Offline Jim Svitko
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Re: A weighty situation
« Reply #105 on: May 03, 2018, 01:11:20 PM »
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"I have also been curious about measuring the line pull in the air. With the proper equipment (instrumentation?) I suppose it could be done if someone wanted to go thru the steps. But, is it worth the time spent? Maybe, for some, but for me, it is easier to do the math. I know the weight (mass) of the plane, the radius of the circle, and with a standard lap time (5 seconds, give or take a few tenths?), I plug the numbers into the formula."
Yes, there was at least one in-situ measurement of line tension. The results were published back in probably the early 1960's. As I recall, it was done by the racing guys. They probably used load cells at the handle, or less likely, strain gages on the lines. (I remember looking at the photos and trying to decide what type of transducer was being used. I couldn't tell, other than there were electrical lead wires to some electronics strapped the pilot's belt.) They plotted the loads and compared them to the straight "centrifugal force" calculation. Their conclusion was that the actual loads were higher than the simple mass/centrifugal force calculation. I would have to find the article to see if they characterized the uncertainty of the test results. The biggest question I would have is whether they grabbed random instantaneous values or were able to collect max/max value.
A few other cautions: pilots often whip at times, which increases the line load temporarily beyond the simple centrifugal force calculation. Also, I suspect that trim can cause a significant offset in the straight and level line tension. Lots of line rake, rudder offset, engine offset, etc. These cause aerodynamic loads that are not accounted for in the simple centrifugal force calculations. Bob mentioned in one of his posts that he could show us how to reduce the excessive line tension--I liked his oil painter metaphor. I used a different one when I tried to describe motor skills and strength to guys at work: try holding a suitcase in one hand and then try to take the cap off a tube of Chapstick and apply to lips. The extra weight demolishes your fine control. Go ahead, no one is looking, try it!
Also note that increased line tension in the wind--which everyone can relate to--occurs from two sources: so-called wind-up where the plane speeds up, and also from the "crosswind drag" on the plane. Planes are not very streamlined along the lateral axis, and you can feel it. So the first one is mass related and the second is aero related.
The team race (F2C) guys experimented years ago with engine in-thrust and banana fuselages to try to reduce the in-flight drag. Not sure how much that achieved--but it made the planes a real #$@%?! to take off. Not sure anyone is still building/trimming that way. However, I know that racers trim for least out-thrust of any of the events that I have participated in. Zero everything and then push the leadouts forward until you have trouble on takeoff. Not sure how the old B-TRs in the study were trimmed, but I would start by assuming a stunter would have more aerodynamically generated line tension.
One of the other posts implied that a centrifugal force calculation wasn't valid during maneuvers. I disagree: the component of force that is solely due to the accelerating mass is always valid--if you put in the correct velocity and other inputs. However, as I tried to list above, it is only one of the vector components.
Dave
Topic: A weighty situation (Read 11968 times)