REFLECTOR:Vortex Generators on XL

[Jim Sower]

Was said:
<... It seems to me that to get a high AOA, the elevator TE needs to be down.  It also
seems that once you are at a high AOA, you would need to put the elevator TE up in order
to bring the nose down.  Are you saying that if I'm at a high AOA and I push the stick
forward I increase the risk of a deep stall or am I confused? ...>

I wasn't very clear.  The amount of elevator TE down that it takes to generate a given AoA
is determined largely by CG position.  Forward CG takes lots of elevator, aft CG requires
less deflection.  Deflecting elevator changes the airfoil into a different airfoil with
different attributes - the more deflection, the more radical the change.  Effective
centerline of an airfoil is the line from the "null" point on the LE to the tip of the
TE.  As you can readily visualize, TE down elevator generates an airfoil with a LOT of
camber, which will stall at a lower AoA (or about the same AoA, but measured to the new
chord line which makes it look a lot higher).  TE up elevator makes for negative camber
and a higher canard stall AoA which can lead to the main wing stalling first ... etc.
That's how you can get into a deep stall by accident - unstable pitch that gets away from
you and the airplane does sort of a hammerhead type maneuver by itself, against your
wishes.

So, neutral, or nearly neutral elevator at lower speeds is a warning of neutral or
negative pitch stability.  The nose staying cocked up (or worse yet, trying to keep going
up) after releasing a nose up control input is a warning of neutral stability or
instability in pitch.  If that happens, you want to GO HOME IMMEDIATELY as someone said
earlier, and probably at moderate speed - like 100-120 kts - using very light control
inputs - and fly the airplane every foot of the way.

<... 1/2" elevator trim to take off ... with 170 lbs load in front seat ...>
Should be OK.  When I'm by myself, TO and landing trim is 3/8" - 1/2" TE down, and I have
positive pitch stability.

As best I can figure .... Jim S.


John Dibble wrote:

> Jim Sower wrote:
>
> >
> ><... 1/2" elevator trim to take off ...>
> >With what kind of load in front seat?
> >
> 170 lbs
>
> >  Just you?  Or you and someone else?  What is
> >your elevator position flat out at 8k ft?
> >
> At 160 k TAS, the elevator TE is about 1/4-3/8" higher than the canard TE.
>
> >  Is it negative?  If so, how much?
> >
> >My best understanding of "stall proof" canard airplanes and "deep stall" is that (at
> >the risk of over simplifying) canard airfoils have more chamber than main wing
> >airfoils and therefore will stall at a lower AoA.  Negative (nose up, TE down)
> >elevator increases effective camber and makes canard stall sooner still.  If the
> >elevator deflection is negative (TE up) the effective camber decreases to the point
> >that the main wing could stall first.  Soooo, assuming that all the incidences are
> >correct, the danger of deep stall occurs when a) you are at high AoA, and b) you have
> >TE up elevator.
> >
> It seems to me that to get a high AOA, the elevator TE needs to be
> down.  It also seems that once you are at a high AOA, you would need to
> put the elevator TE up in order to bring the nose down.  Are you saying
> that if I'm at a high AOA and I push the stick forward I increase the
> risk of a deep stall or am I confused?
>
> John
>