The why of thrust line?

[bob turner]

Steve, on Saturday, mentioned a nose down pitching moment with a power application in the landing configuration. As of right now, I have no information to add to that, except that my friend Gary says that he always got a feeling of either pitch-up or levitation with his T/L Cub.

Today we briefly took the one remaining non-T/L 150 Cub out and did maybe three trials - one at 50 clean, one at 60, flaps one notch, and one at 55, full flaps. Each time, hands off and with smooth power application, the aircraft gently pitched up. Just as we expected it to.

Next week I will do the same in one or more T/L Cubs and report back. I do not expect any difference. There certainly should not be a pitch down, and I don't think 4 degrees is enough to change the pitch up tendency.

 

[bob turner]

Well, what a surprise! Took the lightweight 160 up this afternoon. Full of fuel, but just me. I could barely get it stable at 50 flaps up, possibly because of no passenger in the back? Anyway, I tried hands off full power applications at flaps up, half, and full, and each time I got about twice the pitch-up as with the stock Cub.

Bear in mind that this is a brand new engine, more power, less weight, etc., but I was expecting not much difference. This thing really points itself skyward, and it does so in a hurry.

I believe it is just as safe as a normal Cub, unless a pilot wants to practice go-arounds hands-off. I never realized these things would pitch so much with such a subtle change in engine angle. I rarely approach going from idle to full power, so I had never noticed this difference before. I also rarely fool with much trim, so when the aircraft tries to do something I don't want it to, I just move the stick a bit and it doesn't happen.

 

[Steve Pierce]

Bob, I am sorry but to me your tests are meaningless using two different Cubs, one with the Thrustline mod. and one without. Too many variables to say you are doing an apples to apples test.

 

[bob turner]

Are you saying you would rather I not post my findings? I make no claims of being the world's best test pilot, and anyway, fans of ThrustLine should be applauding my findings. Gary, at least, loves the pitch-up tendency. I agree with you that flying different aircraft and expecting exactly the same results is less than satisfying, but until someone does a rigorous test of this mod, it is all we have.

How about it? Let's hear from someone else on pitch-down with power application.

 

[Steve Pierce]

I never realized these things would pitch so much with such a subtle change in engine angle.

I don't see how you can say this after flying two different airplanes. The FAA engineers said it would pitch up and the FAA's own test pilot couldn't get it to.

 

[Gordon Misch]

Steve, I thinking maybe I don't understand or didn't correctly interpret what you wrote - - Are you saying that with the TL mod, as you add power with no other changes in inputs, the plane noses down - and presumably then descends as a result of the added power? And that without the TL mod the opposite occurs?

 

[Crash]

Wing incident also has a lot to do with how effective "zero thrust line" will work on a particular Cub. Wing incident should be 1.8 (1.843 to be exact)degrees (FAA AC 43-16).

Unless you check both planes and they have the same incident, any tests comparing the two will be inconclusive.

Take care.

Crash

 

[bob turner]

The entire community here has already told me my tests are no good. I am going to keep doing them until somebody else tries this stuff a few times. Stay tuned . . .

By the way, this is a fairly simple test, assuming you have the skill to trim it for hands-off flight, and the courage to add full power without touching the stick. Let us know how that turns out?

 

[AkPA/18]

Bob--Lloyd and all

I have had several people want me to respond to all of this and it gets rather frustrating as the thread goes on as to what to even address.

Lloyd and Clmaxx --- I have never measured a cub TL at 4.24 degrees yet. They are usually between 5 and 5.5. The motor mount has a hard number that is supposed to be 4 degrees and that is what the TL mod change is. This puts the install TL in the neighborhood of 1 to 1.5 degrees down. Just thought I would try and clear that up.

