Angle of attack

[Ed Cesnalis]

FAA Safety Team | Safer Skies Through Education

What’s your angle?

Notice Number: NOTC4886

Want to help reduce GA accidents? Know your angle. Angle of attack, that is…

 

The majority of GA accidents occur when a pilot loses control of the aircraft. This usually happens in airplanes when a wing stalls and the plane enters a spin. The wing stalls when the critical angle of attack (AOA) is exceeded.

 

Remember, the wing (and aircraft) can stall at:

· Any airspeed

· Any attitude

· Any power setting

But, the wing always stalls at the same AOA.

 

The stall speed published in the airplane flight manual is only valid for:

· Unaccelerated flight (1g load factor)

· Coordinated flight (ball centered)

· At (usually) max gross weight

So, a pilot may be surprised to find the wing has stalled above the published stall speed.

 

AOA can be very useful in enhancing safety. Having an AOA meter is the best way to remain aware of your AOA.

 

 

 

If you don’t have an AOA meter, here’s a quick tip on how to manage your critical AOA:

• See it – in the pitch attitude and airspeed indication.
  
• Hear it - in the existing stall warning systems.
  
• Feel it – in the seat of your pants when the wing begins to buffet.
  
• Recover it – by reducing the pitch of the aircraft.
  

Regardless of the aircraft’s attitude, reducing the pitch reduces the angle of attack and recovers from the stall.

 

 

Want to learn more?

• Get out there and practice!
  
• Online courses at FAASafety.gov
  
  • ALC-214: Avoiding Loss of Control
    
  • ALC-63: Performance and Limitations
    
  • ALC-36: Positive Aircraft Control
    

  [*]FAA-H-8083-3A, Airplane Flying Handbook, chapter 4, page 4-3, figure 4-2

  • http://www.faa.gov/regulations_policies/handbooks_manuals/aircraft/airplane_handbook/
    

  [*]FAA-H-8083-15B, Instrument Flying Handbook, chapter 4, page 4-6, figure 4-8

  • http://www.faa.gov/regulations_policies/handbooks_manuals/aviation/media/FAA-H-8083-15B.pdf
    

  [*]FAA-H-8083-25A, Pilot’s Handbook of Aeronautical Knowledge, chapter 4, page 4-22

  • http://www.faa.gov/regulations_policies/handbooks_manuals/aviation/pilot_handbook/
    

  [*]NAVAIR 00-80T-80, Aerodynamics For Naval Aviators, Chapter 1, Figure 1.11

  • http://www.faa.gov/regulations_policies/handbooks_manuals/aviation/media/00-80T-80.pdf
    

  [*]Contact: Kevin Clover: kevin.l.clover@faa.gov

[FastEddieB]

Thanks for that.

 

Maybe a bit nitpicky, but...

 

"But, the wing always stalls at the same AOA."

 

In theory.

 

But, of course, change the wing profile via flaps or slats or the like and the stall angle will change.

 

Surface contamination (insects, dirt, rain or certainly ice) will also change the stall angle.

 

But nice review, nonetheless.

[Ed Cesnalis]

...

 

But, of course, change the wing profile via flaps or slats or the like and the stall angle will change...

 

The 'stall angle' might change but the stall AOA remains the same I believe. In aerodynamics, angle of attack specifies the angle between the chord line of the wing of a fixed-wing aircraft and the vector representing the relative motion between the aircraft and the atmosphere.

 

When we deploy flaps we change the MAC or chord line so we stall at a different attitude but the same AOA.

[FastEddieB]

Ed,

 

I don't think that's right, but headed out the door shortly to head to Racine.

 

I'll research it later unless someone else has the time to.

[Ed Cesnalis]

 

Here is a little refresh on Flap effects wrt AoA and lift.

 

http://www.nordian.n...flight_demo.pdf

 

"Flaps produce the required increase in lift at a lower geometrical angle of attack of the main aerofoil than with a clean wing.

However, the effective chord line and the effective camber changes with flaps selection. Flap deflection results in increased lift and drag at a given angle of attack and increases the maximum CL. The flaps of the trailing edge increases CL max and produce the same amount of lift at a lower geometrical angle of attack than with a clean wing. The lower stall AoA is here measured to the chord line of the main aerofoil. The stall AoA measured from a chord line from the leading edge of the main aerofoil to the trailing edge of the flaps can be higher."

[FlyingMonkey]

AoA can change without configuration changes or pilot input as well, due to gusts, turbulence, or wind changes.

[Andy]

I saw that note from the FAA and went back and looked at it, and flaps actually do slightly decrease the critical angle of attack; you can see it by plotting the points where the lift curve breaks on a clean wing vs flaps/CL vs alpha plot. That's evident in the pdf that was posted as well. However, the shift in lift coefficient at any particular angle of attack more than makes up for the critical angle decrease, which is why you can fly at much slower speeds using flaps than not.

