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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.


F2B: B X-wing: the Ultimate Failure of the HoG line-tension saga

By Supercool

Loyal readers who have been following the HoG (Heart of Gold) saga may recall that I have been trying at attain good line tension at 50 MPH. Line rake, offset thrust-line and radical engine offset have all failed to provide the line tension I need for solid overheads.

The only variable remaining is fuselage lift. Having no fuselage, the HoG would be at a disadvantage if fuselage lift were a necessity for good line tension. The flight of
many models, such as in F3D and F2A, includes knife-edge flight: in these instances, the lift supporting the weight of the model must come from the fuselage, plus a small component due to engine inclination.

To test this hypothesis, I fitted the HoG with centre-mounted stub X-wings. Incidence was made variable by mounting the stub wings on a steel shaft. Hopefully, this adjustment would enable me to adjust line tension to values providing solid tension in overheads. These mods are clearly shown in the photos.

Test flying provided the following facts:

1. Line tension in overheads was not noticeably increased.
2. Drag from the stub wings was high, making it impossible to glide the final lap, as required in the F2B pattern.
3. Increasing incidence only increased the drag.

I am forced to conclude that line rake, thrust-line offset, engine offset and X-wings fail to provide solid, reliable tension in overhead manoeuvres on stunters.

The certain methods are to reduce model weight and fly fast (like 5 second laps).

Methods involving rolling motions, such as tip, weight, differential wingspan and differential flaps can increase tension, but at the expense of wobbly square turns. Motor set-ups (2-4 break) that increase speed during manoeuvres are also effective, at the expense of fast reflexes in the pilot (read “youthful pilot”)

All very sad. Anybody out there got a solution?

In response: from Alwyn Smith

I have read all the different articles that you have written in the A.C.L.N. over a number of years.

My introduction to C/L flying was on the Esplanade in Perth in about 1948, and I still fly C/L.

I was very interested in your latest article about LINE TENSION, as I have been trying to tell C/L fliers, my ideas for about 50 years now.

I have amongst my collection of C/L models in Melbourne, an Aeroflyte Aurora fitted with an OS Max 40 FSR. I acquired this model ready built about 20 years ago. The Aurora is very nose heavy with the 40 FSR, ( I seem to remember it weighs about 11.5 Oz or 330 Gms ) and the model was designed to take an engine like an OS Max S 35 at about 8.0 Oz or 230 Gms.

The model was so nose heavy it would not do aerobatics, and had such a massive line tension, I was not game to fly it level as I felt it might break the lines.

I added a square of lead, under the Tail Plane, and then after a test flight, added another piece of lead, until I had about nine 1” x 1” pieces of lead glued under the T/P

The C/G was moved back until the model would perform aerobatics correctly. At the same time the LINE TENSION decreased, and it became a very nice flying model.

On one of my trips to England I was at Old Warden, and talking to John Stroud, who was at the time, editor of Aeromodeller.

He had been given one of the new PAW 60 Diesels, and built a twice size Aeromodeller A.P.S. Unlimited for it. The photo of this model appeared on the front cover of Aeromodeller.

I helped John prepare the model for flight. In the article in Aeromodeller John had said how the model would not do aerobatics as the model was TOO nose heavy due to the weight of the PAW 60 ( I think about 16 Oz or 560 Gms ), and would pull your arm off.

When I was holding the model for John to start the engine, I asked if it was still nose heavy, and John said “ No” He had added lead to the underneath of the trailing edge, and turned the model over to show me. I commented that the model would also have lost the MASSIVE line tension. John just looked at me and called me a HERETIC.

Ian Smith from Sydney was in Melbourne and contacted me as he asked if he could visit my home to see my collection of models and engines.

We got into a discussion about Line Tension, Control Plate and Pivot point, and Lead out positions. I told Ian I have always thought that ALL the experts were WRONG, and the main point I made was that I believe the model is supported at the PIVOT POINT, not on the wing tip guides. Ian like John Stroud told me I did not know what I was talking about, and all the experts have agreed that the position of the pivot point in the bell crank is unimportant. He mentioned the diagram that has appeared in many magazines, showing a cardboard cut out of a model, and how it will always hang with the C/G in the middle of the supporting line.

I had the same discussion with Russell Wright many years ago, and Russell told me that you must have Offset Rudder and Offset Engine to get line tension. I pointed out to Russell that I had a number of combat models that had NO rudder and No engine Offset, and they ALL had GOOD Line tension, and held out in overhead maneuvers

I claim that the Line Tension comes from the TURNING COUPLE between the C/G and the Pivot Point, and the final line tension comes from the speed and weight of the model. Look at a speed model and have a look at the distance between the pivot point and the C/G. They are very close together. If the C/G was TOO far ahead of the Pivot Point, the nose of the model would YAW out and create DRAG

I have been told that my theory cannot be correct as Paul Turner from Sydney built a model with the Control Plate well back near the trailing edge of the wing, and it made NO difference to the Line Tension. I have not seen this model, or spoken to Paul about his model

I have always built my C/L models with the Pivot Point slightly backward of the position shown on the plan, and my front wire runs parallel to the leading edge of the wing, if the model has a straight leading edge. I have never found it necessary to have my leadouts swept back. I do agree that the trim can be adjusted by having the leadout guides adjustable, but that is due to the nose of the model being pointed OUT, just like modelers claim you need engine offset to obtain line tension, but you tried this and found it not really effective.

I still claim that the LINE TENSION comes from the TURNING COUPLE between the C/G and the Pivot Point, and the SPEED and WEIGHT of the model

You did ask at the end of your article in May 2007 A.C.L.N. No 110 for any other ideas.

Alwyn Smith

Caloundra May 2007

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