Volume III, Issue 3
August, 1998

I wonder if some of our Canadian colleagues would consider doing some research on the Canadian Cirruses to share with us? As you can see from the Directory, several Cirruses were originally imported into the US and then exported to Canada, while others were purchased directly into Canada. How about it, Roman, Tom Foote (I don't have his address) Dave?? (Note that we do have an interesting entry for No. 77, C-GUIL, now owned by Dick Vine, Karl Robinson, and Phil Backman.)

This issue includes updated sheets for No. 67 (N8169), No. 25 (N2791), and No. 101 (N7988). Please swap out these pages in your Directory.

Although Schempp-Hirth had made extensive use of fiberglass in the Standard Austria and SHK machines, the Cirrus was the first all-fiberglass machine they put into production. Klaus Holighaus, a former member of the design team which produced the D-36, directed the design of the Cirrus. The machine is all-fiberglass except for the foam cores used to make the sandwich material for the wing skins and the fuselage bulkheads, and the internal steel frame in the fuselage.

The wings illustrate an interesting compromise between goals of efficient thermalling and high speed interthermal flights. A rather thick Wortmann airfoil (FX 66-196/161) which has a very wide, low drag bucket is used. This yields low wing drag over a wide range of speeds, very gentle stall characteristics, and excellent thermalling performance without having to increase pilot workload with flaps. The slight disadvantage in high speed cruise from the thicker airfoil is offset by using ballast.

With the clarity of hindsight, it appears the Cirrus is to a significant degree "overbuilt". In the sixties, it wasn't clear just how strong these composites were and how long they would last - accordingly, the design was rather conservative. For example, the wing was static tested to 15 G's, and very extensive fatigue tests were conducted to simulate a life of 30 years (about 9,000 flying hours). Good thing, now that several of us are flying Cirruses more than 30years old!! The testing included a total of 6.5 million stress cycles imposed on the wing, running through load factors of +5.75 to -3.45 G units. After the fatigue tests, a static test at temperatures up to 129o F under design load conditions corresponding to a gust speed of about 140 mph.

In addition, the Cirrus incorporates a unique internal tubular-steel frame to tie the wings, pilot seat, and landing gear

together. This frame is bolted to the fiberglass shell and facilitates both the initial construction and any repair, since the highly stressed structure is steel.

The first prototype (see picture) is the only Cirrus with an all-flying V-tail. Klaus Holighaus flew the prototype in the 1967 German Nationals. It is believed to be still flying in Italy.

Schempp-Hirth produced a total of 178 of these fine craft, including the prototype. Of these, 25 were produced in Yugoslavia under license, and are referred to as the "VTC" or the "Yugoslavian variant". (The author has seen one of these ships, in the UK, and it differed significantly in appearance from the other Cirruses.)

The first of the regular production run (all made with conventional tails) to be delivered to North America was No. 4, N1194, to William Foley. (This is the ship that Gleb Derujinsky flew in the 1968 US Nationals, and is now owned by Robert Wicks of Burbank, CA. The current N-number is N49ME.)

Of the total Cirruses produced, one (No. 18) was originally imported into Canada, and 29 into the United States. Subsequently, one of the US ships (No. 23) was exported to the UK, and four (Nos. 15, 42, 69, and 77) were exported to Canada. (None of the Cirruses imported into North America was a VTC. )

Finally, four ships (Nos. 54, 58, 74, and 101) have been destroyed (another, No. 5, was "destroyed" but rebuilt!). This leaves 25 Cirruses in the current North American fleet, 5 in Canada and 20 in the United States, at this writing. Of these, the exact flying/flyable status of five (Nos. 42 and 69 in Canada, and Nos. 44, 64, and 84 in the US) is unknown, leaving 20 believed fying Cirruses on this continent.

References:

1. "German ATC for Cirrus", anon., SOARING, February 1969
2. "Cirrus Test Pilot Report", John Ryan, SOARING, July 1967
3. "The Schempp-Hirth Cirrus", William M. Foley, SOARING, July 1967
4. Selinger, Peter F., "Segelflugzeuge vom Wolf zum Mini-Nimbus"

The Second Annual Midwest VSA Regatta will be held Father's Day weekend - June 18-21, 1998 - at the Mid-America Air Center (Lawrenceville Vincennes Airport), home of the Wabash Valley Soaring Association. This is also the "home club and airport" of Cirrus No. 24, N3441, owned by Fred Fredine!

Peter King (No 83, N6663) [pcking@mindspring.com] writes:

"I wonder if there is a difference in accident rates between old and new gliders. The issue I have in mind is this. New birds have internal control seals. That has a dramatic effect on stall onset. Older birds, e.g., the Open Cirrus, do not. They look very much alike, but there are real differences internally. Read my article on mylar on the SSA web site in the old issues section. My bird had a vicious stall and wing drop when the ailerons were reflexed up 1/8". It became benign when the ailerons trailed properly in flight. But, the stall all but went away when I put mylar on the upper and lower control surfaces. Never mind that the sink rate went down and the L/D moved up. It will rumble, and it will mush, but it will not stall.

