Supercub Thrust Line

TJ, don't tell my wife I'm that old; she'll want to trade me in!!

About 40 years ago, my brother flattened the thrustline on our O-200 powered PA-11. It made a big difference. We never did anything with it. Mark did. More power to him. Mark, you shoulda done it sooner, I'm one of those who thinks getting to treetop height ten feet shorter is a good thing. I'm also glad to see Bob making his data available and wish more data were available. One of these days I'd like to make time to do some testing like Jerry Burr has. Typically, when he states an opinion -- I take it at face value. I've never flown a plane that has Mark's thrustline mod, but I know the concept works, and I'm glad he has made it available for others.
All the best,
JimC

Why doesn't anyone talk about one of the real benefits with the Thrustline mod that don't need any raw data to like.
That is the bigger space between the engine and the firewall. Now I can take the left magneto out without swinging the engine and easier to get to the oilfilter and so on. And I assume that you people that uses other types of exhaust gear likes it to.
And about the other discussion, I just can say again that after flying with my Cub for 24 years and now have put the TL on I directly could feel and notice a difference in handling, TO and landings and that's enough for me.
Thomas
"swedishcub"

TJ - I guess you are right - I didn't directly answer your question. Here goes: Assume angle of attack at liftoff to be 13 degrees (I have no actual numbers, but I am pretty sure the liftoff angle of attack is at least ten degrees). Stock thrust is six degrees lower than the wing , assuming we are measuring at the butt rib or thereabouts. That means thrust at liftoff is acting at an angle seven degrees up from the actual direction you are going in terms of the slipstream. Raise that four degrees and now the thrust is acting in an up direction eleven degrees from the flight path. That does not count wash in or wash out or however the wing may be twisted. Seems to me the up-thrust should help, but I bet there is an angle after which it no longer helps. I have no idea where that angle is - indeed at 90 degrees you have a helicopter, with no ground roll.

Geezer - and all of you - drop in and we'll fly. There are at least three of my very good friends who are convinced that the TL is a dramatic improvement, and we can all drink a beer or something and have a pleasant conversation - or go fly. The natives here are quite friendly. And the tower loves Cubs!

As far as bashing the TL - I have not said it doesn't do what everyone says it does. All I said was I cannot feel a difference (fact) and that I had before and after data. My data was honestly taken, and I have drawn no definitive conclusion with respect to ThrustLine. You may maintain that data should always be taken in zero wind, but you will also note that my data is the only data that specifies the wind. How do you know, for instance, what the winds were when Mark Drath tested? I do not know how Piper's flight department gets data, but I am betting that those posters who maintain that all testing is done in zero wind have not checked with any flight test department. I bet Jerry tests each and every mod that he puts on his aircraft, and I am guessing, but I would guess that he tests in wind.

I stand corrected,

tests should be in similar and consistent wind, not gusts as you described in one of your posts.

Bob, it's a deal - - which airfield are you at?

bob turner said:

You may maintain that data should always be taken in zero wind, but you will also note that my data is the only data that specifies the wind. How do you know, for instance, what the winds were when Mark Drath tested?

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S2D said:

Supercub- +/-150 HP Borer prop, Standard gear, 30" bushwheels VG's
50 deg 5mph wind surface Pavement, 34 gal fuel, 175 lb pilot, 40 lbs junk in baggage.

Before 195 feet
After 159 feet
decrease 36 ft ( 18.5%)

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Bob
If you notice, my data had the wind speed. Now I didn't put it down what direction they were, because I did them with a direct headwind. It would be stupid to do the before with a headwind and the after with a tailwind or crosswind. (which by the way is what you did on some of your tests) and they were done multiple times to rule out errors in technique.

bob turner said:

and that I had before and after data

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and that my friend is pure bull$hit which almost everyone on here agrees with.

SuperCub MD said:

Mark called and said there was some crying going on because I didn't post "hard" numbers in my initial testing of the thrust line. I dug out my notes, so here you go.