The TL was also tested on a 180hp to zero degree---well overgross with 46lbs of lead dive wt on the tailwheel and sand bags etc to be at the 20 in. aft cg line. This was all done on the same day on a before and after test on the same conforming AC. The ACO (AC certification office) was concerned as to pitching component also. After the flight tests---it was a non issue. The TL AC proved to be as good as or superior to the stock Tl in all CAR 3 certification tests ---especially in the go round test in ground effect. This was not done with "no hands" but with force gauges on the stick.

Bob

I have flown cubs that act quite different. When I flew the Redneck Cub with stock length flaps but extended to the fuselage---The nose pitches down when you pull flaps(at least at the load-trim and airspeed that I had that day). The only reason I can attribute that to is that Mike's cub is closer to 2.5 deg angle of incidence. It does have a TL. I don't know why for sure.
ShermBush on this site contacted me several months ago. He had a pitching and trim issue with a TLcub. He asked me if it was just him or was it the TL. It took me a bit but I told him it was neither---The bungee cable was too short at the elevator. Flew fine after Dan's fixed it.
Lonnie lands full nose down trim and holds back to land---Paul Claus trims noseup and pushes fwd to land. Their go-rounds will be quite different. If you trim hands off for landing and add full power the nose will pitch up. Not much of a big deal---just give the AC what it wants. If you come in trimmed hands off, loaded aft, and do a go around with full power and dump flaps---you had better be ready to pull back on the stick forcefully. Again---just give the AC what it needs.

Bob---you are not a test pilot(I am not either). You go up and fly a TL cub at 50mph (we had hours calibrating airspeed) without flaps and say it is unstable. What does that mean? You compare one cub with a TL to one without. These are more like casual observations. Too many variables. Your last round of tests showed that a cub took off shorter with no flaps than with flap. So I really don't want to get into your hard data results again. I have yet to hear any pitching problems over the years we have been doing this. Biggest problem has been that on some cubs we just don't get the results we want.

Lloyd----Have you heard of or have any firsthand knowledge of a problem or were you just bringing this up for sake of discussion from some of your reading?

I know the above is not well written and is rambling--just had to write something. Just gets frustrating to read about this mod in this light without a reply.

 

[SteveE]

Mark,
RIGHT ON!!!!

 

[willyb]

I thought it was very well written Mark.I also agree that testing anything without a very strict test procedure,documentation and no variables thrown in is a total waste of time.
I have been down this road and it is a lot of work to get real proof that what your doing works.
My hat is off to you for what you have done.

Bill

 

[Steve Pierce]

I agree with you that flying different aircraft and expecting exactly the same results is less than satisfying, but until someone does a rigorous test of this mod, it is all we have.

Others have tested it and posted their results over and over again. I think clmax is selling an engine mount with a different angle, why not install it and post your test data?

 

[Lance]

Hey Mark, good to here from you.

Lance

 

[S2D]

All BS'n aside, I find it hard to believe Piper lowered the thrustline for the reason CLmaxx stated. The PA-11 was neither high horsepowered, or flap equipped. So I assume they did it for some other reason. Visibility ??

I spend a lot of time flying the supercub with my attention diverted, and controls unattended in all sorts of loading configurations. I have yet to experience a "scary" situation when going from an idling descent to full power.
(I'm not sure I can say the same for my passengers tho) 8) right steve?

Bottom line, the Thrustline mod was developed by a working pilot for working pilots, Mark probably shouldn't be selling it to pilots that don't know how to use it.

 

[Gordon Misch]

Steve Pierce wrote: "Others have tested it and posted their results over and over again."

Steve, you often find old threads with info the rest of us miss on searches - can you point me to those results? I've been looking and I haven't found good performance comparison data anywhere. Everything I've seen has been an arm-waving argument, anecdotal, or comparisons that are flawed by lack of controlled variables. Even Mark's statement that "the TL AC proved to be as good as or superior to the stock TL in all CAR 3 certification tests" doesn't help a great deal - it gives a qualitative conclusion, but I don't know what "as good as" means?

Steve, how about my question on nose-down pitching with added power?

