Re: Preliminary analysis of aerodynamics of sparrow strainers (or not)


David J. Gall
 

Jay,

 

Nice analysis. I’m heartened to see that someone else has independently concluded what I’ve been saying for years: the Roncz R1145MS airfoil is a better choice than the LS-1. The Roncz airfoil also solves other issues such as eliminating sparrow strainers. I think Rutan broke off his involvement with QAC over this very issue.

 

I note that you’ve asserted that the elevator floating angle of the Roncz airfoil would require mounting the canard more nose down by one degree. I believe this is incorrect. The elevator floating angle you found is not intended to be achieved at “high speed cruise” as compared to your first illustration on the LS-1; rather, the elevator floating angle is intended to give a *low* speed cruise so that in the unlikely event of a total mechanical disconnection of the elevator from both the trim system and the control stick the airplane could still be flown to a destination or diversion airport (long-range cruise) yet that trimmed speed would also be slow enough that a semblance of a “high speed landing” without elevator control could be survived on arrival, using only throttle for climb/descent. See Rutan’s CP59; also see the fate of George Mead’s Piper “Pugmobile.”

 

So I would recommend mounting the Roncz canard at the same angle of incidence as the LS-1, not a reduced angle of incidence. In flight, the “down” trim spring should be able to hold the needed nose-down elevator bias to achieve hands-off trim at high speed cruise, and the “up” trim spring should be able to hold the needed nose-up elevator bias for slow flight (but not stall).

 

Besides having significantly lower drag than both the GU and LS-1 airfoils, the Roncz drag is lower still compared to the LS-1 airfoil because of not needing sparrow strainers and also because of not having that crazy negative lift zone in the spanwise lift distribution that is caused by the LS-1’s sparrow strainers.

 

Another benefit of the Roncz airfoil is that it is almost as thick as the GU airfoil (20.5% vs. 21% for the GU). Structurally that makes it almost a direct replacement for the GU canard, not needing the tubular carbon spar but able to be built using glass and a slightly modified GU canard layup schedule (think: Waddelow canard). For a Q1 Quickie or for a Q2 (max. gross weight 1000 lbs.) it could be a direct replacement; for a Q200 the added engine weight (MGW 1100 lbs.) would require additional structure. Of course, with some of these planes currently operating at 1300 lbs., all bets are on a structural redesign using carbon.

 

Finally, Mike’s point about the lower angle of incidence on the canard is actually countered by the elevator deflection stop limits; one would run out of elevator deflection before achieving stall if the canard incidence were set too nose-down. However, it would be prudent to check the maximum CL and the AOA at which that occurs when selecting the canard installation incidence angle. It might be necessary to shorten the canard chord slightly if the new airfoil is a significantly better performer than the one it replaces, in order to prevent a main wing stall. (Trimming the chord is preferable to trimming the span because the resulting surface has a higher aspect ratio that gives it a steeper slope of the lift curve, so it achieves CLmax at a lower AOA than a lower aspect ratio surface would.)

 

Keep up the good work,

 

 

David J. Gall



 

From: main@Q-List.groups.io <main@Q-List.groups.io> On Behalf Of Mike Dwyer
Sent: Sunday, February 27, 2022 4:15 PM
To: main@q-list.groups.io
Subject: Re: [Q-List] Preliminary analysis of aerodynamics of sparrow strainers (or not)

 

With a lower angle of attack on the canard, wouldn't that reduce the margin of having the canard stall prior to the main wing?

I'd still like to see a better sparrow strainer design.  Move it into the airstream and make it operate in a "not stalled" condition so it could be smaller...

Mike Dwyer Q200

Great work Jay!

 

Q200 Website: http://goo.gl/V8IrJF

 

 

On Sun, Feb 27, 2022 at 6:37 PM Jay Scheevel <jay@...> wrote:

Let me preface this with the statement that the current design of sparrow strainers works. You should not deviate from that design. The following is purely theoretical at this point and is not flight proven.