Average of half dozen take offs with the stock TL,
186.33 feet

Average of half dozen takeoffs with the zero TL,
150.25 feet

This was done in a basic stock mid-sixties PA18-150 with Borer and Bushwheels. It was not my own Cub, but one I had flown enough to feel comfortable with out on the edge. The usual tail up, pop full flap at rotation short field technique was used. Temp was warm, wind was light. Tape was stretched straight into the wind and spotters used to make the measurements. The TL change was done as quick as humanly possible, the before and after tests were about a hour apart in a attempt to keep the conditions the same. If anything, the temp rose slightly and the wind fell slightly between tests so no environmental advantage can be given to the zero TL. I've tried to do a lot of this testing stuff over the years, and environmental changes make a huge difference in these Cubs when you are splitting hairs down to feet and inches. Different Cub, different pilot, slightly different wind/temp and results will always vary.

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Bob, Please carefully read my post, I tried to describe my test as well as I could. I suppose "light" wind is a little vague, but all we have here is a wind sock. I'll call it 3 mph for the before, and 2 mph for the after tests. "The tape was stretched straight into the wind", which means that I did the takeoffs straight into what little wind there was. With typical daytime heating, I'm sure the temp rose a degree or two while we installed the TL kit. The important thing is, as I noted in my post, NO ENVIRONMENTAL ADVANTAGE COULD BE GIVEN TO THE THRUST LINE KIT DURING MY TESTING. I'm sorry if my testing methods are not up to your high standards, but I did the best I could for a big dumb farm boy with a old Cub on a grass strip.

Hey Mark, The sine of the angle....that's the formula I couldn't remember, guess I ain't as smart as a ninth grader..... Thanks to 12 geezer for the good information, great post.

delete

help wanted

If you had a PA-18-160 with 3 inch extended gear, 26 inch tundra tires, and a borer prop which would you do:

1) install vortex generators or

2) Thrust Line mod

help greatly appreciated.

delete

Measure the airplane first and make sure the angle of incidence is within limits for the Thrustline to work right. Then decide. They both make a very noticeable difference in my opinion and I would prefer both.

i'm sure if i searched this forum i could find out what you mean.....but what exactly do I need to measure?? what are the tolerances or measurements I'll be looking for?? thanks for your help.

FlyingY said:

i'm sure if i searched this forum i could find out what you mean.....but what exactly do I need to measure?? what are the tolerances or measurements I'll be looking for?? thanks for your help.

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You need to find a 4' smart level first. Once you have that call mark @ thrustline and he will help you.

Tim

Hey Mark,
Thanks to the nice weather here in Fairbanks Ive been up in my new bird breaking her in. It ROCKS. Thanks for the phone calls and the nice comment on my cub. Your Trustline is staying on. Period. Clean install. I will take some pics with my old doors on, cause its all I got right now. And I don't care, having to much fun flying it. Hey man don't let some body get you down about your mod and all the B.S. some guys got, after all they may have nothing else. SKI time baby!! first day of trapping season Yeah!!
JK

Thrust Line

I see a bunch of conjecture on why Piper chose the 4% nose down angle on the 18. What is documented... during Helio Courrior development, they selected a 4% (yes percent) negative thrust angle to keep the aircraft from requiring nose down trimming every time you throttled up a bit in level flight. Without it you had to make constant trim changes for the smallest increase in power during cruise, a very poor handling issue for any aircraft. I wonder how Thrust line modified cubs are in that aspect, more sensitive?? Of course all a stock cubs nose up when you give a shot of power, but I'm talking reaction to small adjustments. Also, I am contemplating installing Pen Yan Tiger kit on my 18, did they ever modify their angle up as babbled about on the TL web site?? If your wondering, the Helio is exceptionally well balanced, incredible aircraft with gobsmacking, incredible STOL performance...once you spend 200 hours learning to fly it correctly.

Short answer in my experience on different Super Cubs is less or no trim changes from take-off to touch down.

Here are 19 pages of discussion on the Thrustline Modification.

http://www.supercub.org/phpbb2/viewtopic.php?t=3856&highlight=thrustline

Heliodriver is not the first guy to think about thurstline angle and the way it affects pitch trim with power changes.

....also, there's a few things I could fix on a Helio to make it a better flyer by adjusting some angles, and I'm not talking about thrustline.
D

trim

Heliodriver,
I have an experimental cub with a O thrust angle and the engine sets
about 4 inches below the normal center line of an 18. There is no problem
with the trim and even with wild jabs of the throttle there is no pitch up
problems even with my slats open and with or without flaps and droop
ailerons deployed. Wayne

We have stock cubs with no trim problems at all, and Thrustline Cubs with no trim problems at all. We have one Cub that must have trim from flaps up to flaps down - required both before and after Thrustline. Probably has some relationship to bungees.

