AOA indicator

In a skidding turn is the low wing AOA higher than high wing AOA?

Anyone use a side string/s for AOA? DG has an article about it on their website.

On Tuesday, April 12, 2016 at 11:43:26 AM UTC-4, son_of_flubber wrote:
In a skidding turn is the low wing AOA higher than high wing AOA?

In any turn, whether coordinated or slipping or skidding, and with no change in angle of bank, all parts of the aircraft have the same vertical velocity relative to the air mass, and the outside wingTIP is moving faster than the inside wingTIP, so the outside wingTIP has a lower angle of attack. Near the fuselage, there is not much difference in angle of attack, except for some side-flow effects from slipping or skidding.

Dear kristi... and Jonathan,

I have indeed "installed" the mentioned Side String in form of a wool string of 25-30 cm length for the last 5 years on an ASG 29 and ASH 31. It works quite well - once "calibrated" on stall and best glide speed, it's fast and IMHO pretty accurate in straight level flight taking speed and g-load into account. It seems robust in different flap settings (actually it is a great optical reminder for keeping proper flap settings...). When banking/thermaling (especially with a slight side slip) it might loose some of its absolute accuracy due to turbulence and flow changes on the front fuselage but it still provides a good and fast tendency regarding getting closer to stalling the glider (completely in sync with the weakening feeling in the controls when approaching a stall). Drawback might be that attaching the string to the lower canopy creates a turbulence wake on a possibly laminar flow in the front part of the fuselage but given that the bug wipers kill that flow anyway 50 cm behind the string, I recon that this is almost irrelevant...

Best regards
Marc (S2)

I have had two sidestrings on my Pilatus B4.
I works fairly okay, but the location was a bit tricky in my case. In some cases it would not move at all (except during tail-slides :-)).

Here is a video of a spin with my setup:
https://www.youtube.com/watch?v=RWPya_vchDQ

You can see the vortex on the right-hand side which is generated by the high yaw rate of my glider.

I put yaw strings on each side of the canopy of my Ventus Cm a few years ago, and found them instructive. Didn't replace them when they fell off, but will do so in the future, to re-learn the lessons they taught.

I have been casually working on building a low-cost AOA measurement system using a small side vane. This would be easy to install, battery powered so require no digging into your panel to find power. The parts cost does add up faster than I thought so it would still be a few hundred dollars after parts, building and overhead is factored in. My glider gives a very audible vibration approaching stall so I am wondering if it is even worth it. I am a relatively low-time sailplane pilot. Too many skilled pilots continue to spin in which makes me wonder if their ships don't give any hint of approaching stall or what else is going on.

On Wednesday, April 13, 2016 at 10:56:34 AM UTC-4, Soartech wrote:
I have been casually working on building a low-cost AOA measurement system... My glider gives a very audible vibration approaching stall so I am wondering if it is even worth it. ...

I think that if I were impaired by fatigue and dehydration, I might ignore the plane's warning vibration, and I think I would be even more likely to ignore the stall horn. The stall horn is calibrated to go off before the stall, and if I had one, I would have (as an unintended side effect) trained myself to ignore it (on every landing). Plus I think that in a stall spin on turn to final, the buffeting, stall and spin would come quickly while I was decisively increasing the bank, and I would not react to a dropping wing properly in time.

So to mitigate my risk, I'd learn to recognize and foresee my fatigue impairment, have a good pee and hydration system, use high pattern speeds, and for short field landings, I'd reduce airspeed to minimum AFTER the turn to final (and practice this often). In short, develop protective habits.

On Tuesday, April 12, 2016 at 10:43:26 AM UTC-5, son_of_flubber wrote:
In a skidding turn is the low wing AOA higher than high wing AOA?

No, it I lower. This is basic aero, but can be easily understood with a simple visual aid - or a good scale model of a glider.

First, made a simple paper or cardboard wing, about the size of a standard foot-long ruler. Has to be stiff enough to support its own weight.

Hold it at eye level as if it was a glider flying directly at you. Tilt a bit to simulate a flying angle of attack. Now rotate in yaw, and notice that the apparent angle of attack of both wings is the same, despite the yaw.

Now Bend the wing a little in the middle, simulating a reasonable amount of dihedral. Rotate again, and note the difference in apparent angle of attack between the forward and aft wing. That's why you can roll (slowly) using only rudder.

Now, bank your wing as if in a turn, and repeat the yawing exercise. Remember, a skid will have the upper wing forward (too much rudder into the turn) while a slip will have the lower wing forward (not enough rudder).

It should be pretty obvious what is happening to the AOA of each wing and why.

See also the threads about using a bit of a slip to counter the overbanking effect of long wings (the "yaw string to the outside" discussion).

Now the 64 K question - which way and why will you spin out of a skidding turn?

Kirk
66

On Wednesday, April 13, 2016 at 9:56:34 AM UTC-5, Soartech wrote:
Too many skilled pilots continue to spin in which makes me wonder if their ships don't give any hint of approaching stall or what else is going on.

I think a lot of pilots, both low-time and skilled, spend way too little time exploring the corners of the envelope of their gliders - both high and low speed, negative to positive Gs, shallow to steep banks.

Get high and practice trying to kill yourself in the pattern, to the point that you recognize instinctively a dangerous condition and apply the right corrective actions. This is a basic flying skill, NOT an intellectual exercise!

Kirk
66