How many pounds or ounces should it be?

LAST EDITED ON Apr-07-04 AT 02:00 PM (CDT)
 
Is there any target, pounds or ounces, to shoot for on line tension or is it the persons option to have what they like?

I have been thinking about putting a pull type scale in the control line to see exactly how much line tension in ounces I really have.

>Is there any target, pounds or ounces, to shoot for on line
>tension or is it the persons option to have what they like?
>
>I have been thinking about putting a pull type scale in the
>control line to see exactly how much line tension in ounces
>I really have.
>
>It might be a good idea to put a pointer on the lead out
>adjustment and mark several locations, 2oz., 4oz., 6oz.,
>8oz. (or what ever works), then it can be adjusted according
>to what a person wants or needs.
>
>Gary Weaver [email protected]

Well, Gary, it is certainly a person's option to have what they like. And there is no set value that is "right". It will also vary wildly according to weight, line length, air speed, engine power, how things are set up, whatever (You have to get a few "whatevers" in these days if you are going to speak proper English).

And, although there is the "rock on a string" effect (Centrifrugal or centripital force--shall we start another argument on that one?) there is also the flying effect of the airplane along with how you have it set up. A well trimmed half A will be lighter on the lines than a well trimmed .20 which will be lighter than a well trimmed .60, etc. And since we are trying to trim the airplane for flying, not just for pull on the lines, you are not going to get any real value in marking a location on the wing tip for lead out location. Indeed, you can trade off low level tension for upper level tension by moving the leadouts which would make any such markings meaningless. I like to feel the airplane out on the lines and know that it is going to stay there (personal preference), so I have never flown an airplane that I felt had too much line tension (no, I have never flown Paul Walker's B-17).

Although your experiment to measure the tension has merit and would be interesting, what you measure with your plane may (probably will) be different from what I would measure with mine. And what one person may already feel is too much another person may feel is "nice, but I could use more."

Question: How do you propose to attach the fishing scale to the lines and still maintainn control of the airplane while testing?

LAST EDITED ON Apr-07-04 AT 02:16 PM (CDT)
 
Gary,
Peter Soules Line II program figures out LO rake and a bunch of other stuff. It also figures out line tension. Run his program, and input all the data needed and it will spit out the information you're looking for. Do a search on this forum for a link to the program.

Try this:

http://www.clstunt.com/htdocs/dcforum/DCForumID1/7865.html

LAST EDITED ON Apr-08-04 AT 12:56 PM (CDT)
 
I wasn't thinking about all airplanes I was just thinking about one airplane at a time. Suppose a person has an airplane they are setting up to fly. I know there will be more line tension flying flat and lever and less tension flying over the top. Slower airplane will have less centrifical force. I suppose flying over the top or wing over will have the least amount of line tension. I assume 2oz or 3oz line tension on a wing over will have 6oz or 8oz line tension flying flat and level. Or is there some way to get line tension to be the same all the time?

There is an electronic device called a load cell. Its been 20 years sence I have fooled aound with one of these. As pressure is applied to the load cell resistance can be measured with an ohm meter. By knowing the resistance you can calculate the load in grams, ounces, pounds, etc. You can also build an electronic circuit with an IC and a digital read out that will tell you the exact figure in grams or ounces but for a one time experement an ohm meter would work fine.

Allied Electronics 1-800-433-5700, MPX pressure sensors, MPX 2000 series 0 to 29 psi, stock NO. 858-4998, $17.28.

>Is there any target, pounds or ounces, to shoot for on line
>tension or is it the persons option to have what they like?
>
>I have been thinking about putting a pull type scale in the
>control line to see exactly how much line tension in ounces
>I really have.

Here are the formulas that account for line tension due to uniform circular motion:

http://www.clstunt.com/cgi-bin/dcforum/dcboard.cgi?az=read_count&om=5919&forum=DCForumID1

Doing an experiment to *measure* the tension would be very valuable.

Tom Perry and Fred Randell did extensive tests of the strength of lines and terminations a looong time ago. MAN, July/Aug 1960. They found that the calculated pull was a bit less than the measured pull. Around 5-10%. So, if the formula calculates 10 lb. it is reasonably close to what the plane actually pulls.

>They found that the calculated pull was a
>bit less than the measured pull. Around 5-10%. So, if the
>formula calculates 10 lb. it is reasonably close to what the
>plane actually pulls.

