Re: Bruce's tuft study on sparrow strainers

David J. Gall


Thank you for your significant contributions to this discussion, they are much appreciated!

David J. Gall

-----Original Message-----
From: <> On Behalf Of Matthew Curcio
Sent: Saturday, December 21, 2019 12:32 PM
Subject: Re: [Q-List] Bruce's tuft study on sparrow strainers

I would be extraordinarily careful with that setup and my professional recommendation would be don’t fly that. The problem is that unless you have fully evaluated the trim loads incurred during the trim runaway throughout the entire flight envelope, that failure will likely degrade handling qualities such that it would be a catastrophic single point of failure. It’d be nice to have but I wouldnt accept the risk, if not for me for a future owner.

I was just talking about this with another engineer at work (the pitch control on the quickies is pretty terrifying). Where we ended up was that the elevator control linkage on the quickie has single points of failure that are catastrophic (obviously). They would carry a PF of 1e-6 (generic probability of failure / flight hour for any mechanical system) which is kind of stomachable but would never be allowed in a part 23 aircraft. If it’s mechanically controlled, a trimable sparrow strainer introduces another 1e-6 catastrophic single point of failure and in that regard it degrades safety. It doesn’t get rid of a single point of failure it just adds one. If you put an electric sparrow strainer on the PF is going to be appreciably worse unless you had an extremely slow actuator that could reduce the time of occurence. If there is anything more than a simple toggle switch (software) you would need an involved ver-Val test program.

If a reputable aerospace company were to do this we would consider it a land as soon as practical EP, We’d determine the stick loads allowed for continued safe flight and landing, do extensive sims to test the pilots time of response to the failure in the worst phase of flight, add some margin to that time and then ensure that the trim Is slow enough to prevent those loads from developing before the pilot responds. This is not ever the preferred approach of mitigating a catastrophic failure and there’s a few hundred families who buried loved ones last year, due to a very similar scenario, that would likely agree.

For reference Proteus has a trim-able sparrow strainer, everybody that touches the airplane is petrified of it (for good reason) and it’s the subject of a lot of conversation. Just bringing that up to say it’s a fairly well understood dilemma around here.

Matthew Curcio
On Dec 21, 2019, at 11:46, charlie <ffmd@...> wrote:

Way back in the mid '80s when I was building my Q1 with the first customer LS1 airfoil. I built the sparrow Strainers on hinges and drove then with model servos.
Intention was for this to be the trim system.
Being we have not flown this plane yet I have no clue how well this would work. Seems right thought.

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