Elevator take off trim position

[Gary0747]

I can not find in the maintenance manual how one determines if the take off position on the trim indicator is correctly set to indicate the proper elevator position?

[Piloto]

The optimum trim setting varies with the CG. If you are by yourself up trim setting is better, but with 4 onboard setting to the TO range is better.

José

[Hank]

I would think a set of travel boards wojld be required to set everything just right. I also find the upper edge of the Takeoff setting is a good takeoff position for most flights (1-2 people, little to no baggage). Loaded heavy, I center them up.

Seems there was a thread in this last year, and I posted photos of my Flap & Trim indicators for solo takeoff, cruise, final approach and after touchdown.

[Vance Harral]

We just went through this on our M20F after replacing the bearings for the trim jack screw.

The process for rigging the pitch trim is covered in section 6-4 of the maintenance manual.  The trim indicator should match the takeoff mark when the horizontal stabilizer is set to a particular angle of incidence.  I've attached the page of the maintenance manual that shows the correct incidence angle, which depends on the model and serial number.

[Vance Harral]

And yes, it technically requires having the elevator travel board to do the job per the maintenance manual.  There are other ways to measure incidence if you're clever, but we used a borrowed travel board.

[EricJ]

A quick sanity check is to run the trim all the way up and see that the gap between the fin and the fuselage fairing is reasonably small and the piano hinge (inside the cover, you have to take the hinge cover off to see it) is nearly fully extended.   If that happens right when the indicator hits the top, and the indicator hits the bottom when the trim hits the down stop, it's reasonable to assume the indicator is probably good in the middle TO position as well.

At least, this is what I did on my J as a sanity check.   For whatever reason my airplane needs a lot of down trim at cruise and I sometimes run out of down trim in descent.   I was worried it was all goofed up, but as far as we can tell the trim mechanicals and indicator are correct.    

[Gary0747]

Thanks that was exactly what I was looking for.  I have been trying to figure out why I am getting a shimmy only at rotation speeds on take off. Landings are smooth and straight. I can not find anything wrong with the tires or wheels. Tires are only a year old. I was thinking if my trim setting were off that I might be getting up on the nose wheel and the mains may becoming unloaded just before rotation causing the shimmey.  I guess the same thing could happen if I were rotating at too high of speed?  I normally fly out of a small airport in Michigan but have been flying out of KDVT here in Phoenix recently and have experienced the shimmy twice here for the first time.  Not at any other airports.  My first thought was washboard runway surface.  But maybe the big wide runways have me doing something differently.   Any thoughts?

Edited March 8, 2018 by Gary0747

[Vance Harral]

8 minutes ago, EricJ said:

A quick sanity check is to run the trim all the way up and see that the gap between the fin and the fuselage fairing is reasonably small and the piano hinge (inside the cover, you have to take the hinge cover off to see it) is nearly fully extended.   If that happens right when the indicator hits the top, and the indicator hits the bottom when the trim hits the down stop, it's reasonable to assume the indicator is probably good in the middle TO position as well.

No, that's not a reasonable assumption.  The trim "nut" that hits the stops which control the upper and lower trim travel limits, connects to the wire which drives the trim indicator with either a set screw or jam nut.  It's entirely possible for this set screw/jam nut to get loose and allow the wire to slip, such that the trim indicator is significantly off even though the travel limits are correct.  And there is no measurably defined "top" or "bottom" of the trim indicator window you can use as a reference.  The only defined reference in the trim indicator window is the takeoff position mark.  You only know the trim indicator is rigged correctly when it matches the takeoff mark at the defined stab incidence.

I know getting the travel boards and removing the belly panels to access the mechanism is a pain, but please don't substitute shade-tree methods for the procedure in the maintenance manual.  If you have any concern your trim indicator isn't reading correctly, use the correct procedure.

