Terry,

 

It is interesting that we are talking about two different ways to represent AOA.  Another would be the leading edge stall warning tab seen on many std certified aircraft.  I have seen vanes used on many corporate jet aircraft.  Vanes definitely represent aircraft attitude relative to the airstream flowing and, as such, are clearly related to AOA as defined.  It is simple and easy to understand in its operation and implications.

 

Pressure sensing "AOA" sensors are intriguing because of where they are located.  In my case, inboard of the wingtip rib and forward of the main spar.  In a 300 series Lancair this location is where there is significant washout and, in stable flight, the aileron remains somewhat reflexed.  I am not sure how this relates to the defined AOA (Chord to relative wind) in those areas of the wing affected by the flap and its TE that starts above the non-reflexed TE location and eventually drops below it.  The other confusing aspect is the attitude change that occurs as the flaps are extended and how that relates to the webinar discussion of AOA drag as the pitch change is trimmed out by up elevator.  The flap extension attitude change (pitch down) certainly decreases the wing tip AOA throughout the area covered by the aileron.  Good if you don't want the tips to stall before the wing root. 

 

Because of all that fancy footwork (no pun intended), the electronic AOA compensates for aircraft loading and other things that affect lift along with an indication of reaching best glide speed.  The AOA indication changes instantaneously with elevator movement.  It is best to consider the digital readout in tenths - like 00.0 to, say, 15.0 and that may be the reason to ignore the bouncing around in turbulence where the change is no more than + or - 1.0.  Perhaps I will go back out and do similar test only reporting the number and color of the chevron bars - then again, perhaps not since it is too much to write down - Hmmmm, maybe a video?

 

Maybe we should just call my device a lift reserve indicator and leave it at that.  The other piece of data that the device uses is airspeed and I don't know what it would mean to try to pick a device reading and keep it constant while manipulating flaps and airspeed since it is defined for only two configurations, full reflex and partial flaps.

 

The important part is that the nag yells "Angle, Angle, Push" into my headset at an approach to stall - allowing me to utilize another deteriorating sensory channel to the brain rather than overloading just one.

 

BTW,  Aircraft tend to hit a slightly elastic speed wall, regardless of power, by virtue of their design.  Perhaps for that reason the 235's powered by 320's don't seem to reach the expected cruise potential (less than Vne).  The 300 series seems to respond better to added power.  Of course, then we are always cruising in the yellow arc in smooth air.  Be careful out there and note the desirability that the wing stalls before it fails in wild and wooly turbulence.  Stay inside the envelope.

 

All very interesting.  Probably more by relating your "Bacon Saver" and the digital AOA mounted on the same airplane.  Does this experimenting ever end?

 

Scott   

 

In a message dated 10/31/2011 8:04:57 A.M. Central Daylight Time, troneill@charter.net writes:

Scott,

Very interesting, and I envy your performance.  With same power settings and altitude I think our 235/320 is about 15 to 20 KIAS slower than yours.  (I need cowl flaps.)

Sometime after I wrote the detailed article on AOAs in the December 1998 issue of Kitplanes Magazine, Jim Franz called and discussed his plans for making AOAs... which he did. Your installation must be the same as his, so i reviewed the 114 pages of its installation and operation manual.  Really complex, but does not really measure a wing's angle of attack, but instead, the digitally calculated lift available between a 'zero lift angle' and the maxim um lift available. 

 It also performs many additional more functions than my simple relative wind vane does not.  However, it does not perform some that my free vane does, such as show the AOAs above the stall angle, so the pilot can see exactly how much or how little he has to reduce the (stalled) AOA to get flying again.

I didn't understand the large range of numbers (angles?)  digital Franz's AOA displays on the gage ... from zero up to more than 100...?  But I guess you quickly get familiar with what each number range represents, after you go through the calibration procedure. 

To calibrate a simple free-vane AOA I just stuck my Bacon Saver in the mounting tube, flew and noted what angle number the vane is pointing at --  at the stall, the approach, and if it's different with the flaps reflexed or down.

Then I stuck on the small piece of striping tape of the color I like for each condition, and that's the calibration.

I had asked - does your AOA indicator moves instantly with your elevator movement? Or is there lag.

One interesting difference between Franz's AOA system and the simple vane display -- I see where you noted your AOA gage's digital numbers decreased when you held the airspeed constant at 120 kts. and moved the flaps from reflex to profile to 10 degrees down... and this I think seemed to you the opposite to what you expected...  i.e increased lift from deploying a flap.

