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Just now, Dannyvandelft said:

Yeah I didn't make that "1 knot above stall" comment lol. I just assumed that if landing speeds change on weight, so do stall speeds.

P.S. fan of the channel.

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That's my point.  What is the stall speed in an aircraft with a FLCS and AOA limiters? 

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That's my point.  What is the stall speed in an aircraft with a FLCS and AOA limiters? 
We obviously don't know, being mere subsonic ground dwelling mortals

Do the FLCS and AOA limiter make it "unstallable"? Will it automatically level itself reducing its AOA limit to the point where it never stalls?

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14 minutes ago, Dannyvandelft said:

We obviously don't know, being mere subsonic ground dwelling mortals emoji1787.png

Do the FLCS and AOA limiter make it "unstallable"? Will it automatically level itself reducing its AOA limit to the point where it never stalls?

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That's your homework.  😉  If you guys want realism, learn the systems.  

 

If not, then do what the book says.  Fly 11 degree AOA approaches.  Don't flare at 150 feet.  Don't try to find the "stall speed."  Just do the procedure.  

 

As an aside, in general, "stall speed" discussions are for aircraft without AOA indicators.  Speed is irrelevant.  AOA is what matters.  You can correlate speed to AOA, but the "I'm flying" vs "Nope, not flying anymore" is 100% dependent on AOA.  The reason GA pilots use speed is because most do not have AOA indicators.  

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That's your homework.    If you guys want realism, learn the systems.  
 
If not, then do what the book says.  Fly 11 degree AOA approaches.  Don't flare at 150 feet.  Don't try to find the "stall speed."  Just do the procedure.  
 
As an aside, in general, "stall speed" discussions are for aircraft without AOA indicators.  Speed is irrelevant.  AOA is what matters.  You can correlate speed to AOA, but the "I'm flying" vs "Nope, not flying anymore" is 100% dependent on AOA.  The reason GA pilots use speed is because most do not have AOA indicators.  
I appreciate that. You know, wouldn't make a bad series on your channel, to show how the systems work. Definitely for me, visual learning has always definitely been easier than reading about things.



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On 10/9/2021 at 12:45 PM, Mover said:

What is the stall speed of an F-16?  🤔

It's very slow. It doesn't matter much anyway because the touchdown speed is much faster than the stall speed in the F-16 because the AOA at stall requires an attitude that would scrape the ground. Stall AOA in the F-16 in the landing regime is about 35 degrees AOA. As such the F-16 does not stall while landing. Stall landing the F-16 cannot be done and even if it could be the post-stall lift curve doesn't drop off that quickly so the benefit lift/no-lift binary behavior isn't there.

 

Immediately before roundout you want to be on a 2.5 degree visual approach path to the first millimeter of runway. From instrument or VASI/PAPI approaches the approach path will be steeper and intersect farther down the runway. Normally you transition as early as possible to the visual 2.5 as practical. From an approach of 11 or 13 AOA at some point (usually a little after 100' height) you transition to touchdown. I find the best way is to walk the FPM to the horizon so the top of the FPM circle is tangent and under the horizon. This gives a small vertical speed. Always ideal touchdown is 13 AOA.

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57 minutes ago, Frederf said:

It's very slow. It doesn't matter much anyway because the touchdown speed is much faster than the stall speed in the F-16 because the AOA at stall requires an attitude that would scrape the ground. Stall AOA in the F-16 in the landing regime is about 35 degrees AOA. As such the F-16 does not stall while landing. Stall landing the F-16 cannot be done and even if it could be the post-stall lift curve doesn't drop off that quickly so the benefit lift/no-lift binary behavior isn't there.

 

Immediately before roundout you want to be on a 2.5 degree visual approach path to the first millimeter of runway. From instrument or VASI/PAPI approaches the approach path will be steeper and intersect farther down the runway. Normally you transition as early as possible to the visual 2.5 as practical. From an approach of 11 or 13 AOA at some point (usually a little after 100' height) you transition to touchdown. I find the best way is to walk the FPM to the horizon so the top of the FPM circle is tangent and under the horizon. This gives a small vertical speed. Always ideal touchdown is 13 AOA.

Interesting.  How would one get to 35 Alpha in a Viper?

