Mooney tail aerodynamics - not backwards

[carusoam]

Things I don’t recall ever doing in flight...

1) Pushing the yoke all the way in...

2) Trimming the elevator all the way down...

 

Seems like the control range is skewed to limit going too far in one direction...

 

I haven’t really explored being far back in the balance aspect of the WnB curve...

That might change how the control range really works...?

pushing the yoke forwards to avoid an engine out stall on departure... that might get a full push to get light in the seat...  

PP thoughts only, not an aero guy...
 

Best regards,

-a-

[Hank]

15 hours ago, ArtVandelay said:

Didn’t they move the battery at that time? Less nose heavy, better balanced.

My 1970 C has the long rudder, and my battery is on the firewall . . .

[hammdo]

I’ve not seen the long rudder yet, got a pic Hank?

-Don

[carusoam]

2 hours ago, hammdo said:

I’ve not seen the long rudder yet, got a pic Hank?

-Don

@Hank Hank would want you to have this pic...

All nicely straightened and from a similar thread....

It is a beautiful example of a modern M20C...

The tail cone has been included as the rudder extension...

-a-

[Hank]

Sorry, @hammdo, I was sleeping. But Anthony found my picture from a rudder thread a while back, taken for exactly this reason.

[PT20J]

3 minutes ago, Hank said:

Sorry, @hammdo, I was sleeping. But Anthony found my picture from a rudder thread a while back, taken for exactly this reason.

So, did the rudder an elevator changes happen coincidentally?

[carusoam]

How many people are familiar with the elevator range change?

-a-

[Blue on Top]

On 4/3/2021 at 5:07 PM, cliffy said:

 

I'm only looking for a definitive answer as to why Mooney found it necessary to change the angles from 69 on and not make them retro active to the other short bodied that came before.  

Also I wonder what the affect in handling was when the change occurred or was it not felt at all? Could it have had some affect on cruise speed with the lower bungee force with its lower angle set (trim drag?)?

What would happen if we reset an older Mooney to the new settings? How would it fly? No different than the more recent models? 

There has to be a reason why it was done. AND its somewhere in the in flight handling!         I'll bet $50 bucks. :-)

@cliffySome things will never be answered as much time (and many people) have passed.  

Here's a few points to ponder, but I have no definitive answers.

1. The only reason that I know to increase the down elevator throw of an airplane is spin recovery.

2. Flight characteristics will not change if the flight control system gearing did not change.  It looks like travel was increased by changing the length of the push-pull rods (which may explain the trim change(s)

3. Deflection changes in '69 are noted in the S&MMs for the series (attached).

4. I also noted a Service Instruction "Mooney S. I. M20-44" (also attached) which appears to be for both mis-installed push-pull tubes in the tail section ... and to make clearance for larger travels.

5.  Here's an educated guess as to "why?"  There was probably a spin accident that occurred, but the exact CG and loading were not known.  The FAA probably asked Mooney to run some spin testing at aft CG.  I am not sure if Mooney did spin testing for the original M20E since the airplane, weights and CG envelopes were the same as the M20C.  Something was probably found (slow recovery ... 1-turn spin has to recover in 1 additional turn), and Mooney and the FAA agreed to the elevator travel change.  The FAA probably didn't have enough evidence to bring out an AD, and Mooney probably didn't want to put something out that said their airplanes could have an issue.

Blue on Top, Ron

S&MM Elevator.pdf SIM20-44A.pdf

[cliffy]

32 minutes ago, Blue on Top said:

@cliffySome things will never be answered as much time (and many people) have passed.  

Here's a few points to ponder, but I have no definitive answers.

1. The only reason that I know to increase the down elevator throw of an airplane is spin recovery.

2. Flight characteristics will not change if the flight control system gearing did not change.  It looks like travel was increased by changing the length of the push-pull rods (which may explain the trim change(s)

3. Deflection changes in '69 are noted in the S&MMs for the series (attached).

4. I also noted a Service Instruction "Mooney S. I. M20-44" (also attached) which appears to be for both mis-installed push-pull tubes in the tail section ... and to make clearance for larger travels.

5.  Here's an educated guess as to "why?"  There was probably a spin accident that occurred, but the exact CG and loading were not known.  The FAA probably asked Mooney to run some spin testing at aft CG.  I am not sure if Mooney did spin testing for the original M20E since the airplane, weights and CG envelopes were the same as the M20C.  Something was probably found (slow recovery ... 1-turn spin has to recover in 1 additional turn), and Mooney and the FAA agreed to the elevator travel change.  The FAA probably didn't have enough evidence to bring out an AD, and Mooney probably didn't want to put something out that said their airplanes could have an issue.

