Re: Flight characteristics questions


David J. Gall
 

Michel,

Yes.

There is no undercamber on the GU elevator so the aero forces involved are
not large. However, the system does rely on the up-spring to hold the
trailing edge of the elevetor down in flight.

Early on in the pages of QuickTalk there were a couple of reports of trim
system failures ("modified hackwaw blade" failures) in the original Quickie.
The ensuing requirement for holding continuous back pressure was reported,
as well as the fatigue level of doing so for as little as ten minutes. (I'm
not going to look up the exact issue number.)

I'm not planning to re-engineer or suggest that anyone else re-engineer the
system on the GU canard, but be aware that it is a potential problem.

IF<< I were to reengineer the system, I think I'd merely "reflex" the
trailing edge of the elevator ever so slightly, as John Roncz did for the
Roncz 1145MS replacement canard airfoil for the Long-EZ.

IF<< I thought it was a problem of sufficient magnitude and was trying to
retrofit the existing GU elevator, I think I'd add sparrow strainers, though
they would not need to be angled nearly so severely as on the LS(1)
elevator. Of course, exact size and placement aft of the trailing edge would
be a matter of trial and error, and balancing the elevators against flutter
with the additional mass of the sparrow strainers aft of the hinge line is
an additional concern.

Like I said, I'm not planning to re-engineer or suggest that anyone else
re-engineer the system on the GU canard, but be aware that it is a potential
problem in the event of a control system and/or trim system mechanical
failure.


David J. Gall

-----Original Message-----
From: Q-LIST@yahoogroups.com [mailto:Q-LIST@yahoogroups.com]
On Behalf Of Michel Moreau
Sent: Tuesday, October 17, 2006 7:43 AM
To: Q-LIST@yahoogroups.com
Subject: Re: [Q-LIST] Flight characteristics questions

David,

Does this apply to GU as well as LS1? I don't see a "slight
undercamber" on my GU elevator.
Thanks for such a clear explanation of the phenomenon.

MM

----- Original Message -----
From: "David J. Gall" <David@Gall.com>
To: <Q-LIST@yahoogroups.com>
Sent: Tuesday, October 17, 2006 5:42 AM
Subject: RE: [Q-LIST] Flight characteristics questions


Sam,

I think you're thinking backwards here. The described
behavior indicates
sparrow strainers that are too small, not too large; or not
angled enough
(trailing edge up) instead of too steeply.

Without sparrow strainers, when the airplane inadvertently
goes faster
than
its trimmed airspeed, the slight undercamber of the LS(1)
elevator causes
aerodynamic force that pushes the trailing edge of the
elevator up. This
lets the airplane's nose come down and allows the airplane
to accelerate
to
a yet-higher speed. NOT GOOD!

The sole purpose of sparrow strainers is to REVERSE this
pattern. When the
nose inadvertently drops and the speed builds, the sparrow
strainers
should
push the trailing edge of the elevator down so as to raise
the nose of the
airplane, thus slowing it back down. Conversely, when the
airplane slows
down (due to an inadvertent climb) the decreasing dynamic
pressure on the
sparrow strainers lets them allow the trailing edge of the
elevators to
come
up (usually in response to a down-spring in the trim
system), lowering the
nose and restoring the airplane to it's original airspeed.

This is speed stability. It is closely associated with
pitch stability,
but
is not the same thing. Burt Rutan devoted several issues of
his Canard
Pusher newsletter to describing it, discussing it, and
reporting on the
requirements for it in his several homebuilders' designs.

Another clue to incorrect speed stability is light or
"sensitive" pitch
control, as reported in Mr. Q2fun's item #1. True, pitch
sensitivity is
also
a function of CG position, but given two otherwise
identical airplanes
with
the same CG location, the one with greater speed stability will have
less-sensitive pitch feel. That's because the greater
aerodynamic forces
of
the larger or more effective (larger/correct deflection
angle) sparrow
strainers will cause the elevator stick force to be greater
for any given
deflection of the elevators away from their trim position.
It's kind of
like
using a larger paddle on a canoe, you just have to pull
harder to get it
to
move through the water.

