Elevator / Stab position in flight

[Austintatious]

So I have been looking back at my tail feathers in cruise and I have noticed that even with a rearward CG that my elevator is deflected UP a bit.  It would seem to me that this may be causing some extra drag.  With a movable stab, ideally they would be completely in line during cruise...  There is a trim strip along the trailing edge of the elevator.  I would like to adjust this trim strip and try to get the elevator to be in line with the stab during cruise.

 

So the question is... Which way do I bend it?   I THINK that I need to bend it UP.  This would effectively be trimming the elevator for more nose DOWN, for which I would end up trimming the stab more nose Up (leading edge down) to compensate... this should result in the two being more in line during cruise.  

Is my logic correct?

 

EDIT: Sorry, I intended to post this in general.

Edited November 26, 2019 by Austintatious

[PaulM]

My MSC says that is normal.  

The co-incidence of the elevator horns and the rest of the tail is equal only for a certain CG/speed combos.   The Tail is always flying with a down lift force.  I don't know if an aft or forward CG causes up or down horns.. I would have to find out experimentally,  

Check the amount of co-incidence with a forward CG and then with an aft CG. 

The way to think about it: (numbers made up)  if at 150Kts,  a perfectly even tail gives -200lbs of lift but the CG only needs -180lbs of lift for level flight then the tail at 150Kts must not be in the optimum position.  it needs to shed those 20lbs.   So you adjust the trim, and the tail and elevator are now not collinear.

I suspect that the elevator deflection is in the opposite direction, as trimming the tail is the main equilibrium  lift adjuster, and the elevator evens out in the opposite direction.   therefore,  forward CG, trim for more tail downforce, tail angles up, elevator angles down.. shows horns on top.  

The maintenance manual for my M20M lists that the ailerons and rudder are "tweak-able" for a low wing.. but that the elevator is fixed don't attempt to adjust it. 

___________________________________

 

27-94-00 - CONTROL SURFACE TRAILING EDGE

NOTE
Verify flight control rigging is correct before bending trailing edge.

1. Trimming. A special pair of wide nose vice grip pli- ers are to be used to bend the trailing edges as neces- sary (See Fig. 27-22).

RAILING EDGE TRIMMING FIGURE 27-22

A. Aileron. To correct for a left wing-heavy condi- tion, bend right aileron trailing edge down; to correct for a right wing-heavy condition, bend left aileron trailing edge down.

NOTE
The aileron trailing edge MUST NOT BE BENT UP under any circumstance. Any deformed rivets must be replaced after trimming aileron or rudder trailing edge.

B. Rudder. The rudder trailing edge may be trimmed right or left as required.

C. Elevator. A full span tab is built into each eleva- tor trailing edge and is fixed 7 degrees (+/-) 1⁄2 degree trailing edge down. No further tolerance is allowed.

______________________________

 

[PT20J]

Don't mess with the elevator. Short bodies trim with the elevator in trail. Mid bodies trim with the trailing edge deflected down slightly. Long bodies trim with the trailing edge deflected up slightly. This is normal. There are springs in the system that bias the elevator in addition to the incidence change of the stabilizer. The strip on the rear of the elevator increases area -- it's not a trim tab.

Trim drag is primarily the extra induced drag from the wing generating extra lift to offset the tail down force. Any slight misalignment of the elevator out of trail is negligible.

Skip

[carusoam]

Austin,

You are looking at the detail of how/why the trim system works...

Mess with it... the plane potentially stops flying...

There are people that study aerodynamics, and aircraft manufacturing...

There are people that get certificates in maintenance...

It is a bit unusual to take on the role of these other people, as a pilot looking out the window... 

There is a much longer thread of people’s observation on this topic...

You may want to find that thread...

Changing your weight and balance, and observe the effect on the tail feathers would be better... a normal pilot activity...

As far as rigging the controls go... there are maintenance procedures that cover that... And tools that go with that...

I still expect, you will find the trim system works really well for the range of weight and balance you are going to encounter while flying your plane in accordance with the POH...

Is your trim system working properly?

PP thoughts only...not a mechanic...

Best regards,

-a-

[Austintatious]

Gees guys, calm down....  I am not talking a major change here.   It is great to know that the tab should be at 7 degrees with a 1/2 degree allowance... It could be that when it was removed and re bent after the elevator left/right change during paint, that it was not precisely bent.   I will check it and see where it is and will stay within the prescribed limits.

[Austintatious]

20 hours ago, PT20J said:

Don't mess with the elevator. Short bodies trim with the elevator in trail. Mid bodies trim with the trailing edge deflected down slightly. Long bodies trim with the trailing edge deflected up slightly. This is normal. There are springs in the system that bias the elevator in addition to the incidence change of the stabilizer. The strip on the rear of the elevator increases area -- it's not a trim tab.

Trim drag is primarily the extra induced drag from the wing generating extra lift to offset the tail down force. Any slight misalignment of the elevator out of trail is negligible.

Skip

Are you saying that the strip at the TE is bent in the way you describe or that it FLIES in the way you describe?

[N201MKTurbo]

You will probably find that if you get it in trail the plane will go slower. You can find the minimum drag position by mis-trimming the airplane and using the elevator to level the plane. find the combination that produces the highest airspeed. It will probably be pretty close to your present setup.

[PT20J]

3 hours ago, Austintatious said:

Are you saying that the strip at the TE is bent in the way you describe or that it FLIES in the way you describe?

The latter. I've known for years that the J trims in cruise with the elevator trailing edge slightly down. I was curious about how the original design trimmed and in another thread C pilots confirmed that short bodies trim with the elevator in trail. Some long body pilots chimed in that their planes trim with the elevator tailing edge slightly up.

