Accelerated Stalls Explained

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Scott - A2A
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Accelerated Stalls Explained

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Accelerated Stalls – “The Edge” explained
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If you have flown IL-2, Rowan’s Battle of Britain, Mig Alley, etc. you have undoubtedly experienced an accelerated stall (buffet or shake when you pull too hard back on the stick).

In early 2003, when FirePower development began, it was discovered that hidden within the CFS3 flight engine was the capability to model complex accelerated stalls. To our surprise, this same untapped capability even existed in CFS2 when it was released 4 years ago. So why is it that CFS3 is one of the last standing flight simulations that didn't model accelerated stalls with many of its fighter aircraft? The best answer we can come up with is it was a Microsoft business decision to make the simulation a little easier to handle for the average person. Firepower aircraft are designed for the CFS3 pilot who wants a more realistic experience and is ready to step up to the challenge of handling accelerated, high-speed stalls and spins, just like the real pilots who flew these aircraft did.

If you have been a CFS3-only flyer, then you may have developed the old, “yank and bank” approach to flying. Since FirePower models accelerated stalls and spins, the pilot who hasn’t experienced an accelerated stall will undoubtedly notice something new happening in their airplane (usually at the worst time too). This article is designed to talk to that flyer. Below is a brief article explaining what accelerated stalls are:

What is a stall?
In order for a wing to produce efficient lift, the air must flow completely around the leading (front) edge of the wing, following the contours of the wing. At too large an angle of attack, the air cannot contour the wing. When this happens, the wing is in a “stall.”

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Typically, stalls in civilian aircraft occur when an airplane loses too much airspeed to create a sufficient amount of lift. A typical stall exercise would be to put your aircraft into a climb, cut the throttle, and try and maintain the climb as long as possible. You will have to gradually pull back harder on the stick to maintain your climb pitch, and as speed decreases, the angle of attack increases. At some point, the angle of attack will become so great, that the wing will stall (the nose will drop).

Below are some graphical representations of a wing traveling though the air in various conditions:

Level flight – a wing creating moderate lift
Air vortices (lines) stay close to the wing.
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Climb - Wing creating significant lift force
Air vortices still close to the wing.
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Stall
The angle of attack has become too large. The boundary layer vortices have separated from the top surface of the wing, and the incoming flow does no longer bend completely around the leading edge. The wing is stalled, not only creating little lift, but significant drag.
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What is an “Accelerated stall?”
An accelerated stall is one that is “forced” to occur, not due to lack of airspeed, but by forcing the wing into a high enough angle of attack and therefore, force it to stall.

When you pull back on the stick, you are literally using leverage to push your tail down, which forces the main wing into a higher angle of attack, and increased lift. However, there is a limit to how much “force” you can place on a wing before it “gives up” or stalls. If you pull your stick back hard and far enough to pass through the wing’s critical angle of attack, regardless of the speed, you can “force” the main wing into an “accelerated stall.” In a sense you “break” the wing’s ability to provide lift as long as you continue to provide sufficient force to keep the wing in an excessive angle of attack or “accelerated stall” condition.
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QUESTIONS:
Q: Are their any warning signs before an accelerated stall occurs?
A: Yes, but not always. Common indicators are loss of aileron authority (the plane starts to drop or “waggle” its wings), a shaking stick, vibration, or even sometimes a loud crack or bang can be heard on the wings.

Q: How do aircraft react when they are forced into an accelerated stall?
A: That is completely dependent on the aircraft you are in. Some aircraft simply stop producing lift and may just buffet and drop a wing, others may do a sudden, hard “snap” left or right (snap-roll), while others may react violently or even enter into an almost immediate and DANGEROUS SPIN.

QUOTES FROM WARTIME PILOTS ABOUT ACCELERATED STALLS INDUCING SUDDEN, UNEXPEDED SPINS:
"I pulled into a tighter, tighter turn, feeling so many G's I can hardly turn my head. Then the stick goes limp, I'm spinning..."

"If the German fighter (FW190) was pulled into a g stall in a tight turn, it would flick out of the opposite bank and AN INCIPIENT SPIN WAS THE INEVITABLE OUTCOME if the pilot did not have his wits about him"

"he went into a tight turn and as I tried to follow him I found myself spinning out of control again. I repeated this unnerviing experience a couple more times before deciding to give up"


Q: What does “flying on the edge” mean?
A: Flying on the edge is the art of extracting the maximum performance from an aircraft w/o going into an accelerated stall. A pilot must listen closely to his aircraft’s feedback to keep the aircraft flying just on the edge before an accelerated stall occurs to extract the maximum performance out of the aircraft.

Q. How will flying “on the edge” help me in air combat?”
A. Firepower aircraft are built with more inherent ability to maneuver, rather than aircraft that don’t provide this “on the edge” experience. Once you learn to ride that edge – as many current Firepower owners have done – you will find that you can out-turn and out fly many aircraft that aren’t built with this capability. That’s the challenge and the benefit of flying Firepower; it’s not only more realistic, it’s more lethal once you master it.



