Guys,
I'm working through a setup issue on my El D with a Fox 35. The engine holds a fast 4 cycle (with blips to 2) in level flight and wingovers but when doing the 5 inside or outside loops (or 3 eights for that matter) it will break to a 2 cycle and hold until returning to level flight then it goes back (within 1/8 lap) to the 4 cycle. The speed in the maneuvers is slightly faster then in level flight and I would like to get it to do the classic 10 - 2 break to keep a more constant speed (in this case a little slower) through maneuvers.

Details - Fox 35 iron p/c, upright mount, 9.5x6 old TF prop, uniflow tank with pinched outside corner (GMA style), 5%_28% oil Sig, Sig plug, ST NVA, launch rpm 9100, lap time level 4.8 on 60' C to C.

The one area that I am thinking about is the NVA - being a standard ST 51 spraybar it is 0.151" diameter, the normal Fox spraybar is 0.125", this cuts the venture open area down  by almost half. I have read that it should not make that much difference, after seeing the numbers I'm not sure I buy that, but for now it has enough pulling power for this ship. One thing I found that I'm not sure of the effect is that I installed the spraybar with the hole pointing forward across the venturi just below sight level (about where the hole would be with the 2 hole Fox spraybar). I'm wondering if that position is causing some slight airflow differential down the venturi that reduces fuel draw on the round maneuvers and if pointing straight down would get more consistent fuel flow and a better break?

I don't get to fly until Thursday so rather than try something that takes up flight time and may or may not work I though I'd ask if anyone has had this condition and how did you correct it?

Best,    DennisT

Hi Dennis - First, try going back to a Fox NVA.  It seems counterintuitive, but flowing more fuel means more oil, hence better cooling.  The effect of the added volatile components (adding to the heat of combustion) has been outweighed by the added oil whenever I've had one which won't break back from 2 stroke down to 4.
     Second, try a flight with no muffler.  Too much backpressure will tend to 'bottle up' the heat from combustion and prevent the engine from 4 stroking.  See the photo for examples of BIG muffler outlets, albeit on a B40.

Hi Tom,
The problem isn't that it won't go back to the 4 cycle it just breaks to soon and holds until it gets back to level flight. There is no sag, upright and inverted are the same lap time nice solid 4 cycle with blips to 2. I have one of Randy's PA Fox NVA on order but just thought there might be something with the position of the hole on the reduced venturi opening that made it sensitive to air flow into it as it starts maneuvers. Could try one extra head gasket - what do you think?

Best,      DennisT

   I don't think you want more head clearance!  It's absurdly low already. Adding more clearance will just enhance the issue Tom was talking about (where it will hang on to the two for longer). If anything you might want to raise it.

    Try a bit more nitro, or very slightly less diameter. The issue is likely that when you lost venturi by putting in the larger spraybar, you have to run it leaner and thus are closer to a two-stroke to start with.

     Try a tank of 50/50 10% and 5% mixed in the syringe, and set it for the same level flight speed. It should end up running richer in level flight and then break later into the maneuver.

   Brett

Dennis said "it to do the classic 10 - 2 break to keep a " That sounds like a perfect 10-2 to me
Ed

I have a PA NVA for the Fox coming from Randy, I had thought about what you guys suggested and think its the right direction. When I started with the El D I had a 9x6 on the Fox 35 but had some tank issues to work through and tried the 10x6 which yielded the current run condition. On this ship the power does go up when it breaks because in dead calm air the loop and eights are faster, not run a way fast just a bit (doesn't break in the wing over or climb and dive).

If the new NVA doesn't get here in time for the weekend session I will put the 9x6 on it and see if it changes the run characteristics.

Best,         DennisT

OK, tested the 9x6 old Top Flite prop on the Fox 35 with the ST NVA . Pretty much a perfect run, solid 4 cycle in level flight with a touch of 2 at the top of the loops. The 1/64" crush pads have settled in (the spraybar is down a tad) so it broke a little stronger inverted but proved the stock ST NVA really cuts the airflow and makes it sensitive to load (prop size) and tank height. I am waiting for the Randy PA Fox style NVA and think this will allow the 10x6 with the normal Fox break.

Best,          DennisT

Hi Dennis - Did it feel like it improved at all?  My thought is that there should have been a noticeable difference in how quickly the engine settled back to a 4 stroke every time the load was reduced.  Next try a muffler with a BIG outlet, or no muffler at all.  Best, Tom

Hi Dennis - Did it feel like it improved at all?  My thought is that there should have been a noticeable difference in how quickly the engine settled back to a 4 stroke every time the load was reduced.  Next try a muffler with a BIG outlet, or no muffler at all.  Best, Tom

   Is this spraybar is necked down to give the original choke area, or the stock PA diameter? Necked down I would expect little change, perhaps a bit richer in level flight and a bit leaner in the maneuvers, with an original PA diameter I would expect it to be generally a little leaner all the time but break less.

    Restriction in the exhaust could be inhibiting the power, and the response to changes like the venturi. If the venturi is not the limiting factor then the spraybar won't make much difference.

     I still stick with my original comment, I would try more nitro.

    Brett

Brett,
The PA Fox style NVA is turned down in the venturi area to about 0.125" and matches the Fox stock NVA.

Tom,
The change is running a little richer but I do think the muffler back pressure is holding heat a little longer once the motor starts to work in the consecutive maneuvers. That's why I think it needs the smaller prop to hold the needed rpm in the 4 cycle and break at the proper point in the maneuver. I think you could get it rich enough on the ground but would not have enough rpm to have enough speed to maneuver with any confidence. Hopefully the weather cooperates and we find out on Saturday.

Best,       Dennis

  There are a bunch of other things that affect this, like the pitch and airfoil of the prop near the tips. For example, we used to adjust breaks by either sharpening or blunting the leading edge near the tip. And a myriad of other things like the nitro. This is also why we stopped running 4-2 break engines the instant there was something else available!  There are all sorts of trick we learned back in the good old days, that are not needed any more.

    Brett

Guys,
A little update after installing the Randy Fox style PA NVA. The new NVA is very reliable and gave good runs on open uniflow and 9 x 6 props with the muffler. I also tested muffler pressure to the uniflow line. This was the surprise as it worked very good. I did some flights on a 10x5 rev-up and a fairly rich run (gave about 5 sec lap, normal would be 4.85). I think I have the motor happy and just need to get the pitch up a bit to pickup the lap time.

Best,          DennisT