nimbus

Nimbus Elevators and Horizontal


You can see the mod done by Dick Johnson to the elevator in the photo above and below. He used balsa and ceconite to fair over the original undercamber in the elevator. This was done to decrease the nose-up stick force at higher speeds. A small balsa trim tab was added to the trailing edge that restores some of the nose up trim - Dick told me he added that wedge to provide the amount of hands-free stability that he liked. Since the fabric is almost 20 years old now, I plan to remove the balsa and ceconite composite and do the mod with carbon fiber and foam. The challenge will be to keep it light and in balance. Temporary N numbers because I removed them from the vertical during the re-finish.

Both elevators have the small balsa trim wedge for about half their length attached to the trailing edge. I'm thinking about leaving some of the camber in to provide the same amount of nose-up trim that these wedges provide - but how much?



good photo of the balsa rib and fabric covering.


Another photo of the modification. I have to say that this aircraft really handles well and I would really like to leave it just as it is - but having that fabric come loose in flight would not be much fun.


After removing the elevators, close inspection revealed the structure to be in excellent condition. The only real problem is the gel-coat crazing and cracking over the entire lower surface of the horizontal. The trailing edges of the elevators also started to liberate some gel-coat due to cracking. The elevators have no balance weights, so I'll be interested to see where the moments are in its existing condition. They're carbon fiber so they're very light.


I decided to use Poly-Fiber super-light filler. I'm not going to take all of the undercamber out, I'll just leave a smidge to restore that little bit of positive stick feel that Dick created with the balsa wedge he added. First step is to sand all the gel-coat off the surface between the ribs.


4 oz. of Superfil spread on to fill the voids between the ribs and gaps where the foam didn't meet the lower surface profile. I used the large drywall knife to get a flat surface. Next task is to sand it down to the final undercamber shape and prime.


With the foam and Superfil, the elevator weighs 32.4 oz. - one ounce more than the original, as removed condition with the old gel-coat and fabric on. Some of this weight will be sanded off, but it still needs painting, so it will be interesting to see if the un-balance moment falls within limits without any leading edge weights.
First coat of gel-coat reveals small imperfections that grow into large imperfections when sanded down and filled with more SuperLite filler. Sand it again and spray more gel-coat.
After the second coat, it's ready to have most of it sanded back off to provide a very thin layer (.016"), you can see the slight amount of up-trim I left in the trailing edge (red arrow) to replace the balsa filler wedge Dick had glued in place to provide the stick free stability that works so well. This elevator is upside down, so that up-trim really is up-trim.
30 January above - RH elevator (on the left in the photo) is almost done, the gel-coat is sanded flat and it needs one more light coat and final sanding. In this condition, it weighs 31.5 oz. well within the overall weight limit, but will need leading edge weights to get the un-balance moment within limits. The LH elevator (on the right in the photo) has a first application of SuperFil filler (the blue stuff), a second application of filler will fill in the voids and form the lower surface.
Tabby Cat helps out with the balancing using his left rear paw.
17 February - The bad news is the elevators are just too heavy with gel-coat applied to both upper and lower surfaces. Even without the required balance weights, they exceed the maximum overall weight limit. so no choice but to sand all the gel-coat off again and paint with white polyurethane. Both elevators sanded and primed, the one on the left gets a final wet-sanding before priming. This is disappointing and time-consuming, but I live in fear of flutter and these surfaces must be done properly.
13 March, and the elevators have a coat of white Aerothane (left over from 525's ailerons - thanks Dave). With that coat of paint comes a need for balancing. Calculations reveal two rods of steel, epoxied to the leading edge in accordance with the instructions in the Schempp-Hirth maintenance manual will do the job nicely.
The rods are sanded down to give the epoxy a nice grip, laid into a bed of Epon 828 mixed with colloidal silica, the silica really adds strength to the bond, and is covered with a layer of 1 oz. crowfoot cloth, all per the maint. manual instructions.
The finished elevator, shiny white, undercamber removed and within balance limits. That was a lot of work!
Again, here's the small amount of up-trim built in to replace the balsa wood wedge.
Below - re-finishing the horizontal stabilizer.
Elevators removed and the horizontal is set up for sanding. I found I need to sand all the gel-coat off the get to the bottom of the cracks. I'm using a 30x magnifying scope from RadioShack, $6 and it's great for looking at cracks and sanding marks. Most of the dust goes out the tube, the rest gets exhausted by the big fan.
Set up in the furnace room because the rudder was just gel-coated and is drying. This is the lower surface, I'm not going all the way down to the carbon-fiber on the edges because the scope reveals no cracks and there's a lot of filler in those areas of the leading and trailing edges that provide the correct profile.
The gel-coat is very thin on both upper and lower surfaces of the horizontal - much thinner than the rudder and it takes very little sanding to get to the structure. I'm very careful to keep the sander moving because sanding the gel-coat creates a lot of localized heating and if I leave the sander in one place, the surface temperature of the skin gets very hot and I don't think that's very good for the structure. I keep the sander moving and I don't sand into the carbon-fiber - just enough to get the white gel-coat off and that's it.
Gel-coat sprayed on and blue dye painted on. Ready for sanding. This is a big surface and it will take a lot of sanding to get it down flat.
New Year's Day and I'm sanding the lower surface down flat. I've already filled some pinholes with a small brush and some 2481 that remained after he last spray application.
Final contour sanding with a piece of Plexiglas glued to two 1x2 pieces of wood. This will insure a smooth contour with no bumps, undulations or high spots.
With the first coat of 2481 sprayed on, sanded down and all the pinholes filled in and sanded, I carefully sanded the entire surface to insure proper contours and airfoil, now I spray on a coat of 2781 (above) while Dave Piotrowski takes a photo of me wielding the mighty gel-coat gun.
I really like the 2781, flows out very nicely and leaves just a very few pinholes.
January 20th and finally it's time to sand it down through 1200 and polish. With luck, I won't have to spray any more gel-coat on this surface.
27 January and the horizontal is close to being done, it is sanded through 1200 and polished. After polishing, it's easier to see where I didn't get all the sanding scratches out, so I'll have to go back and sand those areas again with the 600, 1200, 1500 grit again, then polish again. In the background are the foam polishing pads, 3M gelcoat polishing compound and a jug of WX-Block from W&W.
after a trial fit, I find I must remove a few thousandths of gel-coat on the lower surface to get the stab to fit into the mounting hole. In the photo above, you can see where the stab is rubbing on the vertical stab close-out rib and preventing proper engagement of the forward mount tab. Some sanding with 220 in the blackened areas gets it down to where the stab fits nice and snug.
Looking much better these days, the rudder still needs some fine sanding to get all the scratches and the right side of the vertical needs a little sanding, some gel-coat touch-up and final 1500 sanding and polishing. The elevators are the main focus now, then I'll put the fuselage in the basement and start on it. a 60°F day on the 27th of January in CT and I feel like I'm running out of time before spring arrives!
March 29, 2002 and new contest numbers are applied.