Stall spin margin

Did something today that got me wondering how close I was to the edge.

First, 74 hours in my -8 and haven't explored much of the edge in a controlled situation. I try to do a bit each time up but haven't explored this situation yet.

I fly downwind to final as a tight continuous turn. I have AOA and know where it's at in the pattern.

So today I'm on final, 80k, have 3 bars green on AOA, and overshot centerline. Without even thinking (!) I found myself pulling it around and uncoordinated.

I had airspeed, pitched down and plenty of reserve on AOA.

It felt comfortable. Seemed well within what the -8 could do without concern but once I realized what I did it got me wondering how close I was.

I'll go out next time and duplicate the scenario at altitude.

In the meantime should I increase my insurance coverage?

Several things:
* In gusty conditions, there are formulas to tell you how much to increase your airspeed for safety. There are no formulas telling you how much to change your AOA based on reported winds and gusts. Do NOT use your still air AOA in gusty conditions -- that's the same as flying with no additional airspeed for gust compensation;
* Do some approaches in which you deliberately overshoot centerline and gently, slowly come back to where you want to be. This will train your inner brain that an overshoot is not a catastrophe that requires immediate, drastic (and potentially dangerous) maneuvering to get back to the comfortable center of the runway. Do those practice overshoots at 500 feet or so a few times before you get lower;
* Just like you don't chase airspeed on final, don't chase AOA. There was a NASA Technical Note (TN D-6210, March, 1971) in which they found that flying AOA on approach didn't work very well in a GA airplane as primary guidance;
* In my RV-8, it's so maneuverable that it's tempting to land out of every approach, and that's a bad habit I've developed, reinforced by lots of good landings from bad approaches. I'm now working on good landings from good approaches;
* Find a really good stick and rudder instructor and go practice some of these situations with that instructor, even if it's not in the RV-8. Don't take my word for it, get somebody to watch you at work. After all, you don't learn to swim by reading a book. (Standard disclaimer applies -- don't try what I recommend without trying it first with a really good CFI).

You sound like you've got a good attitude towards safety! Be careful, practice, and don't push it.

Years ago, there was a poster that showed a DC-8 being lifted out of the water and being placed on a barge. The caption was, "No approach is so bad that it can't be salvaged."

Blain said:

...Without even thinking (!) I found myself pulling it around and uncoordinated...

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Not the end of the world with pulling into the buffet provided you maintain coordination. However, Im wondering why you moved to uncoordinated flight "without thinking"? If you are going to try cure anything, focus on the tendancy to go uncoordinated... Thats what kills.

Id reccomend going up high and practice accelerated stalls in the landing configuration. You will find that the -8 will turn remarkably tight and if it does unhook, the stall is benign - provided the ball stays centered! Let the ball out of the cage and all bets are off. Id also recommend doing sustained 1g stalls and practice keeping the wings level with rudder only. In other words, go nice and high, stall the airplane with the stick locked firmly back and dance ONLY on the rudder pedals to keep the wings level. Its great training for the correct automatic response then things go sideways. Once you have that down pat, try freezing the rudder and working the ailerons to keep it level - but be prepared for a nearly instant wing drop and departure. Its a great (and dramatic) way to demonstate how powerful the rudder is in a stall, and also how incorrect the ailerons are.

Blain,

Foremost--you did a great job debriefing yourself! And, we learn by making mistakes, either our own or observing other's. Eventually, most of us learn that not all patterns/approaches can be salvaged...which is why the first rule is, if in doubt go around.

We definitely want to avoid a "skidding" departure at low altitude, because the airplane will snap underneath with very little aerodynamic warning if you stall it: https://youtu.be/h5OFuQaYs1o. On the other hand, if you stall in an "inside slip" on base (a technique that many folks use to dissipate excess energy in the base turn), the post-stall charteristics are different, and the airplane provides significant aerodynamic warning of impending departure post-stall in the form of uncommanded roll (which is just a fancy way of saying you have to force it to depart): https://youtu.be/uMEVEZMUDXY

Ed is spot on regarding chasing either airspeed or AoA in the pattern--here's more examples of "patterns gone wrong:". https://youtu.be/BCQF8B49tgw. A "slightly fast" AoA is generally appropriate for gusty conditions, but it does take some flight test to figure out what constitutes "slightly fast" with your system. One technique to do this is to establish a normal ONSPEED/Vref condition, note the IAS, add the appropriate gust factor to the IAS, then note the AoA indication/sound to see or hear "slightly fast" in your plane.

The root cause of most of these errors is where you start your base turn. If you take a look in the transition training manual beginning on page 226, you'll find quite a bit of info that you might find helpful: https://drive.google.com/open?id=0B8EIT6g2n8o_NmJNRmRfcGR3dm8. Discussion on pp 231-232 specifically addresses how to plan the base turn point for the type of pattern you are flying.

