Line length, lap time, and speed

[the original Steve Smith]

My Twister flies too fast for me to get much better at flying the pattern more smoothly and accurately.

I am using an OS40-LA that is turning 9800RPM on the ground.  It doesn't 2-4-2 break and if I slow it down to do that it is too slow at about 8500RPM on the ground.  It really likes to run at 9800.  I have a wood 12x4 prop cut down to 11x4.  It loads the engine nicely and has gotten the lap times down to 4.55.  I think I would be better off at a 5+ second lap time.

I am currently using 62' lines and am wondering what effect longer lines will have before I spend the money on them.  I'm sure there is a formula for calculating the lap time based on what I do know now (4.55 second laps with 62' lines) if I just change the line length (say to 63' or 64' or 65'), but I have forgotten my high school geometry.

Anyone got an answer? 

Thanks,

[RC Storick]

1 tenth per added foot. But why not a different prop pitch> Like a 11x .3.75?

[peabody]

I would try a 10 1/2 X 4 1/2 APC....and expect about 5 second laps...

[Lee Thiel]

I had the same problems with my Banshee with a LA40.  Added one headgasket, tried a Miracle prop, but ended up with a 11 X 3. Get good lap times fair break and plenty of power.
Lee TGD

[Bill Little]

Hi Steve,

First, let me say that I have no experience with a LA 40.  But that doesn't matter with the trouble you are having.  I don't tach my engines very often but, when I did, I was launching at around 10,800 with a four pitch prop and getting about 5.4 laps on '65 eye to eye.  Something is wrong here if you are launching at 9800 with a 4 pitch and turning 4.5!  On '62 lines, you should be around 5 to 5.1 if you were launching 1000 rpm faster.  The engine has to be really gaining revs when it's launched, or the tach is reading wrong, or the prop is much stronger than 4 pitch.    I presently launch around 9800 according to what I've been told, and that equates to around 5.4 with a 4 pitch on my present set up.  I would like to know what you come up with!

Good luck,
Bill <><

[Steve Helmick]

Locals have excellent results with the .40LA when using the 10.5 x 4.5 APC, with slower lap times than the 11-4 APC. Some use a stock muffler, some use a MACS muffler, or any other muffler, as long as it doesn't have too much backpressure. Same prop works very well. This prop also works well for the B.40, and they're a lot lighter than the 11-4.  Steve 

[phil c]

circumference of a circle= pi*d,  or c=pi*2r  In this case it would be 3.14*2*62=390 ft.  Divide by 4.55 sec/lap and you get 85.6 feet per second.  if you want to keep the same airspeed(which is kinda high)  figure your circumference for each line length
63=2*63-3.14= 395.4  lap time - 395.4/85.6=4.61
64                   = 402                                      = 4.7


I see some numbers got dropped in the translation.

Those lap times are pretty fast for stunt.

[Bob Reeves]

Boy you can sure tell I have had my head in line calculations

The given numbers are correct in a perfect world but they don't take into consideration things like shorter lines have less drag. Our lines do stretch when flying and longer lines will stretch more. Also the lines are effectively shortened by the sway resulting from line drag in the air. Which changes depending on the air density. Also the ground speed will increase on shorter lines and decrease on longer lines because you have changed the line drag. OK these things are slight but they are real.

[Arlan McKee]

I am using an OS40-LA that is turning 9800RPM on the ground.  It doesn't 2-4-2 break and if I slow it down to do that it is too slow at about 8500RPM on the ground.  It really likes to run at 9800. 

If 9800 is too fast and 8500 is too slow, why not try a little something in between? 1300 RPM is a huge jump. I like around 9200.

I don't know why anyone would want to listen to one of those 10 1/2 x 4 1/2 APC's, but then I hate the sound of tongue mufflers. Most of them are tongue non-mufflers.

[john e. holliday]

If you think a tongue muffler is too loud, just listen when it comes loose or falls off.  DOC Holliday

[Mark Scarborough]

I am curious too as to what type tank set-up you are using. On my twister I launch at 9600 with a modified FP using an 11x4 prop apc. My engine picks up very little from the launch speed, maybe a hundred rpm or so judging by the sound. I get 4.9 or 5.0 lap times fairly normally. I did however have a different tank on it that on take off, as soon as g force picked up (or so was my observation) it would pick up considerable speed.  I eventually narrowed it down to the tank design and fine tuned it with oil content in the fuel FWIW

[bill marvel]

circumference of a circle= pi*d,  or c=pi*2r  In this case it would be 3.14*2*62=390 ft.  Divide by 4.55 sec/lap and you get 85.6 feet per second.  if you want to keep the same airspeed(which is kinda high)  figure your circumference for each line length
63=2*63-3.14= 395.4  lap time - 395.4/85.6=4.61
64                   = 402                                      = 4.7
(SNIP for brevity)

Whoa, Phil.  You lost me.  This is something I have always wondered about but never stopped to try to figure it out.  If Steve's (Wavyhill) ground speed with a 4.55 lap is 85.6 ft. per second (about 59 miles per hour?), don't you have to consider that the aircraft will fly at 85.6 feet per second no matter the line length (within reason, of course, and assuming no change in prop or fuel, etc., and ignoring induced drag, line weight, etc. for the moment).

I was looking for something elegant and simple like Bob Storick's post.  Maybe, the numbers that got dropped answer my question.

Would you mind explaining your solution again a bit slower for someone who hasn't done this kind of math in a long, long time. (Big Grin!!!)

regards,
bill marvel