Sealing the Schweizer 1-35

Shown is Dale Thompson; a long time owner and who provided some great sealing tips.  Many thanks Dale!


Original 10 April 2004. Updated 18 April 2004


First off I want to thank the many SGS 1-35 pilots who provided me some tips on what they have done over the years to seal up the 1-35.  Most tips came from current owners, but some tips came from prior owners, and I really do appreciate all the information you guys have provided.


Each year my goal is to do a little bit better in soaring.  This includes improving my skills, my knowledge, and my glider.  Since buying my 1-35C in early 1998 I did not really do much to improve it’s performance.  The only changes I made early on was to add a good radio, add an audio vario (Tasman), and add mounts for both a Garmin 90 GPS and a PDA.  These all allowed me to fly safer and with more information, and combined helped me do some cross-country work.  I almost finished 2 Diamond Distance flights in 2003, but landed a bit short each time.  It isn’t supposed to be easy – and it isn’t.  Perhaps had my 1-35 been a bit better at glide I could have made it (??).


Late this winter I started to look at what could be done to improve both the performance and the noise level in the 1-35.  In reading George Moffit’s book “Winning on the Wind” he constantly suggests that it is all the little things added up that makes the difference.  In his flying, he tries to shave a second or two off each thermal turn, adding up to many minutes saved over the course of a day.  He keeps his glider is perfect tune, providing him the most advantage he can achieve at the time and with the technology at hand. 


With a factory glider like a SGS 1-35, there are not many big things you can legally do to get more performance, and the 1-35 is already a pretty well designed glider representing the best of 15m metal – but a bit more can be done with little bit of effort at at virtually no cost.


So with this inspiration, I set out to do a lot of little things this spring to seal up my glider.  Each one likely will only yield a small contribution to reduced drag or noise, but perhaps in total they will make a noticeable difference, and spawn more enjoyable soaring.


Here is the list I have pursued, a “12-step program” of sorts.  I will cover each in a little more detail further on:


  1. Seal the 16 small assembly/inspection holes in the rear drag spar [Easy and highly effective] – DONE!
  2. Seal the canopy-to-fuselage gap [Easy and highly effective] – DONE!
  3. Seal the canopy-to-turledeck gap [Easy and highly effective] – DONE!
  4. Seal the towhook opening [Easy and partially effective; mainly keeps feet warmer] – DONE!
  5. Seal the turledeck-to-cockpit opening [Easy and highly effective for noise reduction] – DONE!
  6. Seal the aileron pushrod opening at the root rib [Medium difficulty and highly effective for noise reduction] – DONE!
  7. Seal the flap torquerod opening at the root rib [Medium difficulty and partially effective for noise reduction] – DONE!
  8. Seal the inspection holes in the rootrib spar [Easy and partially effective] – DONE!
  9. Seal the mainspar-to-rootrib gaps (front and back of spar) [Medium difficulty and partially effective] – DONE!
  10. Seal the Flap Hingepoint (bottom surface) [Easy and highly effective] – DONE!
  11.  Seal the Aileron (Top surface) [Easy and highly effective] – DONE!
  12.  Retape the Turtledeck-to-fuselage gaps [Easy and highly effective] – DONE!


Other possibilities: (I likely will not do, but we’ll see!)

  1. Mylar seal the Flaps (Top surface)
  2. Mylar seal the Ailerons (Both Surfaces)
  3. Mylar seal the rudder
  4. Mylar seal the Elevator
  5. Add a Flap-to-aileron dam
  6. Add a fairing to the tailwheel


OK let’s cover each of these:




Interestingly, I never noticed these holes before but they are all the way down the flap and aileron gap on each wing.  Due to the low pressure on the top surface of the wing while flying, air in the wing gets sucked out of these holes and up to the wing top surface causing additional drag (not good!) and poorer roll authority.  The air in the wing is likely replaced with air entering the wing through either the fuselage (not good!) or through the flap or aileron pushrod exit ports (better).  Sealing these assembly holes is very easy using either tape or with a dab of silicon rubber.



I used clear silicon rubber (Dow) as shown.  Apply enough so that it has enough of a surface to grab on to.  Bill Berle feels this is the #1 improvement you can make to improve a 1-35’s performance!!  WOW!





A poorly sealed canopy is a real killer of performance, and is relatively easy to solve.  The 1-35 canopy has a built-in soft gasket that is suppose to seal well to the fuselage, and I’m sure it did originally.  After 25 years you can bet it doesn't seal very well at all anymore.  As a test, I placed a flashlight in my cockpit at night, and shut and latched the canopy – YIKES! – the gaps were embarrassingly huge!  Light leaked out over half of the perimeter.  This had to be fixed. 


