Mooney Trim System

[jetdriven]

Just take a look out the side window next flight. You can see the outside 1 foot of the horizontal stabilizer while flying.

Quote: takair

It should work.  The linkage between the elevator and control wheel shaft is essentially solid. The bungee just biases the position. If you set the trim to the same position you have in cruise and then look at where the mark is, you will know how much the airload moves it.  If you then line up the mark and look at the elevator you will know exactly where it is in flight.  The only thing it won't account for is slop in the system, which should me minimal. I do know that on my E model, the airload does not completely overcome teh spring force, which is significant. 

[201er]

Quote: jetdriven

[takair]

Good photos.  Were these see level max power?  This is turning interesting.  Like many Mooney design decisions, not sure we will ever really know.  My 64E model at max cruise (about 152KTAS) at altitude, has the elevator weights exactly the opposite when looking out the window.  My elevator is still biassed with UP elevator, where these photos show down elevator bias. My theory is that the bias was a bandaid.  I wonder if the early Mooneys just did not have enough jack travel for the stab to move to give enough up elevator.  Will have to try a flight at high power, down low, to see where my trim ends up.  It looks like by the time they got to the 201, things were getting better and soon after they were able to do away with the bias.  Perhaps teh longer fuselage helps.  Will ask some of the flight sciences guys at work to see if they have a theory.  Too bad the Mooney brothers aren't around to grill with some of this stuff.  I often think they locked somebody in a closet with a pile of parts until they came up with some intricate design.

[jetdriven]

The only thing is to move the GC back but the last photo had the max limit of cargo in the rear and the CG was barely in the middle.  I don't see how trimming the tail where the elevator was faired even, soemthing has to keep the plane from climbing.

Stab trim is more efficient than having a trim tab to load the elevator to deflect and cause pressure on the tail.  All commercial jets have stab trim just like a Mooney and don't use trim tabs.

Quote: 201er

[jetdriven]

The first two were the Galveston air race. The last was the Cleveland TX air race. I was not present for either, my wife flew these.  The Galveston photos are 1000' and all knobs forward at 160 KIAS and TAS.  The Cleveland race ends in a dive to the runway and full power, sea level density altitude. IIRC I think the KIAS was 200. Halfway up the yellow arc. With new igniton timing, profiled and balanced prop, and a new Donaldson air filter, we are shooting for something more in the 190 MPH TAS range. There is more to do as well, the gear doors are not rigged proper and the flaps ands ailerons are improperly rigged.

Quote: takair

Good photos.  Were these see level max power?  This is turning interesting.  Like many Mooney design decisions, not sure we will ever really know.  My 64E model at max cruise (about 152KTAS) at altitude, has the elevator weights exactly the opposite when looking out the window.  My elevator is still biassed with UP elevator, where these photos show down elevator bias. My theory is that the bias was a bandaid.  I wonder if the early Mooneys just did not have enough jack travel for the stab to move to give enough up elevator.  Will have to try a flight at high power, down low, to see where my trim ends up.  It looks like by the time they got to the 201, things were getting better and soon after they were able to do away with the bias.  Perhaps teh longer fuselage helps.  Will ask some of the flight sciences guys at work to see if they have a theory.  Too bad the Mooney brothers aren't around to grill with some of this stuff.  I often think they locked somebody in a closet with a pile of parts until they came up with some intricate design.

[Hank]

Quote: tomcullen

I propose an experiment.  For those of you who have in-flight pictures, make a survey and see if you can find any images where the elevators are not "streamlined" with the horizontal stabilizer. 

-dan

[Parker_Woodruff]

My M20J had the counterweights slightly on top of the surface. 

 

My M20K at about 70% power is slightly below. We'll see what it looks like when I'm running a higher power setting LOP. 

[jetdriven]

A wierd theory, but our 3rd partner though the tail was providing a net UP force because the elevator is down, creating a cambered horizontal tail.  But that cannot be.  So, perhaps, the stabilizer is at a negative incidence in cruise flight and the elevator flies 'down" to offset most this negative incidence.  Still a net down force at the tail.  ??   