The bottom line is that if you keep the ball in your cross-check, i.e., keep it coordinated, you'll have options to either fix your energy problems or safely go around.

Good post post/topic for discussion.

Fly safe,

Vac

"Uncoordinated" sounds like the ante-room that leads into "cross-controlled" - a stall from which there is no recovery on short final. You are right to be concerned about what happened, and good on you for recognizing it and chiding yourself to do something pro-active about it. We would all be well-served to have that attitude.

My close in short turn to final is at 65-70 KIAS.
Keep it coordinated and at 1 G, it is safe.

David-aviator said:

My close in short turn to final is at 65-70 KIAS.
Keep it coordinated and at 1 G, it is safe.

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Exactly. At 80 knots, you have about 2.5 available G's. Certainly over 2. You have to be pretty ham fisted to stall in the pattern with that kind of margin.

Since you have AOA it is giving you a realtime reference of how much margin you have above stall. In talking with several people who have added AOA one of the surprising things is that it caused them to fly a more conservative pattern as they discovered how close they were pushing things when trying to tighten up a turn. They had no idea using airspeed alone.
I would do some work at altitude scanning both AOA and airspeed simulating varies turn rates in the landing configuration. Each AOA is calibrated to that airframe so you need to explore the limits yourself.
Everyone should explore both 1G and accelerated stalls at a safe altitude. Watch your AOA as you do this and pick a personal reference AOA you are not going to exceed in the pattern. Set your aural warning just below that point.
G

First thing you need to do is complete phase I flight testing. You obviously haven't done that. Phase I flight testing consists of testing the aircraft throughout its normal range of speeds, throughout all weights and C/G ranges and all maneuvers to be executed.

The aircraft must NOT be signed off into phase II until you have explored the entire envelope.

Toobuilder said:

However, Im wondering why you moved to uncoordinated flight "without thinking"?

.

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Too complacent with the "feet on the floor" (not really on floor) because of the minimal adverse yaw.

Vac said:

Blain,



The bottom line is that if you keep the ball in your cross-check, i.e., keep it coordinated, you'll have options to either fix your energy problems or safely go around.

Good post post/topic for discussion.

Fly safe,

Vac

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You have great AOA knowledge-let me ask a couple questions; I feet like 3 green below the donut is safe and as I said that's what I had skidding through this turn-Is it a false sense of security? Can it degrade and depart so fast that the warning wouldn't have saved it?
And 2nd with the ports on the left wing should a greater margin be made on right patterns?

Mel said:

First thing you need to do is complete phase I flight testing. You obviously haven't done that. Phase I flight testing consists of testing the aircraft throughout its normal range of speeds, throughout all weights and C/G ranges and all maneuvers to be executed.

The aircraft must NOT be signed off into phase II until you have explored the entire envelope.

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Understood Mel. No passengers yet. Still exploring the envelope and getting comfortable with the ship.

Some great advice here. Good on you for the self critique. The overshoot has been discussed here as being one of the most deadly concerns with our RV's. Many round off the base to final leg and you can end up close to the runway without an established approach. Looks cool, but if you establish an approach, you can't overshoot, or you overshoot well in advance and can make simple coordinated adjustments to establish a stable approach.

I doubt you where close to a stall, however....

By the way, after a couple RV accidents attributed to this exact scenario, I went out and practiced with the purpose of making bad things happen. I set field elevation simulated at 3000', lined up a road for my runway, and did all kinds of slips, skids, overshoots, cross comtrolled, uncoordinated final turns, you name it. I was surprised how difficult it was to stall/spin. My particular airplane will made a huge shudder and a lot of noise before she breaks, but that wasn't at gross.

"Beating the system"

Blain,

Good questions...the short answer is I don't know of a situation where a handling technique such as you describe is appropriate in the traffic pattern (or close to the ground). There isn't any reason for uncoordinated flight unless you're are intentially slipping to dissipate energy or cross-controlling the airplane during the round out and flare to compensate for cross-wind. The over- and under-shoot shoot examples in the video link I posted are all flown in a coordinated manner. If conditions warrant, it's safer to simply execute a wing's level go-around and fix heading using coordinated turns after the airplane is safely climbing away from the ground.

As for "three bars" being sufficient energy margin, that depends on how your AOA system is calibrated and performs, which requires flight test. It might be plenty of margin, or it may not. The only way to know is to validate AoA performance against known parameters (generally airspeed derived from conventional flight test) as well as evaluating handling characteristics at different AoA indications (including slip testing as you mention). Every RV is a bit different. FWIW, my RV-4 is almost 20 years old (I've owned it since 2009), I've flown several hundred hours of test and maneuvers validation in it, and I'm STILL testing it and learning--such is the nature of experimental aviation.

