Trim fail

I had a bit of a scare last week.

Just after take off I heard a 'bang' and had to use more back pressure on the stick than normal to keep the climb attitude the same.

I decided to continue to climb up to a safe altitude - 3,000 ft - before investigating what had gone wrong.

After thinking about it for a little while I concluded that one of the trim springs must have either broken or come loose.

The rest of the flight continued without incident and I made a normal landing. The stick forces were not much more than normal - thankfully the Twister does not really need a trim in the first place as the forces are so light.

That evening I investigated and sure enough the rear spring securing bolt had come right out. 

This explains the stick forward bias as only the front spring was acting against the elevator push rod tube - requiring a constant rearward force to overcome it.

I noticed that the bolt thread did not have evidence of Loctite on it so that is a failure on my part. It's taken just over 400 hours for this to happen.

I've used Loctite this time! And also added TorqueSeal to the outside so that at each annual it can be inspected for movement.

Something to be aware of if you have not used Loctite - but certainly an inspection item (with the TorqueSeal) to add to the Annual if you have already not done so.

After removing the baggage tray it was instantly obvious what had happened. The bolt holding the rear spring had worked loose and come completely out.

Closer view.

After fixing with Loctite on the thread of the bolts and also now with TorqueSeal (in Blue) which will aid inspection.

New spinner

It has taken me many months but I have finally finished getting a new spinner installed after the old one 'departed company' in May.

In my usual fashion it has been 'modded'. I always found the original design of spinner to be a bit blunt looking compared to the rest of the aircraft, so this was my opportunity to change it's shape.

My boss loaned me an old spinner from his plane and I used that to make a 'splash mold' part off the front of it. I then combined that part with a replacement spinner that was kindly sent by Matthias.

The result is a sharper look - more Mark 9 Spitfire than the old rounded Mark 1 looking spinner.

The other modification is the addition of a front plate. This has added a great deal of rigidity and security to the attachment of the spinner, I feel quite confident it will not come off again!

The front plate is made using the same schedule as the back plate. A 3mm thick sheet of carbon is cut out and drilled to match the bolt holes in the crush plate. Then an angled flange of carbon is added to the edge, projecting forward, giving a large area of contact with the inner front of the spinner.

The spinner was fitted and aligned using a laser level before all the holes were drilled. Then nut plates were fitted to the inside of the front and back plate flanges. 12 on the back plate and 4 on the front plate. I've gone for metric anchor nuts this time round and button head Allen screws with fibre washers underneath for security as I found the Tinnermans with the sprung plate to be unreliable over time.

Everything runs very smoothly now. However I will still do a dynamic balance to see what the vibration levels are and if they can be improved at all.

Regarding the old spinner - the conclusion was that it was a substandard part. 

It was not made by the factory but supplied to me by Hercules propellors using their own mold. They had not followed the same layup schedule as the factory spinner. They only put carbon on the back inch or so where the fasteners go for the backplate - the factory made spinners have carbon all the way up to and beyond the cut outs for the propellor. Therefore the old spinner was vulnerable to cracking in the prop cut out area. 

I believe this is what must have happened, a crack formed and then quickly propagated on the fateful flight, the spinner ripping open once the high pressure air got inside and popped it off, wrenching the fasteners as it went. We will never know for sure exactly what happened as the spinner was disintegrated but the above seems to be the most likely cause. Please check your spinner if you believe it was made by Hercules as they are not fit for purpose.

Original and new shapes compared.

Front plate for added security, micro on the edge was done to form fit to the inside of the spinner.

Side view of front and back plates

All done and looking good

I think this shape of spinner suits the Twister better

Button headed Allen screws with fibre washers underneath with a front plate. This is the same standard used on certified aerobatic aircraft