The Standard Cirrus was designed when European manufacturers were
relenting from the FAI standard class terminal velocity airbrake standard.
Paul Schweizer, in his book Wings Like Eagles (pg.247), wrote:
"The dive-brake scandal came to light in the United States
when Graham Thompson wrote Eugen Hanle, owner of Glasflugel, ..., requesting that
he sign the SSA form certifying that the Standard Libelle conforms with the
Standard Class air-brake requirement."
"Hanle said that the Standard Libelle did not meet the requirements and that
the LS-1, Standard Cirrus, and the ASW-15 did not meet it either. He stated that
'[the] FAI requirement is nonsense, if the German LFS-requirement, which is
accepted by the FAA, only requires a 45o dive."
A flurry of letter writing followed. In a letter to Graham Thompson, on June 2, 1970,
William Ivans wrote:
"I am somewhat annoyed with Hanle for the arrogance displayed in his letter
to you of May 12th. Specifically, his argument that LFS/FAA approval superseded
the rules laid down by CIVV just isn't valid. ..."
"I am nearly convinced that all of the German manufacturers decided to
deliberately ignore the CIVV Airbrakes requirement, call their ships 'Standard'
nonetheless, and depend upon the number and popularity of ships sold, to bail
them out when a confrontation with CIVV appeared imminent."
"I am indignant for the Schweizers and others who have in good faith (and
no doubt considerable expense) met the CIVV Airbrake requirement, and for all
who have purchased 'Standard' class gliders from German manufacturers."
Nevertheless, Paul Schweizer concludes that:
"The original vertical dive requirements were excessive, whereas the 45-degree
dive tests used by the Germans may not have gone far enough, although they have
proved satisfactory in service"
Effectiveness of Airbrake Modification
People often ask how effective the airbrake modification is. Evidently, todate no accurate measurements before and
after the modification are available. However, everybody seems to think that the modification is very effective.
Derek Copeland comes the closest to having actual numbers to offer. He wrote:
Some years ago we fitted the double panel airbrake mod. to a Cirrus
75. Before fitting, the sink rate with full airbrake at 50 knots
approach speed was about 5 knots, glide angle 10:1 in still air.
After fitting this went up to about 8 knots, glide angle about 6:1.
By the way these measurements were not done very scientifically and
were just based on the vario readings. In practice it made a huge
difference to the glider's approach control.
Technical Note and Comments
Technical Note 278-32 describes a modification to Std. Cirrus airbrakes to enhance their
effectiveness. It appears that when the panel of the standard airbrake lifts
above the wing surface, air flows beneath and its effectiveness stops at that
point and may even diminish if opened further. The modification adds a second
panel that plugs the gap, so that the effectiveness continues to increase until the
airbrakes are fully deployed. This is an issue of concern to all Std. Cirrus pilots, and
everyone wants to know just how much the modification helps. One owner who has run tests
reports the following:
Before installing the doubler plates on my Std. Cirrus, SN 289G, I made a test
from 15000ft to 4000 ft at 52kts indicated, gear down and spoilers full
open. The average calculated L/D was 10.2 to 1. The doubled brakes, tested
under virtually the same circumstances one year later yielded 6.4 to 1. Hope
that is some help.
It involves adding an aluminum plate that blocks air flow under the
original airbrake panel. The cost in 1996 was $530 (USD). The kit comes with good instructions.
Jeff Melin submitted the following photos of his installation on #345G:
#345G Airbrake Mod, Front View
#345G Airbrake Mod, Rear View
#345G Airbrake Mod, Front Detail
#345G Airbrake Mod, Rear Detail
Following are Jens Aarnaes' comments on his installation of the airbrake mod:
I did this over the course of a day with the supervision of an experienced glider A&P.
The first thing I did was to inventory all parts and verify the kit's completeness.
Fortunately, all parts listed were present. I then assured that I had every tool I
needed, including metric measuring devices, a Dremel tool, various metric wrenches,
all the drill bits, grinding bits, etc. and Loctite.
The actual installation does not require disassembly of the spoilers to complete.
I simple extended them and held them in place by hand while working on them.
I proceeded to measure and lay out (in pencil) all part and cut locations on the
spoiler boxes, the spoilers and their actuation tubes and then had my A&P verify
that I had done this correctly prior to beginning any cutting (measure many times,
The first installation step is to grind notches into the trailing edge of the
spoiler boxes to provide clearance for the bolts which hold the spoiler plates
in place. This was a bit nerve wracking as you are cutting into your ship and
any mistakes here would be challenging to rectify. I had planned to use my Dremel
tool for this but my A&P offered his pneumatic "die grinder" which markedly sped up this task.
