Re: Flight characteristics questions
Peter Harris <peterjfharris@...>
David for what it's worth the subject of pitch spring effect came up atoggle quoted messageShow quoted text
couple of years ago. I had complained that when the engine quit there is a
scramble to adjust the trim to hold the nose up. The trim spring made the
nose seem VERY heavy at a time when we do not need that kind of a
distraction and my plans adjuster was stiff and hard to reach with the left
hand. I was not going to let go of the stick! . I was surprised to find
that most of the guys let the trim freewheel unlocked. It is not designed
that way due to the mechanical advantage but if the bearing is made loose in
construction it would freewheel like a buzz saw. Anyhow I redesigned my trim
so it is now operated by a throttle cable control near my left hand and very
easy to disengage. It is a real pleasure to use close to the throttle, I
lock it only when stabilized in cruise, and also I lock it fully elevator up
for final approach and flare. (My throttle is done with a mixture adjuster
cable and that is also nice to use, just set and forget)
At the present time I am dealing with a related problem: Although trimmed
max elev up I need forward pressure to hold the nose down and TE up (3/16"
at 170KTS). Aeleron reflex is 3/8"up. Unless I made a mistake the CG is
same as for the Norton but the Norton line of thrust was 17mm higher. Do you
think that the thrust couple would explain the issue? I am planning to flush
the aelerons again.
From: Q-LIST@... [mailto:Q-LIST@...] On Behalf Of
David J. Gall
Sent: Tuesday, 17 October 2006 7:43 PM
Subject: RE: [Q-LIST] Flight characteristics questions
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
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On Behalf Of Sam Hoskins[mailto:Q-LIST@yahoogroups. <mailto:Q-LIST%40yahoogroups.com> com]
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