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As far as I understand it Mover, maximum desirable AOA upon landing touchdown is 13 and same applies for 2 point aerobraking as maximum... A warning is provided to nearing and exceeding 15 AOA on touchdown due to physically striking some component of the aircraft on the runway (primarily the speedbrakes) but also because of bottom strakes below the tail as well as i'd imagine the exhaust nozzle itself and the horizontal tails, etc... Reading the -1 there are a lot of cautionary texts related to some component of the aircraft either on touchdown or just prior to touchdown potentially becoming physically damaged or causing a drastic change in flight characteristics like the flaperon + horizontal tail movement from an immediate and drastic roll input from the pilot to counter wake turbulence from a prior plane landing, the procedure is to initiate a go around rather than try to salvage an approach which has a high probability of a drastic asymmetric reduction in lift due to movement of the flaperons and the horizontal tails.

 

Then once on the ground, apparently the weight on wheels sensors can possibly be tripped to an "air" state under certain conditions and pilot actions and can reduce the effectiveness of braking by tripping the anti-skid on and off albeit momentarily, etc...

 

LOL as far as 35 Alpha in the Viper, I never heard of that in the landing approach / configuration... Only in the "deep stall" falling leaf type of behavior where to recover requires use of the MPO switch and in phase rocking of the stick in pitch axis with nose movement. Also, apparently using the MPO switch inhibits roll input from the pilot stick, I suppose so you get maximum symmetrical movement of the horizontal tails.

 

 

As far as speed vs AOA discussion... I like primarily relying on AOA (because it is constant and same technique regardless of stores or GW) you can cross check your AOA with a computed final approach speed by using the formula the -1 gives which was something like a base speed in knots plus 5 or 10 knots per 1000 lbs of fuel and stores added together. So, the sum of that gives you a computed final approach speed that should be matching the 13 degrees AOA apparently. If you really want to cross check everything. Me? I just do the fly 11 degrees AOA to touchdown point and when the runway matches HUD frame width I start my flare to 13 degrees AOA and hold that until below 100 kts and lower the nose basically.

 

Apparently the 11 degree approach method is good for added controllability and visibility during the approach but has a farther "float" in the flare than if you did the whole approach at 13 degrees... Which gives you more precision on touchdown point but obviously reduced ability to flare the aircraft with back pressure on the stick so you need to manage throttle to manage sink rate.

 

The other aspect of AOA which is important and why it makes a difference is also having to do with once you are on the ground too, that 13 degrees is like a Goldilocks zone... Too slow can create too high of a sink rate and can physically damage the MLG upon touchdown even blowing a tire, and too fast can cause a whole host of problems too such as runway overrun, riding the brakes so hard you have a hot brakes condition, etc...

 

 

 

You can see here a clear difference in the roll out velocities between the 2 F-16s, presumably they are probably around similar gross weights and stores loads. But one aircraft has an overrun but the other doesn't.

 

It is hard to tell in the video but i'm guessing the first F-16 landed at a higher AOA vs the 2nd one and thus was slower in velocity from touchdown on the runway... Also it looks like its nose is at a higher pitch during its aerobraking (so again higher AOA and slower) when compared to the other.

 

Something I noticed tho is that he closes his speedbrakes when his nose lowers rather than extending them fully once the WOW on the nose wheel sensor was triggered from the strut compression, never noticed that before in other videos... Usually you see them open them fully to that 60 degrees while giving full aft stick. Hard to see but looks like although he closes the speedbrakes he did give full aft stick.

 

At first look, the second jet seems to come in too fast possibly (so too low of an AOA) and then doesn't hike the nose up sufficiently possibly to have effective aerobraking and just eats up runway. Also, as you see him rolling down the runway the speedbrakes don't appear to be extended either.


Edited by Baz000
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The above mentioned 35 alpha I knew of, is seems to be the AOA at CLmax (also known as the stall AOA), according to the 0.6 mach wind tunnel test of a sub-scale model of an early F-16, in NASA TP-1538. This is a test of the base model, which means control surfaces are at 0 deflection in this case (including LEF, which is normally auto scheduled with AOA).

 

untrimmed CL curve.jpg

 

The deep stall AOA however is around 55-60 degrees, according to the Cm curve from the same wind tunnel test.

cm.JPG

EFM / FCS developer, Deka Ironwork Simulations.

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On 9/30/2021 at 2:26 PM, Zergburger said:

Maybe I am struggling to understand what this means in terms of the aircraft's characteristics....

But, shouldn't the nose stay parked where we put it with L/G gains and AoA < 10 ?