[EricJ]

2 minutes ago, Vance Harral said:

No, that's not a reasonable assumption.  The trim "nut" that hits the stops which control the upper and lower trim travel limits, connects to the wire which drives the trim indicator with either a set screw or jam nut.  It's entirely possible for this set screw/jam nut to get loose and allow the wire to slip, such that the trim indicator is significantly off even though the travel limits are correct.  And there is no measurably defined "top" or "bottom" of the trim indicator window you can use as a reference.  The only defined reference in the trim indicator window is the takeoff position mark.  You only know the trim indicator is rigged correctly when it matches the takeoff mark at the defined stab incidence.

I'm not familiar with the indicator in an F model.   In my J the indicator stops at the top of the window when the piano hinge nearly reaches the limit of it's travel in the up direction, and the indicator stops at the bottom of the window when the down stop limit of the trim is reached.   I don't know how those can be correct at the limits and the TO position indication in the middle can be off at the same time.

[Vance Harral]

19 minutes ago, Gary0747 said:

I was thinking if my trim setting were off that I might be getting up on the nose wheel and the mains may becoming unloaded just before rotation causing the shimmey.  I guessthe same thing could happen if I were rotating at too high of speed?  Any thoughts?

Could be due to getting the mains off before the nosewheel, but that's easy to test just by using additional up trim (as others have pointed out, optimum trim position depends on CG) or brute force to raise the nose at a slower - but reasonably safe - speed.  What's more interesting is to see if the shimmy is dependent on ground speed, i.e. the rotational speed of the tire.  If so, it may simply be a balance issue.

[Immelman]

Look at it this way... should you be trimming immediately after takeoff? 100' AGL? 200' AGL? (Basically from liftoff until the wheels are stowed and you're climbing away, before any flap changes).

My answer to that is 'NO'. Trim should be set such that you get the gear stowed and the airplane pitches for your desired initial climb speed with no pilot intervention on the elevator control. What ever trim setting that is... that is what you use, making estimated adjustments for CG changes due to pax/cargo.

I found the spot in my airplane that corresponds to this and set it accordingly before takeoff. Trial and error. It is slightly off from the takeoff 'spot' on the trim indicator.

Edited March 8, 2018 by Immelman

[Vance Harral]

1 minute ago, EricJ said:

In my J the indicator stops at the top of the window when the piano hinge nearly reaches the limit of it's travel in the up direction, and the indicator stops at the bottom of the window when the down stop limit of the trim is reached.

I'd have to see a picture of your trim indicator to understand this statement.  In my F model, there is no "top" or "bottom" of the indicator window.  The window is just a clear section of Lexan, whose only limits are the edge of the Lexan itself.  The indicator slides in a channel behind the Lexan.  If you disconnect the wire that drives the indicator, you can slide the indicator all the way out of the window in either direction - there is no mark or limit that would tell you the indicator is at the "top" or "bottom".  So my concern is you have some shade-tree concept of the top/bottom of the window that's not actually a measurable or defined thing.

To be clear, I'm not the kind of guy who says anything other than the procedure in the maintenance manual must be useless/dangerous/whatever.  But having recently disassembled the mechanism, I'm convinced your sanity check isn't helpful.  Not only are the issues I've already described pertinent, the tolerances are a lot tighter than it seems you may think.  Per the maintenance manual above, the permissible tolerance on the incidence settings are only 1/2 degree.  This is not the sort of thing you can reasonably estimate by how close the vertical stab is to the dorsal fin, or how far extended the piano hinge is.

[EricJ]

4 minutes ago, Vance Harral said:

I'd have to see a picture of your trim indicator to understand this statement.  In my F model, there is no "top" or "bottom" of the indicator window.  The window is just a clear section of Lexan, whose only limits are the edge of the Lexan itself.  The indicator slides in a channel behind the Lexan.  If you disconnect the wire that drives the indicator, you can slide the indicator all the way out of the window in either direction - there is no mark or limit that would tell you the indicator is at the "top" or "bottom".  So my concern is you have some shade-tree concept of the top/bottom of the window that's not actually a measurable or defined thing.

