Stall Speeds Won't Save You - AoA Will

[Ignition]

Ok, So this is going to be a bit of a rant and so I felt it was better off in its own specific thread.

 

http://www.csgnetwork.com/aircraftturninfocalc.html says that at 60 knots, with a 45 knot stall speed, and a 35 degree angle of bank, you can do a rate 4 turn (180 degrees in 15 seconds), increasing the stall speed to 50 knots.

"This calculator DOES NOT take into consideration wind speed or direction, temperature, air density or aircraft performance and assumes LEVEL flight and turns." - ANY of those, along with weight, flap configuration, dirt/ice on the wings will change the so called 'Stall Speed'. The 'Stall Speed' is not a static number and staying a little faster than this won't save your ass when you are low, pulling or pushing the stick, uncoordinated and eventually spinning into the ground.

 

I was asking about the aerodynamics of the stall in this situation, not the numbers on the instruments. I've not flown anything heavier than a Warrior.

I haven't followed the thread closely, however I will jump in here: If you are worried about the 'stall speed', there is a damn good chance you will become another stall/spin statistic.

There is a reason why stall/spins are the number one killer in General Aviation with a significant lead. General Aviation really needs to get this 'stall speed' business out of stalls altogether! An Aircraft stalls because of one thing and one thing alone, exceeding the critical angle of attack.

 

A few points:

 

• You can do this at ANY airspeed, with the nose of the aircraft pointed up, down, or inside out depending on how low you are.
   
   
  
• You can do this with the stick forward (inverted) or back (upright).
   
   
  
• You can do an inverted stall with the ground and sky in the correct position.
   
   
  
• You can do a 'normal' upright stall with the ground above you.
   
   
  
• You can stall in a dive.
   
   
  
• You can stall going 120 kts in an aircraft with the 'stall speed' of 45 kts.
   
   
   
  
• You can fly an aircraft at an indicated airspeed below the 'stall speed'.
   
   
  
• The posted 'stall speed' is at a specific data point - ie. MTOW, Full Flaps, Straight and Level, Nil Wind, Specific Air Density, Specific Temperature, Specific Altitude, Clean Aircraft in the exact configuration it left the blueprints. Change any one of these and you change the 'stall speed'.
   
   
  
• The Critical Angle of Attack is a fixed angle, for most aircraft, it will be around 15 degrees AoA.
   
   
  
• The smallest power off descent rate occurs at a fixed angle of attack.
   
   
  
• The best glide ratio occurs at a fixed angle of attack.
   
   
  
• The best rate of climb occurs at a fixed angle of attack.
   
   
  
• The best angle of climb occurs at a fixed angle of attack.
   
   
  
• The best approach speed occurs at a fixed angle of attack.
   
   
  
• The best L/D occurs at a fixed angle of attack.
   
   
  
• The best range occurs at a fixed angle of attack.
   
   
   
  

 

 

Pilots seem reluctant to get rid of the 'stall speed' idea, I always hear things like: "How can we find out the critical AoA? that's too difficult, a fixed angle! How could I ever avoid that?! I feel much better chucking a few knots on and approaching at 1.3Vso rather than 8 degrees AoA or whatever your specific airfoil will always be optimum at.", the marketing for aircraft "This excellent aircraft cruises at Mach 4 and stalls at 7 kts but not if you change any of the previously mentioned variables and this in reality tells you nothing but makes the aircraft look like an awesome buy so we can make some more cash!" or of course RAAus being limited to "45kts Stall Speed" because it is that heavily ingrained into GA Culture, and the one I love the most "it's not that simple".

 

The only way to reduce statistics and stop a very preventable accident from happening is to teach people correctly and quite evidently what is currently being taught is not working, if it were the stats wouldn't be so horrible; you do not stall an aircraft because of the 'stall speed'. You stall an aircraft because you exceeded the critical Angle of Attack and as a side note an AoA Indicator will show this before any other indicator in the aircraft. It really is that simple.

 

Honestly an AoA indicator with an audible alert is a cheap investment (can get them under $1000 now), if you are able to put one in your aircraft of course. The day it is held with the same regard as Airspeed Indicators as a minimum instrument is the day General Aviation becomes significantly safer.

