GATVC; seat & harness, wheels & brakes, & instrument panel

[JimCrawford]

I have, at last, enough time to sort out the permit renewal on GATVC and I thought I'd tap into the font of knowledge that is the Nipper Forum. There are a few issues I've been pondering and would appreciate comment from those that have experience and ideas, GATVC is a Slingsby Mk111 with no mods.

Seat and Harness.

I find the seat intensely uncomfortable. This is because the seat back ends at the top longeron height and so cuts right across the lower middle of my back. This is made worse by the shoulder harness which goes back to the original securing points on the lower longeron, one bay aft. The harness pulls my back over the top cross-member/seat back edge.
Plan A is to replace the ply seat back with one that extends to the full height of the seat bulkhead. It will require stiffening as the distance between the top cross-member and the top of the bulkhead is too great for the unsupported ply and it flops about at the top edge. I've made up a trial seat back and it seems to do the job.
I know that there is a mod to move the harness attachment points up to the top longeron and so relieve the excessive downwards angle of the shoulder straps. I haven't seen one in the flesh but believe it just copies the lower mounting points to the upper aft cross-member of the first bay. I don't want to do this as it would require access to the fuselage frame by cutting a hole in the fabric to weld on the mounting clips. My plan is to replace the existing 1/8 cables that go to the lower mounts with longer ones that go all the way aft to the last upper cross-member. This is the one that holds the forward tailplane attachment. The cables can be terminated by taking them around the cross-member and swaging. No thimble is required as the cross member is of greater radius than the standard thimble, but the cable could be looped though soft plastic tube to avoid scuffing the cross-member. This scheme has easy access both ends and no welding.


Tyres, Wheels & Brakes.

I’ve been considering replacing the original hubs with Tosts in order to take advantage of the ready supply of tyres at reasonable cost, however the price of Tost hubs is horrendous so I’ll stick with the originals and buy the expensive tyres! I don’t know how long a set of tyres would be expected to last but it may put off the inevitable by kicking the problem into the long grass. My brakes are originals and the discs are dished. I’ve done a preliminary look through the bicycle systems and I’m impressed enough to get a set and do a trial run. I’ve seen lots of comment about this on the forum but no published results so I may be in danger of re-inventing the wheel here (literally!). My nose wheel tyre is a twin contact one which is still available at a price that would make your eyes water. I’ve looked at as many Nipper photos as I can find on the forum and on the web and it seems that many aircraft use the same tyre on the nose as the mains. My TC tyre is in good shape so I’ll leave things as they are for now.


Instrument panel.

Scanning the forum I’ve found a few posts regarding using the wing bolts to mount the instrument panel. There seems to be almost universal comment of “I wouldn’t do that” but no reason why. I’m considering using a piece of 1.5” x 1.5” x 1/8” angle chordwise, the fore & aft thickness of the spar (~3”?), under the usual load spreading bar. The vertical flange then becomes available for mounting the panel.

Any comment on these items would be appreciated, and I’ll post the results of my investigations as they progress.


Jim

[jantar]

Hi Jim I have just been thro all this with ZS-UDT and solved all to my satisfaction but SA CAA inspection still to come.
1/ seat-- I fitted a black polyprop seat shell (ex village hall) secured to orig.seat with 4 fixings and with an oval cutout at "small of back" to position it as far aft as possible. Harness remains as is--- result very good indeed! Pilot may be 12mmhigher than before but I fit just fine.
2/ Tryes 400x4 four ply rating are available for £10-12 including tube and are perfectly adequate. Wheels have much less load than single on a glider.I have a plain rib tyre in front and get no shimmy,but check for play between cable horns and the leg,--there is a "rivetted" connection here which loosens
3/Brakes- Idid much thial and error & now have a working system using cycle hydraulic calipers (Hayes nine) and a motorcycle master. Still some tuning to go as they dont hold against runup yet & dont "park" Had new discs laser cut from s/s 2mm thick & larger dia.to suit my development. Needed 2 new "donuts" turned to replace the disc pillars, which are poor,and 12 new wheelbolts longer than original.As cycle calipers are NOT handed mounting each side differs &clearances are down to 1mm. Remember that you should not have too powerful brakes as tyres will rotate on rims,tearing the tubes.
4/ My old inst. panel was originally "secured" with sticky foam and I didnt like the idea of trying to catch it left handed if it broke loose. Fixing is now more positive. An angle bolted to the panel bracket sides is bonded to 2 strips of foam rubber and these are sandwiched between the spar top surface and a folded sheet alloy Z secured under the the wing bolt clamp plate. Cant see any problem with this. I will post some photos soon but rely on a friend with broadband to do this
Call me evenings on 02825 898085 for more info. Jantar ZS-UDT a Slingsby mk3

[Neil Spooner]

Hi Jim,

Well done with the progress!