But it is a correct indication, because you were holding the airspeed constant instead of holding the actual AOA constant.  I think the reason it appears contra-intuitive is because

Franz's "AOA" is not really an AOA that measures ANGLES, but measures pressure differential ... between upper and lower wing surface ...  and compares that to -- and corrects for ---   what he calls a "zero lift angle" (which one calibrates by flying a zero-G maneuver, and which moves and varies with each wing/flap configuration) and the pressure increment between the top and lower wing surface at an airspeed... all of which must be calibrated.  Pretty complicated.

A simple free-vane AOA just shows the angle between the actual relative wind and the wing's basic chord line, and has very angle-acurate marks on an indicator for the unchanging stall angle, and if desired, the angle one wants for approach, etc. 

I just made an angle scale behind the vane; others use a potentiometer shaft to mount the vane and read that out on a panel gage. 

I look with my eyeball at the AOA and my left hand on the stick adjusts the vane to read the desired angle during approach.  

Either way works; and what's important that with an AOA a pilot can see how close he is flying to his wing's stall angle -- or to its 'maximum lift angle' -- and avoid stalling.

Terry

On Oct 28, 2011, at 3:43 PM, Sky2high@aol.com wrote:

Terrence,

 

Thanks for the information.  Of course you are right that the AOA is affected by the elevator loading up the wing.  Other than that and the worthwhile EAA webinar that David pointed out, I can only report the following as "interesting" on my 320.  I have rigged my ailerons to a slight up position to eliminate any wobble from lash.  Because they are rigged up, the very minimal adverse yaw from aileron deflection (differential bell cranks) is further reduced because the down going aileron has to go further before it is down.  Another factor that cannot be ignored is the eerie reflex flapped wings.  They just don't quite perform the way presented in the EAA webinar.  I have the original Jim Franz digital AOA device (now Advanced Flight Systems) that utilizes the pressure differential betwixt the upper and lower wing along with pitot/static pressure (airspeed).  Beside the fancy chevron, a 3 digit representation of the AOA is also displayed.  The wing pressure pickups are located at the left wingtip.  Today I went for a flight with the following results:

 

My CG was at 25.5 (about 1.5" aft of most forward) at a TOW of about 1540 lbs.

At 6500 MSL (5800 Dalt) and 3C OAT, I was indicating 180 KIAS (195 KTAS), 24.8" MAP and 2480 RPM.

 

At the 180 cruise:

1. The AOA read 16 and 19 in either a right or left 20 degree level banked turn (I used the AP for this test). 

2. 1/2 rudder pushes does induce a slight roll with an AOA of 18.  If one does not attempt to hold level, the roll to the left also starts a dive (no surprise there).

3.  Cross controlled aileron induced 20 degree dutch rolls resulted in AOA readings of 18-19.  If the rudder was not used, the DG would swing about 10 degrees.

 

At 120 KIAS (power appropriately back) and level flight with the ball centered:

1. Flaps in reflex AOA = 42

2. Flaps out of reflex (down 7 degrees) AOA = 27

3. Flaps at take off position (about 10 degrees out of reflex) AOA = 18

Note that this is at variance with the webinar that claimed the AOA, as measured from chord line to relative wind, increased as flaps were deployed.  Not so by my AOA pressure measurements.  Remember that the 200/300 Lancairs have a very strong pitching moment as the flaps are moved.  In my case I required more up elevator as flaps were lowered and more power to overcome added drag and maintain level flight at the steady airspeed.

 

At best glide (106 KIAS), flaps in full reflex:

1. AOA = 58

2. Left 1/2 rudder induced a roll, AOA = 62

3. Right 1/2 rudder induced a roll, AOA = 64

 

Level fight at a lower altitude  in light chop saw the AOA go + or - 10

 

On descent to the airport at 500 fpm and 135 KIAS, the AOA hovered around zero.

 

I did not record data in the pattern as the tower asked that I keep ahead of the following jet.  Besides, my short term memory is not up to the task of remembering multiple data points.

 

Scott

 

 

 

In a message dated 10/27/2011 3:15:16 P.M. Central Daylight Time, troneill@charter.net writes:

Scott, Au contraire mon ami.  

You saw my wing with its free-vane-type AOA, mounted at BL+47.  

