AOA indicator

On Thursday, April 14, 2016 at 6:05:36 AM UTC-5, Bruce Hoult wrote:
On Wednesday, April 13, 2016 at 9:54:17 PM UTC+3, kirk.stant wrote:
Well, you're right that it's basic aero, but you've somehow managed to get it backwards.

Both wings are descending through the air at the same speed (or you'd be rolling). The inner wing is travelling less distance therefore the angle of attack is greater.

Unless you are spinning, I doubt the difference in distance has a greater effect than the effect of yaw on the effective AOA of each wing. Once you are in a spin, then the inside wing is definitely at a higher AOA and stalled (which is pretty much the definition of a spin).

Looking at it another way, the inner wing is travelling slower but has to generate the same amount of lift, therefore its angle of attack must be greater.

Only if you want a constant bank. Which is why you either have to use top aileron against the overbanking tendency, which increases the camber of the bottom wing, or top rudder to increase the AOA via yaw. If you bank and do neither, the top wing will make more lift and you end up in a spiral.

Stable slips or skids and dihedral don't make any difference to this. Of course if you kick in some yaw then at the moment the speeds and distances traveled will be different. Putting in some slip/top rudder will accelerate the bottom wing and (briefly) take it to a lower AoA -- and possibly even out of stall if it was a little bit stalled.

Any wing with dihedral (or sweepback) will roll if held in a steady yawed condition with positive AOA held. Not very effective in the typical glider, but in something like an F-4 or F-15, you can do nice 4-point rolls using only back stick and rudder.

But I'm only an amateur aerodynamicist, any experts out there to explain how I'm wrong? Always willing to learn... ;^)

On Thursday, April 14, 2016 at 12:34:42 AM UTC-5, Surge wrote:
On Thursday, 14 April 2016 05:08:12 UTC+2, Jim Lewis wrote:
My understanding is the glider will spin in the direction of the applied rudder. If that is the case then the glider will spin to the inside of the skidding turn. But, if the AoA of the inside wing is lower than the AoA of the outside wing why would the inside wing stall first and drop so the glider spins in the direction of the inside wing? I have no idea.


Kirk 66 has it wrong.
The angle of attack on the inside wing (low wing) in a banked turn is higher than the outside (high) wing. Both wings descend at the same rate but the outside wing travels further (and faster) than the inside wing.
The AoA of the inside wing is higher which is why it stalls first. Trying to pick up the inside wing with aileron only aggravates the situation because you're increasing the angle of attack of the wing tip (chord line changes) and the additional drag also holds the inside wing even further back which in turn adds pro skid yaw.

A skidding turn makes the situation even worse because now you're holding the slower inside wing back and the fuselage starts to disrupt some of the air flow to the inboard portion of the inside wing which now has to work even harder to provide the same amount of lift. In order to increase lift at a given speed you need to increase the angle of attack and this pushes the inside wing even closer to a stalled state. It all has a compounding effect when added together.

I think too much emphasis is placed on gadgetry and ideas to stop people spinning instead of training pilots to fly safer speeds.
If safe speed control is taught and adhered to then wings won't stall, and if there are no stalls there will be no spins either.
Personally I always fly at about 10 knots faster than minimum recommended approach speed (taking wind into account) in the circuit and only slow down on finals if I need to. I also thermal about 10 knots faster when I'm below 2000 feet AGL. Yes, it's less efficient but it's also safer. What I've found though is that my higher speed habits lower down become my habits higher up too. I tend to fly faster than required everywhere because flying slower feels unnatural unless it's during round out on landing.
I can live with flying less efficiently and my family won't need to hear that they just lost their husband and father due to a silly and avoidable accident.

I think we may be discussing different issues. If you skid a turn, the AOA of the lower wing will initially be lower (due to dihedral), so to keep from rolling, you will have to use a lot of top aileron to increase the lift of the lower wing. Aerodynamically, you have a flapped wing on the inside and a negative flap wing on the top. The camber of the flapped wing is greater, so the effective AOA is greater, and when you finally stall, the inside wing will drop, and with pro-spin rudder, you will probably depart into a spin INTO the turn. My LS6 will only depart (at my weight and CG) in landing flap from a grossly skidded turn, and then it will do a nice spin entry (for a quarter turn, then it transitions to a spiral dive). At any other flap setting, it just drops the nose a little.

Totally agree about maintaining extra speed when low and maneuvering - and strongly feel that pilots need to PRACTICE low speed departure and recoveries so that they know instinctively what is going on.

cheers,

Kirk
66

On Thursday, April 14, 2016 at 10:58:57 AM UTC-4, kirk.stant wrote:
The best way to convey AOA is with aural tones (from personal experience in F-4s that had such a system). Properly implemented, you can control your approach speed precisely without ever taking your eyes off the runway to look at the airspeed or AOA indicator, regardless of bank angle or gross weight.

This sounds worthwhile, and I'd think it would be relatively easy to implement in software.

On Thursday, April 14, 2016 at 8:36:34 AM UTC-7, kirk.stant wrote:
On Thursday, April 14, 2016 at 12:34:42 AM UTC-5, Surge wrote:
On Thursday, 14 April 2016 05:08:12 UTC+2, Jim Lewis wrote:
My understanding is the glider will spin in the direction of the applied rudder. If that is the case then the glider will spin to the inside of the skidding turn. But, if the AoA of the inside wing is lower than the AoA of the outside wing why would the inside wing stall first and drop so the glider spins in the direction of the inside wing? I have no idea.


