Supercub Thrust Line

And when a PA18 is flat-ish like a 12 they are fast, well for a cub anyway...

Thrustline above zero degrees

OLDCROWE said:

And when a PA18 is flat-ish like a 12 they are fast, well for a cub anyway...

Click to expand...

What would say a 1 or 2 degree up from zero do to the cub? Decrease speed? Any adverse effects if just a degree or two? Just curious as to what that change might do. Not an engineer, not wanting to actually do it, just trying to understand what the effect might be for a small up degree from zero vs. zero.
Thanks,
John

Think of this aviation 101 diagram. All else being equal, when you change the angle of incidence of the wing, you are really changing the angle which the fuselage flies through the air. Thus the question should be "What effect will changing the angle of the fuselage 1 or 2 degrees have on the overall drag of the entire airplane?". Will the fuselage generate more or less drag when the angle is changed?

Thrustline

skywagon8a said:

Think of this aviation 101 diagram.

All else being equal, when you change the angle of incidence of the wing, you are really changing the angle which the fuselage flies through the air. Thus the question should be "What effect will changing the angle of the fuselage 1 or 2 degrees have on the overall drag of the entire airplane?". Will the fuselage generate more or less drag when the angle is changed?

Click to expand...

What I was asking was the engine mount thrustline degree change,not the wing. If the engine thrustline was up a degree or two from neutral, what would the effect be? Slower cruise, or?
Thanks,
John

skywagon8a said:

Think of this aviation 101 diagram. All else being equal, when you change the angle of incidence of the wing, you are really changing the angle which the fuselage flies through the air. Thus the question should be "What effect will changing the angle of the fuselage 1 or 2 degrees have on the overall drag of the entire airplane?". Will the fuselage generate more or less drag when the angle is changed?

Click to expand...

Has anyone investigated whether reflexing the flaps, in effect reducing the angle of incidence, reduces drag at cruise? I surmise that in cruise, angle of incidence controls fuselage angle more than thrustline.

sjohnson said:

Has anyone investigated whether reflexing the flaps, in effect reducing the angle of incidence, reduces drag at cruise? I surmise that in cruise, angle of incidence controls fuselage angle more than thrustline.

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I believe later Mauls did that for speed increase.

john schwamm said:

What I was asking was the engine mount thrustline degree change,not the wing. If the engine thrustline was up a degree or two from neutral, what would the effect be? Slower cruise, or?
Thanks,
John

Click to expand...

Let's see if this helps to answer the question. The wing has a normal nose down pitching moment. The horizontal tail is positioned to offset that moment keeping the pitch attitude stable.
Now mount an engine in the nose pointed down thus increasing the nose down moment. To balance this nose down moment the horizontal tail must be angled more leading edge down thus increasing drag. If the engine is then pitched up the horizontal tail LE will be raised to balance the pitching moment. With the tail more in line with the wing the drag will be reduced.

So to answer your question, with all else being equal, yes the plane will be faster. This is more apparent on planes with trimming stabilizers than with trim tabs on the elevators.

sjohnson said:

Has anyone investigated whether reflexing the flaps, in effect reducing the angle of incidence, reduces drag at cruise? I surmise that in cruise, angle of incidence controls fuselage angle more than thrustline.

Click to expand...

I tried this with my 185. I can change the angle of the ailerons electrically in flight. I tried various drooping and raising positions along with neutral. Neutral was the fastest, all other positions slowed the speed.

I thought the PA-18 Thrustline modification was about slow flight improvement. I've never heard anyone talk about top end speed, drag, etc. The best speed mod for a Cub is a Husky, but John knows that better than me.

Actually stewart, it does both. Super Cubs generally are speed limited due to extra drag components which are added such as large tires. Also those with extra low pitch props are propeller limited in top speeds. On a Cub the thrust line change would be more noticeable at the low end of the operating speed range.

Super Cubs that have the thrustline modification installed generally pick up 4-5 mph because the engine is now in line with the wing instead of canted down 4 degrees.

skywagon8a said:

I tried this with my 185. I can change the angle of the ailerons electrically in flight. I tried various drooping and raising positions along with neutral. Neutral was the fastest, all other positions slowed the speed.

Click to expand...

I wonder if you'd have seen something different if you could have reflexed the flaps.

I've read that some folks rig some washout from the wings to improve cruise speeds. Reflexed flaps would do the same without a permanent change in stall characteristics.

My flaps reflex, but I haven't done any serious testing with them. Anecdotally, they seem to add 3-5 mph at cruise when I'm in the 135-140 mph range.

