Trim Setting on Final

[Darrel Starr]

When I am on short final at 55-60 mph, full flaps and with someone in the rear seat,
I am trimmed nearly to the forward stop. My SC has an 0-320, 160hp engine. I have checked everything about the trim; I have full trim travel, etc. Windonhisnose has a nearly identical 160 hp SC and ends up with the same trim settings also. I am assuming that this is "normal" but seems like a design flaw. In the graph below, this condition is points "A" and "B". Just seems like the trim setting should not be so far forward when we are so far inside the allowable weight & balance envelope. Any thoughts?
Darrel
Darrel + 200 lb Passenger.jpg

[12 Geezer]

That's intriguing because I rarely had to touch the trim in my 12, except to adjust for a different weights in the back. I found that changing flap settings alone was sufficient for "all" phases of flight. I don't mind a little bit of stick pressure, so that might be part of it, but I certainly did not have to hold forward stick on landing. The idea of not touching the trim was taught to me by an old-time supercub pilot years ago, before I ever put the flaps in my plane. Now, my 12 was somewhat nose-heavy so that could be a factor, but one would suspect that the trim changes with speed would not be highly dependent on the static balance.

In my rebuild I'm using a short mount and lightweight starter, so my empty CG will move aft some. It will be interesting to see if that affects the need for trim changes with coordinated speed / flap changes.

Do you have the thrustline mod and carry power on final? That would give some nose-up influence I'd think, and I'd expect power changes to have more influence on pitch. Big flaps? More than 50 deg flap travel? I'm anxious to see what others have to say on this.

[Darrel Starr]

Just normal flaps, no thrust line mod, a little power

[aktango58]

Don't look a gift horse in the mouth!!!!

The heavier the load the more aft cg...

[fobjob]

Is the wing at too low an angle of incidence? That would make for less pitching moment from the airfoil, and result in forward trim settings. +1.843 deg at the wing root...

[coxcub]

Mine, Stock PA18-150, is the same - well forward on the trim with full flap - I personally do not have a problem with this and the UK CAA test pilot who has flown it twice (for beaurocratic reasons!!) with a 180lb me in the back did not mention anything untoward - in fact he remarked that it was a "Good SuperCub " Nothing unusual to worry about I would suggest Darrel.

Frank

[texmex]

Mine, Stock PA18-150, is the same

-ditto ......

[mongo]

Mine is a 180hp but full flaps and 45mph on final I am mid trim. With a passenger at cruse speed I am almost full forward trim, I fixed this buy stretching the elevator assist spring.
My wing is not stock
Engine has the stock thrust angle

Sent from my HTC Evo using Tapatalk

[fobjob]

Any conventional configuration aircraft has a few rules about stability/safety re trim settings:
At the nose-heavy/light gross weight end of the loading situation, elevator deflection should be at maximum UP, resulting in maximum down force on the stabilizer (trim indicator to the rear)...
This makes for Maximum pitch stability, making it most difficult to stall/spin.....

At the max rear end of the envelope, maximum gross weight, maximum weight in the rear,
elevator deflection should be NEUTRAL, resulting in ZERO up or down force on the stabilizer
(trim indicator to CENTER)
This makes for MINIMUM pitch stability, making it EASIEST to stall/spin)

AT NO TIME should the elevator/stabilizer be NEGATIVE, as this would result in a NASTY stall/spin, as the tail would DROP when airflow went away.
Cubs can get away with SOME down deflection, because the long tail moment and large stabilizer area gives you some control in a divergent stability situation. BUT, I think you guys that think that a full forward trim indication is normal, are WHISTLING PAST THE GRAVEYARD and need to look closer at this before you have a nasty situation.

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When I got my cub in 1991, the wing was mounted at zero degrees angle of incidence. The lack of pitching moment caused me to fly around with HALF-forward trim indication.... the plane would stall/spin INSTANTLY, and have a SEVEN turn minimum spin recovery. Putting vortex generators on resulted in a decrease to a ONE-THIRD turn spin recovery.
But, I made sure that I NEVER stalled it...a later crash(not because of that) got the plane put in a jig, which got the wing back to it's proper angle, and fixed the problem.

