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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.


The Atmosphere

By Joe Supercool

Lying awake in bed last night, I could here the Easterly wind whining past my window. They say that Perth, Western Australia, is one of the windiest cities on Earth. It sure is a crazy place to live if one has an interest in free-flight model aeroplanes. We have to travel 130 km to the flying field, which is basically a piece of desert sand out in the wheat belt. The crazies here actually held a National Championship at this site in December 2008. The temperature was 43 C and the wind speed for most of the day exceeded 9m/s. I didn’t even know that I could live in such conditions, leaving my models in the car. We normally don’t fly from early November thru to April, precisely because of these strong Easterlies.

Whatever, my thoughts turned to a more amenable place, the turf farm at Richmond in NSW. Bordered by a river and ridge on one side, barbed wire fences and a RAAF base on the others, you would think that was equally crazy. But in the many years that I flew there, I cannot recall a time when it was too windy to fly. I had a model cross the river, but none fell into it.

The thing is, I had plenty of opportunities to observe the motions of the air. Here are some of my recollections, which I hope you may find of interest.

An early winter morning arrival at the field was blessed with dead calm air. There was no dew on the ground, as it was all caught in the spider webs which formed a lace of beads literally everywhere. I believe there is one spider for every 6 square inches of the land surface of the Earth: the spiders on this field sure lived up to that claim.

The air was cold. Starting the Rossi gave a blast of freezing air onto my hands, quite painful in fact. The trajectory of the model was very smooth, no turbulence at all: the glide down was likewise very smooth, like sliding on glass. No thermals, nothing at all.

As the day warmed up, this slowly changed. The spider webs disappeared, the air near the ground warmed, and it became necessary to open the needle a little each flight to stop the development of a lean condition. An engine stop when using VIT is not to be recommended! Interestingly, the model started to rock a little as it glided down into the warming air. In fact, there was a distinct height where the smooth glide broke down into a rocking motion of the wings.

Above this layer, the air was still cold and smooth: below it, turbulence was present.
I decided that this must be the “inversion layer” that I had heard about. Later in the day, this layer got really high, and I figured it must lie at the cloud base .

By mid afternoon, things really got lively. I could see from time to time that wind blown circles formed in the grass as thermals passed thru. There were upper level winds that had no presence near the ground. At one time I noticed that the clouds were blowing in 3 different directions! This was a problem as my F1C had failed to D/T: chasing it in my car with no observer was definitely hazardous. As a result I lost sight of it and that was that for 2 years.

Then I got a phone call from a rather bemused farmer. He had found a nice shiny Nelson 15 sitting in the middle of a pile of weathered, fragmented balsa, all that remained of the model after all that time in the open. He couldn’t figure how the engine got to be there, so differently had the elements treated the Nelson and model!

This turbulence below the inversion layer could be quite extreme. My F1C was hit by lift on one wing only. It literally rolled inverted, dived into a half loop and glided back in the reverse direction! Seeing is believing.

Another time I was in Victoria at F/F contest, sitting back in my folding armchair and watching the efforts of my colleagues out on the flight line. One guy was working on his F1C, turning to get something from his flight box. In the few moments his back was turned, the model rose off the ground, into the air about 2 feet, and then fell back, breaking the tail boom. What a nasty air-God it is that does that with thermals!

Another time in Meckering, by young friend Denzyl had his Pink Elephant A/2 D/T from about 600’. For some reason, this model never did sink gracefully, but came down in a horrible spin that always broke something, usually a wing tip. We spent most of the comp repairing it in time for the next round. But on this occasion, the model got down to just above tree height and went into a flat spin. It stopped losing height, and just spun faster and faster, like a Catherine wheel firework. This went on for a full minute!! Then it crashed, situation back to normal!

On a more serious note, I have seen turbulence that could easily deck a jet airliner. We had a C/L field in the approach path to Perth domestic airport, very nice field it was too. One day I was looking East for a jet to come, but what I saw was alarming. A belt of trees lay in the flight path, and as I watched, there was a commotion about 50 metres in diameter in the top of the trees. Small branches were being ripped out of the top of the trees and flying skyward. What luck that no airliner was going low and slow through that lot!

The next time I was approaching my home in Balga from the Eastern freeway. A mighty black cloud was settled over Balga, and as I watched, a torrent of rain fell from it. Well, it didn’t so much as fall, but plunged violently downwards. When it hit the ground, it burst out sideways at ground level. So that was what they mean by a downburst! No wonder big jets can get hurled to the ground while landing, with that level of violence in the atmosphere.

On a lighter note, while driving on the same section of freeway, I saw a willy-willy crossing the road on a collision course with my vehicle. Quickly I wound down the windows, allowing the vortex to rage through my vehicle, messing up everything. Gee it was good!

Another time I was enjoying an Airshow with a bunch of T6’s on show. Again, I was sitting on my folding armchair, directly behind a T6. Then it started up with a mighty roar, sending a shower of stones and dead grass over me! Heroically, I clung to my chair, refusing to move. Hey, I could be dead tomorrow. Here was a really great piece of fun! I mean, how often do you get a Warbird attack you?

Not quite as funny, I took my aged mother to the Bowral airshow. This is a narrow strip, placing the spectators right up close to the action. Suddenly a Spitfire roared down the strip, I swear its prop tips only a metre from the ground. My poor mother clung in despair to her chair, and bravely rode out the violent passage. But then, not 5 minutes later, a MIG 15 did the same thing. That was too much. My poor mum could take no more. “Take me home”, she cried, her spirit broken.

Well, it seems to me that air we like to fly through is far from placid at times. They say that good airmanship is knowing when to fly, and when not to fly. I guess the best way to learn these lessons is by listening to the old pilots. Not just listening, but hearing!

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