Recline on ACES seat

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Raptorman

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Unread post07 Aug 2005, 20:51

How exactly does the angle on the ejection seat of the F-16 help the pilot withstand G's? Or am I mistaken and it serves a totally diferent purpose?
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Attila

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Unread post07 Aug 2005, 21:51

The aim is to reduce the distance from the pilots heart to his brain in the vertical vector.

If the pilot sits right up and down the dictance is the greatest. So, by tilting the seat the vertical line from the heart to the brain is reduced.
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MATMACWC

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Unread post08 Aug 2005, 01:11

That's incorrect. Talk to ANY Lockheed/Boeing dude and they will tell you the seat was reclined so it would fit!

Thru, it does help a little in G tolerance, but that's not why it's reclined.
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Attila

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Unread post08 Aug 2005, 04:52

They said that: "Research suggested that a pilot's tolerance could be increased by the use of a reclining seat whose back was tilted at angles of up to 65deg. GD engineers compromised by adopting a tilt of 30deg. and by raising the pilot's knees and legs."

I'm just refering to what we were told. If it's incorrect, I'm sure we'll get to know from some of the LM dudes in here.
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F16VIPER

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Unread post08 Aug 2005, 05:05

I remember vividly reading, when I was just a 10 year old boy, an article about the first flight of the YF-16 indicating the many innovations in the plane, including the reclined seat that allowed to increase the tolerance to G forces.

Now, as usual, maybe the real reason may had been that the only way to fit the seat was to recline it, but in a clever move it may had been turned into a technological stunt. Later on I recall seeing at least one article in flight international about g tolerance and angled seats. I will see if I can dig it out.
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Roscoe

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Unread post08 Aug 2005, 05:50

I did my Master's Degree research on this very topic and posted the answer before (see the thread: <a href="f-16_forum_viewtopic-t-3139-start-75.html">Pulling G's - Does it hurt?</a>) but will repeat it here...

Roscoe wrote:Actually, the angle of the seat doesn't buy much; that is urban legend. The actual gain is less than 1 G. G-tolerance is improved by reducing the vertical distance between the heart and the eyes (gray/black-out is the first impact of Gs). So the natural conclusion is that reclining the seat would reduce that vertical distance. Remember, however, that our eyes are in the front side of our head (for most of us anyway :)). This means that as the body reclines, the vertical distance actually increases until the eyes are directly above the heart and then starts to decrease again. Since the eyes are about 10 degrees in front of the heart, 20 degrees of recline would have ZERO effect (first 10 degrees actually maximizes the vertical distance and makes things worse; the next ten restores the original vertical distance).

"Most research suggests that there is no significant increase in G tolerance until the seat is inclined 45 degrees. More recent studies have suggested that increased G tolerance in the reclined F-16 seat is due to greater leg elevation and hip flexion. This body position decreases venous pooling in the legs and increases circulating volume and blood pressure." (Burns JW. Prevention of loss of consciousness with positive pressure breathing and supinating seat. Aviat. Space Environ. Med. 1988;59:20-22. )


The earlier post was correct...the #1 reason for reclining the seat so much was to make it fit the tiny nose of the Viper.
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MATMACWC

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Unread post10 Aug 2005, 03:34

Thank you!
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Attila

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Unread post11 Aug 2005, 20:59

I've been looking in a cockpit without the seat and under the floor.
I would like to know how many inc. they gained by tilting the seat?
Doesn't seem that they would gain much by doing so.

Ok, maby they made the cockpit and canopy, then decided to put in the seat?!

And yeah, I found this article from some acro-dudes.
Great reading....

Next time, we've got a class of students coming, i'll tell them the truth behind the seat :wink:
Attachments
Physio_Effects.gif
http://www.aerobatics.org.uk/repeats/physiological_effects.htm
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Guysmiley

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Unread post11 Aug 2005, 22:39

Code One had an article about this, it IS reclined to make it fit. Wish I could find that article now...

I don't know the height of an ACES II ejection seat, but lets say its 10 units high. Reclined at 15 degrees (like in the Eagle) it ends up being 9.66 units high (cos15 x 10). At 30 degrees its 8.66 units (cos30 x 10). Thats am 11.5% decrease. And I think the Viper drivers here will tell you, there ain't a lot of room between the canopy and their helmet.

The gee tolerance is just an added benefit, but one that General Dynamics talked up quite a bit.
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Attila

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Unread post12 Aug 2005, 04:39

I'll try finding the article, Guysmiley.

I'm sure you're right about the seat. I know that the pilots love the reclined seat when flying CAP. In spite of problems with "Check-six" movements.

Any one know the angle on the seat on the F-22?
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allenperos

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Unread post12 Aug 2005, 13:44

Very good points about "G" tolerance, especially your's Roscoe, didn't know that about the position of the eyes. All are great posts, I sure could use to copy that "G" tolerance/seat angle chart.

Attila - I believe the angle of the seat on the F/A-22 is also 30 degrees. Although I do remember, during the YF-22 prototype years, there was talk of reclining the seat 60 degrees!!
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Guysmiley

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Unread post12 Aug 2005, 14:10

I thought the -22 went back to a 10 or 15 degree seat?
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allenperos

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Unread post12 Aug 2005, 14:18

Could be, don't know? Not certain. From what has been published, I think it's still 30 degrees. It's an ACES II seat, the latest and greatest you know.
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Roscoe

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Unread post12 Aug 2005, 14:28

Loving the seat? Most of the time. Sure makes it hard to use the piddle pack though :)
Roscoe
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Longshot643

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Unread post13 Aug 2005, 09:36

Attila wrote:I've been looking in a cockpit without the seat and under the floor.
I would like to know how many inc. they gained by tilting the seat?
Doesn't seem that they would gain much by doing so.

Ok, maby they made the cockpit and canopy, then decided to put in the seat?!




I think both both sides actually have it about right Atilla. No design team is going to finalize the nose contours and then remember to see if the seat fits. GD's research did seem to indicate that reclining the seat would increase the pilots G tolerence. Seems they were wrong about the actual physiological mechanism, and may have been off about the total net resistance gained, but no matter.

Once they decided to recline it, I'm sure they were modeling different cockpit / canopy configurations and layouts for different degrees of seat tilt. And I'm sure the aerodynamics people were right there giving their input about how those changes in seat angle would affect the Pilot's position and what that would do the the nose and canopy contours and height.

Remember too, Rhinoplasty is a not uncommon thing on fighters. The best example I can think of is the drastic change from the F4H-1 pre-production Phantoms to the F4-B production models. They re-did the nose to fit a larger radar dish ( a 24 in. one to a 32 in. one), drooped it to preserve airflow into the intakes, raised the rear cockpit, and altered the canopy profile to improve visibility. The F-16 had a slightly similar change.

Take a close look at the nose on the YF-16 Prototype, and the nose of the production F-16's. They had to noticeably enlarge the nose to fit the radar in. If needed, it would have been simple at that time to continue that enlargement a bit farther back and set the seat at a more conventional angle if it was needed.

So, The seat was reclined to improve the Pilots G tolerance: Correct, but not to the extent the designers wanted / hoped for. The seat was reclined to fit the tiny nose: Also correct, although IMHO, the reclined seat allowed the design team to chose a slimmer nose, not vice - versa.
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