To be clear, I'm not the kind of guy who says anything other than the procedure in the maintenance manual must be useless/dangerous/whatever.  But having recently disassembled the mechanism, I'm convinced your sanity check isn't helpful.  Not only are the issues I've already described pertinent, the tolerances are a lot tighter than it seems you may think.  Per the maintenance manual above, the permissible tolerance on the incidence settings are only 1/2 degree.  This is not the sort of thing you can reasonably estimate by how close the vertical stab is to the dorsal fin, or how far extended the piano hinge is.

The clear part of the lexan that that indicator travels under is of finite length.   When my pitch trim is in the full up position the indicator is at the top edge of the lexan window.   When the pitch trim is at the bottom travel stop, the indicator is at the bottom end of the lexan window.   The edges of the lexan are definable and measurable.    If the indicator travels the full length of the window over the full length of the trim travel, I don't know how it can be very far off.

This is a pic of a 77 J panel with the indicator at the bottom of the console.

[Hank]

5 hours ago, EricJ said:

The clear part of the lexan that that indicator travels under is of finite length.   When my pitch trim is in the full up position the indicator is at the top edge of the lexan window.   When the pitch trim is at the bottom travel stop, the indicator is at the bottom end of the lexan window.   The edges of the lexan are definable and measurable.    If the indicator travels the full length of the window over the full length of the trim travel, I don't know how it can be very far off.

This is a pic of a 77 J panel with the indicator at the bottom of the console.

That's the same as in my C. There are even helpful lines, just like the one labeled Takeoff, that say Up and Down. While I'm almost never there, I take those as the limits. In your photo, that's about my normal trim setting for takeoff, and where I often see the trim after landing.

[EricJ]

4 hours ago, Hank said:

That's the same as in my C. There are even helpful lines, just like the one labeled Takeoff, that say Up and Down. While I'm almost never there, I take those as the limits. In your photo, that's about my normal trim setting for takeoff, and where I often see the trim after landing.

Same, here, and I've heard that comment before, that the trim winds up in about TO setting after landing for a lot of people.   Mine's always worked that way, so it reinforces the notion that the indicator is correct or at least reasonably so.

[cnoe]

Trim/Flap indicator in a ‘78 J.




Sent from my iPad using Tapatalk

[Hank]

My 1970 C, ready for takeoff:

[Vance Harral]

OK, I think I understand the difference of opinion now.  People are using the dots by the "Nose up" and "Nose Down" lettering as a reference.  Those are not "stops" on the indicator, but anything printed on the indicator can be a reference, of course.

It may be coincidentally true that in a particular make and model that's rigged according to the service manual, that the nose up stop is hit at about the point the indicator is next to the nose up dot and vice versa for the nose down dot.  If so, then sure, I agree you can estimate whether the takeoff trim setting is correct just based on the travel between those points.  Assuming you know one '77J is rigged properly, you can compare it with another, for example.

Here's why I think you need to be careful about using this as a shade-tree rule:

Eric has a '77J, I have a '76F, and Hank has a '70C.  We all appear to have exactly the same trim indicator.  It's probably the exact same silkscreened lexan part, with the nose up/down "dots" and takeoff "trapezoid" in the exact same place.  Best as I can tell from looking in the parts manual, all three of these models use the same worm gear and "nut" (rectangular aluminum piece) mechanism to control the up/down stops and to set the trim indicator.  They all also have the same nose up limit spec: -5.25 to -5.75 degrees.  But the '70C has a nose down limit of +1.00 to +2.50 degrees and a trim spec of -1.25 to -1.75 degrees, while the 76F and 77J have a nose down limit of +0.50 to +1.00 degrees and a trim spec of -2.00 to -2.50 degrees.  It's simply not possible for these different airplanes to all be rigged properly and have their pitch stops correspond to the up/down dots on the same trim indicator.  They can't even have the same range of travel on the indicator, since the '70C must traverse 8.25 degrees of range while the others only traverse 6.75 degrees.  By definition, if our airplanes are rigged properly, they can't show the same indication on the indicator at the pitch stops, and therefore at least one of us can't really use the "stops at the dots = takeoff indicator OK" rule of thumb.