In flight it is my trim tab that maintains the wing's AOA for each airspeed and tail loading (CG vs lift center), and only my light fingertip pressure make corrections for altitude/power). 

This works in my bird in calm or gusty air.

I haven't flown with one of the 'magic lights' or digital AOAs, but the results should be about the same as with my free-vane-type AOA.  

Watch your AOA, -hands off-, in gusty or smooth air, zooming up or diving, and note that the AOA stays as trimmed. 

But with - hands on - the stick,  I am overriding the trim, and control of the wing's AOA, and it moves instantly with my stick movement, as if it were string-attached vane-to-stick.  

If your AOA doesn't react like that, it's got lag in the system.  What kind of AOA do you have?

Operating near the stall, at high AOAs (regardless of airspeed) the wing's AOA is commanded by the stabilizer/elevator/trim unless overridden by the pilot ... who is responsible for his input.

If he deliberately pulls the wing into a stall AOA, (which he can SEE on this AOA) he is deliberately giving away his pitch control.  Then he has to use the elevator-stabilizer to reduce the wing's AOA.  In the CAFE test of a 320/360 the pilot noted that the stall recovery was marginal.  In a professional/military test pilot report at (www.eaa1000.av.org/fltrpts/lanc360/hq.htm)  gives the details regarding high and stall AOA and recovery of the small tail LNC2s, which prompted me to put slots on my horizontal stabilizer.  I wasn't sure how these tests applied to the 235/320, but I thought -- close enough.  The alternative was to be cautious about stalling the plane, and, to also be able to SEE how close I was to the stall AOA.

You mentioned  steep turns to final.  It is not the bank angle but the pulling back on the stick for a balanced turn that increases the AOA.  I watch the AOA vane while pulling the wing into an untrimmed AOA, but keep the AOA vane angle well below the marked stall AOA, (my tab trims only down to about 85 w/ gear down and flaps, rear CG) and the AOA responds just as though I had a string attaching the stick to the free vane.  I keep about 5 degrees in reserve for a little extra lift needed to flare and slow to touchdown. 

 

When first flying with my homemade AOA (1970) while testing my O' Model W, I was delighted to be able to fly very close to the NACA 4415's stall AOA and minimum airspeed with confidence, and thought I'd see if i could land right at the start of the runway and turn off at the run-up area... so when I got right down to the runway to flare and pulled back... there was no more CL at that high angle.  Some airfoils have a gradual peak a little past the top of the lift curve, and I'd been flying in that max lift area.  So I made a 'carrier landing' ... that is, I flew right into the concrete with a big bang..  Fortunately I had made the landing gear nice and strong.  I got out and looked everything over, and no damage, except to my ego.  But I learned to respect the airfoil's gradual max lift peak, add reduced my AOA down to final. 

If (hands off) a plane wants to pitch up or down regardless of the pitch trim setting,  then it is dangerously unstable because the CG is aft of the plane's lift center ... aft of where the CG is not supposed to be... unless you have artificial stabilization.

Regarding slowing in in chop or turbulence, I don't know what would cause that. Maybe some small zone on the plane is just a gentle enough curve that it is low drag unless bumped..?  Some planes have a 'bucket' of low drag in a narrow AOA range, and have to be flown fast enough to get the AOA down into the bucket at cruise power... but if approached slowly by setting the cruise power too early they don;t get up to speed.  So the pilot leaves climb power on until he's in the bucket, and then can reduce power and stay there.

My comments are based on my own experience flying with my vane, in my Model W, in my Magnum Pickup, a Mitchell B-10 Wing, in  a Dragonfly, a Challenger, and in the 235/320.  

I don't want to be stalled either.  : )

Terrence

On Oct 27, 2011, at 10:08 AM, Sky2high@aol.com wrote:

Terrence,

 

Yes, but the statement is too simple and is only true in smooth air under a constant load.  The flying wing AOA (angle between wing and air flow) is not always under control of the pilot.  Operating at an air speed near the stall AOA allows the wing to stall if the air is not smooth such as perturbations from turbulence (wake, wind shear, gusts, etc.) 

 

Pilot control is a component when operating near the stall AOA and loading up the wing without increasing the air flow that increases the AOA leading to a wing stall (i.e. low speed base to final turn where the bank angle is increased and the stick is pulled to line up with the runway on final).

 

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