Forumites will tell you that I have no feel for Cubs, and that is probably true, but I cannot tell the difference. Thrustline did not decrease stability - I spent six hours the last two days in a Thrustline Cub, hands-off.

My opinion is sort of strange - I think you should look at angle of attack, P-factor, and prop efficiency at angles off the perpendicular to flow. Any time the prop is not aligned with the air flow, you have an efficiency loss in the prop, and drag due to rudder input. You want maximum thrust to be acting in the exact opposite direction from the drag.

Bob wrote: "I spent six hours the last two days in a Thrustline Cub, hands-off"

Jeeze, Bob - try it hands-on - - maybe that's why you keep saying you can't feel the plane - - ya gotta touch it first!!!

I'll probably be coming down to San Diego this summer - I'm looking forward to the possibility of doing some just-for-fun-dual with you. BTW, which field are you out of? - seems you might have said something about Ramona once?

12 Geezer said:

Bob wrote: "I spent six hours the last two days in a Thrustline Cub, hands-off"

Jeeze, Bob - try it hands-on - - maybe that's why you keep saying you can't feel the plane - - ya gotta touch it first!!!

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He was probably the instructor. ( in the back sleeping)

S2D said:

12 Geezer said:

Bob wrote: "I spent six hours the last two days in a Thrustline Cub, hands-off"

Jeeze, Bob - try it hands-on - - maybe that's why you keep saying you can't feel the plane - - ya gotta touch it first!!!

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He was probably the instructor. ( in the back sleeping)

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No he was typing data into his computer. :lol:

No data, because I didn't quite know what to record. How about 40 gals, me and cold weather gear, 8500', 2500 RPM, leaned, gear retracted, and wingtip lower surfaces up 5 degrees? I was in the front, heat on. Nobody wants to be in back when it is cold. And it was twelve hours - six per day.

I think a Cub on wheels would have substantially less angle of attack, and be going faster for the same power. And folks who can "feel" airplanes can get them on the step for an additional ten knots.

bob turner said:

I was in the front, heat on.

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Awww come up and help finish up my flights this week Bob.
We spent 6 hrs today at -15. I think my F&G guy would gladly let you have his spot. ( I was hogging all the heat up front.)

Bob's taking my spot the weekend after next Brian. :lol:

I decided to take the plunge and installed the Thrustline kit. I am VERY pleased with the performance, the install, quality and the support from Mark as well. Below is an excerpt from a PM that I sent Mark:

Hello Mark


I'm in New Zealand and have a few minutes so I'll fill you in with what little I can say. The installation was a snap. Your directions are very good and the pictures help idiot's like me to be able to see what is what I had a buddy of mine who happened to be in town from Michigan give me a hand. Total time took us about 3 hours but that was because we had to fabricate a means of lifting (or at least supporting) the engine. And then of course there were the coffee breaks..... Everything went together nicely though. I flew it the next morning and was surprised to see that I lifted off without flaps in what seemed to be a distance about equal to where I normally did with flaps. The tower even commented on the shortness of the ground roll. I had a couple of buddies watching from our hanger and they too thought it got up very quickly. I went out and did some slow flight and found the handling to be very good (read that as much better) than without the kit. I came in and did several landings and was impressed at how much slower I can fly with a little power on for the landing. In my opinion, everything that has been said good about your kit is just that - GOOD! I am really looking forward to getting in and flying more!


I also have to say that I am very impressed with the quality of workmanship on the parts. One of my buddies (our director of maintenance) was equally impressed and he doesn't impress very easily.

Thanks again Mark.

Brian

Did you include a diet? :lol:

Maybe the 'front lobes' that needed flattened were on a different can, I mean cam...

(hey, I resemble that comment)

aktango58 said:

Did you include a diet? :lol:

Maybe the 'front lobes' that needed flattened were on a different can, I mean cam...

(hey, I resemble that comment)

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No my "cam" appears to be the same size. the "Cargo Pod" seems to have a little extra capacity tho