Let's do a thought experiment (easier than bending metal ). The airplane requires a certain tail down force when trimmed to counteract the airplane's inherent nose down pitching moment. This force is generated by a combination of the incidence of the stabilizer (which sets angle of attack) and the deflection of the elevator (which changes the effective stabilizer camber). The incidence is variable by means of the trim system. The elevator floats at a position where there is a balance of aerodynamic forces and the elevator bungee system. Now, suppose we bend the trailing edge up. This will increase the aerodynamic force tending to lower the trailing edge of the elevator and make it possible to get the elevator to trail the stabilizer. However, there will now be less tail down force (due to the decrease in effective camber) and the nose will pitch down. This can be compensated by changing the incidence of the stabilizer by adjusting the trim. So, what's wrong with that? Well, the trim system can only change the stabilizer incidence by about 6 degrees total. By making the above adjustment, you will remove some amount of nose up trim capability and you will likely run out of trim in the landing configuration.

An engineer at Mooney told me years ago that Lopresti noted the elevator misalignment and calculated that the drag penalty was inconsequential and not worth the cost to fix it (which would have required redesigning the empennage to change the fixed incidence of the stabilizer). Evidently, all the engineers that came after him have reached a similar conclusion.

Again, the trim drag is primarily the extra induced drag of the wing to carry the tail down force and to a lesser extent, the induced drag created by the tail in generating the tail down force. The parasitic drag from the slight elevator deflection is inconsequential by comparison at the speeds a Mooney flies.

Skip

[Austintatious]

7 hours ago, PT20J said:

The latter. I've known for years that the J trims in cruise with the elevator trailing edge slightly down. I was curious about how the original design trimmed and in another thread C pilots confirmed that short bodies trim with the elevator in trail. Some long body pilots chimed in that their planes trim with the elevator tailing edge slightly up.

Let's do a thought experiment (easier than bending metal ). The airplane requires a certain tail down force when trimmed to counteract the airplane's inherent nose down pitching moment. This force is generated by a combination of the incidence of the stabilizer (which sets angle of attack) and the deflection of the elevator (which changes the effective stabilizer camber). The incidence is variable by means of the trim system. The elevator floats at a position where there is a balance of aerodynamic forces and the elevator bungee system. Now, suppose we bend the trailing edge up. This will increase the aerodynamic force tending to lower the trailing edge of the elevator and make it possible to get the elevator to trail the stabilizer. However, there will now be less tail down force (due to the decrease in effective camber) and the nose will pitch down. This can be compensated by changing the incidence of the stabilizer by adjusting the trim. So, what's wrong with that? Well, the trim system can only change the stabilizer incidence by about 6 degrees total. By making the above adjustment, you will remove some amount of nose up trim capability and you will likely run out of trim in the landing configuration.

An engineer at Mooney told me years ago that Lopresti noted the elevator misalignment and calculated that the drag penalty was inconsequential and not worth the cost to fix it (which would have required redesigning the empennage to change the fixed incidence of the stabilizer). Evidently, all the engineers that came after him have reached a similar conclusion.

Again, the trim drag is primarily the extra induced drag of the wing to carry the tail down force and to a lesser extent, the induced drag created by the tail in generating the tail down force. The parasitic drag from the slight elevator deflection is inconsequential by comparison at the speeds a Mooney flies.

Skip

Yea, I understand all that quite well...  What stood out in your previous post was that you said Mid bodies fly with the TE of the elevator slightly down.  I am in a 20k ( Mid body no?) and have my TE slightly up (counterweights BELOW the stab LE even with rear CG) which is opposite of what you said (unless I am misunderstanding)

I do appreciate the concern about running out of trim.   That is definitely something to think about.  I don't typically need FULL up trim during landing, not that I should want to have to go full up trim for landing mind you.  I will pay very very close attention to where I have to put the trim to be fully configured and on speed the next time I fly.

[PT20J]

7 minutes ago, Austintatious said:

Yea, I understand all that quite well...  What stood out in your previous post was that you said Mid bodies fly with the TE of the elevator slightly down.  I am in a 20k ( Mid body no?) and have my TE slightly up (counterweights BELOW the stab LE even with rear CG) which is opposite of what you said (unless I am misunderstanding)

I do appreciate the concern about running out of trim.   That is definitely something to think about.  I don't typically need FULL up trim during landing, not that I should want to have to go full up trim for landing mind you.  I will pay very very close attention to where I have to put the trim to be fully configured and on speed the next time I fly.

I haven't flown a K for years and don't recall looking at the tail when I did, so I'm not actually sure where the elevator trims on the K. The K's are generally nose heavy and Mooney changed the trim assist system in the K from the bungies used up through the J to the variable down spring used in the long bodies. With the heavier engines and props in the L and on, they couldn't  balance it without adding moment arm to the tail by lengthening the fuselage, but it worked - barely - for the K with the F/J fuselage. So, I wouldn't be surprised if the K trimmed with the elevator slightly trailing edge up. Maybe @kortopates or @gsxrpilot can weigh in: You guys ever look behind you when you're flying?

Skip

[Austintatious]

10 minutes ago, PT20J said:

I haven't flown a K for years and don't recall looking at the tail when I did, so I'm not actually sure where the elevator trims on the K. The K's are generally nose heavy and Mooney changed the trim assist system in the K from the bungies used up through the J to the variable down spring used in the long bodies. With the heavier engines and props in the L and on, they couldn't  balance it without adding moment arm to the tail by lengthening the fuselage, but it worked - barely - for the K with the F/J fuselage. So, I wouldn't be surprised if the K trimmed with the elevator slightly trailing edge up. Maybe @kortopates or @gsxrpilot can weigh in: You guys ever look behind you when you're flying?

Skip

Gotchya... I am in a rocket after all... that being said, the CG envelope didnt change with the STC.  I am curious what others see in flight.