[Bolter]

Quote: Hank

Note:  finding the elevator over your shoulder with the camera zoomed in isn't as easy as it sounds . . . and without zoom, you can't see the tip of the counterweight.

[bullerbravo]

Hi all, I’m new to Mooneys and to this forum. I have an elevator trim observation and thoughts to contribute to this thread. I have recently purchased my dream personal plane, a 2005 Bravo GX. As great as she is, she has one aerodynamic idiosyncracy:  in level cruise, over a wide range of power settings and weight loadings, she exhibits a distinct elevator up condition (see picture 1 below with elevator horns in an unfared position below the horizontal stab). Paint loss on the leading edge of the horn betrays the longstanding nature of this condition. Simultaneously, the STEC 55X autopilot establishes a strong nose down empennage trim condition with the net aerodynamic result for pitch being neutral. Turning off the autopilot confirms a neutral net pitch trim state.

This condition is undesirable for 3 reasons. (1) increased induced and parasite drag (2) capacity for horn icing leading to elevator lock - note paint loss (3) operating near the limits of nose-down empennage trim in case of out-of-range aft C of G condition, icing, unusual attitude recovery, etc.

I believe the extended elevator of the long bodies is the key to understanding this quirky condition. I gather that the long-bodies were provided extra elevator authority by extending the elevator with a ~1 inch chord full-span sheet aluminum tab along the trailing edge of the previous elevator design. Looking around at similar Mooneys, I see that most of these elevator extensions are deflected slightly downward compared to the chord of the parent elevator. This deflection is at least in part caused by the mounts for the static dissipators which remain attached to the “old” trailing edge and must run across the upper surface of the elevator extension.

This elevator extension design creates, in effect, an elevator tab down condition, leading aerodynamically to an elevator up condition that must be neutralized with opposite empennage nose down trim. As many of you know, elevator tabs are used as the prime elevator control surface in most large aircraft; their position at the trailing edge well behind the elevator hinge axis gives them tremendous leverage such that modest tab deflections give large opposite elevator deflections. This explains how a small error in the angle of incidence of the elevator extension in a long-body Mooney can have a relatively large effect on elevator position. The question is, what’s the best fix? Bending the elevator extension incrementally upward until neutral elevator position is established? Does anyone have experience with this? I know a bit about aerodynamics, but I’m no mechanic.

Chris Buller  Mooney Bravo  C-FDRX

 

 

 

[jackn]

Chris, you bing up an interesting question.. My 'K's elevators cruise orientation looks to be about the same as your Bravo. Surface up, horn down slightly. My trim is set just a little down from neutral. It make sense that you 1 1/2' longer body would require more trim down to counteract the elevator. Why the engineers at Mooney decided to set it up this way is a mystery, for the reasons you mentioned. I don't believe it was just an oversight though, so I wouldn't go bending any metal just yet. I'd like to find out why myself.

[FoxMike]

I have not been following this thread much but I have a recollection that Bob Kromer suggested that the angle of that elevator tab (7d) is critical for autopilot coupled approaches.  I found that tab on my elevator to be set to 3d on one side and 6d on the other.  That turned out to cause quite a handling problem.  I would suggest that you not mess with that tab.  It needs to be correctly set even though it may cause a little drag.  If a little hangar rash has caused it to be bent in places, best to get it straighten and properly set.

[Alan Fox]

The easy answer is that the mooney horizontal has a very shallow camber , it is almost flat , ergo at a level elevator it is basically a weathervane , not really a lifting surface (either positive , or negative)   that is why it is always in some sort of deflection in flight ,

[Awful_Charlie]

My Bravo flies the same - slight horn down, elevator up, but in fairness, my CofG is normally quite well forward too - sometime I mean to do a test with the CofG as far aft as I can practically get it.  Of course, if it was adjustable to make it more efficient, yet still within rigging tolerances, I'd be interested.

Given a fixed load/CofG, the complete empennage is still going to have to make so much downforce, and I would hazard a guess that the drag is more induced than parasitic, so suspect there's not a lot to gain.  Also note the MM section 27-94-00 item C - The tab should be at 7 degrees +/- 0.5, so there's not a lot of tolerance to play with it there. You might have some adjustment in the 'variable downspring' (MM section 27-31-00)

[RocketAviator]

More input on elevator....

