until the nose of the tank is clear of the disturbed airflow directly around the frame and is pulled downward away form the aircraft.

Salute!

Roscoe speaks the truth.

There is a video out there in cyberspace I think. Saw it on TV when a Hornet dropped soething and the doofer "rejoined" on the chase plane and crew ejected a second later.

Stuff does weird things when released in close proximity to larger flying objects.

Was not uncommon for tanks, racks and napalm to "float" under the wings of the A-37, then move forward, then wrap around the leading edge of the wing (We did not have "ejector racks", they were pure gravity doofers). Personally witnessed it happen. Also saw my wingie's nose gun door blown off by gases, then "stick" to the nose until well after he touched down, when it finally fell off.

I would bet a bottle of Jeremiah Weed that the fins are there to help with a) basic loads when maneuvering and b) to ensure clean separation when jetting the things.

Gums wrote:

Quote:

There is a video out there in cyberspace I think. Saw it on TV when a Hornet dropped soething and the doofer "rejoined" on the chase plane and crew ejected a second later.

Yeah, I have the video. The "something" is a bomb (probably Mk82), the chase plane was an TA-4 and it took quite a time for the crew to eject as the wing was smashed and the TA-4 turned upside down immeditely and rotated again a good 5-7 seconds later, when it was already a ball of flames - fortunatly the pilots managed to eject safely.

Forum: F-16 Design & Construction

Fins on drop tanks