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Propeller Dynamics

Essential reading for model aircraft contest fliers. This is the only book on the market explaining propeller theory in non-mathematical terms. A rattling good read, I know, I wrote it.


Disaster at North Oak 


Four hard days drive East of Perth, on the desolate Hay Plain, lies North Oak. There is not much there. Some rundown farm buildings, the sort with doors hanging off and creaking in the wind. Did I say wind? Five continuous days of it, 20 kph and more from dawn till dusk; precious days when I should have been trimming my Open Power ships. Given the red-baked earth, cracked from the long dry and humidity less than 20%, with the brittle wheat stubble testament to a bumper crop 3 months previous, it was just as well I couldn't fly. This was the site of the AFFS Champs 2 years back: luckily, this was the wrong field!

The 'right' field was 5k North toward Narranderra, a healthy little town surrounded by flat land, lots of it, with some native White Oak forest. The flying field was large, just a few trees, with a smattering of dead grass to help lay the red dust from the parched rock-hard surface. No wheat stubble this time, as farming was not possible due to the field being the home of a threatened species, the Plains Wanderer. No, this is not a F/F guy looking for a model lost the previous year. Rather, it is a small bird which favours a ground-based life style, not unlike a quail. There was to be no chasing models on motor-cycles or in cars, as these little guys had machine guns mounted outside their nests, and they knew how to use them!

The Field at AFFS, 2005

Day 2 of the 2005 AFFS Easter Champs dawned calm: the weather then held for all of 2 days. Two glorious days, with unrestricted fly-offs dropping the models back on the field after 40 eye-watering minutes of gliding heaven . So it was time to bring out my 35 year-old Open Power ship, K&B 40 Series 71 and 20% nitro, with a Supercool 9X4 giving a blistering climb. At least, that's what I thought, until I saw Roys' 3-blade, geared F1C rocket up. Truth, I could not believe the speed of that thing!

Anyway, I had a secret weapon to fix my transition problems: this ought to even things up a bit. Yes, a micro-adjust fixed aileron on my starboard wing tip. You see, I had been having problems with a series of stalls off the transition, sometimes known as the ZOOM TO DOOM. In the past, such antics had occurred all the way to the ground, with often not a little damage on the ultimate stall. But I knew the answer, and I wasn't getting caught this time, no sirree, not me.

So I fired up, launching dead vertical into my usual axial rolling pattern of about 4 turns in 10 seconds. Seemed to be a few extra rolls this time, wonder where they came from? But safe, no sign of crashing, which is my usual criterion for a good power pattern. The transition sure enough lead to a series of diminishing stalls, no good at all. So out with the jewelers screwdriver and put some right roll onto my aileron tab.

You see, the idea is to exit the stall by rolling into the right glide pattern with the extra speed from the first stall; ie, washout on the right panel. This is opposite to normal practice, at least when 'normal' was back in the 60's. The idea first came to me when watching Bill East trimming his F1C. I couldn't believe his transitions: straight out of the stall into the glide turn, with practically no height loss. Amazing. So I snuck in a peek at his wing settings, and behold, there was slight washout on the glide side of things. Risky in a thermal I thought, but it works for Bill so I'll try it.

Off home and I added a micro-adjust wing-wiggler to my F1C. Sure enough, it worked a treat. At the 1983 Gouldburn World Champs, my final flight was launched into the teeth of a rain squall that was blinding me, but our thermal man said go so I did. Power pattern started off OK, but then it rolled left, went horizontal and rolled the length of the flight line, bunting straight down at engine shut-off! Ye Gods! Then it zoomed back up, the wing wiggler and VIT cracked in and it transitioned perfectly, good for 2 minutes and 25th place. Not bad for a little fella from Gumnut Road, West Pennant Hills!

So I fired up the K&B again, and off she roared. No better. More Tab. Repeat, repeat. Now she was gliding left!! Now I thought I knew how to trim F/F, but that was before I met Col Collyer. He now explained that in trying to get roll, I was also reducing the drag of the right panel, which meant the model could now glide left with right aileron! Not my day.

By now the rock-hard surface was taking a toll on the model. On D/T landing, the wing spars were cracking: I was repairing after every flight. The poor quality spruce now revealed itself: good enough for D/T onto WA wheat belt sand, but no good on brick-hard clay.

What is more, 3mm off washin had appeared from nowhere on my right panel. It was supposed to be flat. My beautiful vertical roll came from left rudder, not washin: but now it was rotating like a top on the way up. What could be wrong? Had the low humidity warped my fully geodetic spruce/balsa/tissue wing? Surely not. And it hadn't. I discovered a broken lower rear compression spar, right at the root: again, the lousy quality spruce had got me.

It was too late. I was out of auto-rudder travel: out of repair material: but mostly, out of just plain bluddy luck.