Kirk 66 has it wrong.
The angle of attack on the inside wing (low wing) in a banked turn is higher than the outside (high) wing. Both wings descend at the same rate but the outside wing travels further (and faster) than the inside wing.
The AoA of the inside wing is higher which is why it stalls first. Trying to pick up the inside wing with aileron only aggravates the situation because you're increasing the angle of attack of the wing tip (chord line changes) and the additional drag also holds the inside wing even further back which in turn adds pro skid yaw.

A skidding turn makes the situation even worse because now you're holding the slower inside wing back and the fuselage starts to disrupt some of the air flow to the inboard portion of the inside wing which now has to work even harder to provide the same amount of lift. In order to increase lift at a given speed you need to increase the angle of attack and this pushes the inside wing even closer to a stalled state. It all has a compounding effect when added together.

I think too much emphasis is placed on gadgetry and ideas to stop people spinning instead of training pilots to fly safer speeds.
If safe speed control is taught and adhered to then wings won't stall, and if there are no stalls there will be no spins either.
Personally I always fly at about 10 knots faster than minimum recommended approach speed (taking wind into account) in the circuit and only slow down on finals if I need to. I also thermal about 10 knots faster when I'm below 2000 feet AGL. Yes, it's less efficient but it's also safer. What I've found though is that my higher speed habits lower down become my habits higher up too. I tend to fly faster than required everywhere because flying slower feels unnatural unless it's during round out on landing.
I can live with flying less efficiently and my family won't need to hear that they just lost their husband and father due to a silly and avoidable accident.

I think we may be discussing different issues. If you skid a turn, the AOA of the lower wing will initially be lower (due to dihedral), so to keep from rolling, you will have to use a lot of top aileron to increase the lift of the lower wing. Aerodynamically, you have a flapped wing on the inside and a negative flap wing on the top. The camber of the flapped wing is greater, so the effective AOA is greater, and when you finally stall, the inside wing will drop, and with pro-spin rudder, you will probably depart into a spin INTO the turn. My LS6 will only depart (at my weight and CG) in landing flap from a grossly skidded turn, and then it will do a nice spin entry (for a quarter turn, then it transitions to a spiral dive). At any other flap setting, it just drops the nose a little.

Totally agree about maintaining extra speed when low and maneuvering - and strongly feel that pilots need to PRACTICE low speed departure and recoveries so that they know instinctively what is going on.

cheers,

Kirk
66

Something just doesn't ring for me in this. I am easily confused though. I guess I'll just do my best to stay away from trouble!

Poor mans AOA? A string with a nut tied to the end tied to bottom if panel and hanging down with two kines marked on the floor. One for-aft, one side to side drawn at right angles to each other. Works great in a pinch, costs 23cents.

On Thursday, April 14, 2016 at 5:17:04 PM UTC-7, wrote:
Poor mans AOA? A string with a nut tied to the end tied to bottom if panel and hanging down with two kines marked on the floor. One for-aft, one side to side drawn at right angles to each other. Works great in a pinch, costs 23cents.

Will not work under acceleration.

On Thursday, April 14, 2016 at 8:58:15 AM UTC-7, son_of_flubber wrote:
On Thursday, April 14, 2016 at 10:58:57 AM UTC-4, kirk.stant wrote:
The best way to convey AOA is with aural tones (from personal experience in F-4s that had such a system). Properly implemented, you can control your approach speed precisely without ever taking your eyes off the runway to look at the airspeed or AOA indicator, regardless of bank angle or gross weight.

This sounds worthwhile, and I'd think it would be relatively easy to implement in software.

I will write the firmware, if others do the hardware...

Marc

Audio aoa
https://www.youtube.com/watch?v=H81AcnZeaMQ

On Friday, April 15, 2016 at 12:54:52 AM UTC-4, wrote:
Audio aoa
https://www.youtube.com/watch?v=H81AcnZeaMQ

Not quite. This is just a beeping stall horn that goes off at a discrete threshold. I imagined an audible AOA that would produce different tones for different AOA, so that you could use it to gauge AOA that is less than critical AOA. How is that useful? You could use it to hold the plane a notch below critical AOA during maneuvers, like a dive recovery.

It would sound something like an audible variometer. You'd probably want to automatically turn the audible variometer off perhaps when current AOA got within range of critical AOA.

Surely this exists already. There are AOA indicators that provide a visual readout of current AOA over the entire range.

Who cares?

Feel the force, Luke.

On 4/15/2016 7:06 AM, son_of_flubber wrote:
On Friday, April 15, 2016 at 12:54:52 AM UTC-4, wrote:
Audio aoa
https://www.youtube.com/watch?v=H81AcnZeaMQ
Not quite. This is just a beeping stall horn that goes off at a discrete threshold. I imagined an audible AOA that would produce different tones for different AOA, so that you could use it to gauge AOA that is less than critical AOA. How is that useful? You could use it to hold the plane a notch below critical AOA during maneuvers, like a dive recovery.

It would sound something like an audible variometer. You'd probably want to automatically turn the audible variometer off perhaps when current AOA got within range of critical AOA.

Surely this exists already. There are AOA indicators that provide a visual readout of current AOA over the entire range.

--
Dan, 5J