 

I believe surface contamination affects the speed at which the boundary layer separates which affects the stall speed but I don't think it affects the critical angle of attack at all.

 

Frankly, I didn't like the references in that note to AOA control through pitch. Pitch doesn't really have anything to do with it; it's all a function of the relative wind and the critical angle. I understand that in our airplanes equating pitch to AOA control generally works, but I think it's the wrong way to think of it. (I'd tie it more to the velocity vector...)

[Doug G.]

Is it the AoA that changes, or is it the wing cord that changes which creates a different AoA?

[Andy]

Are you asking about talking the airplane's AOA at flap deploy or the critical? The flap deploy changes the chord line so the "effective AOA" the airplane is experiencing changes (as noted int the pdf). But the actual critical AOA does decrease as well, though in the plots I looked at the effect was small.

[Doug G.]

That answered the question Andy. Thanks.

[Jim Meade]

Do you Dynon owners have the AOA displayed on the panel? Do you have the sound hooked up?

[Doug G.]

Yup, the indicator where it is doesn't help much. The sound can.

[FlyingMonkey]

My plane has an el cheapo pitot tube, so no AoA for me. It's on the list to have added at some point.

[Jim Meade]

Surface contamination (insects, dirt, rain or certainly ice) will also change the stall angle.[/size][/font][/color]

 

There is a lot of difference between 2 inches of rime ice and a couple of bugs. I think most of us will not find any difference in AOA and stall characteristics until we get into the realm where the wing shape is changed enough that the turbulence boundary is moved. I've never experienced it myself with rain or snow but did have a couple of encounters with ice where there was a buildup.

[Jim Meade]

Describing angle of attack as being the difference between the relative wind and the chord line is like saying that airplanes fly because of Bernoulli's principle. It is comforting but wrong.

Air movement over the wing is toward the root on top and toward the tip on the bottom. This air movement creates and induced angle of attack which is subtracted from the free air angle of attack. The resultant effective angle of attack is less than the idealistic AOA. The stronger the wing vortices the more induced angle of attack and the less effective angle of attack. Since ground proximity disrupts the vortex air flow, that air flow does not create so as great an induced AOA which means the effective angle of attack is greater - ergo, you have all this nice lift in ground effect.

 

Here's one explanation. If I get time I'll try to find a NASA source.

 

http://www.theairlinepilots.com/forum/viewtopic.php?t=366&sid=87885157d8202dc12aa8af98b8217809

 

NASA has all these little applets to demonstrate this stuff. I don't have time to get into them right now, but here is a referenced to induced angle of attack.

 

https://www.grc.nasa.gov/WWW/k-12/airplane/inclind.html

[Doug G.]

I think I will still use indicated, and sensed AoA.

[S3flyer]

Is the Dynon AOA for the D100 worth it?

[Anticept]

My plane has an el cheapo pitot tube, so no AoA for me. It's on the list to have added at some point.

 

Wait, how do you get a stall warning?

 

As for the AoA indicator: If you do a lot of edge-of-the-envelope flying, then yes it's worth it. It's a good visual representation of how close you are flying to the edge.

[FlyingMonkey]

 

 

Wait, how do you get a stall warning?

 

 

Same way a Cub driver does...from my butt-o-meter.

[Anticept]

Same way a Cub driver does...from my butt-o-meter.

 

Huh. I would have thought EASA and the ATSM standards would have required a stall indicator. I know the FAA requires it for certifying aircraft.

 

Or did you break it and install an el-cheapo?

[FlyingMonkey]

 

 

Huh. I would have thought EASA and the ATSM standards would have required a stall indicator. I know the FAA requires it for certifying aircraft.

 

Or did you break it and install an el-cheapo?

 

Nope, not required on LSA. I have it the way it came from the factory.

[coppercity]

Same way a Cub driver does...from my butt-o-meter.

 

The best stall warning on a Cub is when you leave the lower door down, it always starts raising up when your about to stall

[Roger Lee]

None required for LSA.

[Doug G.]

Andy, it is a kind of sinking feeling, isn't it?

[FlyingMonkey]

Andy, it is a kind of sinking feeling, isn't it?

 

It is!

 

Honestly, I don't feel like I'm missing much by not having a stall indication in the cockpit. The impending stall in a CT is so obvious, with buffet and the control movements getting so large and the feel getting so soft and "wallowy", you'd have to be asleep to not see it coming. About the only way I'd imagine being surprised by the stall would be in a sharp turn where an accelerated stall happens quickly. I don't throw the airplane around like that very much, so avoidance is working for me so far.