Re: Bruce's tuft study on sparrow strainers
David J. Gall
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John Roncz came up with the solution to this many years ago. It’s called the “Roncz canard” in the Long-EZ community. It’s an excellent airfoil, but particularly important is what he did to the elevator. The same technique could be used on the LS-1 elevator to mitigate the elevator hinge moment. It can be modeled fairly accurately in X-foil or in Roncz’s modified Eppler airfoil code that he used back then (freely available on the internet). Just gotta tweak it so the elevator floating angle is stable and appropriate for the airplane’s approximate maximum range low cruise airspeed so that if the trim and elevator control systems accidentally disconnect in flight the airplane will be able to continue flight and land using engine power modulation (and reflex if installed) as primary pitch control. Rutan laid out all these requirements in one of the Canard Pusher newsletters in the 1980s.
David J. Gall
From: main@Q-List.groups.io <main@Q-List.groups.io> On Behalf Of Jay Scheevel
Sent: Monday, December 16, 2019 8:10 AM
Subject: [Q-List] Bruce's tuft study on sparrow strainers
Thanks for the cool tuft study data, Bruce. My nerdiness must be rubbing off on you, or maybe you are just retired and want something time consuming to do.
Seriously, That is an interesting observation. Ideally, you would like your “trim” surfaces, such as the sparrow strainer to be well behaved under all flight conditions, but you have demonstrated that this is not the case. The sparrow strainer flies at a high AOA (negative) all the time, so as to apply a large load to the trailing edge of the elevator, push it into an accelerated condition and boom, you get it to stall. The need for them is entirely the because the aft-1/3 of the LS-1 airfoil has a significant camber on the underside. This camber is by design yielding favorable characteristics when incorporated on a non-articulated airfoil, but when you incorporate an articulated trailing edge (elevator), then the asymmetric force becomes torque on the torque tube and stick deflection instead of into a forward pitching component applied to the rigid wing structure. By countering this effect with the sparrow strainer you actually loose some of the positive characteristics of the pitching moment, and you add a lot of drag. If you could manage it as a pilot, and this is not recommended, the LS-1 would fly better and be more efficient if you could hold the elevator neutral with the stick. But this would require always be pulling lots of aft stick-force. The stick force would be very large in full cruise, and you would never find a hands off condition. It gives you an appreciation for the force that the sparrow strainer supports are experiencing constantly in flight, and why one might potentially depart the aircraft….requiring above average flying skills to recover.
I have modeled the forces on the LS-1 elevator and have also designed a modified LS-1 airfoil with the rear portion of the camber removed, where the forces on the redesigned elevator are balanced, (so the sparrow strainer would not be required). I have thought that sometime I would like to build a new set of these redesigned elevators and put them on my plane, but I am just getting comfortable flying with the plans design. On the redesigned airfoil, the lift profile and polars are essentially identical to the LS-1, but what is the big unknown is how the redesigned elevator would impact that stall behavior. Small changes at the trailing edge can be quite impactful on stall, and my modeling method cannot model the complexity of airflow at stall.
Not sure I really care to know. Either way I will still be aviating in both and won't put any more sound insulation in either one. (more weight)
On a side note I have been doing some tuft testing which might be more informational. I have found out that if I pull up the nose and then unload it by pushing forward on the stick my sparrow strainers are stalling on the bottom side. My TriQ will not recover in most instances until I stall out or slow down the airspeed a bunch so that I can pull the stick enough for the strainers to reattach the air underneath them. I just received some vortex generators to put on the bottom of the strainers to do more testing and will hopefully have them in place this week to test.
I have noticed in the past that when I do an aileron roll this happens. I have been all though the elevators and ailerons to make sure they were tight and they now are very solid. So then I thought that maybe the wingtip vortices were coming up from the canard and rolling down on the main wing. That never really looked like a good theory. So I installed yarn tufts to see just what was going on. When I flew it started to dawn on me that the elevator was coming up every time I experienced the phenomenon. So I had a good idea that something much simpler was happening and it kept pointing to the sparrow strainer. After talking with Paul Fisher, Jerry Marstall, Sammy Hoskins, and even Scott Swing at Velocity I decided we were looking at a stall. Sure enough when I went up with the tufts in place on the canard and the top and bottom of the sparrow strainers I thought "what if I just pull up and then push forward on the stick could I replicate the problem". When I pulled up and then unloaded the canard by pushing over the sparrow strainers held for a bit and then they relaxed to stick forward an inch or so. I looked out and saw the elevators up about and inch or so and videoed the tufts on the bottom of the strainer disappear and the tufts on the top tufts wrap around the back of the strainer and disappear forward under the strainer! I spoke with Paul, Jerry and Scott again and Scott was freaked but agreed that was what is happening.
Bingo! Now I have to fix it so that is where the vortex generator come in. The strainers are a really sensitive trim from dealing with them in the past plus I have them mounted half way outboard on the elevator. I also have 1.2 degrees incidence up on the canard and .8 degrees up on the main wing so we are talking apples to oranges.
Sure I could just fly the Q and not go negative on the elevator enough to stall them but I like to do aileron rolls when I am board. I believe you and I experienced the stall when you were getting time in my Q before your first flight. So we educate and experiment and come out improving the species.
If you have an iPhone it has an app available that measures abient noise level. You could give that a try.
Sent from my Verizon Wireless 4G LTE DROID
I have not noticed which one is quieter. Will try to do that in the future. Trouble is remembering how loud the other plane is between switches.
I will tell Charlie hello for you Dr Mike!
My RV6A is noisy. I have a Clark headset modified for active noise reduction but my new Zulu 3 is quieter. I have a Lycoming O-320 E2D 150 with a metal Sensinich prop. I have problem believing a Quickie 200 is quieter.