 

The alternative is to keep beating the same old drum over and over again and continue to hear the cries about "yet another stall/spin accident" and "what can we possibly do to prevent this from happening time and time again". It's a culture change that is needed as much as an understanding change.

 

/end rant

 

 

[jakej]

If you have a Dynon EFIS you can buy a ( unseated) pitot for $us 200 to get AOA info. The heated version (not needed for VFR) is $us450.

 

 

[facthunter]

Stall warning devices sense angle of attack. There's nothing in ignition's post that every pilot shouldn't know and understand.

 

An angle of attack meter is located inside the cockpit and shouldn't detract you from looking outside so shouldn't be hard to interpret quickly. A string of wool in the freestream will indicate AoA and slip angle in all attitudes, if you could view it from behind . Not easy to achieve but I'm talking principles. Nev

 

 

[Ada Elle]

• The best rate of climb occurs at a fixed angle of attack.
   
   
  
• The best angle of climb occurs at a fixed angle of attack.
   
   
  
• The best approach speed occurs at a fixed angle of attack.
   
   
  

 

 

 

Thanks, I'd read a lot of this elsewhere but it's hard to find instructors, for example, who teach that the elevators control AoA rather than pitch.

 

Does BRoC and BAoC really happen at a fixed angle of attack, in a variable angle prop? I thought that they were dependent on the properties of the prop combined with the properties of the wing/aircraft:

 

- BAoC is thrust available - thrust needed

 

- BRoC is power available - power needed

 

but since power is a speed related variable, as you change weight/CoG/etc the power needed changes and you might change AoA for BRoC.

 

 

[facthunter]

There are several contexts for making statements about what elevators do. Flying by attitude is a common concept and whether done by reference to the natural horizon or an artificially provided one, the principles are the same. In that context elevators control pitch attitude primarily, but if they fail, movement of the CofG, or changed thrust setting will do the job by thrust offset effect if it's there, or speed increase/decrease which will raise/lower the nose, but it's not a practical way of doing it. Only useful in an emergency.

 

You can't make absolute statements in matters flying, without qualification, but revisit the angle of attack question. Certainly no other control has any real effect on angle of attack compared with the elevator . It's the primary way of influencing it. You can stall the wing in both directions and go from max lift to zero in no time at all with elevator. You can unstall a wing when you are well under published stall speed just by pushing the stick forward (fairly positively). That's changing AoA for you...Nev

 

 

[Ignition]

Thanks, I'd read a lot of this elsewhere but it's hard to find instructors, for example, who teach that the elevators control AoA rather than pitch.

Does BRoC and BAoC really happen at a fixed angle of attack, in a variable angle prop? I thought that they were dependent on the properties of the prop combined with the properties of the wing/aircraft:

 

- BAoC is thrust available - thrust needed

 

- BRoC is power available - power needed

 

but since power is a speed related variable, as you change weight/CoG/etc the power needed changes and you might change AoA for BRoC.

A wing is acting independently of the prop, you can not go beyond the optimum performance angle of a particular airfoil. It is not physically possible, so yes, it is a fixed Angle of Attack for the specific airfoil. If it were a maths equation it would probably look like: (optimum/maximum AoA for whatever you are asking it to do) + (maximum prop/engine performance) = (maximum output)

 

 

 

Edit: Of course if you want a specific output, and you have excess power (vs weight) at L/D Max (Optimum AoA), you can maintain the power and reduce AoA (no longer using maximum AoA for what you are asking it to do) [3+7=10 just as 5+5=10) If that makes sense.

 

 

[Ada Elle]

A wing is acting independently of the prop, you can not go beyond the optimum performance angle of a particular airfoil. It is not physically possible, so yes, it is a fixed Angle of Attack for the specific airfoil. If it were a maths equation it would probably look like: (optimum/maximum AoA for whatever you are asking it to do) + (maximum prop/engine performance) = (maximum output)

This still confuses me.

 

A propeller produces maximal thrust at zero speed (static thrust) and has its thrust reduce as the aircraft speeds up.

