PT20J

For comparison, my M20J owner-assist annual was 2 days and $1275. (I don't leave open squawks for annual so this was just the inspection cost). I flew it 140 hours for a total expense of $20,916 or $14,940 per 100 hours. Skip

For anyone interested in the relationship between stick forces and stability, here’s a good article written by a professional test pilot. Enjoy. https://www.eaa.org/eaa/aircraft-building/building-your-aircraft/next-steps-after-your-airplane-is-built/testing-articles/static-longitudinal-stability-steady-as-she-goes Skip

Nice! How did you do the smooth black border? Skip

I'm no a lawyer, but I can't see any legal issue with reverse engineering unless you are infringing on a patent. Years ago Chips and Technology built their first PC graphics chip by photomicrographing a Motorola chip layer by layer and then tracing out the schematic! My comment about liability was meant to refer to product liability if one were to sell the drawings and a part made from them subsequently failed. Probably not an issue -- homebuilders have been selling plans for entire airplanes forever. Skip

Interesting idea. It's not clear who presently owns the intellectual property for the gears. It might be necessary to get some drawings made from a set. The material notes could be added after the destructive testing. That would, of course, add to the cost. It would be up to an A&P to agree that the reverse engineering was good enough to sign off the installation of owner produced parts from the drawings, but I would think that a reasonable person would. Without a PMA you couldn't legally manufacture and sell the parts. Not sure about selling the drawings. Also not sure about liability issues if you sold the drawings.

Those are bevel gears. Doesn’t the Dukes actuator use worm gears? sim20-112_Rev_A.pdf

I think I’m seeing some confusion caused by the term “heavy” with reference to control feel. Clearly a large heavy airplane is going to have “heavier”control feel than a small light airplane all things being equal. Where stability comes into play is in the slope of the curve - stick force per g in maneuvering flight or stick force per knot in one-g flight. In other words, it’s not the absolute force, but the rate of change in force when moving off the trim point that is a measure of the airplane’s desire to return to its trim condition. Skip

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