How many accidents have been prevented because wing roots are sealed, controls are properly set and moving control surfaces are sealed at the factory? "

Peter King has written and spoken often and at length about the rudder dampers in the Cirrus. In pulling together accident and historical data on the ship, I find more and more clues that Peter's concern is not only right on target but quite likely one rapidly getting more critical for the "Classic" Cirrus, many of which are now approaching 30 years of age.

For a long time I assumed that it was the crash of No. 58 led to the US Airworthiness Directive to check and modify, if needed, the elevator drive rod and the balance of the elevator. (No. 58 was destroyed in 1989 by a tail flutter that caused a breakup at altitude. Pilot Fred Blair survived.) My belief was based on the superficial review of the dates - the crash in 1989 and the AD in 1991.

But I know now that is not correct. The US AD references a Schempp-Hirth Technical Note dated April, 1982! Obviously someone knew something about flutter in the Cirrus long before No. 58 broke up.

Now, however, some comments in Soaring back in February, 1969 make me wonder if the rudder might be the more significant worry, particularly in aging gliders. In the anonymous article on page 9 of that issue, the author describes some improvements in the later Cirruses (after the first 40) as follows: "The rudder now has a dynamic mass balance to overcome the flutter tendency that became obvious on Harro Wodl's winning Cirrus [No. 8] during the World Championships [in 1968]. In addition, a damper has been installed in the rudder control system of the fuselage. And ... the trailing edge of the elevator has been extended by a few millimeters in such a way as to obtain a small camber of the chord."

While that cryptic comment is far from definitive, it makes one sensitive to the possibility that rudder flutter was known very early on and led to the installation of the damper. This gets even more critical these days, as these old dampers age and lose their effectiveness. I don't think we'll ever know exactly what "fluttered" on No. 58, but the suspicion that it was the rudder is growing in my mind.

Peter King wrote back in the fall of 1997 he'd actually inspected five Cirruses, including his (No. 83) and Walt Puckett's (No. 105) and found the rudder dampers inoperative.

I sort of accidentally discovered the same problem in my No. 67 in the early nineties.

It's my opinion that owners should check these dampers immediately. In fact, if there's no documented record of their being replaced in the last, say, 10 years, in my opinion it's one that should be replaced immediately. Peter reports that the damper is a steering damper for a Type III VW squareback, about $20 at most auto parts stores. Peter notes that the more familiar Type I (Beetle) damper takes the thrust off center and wears out the internal seals quickly. Also, the angle of the fitting on the rudder bar is not perpendicular to the damper pushrod and requires the head be bent. Not good.

Assembled storage: I know that at least three of us - myself (No. 67) , Peter King (No. 83) , and Walt Puckett (No. 105) enjoy the benefits of storing our birds assembled in a hangar. Does anyone else? I was surprised to learn from a fellow Club member who owned a LAK-12 (22 meter wingspan) that the factory recommends storing the ship in its trailer, and makes the case that storing it assembled puts undue stress on the wing pins and other wing-to-fuselage connections. When I learned that, I got in the habit of putting wingstands under my wings to at least partially unload the wings. Anyone have any good information on this? Suggestions about how much load should be supported?

Finishing: I've noticed that the original finish on No. 67 is in remarkably good shape (except for a slight yellowing) while the repaired areas deteriorate very rapidly in spite of reasonable care. Newer repairs have a strong tendency to blister, requiring frequent light sanding and polishing and rewaxing to restore anything like a decent finish. In talking with other Cirrus owners, I've discovered a wide range of experiences - - and I know at least one (No. 77) is either considering a refinish or is doing it by now. Therefore I was struck by a short note in Soaring, February 1969, page 9: "The paint used on five of the Cirruses [flying at the 1968 Worlds in Poland] built turned out to have something less than the moisture resistant qualities promised by the supplier. This defect showed up on Wodl's ship during the wet practice week in Poland, when small blisters on the wing and fuselage surfaces had to be rubbed down and resprayed. But this situation has been rectified through improved materials and methods." I guess modern repair shops that have worked on my glider have reverted to the poorer materials and methods used by the factory back in the sixties! Pity.

Over the lifetime of this "association", the mailing list has never been very disciplined. We try to make sure we include the hard core of current owners, but haven't been sending a copy to every listed owner in a partnership. A few people who are "special" in the Cirrus world (e.g., Tom Knauff, George Moffatt) and some who are "special" in the old glider world (e.g., Raul Blacksten) received copies of some issues, and the Directory (which was right expensive to produce initially.)

Now I wonder if we should expand or contract or change in any way the mailing list. What follows is a list of the addressees to THIS newsletter. If you would like to be removed from the mailing, please let me know - it would cut costs and make more information available to those who enjoy it. If you know those who should be added, also let me know.

Finally, if there are recipients who do not have a copy of the Directory, and would like one, let me know and we'll get you started.