My personal supposition? I would expect that something close to zero thrustline angle would be best - I always wondered why the engine points down when the plane is supposed to go up. But like Bob, I'd like to see some numbers from controlled experiments. Bob's numbers aren't controlled; they're anecdotal. But he makes no claim otherwise. Mark, I'd be absolutely delighted if you would be able to post some of your pertinent test data, but I understand it might be proprietary info.

 

[ag-pilot]

Mark probably shouldn't be selling it to pilots that don't know how to use it.[/quote]

Brian,,, Good point! Now, its your turn with the weather gods. I did my part Saturday night at the Bucking Horse Sale. Remember how nice it was Sunday morning! Give me a call from the Hell Creek or Ranchers so I'll be ready.
Dave

 

[skywagon8a]

I believe that the test that this discussion is about is the flight test maneuver called the Balked landing climb. It is addressed in the special appendix to civil aeronautics manual 3. CAR 3.85a(c). The procedure is on page 109. Unfortunately that page is missing from my copy. As I recall the maneuver goes something like this. With the flaps extended trim in a glide at 1.3vs. Simultaneously retract flaps and go to full power. The stick force is not allowed to exceed a certain number of pounds. Perhaps one of you has a copy of this and can post it. It may answer the question?

 

[NimpoCub]

It doesn't sound like the folks he's selling it to are the prob.

I'm certainly no engineer (not even much of an aviator yet) but I have gleaned a lot from just reading all the (delete) about this TL mod.

That said (disclaimer), my common/horse sense tells me that a comparison can't be made between 2 different airplanes, nor between 2 different pilot statements like "my airplane does this". (That should be obvious.)

It also seems to me that there can be no such thing as two "identical" airplanes. Differences in the fabric/paint weight & texture, other slight drag differences, where your trim is set (due to CG differences that day) etc, will make a diff. in how the plane flies.

Then there are the biggies like YOUR airplane's AI. Mark has said several times that some cubs' difference is incredible, some cubs' are yawnworthy. Then there all the others in between that. Like everything else, YMMV.

It sounds to me that the thing to do is to hang the TL mod on YOUR plane, let us know YOUR results, and if you're happy then pay for it. If not, not.
How can it get any easier to decide whether to buy something?

When I "recover" from my current necessary spending spree, I know I'll try it on my cub.

Thanks for all the interesting reading, even the (delete).

 

[bob turner]

Geez, you guys. Steve said the aircraft pitched down in the landing configuration. I found the opposite.

I clearly stated that it was no big deal. I have clearly stated that I have had absolutely no stability problems with the T/L kit on any Cub in about three different threads, yet Mark says I am saying that T/L makes the aircraft unstable?

The guy that started this thread is asking why the T/L mod works. That is a valid question that has not yet been answered. Almost 100% of posters are sure that it improves the aircraft in pitch, speed, and takeoff performance, and one has asserted dramatic increase in roll response, yet noone has stated why it has done so. 12 Geezer has come the closest to advancing a plausible reason for enhanced performance.

I too have wondered where all these test results are posted. I have seen only Mark Drath's averaged data, and my own raw data. I have never made any claims of pilot skill or measurement accuracy - I have only expressed surprise that I have not found anyone else's data except Mark's. Shouldn't we gather them all in one spot, so we can see them?

 

[Steve Pierce]

Gordon, I think you have seen the results i speak of. They areon the Thrustline thread with 19 pages. The last two pages especially. Mark Drath, Dave and Brian had some good information to me.

I think there are those who understand the science and it doesn't make sense and there are the folks who don't understand the science but can tell it works and don't care to understand it. The FAA engineers said it would pitch and the test pilots didn't have a problem with it. Sometimes there is some PFM involved. Mark doesn't take your money till your happy so put it on and try it, if you like it pay for and if you don't, take it off and send it back. I was going to suggest we do some experiments at New Holstein with before and after tests with the same airplane and different pilots but I think that would open up another can of worms.