[Darrel Starr]

From the responses, I think this must be a "Normal" condition. I wonder if Piper just never changed the trim system as they added flaps, etc. over the years. I would think the trim should be capable of neutralizing stick force in any flap configuration anywhere within the weight & balance envelope. Just thinking ...

[12 Geezer]

In full nose-up trim, does the front of your stabilizer contact, or nearly contact the upper longeron?

[bob turner]

I fly miscellaneous Super Cubs all the time. One in particular cannot be flown in the pattern without lots of trim when the flaps go down. The others do just fine. I always thought it was due to that spring, but Fobjob's story is truly scary.

We spin them all, and I have never seen a Cub or Super Cub that would go into a spin without really shoving the rudder in there, and they all recover almost instantaneously. The one above that requires immediate trim on flap extension was completely measured by Mark, and it has all the mods. To spin that one took an "over the top" maneuver. It would not enter a spin from a power off stall (I thought it was the Micros).

[Fortysix12]

Trusing that your supercub is like my supercub, try a indicated 40 mph full flap stabilized approach wiith the same configuration. Bet the trim will be different. If thats' a tad slow then try 45mph same config. Only when I'm at gross do I need more than less forward trim.

[Darrel Starr]

Here is the trim mechanism in nose down position.
P1130373.JPG

[12 Geezer]

Sorry Darrel, I don't know up from down it seems.

I just now went out and counted jackscrew threads in full nose-down trim, and it's the same as in your photo - 18 threads under the yoke. And the bottom of the cross tube is approximately even with the bottom of the yoke casting, as in your photo.

[fobjob]

I fly miscellaneous Super Cubs all the time. One in particular cannot be flown in the pattern without lots of trim when the flaps go down. The others do just fine. ......

We spin them all, and I have never seen a Cub or Super Cub that would go into a spin without really shoving the rudder in there, and they all recover almost instantaneously. ...... It would not enter a spin from a power off stall (I thought it was the Micros).

Just a few notes...when my cub had the flat wing, it flew around with the trim half-nose down, and never needed to be changed, even when dropping flaps, OR aft loading. When I did put two turns in for landing one day to try it, I forgot about it , and on take-off, with a friend in the back, I attempted a performance climbout and dang near lost it before I could recover with full forward stick, from the moment of lift-off... When the wing was fixed, it took two or three turns nose-down trim when I put on full flaps, and an over-the-top maneuver to get it to spin. But, it had the Micro VG's on then, too. Without a doubt, the Micro VG's saved my life, and more than once.

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Darrel, it looks like that your elevator should be deflected down with that stabilizer setting....

[Darrel Starr]

fobjob, I too have vgs. Mine are the Cubcrafters type.
Darrel

[fobjob]

Well, according to your photo, the elevator should be hanging down, and stalls should be brisk and exciting. Have you stalled it???

[Darrel Starr]

Stalls are gentle, a non-event.

[fobjob]

Does the elevator hang down, at that setting in the photo?

[Darrel Starr]

With the trim and the stick are full forward, the elevator is deflected about 12 degree down. The exact angle would be the normal elevator 15 degree deflection when the trim is at neutral minus the angle that the horizontal surface angles up when the trim moves from neutral to full nose down.
Darrel

[AkPA/18]

Darrel

I am not sure how to read your post. The normal 15 deg deflection is in relation to the HRL(I guess I am not sure of a measurement for neutral trim). The elevator should be on the stop with approx 5 turns left of nose down trim. If you put all the nose down trim in and your elevator is not at the stop your bungee spring and or cable could be too short. Sorry if I read your post wrong----hard to put into words. Curious as to what yours is like----this is a fairly common problem with the trim issue.

edit: I guess what I would like to know---when you trim nose down and your elevator reaches the stop---how many turns from full nose down is your trim?

Thanks

Mark

[SteveE]

Mark, so now I'm confused. 5 turns down from what starting point?

[AkPA/18]

Ha---like I said---hard to put into words I will try again later--I thought I wrote it about as clear as I could. Sorry man!

Mark


edit: As you roll in nose down trim--the stick moves forward due to the stab moving up and relaxing the pressure on your elevator thru the bungee interconnect. The elevator will be moving down towards the nose down stop. When your stick stops moving fwd---the elevator is now at the stop. You should have about 5 turns left of nose down trim past the point that the elevator is on the nose down stop(stick quits moving).

