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Been thinking ... relationship between back-stick and speed


Echo38
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In every flight sim game I've used which featured accelerated stalls of any sort, pulling back hard on the stick at low speed results in a stall, while it's usually safe to pull hard at higher speed without fear of stall. (Fear of losing your wings, maybe, but not stall.) Lately, I've been wondering about this, the fact that we're able to pull more backstick without stalling, at higher speeds than lower speeds. Is this because of sims' portrayal of virtual stick forces? I.e. pulling back on the (real-world) gaming joystick at high speed results in less pull-back on the virtual aircraft stick than at low speed, because of higher stick forces?

 

Or is this true in the real aircraft, as well, that you can pull back more without stalling at high speed than at low speed? And if so, why? I've been ruminating on all the factors I can think of. Higher speed allows you to pull more Gs at the same stick deflection, right (because the energy hitting the constant-angle elevator is higher), and higher G means higher stall speed--so with that alone, I would expect to be able to pull more back-stick without stalling at lower speed, rather than higher speed. But that isn't the only factor.

 

There's also the relationship between turning speed and turning circle. And--hoo boy, my head is kinda spinning here, but ... a lower speed means a tighter turning circle ... but not at the same elevator deflection? And now I've thoroughly confused myself, and I'm a little shaky on some of these physics things, and can someone who has a good grasp of physics clear this up? I've been telling sim-pilots to pull back less on the stick at low speeds, to avoid stalling, and in sims, this is right. But is it also true in real aircraft? Why (or why not)?


Edited by Echo38
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Stall is related to angle of attack. It's associated with speed, but that's more of a distant relative.

 

You stall when you pass critical AoA. This is probably going to be easier at low speed since just being level requires much more AoA. Eventually you will slow to the point where you have zero travel left on the stick before stalling. But it's not the speed that is the problem, it's the AoA.

 

Effectively, it's always easy to stall no matter what the speed, but lower speeds will probably have you stalling more often because in most situations, you're closer to your AoA limit.

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I didn't attend aerodynamics lectures, but as physics in general is no closed book to me, I'll try to get it right.

I apologise in advance for a long post.

Tried to keep it as short as possible right now.

 

The thing that causes lift with an airfoil in the first place is its shape and the way the airflow travels over the somewhat more curved top side of the wing.

The airflow will naturally tug onto the curvature of the wing and simply said "bending" over the wing, create an area of lower pressure above the wing.

This, together with the mostly unchanged airflow beneath the wing will result in a pressure difference between the top side and the bottom, creating lift.

This effect can be increased with higher AOA.

One could also argue to explain lift with Newton's laws of motion or even Bernoulli's principle, which both would be capable of describing the "phenomenon" too.

 

What happens with a stall is, the area where the air flow on the top side of the wing becomes turbulent, moves towards the leading edge.

The layer of turbulences increases in thickness orthogonally to the direction of flow.

At one point the layer becomes big enough and close enough to the leading edge that the airflow detaches from the top side of the wing.

The result is a stall.

The stall is mostly, but not entirely, AOA dependant and each wing profile has its own critical stallspeed.

Lower speeds consequently give a lower pressure difference, further increasing the "chances" of a stall as higher AOA are needed to compensate for the lower pressure difference.

 

The thing with higher speeds is, that the airflow is also faster.

That means the cone of the turbulent layer is also narrower and thus giving more of a threshold before the airflow detaches from the wing.

However other things come into play with higher speeds.

Such as increased centripetal force in turns with constant angular velocity.

Plus, with higher speeds the reynolds number increases too.

That makes a previously laminar flow more sensible for small irregularities in its path and it easier translates into a turbulent flow, again bad for lift.

 

So when you turn with higher speed, you need more force to keep your craft on a constant circular path.

That increases the demand for a higher pressure difference between the top and bottom of your wing (more lift needed).

Once you overdo it, the plane will stall out, the airflow above the wing will again detach and you have an accelerated (>1g condition) stall.

 

That's why there is an area of best turn performance which isn't either to slow or too fast.

