Saturday, 29 December 2012

Week 71 - Wing pins, cockpit interior and panel

Hours to date: 730.75

I spent a great day helping Pete last Sunday put the finishing touches to his 3rd Twister. He said he should be taking it on it's first flight by mid-February.

I took another 110 photos while I was there (like I didn't already have enough photos of Twisters...).

I'll put some of those soon in a separate post but until then it's back to my build.

The rudder cable mounting points are shown in the photo below from the outside - flock around the space between the washer and the safety cell. I waxed the forked and threaded end on the inside so I could remove it later to make painting the interior of the cell easier.

After that I bonded in the plastic bearings for the rear wing pins - easily done with a bit of flock.

Onto the front wing pins. Which they make no mention of in the manual (at least none I could find). I believe the earlier kits had the front wing pins installed in the factory.

So I just had to figure out how far out they should protrude from the wing root. The bearing they will slide into (bonded into the wing at a later date) have a depth of 25mm so that's what I went for. I noticed from photos that the bearings in the wing are pretty much flush to the edge so 25mm seems like the logical value.

To bond these in you mix up a shed-load of flock after drilling a 30mm hole and then just make sure the pin is properly keyed up for bonding before sliding it in and surrounding it with flock.

I used a technique I borrowed from the French Twister build. Get a block of wood and drill a 30mm hole in it and then clamp that to the side of the wing root to hold the pin at exactly 90 degrees while it sets.

I got around to bonding and glassing in the metal plate on the outside of the cell - it is the control run for the cowl flap. After that I did the same with the thinner plate I made up for the inside.

Onto the panel which again due to my freakishly long legs needs to be modified. I did a test getting out with the standard panel and I always hit my knees on the lower edge - so this would not do if I had to get out in a hurry. So I've cut the knee holes out a bit more to give me good clearance.

I'll also add a tube cut down the middle and placed over the sharp edge later. Hitting a sharp edge like that with your knees every time you get out of the cockpit would become a pain (in the knees!) after a while.

I've also made the lower part of the panel wider at the top to accommodate the GPS I will be fitting. A Garmin Aera 500.

The rest of my time I have spent filling in the gaps in the bonding seams on the cell interior. What a job...

3 days of sanding, vacuuming and the body filling and I'm not done yet. Most of the work is because of the safety cell lengthening mod I did - which has to be smoothed into the rest of the cell and also coated with a layer of body filler to make it ready for painting.

Next up is finishing up that panel and then making up some plywood blocks that hold the seat in place.

Rear wing pin bearings

Rudder cable end attachment - outside the cell

Wing pin - wood block to keep pin at 90 degrees while it sets

Front Wing pin 
Ali plate for cowl flap control bonded onto cell outer

Panel mod - cardboard with duct tape for shaping

Ditto with some flock to shape out the lower section

Cowl flap inner ali plate bonded and glassed into place.

Body filler on seam and cell mod

Rear upper of cell seam

Monday, 17 December 2012

Week 70 - Rudder pedals and cowl flap linkage

Hours to date: 704.0

As promised I will now talk about the 'real' Twister kit - and not just my mod!

But there is still a little more of the mod for those who are interested.

As a result of the safety cell lengthening mod, installing the rudder pedals was a little more involved than it would otherwise be.

I found I had to add some more micro to the block that sits at the very end of the cell as it was still not quite the right shape.

To get the right shape I used a technique I borrowed from Pete. Which is to hold a pencil at the right distance out from the cell - using my finger behind it - then smoothly follow the shape of the cell while marking the edge of the block with the required curve.

Worked out perfectly.

This block and the carbon plate at the other end of the pedals have to be bonded into the cell with flock.

The instructions say to do this without the pedals on the slider tube but I wanted to make sure that everything was square and so I left them on and used some blocks of wood to keep it all square and in the right place while the flock set.

As you can see below I used two spirit levels to get the pedals level. The sides of the cockpit are a level - cross checked by putting the smaller level on the spar tunnel (which we know is level).

Then I put the smaller level on the top of the pedals and got them square.

Everything else should be lined up with the safety cell seam.

The other two little jobs I did in this area were making the holes for the rudder cables - actually the tube that the cables run in - in the sides of the cell.