Any better???

[Darrel Starr]

I'm having a little trouble following the discussion. To clarify my previous post, neutral trim as I am calling it, is when the horizontal tail is level when at the same time the airplane is lifted up to be level, ie when the door sill is level.
At that neutral point, my elevator will travel down 15 degrees. From that position to "max nose down trim", cranking the trim will move the leading edge of the horizontal tail up to the screw thread stop which then leaves the horizontal tail at an up angle of about 3 degrees still with the airplane propped up level.
Darrel

[AkPA/18]

Thanks Darrel.

Where is your elevator at full nose down trim? Is it on the nose down stop?

Mark

[Darrel Starr]

I'll have to look when I get to the plane, but I think it is.
Darrel

[mikeo]

Eaton I'm glad Eddie made it home , hope you an Eddie have a safe trip to Johnson creek!!!! Mikeo

[texmex]

I'm wildly speculating here, but I'm wondering now whether the common problem in not enough forward trim with full flap on final is related to Alex Clark's thread "Horizontal stabs are stuck"
I've just pulled off my tailplane to find the rear stab inner tube frozen with rust. If the stab is ridged at the rear then I imagine the twisting forces on the stab are too much for the stab jack, beyond a limited range. If it becomes frozen in a mostly used position, and has enough flex we may fly around for years not thinking anything of it, except we we apply full flap on final.
It's just got me wondering???

[Darrel Starr]

My elevator was rebuilt along with the rest of the plane not long ago so I'm confident that it is not rusted in place, yet I go to full Down trim when full flaps on final with someone in the back seat. Also I checked the jack screw with the trim in the extreme up & Down positions and it does go through full travel.

[FdxLou]

Darrel, do you have the thrust line mod?
Lou

[Darrel Starr]

No Thrust Line Mod, Lou.

[FdxLou]

Darrel
I too, had lots of trim changes with cg/speed/config changes until I added Mark's Thrust Line....now I set it for takeoff and rarely feel any stick pressures. Sure cuts down on the jack screw movement. I have a fwd cg 180hp Smith Cub....in a word HEAVY!

Lou
heading to the Grand Canyon....

[Darrel Starr]

Thanks Lou. Trim changes are a nuisance ... Seems to me that Piper should have designed in more trim range as the plane evolved.

[StewartB]

If you fly short final at 55-60 mph with the trim dialed forward to maintain the descent, what happens when you reduce the power? The nose drops, right? You have to increase back pressure on the stick to maintain the attitude, right? The pressure would be relieved by rolling the trim back, right? If you're committed to flying those speeds, try flying your final leg with a steeper glide path using gravity to to produce speed rather than power.

I always try to hold my planes on the ground with the tail high to gain as much speed as possible to penetrate unfavorable take-off winds. I've learned that there's a speed where I can't hold the plane down without extraordinary control effort. The same is true to get the plane down.

I'd comment on what my -12 does with full flaps at 60 mph but I can't recall ever having full flaps out while going that fast. And I mean no disrespect by that. Just being honest.

SB

[mdudk]

Mine, Stock PA18-150, is the same - well forward on the trim with full flap - I personally do not have a problem with this and the UK CAA test pilot who has flown it twice (for beaurocratic reasons!!) with a 180lb me in the back did not mention anything untoward - in fact he remarked that it was a "Good SuperCub " Nothing unusual to worry about I would suggest Darrel.

Frank

My SC is exactly the same, really disconcerting at the start.. I felt like I had no elevator authority and i was having to push it all the way forwrd with no extra fwd stick motion available.
Its only an issue with full flap so i rarely use full flap (Im on floats) when I have 2 people on board doesn't feel right. On my own less of an issue
safe landings

[coxcub]

Could it be that the centre of pressure moves forward on the wing as the camber of the aerofoil changes as the flaps are lowered? Thereby making the nose rise which is counteracted by forward stick and the trim wound forward to remove stick forces - this would be exacerbated by a full rear seat. That's why the trim is there.
I am not an aerodynamic expert so please don't shoot me down with heavy calibre!! - Just my thoughts

Frank