Thrustline Mod and Rigging

[acroeric]

Spent 2 nights reading through old discussions on the Thrustline mod and could not find any real input about my question. I do however know a bunch of history on the mod.

I'm finishing up a Javron wide body Super Cub and I've got everything together for rigging. I have rigged many Cubs according to the Piper Service Letter but was wondering if there has been any new thoughts/ theorys on changes to that if one is running a Zero Thrustine mod. I believe Javron calls it the "Inline Thrust 2" lowered" mount.

Appreciate any input/ opinions!

 

[skywagon8a]

My widebody was from Javron when they were building part of the TCOW kit. It has a zero thrust line mount which just tipped the nose up. Nothing was lowered. Everything went together the same as a stock PA-18. The rigging was stock PA-18. It flies like a dream.

 

[skywagon8a]

I believe my mount was the first dynafocal one Javron made with the zero degree angle since I was told they had to make a jig to weld it up. Prior to then, they were supplying the STC'd kit to change the angle. I don't know where the "lowered" came from. I have heard discussions here which seemed to indicate there were clearance issues of some nature with lowered mounts.

 

[acroeric]

I built my cowling fron scratch and used a slightly different nosebowl that we build for the Husky which may account for any clearance issues. With the Vetterman exhaust I have just enough clearance to not need any scoops or fairings. It all fits within the lower cowling.

 

[DENNY]

I believe my mount was the first dynafocal one Javron made with the zero degree angle since I was told they had to make a jig to weld it up. Prior to then, they were supplying the STC'd kit to change the angle. I don't know where the "lowered" came from. I have heard discussions here which seemed to indicate there were clearance issues of some nature with lowered mounts.

The lowered engine mounts have issues with Mag clearance. They Arched the upper support bar on mine so it would clear a Bendix mag on both sides. Backcountry also did this with a Dynamical mount. The electronic mags don't seem to have an issue and that is what Jay recommended for his standard thrust line 4 inch drop mount but I just trust my Bendix when I am away from home.
DENNY

 

[Jeff S]

One thing to consider is how zeroing the thrustline relates to your AOI. On my wide bodied Cub's fuselage, I had the incidence of a stock 12 which is rigged correctly for a negative thrustline. When it was zero'd, the AOI was negative 1 relative to the thrustline. You might say "why would that matter?", and obviously AOA through the airmass doesn't care. But, with a low AOI relative to thrustline, I was "plowing" tail low (presenting a large frontal profile) in level flight and required nose high trim to cruise. That made for a hard working engine and burning a bunch of fuel (ineffecient, obviously). And, the more I tried to climb, the more the airplane was effected. Increase the AOA, the prop would essentially face an inefficiently high attitude to pull the plane. AOA is always going to be exactly how it has to be.... so if your thrustline isn't pulling efficiently, the plane will climb and fly like sh!#. I ended up having Jay make me 1.5" hockey sticks (brackets) to raise my LE 3 dgrees (I have a Javron wing) which put me at positive 2 over thrustline. The fix was instant. Flys like a dream, hands off. Tail is right behind the airplane in cruise (picked up speed at lower RPM, temps decreased, improved handling) and it climbs like it should. Also unexpectedly, I don't even touch the trim any more. I used to have to trim to take off, trim for cruise and trim to land (especially if I wanted to land short). Now, I literally don't touch it. I didn't have to remove the aft wing attachment or de-rig the flaps/ailerons, so besides longer lift struts, getting the wings re-rigged was pretty easy. I did have to shim the top of the fuselage to get the skylight/windshield to match the new wing line, and I made some carbon wing route fairings to blend the LE into the windshield.... don't think the AOI/TL discrepency is a big deal on most 18 fuselages but for a 12, it's a "thing", and I wished I would have known from someone else before having to chase it down. Definitely would be something I'd consider if building. Good luck

 

[phdigger123]

I have the zero/lowered thrust line engine mount on my Javron Supercub. I have a standard nosebowl and the Vetterman exhaust. As far as the cowl is concerned, I would but a bigger “belly” in the lower cowl.
There is no problem with mag fit.

As far as rigging, I just used the Piper info. On trim, when I am loaded to the rear cg, I use full nose down trim. It is the fastest Supercub I have flown, by a good 7 to 10 mph. Part of the speed may be due to the Sensenich ground adjustable propeller.

 

[Steve Pierce]

don't think the AOI/TL discrepency is a big deal on most 18 fuselages but for a 12, it's a "thing", and I wished I would have known from someone else before having to chase it down. Definitely would be something I'd consider if building. Good luck

That is why we measure the PA18 wing incidence before Mark will sell you a Thrustline STC'd kit for a certified PA18. Even factory PA18 fuselages do not have the correct incidence and the mod won't work. Sorry you had to go down that rabbit hole. I had to do the same on an early Smith Cub that had the tail incidence incorrect and instead of cutting fabric to adjust the tail I adjusted the wing like you did.