This makes sense, and is the first information I've seen based on *measurements*. The line tension will be that due to uniform circular motion plus any additional tension due to engine offset and aerodynamic forces such as lift from the fuselage, rudder offset, etc. Given that engine offset and rudder deflection were very common practices then (and now for that matter) the fact that the tensions come out consistently high, but not drastically so, is just what you'd expect. The line tension is *dominated* by the effects of uniform circular motion (centripetal acceleration) with a small positive *correction* for aerodynamic lift and/or engine thrust offset.

I'll buy that!

>>They found that the calculated pull was a
>>bit less than the measured pull. Around 5-10%. So, if the
>>formula calculates 10 lb. it is reasonably close to what the
>>plane actually pulls.
>
>This makes sense, and is the first information I've seen
>based on *measurements*. The line tension will be that due
>to uniform circular motion plus any additional tension due
>to engine offset and aerodynamic forces such as lift from
>the fuselage, rudder offset, etc. Given that engine offset
>and rudder deflection were very common practices then (and
>now for that matter) the fact that the tensions come out
>consistently high, but not drastically so, is just what
>you'd expect. The line tension is *dominated* by the
>effects of uniform circular motion (centripetal
>acceleration) with a small positive *correction* for
>aerodynamic lift and/or engine thrust offset.
>
>I'll buy that!

I would only add that those "extra forces" are there result of things that I would classify as "compromises" in terms of trim. If you have roll that vectors the lift outboard, or some vector of the thrust outboard, when either changes due to maneuvering, then you get uneven line tension. It's my opinion, based on lots and lots of trimming experience, that you want those other forces to be 0, and that if you got exactly cetrifugal force everywhere (and the line tension went down with the elevation due to gravity) that's the optimal trim. It's certainly sufficient at reasonable speeds.

That's why marking a set position of the leadouts for "optimal" trim actually *does* make a whole lot of sense - and other positions to "adjust it" don't.

There's so much, well, bizzare, notions associated with line tension and trim (several of them exhibited in this thread), that it's really hard to get any sound grip on the physics of the situation.

It's not "Magic Line Tension Elves" or breakthrough physics that creates line tension and drives the state of trim, just plain old junior high physics. Which frankly seems to be either beyond many people, or more likely, buried under years of "fencepost wisdom".

I always try for 6 pounds, 8 ounces, 4 grams + or- one gram. Honest.

.

Hey Gary, nobody really addressed your question, even me.

The harder a plane pulls, the less the lines bow and the less they affect the plane in maneuvers. That is one reason for the trend to 64 oz. stunters. It takes that much pull(around 10 lb.) to stretch .018 lines tight enough at typical flying speeds. It's either that, or fly at 90 mph like a fast combat plane.

The real trick is in balancing all the variables so you end up with a plane that flies "nice". To me it seems that every design has a "sweet spot" for weight, airspeed, balance point, and line tension. Going faster makes it pull harder and fly better, but the control forces go up and it gives you less reaction time. Start going slower and you quickly reach a point where the plane feels like it is falling out of the air. A lot of planes don't seem have much of a sweet spot. You're always fighting one thing or another- too much weight and it pulls well but won't turn, too fast and it has good line tension but the controls are so heavy it is hard to turn. Move the balance point back and the control forces go down, but you lose control feel. And when it flys just right it feels like it's going to pull your arm out of its socket.

>The harder a plane pulls, the less the lines bow and the
>less they affect the plane in maneuvers.

Not at all. The higher the line tension, the more they control and are able to supply restoring force in roll and yaw. Hence they have MORE effect on the airplane.

The natural frequency, as a consequence, rises. This may increase or decrease the effects of line whip, depending on where it was to begin with and where it ends up.

> That is one reason
>for the trend to 64 oz. stunters. It takes that much
>pull(around 10 lb.) to stretch .018 lines tight enough at
>typical flying speeds. It's either that, or fly at 90 mph
>like a fast combat plane.

The 55-65 Oz airplanes are a consequence of the size and the construction methods of the airplane. The size is a consequence of the power available and the power delivery characteristics. The construction is a function of the required load-bearing characteristics and the required stiffness. That and the requirements for a 5+ oz paint job, drive the weight. The desired line tension has NOTHING to do with the process, nor do the size of the lines or their dynamics. Never enters into the design thinking.

People can, with heroic efforts, beat this weight, but it's not necessary if you aerodynamics match the expected wing loading.

Of course, the weight has huge effects on the line tension, but I have yet to meet anyone who designs stunt planes to hit a particular line tension, and says, "oh, it needs to be 64 oz to get the line tension I want and I'll build it accordingly".

