Aerodynamics of the Base to Final stall

clearly explained by a leading expert

http://www.nar-associates.com/technical-flying/angle_of_attack/StallSpin_wide_screen.pdf

 

And every PPL pilot should understand this...without the aid of an AoA device.

 

Couldn't agree with you more. Had an FAA inspector come by the other day. I asked him about any FAA ACs explaining it or if he had a simple way of teaching it. He said no to both.

Eyes glazed over...

 

So how is this different from a stall warning horn, other than the addition of visual? (I'm being serious, trying to learn, not bash you).

Could the horn be calibrated go off as early as the AOA? (I realize that the horn is more on-off, while the AOA lets you "see" where you are in the range.)

In a bank scenario, does the stall horn vs AOA give a different reaction depending on left vs right bank, since they are located on one wing?

 

Thank you for asking. Nice to have an adult conversation about this.

"If a Wing could talk it would be in the language of AoA. " that is what the inventor Mark Korin said to me. Great words.

When the stall horn goes off you are now at Cl max (in other words at the peak of Lift and ready to stall)

Now an AoA provides a gauge or ladder of how much lift reserve you have left. Way before it's too late. (Low and slow )

Also the blue donut is the max lift over drag speed at any weight and configuration. The stall horn just tells you when you are going to lose lift.

So by having a lift reserve color bars it gives you precise information.

Technically the AoA should be on both sides. But a more practical approach for GA aircraft is the wing opposite the stall horn. Or biased on the left side since most pattern work is left turns.

 

He's a moron that shouldn't be occupying that position. It's referenced in AC 61-67C and also a requirement by the PTS to get a PPL. If a pilot hasn't been taught this, then they've been short changed on their training. No AoA can replace good training.

 

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Agreed! Good training however can include an AoA !

 

You are wrong.

 

How about explaining WHY you think he is wrong. That way we may all be able to learn something.

 

I thought the stall horn on a certified aircraft was required to sound 5-10 knots above actual stall speed. I know I've flown my Mooney at MCA with the stall horn blaring, turning both directions, even climbing and descending, during Flight Reviews, for 5-10 minutes at a time.

 

Probably better to spend the money on some aerobatic instruction. Once you really understand AOA by thinking like a wing, your need for an AOA indicator is drastically decreased.

In transport category aircraft these devices have great value simply because of the tendency for the pilot to become disconnected from the loop due to the amount of automation which is common these days. There's not much "feel" there. It's purely a numbers game. Good to have AOA in that case.

In a piston aircraft, I could take it or leave it. If anything the emphasis should be on training and education, not installation of aftermarket AOA devices. Certainly I don't see any need to get a movement afoot for AOA indicator installations in piston GA aircraft. Let's move on.

 

Here is the EAA article. You are correct.The horn actuates just before the wing stalls, very much like the other type of stall warning horn - 5-10 mph above the stalling speed. The horn sounds off when a negative air pressure is induced at the wing's leading edge. This causes a reverse airflow through the horn. This typically occurs just after Cl max when lift is degrading.
https://www.eaa.org/en/eaa/aviation...-and-avionics/a-look-at-stall-warning-devices

 

What's the harm in it? You obviously don't need one. There are many pilots that do.
Funny today I flew with an Alaska bush pilot who owns a maule. He said why doesn't every aircraft have this !

 

There's no harm, really, in these doo-dads. There's just a better way to spend the money: on training which involves exposure to high alpha flight beyond simple "slow flight." And that, sir, is aerobatics. Sort of a "give a man a fish, he'll eat for a day, teach him to fish, he'll eat for a lifetime" sort of concept. The indicator might -- might -- help avoid an accident or two. But we'd do far better across the board if we simply gave those pilots exposure and training in flight regimes which they've cordoned off due to anxiety, fear, or whatever else.

Watched a video once of two guys landing gear up somewhere outside of the U.S. The gear warning horn was blaring the entire time. They weren't even aware of it. That's the concern with these kinds of gadgets. If a pilot can't fly a wing referencing AOA he's likely to be easily overwhelmed with extra information. We can't overpower primal human instincts with inexpensive consumer-grade aviation tech. But we can train, and that's what I'd advocate over one of these devices.

 

Thanks for explaining this as I never thought about it. This explains why the stall horn goes off during landing even though the plane has not yet actually stalled.

It really does make sense when I think about it that the horn is meant as a warning. To have it go off right as the plane is staling would be purposeless!

