Line Length Variations - Why No Effect?

[Kafin Noe’man]

Hi,

I'm currently flying P40 with an OS LA 46 ; Prop TT 11x4.5
So far, I've solved the engine break problem I have mentioned on my other thread.
It turned out to be my engine has been in wet-2 all these times and I was told that my engine sounded really happy. (I had shared my other videos with different setting on one of the ZOOM Session some time last week). It was my lack of experience that had led me to a wrong interpretation on how my engine runs.
Never flown a Fox 35 or similar 4-2 break engines in the past, I often times thought that my engine run was in 4-2 meanwhile it was not.

At this moment, I'm still trimming the plane in order to get the best setup to my liking.
However, having various data to analyze, I have found something weird to me that I couldn’t adjust the lap times with the use of different line lengths (please see the attachment for the data I have so far).

The only setup that seems to be corresponding to the line length variation is when I set it up around 9,550 — 9,650 RPM.
Setting it any lower or higher than that RPM range, line length variations won’t have any big impact to my lap time.

Do you have any thoughts on this?
Looking forward to learning more from you guys.


Best regards,
Kafin Noe’man

[Perry Rose]

The rule of thumb is one foot of line length change changes the lap time one tenth of a second. I use the APC 11.5 x 4 prop and I have LA .46's that launch at 8500 to 9500 rpm for the same airplane performance/lap time. Some engines have a nice 2/4 break others  wet a two stroke. I shoot for the best line tension all around and forget about the lap times. If your leadouts are too far aft the fuselage will be sideways to the wind causing a lot of braking action. The P-40 has a fat fuselage. I've settled on 60  foot lines eye to eye for most all my planes. That is plenty long enough.

[Kafin Noe’man]

The rule of thumb is one foot of line length change changes the lap time one tenth of a second.

Yes, that’s what I have been told about and I was actually trying to experiment about it.
But the data shows quite the opposite, it seems there’s no significant impact on changing into significantly longer lines.

[Motorman]

The longer lines load the engine more and make it run leaner in the air.

[Steve Helmick]

I want you to box up all the 11x4.5 TT Cyclone props you can get and ship them to me for proper disposal. 

Actually, my first thought was that your tach reading wasn't correct because you didn't warm the engine up properly before you set the NV. I harp on that ALL THE TIME because when I am judging, I see it happen constantly in the lower skill classes (Beginner & Intermediate)...the engine goes rich just as the flier picks up the handle. Watching Danny Dirt taught me that...bless his heart. Miss him.  :'(

Second thought is that you really should be using the 11.5x4 APC...at least they are available, while the TT is a rare piece, at least over here.

Third, Motorman might have the right idea. Consider that the TT prop on the .46LA might be putting the NV setting in a place on the powerband where a little extra load (longer lines) could bring it to "the tipping point" where it just does whatever it wants. I'd ask if this testing is done with multiple flights at the same setting or only one, and how accurate your stopwatch operator is.

Fourth, I'd wonder if more oil or more fuel consumption might fix this issue. Drilling out the muffler outlet can be a good thing as it lets the engine breathe better and consume more fuel, which means more oil is going through it, = cooler.

Fifth, read through ALL of the pinned articles at the top of the Engine Setup forum. Lots of good stuff in there.  Steve

[Dave Hull]

Assuming all of your measurements are accurate (lap times, line lengths) and no other changes are being made (different prop, fuel, glow plug, different diameter or type of lines, etc.) then:

--Line drag is significant portion of the overall flying drag. And longer lines at the same flight speed have noticeably more drag
--To get the same lap times on longer lines you are flying faster, and with more drag from both the longer lines and the faster speed
--To overcome that extra drag you are making more horsepower
--The engine obviously has the reserve power available, so that is not a problem all by itself
--What you need to find is a combination that makes the engine very stable at a flyable speed, because that makes the pilot very happy
--The comparison of your 60' vs 63' with a ground setting of 9550--9650 rpm seems to be the only configuration where you are getting some of the stability possible from the .46LA.

As noted earlier, these comments assume that all of the measurements are accurate. The stopwatch part is tricky for some....

The OS .46LA has a very broad sweet spot, so lots of different setups are workable. You just have to find one you like. In our area (near sea level, but often hot ~90F), guys seem to be most happy on planes like the P-40 using an 11x5 or 11x6 and running just below the (soft) break. That is much less rpm than I think I hear in your posted video. We generally use something like 10N-11C-11Syn fuel. Going down too far in nitro makes them more fiddly, but 5% works when everything else is pretty standard. Lines of 60' to 63' lines at the eyelets work fine on the .46LA P-40 combo. Using an OS 762 muffler with the stinger cut off and with an opening of around .310" reduces backpressure and broadens the sweet spot. It also seems to provide a more stable run than a tongue muffler. The smaller venturis keep you away from a more touchy setup but they need to be a range known to work. Tanks are important, too. The standard recipe of limiting to 1" height and 2" width helps. Wider and deeper generally means you will struggle with needle settings (ground vs. air) and when other things vary slightly the run quality goes away. You can compensate, but everything gets fiddly, and you end up with lots of flights where the power isn't right. You can run muffler pressure but it isn't essential as long as the tank is a standard size (1" x 2" wedge) and has clean air to the vents. We have run them with either standard vents or uniflow. A well-made 6 oz. clunk tank on uniflow with pressure works good. Again, the .46LA has a broad sweet spot and lots of setups work well.

[Kafin Noe’man]

Hi all.
Thank you for your responses.
It took me quite some time to process and to experiment more after reading your suggestions.

I just found out there has been a leak on my fuel system, specifically on the uniflow tube while the other two seem to be okay. No matter how high or low RPM I set on the ground, the engine will eventually gaining more RPM as soon as it gets in the air.

Can this be the culprit?

If I try to remember, I have experienced this problem after I had a prop strike accident that also affected my fuel tank holder. Before the accident, everything seemed to be working fine.


Best,
Kafin

[Dan McEntee]

Hi all.
Thank you for your responses.
It took me quite some time to process and to experiment more after reading your suggestions.

I just found out there has been a leak on my fuel system, specifically on the uniflow tube while the other two seem to be okay. No matter how high or low RPM I set on the ground, the engine will eventually gaining more RPM as soon as it gets in the air.

Can this be the culprit?

If I try to remember, I have experienced this problem after I had a prop strike accident that also affected my fuel tank holder. Before the accident, everything seemed to be working fine.


Best,
Kafin

      I would say this is most likely. If you can not reliably speed up or slow down the model speed and the engine repeatedly does what you describe. repair as necessary or replace the tank. If everything in you fuel system is correct, from tank position to no leaks anywhere from the pick up to the spray bar, your engine run should settle into a slightly richer setting in the air after a lap or so. If it continuously speeds up in the air, something isn't right. I have experienced the old style wide wedge tanks doing this if the pick up tube is way outboard. replumbing the tank and moving it closer to the inside helps. Shimming the tank outboard is actually a trimming tool for profile speed modelsto make sure they don't get too rich in flight.
     Type at you later,
      Dan McEntee