The "feeling" that you can pull harder on the stick is subjective as with higher speeds you reach higher Gs with less throw

(refer back to increased centripetal force at higher speed with constant angular velocity).

Besides that you will have a harder time deflecting the control surfaces at higher speeds anyway and I don't quite know to what degree it is modeled.

But if my mind serves me right, I read somewhere that it is modeled so the joystick movement and the virtual stick movement may not be the same at all speeds.

 

Hope I could shed some light onto this.

Please correct me if I'm wrong on some parts or left out any vital parts.

In my defence, I didn't major in aerodynamics and the fluid mechanics lectures are quite some time ago.

So I don't guarantee for 100% accuracy of description of the stall phenomenon and all the physics incolved.

 

Greetings

MadCat


Edited by -=MadCat=-
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The thing that causes lift with an airfoil in the first place is its shape and the venturi effect.

Well this is not THE thing. Lift can be expressed in different but all equally real ways. Velocity is one.

 

I myself prefer looking at momentum exchange. The wing deflects air down. By Newton's Third law, the plane is help up.

 

What happens with a stall is, the area where the air flow on the top side of the wing becomes turbulent, moves towards the leading edge.

Turbulence can actually fight stall. Turbulent air is more resistant to flow separation because there is a greater transfer of momentum downward into the boundary layer toward the surface. In laminar flow, momentum moves mostly along the streamlines (lengthwise) and not up or down toward the surface. This makes it much easier to develop a reversed velocity profile, which is what leads to separation and stall.

 

Basically, the wing is pushed to a point where it needs to work so hard that it's pulling air backwards.

 

The thing with higher speeds is, that the airflow is also faster, meaning that the cone of the turbulent layer is tighter and thus giving more of a threshold before the airflow detaches from the wing.

Incompressible flow is not speed dependent, so up to a maximum of around Mach .3 to .5 the flow structure around the wing isn't significantly different. From .5 to .8 you get compressibility effects, but this can be represented by simple scaling of the pressure contours around the wing. They become shorter/taller.

 

Now the boundary layer and the point of transition from laminar to turbulent does change with speed but this only really effects drag numbers, the overall flow structure can ignore these features and the biggest difference comes from whether or not you need to account for compressibility.

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If I recall correctly, DCS doesn't model stick forces, meaning you can pull harder than someone should be able to and so you're forced instead to self moderate based on experience and judgment to fly the airplane within parameters differently than a real pilot would who would be fighting stick forces.

Warning: Nothing I say is automatically correct, even if I think it is.

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So, then, in a real aircraft, can you pull more back-stick without stalling at high speed than you can at low speed, or not? Still confused on this point.

 

At high speed, you have more lift, which means you can ... pull a higher AoA than when you have less lift at lower speeds? Or does the amount of lift not affect the [max Alpha at any given moment]?

 

I guess my grasp of the nature of turning in virtual dogfighting is more intuitive and less analytical than I'd like--I learned mostly from trial and error, although I did do a lot of reading from real-world sources on the subject. In trying to share this knowledge with others, I've realized that I sometimes can't explain the "why," even on a simplified level, even when I can explain the "how."


Edited by Echo38
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At high speed, you have more lift, which means you can ... pull a higher AoA than when you have less lift at lower speeds? Or does the amount of lift not affect the [max Alpha at any given moment]?

 

The max AoA is the same for low and high airspeeds.

 

You don't stall as easily at high speed because the aircraft's vector stays much closer to the direction the nose is pointing.

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So, then, in a real aircraft, can you pull more back-stick without stalling at high speed than you can at low speed, or not? Still confused on this point.

 

I don't think thats it. I think its that in a real aircraft you have to fight stick forces that prevent you from being able to yank as hard as you can in game. You're still deflecting the control surface as much as in real life, you're just doing it in game more easily than you would be able to in real life, and as such you have no feedback to tell you whats happening based on 'feel' like you would in real life. Think of it like pounds of force necessary to provide X deflection. At 1 G at X airspeed it will be much easier than at 5 G at Y airspeed (or even X airspeed).