Back to your original post- I appreciate this too. I went with a CFI during my last BFR who suggested that pilots should fly more of a rounded pattern and instead of the standard rectangular pattern. He stressed this was most important on the base to final turn as increased banking is not good at this point. Since he explained this and showed me the benefits I've now adopted the more rounded pattern. To me, the FAA should change their recommended patern from a rectangular pattern to a rounded base to final pattern. As it stands now, the way I fly a pattern may mean I could never get my PPL as I would not be meeting the proper pattern technique even though I believe it is a much safer way to fly. It seems the FAA should re-evaluate their position on this issue.

 

The stall warning is sensitive to the movement of the stagnation point. As AoA increases, the stagnation point moves from the leading edge back under it, and the flow upward from it and over the top causes a low pressure at the stall port, or pushes the stall switch vane up.

That "leading expert" in the first post is ill-informed. A receding or advancing wing has a very small differential in airspeed compared to the overall airspeed. A 36-foot wing's tips will be rotating, in a turn twice rate one (or 360 degrees per minute) at 113 feet per minute, with the differential of 226 feet per minute. The airplane, if moving forward at 60 knots, is doing 6013 feet per minute. Each wing tip, therefore, is either faster or slower by 1.9%. Miniscule. ANd that's at the tips; inboard it's far less.

The real problem is the helix describeed by each wing in a descending turn. The inside wing is descending a little more steeply than the outside wing because it's path is a bit shorter, being on the inside track, so its AoA is higher by a fraction of a degree. A tighter turn increases the AoA differential. And the pilot's cross-controlling really makes a big difference, because introducing down-aileron on the inside wing increases the wing's AoA over the section of wing occupied by the aileron. Remember that the chord line runs through the leading and trailing edges, and when you drop the trailing edge, you move that chord line, and therefore the angle of incidence, and therefore the AoA on that section if the rest of the wing's AoA is unchanged.

 

I don't disagree. However some pretty well trained pilots have crashed because they had no sense of AoA. Asiana, Air France 447... but I digress.
Because I can't spin a Cirrus, I send my students to an aerobatics instructor so they get the spin training. So yes I'm a believer. But in the fancy glass and smooth aircraft skins we have today it's not like a Cessna.
Now in 2017 there is an AoA in the PFD of Cirrus Garmin as standard.
I predict in 5 years it will be required equipment for certification.

 

Hmm. The leading expert is Professor of Aeronautical Engineering at the US Naval Academy.

Let me understand what you are saying please. And first let me say I appreciate you chiming in with a very intriguing perspective.

I'm having trouble with the 3rd paragraph 2nd sentence. Can you dive into that? The wing stalls from the root out to the aileron. And if the CG is the mid point, is not the wing tip moving slower than the root? Rotational inertia. If so how does it reconcile with what you said? https://en.m.wikipedia.org/wiki/Moment_of_inertia

 

The leading expert bio who wrote the paper
http://www.nar-associates.com/technical-flying/biodfr_web.html

 

Wouldn't hurt to get both an AOA indicator and some acro training.

Both are cheap compared to a set of Bose headsets and an iPad as far as return on cash spent (my opinion).

I would hope that if I ever got a plane of my own and installed one that I would use it to "feel" the envelope as a training aid. Similar to how a stall horn is a tool, I would not want to bet my life on an AOA indicator and fly by it alone.

Last thought - I wonder what point in fatal stall-spin accidents that are too close to the ground to recover did the stall horn go off, and would the pilot have had adequate warning with an AOA indicator? We will never know. But someone with an AOA tool can do a spin test and see how quickly they can get themselves into trouble (or not) before the AOA warnings go off. Maybe its not how close you are to the stall, but how quickly it can alert before conditionz deteriorate into a quick stall.

 

Here is an FAA video addressing your question. And I agree with what you said.

 

The wingtip is moving 113 feet per minute slower than the root, compared to the forward airspeed of over 6000 feet per minute at 60 knots. Adding yaw (too much rudder) will increase that a bit, but it's still a small fraction of overall airspeed.

A 60-knot forward speed, turning at twice rate one (or 360 degrees per minute; rate one is 360 degrees in two minutes), has a radius of nearly 2000 feet. That 36-foot wingspan is a tiny fraction of that radius, and the difference in airspeed across it is similarly tiny. The descending turn's helix differential is what makes base-to-final turns sensitive, and introducing a skid increases that differential; adding inside aileron (cross-controlling) increases it far more. All the pilot has to do now is get too slow.

 

Dan, I have to admit I have never heard your explanation. I searched some textbooks and online and do not find a reference. I am very intrigued with your premise and if you have some reference material to direct me to, that would be great.

