| Scott, you're right. Thermal or turbulence bumps would pop the airflow to a higher AOA for a brief moment. Yes, according to my old Abbott and Doenhoff, there are slightly different lift/drag curves for 'standard roughness', and also for different reynolds numbers. But -- I believe the majority of fatal stall/spin accidents are caused by the pilot pulling his wing up past the stall AOA, unintentionally, of course. Light controls contribute. I also note that some airfoils have a kind of plateau near the max Cl, where the angle can be increased maybe 5 or more degrees, but no increase in lift. ... as in trying to flare for a short field approach. But I think the Lancair airfoil (NLF(1)-0215F ? ... I don't recall) has a definite break angle, and I allow myself about 5 degree for flare (and turbulence/bumps. Buggies on the leading edge also cause little turbulent V-wakes behind themselves and that increases the drag a little. John Roncz counted the bugs on the Voyager's leading edge and tried to calculate the effect on their range estimates.
I guess I don't have enough time in the plane yet to have noticed if it slows in turbulence. I see in this airfoil's L/D curves the Cl doesn't change with standard roughness... just the drag increases. Regarding the stall AOA with flaps, the stall angle reduces as flaps and the max Cl increases. Like, 30 degrees flaps would move an airfoil's stall AOA from, say, 16 degrees back to 14 degrees. ... but remember that the whole wing isn't flapped. Part is inside the fuselage, and usually the tips are unflapped.... so that reduces the change in stall AOA that otherwise would be caused by flap.
About the marginal nose-down pitching moment at AOAs in the stall range that some very pro- test pilots have reported, my own opinion is that it results from the wide engine cowling and the small fuselage cross section at the tail end. It's the whole airplane that contributes to these moments. So I put slots on my horizontal stabilizer, which was easier than adding stabilizer area or strakes, and also gave me better airflow over the rudder at stall-zonel AOAs, for spin recovery (perish the thought). I got into this pitching moment problem when I decided to buy the Dragonfly, and I knew that Pug piper's canard had crashed being unable to recover from a stall... and that NASA had done some special wind tunnel tests on tandem wing aircraft. I called NASA at Langley and talked to those guys, and they sent me copies of the reports. Short story is that the whole plane would "trim" (tend to not pitch down) at around 30-40 d AOA, and this got more critical as the CG went aft. In the Dragonfly's case it mattered whether the front wing was mounted high or low, and the rear wing opposite... low in front was best. So I always flew with the CG definitely forward of the design CG aft limit.
ABout trimming, I think of it by imagining the CG and aerodynamic center stuck together, and then think what the tail is doing. The tail is way back there, with a long moment arm and can do whatever it wants. If it develops a little camber, it will swing like a rudder on a windmill until it isn;t being pushed one way or the other any more.... it goes to Zero force.. And its leverage forces the balanced wing to go to that moment arm's angle. The ANGLE is the thing... and when you trim the tail to force the wing to that angle, it will hold it at that angle, no matter how fast or slow the air is going by. And that's why we change altitude with power, and leave the trim alone if we want to avoid having to retrim, for the final indicated level-out speed.
Caveat: I'm not a professional aerodynamicist... this is just a compendium of what I picked up since I started reading Frank Zaic, and CG Grant in Model Airplane News in 1945, and a year in Aero at Notre Dame before switching to Journalism and graduating before the Draft could send me to Korea, and enlisting as a NavCad. Also picked up some from the engineering files I bought from Waco when I acquired the AristoCraft., and then designing my Model W for an FAA TC. It's been fun. : )
Terrence O'Neill
Terrence,
It doesn't always take the pilot to pull the wing beyond the critical AOA -
The air is not always smooth as in a stable fluid body - turbulence, wind
shear, slow flight over different heat radiating bodies in the summer (green
fields vs black dirt), crosswinds that tumble over trees on
a strip carved out of the woods or over nearby hangars, etc. The margin
above the stall AOA may disappear in a slow highly banked turn or even on a
straight in if the air is not compliant regardless of the
trim.
I have been impressed with our 200-300 series reflexed laminar flow wing -
have you noticed a slow down (loss of laminar flow) in turbulence? Is
that just drag or is lift also affected? Does the critical AOA change if
the laminar flow is disturbed? Is this more important when the wing is
already at a high AOA when slow and is no longer in reflex (flaps partially
deployed)? Note that if the speed changes, the trim is no longer
correct.
I believe that the 300 series Lancairs are unstable at low speed high
AOA because the margin may be too narrow. It may not be the pilot
unintentionally pulling past the critical AOA, but an abrupt change in the
airflow that causes the angle to be exceeded. Even though the AOA was
calibrated in clean air, the calculated margin speed above stall by formula
(1.15 x stall) may not be enough for these high
performance wings.
Scott Krueger
If I
may, comparatively briefly:
We all know the word 'stall' refers the wing's airflow breaking away at a
certain angle.
You, the pilot, controls the angle at which the airflow approaches
the wing. Hands off the pitch control that angle stays where you trimmed
it, regardless of attitude. You probably don't really believe that. but it's
true -- unless -- you are too far aft CG, or the total airplane has a pitching
moment that increases 'nose-up' as the AOA increases.
Many (military) aircraft with artificial stabilization are designed to
use all lifting surfaces to get the best L/D, for more performance -- at the
cost of stability.
GenAv planes don't. Some Experimentals are marginally stable or even
unstable when at high AOAs. Add to that, all the fatal GenAv accidents
are caused by unintentional stalls, a quarter to a third of all
fatals.
Because the pilot pulled his wing past its stall AOA ..
unintentionally.
Unintentionally, because he can not SEE the air-to-wing angle.
Because he does not habitually reference how he is 'planing' his wing.
He uses airspeed, a very vague, inaccurate reference to the wing-wing
angle.
For GenAv planes, the FAA persistently, ignorantly, does not require all
airplanes to have an AOA indicating vane right in the pilot's field of vision,
and require him to demonstrate using it, to get his license.
The great majority of pilots are not aerodynamics guys who have designed
an airplane, built their own design, and then got in it and risked their life
on their own design knowledge. Most know about AOA, but don't
understand it -- or use it.
I was ignorant too, in spite of my Navy wings and five more year of GenAv
flying. Until 1960 when I bought Waco's last prototype Experimental and
restored and flew it, and then conducted a FAA Type Certification program for
my own similar design, a 6-seater ... then I realized I needed to really
understand what was going on at high AOAs, and started making my own AOA
vanes, so I could see the angles, down to one degree, and watch them as I
maneuvered the plane, stalled and unstalled the wing, watched the vane move as
I moved the wheel in and out, like there was a string attached to the AOA
vane.
It makes me so sad, every time I see a stall-related crash ... friends
and compatriots, year after year, hundreds of great aircraft and wonderful
people, die unnecessarily.
Why?
The GenAv survivors should blame the FAA and manufacturers' marketing
departments.
We Experimenters have only ourselves to blame. We don't believe in AOAs.
We don't want to take the time or spend the money. We don't want
to make our beautiful planes ugly. We don't want to degrade performance
one mph. We don't want to learn new tricks. We're great pilots,
and we'll never make that mistake. But a few of us do.
Please excuse my continued harping on this. It's just 'tough
love'.
Terrence
L235/320 N211AL
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