 

Certainly under certain circumstances you would be able to in game deflect a control surface to a degree not possible, or plausible, in real life owing to the fact that in that circumstance in real life you'd be under so many G that no normal pilot would be able to do that.

 

Of course, I'm not accounting for any type of hydraulic or electronic systems that may be immune to this. I know that systems that are more than just physical linkages often have "artificial feel" devices to give pilots that feedback they lack thanks to the systems basically being strong enough to overcome nearly any force acting against them. In an F-16 for instance, the stick functions by barely moving but tracking the force of the inputs the pilot makes to translate that into control commands. Of course this is not really relevant to our case here because the F-16 is not flown by normal measures of control, that is to say the computer does a lot of filtering of the pilot's inputs, and the design of the stick negates any sort of stick force limitations.

 

Its all a bit complicated and confusing. I do know that the lack of physical feedback us simmers have changes how we cope with flying and the associated decision making compared to any real pilot.


Edited by P*Funk

Warning: Nothing I say is automatically correct, even if I think it is.

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I do know that the lack of physical feedback us simmers have changes how we cope with flying and the associated decision making compared to any real pilot.
I agree completely! I don't have any aerobatic time, but I do have a few hours in Cessnas, and I'm one of the loudest when it comes to complaining about our little plastic gaming joysticks. The difference in normal flight alone is cosmic, let alone anything like aerobatics or combat maneuvers.
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In every flight sim game I've used which featured accelerated stalls of any sort, pulling back hard on the stick at low speed results in a stall, while it's usually safe to pull hard at higher speed without fear of stall. (Fear of losing your wings, maybe, but not stall.) Lately, I've been wondering about this, the fact that we're able to pull more backstick without stalling, at higher speeds than lower speeds. Is this because of sims' portrayal of virtual stick forces? I.e. pulling back on the (real-world) gaming joystick at high speed results in less pull-back on the virtual aircraft stick than at low speed, because of higher stick forces?

 

Or is this true in the real aircraft, as well, that you can pull back more without stalling at high speed than at low speed? And if so, why? I've been ruminating on all the factors I can think of. Higher speed allows you to pull more Gs at the same stick deflection, right (because the energy hitting the constant-angle elevator is higher), and higher G means higher stall speed--so with that alone, I would expect to be able to pull more back-stick without stalling at lower speed, rather than higher speed. But that isn't the only factor.

 

There's also the relationship between turning speed and turning circle. And--hoo boy, my head is kinda spinning here, but ... a lower speed means a tighter turning circle ... but not at the same elevator deflection? And now I've thoroughly confused myself, and I'm a little shaky on some of these physics things, and can someone who has a good grasp of physics clear this up? I've been telling sim-pilots to pull back less on the stick at low speeds, to avoid stalling, and in sims, this is right. But is it also true in real aircraft? Why (or why not)?

 

In fact, both in DCS sim and in RL the stick position to get the certain AoA is the same up to compressibility area (approximately more than 0.7-0.75 M) because as M increases from 0.2 to these numbers there are no changes in dCL/dAoA derivatives or CL slopes both for the wing and the stab and their own moments are not changing, too.

But there is an effect of local Mach number because of circulation around the wing. It means that the local air velocity over the wing can be much higher than the incoming air velocity. The temperature of the accelerated airstream decreases making local speed of sound less than for the ambient air temperature.

Thus, the local M increases with lift and the point is that maximum CL wing can produce begins to diminish from M=0.2 for the typical non-swept wings.

That's why at the high altitude at the same IAS you can not pull the same G and the stall bites you at the stick position that was safe near the ground.

 

I suggest that DCS is the first sim that takes this effect in account since A-10C was released.

 

And, as a matter of fact, the Mustang's laminar airfoil was way better than NACA 230, for example, at moderate M (0.5-0.6) providing higher CL max.

Ніщо так сильно не ранить мозок, як уламки скла від розбитих рожевих окулярів

There is nothing so hurtful for the brain as splinters of broken rose-coloured spectacles.

