Re: Flight characteristics questions
Peter Harris <peterjfharris@...>
David for what it's worth the subject of pitch spring effect came up a
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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. Peter _____ From: Q-LIST@... [mailto:Q-LIST@...] On Behalf Of David J. Gall Sent: Tuesday, 17 October 2006 7:43 PM To: Q-LIST@... 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-----[mailto:Q-LIST@yahoogroups. <mailto:Q-LIST%40yahoogroups.com> com] On Behalf Of Sam Hoskins[mailto:Q-LIST@yahoogroups. <mailto:Q-LIST%40yahoogroups.com> com] On Behalf Of q2fun
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