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BackgroundThe 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:
A flurry of letter writing followed. In a letter to Graham Thompson, on June 2, 1970, William Ivans wrote:
Nevertheless, Paul Schweizer concludes that:
Airbrake Modification
Effectiveness of Airbrake ModificationPeople 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:
Technical Note and CommentsTechnical 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:
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:
InstallationFollowing 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, cut ONCE!). 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)
Airbrake AdjustmentTechnical Note 278-15 added the following text to the Standard Cirrus Service Manual:
NOTE: 15-20 kp (killopond or kg-force) is 33-44 lb. Locked AirbrakesAfter 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 mystery.
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. |
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