Re: Flight characteristics questions
Davetoggle quoted messageShow quoted text
As a matter of interest, an old method of achieving trim was to 'dope' a length of string along the airfoil trailing edge, varying the length to achieve the desired effect.
Do you think that this method could be used underneath the GU elevators as an alternative to camber, if only as a temporary explorative measure?
----- Original Message -----
From: David J. Gall
Sent: Tuesday, October 17, 2006 6:22 PM
Subject: RE: [Q-LIST] Flight characteristics questions
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
David J. Gall
> -----Original Message-----
> From: Q-LIST@... [mailto:Q-LIST@...]
> On Behalf Of Michel Moreau
> Sent: Tuesday, October 17, 2006 7:43 AM
> To: Q-LIST@...
> Subject: Re: [Q-LIST] Flight characteristics questions
> 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.
> ----- Original Message -----
> From: "David J. Gall" <David@...>
> To: <Q-LIST@...>
> 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
> > 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@... [mailto:Q-LIST@...]
> >> On Behalf Of Sam Hoskins
> >> Sent: Monday, October 16, 2006 5:02 PM
> >> To: Q-LIST@...
> >> 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@... [mailto:Q-LIST@...]
> >> On Behalf Of q2fun
> >> Sent: Monday, October 16, 2006 6:28 PM
> >> To: Q-LIST@...
> >> 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
No virus found in this incoming message.
Checked by AVG Free Edition.
Version: 7.1.408 / Virus Database: 268.13.4/477 - Release Date: 16/10/2006