>
>The real trick is in balancing all the variables so you end
>up with a plane that flies "nice". To me it seems that
>every design has a "sweet spot" for weight, airspeed,
>balance point, and line tension. Going faster makes it pull
>harder and fly better, but the control forces go up and it
>gives you less reaction time. Start going slower and you
>quickly reach a point where the plane feels like it is
>falling out of the air. A lot of planes don't seem have
>much of a sweet spot. You're always fighting one thing or
>another- too much weight and it pulls well but won't turn,
>too fast and it has good line tension but the controls are
>so heavy it is hard to turn.

Mostly right.

>Move the balance point back
>and the control forces go down, but you lose control feel.

Not really. Move the balance point back, and trim accordingly, and the control feel *improves* because the reduced control load reduces the effects of compliance in the lines, handle, controls, surfaces, etc. If it's designed to fly at the aft CG, there is no degradation in the tracking, and in fact most modern stunt planes track better with the CG further aft, and wander if the CG gets too far forward.

Fairy God mother;
Please give me a plane that flies at 40 mph and pulls so hard that I have to have bricks in my pockets on the wingover and square eight.
If you do I promise I won,t do that ANNNY more.
Please please please.

>Fairy God mother;
> Please give me a plane that flies at 40 mph and pulls so
>hard that I have to have bricks in my pockets on the
>wingover and square eight.

You can gets tons of line tension at 40 MPH - but it will mess everything else up so much you'll get *killed* in the square 8. Line tension isn't the only requirement.

In the legalese of the space biz - Enough line tension is a necessary but not sufficient condition to ensure good performance.

if you can build that 40 mph plane at 100 ounces and add enough wing area to maneuver, it will be awesome!! at least in calm weather. At 40 mph you can nearly count the inches when you try to time a pullout.

We used to do that with some sport half A combat planes. At 45 mph(3.3 sec laps on 35 ft lines) you can do reverse wingovers and literally clip the dandelions, almost at will. It gets exciting when the lines catch one though.

Brett, not trying to argue here, but I think is a pretty well-proven fact- If you slow a plane down the lines bow more and when this "bow" swings around in maneuvers it interacts with the plane and can whip it into ungodly attitudes if the pilot doesn't take it into account. I've seen many intermediate pilots literally put planes into the ground from this. Experienced pilots make corrections for this effect almost subconciously by how they feed in the control input and how they hold their hand relative to the plane.

More line tension reduces the angle at the leadouts and the bow in the lines. The smaller curve reduces the whipping effects. The current crop of planes may not be designed to a weight, but a side effect of the weight is that it reduces the bow in the lines, making the plane fly better.

The 10 lb. number is just an observation from the flying circle. A fast combat plane pulls about 10 lb. and flys with little regard to lines whip, at least until the plane slows down. The 60 ounce stunter pulls roughly the same on the same 018 lines at a much lower speed and behaves similarly. The bow in the lines has relatively little effect as long as the plane keeps up its speed.

>Brett, not trying to argue here, but I think is a pretty
>well-proven fact- If you slow a plane down the lines bow
>more and when this "bow" swings around in maneuvers it
>interacts with the plane and can whip it into ungodly
>attitudes if the pilot doesn't take it into account. I've
>seen many intermediate pilots literally put planes into the
>ground from this.

More line tension means more resorting force - meaning line tension has MORE influence on the airplane, not less. Taken to the extreme, no line tension means that the lines have NO effect on the airplane.

The reason that airplanes that are flying too slowly and have too little line tension have problems is that with less line tension, there is less tolerance to out-of-trim conditions, and thus small trim errors have far greater results. Because with less line tension the lines do not have enough influence to keep you out of trouble.

Same observed effects, but different explanations.

You seem to be talking about something else entirely. When the plane changes direction abruptly the sag in the lines goes down the lines to the plane in a curling wave. It is most obvious when the plane reverses direction. The plane switches direction of the turn and the bow in the lines doesn't follow immediately. When the wave hits the plane it can swing and roll it as much as 15-20 degrees. It looks alot like kids trying to get a jumprope going. As you slow the plane down, either through maneuvers or with a slower engine setting, the bow gets bigger and has more effect. Flying the plane faster, or adding more weight, generates more line pull and reduces the bow and its effects. If the plane flys faster than a certain speed range(for a given plane/engine/lines) the bow gets to be minimal and has minimal effects. Most stunt planes are flying in this range so you only see the trim effects of the plane.