Gavin Magills Online KR2S Build Log

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Horizontal Stabiliser - Version Two

This is my second attempt at building a Horizontal Stabiliser after my building mentor recommended I use the standard KR hinge setup rather than the Dr Dean hinges. I orginally considered just modifying my original HS but after looking at what was required I decided it would just be easier to build a new one from scratch. The new HS will have the following features.

1. The HS will incorporate the NACA63009 profiles sourced from Mark Langfords web site.
2. The HS will be 7ft 6" wide.
3. The HS will incorporate 3" aerodynamic balance horns.
4. Spacing between the balance horns will be the same as for the Cessna 152 (1/4" at the side and 3/8" at the front.)

Please note that I haven't detailed all the steps I have taken in building this second HS if the step has been detailed previously in the original Horizontal Stabiliser page.

24th Februray 2010. Today I went out to the local airfield to measure the gap between the balance horns of a Cessna 152 so I could incorporate this spacing into my HS. My building mentor recommended increasing the gap on my HS so I decided to use the Cessna dimensions as a proven template to work from. The spacing at the side of the balance horn is 1/4".

The spacing at the front of the balance horn is 3/8".

4th March 2010. After a number of attempts at making spars for the new HS without much success, today I went to BBS Timber in Henderson and purchased a piece of 6" x 2" dressed Yellow Cedar. I had originally purchased rough sawn timber but the rough surface often hides flaws in the wood which only show up when the timber is dressed. By purchasing the dressed timber I was able to buy a piece with the straightest grain I could find.

After getting the timber home I cut and dressed it into four spar blanks, (one extra for just in case.)

7th March 2010. Tonight I spent a couple of hours drawing the dimensions onto the spars. Once again I used Darren Cromptons dimensions from his web site which can be found here. I have listed them again below.

FORWARD SPAR
------------
Total Length                 = 90 inches
Fwd Face Height At Inner Rib = 59mm less 3.9degrees = 56.8mm
Fwd Face Height At Outer Rib = 32mm less 7.8degress = 27.6mm
Aft Face Height At Inner Rib = 59mm
Aft Face Height At Outer Rib = 32mm

CENTRE SPAR
-----------
Total Length                 = 84 inches.
Fwd Face Height At Inner Rib = 59.5mm
Fwd Face Height At Outer Rib = 42.2mm
Aft Face Height At Inner Rib = 59.5mm less 3.0degrees = 57.8mm
Aft Face Height At Outer Rib = 42.2mm less 2.9degrees = 40.6mm

REAR SPAR
---------
Total Length                 = 90 inches.
Fwd Face Height At Inner Rib = 52.8mm
Fwd Face Height At Outer Rib = 35.6mm
Aft Face Height At Inner Rib = 52.8mm less 5.1degrees = 50.0mm
Aft Face Height At Outer Rib = 35.6mm less 4.5degrees = 33.0mm

4th April 2010. Today I measured and cut a piece of 6mm ply for the middle spar doubler. While checking the length of this doubler on the plans I realised that the doubler is exactly half the width of the original middle spar and as such I will need to widen mine to 45" as I am widening the HS to 7' 6".

As you can see 45 inches on the button.

I then setup and glued the doubler into place.

After glueing the doubler into place I then spent some time trying to work out how the KR hinges I have purchased match up into pairs. This is the complete set of parts as they arrived from the supplier.

4th - 5th April 2010. I then moved onto shaping the spars to the dimensions required. Here is the result of my efforts.

6th - 11th April 2010. Cut out the aerofoil templates for the HS.

I have decided to use the thicker 3/8" template as called out for in the plans at the center rear position. Here you can see the thicker template. This is made from thicknessed yellow cedar instead of the 2.5mm ply used for the other templates.

2nd May 2010. Today I laid the hinges out on the center and rear spars to check for alignment. I am not overly happy with the fit of the hinges as the spars for the NACA63009 profiles are deeper than the standard KR spars. The centre hinge is wide enough to cover approximately 3/4 of the depth of the spar but the middle and outer hinge pairs cover less that half the depth.

I have also found that the outer pair of end hinges appear to have a manufacturing fault in that the hinge hole in the rear hinge channel has been drilled 11/16" from the spar face instead of 5/8". As a result the overall hinge width between the spars is 1/16" wider than the space available when the spars are mounted in the templates. Also the mounting holes for all the hinges have only been drilled to take an AN-2 bolt where the plans call out for AN-3 bolts. I have drilled the holes out to take AN-3 bolts but this does not leave much space for the head of the bolt to fit between the hole and the inside curve of the aluminium channel. I also managed to stuff up a couple of the hinges when drilling out the holes so have had to bin them. All in all I have to say I am not overly impressed with the quality of the hinge kit I received especially given the purchase price.

The net result of the above is that I have decided to manufacture my own hinges. I will still use the center hinge I purchased as this is ok and at 1.5" wide is sufficiently wide for the spar. I will however make a new set of hinges which will be 1.25" wide for the middle set of hinges and also make a set of hinges 1" wide for the outer hinges to replace the ones I stuffed when trying to drill out the holes. With this plan in mind I ordered some Aluminium channel from a local parts supplier. I asked for 6061-T6 channel and the supplier sent me 6063-T5 saying this was equivalent to the 6061-T6. Not knowing any better I set about manufacturing the new hinges. (Note I found out later that 6063-T5 is considerably weaker in Yield Strength than 6061-T6 and is not an acceptable replacement.) To manufacturer my own hinges, the first thing I did was purchase an Aluminium cutting blade for my drop saw.

I also purchase some cutting fluid at the same time.

Apart from the mess, the blade makes it very easy to cut the Aluminium channel stock. I would recommend wearing gloves when cutting any metal with a blade as the swarf that comes off can be very hot.

To shape the hinges I used a Dremmel tool to roughly shape the channel to the right shape.

This is what the hinge looks like after rough shaping.

I then used files and emery tape to smooth the hinges down to the final shape and surface finish. Here is the finished result. Note that I will not be anodising my hinges like the KR supplied ones as there appears to be some debate as to the benefits or otherwise of anodising so I will just be giving them a coat of zinc cromate or similar instead.

As I said earlier I have since found out that 6063-T5 Aluminium has an inferior shear strength than 6061-T6 and as such I will not be using the hinges I have made either. I have been trying to source 6061-T6 here in NZ but there does not appear to be a local supplier so I will need to import this from the US. I have also secured the help of a machinist friend to manufacture the hinges I require on a CNC mill so I won't be making them by hand either. As you can imagine it has been a very frustrating exercise to switch to using the correct hinges and I am somewhat unhappy with where things are at. However that said I will continue to battle on and get the hinges made. The image at right shows the spec sheets for 6063-T5 versus 6061-T6.