The instructions say to cut a big hole - 65mm long but I noticed Pete just drills a hole of the right diameter (8mm) at the appropriate angle and this is what I did. A lot less messy methinks.

Then there are the two attachement points for the ends of the cables that need to be fitted - again in the end of the footwell - in line with the outside bottom of the pedals.

I'll flock in the washer and bolt on the outside of the cell (as per the instructions) then I'll remove the forked connecters when I go to paint the inside of the cockpit.

I can't do too much more of this job until I get hold of a Nicopress tool - this tool crimps the cable ends and is very expensive - over £200 so I will try to borrow one rather than splash out.

Onto my favourite subject - the mod!

As you can see I have bonded in the cowl flap and made up a connecting linkage for the cowl flap control that will go inside the cockpit.

Very time consuming making the ali plate - all done by hand and with a file...

Still the brass bush slides very nicely inside it now and locks into it's little notches so I'm pretty pleased with it.

I made it from the only Ali I had lying around which is 3mm thick - I will make the one on the inside of the cockpit with thinner ali as 3mm is a bit overkill.

I also want the one on the inside to be etched with some markings. Oil Cowl Flap, Open and Closed.

I got the linkage all set up as a test and it works well - I found I just had to shorten the connecting rod by 3mm to make it work right - so I'm pretty pleased.

Micro sanded before final layer of kevlar

Levels to get pedals lined up square

Blocks to make it set in the right place

Cable attachments in place

Hole for cable tube

Kevlar done - just got to body fill and sand smooth now.

Cowl flap bonded into fuse lower
Cowl control linkage - cockpit control end
Cowl control linkage - flap end

The full monty

Ali plate for outside of cell - brass bush at right end

Plate over hole in cell.

Control inside cockpit - still got to make ali plate to fit over here

Linkage and plate plus oil fitting bulge outer

Linkage fitted to flap and exit duct

Saturday, 15 December 2012

Pilots don't get much better than this...

The legend that is Bob Hoover.

Still going at 90!

I had the privilege to see him fly this routine at Oshkosh in 99' - unforgettable.

Note the cowl exits on the top of the wing...

Friday, 14 December 2012

Twister cowling

I thought I'd make my last post clearer.

 Below is how the cowl I'm proposing will look.

There will also be some fairings inside the cowl to smooth the airflow to the outlets.

These will just be a simple thin fiberglass sheet formed to the exits and covering the messy parts under the cowl with a smooth bowl (in between the engine and firewall area).

Wednesday, 12 December 2012

Cowling study

Just been watching this remarkable video:

It's Sean D Tucker in his Challenger III Biplane.

This aircraft has a unique cowling on it so I thought I would talk about what I think is going on here and why it works so well for him.

First of all his engine is heavily modified and produces over 400hp.

Plus as you watch the video you can see that he spends some time hanging off the prop on full power at high angles of attack.

Let's look at some photos and comment on each one.

Firstly the front.

As you can see the inlet is not in the usual place.

As he spends most of his time in a high angle of attack then he needs to make the best use of the high pressure air that is forms underneath the cowl.

If the inlets were on the side as usual they would be masked off by the lower cowl from that air.

I'm a little surprised at the prop choice - as it is not providing any high pressure pulses of air to that inlet. At the props root ends you can see it is more of a plain shaft than an airfoil.

There are props that exist that have the airfoil all the way in to the spinner. That type of prop would aid cooling when the plane is in the condition of slow forward speed and on a high power setting.

Also note the inlet size - remembering 400+ hp.

Onto the sides.

He's taking full advantage of the low pressure area here on the sides of the cowl to exit some air.

Onto the lower cowl.

A smaller than usual exit here and it is well back on the cowl - it does have a big lip on it though which helps to make a lower pressure area behind it helping to suck the air out.

Finally the top.

2 more small exits at the top of the cowl just in front of the wing.

As you can see there is a lot of exit volume compared to the inlet volume.

I plan to do a similar thing on the Twister with regards to the side exits, taking advantage of the existing tapering shape of the fuselage sides. I will also have a lower exit which I can easily change the size of should the 'Mark I' version not be correct.

