Takeoff trim tail gap measurement?

[jetdriven]

Can somebody set  their M20 J trim at takeoff position and then measure the gap at the bottom between the tail  assembly and the back edge of the fuselage skin? Curious if mine is right 

[N201MKTurbo]

I’ve got 4” at the bottom of the tail.

[Mjknick@gmail.com]

How does she feel on takeoff as far as control pressure. If okay I’d say your good.


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[jetdriven]

Where it feels OK the trim indicator is quite a bit nose up from the takeoff mark 

Edited November 6, 2019 by jetdriven

[Zulee]

18 hours ago, jetdriven said:

Where it feels OK the trim indicator is quite a bit nose up from the takeoff mark 

Totally agree, I run it up 1/2 of indicator above the indicator. 

[PT20J]

With just me in the airplane I set trim like this. With a further aft CG, I set it more nose down.

With trim set this way and takeoff flaps, a steady 5 lb. (approx.) pull on the yoke during the takeoff roll will cause the nose wheel to lift and the plane to fly off when it's ready. After it rotates about 5 degrees, (ten degrees on the AI since the plane sits about 5 degrees nose up on the ground) releasing the pressure stops the rotation and the airplane is usually pretty well in trim until flap retraction.

With the trim set this way, the gap is 4.5":

Also, with takeoff trim, the leading edge of the fin is tilted 1-deg. aft of vertical and the bungees hold the elevator in trail:

For reference:

Full nose up trim gives a gap of 5-1/4", the fin is tilted forward 1-deg., and the bungees hold the elevator trailing edge up 10-deg. 

Full nose down trim gives a gap of 3-1/2", the fin is tilted aft 5-deg., and the bungees hold the elevator trailing edge down 10-deg.

These measurements were made with the airplane sitting on the gear. The nose was 4-deg up in this condition measured at the leveling screws.

As I'm sure you are aware, the trim indicator cable can be adjusted independently of the trim jackscrew position, so it may or may not be set correctly. The important thing is that the elevator should be in trail with the stabilizer in the takeoff trim setting. 

Skip

 

[jetdriven]

Excellent info,. Thank you

[larryb]

In my J best feel was also quite above the takeoff mark. But in my K best position is just below the takeoff mark.

[carusoam]

Great detailed response, Skip!

 

That much detail allows somebody with concrete chunks, where their jack screw grease used to be, to have a fighting chance of identifying how bad the concrete has become...  

Best regards,

-a-

[skykrawler]

When solo, My airplane will rotate with less force with the trim indicator up by the width of the bar.  But once it hits 85kts it needs down trim.  So I now I just leave it set to the take-off position and apply more force to rotate and only need to trim when I raise the flaps.

[EricJ]

With mine, 77 J, 24-0077, at the takeoff trim marker it measured 4 5/16".   I didn't run it to the limits, but I'll do that next time I'm in the hangar if I remember.   I'm not sure my takeoff indicator is in the right place, though.   It's comfortable for takeoff, but I run out of down trim in descents all the time, which I'm told is unusual, so I think something is not where it should be.

 

[carusoam]

Running out of down trim...

Could mean...

1) Something May be blocking the trim screw’s motion at the end... often aged solidified grease has been known to shorten the useable or effective length of the trim screw...

2) If the trim runs out of screw at the down end... it may have excess at the up end...

3) accidentally over trimming a Mooney in the up direction can lead to a stall... as the plane is pulling back on the yoke for you...

4) The WnB may be pretty far aft...

Probably some things you can look into yourself...

Definitely something your mechanic can check...

PP thoughts only, not a mechanic...

Best regards,

-a-

[EricJ]

10 hours ago, carusoam said:

Running out of down trim...

Could mean...

 

2) If the trim runs out of screw at the down end... it may have excess at the up end...

 

Without having looked into it in detail (it's not really broken), this is my suspicion based partly on tales and inputs from others here.   The stops are at the gearbox, and the indicator controlled from there as well, so I think something is just not rigged exactly right at the gearbox.   I've never come close to using the up trim limit.

[PT20J]

52 minutes ago, EricJ said:

Without having looked into it in detail (it's not really broken), this is my suspicion based partly on tales and inputs from others here.   The stops are at the gearbox, and the indicator controlled from there as well, so I think something is just not rigged exactly right at the gearbox.   I've never come close to using the up trim limit.

