PT20J

Some excellent points. It’s always great to get info from someone with first hand knowledge! Several sources have described the spring used during demonstrations at 40 lbs - but who knows - they may all be quoting each other. Do you know if it is really that strong? Seem like a lot just to remove the dead zone. Good observation about speed increases over the years. The control surface hinge moments that are felt as stick forces vary with the square of the airspeed. Skip

I asked Corrine about this part a while back and she said that the machinist they used to make these for them had retired. The part was apparently expensive to make (hence the price) and they concluded it didn’t offer significant advantages over the refurbished parts which were less expensive. Skip

Here’s an interesting example to ponder. Jet fighters have light controls and high maneuverability (the controls are hydraulic and the feel is artificial, but that’s not really important here). The Blue Angels found that the F/A-18 wasn’t stable enough to permit the super precise control necessary to fly in such tight formations until they added a 40 lb spring. Skip

An excellent question. Control and stability go hand in hand. The pilot needs to be able to control the airplane. Stability is adverse to maneuverability. And, stability communicates itself to the pilot through control forces. In a sense, when you try to maneuver, the stability fights you snd you feel this in the controls. An aerobatic airplane will have low stability, high maneuverability and low stick forces. A good IFR plane will have higher stability, lower maneuverability and higher stick forces. On the other hand, sometimes you don’t want to maneuver. You just want to cruise along at a steady airspeed and altitude. In this case, the stability helps you. Consider flying along trimmed in cruise in turbulence. A measure of stability in this condition is called airspeed stability. It will generally be easier to control the airplane around the trim point if it is more stable in which case the force gradient (lbs per knot) will be higher than that of a less stable airplane. Another measure of stability in maneuvering flight is stick force per g. You want the controls to get heavy when you pull g’s because if the controls are too light, it is easy to over stress the airplane. Control forces really get into an area of aircraft design and evaluation known as handling qualities which is to an extent subjective. Skip

I may have not been clear in making my point. Let’s just consider longitudinal stability for the moment. It’s simpler because only one axis is involved whereas there is generally coupling between roll and yaw axes. For an airplane with reversible (mechanical, unboosted) controls, as you deflect the elevator with the stick what you feel is the aerodynamic force at the elevator pushing back. Let’s say you are trimmed in level flight. A stable airplane will return to its trimmed airspeed when disturbed. The degree a stability (unstable to neutral to positively stable) is sensed by the pilot through the control pressures. Now let’s say that the pilot wants to reduce airspeed. If it takes a push to slow down, the airplane is unstable. If there is no force required to change speed, it is neutrally stable. If it requires a pull it is positively stable. The amount of force is proportional to the stability. Airplanes are less stable, and the control forces get lighter, as the CG moves aft. Adding gadgets to the control system such as downsprings and bobweights is a standard way of increasing the stability as sensed by the pilot. The Seneca has downsprings which improve stability with aft CG (to increase CG range) but the penalty is very heavy pitch forces with forward CG which always made my landings interesting until I learned to only flare enough to stop the descent. Edit: Of course, lots of factors determine control forces, and I’m not intending to imply that stability is the only one. For instance, Mooney’s have a relatively small control wheel angle of rotation (equivalent to a short stick) fairly large chord ailerons that lack aerodynamic balance which contribute to somewhat heavy roll forces. At some point the trailing edges of the ailerons were bevelled and this would have reduced control forces without an appreciable change in stability. My point is simply that the airplane’s stability communicates itself to the pilot through the control forces the pilot feels. There’s a much more eloquent discussion of stability in Aerodynamics for Naval Aviators. Skip

Well, this is getting interesting. The drawing from the IPC shows the bolt installed head down in the retracted position. I looked at mine (1994 M20J) and it is installed in reverse with the head end up when retracted. It is secured with a castle nut and cotter pin. The pictures posted by @larrynimmo show the bolt installed head down (when retracted) but a self locking nut rather than a castle nut with cotter pin. I would think that bolt head up would be better but the most important thing would probably be to use a drilled bolt and a castle nut and cotter pin. See the attached chart for the length of an AN4-21 bolt (Your A&P probably has them -- it's standard shop hardware). an3_thru_an20.pdf Skip

The heaviness is the airplane’s stability feeding back to you through the controls. A Pitts has very light controls and a wicked roll rate. You wouldn’t want to fly it IMC. Stability (heavy controls) and agility (light controls) are opposite design constraints. Designers try to strike the best balance for the intended mission. Skip