 

Here are my observations so far.

At 2 degress AOA, which is about where the Q2 is at high speed cruise, the standard LS! Airfoil produces a Cl of 0.461 and Cd of 0.0191 as shown below

In order to reverse the effect of the elevator deflecting upward in flight, sparrow strainers are installed. As a result, the span that contains the sparrow strainers ends up with an overall NEGATIVE lift coefficient, Cl of -0.333 and almost double the drag of the clean LS1: a Cd of 0.0317  (shown below). The extreme pressure field of the sparrow strainer, because of its high angle relative to the streamlines, actually kills the lift of the LS1 airfoil over the length of span that is occupied by the sparrow strainer. The result is a negative lift coefficient over that portion of the span. So roughly 1/8 of the total canard span (the portion of the span with the sparrow strainer installed)  is actually providing negative lift at an AOA of 2 degrees, and is creating lots more drag than it would if there was no sparrow strainer present.

 

The catch 22 is that we know the sparrow strainer is necessary for the LS1 profile, as we can demonstrate as follows:

 

If the sparrow strainer is omitted and no stick force is applied (hands off), the elevator portion of the canard will tend to reflex up in flight. But, how much would it reflex up? 

 

Well, it looks like it would reflex up 20 degrees before the pressure field is in equilibrium on top and bottom of the elevator. This would result in the overall “hands off” configuration having a negative Cl = -0.420 over the entire span occupied by the elevator. This would result in a severe “tuck”.

 

If the elevator portion of the LS1 is redesigned so as to not deflect up or down in flight and to be in trail throughout most of the AOA range, then the a result is shown below. This design should acheive a “hands off” Cl of 0.434 (slightly less than the bare LS1 of 0.461) and a Cd is 0.0198, very slightly higher than the bare LS1. No stick force or sparrow strainer would be required for this elevator profile if it performs as modeled, so the draggy and negative lift portion of the span would not be nescessary.

 

But what about a different airfoil, such as the the  Roncz 1145MS airfoil? R1145MS would result in the same Cl as the modified-elevator LS airfoil, but at a lower Cd of 0.01393. As shown below.

However, the Ronz airfoil, if fitted with an articulated elevator in the Q2 configuration, would tend to see the elevator deflect downward before achieving aerodynamic balance with no springs, stick force or sparrow strainer applied. Equilibrium would be achieved with the elevator deflected downward approximately 2 degrees as shown below. This deflection results in a higher Cl and would require that the overall airfoil be installed 1 degree nose down to the LS1 to perform aerodynamically similar to the LS1 equipped Q2. Doing so, would  yield a Cl of 0.458 and Cd of 0.01372 at an AOA of 2 degrees.

 

So we would mount the Roncz airfoil 1 degree lower angle. If we were to do this on a Q2, then the Cl would be similar to the LS1 airfoil, but the drag would be about 30% lower than the LS1: Cd 0.014 at and AOA of 2 degrees.

 

Note that none of these models include the effect of a turbulent stall bubble on the trailing edge of the elevator, so the numbers would be slightly different on a flying aircraft.

 

In summary, we see that the Roncz 1145MS airfoil would probably have been a better choice than the LS1 for a Q2, but it may not have been available at the time the decision of QAC was to go with the LS1. We can also see that the sparrow strainer solves the problem of preventing the LS1 elevator floating up in flight, but at a great drag cost and a reduction in overall canard lift over the span to which the sparrow strainer is applied (roughly 1/8 of the total span of the canard) with the 11.5” sparrow strainer, higher if using the 17.5’ sparrow strainer).

 

As shown above, it is possible to design a more benign elevator shape for the existing LS1 canard that will solve the problem of the elevator floating up in flight without the need for a sparrow strainer, but this solution is not as efficient with respect to drag as would be use of an entirely new airfoil as was done by John Roncz for the Long EZ.

 

This is my analysis so far. I will continue to work on this as time permits.

 

Cheers,

Jay

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