If someone disconnected the trim shaft for some reason and rotated the trim wheel before reconnecting, it can cause such a problem because the stops are at the forward gearbox, not the tail jackscrew. The trim indicator cable also attaches at the forward gearbox and can be set (by loosening a setscrew and sliding the cable) independently of the of the actual position of the empennage. 

Proper set up using the service manual procedure requires leveling the airplane and using travel boards. However, mine seems to be rigged properly and you can use my measurements above to check yours.

Skip

[EricJ]

5 hours ago, PT20J said:

Proper set up using the service manual procedure requires leveling the airplane and using travel boards. However, mine seems to be rigged properly and you can use my measurements above to check yours.

Skip

In my manual there are angles spec'ed relative to the thrust line.   I'm not going to bother with it until/unless something indicates that I really need to.

Meanwhile, I checked my gap measurements:   Full down trim 3 7/16",  Full up trim, 5 3/16",  Indicating takeoff, 4 5/16".

So my limits are the same as yours less 1/16", which isn't much difference and within reasonable measurement error.   My t/o setting is 3/16" less, though.  I doubt @N201MKTurbo's airplane is seriously out-of-whack, but he's indicating a quite a bit less, still.

So there seems to be some variance.  Can't say I'm shocked. 

[PT20J]

I suspect some variation in takeoff trim setting is a personal preference issue. If set above the top of the takeoff range (forward CG) it lifts off with very little back pressure. But, you will need to trim nose down after it becomes airborne (before retracting flaps). If set within the takeoff range, it takes more pressure to lift off, but it will be in trim until you raise the flaps. I prefer the latter because I don't want it pitching up after liftoff at low airspeed close to the ground if I get distracted. 

Skip

[PT20J]

23 minutes ago, EricJ said:

In my manual there are angles spec'ed relative to the thrust line.   I'm not going to bother with it until/unless something indicates that I really need to.

Meanwhile, I checked my gap measurements:   Full down trim 3 7/16",  Full up trim, 5 3/16",  Indicating takeoff, 4 5/16".

So my limits are the same as yours less 1/16", which isn't much difference and within reasonable measurement error.   My t/o setting is 3/16" less, though.  I doubt @N201MKTurbo's airplane is seriously out-of-whack, but he's indicating a quite a bit less, still.

So there seems to be some variance.  Can't say I'm shocked. 

Sounds like your measurements are pretty close to mine. When you run out of nose down trim, are you forward or aft CG and what's your airspeed? I've never run out of trim, but I've never looked at the indicator position in descent, so I don't really know how close to the limit it is. I'll try to remember to look next time I fly.

Skip

[EricJ]

19 minutes ago, PT20J said:

Sounds like your measurements are pretty close to mine. When you run out of nose down trim, are you forward or aft CG and what's your airspeed? I've never run out of trim, but I've never looked at the indicator position in descent, so I don't really know how close to the limit it is. I'll try to remember to look next time I fly.

Skip

It's actually about in the middle of the envelope, based on the original forty-two-year-old-and-n-times reworked WnB, so who knows where it is actually.    I plan to re-weigh it, but not until I finish with avionics mods and such.   I don't run out of down trim except in fast descents, about 160kias, which is significantly cranking it down, but there have been a number of times when I wanted it down further and there just wasn't any more.   Otherwise flight characteristics and such seem fine, so it's not been anything I've worried about, just an occasional annoyance.   Don Maxwell flew it to check rigging during my pre-buy and thought it was fine and "pretty fast", so I haven't worried about it.

[carusoam]

Something to consider when you get to it...

(1)

An E-descent is performed with the gear down... and speed back up to Vgear down... probably 160kias...

these are trimmed speeds..

 you will be busy problem solving while this is going on...

Set the ASI using trim, then get on with the problem solving... scanning instruments as you go...

 

(2) 

A fast descent is performed in smooth air around Vne...

Set the ASI using trim, then get on with snapping pictures of the ground speed monitor... also scanning instruments as you go...

 

You shouldn’t run out trim unless you are passing Vne...

So... it sounds like something may be getting in the way...?

Could be decades of not flying near Vne...   

 

PP thoughts only, not a CFI...

Best regards,

-a-