I first planned to just dig out the old gasket from the canopy and glue in a new one, but the existing one was pretty tough to dig out and I wasn’t sure what I would replace it with.  I then thought that even if I replaced it that it likely would get beat-up again since my canopy is removable (not hinged) and routinely rests on the ground.  I thought it smarter to put the seal on the fuselage surface where it would get less damage and would be easy to inspect and replace.


After looking at some weather-stripping type foam seals at Home Depot, I bought some closed cell (does not let air through) ¼” thick stuff that seemed workable but turned out to be far too thick for this application (canopy wouldn’t close).  I ended up using some nice “Wing seat tape” used by R/C modeler’s which is also closed cell and only 1/16th inch thick.  It comes in Black and is shown in the picture above.  Like the weather-stripping stuff, it has adhesive backing on one side so it is a snap to apply.  I bought it here:  It is $1.29 for 3 feet, and you’ll need 9 feet for this section.  [Part number LXK197].  I applied it, tested it, and NO MORE GAPS!





Here I used the Home Depot 1/4th inch thick weather-stripping mentioned in #2 above.  The canopy closes “OK” but it is a bit tight.  I may have to use the 1/16th inch thick stuff for the side turtledeck pieces and use the 1/4th inch thick stuff just for the top turtledeck piece – or may have to use the 1/16th inch thick stuff all the way around.  The heat of the summer will cause the canopy to expand and so I have bought enough of the black stuff to use all the way around if the canopy is too hard to open/close with the thicker gray stuff in use.  Either way, the canopy is now COMPLETELY SEALED!  YES!





OK – first let me say that the chin of my glider is an embarrassment I know – but can you believe I have thoroughly cleaned this??  Here’s the story – in May last year I landed-out in a newly plowed field and the nose dug in several inches as I ground to a halt.  The gravel in the soil just scoured deep into the chin paint causing some unhappy cosmetic damage.  It looks worse in this picture as this is just a small area showing.  Anyhow, back to the towhook hole.  My towhook is the retractable kind, and whether it is “out” as shown, or retracted, it still lets in a fair amount of air.  As a first try, I have taped up the rear half just behind the pivot point (hard to see) plus aligned tape to run very close to the edge of the towhook cover when it is closed.  It isn’t perfect but it is much better than it was.  I did look at sealing the towhook from the cockpit side but I wasn’t confident that I could do anything that would still allow the rotating towhook to work reliably. 




  Several pilots have mentioned that a seal here really cuts down the noise into the cockpit area.  I simply made a cardboard template (half-moon shaped) using the inside of the turtledeck (behind the pilots head) as a guide to derive the size and shape to cut a 2” soft foam piece.  I had some leftover “Memory Foam” lying around and it was perfect for this job.  I simply cut it to shape and used double sided tape to adhere it to the metal “Y” framework of the turtledeck just behind the pilot’s head.  I wanted the foam to stay in place because a loose piece could foul-up the aileron mechanism just inches away. The lower portion of the foam will be in front of the main spar so this is added protection for it to stay in place. The double-sided tape seems to hold well enough but I plan to check it often.





If you look at the root rib, there are 4 areas where air can migrate between the wing and the fuselage.  The main one is the 3” aileron pushrod opening, the 2” flap torque-rod opening, the dual gap between the spar and the root-rib, and then several assembly/inspection holes. 






The aileron hole requires a fabric cone be made that is glued to the root rib using contact cement and then cable-tied to the pushrod.  I used some chamois material as it is sealed but had some stretch to it – others have used parachute material.  After some careful planning and measuring the fabric cone is built and installed as shown.  Care must be made to allow no restriction in pushrod travel due to this add-on.  Make sure you fully inspect your system prior to each flight.  This seal also should keep the majority of the wing noise from reaching the fuselage – COOL!





The Flap torque-rod is a bit trickier since it needs to be free to center itself and to rotate 90 degrees.  The only idea I had was to use some rubber material (from a thick rubber glove) and cut it to shape.  By undersizing the hole in the rubber material it fits snug around the torque-rod providing a pretty good seal.  I created a template then cut the rubber material to size.


The rubber glove was black rubber on the outside and a white fabric type surface on the inside.  Since I wanted the rubber to glue to the root rib, and the rubber to stretch over the rod, the material is glued “white-side-up.”  I used silicon rubber to glue this in place and it seems to be a good fit.  Again – inspections are needed to assure these changes have lasting integrity.  I found you have to wrap the material along the bottom and glue it to the rib flange as there is not enough material on the root rib in this area to get a proper seal.  There are also rivets there that requires enough silicon rubber material to overcome these bumps in the surfaces in order to get a good seal.








