News:


  • March 28, 2024, 04:22:17 PM

Login with username, password and session length

Author Topic: Line length and engine tune.  (Read 4999 times)

Offline Curare

  • 2014 Supporters
  • Trade Count: (0)
  • Captain
  • *
  • Posts: 779
Line length and engine tune.
« on: December 14, 2014, 01:11:49 AM »
Hey guys, just trying to figure out whether I noticed a bonafide phenomenon today, or whether I'm just crazy.

I pulled my chipmunk off the wall, which is slowly getting dialled in, tank position is now right so the tune doesn't change inside to outside.

I was thinking that at 62 feet eye to eye, my lines could do with maybe being a foot longer, so I fitted 12" extensions in to test the theory, the first flight was done on 62's.

After taking off I noticed that engine sounded leaner, and sure enough if I tried to do anything with sustained vertical, like a vertical 8, it would sag on me.

I hadn't touched the needle between flights, the only change was the line length.

It got me thinking, would the reduced centrifugal force be changing the tune? Sounds logical as motors go rich after launch, unless the tank is inboard.

Am I crazy?
Greg Kowalski
AUS 36694

Offline Paul Smith

  • 24 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 5793
Re: Line length and engine tune.
« Reply #1 on: December 14, 2014, 07:47:34 AM »
You may have point.
When a model flys on longer lines to achieves a higher true airspeed due to flying closer to straight line.  So the prop might unload more RPM and change your setting.
Line drag is not all that significant. According to Netzeband's theories, the signficant part of line drag occurs in the outer 1/3 to 1/4 of line length, so the extra 2 feet of wire is trivial compared to the bigger circle.
Paul Smith

Offline Keith Miller

  • Trade Count: (0)
  • Commander
  • ****
  • Posts: 210
Re: Line length and engine tune.
« Reply #2 on: December 14, 2014, 11:59:50 AM »
I did some quick math.
Assuming that your plane flies at the same linear air speed, changing only line length by 1 ft (from 60 to 61 ft) would reduce the centripetal force by ~.84%.  So for the short column of fuel in the tank, assuming density of methanol and a 2" deep tank, the change in pressure at the venturi would be only about -0.000463487 psi.
I'm not thinking your engine would notice that.

Just my thinking....

Offline phil c

  • 21 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 2480
Re: Line length and engine tune.
« Reply #3 on: December 14, 2014, 12:54:15 PM »
You may have point.
When a model flys on longer lines to achieves a higher true airspeed due to flying closer to straight line.  So the prop might unload more RPM and change your setting.
Line drag is not all that significant. According to Netzeband's theories, the signficant part of line drag occurs in the outer 1/3 to 1/4 of line length, so the extra 2 feet of wire is trivial compared to the bigger circle.

I'd agree with Paul.  If you are looking for an explanation the plane will fly a bit faster.  You mentioned it sounded leaner which would go with the motor working a bit harder.   If you're running a uniflow tank the change is fuel pressure would be very low.  If the line extensions were at the handle they'd cause very little drag.  If at the plane the combination of extra line clips and extra line would cause some extra drag.

More experiments, please.  If you feel like it.  Most people can feel a difference of 6 in. in line length, so start a couple feet long and cut the lines down 6in. at a time until the feel on the lines is solid, the plane stays out well in maneuvers, and wind bothers it least.  The judges will notice if the plane is flying nicely more than they will notice that the lines are a bit shorter than most.
phil Cartier

Offline Howard Rush

  • 22 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 7805
Re: Line length and engine tune.
« Reply #4 on: December 14, 2014, 01:27:47 PM »
I'd agree with Paul.  If you are looking for an explanation the plane will fly a bit faster.

Why would it fly faster?  Stop waving those arms and get out a pencil.
The Jive Combat Team
Making combat and stunt great again

Offline john e. holliday

  • 24 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 22752
Re: Line length and engine tune.
« Reply #5 on: December 14, 2014, 01:44:27 PM »
No matter the line length the plane will have a happy speed  it likes to fly at.   I was told at one time between 60 - 65 mph a aerobatic plane is happy.   Speed to the pilot or lap speed is what the pilot needs to adjust the line length for.   Never can get that through to people.   Like Howard says get a pencil and paper and start doodling.
John E. "DOC" Holliday
10421 West 56th Terrace
Shawnee, KANSAS  66203
AMA 23530  Have fun as I have and I am still breaking a record.