Mark, would pls tell me if a PA-18 with slats like the Dakota Cub or Piper YL-14 have with your trustline can take advantages with it installed? I am aware that Wayne Mackey's Stolquest SQ2 has a different trustline. Is it the same or similar yours?

Regards

Mark, let me ask something about your trustline on PA-18. Everybody talk about visibility issue. Reading about Stolquest SQ2, its engine's mount is lowered 3" and zero-degree trust-line but it engine cowl is very different.
My question is. Could we, in a new engine instalation, using the original top cowl, to lower the engine mount with your trustiline mod enough to keep the visibility as it is in original PA-18?

Best Regards

AC Muller

carlbrun said:

Mark, let me ask something about your trustline on PA-18. Everybody talk about visibility issue. Reading about Stolquest SQ2, its engine's mount is lowered 3" and zero-degree trust-line but it engine cowl is very different.
My question is. Could we, in a new engine instalation, using the original top cowl, to lower the engine mount with your trustiline mod enough to keep the visibility as it is in original PA-18?

Best Regards

AC Muller

Click to expand...

No, ring gear for starter would hit cowl, plus hole for prop.

I didn't notice much, if any change in visibility over the nose after installing the Thrustline mod. We I attached the lower cowl rails I put a 3/8" block on the top of the ring gear and dropped the nose bowl down on top of it. It actually lowered the nose bowl 5/8" from center of the crankshaft. I have 3" extended gear on 31" BWS and a Baby Bushwheel tailwheel.

You can see where my spinner had rubbed the nose bowl sometime previously before I installed the Thruse line modification.


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carlbrun said:

Mark, let me ask something about your trustline on PA-18. Everybody talk about visibility issue. Reading about Stolquest SQ2, its engine's mount is lowered 3" and zero-degree trust-line but it engine cowl is very different.
My question is. Could we, in a new engine instalation, using the original top cowl, to lower the engine mount with your trustiline mod enough to keep the visibility as it is in original PA-18?

Best Regards

AC Muller

Click to expand...

I didn't notice any difference in visibility. I only have 26" goodyears.

I did notice the cub felt much lighter after the thrustline mod. I think the downward cant of the engine was for stability...i have micro vgs that added low speed stability and the thrustline added responsiveness.

Great modification for a 51 Supercub and Mark was just a really nice guy to deal with.

Steve, I am thinking in put 31" BWS and Baby Bushwhell too. Could you post a picture of yours?

Regards

AC Muller

This is before I cut the new lower cowl rails. You can use the extensions Mark sends as well.

This is after installation.



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i've used used mark's thrustline mod on three different super cubs i've owned over the years,#1 cub mod imho,mark has always been great to work with!!!

jr. :smile:

Thanks Steve. Amazing flying machine. Congrats.

Thrustline mod for the rest of us

Amazed at the amount of discussion my recent vg thread has produced, would like to get some thoughts on the thrustline mod for those of us south of Alaska. Although i understand the concept of this mod, i have zero experience with it. Assuming an airframe is in such shape as to benefit from the mod, what benefits other than cruise speed should someone expect with a light weight, low horesepower, stock cub on 8.50's ? Most of what i have heard pertains to highly modified 180hp, extended gear, 31's etc. Considering what some of us have spent trying to squeeze a couple more knots out of a cherokee, the speed increase alone if real is worth it. Aside from that, is this another "you got to fly it on the edge to see any benefit" mods ?

VG's go one EVERY plane I Build.....
but only a couple had thrust line on it... but it was owners preference... not saying anything bad about it...

You should do a search instead of starting a new thread. VGs have been discussed for years as has the Thrustline modification. Lot's of great information posted here, read it and then ask your question.

I merged the Thrustline Modification threads together. Hope you have some spare time, 18 pages of posts. Very good information. I will comment on my experience shortly.

So I added the Thrustline modification to my Super Cub last winter. It is a 1972 model that spent it's life flying powerline patrol until I got it 3 years ago with just over 12,000 hours total time. Never been wrecked and the only modifications were VGs and Dakota Cub brakes. I have added 3" Atlee Dodge gear with 1 1/2" axles, 31" Bushwheels, Borer 8243 prop and a Baby Bushwheel tailwheel. Prior to installing the Thrustline modification I was flying almost everyday which normally includes bar hopping on 300-800 foot gravel bars on the Brazos River about 7 miles from the airport. SJ had given me my bi-annual flight review the previous Fall and demonstrated how to slow down, pull a notch of flaps, slow down more and pull more flaps and use power and never have to trim the airplane. This method works well in his airplanes and others I have flown but after a lot of experimenting I went back to my two rolls of nose down trim prior to landing. He came down a week or so before I installed the modification and we did some bar hopping with him in the back which was good because a lot of times I am empty with 8-10 gallons of fuel on my nightly river runs.