 

An aircraft that is climbing at an angle S has:

 

- thrust aligned with oncoming air

 

- a drag component of weight (W sin S)

 

- lift orthogonal to the oncoming air

 

- lift equal to W cos theta

 

- aerodynamic drag (induced + parasite)

 

where thrust = aerodynamic drag + W sin S

 

Best angle of climb occurs at maximal S, which is at an angle and speed where thrust minus drag (thrust available) is maximised.

 

See, for example, http://code7700.com/images/climb_performance_thrust_atc_51-3_figure_2.21.jpg (taken from http://code7700.com/v-x.html).

 

Changing the aircraft weight will shift the drag curve to the left (because of decreased induced drag).

 

I can't see that the maximised thrust available occurs at a single AoA.

 

 

[Guernsey]

Critical angle of attack to stall the wing can occur at any speed.

 

Reduce angle of attack (stick forward) unstalls the wing.

 

Quite simple really.

 

Always worked for me.

 

Alan.

 

 

[Ignition]

I can't see that the maximised thrust available occurs at a single AoA.

Forget the engine and prop and take a look at a glider, the wings do not produce thrust (see below regarding gliders), the thrust is an additional component of the powered airframe. This is especially important when we are talking stalling, because once the engine fails, you are effectively in a glider and the wing will be the only thing you are relying on for the best performance.

 

http://www.grc.nasa.gov/WWW/k-12/VirtualAero/BottleRocket/airplane/glider.html

 

Here is another link from the website you referenced also regarding L/D Max: http://code7700.com/l_over_d_max.html

 

 

[Roundsounds]

This calculator DOES NOT take into consideration wind speed or direction, temperature, air density or aircraft performance and assumes LEVEL flight and turns." - ANY of those, along with weight, flap configuration, dirt/ice on the wings will change the so called 'Stall Speed'.

I'm pretty sure some of the factors quoted here do not alter the indicated stall speed. ie: wind velocity, air temperature, air density and I'm not sure what is meant by aircraft performance?

The critical one missed is load factor, it tends to be the killer of pilots who blindly rely on IAS as a stall predictor.

 

I believe ALL instructors should hold an aerobatic flight activity endorsement. There's no other training that will allow you to fully appreciate how an aeroplane can stall at any IAS and attitude or how to recover from the stall, or more importantly the impending stall. The number of instructors I've flown with who are tentative (or simply scared) of stalling is frightening.

 

I believe the stick position is by far the best stall warning. Every pilot should stall a new type when converting, including clean, landing configuration, power on, power off, level flight, turning and various combinations of these configurations.

 

 

[Roundsounds]

Critical angle of attack to stall the wing can occur at any speed.Reduce angle of attack (stick forward) unstalls the wing.

Quite simple really.

 

Always worked for me.

 

Alan.

Try this recovery method in a stall turn in a Pitts, you'll find yourself in an inverted spin 9 out of 10 times. You need to install the wings, which may require back stick.

 

 

[Ada Elle]

Forget the engine and prop and take a look at a glider, the wings do not produce thrust (see below regarding gliders), the thrust is an additional component of the powered airframe. This is especially important when we are talking stalling, because once the engine fails, you are effectively in a glider and the wing will be the only thing you are relying on for the best performance.

A glider has no best rate of climb or best angle of climb - these are only factors for powered aircraft.

 

I'm agreeing with you on the whole - we fly a wing, and a wing has an AoA. I just don't see that BRoC / BAoC occur at fixed AoA.

 

 

[Guest ozzie]

A publication i never hear recommended these days is The Naval Aviators Handbook renamed now I think as Aerodynamics for Naval Aviators.

 

 

[Roundsounds]

Kermode's Flight Without Formulae is also a great reference.

 

 

[Ignition]

I'm pretty sure some of the factors quoted here do not alter the indicated stall speed. ie: wind velocity, air temperature, air density and I'm not sure what is meant by aircraft performance?

Indicated maybe not, but true airspeed will change. That part was simply a quote from the website that was linked with the calculator.

Aircraft performance, I assume they are referring to engine/power settings/no engine and how the aircraft is being operated (diving, climbing, etc).