Great thought! I re-read the original post and looked at the pictures again. The bolt (34 in the diagram in Clarence's post) is missing. It looks like if the nut came off, the bolt would be vertical and head down when retracted and could fall out. So if the bolt fell out when retracted, things might jam up on the subsequent extension. The missing bolt and nut would have fallen out when the gear doors opened. Think I'll check the torque on my bolt. Skip

I wonder what caused the stud to shear? Doesn’t seem like it should be heavily loaded; it’s just a pivot point in line with the centerline of the nose gear truss. Corrosion? Fatigue due to shimmy or some other vibration? Lack of lubrication? Has anyone else seen this failure? Skip

Ross @Shadrach has done a lot of studying and thoughtful experimenting and I think his posts about how he operates a NA engine make a lot of sense and are based on a thorough understanding of the processes involved. Maybe he’d be willing to summarize here. Skip

My M20J IPC calls out 2 different studs depending on serial number (I have no way of knowing what the difference is but a MSC might be able to tell from the Parts Portal) and the K is probably similar. If it were me I would measure the pieces and have one fabricated since the factory is closed and it doesn’t seem like a part that a MSC would stock. It’s welded to a structural tube, and the geometry is going to be critical, so I’d want to make sure that I got someone that knows what they’re doing to replace it. I’d consult with Don Maxwell or Tom Rouch - they’ve seen just about everything. Skip

Agree with Byron. My Shadin generally shows about 17.2 gph on takeoff roll at sea level. I extrapolated data from the Lycoming test cell data on my factory rebuilt IO-360-A3B6 and it was about 18. But, that’s with optimized intake and exhaust plumbing. I suspect anything between 17 and 18 gph is good installed on a Mooney. The fuel servo measures the volume of airflow. What you really want is the mass of airflow because it’s the mass ratio of fuel to air that sets the mixture. But for an incompressible fluid, the mass flow is proportional to the volumetric flow adjusted for air density, so it works out But, as you climb, the air density decreases which changes the relation between air mass and volume and that’s why you have to lean as you climb. Skip

It sounds like a few things might be going on. The fuel pump high pressure problem happened a few years back. Lycoming had solved it by the time I got my A3B6 in October 2018. Anyway, it appears that issue is corrected by replacing the fuel pump. Fuel flow transducers can get flakey after 20 years or so. Was it replaced at the time of the engine replacement? I would check the actual fuel burned from the tanks with the Shadin fuel used to see if it is reading correctly. It sounds like it is reading zero during priming when in fact fuel is flowing and flooding the engine. At some point, Lycoming switched from Precision Airmotive RSA fuel injection systems to Avstar (which is a copy of the RSA). I have the Avstar on my factory rebuilt and it has worked fine. But, it sounds like the idle cutoff at a minimum is a problem. This should have been a warranty issue with Lycoming, but it looks like it is out of warranty now. I'd find a good shop (maybe not the last one you used) and have the system thoroughly checked out on a flow bench. Good luck, Skip

I used to fly Cherokees a lot. The trim always seemed to take a bit of fiddling to get just right. There just wasn't a lot of control force feedback near the trim point. Is that a characteristic of stabilators? Skip

Clarence @M20Doc, Since the Comanche layout, size and wing are similar to the M20, but the control surfaces are different, I'd be interested in your comparisons of handling and especially control forces between the two airplanes. Skip

Interference drag?

Gee, wouldn't it be fun to be a test pilot? It's a PA30. Not sure if a PA24 would do the same thing and the test conditions aren't stated; maybe they were way over Vne or something. Stabilators must be mass balanced just like any control surface to give margin against flutter. There is a big balance weight on a tube out ahead of the stabilator inside the fuselage on a Cherokee. I assume that the Comanche had a similar balance. Maybe @M20Docknows about this. One thing I always liked about the rectangular stabilator on a Cherokee is that it was just the right height for a nice picnic table if you brought along a couple of folding chairs. Just rotate it level and throw a table cloth over it Skip

This is interesting. I ran down the specs on the PA24-250 at https://www.skytamer.com/Piper_PA-24-250.html. Since the dimensions of the M20J and the PA24-250 are almost identical except for the tail feathers, it is interesting to compare. The Mooney horizontal tail is 2 ft2 larger than the Comanche (Mooney 34.5 ft2, Comanche 32.5 ft2) The Mooney vertical stabilizer + rudder is slightly larger than the Comanche (Mooney 14.15 ft2, Comanche 13.4 ft2) So, the Mooney's empennage design (swept forward surfaces and trimmable stabilizer with trim assist bungees) didn't end up smaller (less drag) than the Comanche's empennage (stabilator and swept-back fin). Skip