Tuesday, 11 December 2012

Week 69 - Oil cooler install

Hours to date: 691.25

The flanges at the other ends of the ducts worked out well and are now finished.

I've made the decision to leave the installation of the nut plates on those flanges until after the fuselage lower panel is bonded in place.

I would never be able to line up the holes, so it's best to wait till it's bonded on and then just drill right through the fuselage lower and flange - then remove the ducts and attach the nut plates.

I've also started fixing up the hole made by the inside oil cooler fitting.

I started by filling in the space around the fitting with plasticine - then put a layer of Kevlar over that and onto the edges. This is followed by a large amount of micro to smooth out the shape and make it blend into the rest of the fuselage side. Micro is light and easy to sand so shouldn't add too much weight.

I spent quite a bit of time working the micro with my thumb (Nitrile glove on of course) and I think it gives a much better result as I was able to get a nice shape and it was pretty smooth overall - so this will mean much less sanding when it is hard.

I will finish up with another layer of kevlar on the top - the micro will then act as a core to the kevlar sandwich - giving the same strength as the rest of the cell.

I also did a little trial fitting of the hoses and as you can see from the below photo there is plenty of room for insulating them and making sure the heat from them does not get to the spar tunnel.

I've started the rudder pedal install but will leave that to another post next week.

Test fit before final finishing

Everything in place - cowl flap closed

Trial fit of hoses

Loads of room for insulation here

Flanges before final thin coat of gelcoat

Ducts flanges all done

Oil cooler fitting coming into cockpit

First layer of Kevlar over plasticine in centre and before micro

Micro to smooth out shape

Sunday, 9 December 2012

Pete's UL260 install

I thought I'd share these photos I took last year of Pete's Twister.

As you can see he has made a nice neat job of installing the beefy UL260iSA in the limited space available under a Twister cowling.

As always click on the photos for a much larger view.

If anyone wants high res versions of these then let me know in the comments section.

Sunday, 2 December 2012

Week 68 - Ducts...

Hours to date: 677.75

Yes, I'm still working on them...

It's well over 100 hours now on this mod.

I finally got the flanges to fit nicely to the oil cooler.

Then I put all the nut plates in place only to discover that I had gone overkill on nut plates. The ducts fit so well top and bottom now, that only the nut plates on the sides are really necessary.

I also bought some HiTemp RTV which I will use when finally installing everything to make sure the edges are air tight on the ducts.

I put the strip of door sealing foam all round and everything is feeling pretty snug now.

I moved onto fitting the cowl flap - which has a long arm on the inboard side which will connect to a forked rod end and then onto a control arm before finally connecting to a yet to be decided control in the cockpit!

Actually I have done quite a bit of thinking about this and am leaning towards the simplest solution for the control - which is a metal gate with a knob on the cockpit side which is screwed onto a threaded rod which goes through the cell to the outside and connects to the control arm via a rod end bearing (I think.)

The metal plate will just have a notch at either end which the knob and rod will slide into to lock it in place - either open or closed. I don't see the need to have any other settings at this stage but can easily put some more in should I need to in the future.

I think I may also need a spring in the system to provide some resistance when the flap is open - you would think it would be the other way round but I have read online about someone else who had a cowl flap and this is the way it works. Easy to open - hard to close.

Due to the arm on the cowl flap I had to make a slot in the outlet duct - to seal up this gap as best I could I used a piece of rubber and contact cemented it onto the outside of the duct. Then cut a thin slice in the middle with a sharp blade. This should seal up most of the gap. (see photo below).

Finally I moved onto laying up the flanges at the other ends of the ducts.

I think I've made a better job of them this time round - I could hardly do worse than last time!

I used the fuselage bottom itself as a guide for the outlet duct flange. First off taping the area with Duct tape so it will release when set.

The same principle applied with the inlet duct and I used the bottom of the air scoop as a guide for the lay up.

We'll have to wait till tomorrow night to see if I have done it right this time and hopefully everything releases okay.

Fitting pretty good now

Cowl flap open

Ditto - you can see the control arm and slot

Flap closed - a little extension is required on the outlet duct...

Gap closed up with rubber contact cemented to outer

Outlet duct ready for laying up

Inlet duct clamped to air scoop ready for laying up too