My seat back is 3 ply, and is extended above the crosstube in the center, it supports my back very well. The upper shoulder harness is attached to the same crosstube behind the seat, as it gives me much better support for inverted flight. It proved to work in my forced landing, the top of the seat broke, but it still kept me well away from the instrument panel and I was completely uninjured. The top harness is fitted to the crosstube with 4180 P clips. I also use an extra lap strap to the 4 point harness, this helps spread the load on the lap straps during negative G maneouvers. The seat is fitted with made to measure foam filled covers, and is very comfortable.

The tyres are available, I think I got my new nose wheel from Skysport. The twin contact tyre is very heavy. I have had no problems with shimmy with a standard tyre.

There are some very good hydraulic mountain bike brakes out there, I have a pair of Formula 4 calipers and brake lever/master cyl' which is destined for my a/c. It is quite acceptable to mount the lever/master cyl' on the control column.

My instrument panel is fitted via rubber mounts to wooden pads glued to the main spar. What ever you do don't screw anything into the spar!!
I have included some photos to show where the 'panel mounts are, to these lugs is a "cotton reel" rubber mount which is fitted to a wooden pad stuck to the spar.

Good luck,

Neil

[jantar]

Hi Neill thats quite an office you have Im afraid mine is more basic. Finally managed to post some pics today 12/6/11 Tom

[JimCrawford]

Progress report

First a rather embarrassing error – I typo’d the registration of my aircraft in the thread title. It should read GAVTC, as it does in large red letters on the side of the aircraft!

The seat back and shoulder harness mods were discussed with Francis at the LAA test pilot lecture and verbally approved. I’ll be submitting the proper paperwork but am also confident enough to progress the work. The seat back gets a second trial fit this week and the harness is waiting for the cable and nicopress sleeves to turn up. I need to find out how to publish photos on the forum to show how it is going.

I’ve replaced all the wheel bearings, one original was u/s and the others varied between worn and tired. At £5 each it was a simple matter to strip the wheels and replace them. I also was able to inspect the tubes and inside walls of the tyres with the hubs dismantled so there is confidence that all is well there.

The remaining problem I’m attacking is the wheelbrake arrangement. I need to rig up a trial fit. I either need to get some new standard discs, mine are very bowed, or I need to fit a bike disc and caliper. The difficulty with the bike disc is that the pcd of the standard bike disc mounting bolts is smaller than the pcd of the wheel bolts. This will require a complex mounting donut as Jantar pointed out. I agree that the disc pillars are poor, mine have some evidence of bruising. An adaptor donut is critical in dimensions to avoid the disc bolts from hitting the undercarriage leg structure. Again, as Jantar points out, the callipers are both unhanded, requiring a different mounting for each side, and there isn’t much space between the caliper and the tyre. I may try and get some standard discs for the existing system to get the aircraft flying and sort out better brakes later.

Thanks for the photos Neil. I see you have the luggage area ‘boarded out’. I guess you have to be careful with weight and balance with the locker being so far aft, but a sleeping bag and a toothbrush should be fine. How is it all fitted? It looks as if it would be quite easy with an uncovered fuselage, but difficult to get to the aft bulkhead in a covered structure.

Regards

Jim

[Neil Spooner]

Hi Jim,

To post photos: Look just below the "submit" button under the text box and you will find "Upload Attachment" in an orange colour (it doesn't stand out very well), just below that to the left you will see a "choose file" box. Click on that and it will open a dialogue box, that dialogue takes you into your computer, and you now need to seek out the file you wish to publish. The file is where you store the photos from your digital camera, when you have located the photo just double click it which will select it, the file name will now appear just to the left of the "choose file button". Now click on the "Add the file" button and a little box will open up to show the upload progress. Once the upload is complete the upload progress box will disappear and your photo details will appear just below the text box you are typing in. Move the cursor to where you want that photo relative to the text you have typed and click the "place inline" button next to the photo file details............and Voila! All you will see until you publish the post is a line in brackets detailing the attachment. You can upload more photos in the same post by doing the same again.