Offline Chris Wilson

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 1710
Re: Line length and engine tune.
« Reply #6 on: December 14, 2014, 01:52:11 PM »
Extra line length is more weight, drag and sag and where are the extensions placed? At the model end or with the handle.

I am betting that the model actually flies more slowly and the engine needs more fuel or energy to compensate and keep the same 'sound' as before.
MAAA AUS 73427

You have enemies? Good. That means you've stood up for something, sometime in your life.
 Nothing in life is so exhilarating as to be shot at without result.  It's not enough that we do our best; sometimes we have to do what's required

Offline Curare

  • 2014 Supporters
  • Trade Count: (0)
  • Captain
  • *
  • Posts: 779
Re: Line length and engine tune.
« Reply #7 on: December 14, 2014, 03:17:43 PM »
I was extending to see what difference it would make to line tension and lap time. While not acutally timing it, it was a question of feel. On 60' lines the wingover was over and done with before I had time to think!

I'm now beginning to think that maybe the fact that the 62' flight was early morning, and the 63' flight was mid morning, there was some temperature increase. I can't see why that would have made it leaner, I would have thought richer, stoichiometry and all that.

It may be over propped and undercooled, but it's a 46LA with an 12x3.75 APC, and open cowling!
Greg Kowalski
AUS 36694

Offline Igor Burger

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 2165
Re: Line length and engine tune.
« Reply #8 on: December 14, 2014, 04:12:55 PM »
I bet that longer lines will cause another yaw angle, nose will be more inward to the circle due to straighter air flow (circular air flow will not push nose out so much) and more line drag. Difference between ventury and tank can easily cause that pressure difference. Try little rudder out when you fly longer lines and I think it will come back.

Offline Curare

  • 2014 Supporters
  • Trade Count: (0)
  • Captain
  • *
  • Posts: 779
Re: Line length and engine tune.
« Reply #9 on: December 14, 2014, 05:26:35 PM »
 Rudder out or lines moved further aft?
Greg Kowalski
AUS 36694

Offline Ron Cribbs

  • Trade Count: (0)
  • Commander
  • ****
  • Posts: 330
Re: Line length and engine tune.
« Reply #10 on: December 14, 2014, 06:22:17 PM »
You'll have to excuse my ignorance, but I have a hard time understanding how line length has a profound effect on an engine run.

You mentioned that you hadn't touched the needle since your last flight, perhaps the temp, humidity, or atmospheric pressure was different. Maybe the needle needed a tweak?

In my limited experience, the needle is always close between flights, but always requires a minute adjustment.

Ron

Offline Douglas Ames

  • 2014 Supporters
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 1299
Re: Line length and engine tune.
« Reply #11 on: December 14, 2014, 07:52:37 PM »
Airspeed is the same. Since the circumference is bigger it takes a longer distance/time to travel the circle at the same airspeed.
Centrifugal force would be less (less arc) which might be causing your mixture change? Just my 2c.
AMA 656546

If you do a little bit every day it will get done, or you can do it tomorrow.

Offline Igor Burger

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 2165
Re: Line length and engine tune.
« Reply #12 on: December 15, 2014, 01:21:26 AM »
Rudder out or lines moved further aft?

Yes, rudder, it will directly compensate both, also line drag, also different circular flow and it will keep fuselage at the same angle as before. If you move LO, it can solve the same problem, but model will be in another trim, so it can act different in maneuvers (when the line tension changes).

Offline Dennis Moritz

  • 22 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 2464
Re: Line length and engine tune.
« Reply #13 on: December 15, 2014, 06:06:19 AM »
Open the needle a click. Or two. Or turn the needle out a quarter to half a turn. 2 stroke model airplane engines are simple mechanically but touchy. Numerous variables can alter tune. Key question is whether or not engine will power through the maneuvers with authority. Extra line length can have a dramatic effect on feel and ability of the plane to go up and over and stay out on lines when doing overheads. Even horizontal maneuvers are affected. I've had models dramatically change how they fly when line length is increased or decreased.

