Line Tension conundrum

[pipemakermike]

There is something that I don't under stand about line tension.  A friends model - well setup and very competitive - has demonstrated something that we can't explain.
The model was lacking line tension entering the cloverleaf.  The model is always flying at close to 5.5/lap.  Increasing the compression increases the power and you would expect that to increase the speed and thus the line tension but by adjusting the pipe and the venturie the model was slowed back to the 5.5/lap but now has more line tension.  How can that be?
same model, same weight same speed but more line tension!
Another friend has stopped flying 4 strokes as he is sure that they give so much more line tension that his back starts to play up.

I have been hearing these tales for some years and always assumed that there was some simple explanation but what it is

[Tim Wescott]

Perhaps you are seeing a more constant speed, or even a speed increase in the "hard parts"?

Just because you're flying equal-timed laps in level flight with two different setups doesn't mean that one isn't doing a much better job of achieving a constant speed than the other.

[Steve Helmick]

The combined changes probably make the engine kick in harder above 45 degrees. I suspect Tim is correct, at least to some degree (pun intended). Seems like it would make faster lap times at 45 deg. or do V8's a bit faster, or do OH8's quicker, etc., but I don't have any idea how one could actually time that, given human inconsistencies in estimating 45 deg., as well as duplicating any suitable tricks close enough to give any validity to a timing.

I would like to know whether the pipe was shortened or lengthened, and if the venturi larger or smaller. My current ride kicks in some extra boost up top, but I don't know why. It needs every bit of it, tho!  Steve

[Mark Scarborough]

to add to this interesting observation,, same airplane, same engine, same fuel, same everything but the prop, establish rpm to provide exactly the same lap times with two seperate props,, ( both or in my case all 4 different props are all the same measured pitch and same diameter with differing airfoils,,and blade planforms),, all four props going the same ( down to the second decimal, or more accuratly than you can actually measure) lap times,, have vastly different line tensions in LEVEl flight, not to mention in overhead manuevers,,

I cannot explain it with any real physical facts,, I just know it is different,, hence the need to theorize,, then FLY it to find the prop that works best,,

[pipemakermike]

to add to this interesting observation,, same airplane, same engine, same fuel, same everything but the prop, establish rpm to provide exactly the same lap times with two seperate props,, ( both or in my case all 4 different props are all the same measured pitch and same diameter with differing airfoils,,and blade planforms),, all four props going the same ( down to the second decimal, or more accuratly than you can actually measure) lap times,, have vastly different line tensions in LEVEl flight, not to mention in overhead manuevers,,

I cannot explain it with any real physical facts,, I just know it is different,, hence the need to theorize,, then FLY it to find the prop that works best,,

Yes that is the effect that has been seen.
I agree that a change to the engine charactistics that gives an increase in power when the engine is called on to work harder may improve line tension in some of the more demanding parts of the pattern but changes in the tension in level flight withut a change in the parameters that create the centrifugal force (speed and weight) or aerodynamic changes like engine or rudder offset that needs an explanation.

[john e. holliday]

Time your individual maneuvers like we used to do in pit stops for F2C.   Start your watch one lap before the maneuver and stop it one lap after the maneuver.   Do it only for the maneuver you are questioning.   You may surprise yourself with the out come.   In F2C it is the combination of plane, pilot and pit man doing every thing right to get those quick pit stops.  We would fly five laps with a pit stop in the middle.   That is when you learn to do 1/4 lap shut downs and hopefully one flip starts.

[Brian Massey]

Could it be that the adjustments you've made keep the engine in it's powerband better; on the pipe during manuvers so to speak, thus giving you the line tension? That would be my guess.

Brian

[James Strickland]

By moving things around, you may have un-knowingly moved the CG. forward enough to result in increased line pull, sorta common when shortening the pipe for more power as you said at first.   James

[Bob Reeves]

By moving things around, you may have un-knowingly moved the CG. forward enough to result in increased line pull, sorta common when shortening the pipe for more power as you said at first.   James

I believe this is the answer to increased line tension in level flight. Could be wrong but I really don't think a prop change will make any difference on the same airplane with the same lap times in level flight unless it changes the cg as stated by James.

[Mark Scarborough]

well, in MY experience, it did change it with no change in CG,, at least no measurable difference,,

[Steve Helmick]

Agreed, Mark. I changed the pipe length about 3/4" on my PA .51 to make it work right. Set it per Randy Aero's chart, of all things! Didn't notice anything except a better engine run.   Steve