Line Pull

[KenP51]

OK guys, I was really curious about the line pull question raised by a previous post I made. http://stunthanger.com/smf/index.php?topic=24879.msg239472#msg239472
(Really cool plane by the way)
I have previously tried to calculate line pull before. I really don't think I know what I am doing 
So not being an engineer or having the math abilities myself I went to the following websites to try to figure out line pull.http://www.easycalculation.com/physics/classical-physics/centripetal-force.php
And used the formula f=mv²/r    where f= force in Newton’s and m= mass in kilograms and v=speed in meters per second
I also used the following sites to convert weight and speed to metric and back
http://www.towerhobbies.com/help/convcalcs.html
http://www.unitconversion.org/unit_converter/force.html and selected from Newton [N] to pound-force [lbf]
I then made a spread sheet to convert the weight, speed and radius to metric . Then work the Centripetal Force equation. I then converted back from metric.
I think I goofed it up. Not just the spreadsheet. I think my method is wrong.
Anyone know what I did wrong or did I actually get it right?
I can't upload the spread sheet I made. That type of file is not allowed. Any way to put my spreadsheet up?

[Tim Wescott]

f = m * v^2 / r will work in any units, as long as they're consistent.

So I'll use the English units, with which we're all familiar  .

Your referenced thread was for that mondo B-36, and you mentioned that it is 15 pounds.  So, for a 15 pound airplane with a wingspan of 64", on 61 foot lines, flying 6 second laps, we get:

Mass = (15lbm)/(32slugs/lb) = 0.47 slugs (nice easy Imperial units -- remember?)
effective R = line length + 1/2 wingspan = 63.7'
velocity = circle circumference / speed = (63.7' * 2 * pi) / (6 seconds) = (400')/(6 seconds) = 66.67 ft/sec

f = (0.47 slugs) * (66.67 ft/sec)^2 / (63.7 ft) = 32.8 slug-ft/sec^2.

That's not a useful measure, but remember from high-school physics (assuming you had a demented physics teacher) that by definition one slug accelerated at 1 ft/sec^2 exerts a pound of force, so

f = 32.8 lbf.

That's not terribly bad.

If you're too Euro for good old fashioned Imperial units, then you can go

m = (15lb)/(2.2lb/kg) = 6.8kg,
v = (66.667 ft/sec) * (.0254m/in) * (12 in/ft) = 20.3 m/s
r = (63.7 ft) * (0.0254 m/in) * (12 in/ft) = 19.4 m

So

f = (6.8 kg) * (20.3 m/s)^2 / (19.4m) = 144 kg-m/s^2 = 144N

f = (144N) * (0.225 lbf/N) = 32.5 lbf.

Given that I was playing fast and loose with my precision, these match up pretty well, and they're within the realm of possibility for flight.  Your arm might fall off after the end of six minutes, and that may be why the thing was sitting in an FBO instead of being all worn out from fun flying, but I think you could do it.

[bill bischoff]

I remember the equation {.0668 x pounds weight x MPH x MPH / radius in feet} from back in the day when I was CLCB chairman and we were working on pull tests and line sizes. The coefficient .0668 allowed the use of normal values of pounds, mph, feet. This would give a pull of 35.8 lbs  for 15 lbs weight/ 50 mph/ 70 ft

[KenP51]

thanks guys now I am getting numbers that make sense

[Howard Rush]

Speaking of line pull, I got one of these cute digital pull test scales from Amazon: http://www.amazon.com/Portable-Digital-Hangging-Luggage-Fishing/dp/B002GXO5E6/ref=sr_1_1?ie=UTF8&qid=1326960053&sr=8-1  (first picture below).  Naturally, I took it apart.  That's what I do with things (second picture).  It looks like a strain gauge doodad on that bar in the middle.  Four wires come out.  Wouldn't it be cool to put one of these on each line at the handle and measure line tension? 

