Blackbird
Topic Author
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### F-16 Deep Stall Question

I've heard about issues the F-16 had when it came to "deep-stalls" which appear to be due to one of the following

- The chine produces more lift at high AoA than the wings do (even though the vortex off the chine covers the wing) producing an unusually forward C/L resulting in an unstable configuration in which full-down elevator cannot get the plane out of, until a higher AoA is achieved at which the chine starts producing less lift which produces a more "reasonable" C/L position at which full down elevator can make the nose and AoA drop with hopefully enough momentum built up during this nose down movement to get it below the AoA at which the "deep-stall" occurs at...

- The wings lose lift at high AoA faster than the chine does resulting in an unusually forward C/L configuration, resulting in an unstable configuration in which full-down elevator cannot get the plane out of, until a higher AoA is achieved in which the chine starts producing less lift which produces a more "reasonable" C/L position in which full down elevator can make the nose and AoA drop with hopefully enough momentum built up during this nose down movement to get it below the AoA at which the "deep-stall" occurs at

- A combination of both descriptions

Assuming it's not classified

- Which one of these explanations is correct?

- Which aerodynamic characteristics contribute to this aircraft's deep-stall tendency?

Starglider
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Joined: Sun Oct 01, 2006 12:19 am

### RE: F-16 Deep Stall Question

Deep stall on the F-16 occurs between 50 and 60 degrees AoA when at or near zero pitch rate. If the pilot somehow reaches this AoA range (airspeed too slow) at near zero pitch rate the F-16 most likely will deep stall.

The F-16 is not statically stable (that is why it is so maneuverable), so as you move through different AoA, the center of rotation moves relative to the center of gravity because the amount of lift changes over various parts of the airplane. These changes prevent the elevons from pushing the nose over enough to get the airplane to pitch down. At zero pitch rate (between 50-60 deg. AoA) the aircraft will just stay in that position and drop like a rock. So trying to lower the nose with the elevons will not work. However, there is a way out if altitude is available.

Although there is no nose down moment available to overcome the situation, there still is a nose up moment according to the graphs in the flight manual. By selecting the MPO (Manual Pitch Override) switch, overriding the FBW black boxes and pulling nose up there is enough moment available to move the nose further up. Once above 60 degrees AoA (you'll have to use your senses because the AoA indicater is pegged in the upper region), some nose down moment over the elevators becomes available again, enough to increase the pitch rate nose down through the critical 50 to 60 degrees AoA range and rock your way out of a deep stall.

Starglider.

[Edited 2009-06-24 12:17:19]

Blackbird
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### RE: F-16 Deep Stall Question

Starglider,

I was under the impression that center of rotation and center of gravity were always the same thing?

Starglider
Posts: 658
Joined: Sun Oct 01, 2006 12:19 am

### RE: F-16 Deep Stall Question

Blackbird,

Center of rotation is the aerodynamic center, this is not the same as the center of gravity.
A cambered or non-symetrical airfoil produces both a translational force or lift, and a moment. The aerodynamic center of the airfoil defines the center of rotation of the moment, as well as, the center of lift.

Center of gravity is a mass/weight issue, not an aerodynamic force. Depending on changes in AoA, the center of lift shifts relative to the center of gravity.

Center of gravity will be affected by weight and balance issues. For example location or shifting of payload and/or fuel distribution/consumption.

Starglider

johns624
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### RE: F-16 Deep Stall Question

Blackbird
Topic Author
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Joined: Wed Oct 06, 1999 10:48 am

### RE: F-16 Deep Stall Question

Starglider,

I always thought an object rotates around the center of gravity...

Johns624,

Well how else do you learn if you don't ask questions?

Blackbird

mig21umd
Posts: 275
Joined: Wed Feb 16, 2005 6:30 pm

### RE: F-16 Deep Stall Question

 Quoting Blackbird (Reply 5):I always thought an object rotates around the center of gravity...

Yes, if it is due to weight/gravity but we are talking about an aerofoil which is producing lift which can be 'averaged' out to be working though one point on the particular aerofoil. Usually this lift force is working in an up (depending which way the aircraft is flying) direction.

On an aircraft you have 4 forces which an aircraft can rotate about:
1. Thrust, working in the forward direction,
2. Drag, working opposite to thrust i.e. in a backwards direction
3. Weight, which is always working towards the centre of the earth (gravity)
4. and lift.

These 4 force’s do not always work through the same particular point but average out to be very close to the centre of gravity and also influence the position of the centre of gravity.

Hope this helps.
Once you have tasted flight, you will forever walk the earth with your eyes turned skyward, for there you long to return

johns624
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Joined: Mon Jul 07, 2008 11:09 pm

### RE: F-16 Deep Stall Question

 Quoting Blackbird (Reply 5):Well how else do you learn if you don't ask questions?

Well, you could go to college and take up aeronautical engineering since such esoteric information seems to really intrigue you.

Blackbird
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### RE: F-16 Deep Stall Question

Johns624,

Actually that isn't all that crazy an idea. I'd have to take a remedial math course to get me caught up. It's been years since I've used any complex math (anything above basic algebra)

Blackbird
Topic Author
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### RE: F-16 Deep Stall Question

What characteristics would prevent a deep-stall characteristic like that from happening?

I mean the YF-17, F/A-18, F-14, Su-27, and F-22 don't have these problems and they can all pull extroardinarily high alphas...

