Re: Bruce's tuft study on sparrow strainers


Richard Thomson
 

Yay Jay, I think we already discussed, I have been beating my 2 brain cells together over the situation that I only have the elevator helper spring fitted (that was in when I bought FN) and trying to work out if I needed the full elevator trim fitted. So your stick rests around neutral in cruise without the trim ?

Thanks for the tips you sent earlier on testing set up for the reflex, I have copied those off to add to my notes.

Best present yet this year, merry Christmas. :-)

Rich.

On 22/12/2019 19:12, Jay Scheevel wrote:
I do not have the elevator trim system installed. I have been using my aileron reflexor for pitch trim since I first flew my plane a year ago. The elevator seems fully stabilized without the trim system. I think that the sparrow strainers have a significant damping effect.

Cheers,
Jay

-----Original Message-----
From: main@Q-List.groups.io <main@Q-List.groups.io> On Behalf Of Matthew Curcio
Sent: Sunday, December 22, 2019 11:57 AM
To: main@q-list.groups.io
Subject: Re: [Q-List] Bruce's tuft study on sparrow strainers

Jay, I think to evaluate this conservatively you’ll have to disconnect the pitch trim springs since those help to stabilize the system in some ways.

Matthew Curcio
419-290-3773
On Dec 22, 2019, at 10:44, Jay Scheevel <jay@scheevel.com> wrote:

Thanks for the more complete explanation. I have a manual "stick" type of control on my aileron reflexor. It fits into my left hand and feels and manipulates like a second stick (absent roll control obviously). I will have to try to see if I can fly the airplane using that reflexor control for pitch and the actual stick for roll. The sparrow strainers hold the elevator pretty much in trail for most AOA's (in the event that elevator continuity was absent), so I think it may be possible to fly the airplane with reflexor and aileron input only. I will try this out at a safe altitude and let you know.

Cheers,
Jay

-----Original Message-----
From: main@Q-List.groups.io <main@Q-List.groups.io> On Behalf Of Matthew Curcio
Sent: Sunday, December 22, 2019 10:46 AM
To: main@q-list.groups.io
Subject: Re: [Q-List] Bruce's tuft study on sparrow strainers

Yes but a lot more than that. This may be common knowledge for you but I’ll explain in some detail since I think most people aren’t fully aware. Taking a step back, part 23 and part 25 both say that you cannot have any single points of failure in flight control systems that are catastrophic (airplane is a total loss and more than one fatality). Look at a 172. If a pushrod or cables breaks going to the elevator (one or both panels), you can still fly with the trim tab, if you lose aileron authority (both panels) you can take advantage of adverse yaw to maintain roll authority - the quickie can’t do either of those things. A lot of pilots are aware of this but see it as a neat trick - they don’t realize it’s actually a requirement. The FARs also state stick load limits, with time functions for failure cases (This is why most twins end up with rudder trim and roll trim to deal with control loads after an engine failure) Additionally, if a 172 loses signal to one of its elevators the pilot would have ~50% elevator authority and the associated rolling moment would easily be reacted by the ailerons.

If a quickie loses signal to one elevator (hand wavey assessment here) you’re not going to have enough roll authority to react the rolling moments for continued safe flight and landing, assuming one elevator is faired and the other is being used to control pitch attitude. Its an issue of moment arms, the elevator’s arm about the longitudinal axis is so close to the CG that they require huge authority to drive a pitching moment, hence the full span and associated high stick loads that drive the requirement for sparrow strainer (it’s a band aid) - but their arm from the cg in the lateral axis is bigger than the ailerons arm. Hence, I argue that while the q2 has mostly separate load paths going to each elevator if either one fails it’s still not likely going to be survivable. I’d be thrilled if someone can prove me wrong or point to a case where that happened and it wasn’t catastrophic. There is nothing you can do to fix it, it’s just a flaw inherent with the design. Elevators should hav a big arm from the cg in the longitudinal axis and short about the lateral axis if you want to have a stable airplane, with benign failure modes.

That said, clearly if the bearing that supports the stick falls off your in a bad spot (BTW It should have a >1” long taped gusset in the x-y plane to react the loads in x that react the pilots pitch inputs, fiberglass tapes are terrible in reacting out of plane moment), if the bolt that connects the stick to elevator pushrods fails that’s single point and based on my assessment above, all the other linkage going to each surface is single point catastrophic failure.

If this were a part 25 airplane that bolt connecting the pushrods to the stick would be hollow with a second bolt running through it, the pushrods would have a second pushrod inside, as well as the torque tubes and horns a would all have parallel, independent load paths. Part 23 airplanes don’t usually end up requiring this level of complexity and the quickie just can’t be certified due to this reason and other. Im stating all of this as fact but it’s mostly hand wavey assessment (based on my experience from a couple years of designing and evaluating flight controls to part 25 requirements) so feel free to poke holes!

Matthew Curcio
419-290-3773
On Dec 22, 2019, at 08:55, Jay Scheevel <jay@scheevel.com> wrote:

Hi Matthew,

When you state the that the elevator control linkage has single points of failure, are you referring to the fact that there are independent (left-right) control arms with separate push rods returning to a single point on the stick?

Cheer,
Jay N8WQ

-----Original Message-----
From: main@Q-List.groups.io <main@Q-List.groups.io> On Behalf Of Matthew Curcio
Sent: Saturday, December 21, 2019 1:32 PM
To: main@q-list.groups.io
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
419-290-3773
On Dec 21, 2019, at 11:46, charlie <ffmd@vermontel.net> 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.
CharlieN















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