Like the drag spar assembly holes (#1), these root rib holes can be sealed with silicon rubber or tape.  There is one large hole that I used tape on; the others have a dab of silicon rubber. There is also a small gap right at the leading edge you can also seal up with silicon rubber. Check for other similar gaps in the root rib and fill them all.





There is a 1/4th - 3/8th inch thick gap on each side of the spar allowing air to flow from the wing to the fuselage.  I decided to use some closed cell foam sheet I had to build up layers of a “plug”, custom fit to each gap.  This took some time.  I then silicon glued each piece carefully into place making sure the silicon “gasketed” the edges and surfaces of each foam piece to form a seal.  I am happy with the results.



This was just a redo of the old tape already there.  After removing the old tape and cleaning up the surfaces, I ran one piece of 1” wide Bowlus Supertape span-wise, then overlayed it with 2” wide clear 3M plastic tape spanwise.  Make sure the flap is fully “up” when you do this so that full motion is maintained.



In the picture here I show the original 2" clear tape which was beginning to crack in the seam after several years of use. I redid it this time by removing the old tape and cleaning up the surfaces, then I ran one piece of 1” wide Bowlus Supertape span-wise.  Make sure the aileron is fully “down” when you do this so that full motion is maintained.  While working that end of the wing, make sure the vertical wing tie-down holes are taped (top and bottom) as well as the wing wheel holes (bottom).  The wing should be sealed about as good as you can get it if you did all associated steps above.




No pictures on this one, just a redo of the old tape already there.  I ran one piece of 1” wide Bowlus Supertape along the adjoining surfaces on the turtledeck and the fuselage.  This prevents the paint on each surface from being pulled-up due to repeated pulls of gap seal tape after each flight.  After final assembly on flight -day, run pieces of 1” wide Bowlus Supertape across this gap.  This greatly improves performance and noise reduction.  Always use the same type of tape throughout as if 2 different tape types (Bowlus and 3M for instance) are used they will expand and contract at different rates and either the gap tape or the stationary tape will come loose.  I learned this lesson the hard way!



Well that’s it.  It is still too cold to fly up here in Minnesota at present, but me and my trusty 1-35 are ready.  I will report back on my impressions of this “12 step program” above in a few weeks. I expect the noise reduction to be fantastic and the glide performance to improve maybe 5%. I have successfully used SeeYou to roughly measure the L/D at various speeds in calm morning-air conditions, and my glider is at 34/1 at 52mph currently. 36/1 is the factory rating with an optimized CG for the 1-35C. My 200# carcass shifts the CG forward enough that I lose a bit of performance vs the factory rating. I'd be happy with getting to 36/1 at 52 mph, and to get a flatter glide performance at higher speeds where the sealing likely will really show it's worth. [Drag goes up with the square of velocity]. With my new Colibri logger, my performance comparisons will be better as well since it records the more stable pressure altitude (GPS altitude is unstable even at ground level - see the Colibri link on the main 1-35 page for a discussion on this).

  Anyhow, stay tuned - and feel free to email me if you have more tips! [] ................. TOM


Well - I flew the sealed up 1-35C for the first time on 4/17/02004 at Stanton Airfield; a blue day but I managed over 3 hours as did others. I am happy to report that the sealing efforts have made the 1-35 almost dead-silent, WOW!. The only noise comes when you open up the vent or the window for cooling. When these are shut it is amazingly quiet and I'm thrilled! It is easier to relax when you are not distracted by every little rattle or leak. The items that likely had the most impact on the noise reduction are - canopy seal, turtledeck "headrest" foam block, and then the aileron cone seals. Below are some preflight pics of some of the items discussed above. They will give you a better sense of how things look as assembled. One note I want to make is I was a bit suprised at the motion the airleron pushrod "sweeps" while swinging from one extreme to the other. No wonder the opening on the root rib is so big. The key here is to assure this sweep does not stress the cone seal as it might come loose over time, and secondly, if you use a cable-tie to tie the cone-seal to the aileron that you position the head of the cable-tie in a position that will not snag on the root hole edge. I positioned my cable-tie head on the backside of the pushrod and it clears well. Yours may be slightly different. I also want to point out that the double-sided tape I used to hold the turtle-deck foam block in place does not hold very well to the foam. To avoid problems make sure you size your foam such that a good length of it is wedged in front of the spar and that it is not going to move on it's own while wedged (make it thick enough). Mine is about the right size and shape but I plan to find a better way to attach it to the turtledeck, maybe just need better double-sided tape (like automotive trim tape).

The post-flight inspection of all the seals shows that they have held up well after 3 hours. The flap torque-rod seal looks like a keeper. Study these pictures and good luck on any sealing projects you may have underway. I will now try to measure some performance aspects but that might take a while.

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Updated 18 April 2004