The slight nose down pitch when landing and adding power seems like the airplane is pitching around the center of gravity. These were someone else's experiences that came up in a Thrustline conversation. I have never really paid attention to the pitching either way, I just make the airplane do what I want it to, and haven't noticed any bad habits.

 

[bob turner]

It doesn't have any bad habits.

 

[AkPA/18]

Well, what a surprise! Took the lightweight 160 up this afternoon. Full of fuel, but just me. I could barely get it stable at 50 flaps up, possibly because of no passenger in the back? .

I have clearly stated that I have had absolutely no stability problems with the T/L kit on any Cub in about three different threads, yet Mark says I am saying that T/L makes the aircraft unstable?

 

[Steve Pierce]

Bob, Is there any way possible you can come to New Holstein? I have a volunteer with a late model original Super Cub that we can do before and after tests on. I think it would be fun and education for all of us to fly and theorize around the camp fire.

 

[Crash]

The CC Sport Cub is "zero thrustline" according to Jim Richmond.

Crash

 

[bob turner]

Steve - I would love to do that. You are one of my favorite folks here; we could swap stories . . . I will try. As an ex-airline type, I absolutely hate to go commercial air, even though it is free - but I have family in Milwaukee.

Mark - my apologies. I sort of didn't post that comment clearly. For my pitch-up test, I want the aircraft in stable flight, hands-off. That requires enough trim authority to do so; it has nothing to do with T/L. I had never actually run out of trim on a Cub, but for the slow speed stuff with full fuel and me the trim was almost full aft.

I have never seen an unstable Cub in pitch - T/L or not. I was using the word incorrectly in the post you cite - I was using it more as in "trimmed for steady hands-off flight". I will try to find a better term.

True instability is when you let go and the thing starts to oscillate, then the oscillations do not dampen. That simply doesn't happen in T/L Cubs - or any other Cub that I know of.

More later today . . .

 

[bob turner]

Did the same test on the AMD -VG-equipped Cub - heavy, belly tank, 26" Goodyears, 160/Borer. Almost identical pitch-up to the light Cub above.

Also tried the J-3 - somewhat milder pitch-up, but more than the stock Super Cub.

I know I am repeating myself, but I consider this not a detriment. I never noticed it before, because it is a maneuver that I simply never felt the need for. Nominal stick forces are all that is involved in maintaining level flight in any Cub from idle to full power, and by nominal, I mean I have never felt the thing push back.

The only reason I even tried this is - I have never flown an aircraft that would pitch forward with power application. Obviously I have never flown a Lake, and comments last week will definitely help if I ever do . . .

You do not have to believe that I am telling you what actually happens, but this one is so trivially easy to verify. Please - somebody try this, and tell me I am wrong.

 

[clmaxx]

What a great amount of good information from from so many people. I don't know about the rest of you but I learned alot. I'm going to try to add some more detail applying to all of this in another posting.

Thanx
Clmaxx
"In Thrust We Trust"

 

[S2D]

What a great amount of good information from from so many people. I don't know about the rest of you but I learned alot.

You should see how smart we are before we start drinking

I thought this thread started out with good intentions and information, but quickly degenerated to a "why the thrustline mod shouldn't be installed"

 

[NimpoCub]

Aawww, STD (I mean S2D)
Only a couple of doubters, mostly good BS.
I'm still gonna do it when I get caught up with all the stuff I HAFT'A do.

This is one of the good threads lately!
[sip] waiting for another one...

 

[Gordon Misch]

I'm thinking it's more like questioners (wonderers) than doubters. I've talked with this thread's main-man-wonderer-in-chief (Bob) quite a bit personally about the topic - that's why I think that.

Some years ago my oldest son and I were arguing vigorously about something - don't remember what, and somebody clearly won - don't remember who (probably him?). But I do remember jointly coining an aphorism that I like - "To lose an argument on its merits is to win". The idea being that the 'loser' learns something, while the 'winner' doesn't. Of course an argument 'won' by putdown doesn't count - -

Yeah Nimp, good plan - just got 'nother un.