MCIGuy
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Joined: Sun Mar 26, 2006 8:15 am

### RE: F-16 Deep Stall Question

F-16 will oscillate in a flat stall and it's pretty simple to recover by simply matching the oscillation with the stick and "rocking" yourself out of the stall. This is of course assuming you have enough altitude.
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Blackbird
Topic Author
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### RE: F-16 Deep Stall Question

MCIGuy,

I didn't ask how hard it was to get out of the stall... I simply wanted to know what characteristics would have prevented the deep-stall characteristics from occurring?

I'm curious because there are other planes that can fly at very high alphas and can get themselves out of it

mig21umd
Posts: 275
Joined: Wed Feb 16, 2005 6:30 pm

### RE: F-16 Deep Stall Question

Blackbird

Maybe leave that one for Starglider to answer but I'm assuming it’s just the way the aircraft was designed. Its like asking why did the British trident would happily get to 'coffin corner' stall and fall out of the sky with very little chance of recovery and the B727 didn't.

Remember, the F16 is a highly manoeuvrable aircraft and can pull more G's then an F18. I think the F16 is rated at 9.5 or 10 G's and the F18 is only rated at 7.

So, for an aircraft to be moveable it has to be designed to be unstable in flight.
Once you have tasted flight, you will forever walk the earth with your eyes turned skyward, for there you long to return

MCIGuy
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Joined: Sun Mar 26, 2006 8:15 am

### RE: F-16 Deep Stall Question

^^Good post^^

Also, the F-22 and other thrust-vectoring aircraft are a whole other animal and should probably be left out of any comparison. Anything with thrust-vectoring control can simply swivel the nozzles and power out of a stall.
Airliners.net Moderator Team

Starglider
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### RE: F-16 Deep Stall Question

 Quoting Blackbird (Reply 11):I simply wanted to know what characteristics would have prevented the deep-stall characteristics from occurring? I'm curious because there are other planes that can fly at very high alphas and can get themselves out of it

The answer to your question is simple: Pilot training, which includes reading the flight manual (Dash One).

a) The F-16 will not get into a deep stall situation if the minimum maneuvring airspeed limits are respected as recommended in the Dash One. In other words, if one finds him/herself in a deep stall in an F-16, one has departed the flight envelope and after departing, did a few other tricks to get oneself into that situation.

b) If departed, one has to do several other things that are not tought in flight school in order to find oneself in a deep stall. Such as (AT A TOO LOW AIRSPEED) abruptly pulling back on the stick or rolling rapidly while simultaneously pulling back on the stick or kicking in a boot full of rudder in the process to increase roll rate. In this situation it is simply "not done!"

Other aircraft types you have mentioned have limits to adhere to, since each one also has minimum manuevring airspeed limits recommended in their respective flight manual. In that respect the F-16 is no different. It comes down to knowing/respecting the aircraft's limitations . . . be one with the machine . . wear it like a glove!

Regards,
Starglider

Blackbird
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### RE: F-16 Deep Stall Question

I was guessing the chine configuration played a role in the deep-stall issue

Starglider
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Joined: Sun Oct 01, 2006 12:19 am

### RE: F-16 Deep Stall Question

The chine is a design feature which generates vortex lift. Since some of that lift is produced forward of the aerodynamic center it produces a lifting force that needs to be countered by the horizontal stabilizers/elevons, certainly at extreme high AoA. The YF-16 and early production F-16s had insufficient horizontal stabilizer area to effectively counter such forces in a deep stall situation. The problem was fixed (as much as possible without fiddling with the overall design) by increasing the horizontal stabilizer area by approximately 25%. This solution was incorporated in block 15 and higher production F-16s and retrofitted to many earlier block 1, 5 and 10 aircraft.

Other issues that can park the plane in a deep stall are center-line stores and asymmetric stores. Center-line stores make the deep stall more oscillatory in pitch. Asymmetry means additional workload at high AoA.

Starglider

Blackbird
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### RE: F-16 Deep Stall Question

Starglider,

I'm wondering what characteristics the Su-27, YF-17, F/A-18 A-D and F/A-18 E-F's chines had over the F-16's that allowed them to not have deep-stall like problems?

Blackbird

Starglider
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Joined: Sun Oct 01, 2006 12:19 am

### RE: F-16 Deep Stall Question

The aircraft you mention have a different fuselage/wing plan-form compared to the F-16. Without going into specifics my hunch is that the wider aft fuselage (housing 2 engines) acts as an aerofoil which probably has some influence on the stall characteristics of these types.

Starglider

spudh
Posts: 342
Joined: Fri Jul 17, 2009 11:00 pm

### RE: F-16 Deep Stall Question

Su 27, Mig 29, and F14 all have widely spaced engines with an aerodynamic 'pancake' between the engines. The wings themselves will be a relatively high aspect ratio when compared to the fuselage.
What this gives is a large area of very low aspect aerofoil which doesn't stall per se. The wings of each of these aircraft will be completly stalled (wings spread in tomcat case) at say 35 deg AoA but each of them will happily fly along on the lift generated by the fusealge. This is why these aircraft in particular excel at high alpa, they are virtually unstallable in the traditional sense.
I would guess that the large area LERX's on the YF17, F18 A-D act in much the same way. The LERX on the F18 E-F are huge and have to be acting in this manner.
I think the chines on the F16 are more traditional vortex generators and wouldn't generate direct lift in the same way.
I believe the F16 deep stall is a unique case where it gets stuck between 50 and 60 deg AoA because of an eccentricity (if you can call a potentially fatal flaw such) in the FBW programming. From what I've read the airplane physically still has some pitch up authority but the computer won't allow the pilot use it as it is trying to recover the plane by pitching down, it has to be manually over-ridden to allow the pilot recover.

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