According to some, the Piper Comanche was copied from the M20. They certainly have almost identical wings and planforms, so it is interesting to compare them. One notable difference is the tail. The Comanche has a stabilator. In John Thorpe’s patent, he claims advantages for the stabilator. So, I’m wondering what are the pros and cons of the stabilator versus the Mooney design? Skip US2563757(1).pdf

Sorry, no. I looked in the IPC, but it’s not clear because there are retrofit kits for the earlier models. Skip

Oh, I don't think we'll ever run out of work for test pilots But my purpose in using tail volume is simply to compare the Mooney stabilizer sizing with other airplanes to test the hypothesis that the horizontal tail is smaller for less drag. So far, I have found it to be smaller than a C-182 (0.5 compared to 0.7). The Mooney stabilizer also appears smaller than average as shown in this excerpt from DeRaymer, Aircraft Design: A Conceptual Approach. So, I think there may be something to it. Skip

Ron, I was out by the hangar today, so I made some crude measurements. It's definitely a symmetrical airfoil, thinner at the tips and thicker at the root. I made the following measurements at the inboard rib rivet line: Chord: 44" Location of max thickness: 14" aft of LE Max thickness: 5" So, that would put the max thickness at 11.4% located at 32% of chord.. Given the crudeness of my measurements, that matches a NACA 0012 pretty well http://airfoiltools.com/airfoil/details?airfoil=n0012-il And, now when I look at it, it does look thick Skip

OK, so now I am not so sure about where the elevators rest when trimmed on all models But, it doesn't really matter -- the airplanes fly just fine and we decided that the drag is not significant Now, an interesting question is, "Why are the bungees there?" First, they are not an Al Mooney invention. The earlier Piper Supercub has a trimmable stabilizer with up and down bungees that are variable with trim position similar to the Mooney design (but much less elegantly implemented!). There are probably other examples. In another thread I recall a comment attributed to Bill Wheat that Al Mooney had said that the design allowed making the stabilizer (20%?) smaller which reduced drag. I've been thinking about that. The purpose of the tail is stability and control. Looking only at stability for the time being, the larger the tail the more stable the airplane is (and the wider its CG range). If the elevator floats (like say a C-172) then the tail loses some of its effectiveness and the stability is reduced. If the bungees acted as centering springs, they would reduce elevator float improving stability. This could allow for a smaller stabilizer. The standard method for comparing the stabilizer size between airplanes is a dimensionless coefficient called a "volume coefficient." The beauty of this abstraction is that it normalizes physical dimensions so that airplanes of different sizes can be compared. It is simply calculated: Horizontal tail volume coeff. = ((distance from CG to stabilizer 1/4 chord) x (stabilizer area))/((wing mean aerodynamic chord)x(wing area)) From the Mooney M20J service manual and POH: Horizontal tail volume coeff. = (155 x 21.5)/(59.18 x 174.786) = 0.322 Different sources have different suggestions about what this coefficient should be for a Mooney-type airplane, but the range seems to be about 0.3 to 0.7 so this is definitely on the low end. Edit: I looked at this again and I think there is an error here. From some measurements I took, it appears that the stabilizer area listed in the manual does not include the elevator area. If the elevator area (13.0 ft^2) is added to the stabilizer area (21.5 ft^2) the above calculation comes out to a coefficient of 0.517 which seems more reasonable. I found a reference that calculates a C-182 horizontal tail volume coefficient at 0.7, so this is still low. Ron @Blue on Top, any insights? Skip

The horizontal stabilizer is symmetrical -- I think it might be a NACA 0012. My thought about changing the tail incidence being necessary to make the elevator trail follows this logic: The amount of tail down force is fixed by the airplane pitching moment. If you were to mechanically lock the elevator in trail with the stabilizer, there would be an incidence that would provide the angle of attack for the stabilizer/elevator to generate the appropriate tail down force. The center point of the bungees would also have to be adjusted to make the hinge moments zero when the mechanical elevator lock was removed. The reason for changing the fixed incidence of the stabilizer rather than just adjusting trim is to preserve the trim range in both directions. If the original design trailed, then something changed when the fuselage was lengthened that causes it no longer be in trail. I didn't consider the difference in drag from camber change vs AoA change. But, on the J, the trailing edge is down making for positive camber which means that the camber is decreasing the tail down force generated by negative AoA. BTW, why are symmetrical airfoils chosen for horizontal stabilizers? Is it aerodynamics or structural (like a helicopter rotor blade)? Skip

Yeah, what I'm worried about is that if the effect is only a couple of knots, would I be able to hold altitude precisely enough to observe the effect? The bungees increase stick forces when out of trim so there's a bit of a pull to maintain.