 

It should, as -1 put it the Take-off & Landing Gains is a pitch-rate command system until 10 deg AOA (meaning AOA < 10). Neutral stick means zero pitch-rate when AOA < 10.

 

When MLG WOW, there's some modification to the control law:

1. AOA input/feedback sets to 0. Meaning you won't get that pitch down command when AOA > 10.

2. Pitch integrator stops integrating and gradually returns to 0, turning the pitch axis from a PI controller to a P controller. Meaning any uncommanded pitch-rate would be cancelled out to a lesser extent. Steady state error between command and feedback could persist.

3. Pitch command doubles.

4. Other minor changes such as additional structural filters.


Edited by LJQCN101
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EFM / FCS developer, Deka Ironwork Simulations.

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I hear the stall horn for sure at 140kts, but I think it starts at 150kts.  Weight is definitely a factor for approach speed.  I find myself keeping my approach speed higher if I am coming in with over 3500lbs of fuel.  I never think about AOA in the Viper.  I just fly the "hook".  I like to keep my FPM floating just over the hook until flare at the threshold.  Once I enter the flare I center the hook with the FPM and put the FPM on the end of the runway with the horizon line.  At this point I throttle back and settle onto the runway with the main gear.  If I'm high in the flare I may hear the horn just at touchdown.  If all goes well, when the main gear touch you can simply hold the flare as long as you can.  

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4 minutes ago, Glide said:

I hear the stall horn for sure at 140kts, but I think it starts at 150kts.  Weight is definitely a factor for approach speed.  I find myself keeping my approach speed higher if I am coming in with over 3500lbs of fuel.  I never think about AOA in the Viper.  I just fly the "hook".  I like to keep my FPM floating just over the hook until flare at the threshold.  Once I enter the flare I center the hook with the FPM and put the FPM on the end of the runway with the horizon line.  At this point I throttle back and settle onto the runway with the main gear.  If I'm high in the flare I may hear the horn just at touchdown.  If all goes well, when the main gear touch you can simply hold the flare as long as you can.  

 

The F-16 does not have a stall horn.  What you are hearing is a "low speed warning tone" which comes on with the gear down at 15 degrees AOA or greater.  If you are hearing this at touch down, you may strike the ventral fin, nozzle, or grind down the speedbrakes.  

 

Again, this is not a Cessna.

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On 10/10/2021 at 6:54 AM, Mover said:

You are correct that the speeds correlating with the appropriate landing AOA change based on fuel and stores weight.

 

My question was more of this notion of "stall speed" with regard to the F-16, and how/why anyone would want to be "1 knot above it" for landing.  

 

It's an academic discussion more than anything on how the FLCS works, methods for flying the pattern in a fighter, etc.  The F-16 is not a Cessna.

@MoverMy comment about ideally landing 1 knot above stall speed is idealistic.  In real life plane landing, the closer you are to the stall speed the less likely you are to bounce - plus you can apply more braking force faster (slower landing roll out speed = less lift from wings during roll out = greater braking effect)- all resulting in a shorter roll out after landing (i.e safer roll out).
When learning to fly real planes, when landing (in ground effect) you are often told to keep pulling the stick/yoke back and back as you are in ground effect just before touching down (kind of like slowing and stalling just before touchdown).
In the F-16 or other fighters you fly the plane to the ground (i.e. keep some power applied (to avoid excess sink rate = they don't glide very well).

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F-16 stall speed is:  with 1000 lbs fuel, gear down, no bags, having fired 6 120Cs, 200 Above Sea Level, no wind = about 130 knots.
(Just start in ME at 175 knots, drop the gear/bags, fire the missles - and see how slow you can go and still maintain a CONSTANT altitude.

Why "about" 130 knots, is because the digital flight controls don't want the plane to stall, so it gets really nose high and mushy below 135 knots.  At about 130 knots you cannot pull the stick far enough aft to maintain altitude.

5600x, EVGA 3070 FTW, B550 Tomahawk, M.2 Samsung, 32GB CL16, AIO 240mm
VKB Gladiator Pro, Freetracker IR 3d printed, TM MkII HOTAS circa 1985 w/USB
Asus 27" 2560x1440 60fps (so constrain DCS to 60fps)    F-16, F-18
       2021 = First year on DCS:
 

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10 hours ago, ACME_WIdgets said:

@MoverMy comment about ideally landing 1 knot above stall speed is idealistic.  In real life plane landing, the closer you are to the stall speed the less likely you are to bounce - plus you can apply more braking force faster (slower landing roll out speed = less lift from wings during roll out = greater braking effect)- all resulting in a shorter roll out after landing (i.e safer roll out).
When learning to fly real planes, when landing (in ground effect) you are often told to keep pulling the stick/yoke back and back as you are in ground effect just before touching down (kind of like slowing and stalling just before touchdown).
In the F-16 or other fighters you fly the plane to the ground (i.e. keep some power applied (to avoid excess sink rate = they don't glide very well).