 

Try this recovery method in a stall turn in a Pitts, you'll find yourself in an inverted spin 9 out of 10 times. You need to install the wings, which may require back stick.

Exactly, shove the stick too far forward and you'll be in an inverted stall requiring back pressure. The safest bet is to centralise the control column.

 

A glider has no best rate of climb or best angle of climb - these are only factors for powered aircraft.

If a glider doesn't have a best rate or angle, how do they obtain maximum performance out of the wing? Think about it for a bit...

 

 

[Guest ozzie]

Kermode's Flight Without Formulae is also a great reference.

My first aviation book back in 76

 

 

[Ada Elle]

If a glider doesn't have a best rate or angle, how do they obtain maximum performance out of the wing? Think about it for a bit...

What do you mean by maximum performance?

 

Best glide is at maximal L/D AoA.

 

Best rate and best angle are thrust based parameters. They don't mean anything to a glider that doesn't have thrust. They are wing+prop, not just wing.

 

 

[SDQDI]

A glider has no best rate of climb or best angle of climb - these are only factors for powered aircraft.I'm agreeing with you on the whole - we fly a wing, and a wing has an AoA. I just don't see that BRoC / BAoC occur at fixed AoA.

A glider is still a powered aircraft Ada. Gravity is the motor.

So in an effort to further complicate things as simply as I can.

 

A glider (or any plane really) will have different gliding speeds, two of which are important (for this discussion).

 

1. Penetrating glide (glide that will get you the furthest although you may reach the ground quicker)

 

2. Loiter glide (glide which will keep you aloft for the longest period (lowest sink rate) but may not get you as far across the ground.

 

Add an engine and full power and the broc or baoc should still be similar (within reason) in their AOA

 

Lots of replies while I wrote this, hope this complicates things a little more..........

 

 

[Guest ozzie]

Wing loading?

 

 

[Ignition]

What do you mean by maximum performance?

Maximum Lift, Minimum Drag.

 

 

[Ada Elle]

Maximum Lift, Minimum Drag.

You fly at the maximal L/D AoA.

 

How do I fly best rate of climb in a glider without an iron thermal?

 

 

[Ignition]

You fly at the maximal L/D AoA.How do I fly best rate of climb in a glider without an iron thermal?

Your rate of climb can be in the negatives (aka descending), it is not limited to a positive rate of climb only, and any attempts to raise or lower the AoA from the L/D Max will result in an even lower climb rate.

 

 

[Ada Elle]

Your rate of climb can be in the negatives (aka descending), it is not limited to a positive rate of climb only, and any attempts to raise or lower the AoA from the L/D Max will result in an even lower climb rate.

Gravity doesn't have a velocity dependent thrust. This is similar to jet planes, and jets can fly BAoC/BRoC at the maximal L/D.

 

In prop planes with airspeed dependent thrust, how does AoA for BRoC/BAoC stay constant, given that thrust varies depending on airspeed?

 

 

[Yenn]

I realise from all this high falutin theory that I know absolutely nothing about flying. I wonder how I have survived for 47 years as a pilot without knowing my angle of attack. I do know that it appears awesome when I pull the stick hard back, with full throttle and go up. The wing appears to be at about 45 deg to the horizon and I still keep going up until the CHT gets too high.

 

 

[Ignition]

Gravity doesn't have a velocity dependent thrust. This is similar to jet planes, and jets can fly BAoC/BRoC at the maximal L/D.In prop planes with airspeed dependent thrust, how does AoA for BRoC/BAoC stay constant, given that thrust varies depending on airspeed?

This is just gonna go around in circles because neither of us are talking about the same thing.

When the engine fails, your best rate of climb is going to be L/D Max, because you won't be climbing and you will be wanting to obtain max lift/min drag. This will also be your max glide. This is at a fixed AoA. Just because you have an engine on the front does not mean that the airfoil will operate any differently, it is still operating under the same laws of physics as a fixed wing aircraft, the exact same as a glider.

 

This thread is specifically about stalling however so I would much rather keep it specifically on that as your other thread is already discussing Vx/Vy.