Now on to business:

Make sure you get your paperwork off to Francis asap, the LAA take forever to progress paperwork.

Don't bother with standard discs, they are too weak. Get some cut by a laser cutting outfit or cut them yourself by "chain drilling" or plasma cutting the center out so that three "lugs" protrude into the center and the area that is not a "friction" area is kept to an absolute minimum. On the standard discs it is the large area that is not heated by brake application that causes the disc distortion. One of the Nippers is running discs cut from 4130 which work fine, most are stainless. I would suggest using a thicker material also, about .025" is about right. I don't have a pic' on my confuser at the mo', but will take some and post them.

Mounting the calipers is a difficult job, one that you may want to make temporary brackets from light alloy until you have got the size/shape right, then make them from 4130. I am still running std calipers with the modified discs. They work fine but will not stop you in a hurry......they never did!!

Make sure the pad material you use is a sintered metal type, the "organic material" ones are useless in the wet and fade very very quickly.

When I was restoring the a/c I thought how messy the std cockpit fabric back wall was, so welded some lugs to the fuselage longerons and the second turtle deck hoop. The lugs have captive nuts fitted, so the very light ply I used was fitted with a little cubby box with a lockable lid to stop everything falling out when inverted. You are absolutely right, the area behind me looks like you could put a kitchen sink in there, but in fact is useless as a load carrying area in terms of W&B. The cubby box is where the original fabric one was, so no W&B change there. The area behind me now looks much nicer, and is lighter than the leather/fabric original. You could replicate this using "P" clips for attaching the ply to the longerons and second hoop after the fuselage is covered.

Good luck,

Neil

[JimCrawford]

Neil, thanks for the instructions on posting pictures - I hope to have something worth viewing soon!

I am working as fast as I can on the LAA mod papers but they are not on the critical path as the aircraft can be flown for it's permit renewal unmodified. By far the most critical part is the wheelbrake system and that is a problem as something has to be made, which at the moment is an unknown issue.

I would prefer to use bicycle discs as the material and manufacture method will have been optimised for flatness and resistance to warping, rather than cutting a 'standard' set from sheet. Also it seems more coherent to have the wearing part as an over the counter spare from Halfords, rather than as a bespoke part. The big disadvantage is that I have to get the donut or spider made to adapt the bike disc to the original hub. I feel a social with my old drinking buddies from the machine shop at Uni appearing over the horizon! Twenty years on and they are still turning out 'homers' for my aeroplanes!

Alternatively I'll see if they can make up a 'standard' disc, although with the cut outs you suggested on this thread and on the the other one which went into quite some detail. I'd copy the shape of the Shimano disc I'm thinking of using.

Either way, drawing the part out in AutoCad produces a file that the guys can slap on the numeric controlled mill over lunch.


The plot thickens

Jim

[RobW]

I'm also in the market for new brake discs - Raymond has kindly sent me some old discs to use a template (6 hole type). I'll try to get 2-3mm thick discs laser cut in the next couple of months, but I'm also looking for some moped discs (160mm diameter is relatively common) that can be modified. My calipers will happily accept thicker discs (as long as w&b not upset!)

In the meantime I contacted LAA to find out if any brake mods are registered, or wheel changes (since I understand that Jabiru wheels will fit). Here's Andy Draper's reply:

"I found 4 mods on brakes, but only two recent ones. One was to fit brakes made by Hope Brakes Ltd. (Nipper G-ASZV Mod No. 12549). I see from the notes that the calliper mounting plates have to be home manufactured but these are not complicated. The other was to fit mountain bike brakes. I could find no mod for changing the wheels, though."

Hope that helps,
Rob

[Neil Spooner]

Hi Rob,

Did you ask if we needed to make a mod' application? ie is it mandatory?

Neil

[RobW]

Didn't ask- mod app required I think for any significant deviation from original spec, but it's easier if the mod has already been applied to another nipper. There's a link on LAA site. I'm new to all this but I'm sure many on this forum have been down that avenue...?

