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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.


Fools Rush In


F2B is probably the last area of aeromodelling that I should brave to comment on (on which I should comment!). My list of bad engines and clunky aeroplanes would fill quite a few pages.

Others, Al Rabe, Bob Hunt etc have certainly filled a few pages with their successes. However, being a failure has never stopped me before, so here are some reminiscences and thoughts for the future on F2B Stunt.

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My interest in Stunt was first piqued in about 1960, when I visited Sydney's Centennial Park to watch a stunt comp. I was stunned by the gorgeous Thunderbirds of Ian Brown and Ron Diprose, not to mention Paul Turners beautiful "Peter" Commanche style-airplane. I was hooked, so it was off to Hobbyco for a  Mk6 T'Bird and OS35 Max3.

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This model had a chequered career. It think I got a third place in one comp, but I could never get that motor to run right. The model suffered various indignities, including Gary Lynch taking off the inboard wing with a combat model. Still, the wing glued back on all-right; the model meeting its ultimate end at the 1964 Victorian Nats when both lines broke and it went through the rear window of the passing garbage truck. I thought the driver might get mad, but he just sat there and didn't move. He was still sitting there an hour later. Maybe he was dead.

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By the way, that was the era of the gorgeous Victorian giant stunters, all beautifully finished with Olde English lettering and very swank.

"Angelique" was number one by me, but others were the "Shark 45", and John Hughes "Rebel", with Kyowa 45 engine. The latter model flew at a beautiful constant speed, leaving a strong impression on me. If only I had a motor that ran like that!

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But returning to the Thunderbird: Palmer had introduced differential flaps, which used independent horns on each flap. By using a shorter horn on the inboard flap to give more deflection, the effect was to roll the model out on the lines during manoeuvres. This worked a treat; you could wind in the overhead eights really tight. Its rare these days to see overhead eights about the 45 degrees required. But with the T'bird, you could lie on your back and do them till the tank ran dry. You were in trouble then, of course, but being younger we could jump up and land safely. These days, just a loop gives me a heart attack.

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In the late '60's, I was impressed with Dick Mathis "Chizler" design, which he claimed to fly very slowly, something like 45MPH. Now our T'Birds were screaming around at 60 MPH, and you really needed good reflexes. Cutting furrows in the grass with the fin was not uncommon.

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But the T'Bird would not take to being slowed down, at least for me. The problem seemed to be the flaps. While the model rolled out nicely in the loops, it over-rolled in the squares. At high speed, you didn't see this. So in the late '60's I came up with my "Rivets" design, which had "constant differential" flaps.

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The problem with the Palmer differentials was that they were progressive.

More deflection gave more differential between the flaps.

In my system, again two horns were used, but this time they were the same length. However, the outboard horn had a horizontal slot instead of a circular hole. This way, the inboard flap deflection always lead the outboard flap by a fixed amount.

This remains the sweetest flying model I ever had, and I remember with affection the days flying in Centennial Park with Reg Towell, Ron Nyberg, Will Sutton, Ian and Robert Mooney, Ian Korner, Tom Barry and others. We always had a lineup of 6 to 8 stunters every Sunday. Actually it was Ron Nyberg who sold me the best Stunt motor I ever ran.

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He pulled this beat-up looking Enya 35 6001 out of his pocket and sold it to me for $3. This was really a dubious buy, it had bits of araldite stuck to it and looked horrible. But it had exhaust lugs that were strong and you could hang an OS "Jetstream" muffler off it no trouble at all.

But mostly, it ran beautifully! You set it rich on the ground, with just enough power to get the model rolling. This gave beautiful long take-off rolls which were very impressive. Once in the air, it lumbered around slowly picking up revs until by the time the reverse wingover was called the power was there. Every time!

That is, until the Northam Nats, when the rod let go. That motor had so much running the crankpin was worn triangular!

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At about this time (1973), Dave Campbell built the first "Anna Domini", which was published in "Airborne" magazine. This was the first of the long-fuselage stunters. The earlier designs, including "Angelique", were short moment models, reflecting I feel the Palmer and Aldrich set-ups. But "Anna Domini" was more than 48" long, a trend continued by Reg towells "Caudron C-460" series and Brian Eathers "Firecracker"'s.

In "Anna", I abandoned differential flaps in favour of a new system. The problem with differential flaps, of any type, was that the increased lift on the inboard wing also produced increased drag. So while the model rolled out nicely, it also yawed inwards. This compromised the line tension, especially in the square eights. That is not something you want on a slow flying model. 

The solution seemed to be some sort of outboard spoiler. The spoiler would reduce lift, thus rolling the model out: and at the same time increase drag, thereby yawing the model out as well. This is the best of both worlds. Furthermore, the spoiler can be retro-fitted, as the mechanism is all external.

I flew this set up at the '74 Camden Nats, but my luck was really out that year. Firstly, I had an undercarriage leg break off in practise. Then in the first round, I had my silicon fuel line split where it entered the venturi, stopping he motor dead after the loops. Finally, the heat affected my black and red finish, so that the spoiler jammed over-centre, leaving me with a model flying severely wing-up. Sheesh, some days you should just stay in bed!

Now in the year 2001 I have another shot at this idea. Recently I have retrofitted my Eather "Firecracker" with an outboard "anti-flap". I didn't want the spoiler, due to the over-centre problem. But the new method won't do this. Check out the photos.

A small flap is mounted near the tip of the outboard flap. A simple linkage drives this flap in opposition to the main flap, thereby always rolling the model out, and also yawing it out. Test flights so far have been very good, plenty of line tension everywhere, and no possibility of jamming.

Now I've just got to fix my tank problems!!!!!!

Finally, there is a secondary trimming problem I want to address. The fore-and-aft position of the tip weight has always worried me. Should the weight be near the leading edge, on the C/G or near the trailing edge??

When a model turns, one would assume it turns about an axis that passes through the C/G. But where does the rest of that axis pass? If it passes thru the inboard tip leading edge, the model will yaw outwards. Conversely, if it passes toward the outboard tip leading-edge, then it will yaw inwards.

Maybe the tip weight position needs to be adjustable fore-and-aft for trimming purposes.

Well there you are folks. Would you like to hear about all my dud stunt engines next?

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