I don't actually know where the trim indicator stops relative to the up/down dots at the max travel limits on my airplane.  But I submit it's irrelevant, except maybe as compared to exactly the same make and model - and even then you'd be inventing a "reference" that Mooney didn't use.  More importantly, there certainly isn't any rule of thumb that applies across different models, since they have different stops and trim settings, and in some cases even a different indicator.

All that said, it's not like having the trim indicator 1/4" off from where it's supposed to be is a life-and-death event.  As has already been mentioned, the optimum practical setting for takeoff varies both with loading and personal preference for control feel.  I don't think Eric, Hank et. al have dangerous ideas here.  But I hope the details above illustrate why the shade-tree method can give you non-trivially different results than the procedure in the service manual.

[Hank]

Anither difference is the indicator. Mine is nice and wide, white plastic, but the 78J has just a thin wire. Can't see much detail in the 77J photo . . . .

[Yetti]

my indicator bar is centered in the Take Off position for take off.  For landing with half flaps (normal) the indicator will be about mid window.   For full flaps landing the indicator is about 3/4 up the window.

[carusoam]

Somebody working at the factory took into account the thing we are discussing today... and they did it for each model.

My C and my O use the same indicated trim positions, as you fine gentlemen have described...

slightly above the range for front seats filled only... in the range whith the back seats filled...

The big differences of these Mooneys, the long lever of the extended fuselage, and LED indicators.

They must have been chuckling while designing the trim screw, as it had been done so thoughtfully before...

I was always impressed by Al Mooney’s design.  The trim range runs out at the same time the indicator runs out...  the trim range runs out, more or less, in the Same fashion...

Flaps at maximum, starts to bump into a HP limitation, when fully loaded...

Go arounds are equally a handful... wether a short body or a long body with 310hp.

Somebody else put in a similar amount of effort when designing the ASI...  each 90° seems to have an implied meaning....

Either way, i’m More of a follow the procedure kind of guy.  Just find it interesting that the ranges kind of match the available space.

Mooneys got bigger and faster through the years, but they were as precise in 1965 as they were in 2015... 

Adding a tail trim, you find how balanced the Mooney is when the P-factor changes in smooth air from climbing nose up, to descending nose down.  The tail trim nicely centers the ball...

Go Mooney!

Best regards,

-a-

[Hank]

1 hour ago, carusoam said:

Somebody else put in a similar amount of effort when designing the ASI...  each 90° seems to have an implied meaning....

Let's see, ASI quadrants . . . .

• Quad 1:  on the ground; 90° is pushing rotation
• Quad 2:  climbing for altitude
• Quad 3:  cruising to get somewhere
• Quad 4:  power on descent at 500 fpm

Is it the same for your O?

[carusoam]

That is probably helpful knowledge for a CFI that is jumping plane to plane...  on final approach, glancing over at the ASI to see it sitting level... scanning everything else including the student. 

More cool than memorizing a half dozen Electronic displays with various ranges encoded in it...

Flying only one plane, there isn’t a real need to have a standard panel layout (six pack), to mount the standard ASI into... 

Best regards,

-a-

[cliffy]

Just a note for information - If the trim isn't set correctly it can lead to running right off the far end of the runway not being able to rotate.

Its happened twice at Stockton airport that I know of. Trim was set full nose down and nose could not be rotated with full up elevator. 

Level the airplane, check and set with a travel board and spirit level is the only way of doing it correctly. 

[cliffy]

There was a Jetstar corporate jet many decades ago that went off the end of the runway in Chicago (IIRC) with the elevator trim set wrong. Killed all on board.  It happens. 

On corporate stuff some of us refereed to them as "the killer items" - trim, flaps, speed boards, as the last items we double checked as we pushed the knobs forward. This was of course, old school, many years ago. It still works today, even on small stuff like Mooneys.

Edited March 9, 2018 by cliffy
grammar