Newb Question: cessna 150 trim wheel

[FMR]

Can anyone tell me what these lines on the c-150 trim mean, which one is the take off position trim? Thanks for the help!

 

[martym]

the "fat" line on the left is the takeoff range. It is not exact, only in the ballpark. The indicator moves with the trim wheel.

 

[FMR]

the "fat" line on the left is the takeoff range. It is not exact, only in the ballpark. The indicator moves with the trim wheel.

Thanks for the reply Martym, do you know what the horizontal line stands for; where the indicator is on the picture. Could it have anything to do with level flight or maybe the mid range of the trim? thanks again for the help!

 

[Rgbeard]

The other horizontal line looks as if it's mid-range.

Trim isn't this much of a science, by-the-way.

I guarantee you that's not the precise-point where level flight always occurs. That's dependent on a number of factors, some of which are within your control, and some which are not. Such as: weight and balance, density altitude and power settings.

 

[martym]

The horizontal line was put there by someone for unknown reason. Not factory. Maybe that's their own private takeoff setting. The trim for level flight is never exactly the same and you're not going to fly the plane trying to adjust the trim indicator to an exact position anyway. That's not how it works. When you fly the plane it will take just a few minutes to figure it out without looking.

 

[Dan Thomas]

I guarantee you that's not the precise-point where level flight always occurs. That's dependent on a number of factors, some of which are within your control, and some which are not. Such as: weight and balance, density altitude and power settings.

And control rigging. Old airplanes are so often far out of rig. Mechanics often don't have, or don't read, the service manuals, and just rig it to what they think is about right, and it's usually not right at all. I have found the trim rigging off a lot, sometimes so badly that the jackscrew comes close to disengaging in the nut inside the actuator.

 

[dtuuri]

The best way to check is to pull the elevator back and turn around until you can see if the tab is streamlined with the elevator. Easiest to do during preflight.

 

[Let'sgoflying!]

It’s a 150. The great thing is, you won’t likely notice minor changes from having the trim wheel here, or there.

 

[FMR]

Thanks to everyone that has replied, it has helped me see that the horizontal line is not very important and that I should use the fatter line for take off reference and trim the airplane for level flight as I see best (do to the many variables weight, balance, etc.). Appreciate all the help!

 

[MauleSkinner]

Depending upon the age of the airplane, it might have been a mark for the old ASF inadvertent IMC technique.

Set Trim Tab. The trim tab is rolled to a predetermined and premarked position which will give the desired slow flight speed

 

[Wagondriver]

I would suspect that the forward line is one someone added as their preferred take off setting. In my 180 I never take off with the indicator at the take off mark. I roll it 1/8 to 3/16" past, through experience I know thats where I need it.

 

[Let'sgoflying!]

I wonder if a previous owner put that on to allow for the parallax issue.

 

[Dan Thomas]

I wonder if a previous owner put that on to allow for the parallax issue.

More likely the thing is, or was, out of rig, and the new mark was put there to correct it. Should have been rerigged.

And rerig as per the service manual. Not all Cessna trim tabs are streamlined in the takeoff position. From the 150 manual:






Nowhere in there does it say to set the tab streamlined with the takeoff mark lined up. If it's rigged right, the marks will be correct.

The '69-76 172 manual says the same thing. No mention of streamlining at takeoff position, and it won't be streamlined.

Now, the '69-76 182 manual has this in it:



See, that streamlined tab position in takeoff apples to the 182. Mechanics might see that and assume it applies to all the models. It doesn't. That's what service manuals are for.

 

[dtuuri]

Not all Cessna trim tabs are streamlined in the takeoff position. From the 150 manual:
...
Nowhere in there does it say to set the tab streamlined with the takeoff mark lined up.

To check for a 2° max offset you'd need an inclinometer and know how to use it. Checking for streamline will be a more reliable way to ensure a safe takeoff trim setting than trusting a mark by the trim indicator, imo. Not saying this works in other planes.

 

[Dan Thomas]

To check for a 2° max offset you'd need an inclinometer and know how to use it. Checking for streamline will be a more reliable way to ensure a safe takeoff trim setting than trusting a mark by the trim indicator, imo. Not saying this works in other planes.