FirePower Wind Tunnel Technology
“Flying on the edge”
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CREDITS - NASA

By using authentic, high-resolution wind-tunnel data for a specific wing, this new process has created a much more accurate, smoother, and detailed response from the wing, especially when “riding on the edge.” What this means is a more natural feel and feedback.

Two benefits:
1. Real NACA airfoil data files for the actual aircraft being modeled are imported and tested in a computer wind tunnel program to reproduce the true character of that particular aircraft’s wing.
2. A higher resolution lift curve on the front side of the lift curve means more natural feel and feedback, especially when flying on “the edge.”

Higher Resolution:
Below are two simple examples of a wing made with and without the Wind Tunnel Technology upgrade. Not only does each wing get its own specific character, the new process makes each wing with a more natural, and higher resolution.

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Notice on the wing created with the wind tunnel data, there is a higher resolution and a more natural curve as the angle of attack increases. The area where this is most important is on the front part of the curve, which is where you’re your flying is done, and on “the edge,” or peak, which is the area that exists just before the wing’s angle of attack breaking point. The stall occurs more gradually (if this is in fact the characteristic of the plane being modeled). This kind of resolution affects everything including takeoffs, landings, and just normal cruise flight.

There is a feature in FirePower called "G-view" that simulates the pressures a pilot experiences in the cockpit. This view helps give the pilot critical visual feedback to help them more accurately fly on the edge.

http://www.a2asimulations.com/firepower ... /gview.zip
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From "airfileguy"

POST 1:
There's been some discussion about comparisons of the IL-2 aircraft with Firepower aircraft, with respect to stall and spin characteristics. Scott mentioned that, after the Firepower package was wrapped up and ready to go, he found some time to fire up IL-2 and do a little flying. The result of this was that he found the stall/spin behaviors of some of the Firepower fighters to be remarkably similar to the IL-2 fighters.

This was not intentional -- in other words, when the flight models were being developed, we didn't say, "well, lets play IL-2 and then try the Firepower stuff and really get it matched." The way it came about was much more natural and evolutionary, and was based on a lot of different experiences and research. But the result, in our estimation, was quite pleasing.

Scott and I are both pilots in real life, as well as being avid flight simulator fans. We think we have some understanding of the difference between flying a real aircraft and flying a computer. As one friend of mine put it, who's an aerobatic pilot and aeronautical engineer, the biggest difference is that "you can't hit the pause button." Obviously there is much more to it than that. Scott and I had many discussions about this, and, aside from the fact you can't hit the pause button, we came up with a few observations about the difference between computer flight simulators and real airplane.

The biggest difference is the "seat of the pants" feel you get from a real plane. In VFR flight, at any rate, you really do rely on the sensations of motion and pressure you get from your butt, along with the visual references from the outside. That's the second biggest difference. No matter how hard we try, we can only get a small slice of the outside world to appear in our cockpit view at any given time. There is just very little outside reference, which makes PC flight simulators much harder to fly than real aircraft. It's pretty much like flying on instruments all the time, at least with respect to maintaining level, coordinated flight. That's why takeoffs and landing are so tough in the simulator; you can't see much, which is why the games always give you a HUD view without the panel, which really helps.

What we wanted to do was to provide the most analogous experience possible. One way this was accomplished was through the g-view which is describe in another thread. The second way was the method we used to model stalls and spins. There has been and always will be a lot of talk about "realism." Just what is "realism," anyway? We decided it was a simulated experience that would leave the person feeling as though they had really "been there, done that." The problem is this is different for every individual. And most computer gamers have never flown a real aircraft and probably never will. They just want to have a great, immersive experience playing the game. But, at the same time, there are quite a few folks that like to at least believe that some effort has been made to get the game as "close to reality" as possible, or as Microsoft puts it, "as real as it gets."

That's where the slow flight, stall, and spin behavior really come into play. I'll follow up on this discussion a little later with another post, explaining just how we went about putting together our version of "as real as it gets."





POST 2:
It's just certain characteristics that seem to be shared between the two flight models that we find interesting, and don't worry, there won't be any flame wars here, that's not the purpose of this forum. It's to discuss the Firepower product, and help Firepower customers get the most out of their purchase.

In truth, the best flight model is going to be the one you like the best, but we also realize there are different schools of thought about what makes a good flight model. One thing that is worth responding to is the idea of a particular aircraft being "easy" to fly. We have to remember that "easy" is a relative term here. Most of the WWII taildragger fighters were somewhat tricky, if not downright nasty, on takeoff and landing. But once in flight, a pilot could certainly fly for many hours and never come near exceeding the plane's capabilities. In fact this is just what happened most of the time; most of the pilots who flew these aircraft never shot down another plane. They just did their jobs and flew conservatively. It was only the elite few that became aces. The vast majority were wingmen or flew in other supportive roles.

But we have to consider the circumstances of dire air combat to begin to understand where an aircraft that might be "easy" to fly 99 percent of the time suddenly becomes a vicious machine. A good way to compare this is to consider motorcycle road racing, or perhaps auto racing. If you watch a national or world championship motorcycle race, like the Moto GP series or the World Superbike races, 99 percent of the time it looks like a ballet. These guys are so good, they make it look easy. But when they're battling for the lead, they're on the absolute ragged edge a lot of the time. What is so deceptive is that they are so smooth and highly skilled that they make it look easy, leading people to think, "Hey, I could do that!" But we know it's not that easy at all -- otherwise everyone would be doing it.