Offline Bob Reeves

  • 2016 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 3415
    • Somethin'Xtra Inc.
Re: Line length and engine tune.
« Reply #14 on: December 15, 2014, 06:33:49 AM »
Agree, changing the line length had nothing to do with the engine tune.. More like the needle needed to be tweaked in the first place due to a temperature or atmosphere change.

Longer or shorter lines will require a re-trim, mainly lead out position and tip weight.

Offline bob whitney

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 2244
Re: Line length and engine tune.
« Reply #15 on: December 15, 2014, 07:27:44 AM »

 Diff but the same .i fly my deisel F2C team racer on 52 ft lines  and it is a full time job keeping it at top speed without over heating,  i also fly it in Rat Racing on the same size lines .014 at 60 ft  the engine acts like a sport engine for the whole 140 laps , i am often asked why i cant get it to run like that in F2C
rad racer

Offline Paul Smith

  • 24 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 5793
Re: Line length and engine tune.
« Reply #16 on: December 15, 2014, 08:47:37 AM »
When they make the lines longer in speed and racing the true airspeed always increases.  The goal is to reduce line pull and slow down rotational speed.  It always partially achieves those goals, but not as much as simple arithmetic would suggest.

On a stunter, the time to fly the pattern will increase, as well as the time do each loop.   The bigger loops will result in drastically reduced G force during the maneuver and noticeable change in engine performance.

Paul Smith

Offline Norm Furutani

  • Trade Count: (0)
  • Commander
  • ****
  • Posts: 212
Re: Line length and engine tune.
« Reply #17 on: December 15, 2014, 10:34:25 AM »
Maybe I'm missing something, but I would try flying extensions on, extensions off several times and see if you get the same results?

Norm

Offline John Park

  • Agricola
  • 2017
  • Trade Count: (0)
  • Captain
  • *
  • Posts: 458
Re: Line length and engine tune.
« Reply #18 on: December 15, 2014, 10:43:29 AM »
It's nothing to do with changes in mixture or G force.  It's all to do with LOAD, which is why an engine set up for a typical stunt run 'richens up' and starts four-stroking when it takes off and gets up to flying speed, and why it 'leans out' to a two-stroke when you increase the load by pulling the model up into a manoeuvre.  You can experience the same phenomenon on a small two-stroke motorcycle - ride it slowly on a level road so that it four-strokes, and feel it 'lean out' to a two-stroke when you encounter an incline, with NO change to the throttle setting.  The apparent 'leaning-out' in the case under discussion is, I submit, simply because of the small amount of extra drag imposed on the model by the longer lines.
You want to make 'em nice, else you get mad lookin' at 'em!

Offline Chris Wilson

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 1710
Re: Line length and engine tune.
« Reply #19 on: December 15, 2014, 01:08:28 PM »
It's nothing to do with changes in mixture or G force.  It's all to do with LOAD, which is why an engine set up for a typical stunt run 'richens up' and starts four-stroking when it takes off and gets up to flying speed, and why it 'leans out' to a two-stroke when you increase the load by pulling the model up into a manoeuvre.  You can experience the same phenomenon on a small two-stroke motorcycle - ride it slowly on a level road so that it four-strokes, and feel it 'lean out' to a two-stroke when you encounter an incline, with NO change to the throttle setting.  The apparent 'leaning-out' in the case under discussion is, I submit, simply because of the small amount of extra drag imposed on the model by the longer lines.
I am in total agreement with this.
Thanks for wording it in such an easily digestible form.
MAAA AUS 73427

You have enemies? Good. That means you've stood up for something, sometime in your life.
 Nothing in life is so exhilarating as to be shot at without result.  It's not enough that we do our best; sometimes we have to do what's required

Offline Paul Smith

  • 24 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 5793
Re: Line length and engine tune.
« Reply #20 on: December 15, 2014, 03:18:46 PM »
Maybe I'm missing something, but I would try flying extensions on, extensions off several times and see if you get the same results?

Norm

Spend a few bucks and make some genuine 60, 63, and 65-foot lines.  This extension business is messing up the data.  You can always shorten the lines that prove to be too long.
Paul Smith

Offline Howard Rush

  • 22 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 7805
Re: Line length and engine tune.
« Reply #21 on: December 15, 2014, 03:27:57 PM »
I am in total agreement with this.
Thanks for wording it in such an easily digestible form.

