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Author Topic: Position of rubber motor peg  (Read 807 times)
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Snaky Stringer
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« on: December 27, 2017, 01:08:51 PM »

I notice that most if not all of the top modellers in Britain now use a very forward motor anchorage. Is this just for indoor flying and scale like flight or is it also valid for outdoor flying? Back in the 'seventies or 'eighties the consensus seemed to be that the anchorage should be as far back as possible, giving the longest possible motor. Apart from cunning schemes such as Rupert Moore's diaphragm to keep most of the motor weight ahead of the  c.g., of course.  As far as I remember, having experimented a bit with more forward anchorages, but not as far forward as seems to be the vogue nowadays, my conclusion was that the rearmost point was usually best, but then I was never an expert, merely a lowly hacker.

I would be interested to hear what the resident experts have to say. Does the reduction in ballast more than compensate for the loss of possible turns?
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« Reply #1 on: December 27, 2017, 01:39:38 PM »

I go with a very forward position on all of my designs, usually and as far as possible equalish around the CofG so that motor changes keep noseweight changes to a minimum.

I have found that you can get plent of rubber in, and it reduces the requirement for noseweight in a big way, reducing the AUW and making for better slower flights.

Since I don't fly for duration it is a lesser concern for me, but I would hazard a guess that there would be some sort of knee point on the graph where the best effect is gained, certainly not right at the back though!

Andrew
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tom arnold
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« Reply #2 on: December 27, 2017, 01:53:10 PM »

The answer is "Yes, in practically all cases". If you are contest flying under the FAC rules (in which we colonists have a fetish for long flights) the answer is a resounding "yes". The biggest killer of long flights is weight, and a long motor back to the tail, while yielding a lot of turns, requires a ferocious amount of ballast in the nose which kills any glide which, in turn gives a shorter than desired flight. Get rid of the tail weight and you get rid of the nose weight simultaneously which improves the glide tremendously. However, if you have shortened your motor, you have less turns, correct? If you are fixated on a high number of turns that is bad, but forget turns and instead think of "energy expended" to swing a prop and go for a thicker motor and a higher pitched prop. Big slow turning props can get just as high as a fast smaller turning prop and you have a lightened plane when that thicker motor finally runs down. The competition free flight guys do this all the time. The move to a thicker motor is not even necessary as guys have put motors up to 3 to 4 times the hook-to-peg distance combined with a moveable rear peg so the "shortened" motor is kind of a moot point anyway.

If you are flying under UK rules where time is not such a factor, it is still a "yes". By moving the peg forward to fly, the least you have is a lighter model which always trims better and flys better no matter what the rule system. Now just to tickle your funny bone, Clive Gamble, a master modeler, fine gentleman, and UK transplant has flown a Sopwith Camel with the peg just clear of the trailing edge of the wing for over a minute as a matter of course. His motor was 3 to 4 times that short hook to peg distance.

As to the exact location of that peg is a matter of personal preference. Many flyers like to put the half point of the motor directly over the CG as then there is no CG change as the various motors are put in. Others put the peg back of the CG the same distance as the CG to the nose tip distance. This requires a little nose weight for an accurate balance but it, at least, is at the far forward end of the balance beam, so to speak. Then there is the final method and that says "put the peg where your blast tube will fit".
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lincoln
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« Reply #3 on: December 27, 2017, 07:23:26 PM »

Clive Gamble did a Sopwith Camel with a rear peg that was in front of the trailing edge of the lower wing. Apparently, according to Mike Stuart, it was a good flyer. He wrote that it had a long, "heavily braided" motor.
http://www.ffscale.co.uk/page3pp.htm

I like the hook and the peg to be an equal distance from the anticipated c.g. This way, I can change to a heavier or lighter motor without much re-trimming.
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Walt
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« Reply #4 on: December 27, 2017, 11:08:35 PM »

I agree with these guys.  Keeping the motor at 50/50 works well.  My mentor Don Srull advised me that you get diminished returns if you go more than 40/60.
Wally
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Snaky Stringer
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« Reply #5 on: December 28, 2017, 07:30:18 AM »

Thank you all, gentlemen. I'll definitely fit a fairly forward rubber anchorage in my next test flight subject, which should be quite soon, weather permitting. At the moment the weather is not very favourable to test flying. I have made contact with a local RC group who fly indoors with small electric models, something I am tempted to try but so far have not ventured. They may permit an outsider flying small rubber models if the air isn't too crowded. On the other hand they may cringe in terror from the possibility of collisions with a non guided missile. One has to be a member of the BMFA to fly in their sessions and I intend to re-join in the New Year, having been dormant, Rip Van Winkel like, since about 1990.
 
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