That's a 2° overtravel, not a 2 degree offset. The 150's tab travel is up 10° and down 20°, from the streamlined position. The manual tells you to disconnect the tab link rod. Loosen the cable stop blocks, and run the trim all the way down. Then you adjust the actuator's screw to get that 10° up plus a bit, max 2° degrees overtravel, or 12° total, then connect the rod. Then the trim wheel is turned to get the tab at 10° and the up stop blocks secured. Run the trim up to get the tab down 20° and secure the up stop blocks. If the indicator needle stops at equal distances from the slot ends, all is correct. With the needle at the takeoff mark, the tab will be down several degrees, maybe 7 or 8°. Definitely not streamlined.

 

[dtuuri]

That's a 2° overtravel, not a 2 degree offset. The 150's tab travel is up 10° and down 20°, from the streamlined position. The manual tells you to disconnect the tab link rod. Loosen the cable stop blocks, and run the trim all the way down. Then you adjust the actuator's screw to get that 10° up plus a bit, max 2° degrees overtravel, or 12° total, then connect the rod. Then the trim wheel is turned to get the tab at 10° and the up stop blocks secured. Run the trim up to get the tab down 20° and secure the up stop blocks. If the indicator needle stops at equal distances from the slot ends, all is correct. With the needle at the takeoff mark, the tab will be down several degrees, maybe 7 or 8°. Definitely not streamlined.

No doubt you're right, I never rigged a 150 although I spent two years rigging flight controls as an AF mechanic. But I bet I've got over 1,000 hours flight instructing in 150s and what I said is sage advice I was taught and have always passed on to my students. Never experienced a problem. YMMV.

 

[JAWS]

Streamlined elevator and streamlined tab = neutral (0 degrees). Indicator should = 0. If not, adjust it to make it so. "Neutral is neutral".

The stab may have a + or - angle of attack relative to the airframe, but that shouldn't matter 99% of the time. To check the stab, you usually have to level the aircraft longitudinally and laterally and check the stab in the correct spots for the correct angle. If the aircraft has damage history or it just won't trim out correctly in flight, you got bigger issues and a full check is warranted. Watch for twisted stabs and elevators.

Takeoff range on the indicator should be 2 or 3 degrees tab down (just a touch trailing edge down from the elevator). If more than a calibrated touch, something is off, most likely the indicator. I have seen things way off on many different aircraft. Time to go back to "neutral is neutral".

 

[JAWS]

View attachment 104763Can anyone tell me what these lines on the c-150 trim mean, which one is the take off position trim? Thanks for the help!

Lines = neutral (0 degrees relative to elevator). Set to neutral and do preflight. Tab should be even(ish) with elevator. If not, something is amiss.

Takeoff range = thick line/bar/whatever you want to call it. Should be slightly tab down in this area.

 

[Dan Thomas]

Takeoff indexes DO NOT mean tab neutral. They mean takeoff position for the tab which, depending on model, might be streamlined and often won't be streamlined. It's why the service manuals have instructions specific to the various models. If they wanted that tab streamlined in the takeoff position they would say that explicitly. Most of the time, in Cessnas, they don't. See if you can find such a statement in a Cessna 150 or 172 manual, for instance.

Mechanics are too often ignoring the manuals and just ad-libbing the rigging. It's why I found so many airplanes way out of rig on just about everything. And so they flew like dogs, and the pilot had to set things like trim in places other than the designated positions.

Edit: here's an example from a Cessna 210 manual:



Look at that. It tells you to put the tab at 10° up, then set the indicator at the takeoff mark. In other words, if it was set streamlined at the T/O mark it would be way off and the pilot would have to immediately push forward when the airplane lifted off and scramble to reset the trim. That's where pilots make extra marks on the pedestal to use for takeoff: because of a mechanic's mistake.

There is something else about a lot of 210s: the elevator rigging.



That differential is there to counter torque roll.

Manuals are important. Really important.

 

[JAWS]

Manuals are important. Really important.

That is very true. My statements were more of general guidelines. There will always be exceptions to the rule!

In the end, FOLLOW THE MANUAL!