Easy for you.  I get indigestion from seeing these "explanations" with no quantitative substantiation.
The Jive Combat Team
Making combat and stunt great again

Offline Chris Wilson

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 1710
Re: Line length and engine tune.
« Reply #22 on: December 15, 2014, 05:03:23 PM »
Easy for you.  I get indigestion from seeing these "explanations" with no quantitative substantiation.
Howard, can can you have an accurate output when there is no accurate input?

You can only use reason and extrapolate on these kinds of questions.

Please don't get indigestion mate.
MAAA AUS 73427

You have enemies? Good. That means you've stood up for something, sometime in your life.
 Nothing in life is so exhilarating as to be shot at without result.  It's not enough that we do our best; sometimes we have to do what's required

Offline Curare

  • 2014 Supporters
  • Trade Count: (0)
  • Captain
  • *
  • Posts: 779
Re: Line length and engine tune.
« Reply #23 on: December 15, 2014, 06:38:35 PM »
Oh I do love it when I open a can of worms.  ::)

Greg Kowalski
AUS 36694

Offline bob whitney

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 2244
Re: Line length and engine tune.
« Reply #24 on: December 15, 2014, 06:41:33 PM »
Spend a few bucks and make some genuine 60, 63, and 65-foot lines.  This extension business is messing up the data.  You can always shorten the lines that prove to be too long.

if the extentions are at the handle what diff would there be from one solid line????
rad racer

Offline Chris Wilson

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 1710
Re: Line length and engine tune.
« Reply #25 on: December 15, 2014, 06:52:47 PM »
if the extentions are at the handle what diff would there be from one solid line????
Probably no difference will be noted if the extensions at the handle end but the point is that if they are at the model end it will be far more critical.
MAAA AUS 73427

You have enemies? Good. That means you've stood up for something, sometime in your life.
 Nothing in life is so exhilarating as to be shot at without result.  It's not enough that we do our best; sometimes we have to do what's required

Offline Steve Hines

  • 21 supporter
  • Trade Count: (0)
  • Captain
  • *
  • Posts: 495
Re: Line length and engine tune.
« Reply #26 on: December 15, 2014, 11:22:03 PM »
Did you take them back off and then try to fly it again. Same temp same fuel. This is the only way to see if it was the lines. How long was this engine been sitting with out running.

Steve

Offline Randy Cuberly

  • 21 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 3674
Re: Line length and engine tune.
« Reply #27 on: December 15, 2014, 11:25:01 PM »
Easy for you.  I get indigestion from seeing these "explanations" with no quantitative substantiation.

GIGO Howard...too many variables that are totally unknown if you ask me!  ~^ n~

Randy Cuberly

PS:  Richen the needle a bit and try it again!  Then do what Igor told you to above!

Randy Cuberly
Tucson, AZ

Offline Howard Rush

  • 22 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 7805
Re: Line length and engine tune.
« Reply #28 on: December 16, 2014, 11:38:06 PM »
Howard, can can you have an accurate output when there is no accurate input?

So you are "in total agreement" with an assertion that the extra drag of a foot of lines suffices to lean out the engine, causing it to sag in a vertical eight, having come to this conclusion from "no accurate input".

Here's how I would go about evaluating some of the engineering claims above.  First, do some rudimentary calculations to see if an assertion is anywhere in the ballpark.  For example, take the assertion that the extra drag of an extra foot of lines causes the engine to lean way out.  In this case, there are indeed some unknowns: airplane speed, the length of Greg's arm and handle, and air density.  I could have asked Greg, but reasonable assumptions suffice for now.  They can be refined later if needed.  I assumed airspeed to be constant: that's the worst case.  It would result in the greatest drag difference. If the answer you get for the difference in drag is trivial, you can dismiss the assertion and not embarrass yourself by stating it or not be led astray by accepting it.  I did some calculations on the attached spreadsheet.  Assumptions are in yellow.  You can see the formulas in the cells.  Tell me if I made any errors.  Looks like drag is .0048 lb. more with 63' lines than with 62' lines.  A lap time difference of 40 milliseconds would give the same drag increment.  This wouldn't cause your engine to lean way out and not make it to the top of a vertical 8.  Another equivalent drag increment would come from turning a Supertigre-type needle valve from fore-aft alignment to sideways if the bent part is .72 inch long (not counting the effect of changing the mixture to the engine).  

