USMCFLYR

I received an AOPA Essential Aerodynamics/ Stall and Spin Safety CD in the mail. The last lesson was a PDF with key points to remember and reference. I thought I would share it with the masses. Btw - I enjoyed the CD. Thanks AOPA.

http://flash.aopa.org/asf/aerodynami..._KeyPoints.pdf

USMCFLYR

RVSM Certified

Thanks for the link.

Let's talk about the Stalls section for moment.

It is indicated in the document that a stall could occur with an onset of g's. Now, at first, I had to scratch my head on that one, sit back and think about it for a while.

Let's say I'm flying something very high-performance, with a very high positive g-load capability - something built with a +13/-8 g structural load limit. Now, let's say the aircraft is cruising straight and level at altitude doing 300 kts. The pilot pulls into a climb putting on 11 initial [impulse] g's as the AoA goes slightly beyond the critical angle of attack necessary to produce a power-on stall.

Now, this aircraft was doing 300 kts - nowhere near its stall speed of 91 kts. Assuming the pilot has the required g-tolerance to sustain the initial onset of +11g, "why" does this aircraft still stall? And, will the climb the pilot entered merely become a "ballistic climb" [until all the upward energy has been bled-off] as opposed to an "aerodynamic climb" where the aircraft is climbing because the Coefficient of Lift (Cl) is maintained through the linear increase of AoA.

I can see from looking at the coefficient of lift curve, that at some point, if AoA continues to increase, lift tapers off dramatically - I get that part. What I'm trying to understand is how this fits into a power-on stall caused by an increase in back-pressure leading to an initial climb that results in the initial onset of 11g's. [I'm just using a 11g as an illustration of a g-load capable of causing a power-on stall]

Is it because the 11g initial climb raises the index weight of the aircraft to 11 times its pre-climb weight, requiring the wings to be able to produce enough lift to support an aircraft weighing 11 times more, which would put the airspeed requirement well beyond Vne, Vfc, Vno, Vmo and every other do not exceed 'go straight to jail' max airspeed in the book?

USMCFLYR

[QUOTE]
I don't know the EM diagram for this mythical aircraft of yours but let's assume that most high performance aircraft in your scenario above would not be able to hit that 'g' before exceeding the lift limit AOA first.

It doesn't. Your aircraft would hit its' max 'g' first before hitting a speed where it would transition to its' lift limit. Think of the max performance turn you see a fighter at an airshow. That aircraft is maintaining an airspeed above a certain limit to enable him to maintain a high 'g' force before coming anywhere close to its' max AOA.

Aircraft with performance that you mention don't behave necessarily as you mention in your post. In high performance aircraft you will hit the max 'g' before a stall until a certain airspeed and then you can't maintain max 'g' and you will hit your AOA limit which if exceeded will cause a stall. Check out the book 'Aerodynamics for Naval Aviators' - a classic.

USMCFLYR

NoyGonnaDoIt

I'm not sure what kind of an explanation you are looking for. If it's a mathematical proof using a series of equations of aerodynamic principles, you might start here: Pilot's Web The Aviators' Journal - Load Factor

But the bottom line is that the quoted language from your post is essentially incorrect, although I'm thinking that you probably know that since it's pretty basic that stall speed increases with load factor. The correct statement would beIf you follow the equations in the article down, you ultimately get to the relationship between stall speed and load factor:

Accelerated Stall Speed = Unaccelerated Stall Speed X Square Root of Load Factor.

Your scenario involves an 11G load factor. SqrRoot of 11 is 3.32. That multiplied by your postulated 91 kt unaccelarated stall speed gets you a stall speed at 11G and max gross of just a hair under 302 kts.

RVSM Certified

Precisely why I selected those fictional numbers. The initial entry into the climb was 300kts and I did my calculations in my head, so I was a little off - by bad.

I realize they are fictional and I used them to try to better understand my own question - not that an aeronautical engineer would design such an aircraft, or that my fictional aircraft met fundamental modern aircraft design rules and traditions.

Ok, let's drop the fictional numbers and get back to the basic idea of AoA relative to Cl and a power-on stall.

USMC, please jump in here, too - I did read your reply, thanks.

What brought all this up in my mind, was the SU-27's/Mig-29's ability to maintain what appears to be controlled forward motion [in flight] after clearly pitching the nose up, well beyond the critical AoA during the so-called "Cobra."



I have no idea what the entry speeds are for this maneuver, but my first thought was that these Flanker and Fulcrum drivers must be stalling the wing [as they are literally flying backwards at one point] and relying upon inertia and the remaining elevator effectiveness to return the nose back to the horizon, without the aircraft first tumbling backwards tail first.