A more effective sparrow strainer will also need to be met
with greater
tension in the pitch down-spring of the spring-type pitch
trim system at
higher airspeeds. If the down spring of the trim system is
only lightly
loaded at cruise speed or if the up-spring is loaded and
the down spring
is
slack at cruise speed (heaven forbid!) then there is a real
problem and a
potentially dangerous situation.

What happens if the up-spring is loaded at cruise? If the
nose drops, the
aerodynamic forces get stronger while the spring force
stays the same, so
the aerodynamic forces "win." The up-spring was resisting
the aero forces
that want to make the airplane dive, but those forces now
win and the
airplane dives. Conversely, if the down-spring is loaded at
cruise, then a
nose drop again means larger aero forces, but those forces
(from the
sparrow
strainers) act to RAISE the nose, not lower it further.
Raising the nose
restores the airplane to level flight.

Rutan advised his builders that his airplane designs should be
AERODYNAMICALLY trimmed to a low-cruise airspeed when flown
hands-off and
with the pitch trim system disconnected. His rationale is that if a
crucial
part or connection in the pitch control system should fail
thereby leaving
an elevator completely disconnected from the trim system
and the control
system, it does not automatically kill the pilot. I do NOT advise
disconnecting the trim system in a Q2 or Q200 or even a
Quickie because of
the risks involved, but you can certainly look at your trim
springs and
simulate a disconnect by moving the trim control to make
them as equally
slack as possible. By this method one might be able to get
a relative idea
of what the sparrow strainers are doing and what the airplane's
aerodynamic
trim speed is.

In summary, I think there is a dangerous situation being
described by Mr.
Q2fun and I hope he gets his sparrow strainers and trim
system looked at
by
one of the "old heads" on this list to see if it is rigged
right or if
there's something amiss. I'll wager that the up-spring is
doing a lot of
work at cruise speeds and just loses the battle against
aero forces at
speeds above 150 mph. Not good -- not safe!

Just my worry-wart two cents worth,


David J. Gall
BSAE
Sacramento, CA

-----Original Message-----
From: Q-LIST@yahoogroups.com [mailto:Q-LIST@yahoogroups.com]
On Behalf Of Sam Hoskins
Sent: Monday, October 16, 2006 5:02 PM
To: Q-LIST@yahoogroups.com
Subject: RE: [Q-LIST] Flight characteristics questions

1. I think pitch sensitivity is normal, especially with aft
CG and at high
(8,000+ft) altitude.

2. My plane does the same. The rudder doesn't control bank,
the ailerons do.

2. This sounds a little odd. Maybe the sparrow strainers
are too big or have too steep of an angle. It could also be
that your canard needs to be adjusted up a degree or so.
Maybe try installing adjustable sparrow strainers first.
That would be an easy experiment.

Sam Hoskins Q-200 1,600+ hrs.

Murphysboro, IL



_____

From: Q-LIST@yahoogroups.com [mailto:Q-LIST@yahoogroups.com]
On Behalf Of q2fun
Sent: Monday, October 16, 2006 6:28 PM
To: Q-LIST@yahoogroups.com
Subject: [Q-LIST] Flight characteristics questions



Hi All,

I have some questions about the flight characteristics of the Q-2.
Pitch sensitivity. With an after c/g my Q2 seams pitch sensitive.
My Q2 has the LS1 canard, Revmaster 65hp, tail dragger. Is it
normal to get more pitch sensitive the more aft the C/G gets?

Rudder to control bank. My rudder does not do anything for bank.
If the wing is down 5 degrees and you use the rudder to bring
it up, all that happens is the plane will yaw and the bank
may even get steeper. Is that a normal characteristic of the
Q design?

With an increase in airspeed above 150 mph indicated the
airplane wants to pitch down and farther increase
airspeed/pitch down harder. Is this normal? Sparrow strainers
to small for this airspeed? I have the standard pitch trim system.

I have been flying my Q since April and now have 62 hours in
it. It is a joy to blast around the pattern and it is even a
joy to put gas in it after a long cross county. It is just a
little too slow in cruise 130 kts.

Thank You All.
Tim Bryant
KUNV
N86TB

Join main@Q-List.groups.io to automatically receive all group messages.