The next step would be to vary the assumptions to see if they matter.  We do this systematically in airplane trade studies to see which inputs we have to refine and which we don't need to mess with-- Google "tornado chart".  For example, what if Greg's arm is 4 feet long?   Lap time difference to get the same incremental drag would be 39 milliseconds, rather than 40.   You can fiddle with the inputs on the spreadsheet to see what happens, but I think the extra-drag explanation can be dismissed.  

When a model flys on longer lines to achieves a higher true airspeed due to flying closer to straight line.

From the spreadsheet you can see that the incremental angle from the different circle curvature at the prop disk is .01 degree, equivalent to an incremental crosswind of .017 ft./sec.  The incremental angle from the different circle curvature at the rudder is .02 degree, equivalent to an incremental crosswind of .028 ft./sec.  I don't know about your engine, but I don't think .028 ft./sec. of wind would make any of mine go nuts.  Maybe you think that the curvature has some kind of dissipative effect, such as that which will cause the earth to crash into the sun in a couple years.  

The bigger loops will result in drastically reduced G force during the maneuver and noticeable change in engine performance.

Drastically?  Come on, Paul, you're an engineer.  You'd get an equivalent G force reduction from going 39 milliseconds slower per lap.  The engine wouldn't notice.  

Igor's rudder compensation would require an incremental .05 oz of side force on the vertical tail.  Don't overdo the rudder tweak.  
The Jive Combat Team
Making combat and stunt great again

Offline Howard Rush

  • 22 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 7805
Re: Line length and engine tune.
« Reply #29 on: December 16, 2014, 11:48:50 PM »
Oh, I assumed that lines and needle valve have a Cd of 1.  Refine that from Line III or Schlichting's book if you want.
The Jive Combat Team
Making combat and stunt great again

Offline Dennis Moritz

  • 22 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 2464
Re: Line length and engine tune.
« Reply #30 on: December 17, 2014, 01:05:16 AM »
Open the needle a click or two. Or do the equivalent without the clicks, if it's a Super Tigre needle. If you're still running over lean, check for crud in the filter, leaks in tank or tubing. And so on. Howard is saying, if you're engine is going berserk, it's not the lines. On the other hand, I have had the feel of planes change noticeably when altering line length. Even changes as small as one foot, felt like it had an effect.
« Last Edit: December 17, 2014, 06:57:56 AM by Dennis Moritz »

Offline Igor Burger

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 2165
Re: Line length and engine tune.
« Reply #31 on: December 17, 2014, 01:14:23 AM »
Igor's rudder compensation would require an incremental .05 oz of side force on the vertical tail.  Don't overdo the rudder tweak.  

Right, I did not do math, and I say t is only my bet if everything else is equal, because there could be several reasons which could be diffent between flights, for example mentioned temperature or any other atmospheric difference ... and I did not do it because it is too complex. There are several thing coming to equation:

1/ drag of lines which is higher on longer lines so makes inward moment

2/ fuselage moment (out of circle) which is lower on longer lines so makes inward moment

3/ unequal wing drag on left/right is lower on longer lines so makes inward moment

4/ prop moment of angled prop disc which is lower on longer lines because air comes more perpendicular so makes lower outward = relative inward moment

yes all those things caused by different radius are very small, in range under 1% (and some of them far under), but almostnothing + almostnothing + almostnothing + ... + almostnothing + almostnothing = something  ;D

plus if you compensate such moment only by lines (in the case that fuselage moment does not change with yaw angle), it can make visible difference in yaw angle additionaly all multiplied by centrifugal force / G force (relative to ground stand). To that conlusion leads me one experience happened during initial trimming of new model with OS .46 - I lost 5 g weight during flight from tip weight (temporarily fixed by tape) ... motor went immediatelly rich ... when I installed it back, motor worked as before - all without needle adjusting.  