Visually, it looks like the aircraft is stalled, either fully [temporarily] or partially - yet - at no time does the aircraft seem out of control in either axis, nor does it ever appear out of control [departed] in the horizontal flight plane. And, the elevator obviously has enough effectiveness to return the nose to the horizon and then stop the nose from plowing to far below the horizon. In other words, these guys seem to do what the text book says you are not supposed to be able to get away with regarding exceeding the critical AoA at any airspeed.

So, are they stalled, or are they not stalled? If they are not stalled, then how can one account for the massive AoA assumed by the Cobra - it actually appears to go further than 90-degrees positive AoA at one point. It is almost as if you can take the Cl and AoA curve, or the Clmax and throw those concepts out the window with this aircraft. Is it just ballistics [forward inertia] and residual aerodynamic effectiveness on the elevator that give the illusion of the nose smoothly moving down to the horizon under "controlled flight" at the end of the Cobra? Or, have these aircraft re-written the books on AoA relative to Coefficient of Lift?

If you tell me that they are stalled and then begin flying backwards, ok - I'm fine with that. But, where is the associated aerodynamic instability that you would expect from having BOTH wings stalled? There is no dip to the left, no dip to the right - these aircraft remain on a horizontal plane with forward momentum and return the nose to the horizon as if no stall ever took place. And, they are not using vectored thrust as the Raptor does to help it accomplish the same/similar maneuver.

USMCFLYR

I don't know the flight characteristics of the two Russian fighters that you are mentioning so I won't comment on their numbers; but the era of fly-by-wire controls and excess power and in some aircraft thrust vectoring and other advancements have indeed rewritten the manual in which aircraft can be operated - doubtful the actual aerodynamics. For instance - I might be in a stalled condition when at military power and then be able to power myself out with use of afterburner. I can fly my aircraft above my max C/L but I'm falling out of the sky while I'm doing it but I have control authority past it I'm not not optimizing my flight characteristics. You mention no left of right turning. Not required in a stall. If you keep the aircraft coordinated it will just buffet and fall forward. If you stall uncoordinated then you risk inducing a spin.

USMCFLYR

RVSM Certified

Thank you very much, USMC. I've been out for a few days, but I do appreciate your response.

nciflyer

Some of those AOPA ASF tools are great. The Essential Aerodynamics / Stall Spin Safety Course and other courses should be available at AOPAs site Interactive Safety Courses It does require registration but you don't have to be an AOPA member.

RVSM Certified

Pitch/Power/Trim:

Let's keep the thread going.

I've been asking around for some help on whether or not I should obtain from my instructor [when I begin actual dual training], the Pitch/Power/Trim settings for my training aircraft, so that I can use that information to better handle the aircraft early on in my initial flight training, as a way of getting up to speed faster and as a way of learning good "control habits" once I make the move into the single pilot certified light business jet. I've gotten feedback from some here and I thank you for that. While that feedback was coming in, there seemed to be a split between some pilots here on whether or not doing such a thing would be beneficial to me at all. Some said yes and some said no.

So, I began looking for other examples of multi-engine jet pilots and taking a close look at what they do in this regard. What I found was this video showing a commercial crew making a 12 minute ILS approach to landing.

Notice at time stamp: 2:52 [2 minutes and 52 seconds into the video] the FO calls "Pitch 180" and the Captain confirms. If you listen closely, this "pitch" call from the FO comes after Control directs the Captain to descend down to "four zero" [4,000 ft]....then later Control directs the Captain to "Reduce speed to 180."

Now, if you pay attention to the video from the very start, you can see that they were already well into their descent from cruise altitude profile at somewhere around the 7,000+ ft level on their way lower.

Remember my initial question about this subject? Yeah, that one. Whether or not "Real Pilots" used specific Pitch/Power/Trim settings for the six (6) basic flight segments: Take-Off, Climb, Cruise, Descent, Approach & Landing? Yep, that one. Well, call me crazy - but - is this crew not doing exactly that? Furthermore, does this video resolve the question of whether or not you use pitch for airspeed and throttle for altitude? Sure does seem that way here when the FO calls "Pitch one eight zero." Heck! Even the cockpit was set-up to display the words: Pitch, and the number 180, as a visual panel output reference. So, not only does it appear that "real pilots" use these "configurations," but it also appears that at least some aircraft manufacturers design the cockpit to visually confirm for the crew that a specific Pitch has been set for a specific Airspeed.

I'm I viewing this the wrong way? Again, I ask only because I want to start early in the training cycle, doing as much as I can to get ready for the SJ30. The FO and Captain seem to be very smooth here, very controlled. They are not fumbling around trying to figure out how to get the aircraft down to 4,000 ft, they just Pitched to a specific airspeed and that was all she wrote.

What am I missing?

As usual, you feedback is always greatly appreciated!



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