Ничто так сильно не ранит мозг, как осколки стекла от разбитых розовых очков (С) Me

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I agree completely! I don't have any aerobatic time, but I do have a few hours in Cessnas, and I'm one of the loudest when it comes to complaining about our little plastic gaming joysticks. The difference in normal flight alone is cosmic, let alone anything like aerobatics or combat maneuvers.

 

Did you try a Tiger Moth? ;)

Ніщо так сильно не ранить мозок, як уламки скла від розбитих рожевих окулярів

There is nothing so hurtful for the brain as splinters of broken rose-coloured spectacles.

Ничто так сильно не ранит мозг, как осколки стекла от разбитых розовых очков (С) Me

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@ Exorcet

 

Thanks for not ripping apart my entire post :D

 

The first thing about the venturi effect actually was edited shortly before your post, my mind isnt all the fastest that late :D

 

As said aerodynamics isn't my major and honestly I even hated flow mechanics back then.

Now that you mention reverse flow patterns, some more things come to mind again.

I still hated flow mechanics. :lol:

 

Hope the rest of my post wasn't total rubbish or I may be better off editing all to "I don't have ANY clue". :music_whistling:

 

Greetings

MadCat


Edited by -=MadCat=-
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So, then, in a real aircraft, can you pull more back-stick without stalling at high speed than you can at low speed, or not? Still confused on this point.

 

I think you should ignore the stick, especially if this is for the general case. Stall is based on AoA.

 

To answer your question though, the stall AoA is basically constant, so that's not really a moving barrier. However the stick position for level flight a a given altitude moves back as you slow down. This reduces the stick travel you have until you stall. So as a rule of thumb, you could say that pulling back at low speed is more dangerous.

 

At high speed, you have more lift, which means you can ... pull a higher AoA than when you have less lift at lower speeds? Or does the amount of lift not affect the [max Alpha at any given moment]?

Max AoA doesn't depend on speed. You have more lift, but the stall AoA is the same.

 

300px-Lift_drag_graph.JPG

That graph is reliable for most of the speed range up until Mach 1. The zero lift AoA and the stall AoA stay where they are, but as you go slower, the AoA for 1 g level flight moves up the blue line. The amount of stick motion you have available is the distance from the 1 g AoA and the stall AoA.

 

@ Exorcet

 

Thanks for not ripping apart my entire post biggrin.gif

 

The first thing about the venturi effect actually was edited shortly before your post, my mind isnt all the fastest that late biggrin.gif

 

As said aerodynamics isn't my major and honestly I even hated flow mechanics back then.

Now that you mention reverse flow patterns, some more things come to mind again.

I still hated flow mechanics. lol.gif

 

Hope the rest of my post wasn't total rubbish or I may be better off editing all to "I don't have ANY clue". music_whistling.gif

 

Greetings

MadCat

 

No, it was pretty good. I'm probably off somewhere too, I don't use this day to day, and usually focus on a more zoomed out view of aerodynamics. Bernoulli Effect isn't wrong for describing the generation of lift, and it's pretty dominant outside of the boundary layer, but it doesn't tell the whole story by itself.


Edited by Exorcet

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Its addicting isnt it? :)

 

Yes it is, and delightful, both this threads and everything about the flight and systems modeling in DCS.

 

I believe one day I will finally be tempted to try the military side of the sim... maybe online (?), but up until now I only start it to feel in piece with myself regarding having *** finally *** found the best investment of my life in terms of simulation of FLIGHT!

 

The P51d was my entry aircraft, and although I have all less FC3 and CA on my account, including the superb UH-1H, it is still the model I use the most :-)

Flight Simulation is the Virtual Materialization of a Dream...

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Max AoA doesn't depend on speed. You have more lift, but the stall AoA is the same.

 

300px-Lift_drag_graph.JPG

That graph is reliable for most of the speed range up until Mach 1.

 

You are wrong. And your simple way to consider max AoA and CL constant UP TO MACH 1 (!) is DEEPLY WRONG!

 

If you try to google "lift carpet" term you can find many interesting facts and reports... the best source I know is NACA report about Re and M dependance of several airfoils where you can find a lot of facts making your point of view not so simple. :)

 

Typical diagram for L-39 tells us that CL_max is 1.3 @0.1M, 1.0@0.5M and 0.8@0.6M... and as the derivatives are still the same, AoA decreases proportionally.