Offline John Park

  • Agricola
  • 2017
  • Trade Count: (0)
  • Captain
  • *
  • Posts: 458
Re: Line length and engine tune.
« Reply #32 on: December 17, 2014, 07:36:39 AM »
The point I was trying to make in my previous post was that, in the set of circumstances described, there isn't any 'rich/lean' - just a lightish load, which allows the engine to four-stroke, and a slightly heavier load, which just tips it over the edge into a two-stroke.  This is for a traditional 4/2/4 run: if you set the needle richer, for a constant four-stroke through all manoeuvres, you may hear the occasional 'snarp' of two-stroking if you add just a little extra load by pulling the second or third corner of the hourglass a bit too tight.
You want to make 'em nice, else you get mad lookin' at 'em!

Offline Dennis Moritz

  • 22 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 2464
Re: Line length and engine tune.
« Reply #33 on: December 17, 2014, 10:05:16 AM »
I wouldn't be surprised if changing the line length does effect the way an engine runs. Everything else effects these engines to one extent or another. Some setups no doubt more sensitive than others. Also. Flyers differ widely in their ability to gauge an engine's tune as signified by the engine's sound. Changing line length does effect engine load and flying trim. How much. In what way. I'd trust my ears, the feel at the handle and what I see the plane doing as I fly it. Factors involved are probably too numerous and various for effective computer modeling. Speculations. Discussions. Ernest debate. (Pissing contests  ;D) Indicative of our species.  Also, defying effective computer modeling.

Offline Larry Fernandez

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 1272
Re: Line length and engine tune.
« Reply #34 on: December 17, 2014, 10:49:56 AM »
Do you have a Tach????

All of this guessing. A click or two in, a click or two out, will get you no where.
Put the short lines on. Fire it up,check the RPM, fly the plane.
Lenghten the lines . Fire it up, set the RPM as before and the plane.

Now you can compare apples to apples.

I get a kick out of some fliers who will tell me, " I haven't touched that needle in eight months"  then I watch them fly and it becomes quite apparent that they haven't done any tweeking for some time.

All  motors run leaner in the morning when it is around 65 degrees at 0900 than they do at 1400 Hrs. and its warmed up to 95 degrees. Its physics boys and girls. The colder air is more dense, so we need to open the needle to get the correct air/fuel ratio. As the air warms up in the afternoon, it becomes less dense, so we close the needle to compensate.

I cant see how anybody can fly with the same setting through the varying conditions during the day.

Larry, Buttafucco Stunt Team

Offline Dennis Moritz

  • 22 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 2464
Re: Line length and engine tune.
« Reply #35 on: December 17, 2014, 11:09:32 AM »
Tachs don't tell you what's happening as the plane flies. Windy always said throw the tach away. His engines ran fabulously well. Ears.

Offline john e. holliday

  • 24 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 22752
Re: Line length and engine tune.
« Reply #36 on: December 17, 2014, 11:41:11 AM »
But, some people can't hear.   I have had settings using the cheapo tack that would not even think of flying as the engine was too lean or too rich.   Same with the stop watch, leave it in the box until you get consistent flights on the plane.   It like the tack is only good to see if something drastic has changed.
John E. "DOC" Holliday
10421 West 56th Terrace
Shawnee, KANSAS  66203
AMA 23530  Have fun as I have and I am still breaking a record.

Offline Howard Rush

  • 22 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 7805
Re: Line length and engine tune.
« Reply #37 on: December 17, 2014, 01:18:52 PM »
Factors involved are probably too numerous and various for effective computer modeling.

They are for those unable or unwilling to do it.
The Jive Combat Team
Making combat and stunt great again

Offline Chris Wilson

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 1710
Re: Line length and engine tune.
« Reply #38 on: December 17, 2014, 02:13:34 PM »
So you are "in total agreement" with an assertion that the extra drag of a foot of lines suffices to lean out the engine, causing it to sag in a vertical eight, having come to this conclusion from "no accurate input".