The first parts I installed were some plexiglas "wedges" which screwed directly
to the fiberglass spoiler structure. These act as guides for the plates and
prevent their hanging up on the existing spoiler structure. Installation consisted
of marking holes (two for each guide), drilling and counter-sinking them and then
screwing the "wedges" into place.
The next step is to cut recesses in the TE of the spoiler fiberglass (the "U"
shaped under-structure, not the surface) to accommodate the clamps which the
bolts noted above attach to. This, I did with the Dremel tool as I felt that
I had more "deft" control with this than with the die grinder and the material
is easily cut.
The final step is to bolt the attachment clamps to the spoiler actuating tubes,
while simultaneously bolting the spoiler plates to these clamps (The plates
attach to one of the two bolts which close the clamps around the tubes).
After some adjustments (some additional grinding of fiberglass was required),
the installation was complete. Total time was around 8 hours. Remember, I am
slow and methodical. I think my A&P could easily have installed this kit in half the time.
In my first flight test with the "new" spoilers I have noted that the spoilers
are quite a bit more effective with little noticeable change in the forces
required for deployment. I would now rate my spoilers as "good" where before
I would have hardly have rated them "satisfactory". Using my normal, high
approach I was able to put it on the numbers with no slipping. In fact,
I didn't even use full spoilers at the end of my approach because they were too
effective. A VERY POSITIVE CHANGE! So far, the only "negative" I have found
is that with the spoilers being effective throughout their deployment range,
I now can actually use them when fully deployed. Before the modification, full
deployment only decreased their effectiveness due to the gap which opened between
the bottom of the spoilers and the top of the wing. Herein lies the "problem".
I have longish arms and now with full spoiler deployment, my arm is jammed into
the seat back. I'll have to rearrange some things in the cockpit to make room for this.
--- Jens Aarnaes
Documentation (click for larger image)
English Translation of
Last Two Pages (part 1)
English Translation of
Last Two Pages (part 2)
Technical Note 278-15 added the following text to the Standard Cirrus
Adjustment of the airbrakes
If the top of the airbrakes should open at high speeds, check at first the
wings for excessive tangential backlash and eliminate it if necessary.
Adjust the airbrake control rods thereafter.
Before the airbrakes are fully retracted the spring-loaded top of both brakes
must have the same distance to the upper wing surface to warrant an equal spring load
acting on the top. If an adjustment of the distance to the wing surface should
be necessary, turn the control rod connector of one wing about one rotation --
turning in means extending the brake. If one rotation of the connector is changing
the length of the rod too much, keep the connector in its original position and turn
the tube using a pipe wrench until the correct distance to the wing surface is attained.
Adjust then the force required to lock the airbrakes, i.e. to tighten their
seating by equally changing the length of the control tubes of both wings, equally
shortening means a decrease and equally lengthening an increase of the locking force.
The locking force is correct if a pilot's effort of 15 kp to 20 kp is required
to close the brakes, while the operating rod in the cockpit should overtravel the
center lock about 10 mm. A shortening of the cockpit rod means a greater travel
over the center lock.
Never adjust the airbrakes too tight by which in course of time a damage of the
tube connector catches and the balls may occur.
NOTE: 15-20 kp (killopond or kg-force) is 33-44 lb.
After refinishing #60, the left airbrake developed a habit of sticking closed
when there was a positive load on the wing. A query to Schempp-Hirth cleared up the
"...we suppose (from experience of similar cases), that the airbrake top at the
outer end touches too close the wing shell and jams when opening. Make sure, that
the gap at this point is large enough, specially when the wing is bended."
"Because of a certain elasticity of the airbrake mechanism it is normal
and wished for, that the top plate touches the wing surface at the outer
first and a bit earlier than the inner section, in order to obtain an equal
pressure downward all over the airbrake length. ..."
Sure enough, it turned out that the airbrake was hung, outboard end down, for
painting and excess paint had collected there. This was enough to cause the problem.
A little sandpaper applied to the outboard end solved the problem. It's hard to believe
that a little excess paint can have such an effect. But, when the wing bends upward, the air
brake cap slides a little outward, reducing, and possibly eliminating, the end clearance.
If contact is made, enough force can result to effectively lock the outboard end
of the airbrake.