The modification took me 3-4 hours with Bill Duncan assisting. Don't remember that being a hindrance though some people can be. Did have to cut off the upper left bolt head as it was stuck in the fuselage fitting. The angle drill saved a lot of time and aggravation getting what was left out of the firewall fitting. I have installed several Thrustline mods on flying airplanes over the years and feel like I have a pretty good system. I use a 3/8" plywood shim on top of the flywheel and drop the nose bowl down on it which actually lowers the nosebowl from being centered over the crank flange. The nose is still higher but not as high if left centered. I installed the new longer lower cowl rails instead of the cowl rail extensions Mark includes in the kit. As usual on my own projects I started this project late on Sunday afternoon and didn't get done till dark so had to wait till the next day to fly it.

First thing I noticed after installing the TL mod was the stick pressure when taxiing. From learning to fly in my Clipper I always hold the stick back unless there is a screaming tailwind. I noticed immediately an increase in back stick pressure while taxiing. First couple of landings seemed normal until I started forcing it off before I thought it was ready and she was ready and flew. Landings were more noticeable. I was overshooting my spot. It took a few landings to get back to hitting my spot and never really figured out what I am doing different now but I am back to hitting my spot. I did notice a 3-4 mph increase in speed but the biggest thing I noticed was the take off. The airplane was ready to fly sooner. This really became apparent when I took off from a gravel bar I had used a week prior with SJ in the back. I had a passenger in the back of similar weight and horsed it off a little earlier than I thought I should and she came off and climbed. That was a surprise as I was expecting a mush. The other thing I noticed was the trim. I no longer need to trim nose down in the landing configuration. It is quite comfortable to slow down and pull flaps and not have to play with the trim. I do not like working against the controls and usually trim for hands off but no longer have to mess with it. In fact the only time I trim now is for a different load. I have heard some people say they lost visibility over the nose but I never really noticed any difference. I know the nose is up a bit higher but I never noticed it from the pilot seat. I am happy with the mod and have yet to find any negative to it. Mark is always great to work with and Super enthusiastic to hear what you think. In my opinion if you truly fly your airplane and strive to get all you can out of it and yourself it is a great modification.

Raising the nose didn't seem to effect my paint scheme either. That is a big deal to some.

Steve thanks for the detailed report on Thrustline. In the last photo, it appears that the oil cooler sits a little higher relative to the nose bowl opening: Did you notice any effect on cooling? And which flavor oil cooler are you running? Also notice no gap seals between nose cowl and oil cooler. Hope this doesn't lead to thread drift [please separate threads if appropriate], but there are sometimes unanticipated effect from good mods like this. I'd be interested to hear others experience with this as well, including Mark's.

Thanks. cubscout

cubscout said:

Steve thanks for the detailed report on Thrustline. In the last photo, it appears that the oil cooler sits a little higher relative to the nose bowl opening: Did you notice any effect on cooling? And which flavor oil cooler are you running? Also notice no gap seals between nose cowl and oil cooler. Hope this doesn't lead to thread drift [please separate threads if appropriate], but there are sometimes unanticipated effect from good mods like this. I'd be interested to hear others experience with this as well, including Mark's.

Thanks. cubscout

Click to expand...

I was concerned as well but noticed no difference. I am using a Niagara oil cooler that is PMA'd for the Super Cub.

Steve Pierce said:

I merged the Thrustline Modification threads together. Hope you have some spare time, 18 pages of posts. Very good information. I will comment on my experience shortly.

Click to expand...

Thanks for the very detailed description of your before and after and not just telling me to look elsewhere. The kind of response i was looking for.I had done searches and have read all 18 pages of this particular thread. As with most, alot of drift from topic but entertaining none the less. The comments that come closest to what i had in mind were a few about stacking washers under the mount on a PA-11.But your right, i really asked the wrong question. should have been,
For those of you that have experienced the thrustline mod on mostly stock 90-115hp cubs in the 800-900 lb. range, wich response best descibes your experience ?
A. Holy Crap ! its like a totally different plane now, you gotta get this
B. Yeah, i think maybe i see a difference. probably worth it to guys that need all the extra they can get
C. Should of spent it on avgas