Now consider heated air combat, where you are fighting for your life. The pilot who can turn the tightest, shoot the best, and in short, get the absolute maximum performance out of his aircraft is the one who is most likely going to survive. That means, in no uncertain terms, that he's going to have to take that aircraft to the ragged edge of control, with the stick shuddering and the control surfaces buffeting and fluttering, and hold it there longer than the other guy, without going "over the edge."

Any plane, no matter how well designed, can be stalled. Some spin more easily than others, but no plane has ever been "spin-proof." This comes down to a couple of factors (ignoring the center of gravity for now), the first being elevator authority, and the second being airspeed and the strength of the pilot. The higher the indicated airspeed, the harder it is to pull back on the stick. This second phenomenon is only critical for most fighters at very high indicated airspeeds, such as pulling out of a dive. The first one is the one that makes the real difference in the speeds encountered in combat. To illustrate the point, here's a quick quote from "America's Hundred Thousand," a book that could be considered the "bible" when it comes to facts and figures on American fighter aircraft. This concerns the P-51D Allison-powered Mustang:

"In turns, elevator control...was sufficient to develop either the allowable 8g limit load factor or maximum lift coefficient throughout the speed range. The aircraft was sensitive to small stick movements but stick force gradients were satisfactory."

Regarding the Merlin-powered Mustang, the book states the following:

"Turns above 250 mph IAS and above four g were particularly dangerous. In these cases a pilot not equipped with an anti-g suit could be at least partially blacked out, and consequently less alert..."

Here's another tidbit from Eric Brown, on the Fw 190A:

:...if the German fighter was pulled into a g stall in a tight turn, it would flick out into the opposite bank and an incipient spin was the inevitable outcome if the pilot did not have his wits about him."

Another thing to consider when evaluating the ease with which these planes could be operated is that the stall speed increases with bank angle. In other words, the more the wings are banked, the less vertical lift is available and the higher the stall speed is, so it becomes much easier to inadvertently stall the aircraft. This is especially true if the pilot is partially blacked out because of g forces or has his attention on an imminent stream of cannon shells about to be unleashed on him!

One final note on g forces: we can't feel them when flying our PC, but the pilots of real planes sure can. Anything beyond 6 gs required an anti-g suit, so the earlier fighter pilots could not be expected to comfortably pull more than a maximum of about 6 gs, but 4gs -- as mentioned in the quote above -- was sufficient to cause partial unconsciousness. With the advent of the anti-g suit, a pilot could pull up to 8 gs but this was still not something that happened very often.

POST 3:
Anyone doing flight models will hopefully do their best, based on whatever knowledge and information they have available at the time. One limiting factor for any flight sim is the simulation engine itself, and the way it models changes in air density from high to low altitude and the physics of lift, thrust and drag. These things change quite a bit with airspeed and air density and the equations are very complex. In fact there are still plenty of unknowns, and the flight simulators we have on our computers were never intended to address the cutting edge of aerodynamics...they were meant to be a game. So we're up against this as well. Then there is engine performance, which is not modeled in exactly the same way the real aircraft perform. Some engines perform better at high altitude, others at lower altitudes, depending on how the supercharging is set up.

We always try to keep in mind that great gameplay is the most important thing, but we also tried to create flight models that had parallels with their real-world counterparts. Hopefully we have succeeded in that to some degree.

Scott.
Last edited by Scott - A2A on 09 Jul 2008, 12:53, edited 2 times in total.
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sspaceguy5
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Post by sspaceguy5 »

What is an accelerated stall?

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Scott - A2A
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Post by Scott - A2A »

As air travels over and under a wing, it provides lift. The lift is created by creating sort of a vacuum above the wing. This vacuum is created because the air that passes over the wing is forced to travel further than the air that passed below it, so as it is rushed along, it sort of pulls the wing upward.

Now, if you alter the pitch (angle of attack) a bit, lift increases. This is done by pulling back your stick (moving the rear elevator). This movement pushes your tail down, and this force changes the main wings angle of attack.

Quite simply, there is an angle of attack where the wing can no longer create lift, and this is when we say a wing is in a stall. Typically this happens when an aircraft gets so slow that there isn't enough air passing over the wing to keep the aircraft flying, and it begins to fall.

An accelerated stall happens when you "force" that angle of attack upon the wing at speeds where it could continue to fly otherwise. Pull back so hard on the stick, apply so much downward pressure on the tail so the main wing is forced into an angle of attack that it cannot create lift, or "stalls."

This is why when you are traveling fast in a high performance plane, and you pull back really hard on the stick, the plane "snaps" or "spins." The "snap" is caused by one wing abruptly "snapping" or "unloading" and that side drops suddenly. In many aircraft, this snapping action results in a subsequent spin.

Scott.
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sspaceguy5
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Post by sspaceguy5 »

Is that why if I go strait up at 500 MPH, I start to stall?

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