[Mark Scarborough]

Howard, I would LOVE to see that, especially if you could log the output into some sort of graphical, or even database on say, a sampling rate of 10 times a second, or even higher ...
It would be fascinating to know what actually happens realtime with loads.. Many debates about whether a single line carries the load during control input would be put to rest,,

so when are you going to have it ready 

[KenP51]

That is cheap enough to allow experimentation with.
The electronically savvy types could buy a suitable strain gauge and add the electronic to run it but I bet not as cheap as just salvaging out a pair of those.
only $10.16

As a slight side trip (but related) to this thread could the lines on a large control liner be longer than 70 foot just for sport flying? Correct me if I am wrong but I think 70' is the longest you can use in sanctioned events.
I know line drag would go up (I bet a bunch) and that would quickly set a upper limit. But where (how long)?
There is a 1 to 1 line pull in #s ratio involved with the line length and also in model weight, i.e. double line length and halve the pull or halve the model weight and halve the pull. Speed has a lot more effect.
Just curious. I am not thinking of try beyond 70' lines now,(or proposing that any else do it) but just wondering if it could be done and what the limits would be. A mental exercise if you would.
I do not own a flame suit so no    

[bill bischoff]

Speaking of line pull, I got one of these cute digital pull test scales from Amazon: http://www.amazon.com/Portable-Digital-Hangging-Luggage-Fishing/dp/B002GXO5E6/ref=sr_1_1?ie=UTF8&qid=1326960053&sr=8-1  (first picture below).  Naturally, I took it apart.  That's what I do with things (second picture).  It looks like a strain gauge doodad on that bar in the middle.  Four wires come out.  Wouldn't it be cool to put one of these on each line at the handle and measure line tension? 

We did this in the early 1990's when I was CLCB chairman. Most of it was home-made then. We found that direct reading was next to useless, as the readings were constantly changing. We ended up recording data on cassette tapes, and analyzing it on a PC after the fact. The conclusion that we reported to the AMA was that the maximum pull recorded was about 20% higher than theoretical. This would indicate that maximum instantaneous velocity was about 10% above average velocity over 1/2 mile. I think the rule book shows the calculation I mentioned above, with the coefficient of .080 instead of .0668 to represent the 20% higher figure.

Bill Bischoff

[Jim Thomerson]

Many years ago, at the university, I asked one of the physics majors for the equation for centrifugal force. He said he didn't know, but would derive it for me.  He did so in a minute or two with vector diagrams on a piece of paper.  I was impressed, much more than if he had recited a memorized formula.

[Howard Rush]

Many years ago, at the university, I asked one of the physics majors for the equation for centrifugal force. He said he didn't know, but would derive it for me.  He did so in a minute or two with vector diagrams on a piece of paper.  I was impressed, much more than if he had recited a memorized formula.

Reminds me of a story, of course.  We aeros were required to take a class in electrical engineering.  It was a lot of fun with the aeros and teacher teasing each other.  He gave us a mnemonic for power: "Remember the little poem, 'Twinkle, twinkle little star.  Power equals I squared R.'"  We were all pleased.  We could use this on the exam and get a quick answer without having to derive the formula.  Then he went on to say, "Do not remember the little poem, 'Little star up in the sky, Power equals R squared I', because it's wrong."  A unanimous groan arose.  We knew we couldn't remember which one was right, so we'd have to derive it.  

[Larry Cunningham]

Speaking of line pull, I got one of these cute digital pull test scales from Amazon: http://www.amazon.com/Portable-Digital-Hangging-Luggage-Fishing/dp/B002GXO5E6/ref=sr_1_1?ie=UTF8&qid=1326960053&sr=8-1  (first picture below).  Naturally, I took it apart.  That's what I do with things (second picture).  It looks like a strain gauge doodad on that bar in the middle.  Four wires come out.  Wouldn't it be cool to put one of these on each line at the handle and measure line tension? 

"That was a priceless Steinway!"

Inspector Clouseau: "...Not anymore"

L.

PS - it would be VERY COOL.. Wish you would do it

"Don't touch my men-kee.." -Dieter, in Sprockets

[Larry Cunningham]

Reminds me of a story, of course.  We aeros were required to take a class in electrical engineering.  It was a lot of fun with the aeros and teacher teasing each other.  He gave us a mnemonic for power: "Remember the little poem, 'Twinkle, twinkle little star.  Power equals I squared R.'"  We were all pleased.  We could use this on the exam and get a quick answer without having to derive the formula.  Then he went on to say, "Do not remember the little poem, 'Little star up in the sky, Power equals R squared I', because it's wrong."  A unanimous groan arose.  We knew we couldn't remember which one was right, so we'd have to derive it. 

From Thermo.. "Fishy, fishy in the sea. PV = nRT"

L.

