ARE YOU ALL WASHED OUT? – YOU SHOULD BE! Mike Golby

 No, I am not talking of being knackered working in defence of The Realm or as one of Bush’s mercenaries (can I say that?) but read on and all will become clear!

Late in 2004 I crashed my Zlin 50L, a Czech aerobatic aircraft dating back to the mid 1970s, following a disastrous decision to turn back following an engine failure. Yes, I know I’m a plonker but I really did think……… However, the gusty conditions, lack of speed and height, and bad piloting meant I could not complete the downwind turn and it all ended up in a bit of a mess with the wing being badly damaged, a smashed cowling and few chunks taken out of the fuselage. As I had a love - hate relationship with the aircraft I nearly binned it but hung on to the wreckage to make a decision when I was in a better frame of mind.

I say love – hate because it was my first own design scale job and it looked good. It was a charm to fly but when it came to landing it all got a bit traumatic. The reason behind this was that I decided not to incorporate wing washout as I felt it unnecessary because of the wing’s low aspect ratio. How wrong can you get! When it stalled the whole wing stalled without any wobble, wing drop or any other indication and the aircraft then emulated an Otis high speed lift in descend mode. If I was lucky and had enough height the resulting panic throttle increase would pull me out but not always.

This resulted in some heavy arrivals over the years which caused the wing to break along the stress raiser between the aileron cut out and a cut out in the leading edge to accommodate the undercarriage mounting. This was repaired on at least two occasions using local reinforcing sub-spars and loads of epoxy with the inevitable increase in weight.

After discussing the possible rebuild with Colin Waite, your friendly and good looking editor, he offered to make up a new set of foam wings for me, this time incorporating 2 degrees of washout. (I may need another set of wings one day, hence the flattery.)  We discussed how I could rectify the wing’s weak point and within a few weeks the most gorgeous set of balsa covered wings arrived. The repairs got underway during 2005 interspersed with other projects. Weight of the repaired aircraft was much the same as before because the wing had been beefed up using a full depth ply sub-spar at the front and full span spruce spars on the aileron hinge line. I also suspect that Colin used proper foam (Virgin Bead…… Ed) for the cores as opposed to the original builder’s foam that I used! Total weight was 11lbs which gave me a little cause for concern as it is only 72 ins span.

The aircraft was test flown earlier this year and it was a delight. The new wing was so accurately cut that it only required one click of aileron trim for level flight and tracking through inside and outside loops was absolutely straight. The inclusion of washout has made the stall predictable as the aircraft now wobbles significantly before gently dropping a wing. This means that I am a lot happier flying it slowly and I do not have to land it on the fast side to be safe. This together with the high and stalky undercarriage used to result in some horrible yet entertaining bounces if touch down was less than absolutely perfect.

 However, one problem remains. The aircraft still has the strange characteristic of the elevator becoming more sensitive as it slows down making approaches and the flare out a bit tricky. I have partially overcome this by increasing the elevator exponential but why it does this, defeats me. My basic aerodynamics and experience indicate the opposite is usually the case with elevator authority getting less as speed decreases. Can anybody out there offer an explanation? Is it anything to do with the short couple between the wing and tailplane?  Does the C of P move towards the C of G with a decrease in airspeed even though the aircraft’s attitude is the same resulting in less pitch stability?

Notwithstanding this niggle, the effort of repairing the aircraft was really worthwhile, considering I nearly binned it. I cannot thank Colin enough for making up the new wing, complete with washout of course!

Mike Golby