Edited by Yo-Yo

Ніщо так сильно не ранить мозок, як уламки скла від розбитих рожевих окулярів

There is nothing so hurtful for the brain as splinters of broken rose-coloured spectacles.

Ничто так сильно не ранит мозг, как осколки стекла от разбитых розовых очков (С) Me

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If you try to google "lift carpet" term you can find many interesting facts and reports...

 

Mostly concerned with moving your carpet around :)

 

https://www.google.ca/search?q=%22lift+carpet%22&ie=utf-8&oe=utf-8&rls=org.mozilla:en-US:official&client=firefox-a&channel=rcs&gws_rd=cr

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You are wrong. And your simple way to consider max AoA and CL constant UP TO MACH 1 (!) is DEEPLY WRONG!

Well yes it is simple, to keep the post shorter. I didn't say up to Mach 1, but for the most of the range (up to transonic) the graph should be OK.

 

You do agree with the point I was trying to illustrate regarding stick motion though, or no? The reason that pulling harder at low speed generates a stall more often is because you're closer to the limit of the airfoil?

 

Typical diagram for L-39 tells us that CL_max is 1.3 @0.1M, 1.0@0.5M and 0.8@0.6M... and as the derivatives are still the same, AoA decreases proportionally.

Well I don't have specific data to reference for high Mach, but that variation is a bit more than I thought you'd see. In any case DCS seems to work right so I'll trust your numbers (and hopefully if you stop posting F-15 AFM gets done faster lol).

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BTW: Just out of curiosity,

 

Mach tables for Delta (CL0, CD0, Clde, Cladot, CLq, Clih, Cmde, Cmadot, Cmq, Cmih, Cm0, Cyb, Cydr, Cyr, Cyp, Clb, Cldr, Clda, Clr, Clp, Cnb, Cndr, Cnda, Cnr, and Cnp), due to Mach, have long been used, since Microsoft FS9 and in the Combat Flight Sim series, being tuned and added in FSX, now P3D...

 

Very few authors have made use of it, either because of total ignorance of how to use them or simply because they do not have access to aero data for covering this parameters.

 

There are a few perls for FSX / P3D, that show how detailed and accurate the old MSFS FDM can be, if used in it's plenitude...


Edited by jcomm

Flight Simulation is the Virtual Materialization of a Dream...

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Well yes it is simple, to keep the post shorter. I didn't say up to Mach 1, but for the most of the range (up to transonic) the graph should be OK.

 

You do agree with the point I was trying to illustrate regarding stick motion though, or no? The reason that pulling harder at low speed generates a stall more often is because you're closer to the limit of the airfoil?

 

 

Well I don't have specific data to reference for high Mach, but that variation is a bit more than I thought you'd see. In any case DCS seems to work right so I'll trust your numbers (and hopefully if you stop posting F-15 AFM gets done faster lol).

 

THis graph IS NOT OK starting from M=0.2 (for the reason I wrote above). It's a simplification that most of sims or projects (as jcomm wrote) suffered from. And not only sims as well - P-51 G-load diagram in flight manuals used IAS^2 based curve that gives wrong G-load values at high speed. The diagram for A-10, though, are more right, for example, so intially I used it to determine max CL at various M. Later I got a lot of diagrams for A-10 including CL_max vs M and the determined curve was very close to it.

Ніщо так сильно не ранить мозок, як уламки скла від розбитих рожевих окулярів

There is nothing so hurtful for the brain as splinters of broken rose-coloured spectacles.

Ничто так сильно не ранит мозг, как осколки стекла от разбитых розовых очков (С) Me

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Add aircraft, wind tunnel and tests... :) You could never imagine what "G-" based searches drag to the beach...

Ніщо так сильно не ранить мозок, як уламки скла від розбитих рожевих окулярів

There is nothing so hurtful for the brain as splinters of broken rose-coloured spectacles.

Ничто так сильно не ранит мозг, как осколки стекла от разбитых розовых очков (С) Me

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