PS - and, ELI the ICE man

"He has a 110 ohm nose.." -- said of a sycophant

[Larry Cunningham]

Many years ago, at the university, I asked one of the physics majors for the equation for centrifugal force. He said he didn't know, but would derive it for me.  He did so in a minute or two with vector diagrams on a piece of paper.  I was impressed, much more than if he had recited a memorized formula.

Pi factors right out.

L.

"Progress is not created by contented people." -Frank Tyger

[Howard Rush]

PS - and, ELI the ICE man

and the promiscuous Violet

[Eric Viglione]

She like's bad boys...

[Larry Cunningham]

..for Silver and Gold

L.

PS - I don't know what the big deal on Star Trek is with phasers. We studied them in 1967 in Dr. Crosno's Electrical Machinery class.

"The greatest right in the world is the right to be wrong." -William Randolph Hearst

[phil c]

Howard, I would LOVE to see that, especially if you could log the output into some sort of graphical, or even database on say, a sampling rate of 10 times a second, or even higher ...
It would be fascinating to know what actually happens realtime with loads.. Many debates about whether a single line carries the load during control input would be put to rest,,

so when are you going to have it ready 

The speed fliers did exactly that back in !~1957.  The article was published in Model Airplane News.  The gist of it was that plain old calculations were within the experimental error of the measurements.  Unless you do something really out of the ordinary with engine offset, leadout rake etc. the line pull calculations are close enough.

The one proviso is that it is hard to calculate what the line pull is when the plane snaps in and cuts across the circle.  That can generate some pretty high accelerations.  So if that does happen to you put your hand straight up over you head and hope for the best.  When the plane hits the end of the lines your arm won't be able to straight and will cushion the shock.  From experience, holding your hand out in the normal flying position won't absorb much shock.  You can easily snap .018 lines even with a small light plane..

[Howard Rush]

This exercise is to measure differential line tension due to control surface hinge moment.

[Paul Smith]

Yesterday we had winds gusting to 54 MPH here in Sterling Heights Michigan.

I was carrying a wood plank from Home Depot to my car.  The plank has a wing area of 768 square inches but not much of an airfoil.  It came very close to lifting me of the ground.

The rock-on-a-string formula works perfectly for a streamline rock on a thin string.  I can't do the math, but I feel that the downwind force on a model is greater than the force computed by the simple formula.

[Serge_Krauss]

To get a line tension in pounds, you can also simplify the F_c = mv²/r expression to

F_c = 1.226rW/t², for W in pounds,

or

F_c = .0766rW/t², for W in ounces,

where r = radius in feet, and t = lap time in seconds.

SK

[Mark Scarborough]

Guys, you are missing the point of this thread I beleive,,
I think you will find that the POINT of this was to measure the line tension on EACH line as the plane manuevers to compare the differential tension, not the overall tension.. so an effective analysis of the tension with respect to handle input could be made. Especially since the topic of "hanging on one line" comes up so  often

[Howard Rush]

I think Ken wanted to calculate total line tension, and I changed the subject.  Sorry.

[Mark Scarborough]

I guess that puts me at the chopping block too cause I followed you right on down that path,,
( although, I find the differential question rather intriguing myself)

[Howard Rush]

Then there was the detour through mnemonics.  Here we go again.

[KenP51]

I think Ken wanted to calculate total line tension, and I changed the subject.  Sorry.

I started to reply yesterday, Then my computer went belly up. Bad drive.
I am rebuilding now.

No need to apologize guys.
I rather enjoyed were the discussion went. I became real curious about line pull from a discussion with some one and remembering a .piped 61 plane that I had a one time that pulled like a mule. Wore me out to fly it.
I now realize I most likely could have tamed the beast down by bringing the lead-outs forward plus a little tweaking on the engine offset and rudder.
Thanks for all the info.

[phil c]

Guys, you are missing the point of this thread I beleive,,
I think you will find that the POINT of this was to measure the line tension on EACH line as the plane manuevers to compare the,  tension, not the overall tension.. so an effective analysis of the tension with respect to handle input could be made. Especially since the topic of "hanging on one line" comes up so  often

Since the max force you can apply is the centrifugal force that is a key li,mit.  short pf jerking on  the handle there is not  much  more to be  had.  Wild Bill'ss various equations and nomographs  are adequate calculating the pushrod forces if you want to go to the bother o figuring intermediate deflections.

Paul Smith's point about wind is a good one.  A 20 mph breeze can easily add  enough force to break an .018 in. line.