 

It's not idealistic.  It's wrong, sorry.  

Does not apply to this aircraft.  This is not a Cessna.

And you do go to idle in the roundout/flare in a Viper.  It floats significantly if you don't.

There is no stall speed.

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There technically is a stall speed and AOA but you can't reach it unless the FLCS fails, and if it does, you'll depart either way.

Probably a very violent way of leaving controlled flight. 

So no, there is neither a stall horn nor a stall speed in the F-16. There is a AOA limitation inside the FLCS that will limit the AOA to prevent departing which will block you from stalling, departing and other nasty things. 

You can't compare an aerodynamically unstable airframe to a Cessna. There is a computer that makes the airframe do things that a Cessna can't. And it can't fly without. 

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1 hour ago, TobiasA said:

There technically is a stall speed and AOA but you can't reach it unless the FLCS fails, and if it does, you'll depart either way.

Probably a very violent way of leaving controlled flight. 

So no, there is neither a stall horn nor a stall speed in the F-16. There is a AOA limitation inside the FLCS that will limit the AOA to prevent departing which will block you from stalling, departing and other nasty things. 

You can't compare an aerodynamically unstable airframe to a Cessna. There is a computer that makes the airframe do things that a Cessna can't. And it can't fly without. 

Sooo... there's no stall speed.  😂

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I think you're misunderstanding, idealistic means theoretical. Stall landing the F-16 would theoretically be a good (although less useful due to the soft CL curve) method if it was possible for the various reasons. The F-16 always has a stall speed because it is an aerodynamic property.

 

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People seem to be confusing these two concepts:

  • stall, which is a disruption of airflow over the wing that reduces lift, and is almost purely an angle of attack phenomenon independent of weight, and only very mildly dependent on airspeed
  • the condition of flying too slow to maintain level flight at an unstalled angle of attack, which is directly dependent on weight

"Subsonic is below Mach 1, supersonic is up to Mach 5. Above Mach 5 is hypersonic. And reentry from space, well, that's like Mach a lot."

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vor 7 Stunden schrieb Mover:

Sooo... there's no stall speed.  😂

If life was a wind tunnel... But it isn't. 

So there is none. That's what I wanted to say. 

(English isn't my mother language so sorry if I sounded otherwise) 


Edited by TobiasA
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vor 10 Stunden schrieb Frederf:

I think you're misunderstanding, idealistic means theoretical. Stall landing the F-16 would theoretically be a good (although less useful due to the soft CL curve) method if it was possible for the various reasons. The F-16 always has a stall speed because it is an aerodynamic property.

 

I imagined an F-16 with auxiliary wheels on the nozzle slamming on the runway with 35 degrees AOA just right before my first coffee and it made me smile. 

I understand you. 

Let's just say that the shape of the F-16 in a wind tunnel has a stall speed. The F-16 as such has not, because at least the weight distribution would make you leave controlled flight way before reaching it.

You reach a point of no return there, that's why the FLCS prevents it. 

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15 hours ago, Frederf said:

I think you're misunderstanding, idealistic means theoretical. Stall landing the F-16 would theoretically be a good (although less useful due to the soft CL curve) method if it was possible for the various reasons. The F-16 always has a stall speed because it is an aerodynamic property.

 

I think there's a basic misunderstanding in how aerodynamically unstable jets with LEFs/TEFs and FLCS work and this obsession with speeds.

The bottom line - fly 11 degree AOA approaches.  

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Hey @Mover,

so I hope I get it right - please correct me:

Approach and throttle for 11 Alpha, once reaching the treshold (feet altitude?), throttle to idle and flare to 13 alpha and let it settle on the runway?

thank you!

 

Edit: Would you typically land with airbrakes or without, or does it depend on weight?


Edited by Hiob

"Muß ich denn jedes Mal, wenn ich sauge oder saugblase den Schlauchstecker in die Schlauchnut schieben?"

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