The other thing to note is that there hasn't been a flight control or warning system built that a pilot can't beat (just ask the folks that work the flight control logic in fighters!). In the case of an RV, two things can happen: first, it's possible to move the mechanical flight controls so fast that the airplane doesn't have time to respond to the input, and second, you can generate sufficient turn rate/G to "beat" any CAWS (caution and warning system). For example, my AoA system is only capable of keeping up with G onset rates of 2 or less G's per second. So if I stall at high G using and abrupt control input (> 2 G/second) I'd likely stall before I heard a stall warning: https://youtu.be/3avXLYp68HY. Note that in this example, you heard the slow tone, but not stall warning before the nose stopped tracking (which is what an accelerated stall looks like in an RV). In the next example, I "beat the system" for the first stall, but it caught up for the secondary stall: https://youtu.be/DLtamTAh-Is. This is one reason why there is a consistent strain of SMOOTH flight control input throughout all of Van's writing on the subject.

Fly safe,

Vac

Blain said:

Too complacent with the "feet on the floor" (not really on floor) because of the minimal adverse yaw...

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Good on you for identifying the problem.

I took an experienced, but rusty 172 pilot for a fam flight in the -8 last weekend and demo'd stalls and simulated base-to-final stalls. In the extended stall sequence (stall held for 2000+ feet), I gave ample demonstration on how the rudder was the primary wing leveler control and also how aileron use exacerbated wing drop. Despite his nodding in apparent understanding, he could not get his feet to do anything, and could not break the habit of attempting to pick up a wing with the stick.

Everyone latches on to certain things differently as well as trains differently, so I can't offer you any specific advice other than to figure out how to train yourself to ALWAYS work the rudder correctly when low and slow. Improper rudder use is the real killer in these "base/final, stall/spin" scenarios, so do what you have to get those feet trained.

Rudder control

Michael got me thinking about my training.

I came from a Robinson R22. Working the feet is pure yaw and not a means to level the rotor. So there is something to overcome. With the doors off (my favorite way to fly the -22) the VOX mic became an aural yaw warning. If not tracking straight the wind would break squelch. (if you miss the yarn as described below)

One advantage that it did give me is smooth, light and not abrupt use of controls.

Frank Robinson is a master at reducing weight. Instead of inclinometer or gyro he used a simple piece of yarn at the center of the bubble. Sort of a crude "Heads up" display. I think I'll procure a piece of yard from the wife's craft supplies

Back in April I spent 5 hours of training with Patty Wagstaff and her instructor Allan down in Florida. 4 hours was in the Super D, the 5th in the extra 300. One of the last things Allan showed me was this very "Trap". Leading up to it, we obviously spent some time doing stalls, spins, loops, rolls, etc. He got on the radio and told me he was going to show me what is killing the most pilots and he would do all the flying and to just watch.

Sure enough at altitude he setup for base to final, got slow, then put in some wrong rudder and the plane stalled and flipped on its back quicker than I could say Vans. At this point he got on the radio and declared "YOUR AIRPLANE". As a low time pilot with lots of time looking up while building it was eye opening and something I will always remember. Not to discount my initial training prior to my license, but for sure life changing and I suspect will be very valuable now in keeping me safer.

I suggest everyone SAFELY do this with instructor, etc. and get yourself in a bad situation at altitude and learn to get out quickly and safely.

RV8JD said:

Won't do much good on our RV's with the tractor engine arrangement and the prop's rotating slipstream.

("Step on the ball, step away from the string!")

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I guess I didn't think that one through......

ty1295 said:

Back in April I spent 5 hours of training with Patty Wagstaff and her instructor Allan down in Florida. 4 hours was in the Super D, the 5th in the extra 300. One of the last things Allan showed me was this very "Trap". Leading up to it, we obviously spent some time doing stalls, spins, loops, rolls, etc. He got on the radio and told me he was going to show me what is killing the most pilots and he would do all the flying and to just watch.

Sure enough at altitude he setup for base to final, got slow, then put in some wrong rudder and the plane stalled and flipped on its back quicker than I could say Vans. At this point he got on the radio and declared "YOUR AIRPLANE". As a low time pilot with lots of time looking up while building it was eye opening and something I will always remember. Not to discount my initial training prior to my license, but for sure life changing and I suspect will be very valuable now in keeping me safer.

I suggest everyone SAFELY do this with instructor, etc. and get yourself in a bad situation at altitude and learn to get out quickly and safely.

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I think the instructor demonstration in this video makes your point, it was discussed here about a year ago: https://m.youtube.com/watch?v=8wPCaazpU5k