Yep, because it seems to me to be the most reasonable outcome - so I agree with it

And even with the most accurate input there seems to be no more accurate answer also -but that is not to say that I do not stand in awe of your time and skill spent on this, thankyou.
MAAA AUS 73427

You have enemies? Good. That means you've stood up for something, sometime in your life.
 Nothing in life is so exhilarating as to be shot at without result.  It's not enough that we do our best; sometimes we have to do what's required

Offline Howard Rush

  • 22 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 7805
Re: Line length and engine tune.
« Reply #39 on: December 17, 2014, 02:27:24 PM »
Right, I did not do math, and I say t is only my bet if everything else is equal, because there could be several reasons which could be diffent between flights, for example mentioned temperature or any other atmospheric difference ... and I did not do it because it is too complex. There are several thing coming to equation:

1/ drag of lines which is higher on longer lines so makes inward moment

2/ fuselage moment (out of circle) which is lower on longer lines so makes inward moment

3/ unequal wing drag on left/right is lower on longer lines so makes inward moment

4/ prop moment of angled prop disc which is lower on longer lines because air comes more perpendicular so makes lower outward = relative inward moment

yes all those things caused by different radius are very small, in range under 1% (and some of them far under), but almostnothing + almostnothing + almostnothing + ... + almostnothing + almostnothing = something  ;D

plus if you compensate such moment only by lines (in the case that fuselage moment does not change with yaw angle), it can make visible difference in yaw angle additionaly all multiplied by centrifugal force / G force (relative to ground stand). To that conlusion leads me one experience happened during initial trimming of new model with OS .46 - I lost 5 g weight during flight from tip weight (temporarily fixed by tape) ... motor went immediatelly rich ... when I installed it back, motor worked as before - all without needle adjusting.  

2, 3, and 4 are almost nothing. I didn't calculate 3, but shee-it, Igor.  That's only three almostnothings, not enough to count, and not as significant as a little bit of wind, which has a trivial effect.

I hadn't calculated #1, except the amount of rudder force needed to balance it.    Here is a revision to the spreadsheet that has incremental leadout position needed to compensate for the yaw increment of the extra line drag.  It's .032 inch (0,8 mm to you).  The least adjustment I can make to my leadouts is four times that amount.  I never noticed that affecting engine run.  I calculated this for level flight.  The amount of leadout increment needed for maneuvers may be another .01 inch.  You are welcome to calculate the exact amount.  
The Jive Combat Team
Making combat and stunt great again

Offline Curare

  • 2014 Supporters
  • Trade Count: (0)
  • Captain
  • *
  • Posts: 779
Re: Line length and engine tune.
« Reply #40 on: December 17, 2014, 06:07:20 PM »
Geez guys, this is getting in depth!

It was something I noticed, and yes you're all right, a tweak of the needle would have fixed the problem. It was more a thought experiment as to why it should have changed.

Larry, as I said the first flight was on 62's and the second (later in the morning) was on 63's. The increase in temp from flight one to flight two should have actually richened the mix, but still it was sagging!

Just to add more fuel to this hypothetical fire, as stated the engine is a 46LA, set up as per Brett Bucks method, with the addition of a Randy Smith NVA and muffler. It runs a wet 2 in level flight. I understand how load effects 2 stroke motors, and have experienced the change in under load of motors 4 stroking to 2 stroking. In this instance that wasn't happening. I don't really recall there being much change in take off RPM between the two flights (it was now 6 days ago so I may be wrong).

thanks for the spreadsheet Howard, that's interesting stuff! Well,the outcomes anyway, how you got there is looks like black magic to me :|
_
Greg Kowalski
AUS 36694

Offline Randy Cuberly

  • 21 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 3674
Re: Line length and engine tune.
« Reply #41 on: December 17, 2014, 09:27:52 PM »
They are for those unable or unwilling to do it.

Or simply too lazy to spend that much time on a silly question that is very easy to answer empiracally!  LL~ LL~ LL~

Randy Cuberly
Randy Cuberly
Tucson, AZ

Offline Curare

  • 2014 Supporters
  • Trade Count: (0)
  • Captain
  • *
  • Posts: 779
Re: Line length and engine tune.
« Reply #42 on: December 17, 2014, 10:29:54 PM »
Riiight.
Greg Kowalski
AUS 36694