Link here: http://www.lightaircraftassociation.co. ... table.html

[JimCrawford]

It's been over a year since I started this thread so a progress report is long overdue. Gider maintenance at Windrushers and life in general have thrown considerable delay into plan A but the end game is in sight.
I'll put all my information in a file for anybody that would like it when the mods get approval however, to date:

Seat and Harness:

I've built a new, full depth seat back and am now in the detail fitting stage. The seat back is 4mm ply, the same as the original 'short seat', but cut to the bulkhead profile. This wasn't quite as simple as it sounds as several iterations with cardboard templates revealed that the profile isn't very symmetric!
The ply, when cut to a satisfactory match to the aircraft, was used to cut a plug from 50mm biulder's foam. The sequence then went;
1. One layer of glass twill laid up on the plug and vacuum bagged
2. ply seat back attached to glass
3. ply edge sanded and filled to a fair edge.
4. layer of glass twill laid up overall and two additional layers around perimeter, all vacuum bagged to consolidate.

So far the seat back has been removed from the plug and test fitted. It is remarkably comfortable. Next is to fair the upstand edge, drill the attachment bolt holes and cut an aperture for the harness to go through.

P1050006web800x600.jpg (1.2 MiB) Viewed 20421 times

The final vacuum bag consolidation is shown. The layers in the set up above are;
foam plug
first glass
4mm ply
second glass
two edge reinforcing glass
peel ply
breather film
breather felt
vacuum bag

there is also rather a lot of tape that was used to catch an elusive leak!

The trial fit before detail fitting

P1050013web800x600.jpg (72.49 KiB) Viewed 20387 times

This is the harness attachment to the aft crossmember. The excess 'tails' will be cut off when I find my cable cutters!

I'm not sure how big a forum submission can be before it gets rejected so I'll do the wheelbrake on a seperate post

Jim

[JimCrawford]

Well that was only partialy successful. I'm not sure why the reduced photo was still so large and drove the width of the post. Also some text and a picture was lost.

missing text;
To finish the seat back;
trim the unfinished upstand edges
drill the hole for the attachment bolts (same as original)
cut an aperture for the harness.

missing picture, rear harness attachment;

P1050017web800x600.jpg (72.9 KiB) Viewed 20344 times

Wheelbrake mod;

After much experimenting with standard 6 mounting hole discs I couldn't get them to fit without a complicated fitting to avoid the mounting bolts fouling the undercarriage leg. The solution, with a blinding flash of simplicity, was to choose a disc with a different mounting layout, but one that was still in production - not obsolete.

In the picture there is a standard (but not common) Rolhoff hub fitting. This is easily obtainable but usually not off the shelf. There is enough material on the fixing ring to accommodate the Nipper bolts and stand-offs whilst ignoring the 4 Rohloff holes. I drew the Rohloff and Nipper holes on a CAD system and used that to produce the 1 to 1 drawing you see. This was spraymounted onto the disc and used to position the drill for the Nipper holes. Worked a treat.

P1040944web800x600.jpg (50.71 KiB) Viewed 20331 times

The disc fitted;

P1040937web800x600.jpg (121.42 KiB) Viewed 20356 times

There are a couple of points about the calipers,

They are not available 'handed' as they always fit to the left on bikes. This means that the mounting bracketry must be made especially for each side. I have been through a few cardboard and scrap metal iterations and am about to make the real thing. Pictures to follow. The second point is that the tyre is extremely close to the caliper if the wheel is the 4.95x3 1/2 type. I only found one caliper slim enough to miss the tyre by ~ 2mm. I believe the 400x4 wheel would give more clearance and allow the use of a hydraulic caliper. The original calipers on my aircraft were not re-useable.



Jim

[Neil Spooner]

Hi Jim,

Thanks for posting this info'........just a thought. I believe the discs have to "rotate" the correct way, the picture has the disc rotating the wrong way for the spokes to work in tension?

Neil

[JimCrawford]

Neil,

Thanks for the comment, it reminds me of a conundrum I haven't been able to solve. The discs I've used are unmarked but, if you look at any which are marked with the direction of rotation, they appear to put the spokes in compression which is very counter intuative. I would love to know why this is so.
If you check the cycle stores websites you can find many images of discs with the direction of rotation marked and I just don't understand the reasoning.
I've ignored the problem as the discs are not 'sided' by any asymmetry so I can always take them off and reverse them!

Jim

[Neil Spooner]

Hi Jim,

How strange......you are right. Here is an ebay item which shows DOR: http://www.ebay.co.uk/itm/Avid-Disc-Bra ... 420wt_1344

Keep up the good work!!

Neil