Angle of Attack

Hello
Anybody could help me to understand how to calculate the resulting Angle of Attack of the wing for a given airspeed?
Thanks in advance.

On Friday, October 23, 2015 at 11:09:46 AM UTC+3, wrote:
Hello
Anybody could help me to understand how to calculate the resulting Angle of Attack of the wing for a given airspeed?
Thanks in advance.

Sure.

First you need to know the weight being supported, and the effective wing area.

And then you need a chart or formula of the coefficient of lift at each angle of attack, and find the angle that generates enough lift from the given wing area to support the given weight.

For small angles of attack you can approximate the lift/AoA relationship as linear, bearing in mind that for common airfoil shapes the zero lift point is at a slightly negative geometric angle of attack not at zero.

(geometric angle of attack is relative to the wing chord, which is defined at the longest line that can be drawn, starting and ending on the skin of the wing)

Didn't the SSA have a contest to develop a working AOA indicator for gliders? What ever happened to that project, has a practical AOA indicator ever been developed for gliders?

At 15:09 23 October 2015, Jonathan St. Cloud wrote:
Didn't the SSA have a contest to develop a working AOA indicator for
gliders? What ever happened to that project, has a practical AOA
indicator ever been developed for gliders?


Yes, you sit in it

On Friday, October 23, 2015 at 9:09:47 AM UTC-6, Jonathan St. Cloud wrote:
Didn't the SSA have a contest to develop a working AOA indicator for gliders? What ever happened to that project, has a practical AOA indicator ever been developed for gliders?

Not the SSA but OSTIV - I think. IIRC, several workable designs were proposed but none were adopted. Most involved some sort of external vane which was considered too fragile and complicated for gliders.

When discussing AoA instruments, it's important to make the distinction between simple stall warning systems and full AoA instruments. Stall warners just set off a flashing light and noise maker when the AoA exceeds a pre-set value. AoA instruments display the actual angle of attack throughout the whole operating envelope and are used to optimize performance. Stall warning can be added to an AoA instrument and the trigger point can be adjusted.

In recent years, NASA has developed several AoA systems that don't use vanes. Their "5-hole probe" not only measures AoA but yaw and stagnation pressure as well. For gliders we would only really need a 2-hole probe - essentially 2 pressure taps, one on the upper nose forward of the canopy and one on the lower. A vario-like instrument could measure the pressure differential between the ports and derive the AoA.

Of course the 5-hole probe could replace the yaw string with an instrument indication.

On Friday, October 23, 2015 at 9:21:38 AM UTC-7, wrote:
On Friday, October 23, 2015 at 9:09:47 AM UTC-6, Jonathan St. Cloud wrote:
Didn't the SSA have a contest to develop a working AOA indicator for gliders? What ever happened to that project, has a practical AOA indicator ever been developed for gliders?

Not the SSA but OSTIV - I think. IIRC, several workable designs were proposed but none were adopted. Most involved some sort of external vane which was considered too fragile and complicated for gliders.

When discussing AoA instruments, it's important to make the distinction between simple stall warning systems and full AoA instruments. Stall warners just set off a flashing light and noise maker when the AoA exceeds a pre-set value. AoA instruments display the actual angle of attack throughout the whole operating envelope and are used to optimize performance. Stall warning can be added to an AoA instrument and the trigger point can be adjusted.

In recent years, NASA has developed several AoA systems that don't use vanes. Their "5-hole probe" not only measures AoA but yaw and stagnation pressure as well. For gliders we would only really need a 2-hole probe - essentially 2 pressure taps, one on the upper nose forward of the canopy and one on the lower. A vario-like instrument could measure the pressure differential between the ports and derive the AoA.

Of course the 5-hole probe could replace the yaw string with an instrument indication.

Any idea what the shape of the two hole solution might look like? I would like to explore putting it in the nose of my Ventus C

On Saturday, October 24, 2015 at 9:37:46 AM UTC-6, Matt Herron Jr. wrote:

Any idea what the shape of the two hole solution might look like? I would like to explore putting it in the nose of my Ventus C

NASA has published several papers which are available on-line. The nose of a Ventus isn't too different from the probe shape.

On Friday, October 23, 2015 at 10:09:47 AM UTC-5, Jonathan St. Cloud wrote:
Didn't the SSA have a contest to develop a working AOA indicator for gliders? What ever happened to that project, has a practical AOA indicator ever been developed for gliders?

Easy to install, just put a yaw string on either side of the canopy, as low as possible. Works great EXCEPT if you have ANY yaw, then you will see a large difference! So, pretty useless in practice.

There was an interesting article in an old Soaring (mid 70s?) that describes how to build a pneumatic CLmax indicator; it had a pellet in a calibrated tube that would show when you got to CLmax - another way to get AOA info, really.

Seems it would be easy to build-in symmetrical pressure ports in the wings to get AOA info - nowadays you could even get the data via bluetooth to avoid leaky connectors. Hmm, maybe a little "blister" could be made, self-contained with pressure sensors, microprocessor, bluetooth, and battery, that could be slipped on the leading edge of each wing and provide the necessary data to a small display in the cockpit? External, or recessed flush with the leading edge (after cutting out a small chunk of leading edge?

Kirk
66

If it's on the side of the canopy is it still a yaw string?

Sorry - couldn't help myself.

On Friday, October 23, 2015 at 4:46:53 PM UTC-7, Dave Springford wrote:
If it's on the side of the canopy is it still a yaw string?

Sorry - couldn't help myself.

Nope, AOA-string.
Had a Mod-1 on the side of the DG-1000. Not very useful if you wanted to see where you are going and where the trouble might come from. Worse than any flight computer.
Heinz