Offline Steve Helmick

  • AMA Member and supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 9920
Re: Line length and engine tune.
« Reply #43 on: December 18, 2014, 07:12:13 PM »
Sometimes the thing(s) not mentioned are important and relevant. Most of us have started flying our IC engine early AM and found that as the day heats up, the NV has to be screwed in 1/4 turn or even more, as the day warms. IF Mr. Curare happened to start at 3pm and then space flights out until 8pm, the reverse would happen...only the tachometer will tell the truth. I'm saying that without knowing that the launch rpm was the same for each flight, you can't be 100% sure that it's caused by the line length. And I gave up on "clicker" NV Assys. about 15 years ago. If you're using a clicker type, trashbin them and get yourself some infinite adjustability...Randy Smith's are the best!   y1 Steve
"The United States has become a place where professional athletes and entertainers are mistaken for people of importance." - Robert Heinlein

In 1944 18-20 year old's stormed beaches, and parachuted behind enemy lines to almost certain death.  In 2015 18-20 year old's need safe zones so people don't hurt their feelings.

Offline Curare

  • 2014 Supporters
  • Trade Count: (0)
  • Captain
  • *
  • Posts: 779
Re: Line length and engine tune.
« Reply #44 on: December 18, 2014, 08:33:28 PM »
As I've said before, more than once now, the flights were in the morning, not the afternoon.

Also the engine IS fitted with a Randy Smith NVA.

I know how to correct a lean mixture, I was never asking for the HOW, I was asking for the WHY.

Greg Kowalski
AUS 36694

Offline Igor Burger

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 2165
Re: Line length and engine tune.
« Reply #45 on: December 19, 2014, 12:26:37 AM »
2, 3, and 4 are almost nothing. I didn't calculate 3, but shee-it, Igor.  That's only three almostnothings, not enough to count, and not as significant as a little bit of wind, which has a trivial effect.

I hadn't calculated #1, except the amount of rudder force needed to balance it.    Here is a revision to the spreadsheet that has incremental leadout position needed to compensate for the yaw increment of the extra line drag.  It's .032 inch (0,8 mm to you).  The least adjustment I can make to my leadouts is four times that amount.  I never noticed that affecting engine run.  I calculated this for level flight.  The amount of leadout increment needed for maneuvers may be another .01 inch.  You are welcome to calculate the exact amount.  

Well .. I spent lot time building my spread sheet years ago, but I stopped at ~60 input + ~60 output values, then I found almost everything affects everything :- ))) .. the real value of that were not values, but understanding allowed me to separate truths and myths ... for example Line X program works just like described, and gives true values, but its aplligation for stunt model and believing that it is fact it myth :- )))) ... but back to calculation, for example that point 4 is not so easy to estimate, yaw moment or side force of prop is function of yaw, so you cannot just put it to simple equation, it is little bit self amplifying ... I am not writing that different radius is enough, I say it can be reason, values are really small, but it IS here and so it CAN cause such thing.

so the solution for Curare:

it is very easy, whatever we say here, the only way to know is test ... I have markers on inner wing tip and on fuselage visible from center of circle as points one over the other, it is showing me actual yaw. If you want be sure, make such marking and test. If you can see different yaw angle (and such marker will show you less then 1deg difference) and if you recover it back by rudder and you will see constistent run change regarding yaw, then it is caused by differen fuselage position, if you see clear run change but no yaw change, then it is caused by prop load ... and if those changes are relative to temperature difference, then it is also clear :- )) ... and may be you will find that they act all together ... or that it was just some problem with fuel :- )))))))))))))

Offline Curare

  • 2014 Supporters
  • Trade Count: (0)
  • Captain
  • *
  • Posts: 779
Re: Line length and engine tune.
« Reply #46 on: December 19, 2014, 02:14:21 AM »
Igor thanks for that, I now believe that what I experienced was not caused by what I thought it was initially. As you rightly say, it's trying to subtract myth from fact. I have in my F3A career seen many myths busted, and new ones to spring into play. It's very easy to see an effect, and misdiagnose a cause. One such thing is spiral slipstream. Some guys were running different decalage on left and right elevator halves to 'compensate'. I don't neccesarily think that they'd found THE cure, but they'd found A cure. What the actual problem was may not have been spiral slipstream, or maybe it was, I don't know!

Thanks for all your input gents, even if the fix is simple, I like to think that knowledge of whats causing the issue should lead to better fixes later on. As the old saying goes, if all you have is a hammer, every problem looks like a nail.

Greg Kowalski
AUS 36694


Advertise Here
Tags:
 


Advertise Here