[bd32322]

I dont have an answer about why the elevator hangs slightly in flight, but regarding the yoke/elevator movement with trim : I have noticed this especially after landing when I set the trim for takeoff. The trim goes from very nose up to very nose down and the yoke moves quite a few inches forward.

I wonder if there are two separate pivot points , one for the stabilator and one for the elevators. Maybe an A&P could chime in here. But if there are indeed separate pivot points, trimming and hence pivoting the stabilator will physically lengthen or shorten the distance between the yoke and the elevator pivot point as the entire assembly moves towards or away from the pilot / yoke. Since the elevator pivot and the yoke are connected by a rod, movement in the stabilator will cause movement in the yoke position.

However, movement in the yoke position is not the pressure we feel on the yoke when out of trim. That should be purely from flight loads or any bungees what not attached to the elevator.

[cliffy]

I posted the following on another thread about Mooney elevators. Maybe it will help here. 

 

Any Mooney with spring bungees on the elevator is rigged to a certain amount of UP elevator at a specified stabilizer setting. You can read what it is by model on the TCDS sheet. The actual angle observed, at rest, will vary with whatever stab (elevator trim setting) setting the airplane landed or parked with. 

Per Bill Wheat (when I asked him last summer), Al Mooney used bungees to help provide the correct amount of elevator force required by design specs while keeping the tail surfaces as small as possible to reduce drag. He said they allowed him to use surfaces 25% or more smaller than similar airplanes and still have the require control authority. 

Later models did away with the bungees. 

[cnoe]

FWIW I have a pic of my '78 J's tail showing results similar to those of Byron's. The pic was from about a year ago and shows the horn slightly up (elevator slightly down). I was in level cruise flying solo (145# pilot) with approximately 50 gallons of fuel aboard. There was about 35 pounds in the baggage compartment. Altitude roughly 7,500 MSL with airspeed likely around 150-155 Kts.

 

To the best of my knowledge I have no known rigging issues.

 

Chuck

[Hank]

Very often when I land, the trim wheel is very close to the takeoff position. I don't bother adjusting it until I'm at the hold short for runup.

Maybe Al planned it this way?

[Hank]

Chuck, somewhere here there's a whole thread about elevator position in cruise, with many similar photos. I posted my own in level cruise at 9000' in my C, with me, my flight bag and full fuel (52 gals).

[garytex]

My F shows the bob weight all the time. Which means that the elevator is generating lift. So if I load Aft CG does that mean that I need more downward elevator deflection and go slower? That doesn't seem to be the case because the amount of bob weight showing seems to be really consistent no matter what the loading is. I suspect that the whole tail moves to balance the CG loading and the elevator retains its relative position to the tail.

So the big question remains: is the plane faster with an AFT CG?

[Hank]

Gary: yes and yes.

[cnoe]

Hank,

 

I vaguely remember reading that post back when I was partner in an E. The pic I posted was taken on the return trip (Michigan to the Texas coast) from picking up my J about a year ago.

 

It seems to me like the horizontal stabilizer must be providing at least a little "downforce" or the elevator wouldn't be down a bit. Next time I've got the back seat and baggage compartment loaded I'll take another picture to note the difference. I'd expect the elevator to be more neutral then.

 

But if the rear stabilizer is "lifting" then the elevator should be even lower with an aft CG (which would cause more drag).

 

Does this sound right guys?

 

Chuck

[Hank]

The rear stabilizer is a thin airfoil, and it generates lift. It is upside down, so the lift it generates pulls the tail down and the nose up [i.e., it's making downforce all the time]. As you trim Nose Up, the angle of incidence on the horizontal stabilizer increases, so it makes more upside-down lift, more downforce.

 

I've only heard of it happening with ice, but the tail can stall, forcing the nose down whether you want it to or not. This is a danger of flying in icing conditions, and can bite hard on approach. Me no fly in ice!

[jetdriven]

It looks like a pretty symmetrical airfoil to me, I'm not sure which way it produces force but Id guess the total net force with the elevator included in this calculation is a net force in the down direction.