 

I thought it was common knowledge. Look at any explanation or graph of CLmax, for example, the current PHAK starting on page 5-3 under the subheading "Lift" and continuing on to page 5-4.

The wing stalls at (or infinitesimally beyond) CLmax. The angle of attack for CLmax and that for a stall are indistinguishable. To say the stall warning device is not activated until CLmax is silly.

 

LOL. "Common Knowledge" isn't always so common. I just wanted you to clearly define the argument.

 

Eman1200 posted this video recently in another thread and I thought it was a great help to me to explain the dynamics of this.
I'm a new student and I felt his explanation put it all into place.
I'm trying to remember, I think if I recall correctly in this instance the stall doesn't happen from the root of the wing out, but at the tip?

 

So, what are the recovery control inputs? Obviously, trying to lift the stalled wing will increase the AOA, making things worse. Prevention is preferred, but that doesn't work, what do you do to recover?
I once entered an inadvertent stall once. Fortunately, my instructor was old school and had taught me spin recovery.

 

I flew an an airplane with an AOA indicator day before yesterday. A C172 with an Alpha Systems indicator. It was the "donut and chevrons" style mounted above the glare shield a little to the left, about in line with my left shoulder. It was intuitive and easy to use. The sensor was on the right wing pretty close to the fuselage, I think in the first inspection panel out. I thought it was closer in than should have been, but it was at least out of directly behind the propellor. I did do a forward slip on one approach to get down when I did a short approach. A right aileron, left rudder slip, maybe about 10-15 degrees left of straight ahead. It was easy to stay on the "blue donut" without any significant change in elevator pressure. It still seems to me that being in uncoordinated flight should make for some error in what these things indicate. Next time I take that plane out I'm going to get up to altitude and do some full rudder deflection slips both into and away from the sensor and see what it looks like. I expect that things might not be as they seem when the fuselage gets in the way of the sensor when slipping with full right rudder.

 

How do you factor in wind to get max range over the ground?

And does your AOA system provide an aural warning of the impending stall during the base to final turn?

 

I'm pretty sure wing configuration changes things. Alpha Systems acknowledges this. It's fairly negligible for some airplanes, but for others their system does compensate for flap settings.

That the "practical approach" is to install it on the wing opposite the stall horn, and that it be "biased on the left side" implies that there is a difference in its indications. I would sure like to see the numbers on this. Something like when skidding to the left/right, stall will occur 1/2 of a "bar" earlier/later

 

Man, don't know how I made it as a student pilot, no AOA, no stall horns or lights, just common sense.

If you keep your nose down in the pattern, as you will flying a tight oval pattern, it's MUCH harder to do this boogie man base final thing all these folks try to sell.

 

Base to final turns are not a problem. Uncoordinated turns at slow speed are a problem. Don't do uncoordinated base-to-final turns... no problem.

 

While it is possible to stall in any attitude (including nose-low), after flying with some not-so-great and rusty pilots, I am led to believe that stall-spin accidents in the pattern are caused by a severe lack of awareness of what the airplane is doing as far as attitude and airspeed; in other words, they have psychological tunnel vision. An AOA indicator is not likely to help that; nor is it likely to help in a situation in which the AOA is suddenly increased.

 

Stepping on a rudder to reduce a turn radius is common when close to the ground with clear ground references. It's all about coordination. Any pilot should be able to configure for slow flight and fly the edge of a stall at 20-30* banks. Lower the nose or add some power. No problem. Push in some rudder? Bad idea when close to the ground.

 

I would argue, then, that they weren't very well trained.

 

This...I see a fair number of inadvertent stalls. The AOA indicator is just one of many things they'd not looking at while they fixate on the distraction of the moment.

 

While the AOA indicator is a tool that provides information, I don't think it will help with accidental stalls while turning. The people who do this would probably not look at the AOA indicator at that moment even if it was there.

 

Neutralize ailerons, lower nose, opposite rudder and full power. Simultaneously, if possible.
Maybe the way to ingrain this is for an instructor to do an intentional skidding turn/stall at altitude, to really show the effects of trying to save a base-to-final overshoot with a ton of rudder. That is a lesson that would stick! I'm a big fan (in a mild overshoot) of increasing the bank angle and diving for airspeed and not touching the rudder. (Maybe not the right solution for all aircraft/pilots, but RVs have next to no adverse yaw).

 

That's a nice theory. But at pattern altitude, you don't have the altitude necessary to recover if you get into a spin, even if you are expecting the spin to occur. Add in the surprise factor when someone inadvertently enters the spin, and there is no chance of recovery.