Blue on Top, Ron

S&MM Elevator.pdf 12.77 MB · 0 downloads SIM20-44A.pdf 1.26 MB · 0 downloads

I'm in full agreement with your postulation. More down = better pitch out of a spin  Again memory fuzzy that far back BUT now that your mention it - some of the fuzz is falling away. It didn't make much news even back then and I think it was portrayed as "it just barely meets spec" so a change was made.

Bill Wheat also told he got caught in a 5 turn spin from which he almost didn't get out of and said he'd never do that again but I don't know what year airplane it was in. 

I'm still left to wonder about the change to the bungee spring angle and what that meant to the trim drag as Mooney put that on to lessen trim drag IIRC. What affect would increasing the down elevator angle have on bungee setting? I can't quite visualize that one. 

My areo engineering is self taught by reading dry texts but mostly by designing model aircraft and playing with tail moments, tail feather area percentages, some airfoil work, decalage angles (geometric rather than aerodynamic), nose moments, CG settings etc. Rather simple stuff that I could "visualize" the air flow and forces at lower speeds. Some of the "higher math" aero stuff is out of my venue :-) The mechanics I can easily follow.

[PT20J]

I cannot help with spins, but I can shed some light on the trim bungees. For every flight condition, the airplane (excluding the tail) develops a pitch down moment. This moment varies with flight condition which is why the trim is adjustable. To balance the moment, the tail creates a down force and the amount of force is adjusted by the trim. The stabilizer only has about 6 deg of incidence adjustment via the trim system, and the gearing is such that it takes many revolutions of the trim wheel to move it from full up to full down. To be effective, it needs some help.

When stationary on the ground and holding the yoke in a fixed position, you will notice that rotating the trim wheel nose down increases the nose down force on the yoke and rotating the trim wheel nose up increases the nose up force on the yoke. Therefore, the bungees act to add an extra increment of force into the control system so that trimming does not require the stabilizer incidence to change enough to null out all the force on the elevator aerodynamically. The bungees also act as centering springs which increase the airplane's airspeed stability.

A lot has been made of whether the elevator trims in trail with the stabilizer and if not, is this causing extra drag. Generally, short bodies trail, mid-bodies (except the K) trim elevator trailing edge down and the K and all subsequent models (which have bob weights and variable down springs rather than bungees) trim with the elevator slightly trailing edge up. All this "misalignment" does is increase the camber of the stabilizer. The drag increase should be negligible because the air is flowing smoothly over the tail. Most "trim drag" is induced drag: some from the tail and some from the wing which must generate extra lift to carry the tail down force. One "proof" that the drag is not significant is that when Roy Lopresti created the J, he was looking for low hanging fruit in the drag reduction area. Almost certainly if the tail were creating enough drag to be worth fixing, he would have done so.

Skip

[cliffy]

Agree and understand the above with respect to how the Mooney tail and bungees work.  Including the elevator counter weights being on different sides of the stabilizer (in cruise) on later models.  Ingenious. 

My question deals with the change they made when they changed the throws AND then changed the amount of up elevator on the bungees at the same time  (at the same stabilizer depression of 3 1/2 nose down)They cut 9 degrees out of the up mode. 

I'm wondering what they "found" that let them lessen the angle and still have enough authority. I'm wondering what changed aerodynamically to allow them to do that just by changing the throws. Down angle shouldn't have affected nose up requirement at least in my thinking. That force requirement should have remained the same???????

I guess a bigger question would be What governs how much bungee UP a certain model needs? As there are different settings in the TCDS. 

If the pitching moment of the airframe combination needs 19 degrees how can just changing the DOWN throw change that pitching moment?  

[PT20J]

The stabilizer incidence was changed to give more nose up trim range coincident with the elevator travel change. Changing the incidence would affect the trim bungees since they are connected. 

Mooney made changes to various control deflections and trim bungees over the years. For instance, M20Js below SN 24-1038 have a max up aileron deflection of 17 deg. whereas later aircraft have a maximum of 14.5 deg. There’s probobably no one left at Mooney who knows why these various changes were made, and as you discovered, even if they do know, they may not tell you.

Skip

 

[Blue on Top]

Bungees will only change (out of trim) forces and help stability ... stick free and stick fixed become closer to the same.  Control of the aircraft will not change with bungees ... only the control surface stops will change the performance of the airplane.  In addition, when the elevator down travel was increased, no new parts were made (some clearance modifications were made), which means that the gearing wasn't changed (majorly.  minor gearing changes will happen due to a different (or in this case larger) portion of the sector is being used.  The differential in the aileron system uses this type of geometry to make the up-going aileron travel more than the down-going aileron.