PT20J

Th J has the fuselage cavities on both sides - the right for exhaust and the left for drain tubes. So yes, there is some airflow with the cowl flaps closed. The later Js just have more out the left side. I don’t know why this was done. Ron suggested drag might be a little less. Bob Kromer has suggested that the plane flies a couple of knots faster with the cowl flaps open a couple of inches. I’ve tried that in my ‘94 J with the the later style cowl flaps and didn’t notice speed difference but the CHTs were lower. Skip

The placement is important, also. Lopresti's designs placed the inlets as far outboard as possible to capture "ram" air from the prop. The blade shanks nearer the hub don't produce much thrust. On the early M20J, the left cowl flap was flat and closed off most of the airflow on that side when closed. Later M20Js have a rounded cowl flap similar to the right one that lets some air out even when closed. The Reno race P-51s put out more heat than the radiator can handle and they solve the problem by spraying water on the radiator to cool it. You can often see the vapor trail behind the airplanes. Another Kerchenfaut innovation was to realize that it wasn't the water that cooled but the evaporation of the water. He designed a system using fine spray nozzles in the inlet duct of Strega well ahead of the radiator so that the water would evaporate and cool the air before it got to the radiator. The result was more efficient cooling and a reduction in weight from carrying much less water. By the way, the Meridith effect is real, but it doesn't turn the cooling system into a little jet engine producing net thrust like some people think. But enough thrust is produced to reduce the cooling drag from 6%-10% of the total drag down to about 3% according according to David Lednicer's calculations. (https://arc.aiaa.org/doi/pdf/10.2514/6.1991-3288) Skip

Lee's got a good point. If they are getting brittle, it might be best to bite the bullet and beef them up before they get worse. The panels are made from thin ABS plastic and it gets even thinner at the radius' when vacuum formed. A little light weight fiberglass cloth and hardware store ABS cement (or Plane Plastics sells ABS chips in a can that you can dissolve in MEK to make cement) can make the panels better than new. SEM Color Coat comes in a number of whites and works great and is easy to apply. You'll want to do the whole interior to get a consistent color, but as the original ABS tends to yellow over the years, it's amazing how a fresh coat of SEM improves the look. I did this on my '78 J and am planning to do it on my '94 J soon. Skip

Well, I'm not 100% sure either. I think I know, but I've been wrong before. Where's a CFD guy when you need one?

This is why physics has two disciplines: theoretical and experimental. The theorists have to wait for the experiments that prove (or disprove) their theories. The old saw that something works in theory but not in practice is not correct. If it doesn't work in practice, the theory is wrong

Maybe. But, since air is incompressible at freeway speeds, after the pressure stabilizes wouldn't the air at the opening be primarily stagnant to first order approximation?

Details at https://cafe.foundation/v2/pdf_cafe_reports/localflow2.pdf

Here's an interesting experiment. Take your favorite brand of canned soup and remove one end with a can opener. After a tasty lunch, take the can out to your car and drive down the freeway. Roll down the window and hold the can open end forward and horizontal out in the airflow and note the force (drag) you feel. Now, turn the can around so the closed end is forward. Is the force different? Skip

I'm still curious about the comparison between the Mooney and the Comanche (especially since lore has it that the Comanche layout was copied from the Mooney). The wings of each have nearly identical airfoils, span and planform. The flaps on the Mooney are 123" long and the ailerons are 63" (186" total). The Comanche flaps are 111" and the ailerons are 75" (186" total). So the Comanche has a foot less flap span on each side, but similar stall speed reduction as the Mooney (POH numbers indicate it might even be 1 kt. better, but that's probably splitting hairs given the uncertainty of airspeed calibration at high angles of attack). The extra foot gets added to the Comanche ailerons and this allows the ailerons to have the required area with the same chord as the flaps. I've never had the pleasure of flying a Comanche, so I don't know if he roll control is crisper and the forces lighter than the Mooney or not, but I'm sure @M20Doc would be happy to opine on this Ron, I don't have a good feeling for the effect of flap chord. A lot of the old NACA wind tunnel tests just compare different types of flaps, all with the same (frequently 25%) chord. Any insights on the effects of chord? Clearly Cessna favors fat ones and Mooney skinny ones. Skip

Hey Charles, I think we really agree on more than we disagree, and I learned something about race cars (of which I know very little with the exception of what I remember from a Skip Barber class years ago where I discovered that there's a lot of physics involved in going fast around a track). Years ago, I had an interesting conversation with Bill Kerchenfaut who was then crew chief on the P-51 race plane Strega. Kerch designed most of the aerodynamic improvements on Strega. The stock P-51 belly scoop is not flush with the belly which creates a dead air space increasing drag. Bill tracked down an engineer that had worked at North American on the P-51 and it turns out that the original design for a flush scoop took in disturbed air and created a duct rumble. They couldn't figure out how to fix it so they lowered the scoop below the boundary layer. Bill figured out how to get it flush (he was a little coy about how he did it, but I think he got a CFD guy to model it). Cheers, Skip

I agree this is not a particularly good design. The bellcrank at the outboard end causes the push-pull tube to have a fore-aft motion which causes the tubes to rub against the guide blocks and increases friction. On some planes (including my J) you can hear it from outside the airplane if move the aileron through the extremes of travel. I recall hearing from Roger Hoh that around 1990 Mooney thought to enter the JPATS competition and built a prototype for testing. At high speeds, the ailerons floated up and flexed the tubes enough to bind. The stick would stay wherever it was displaced. I’ve often wondered if something was beefed up in the big engine Mooneys (that fly faster than the J) to solve this. Skip

I would expect mis-rigged gear doors to cause a yaw rather than a roll and even at that nothing of the magnitude you describe. The aileron control tubes run close to the gear bellcrank. Should be easy to figure it out if you put it on jacks and remove the belly and observe the gear motion.

Cool. I didn’t know that! I just looked up the specs for my iPad mini 5 and it supports Assisted GPS (evidently this uses the cellular network to provide an initial position for faster acquisition) GLONASS, Galileo, and QZSS. Skip

The OP's M20J tip/lens was a factory installation on later Js. Your C tip is a retrofit -- different part.

One thing that helps me understand airplanes is to look at the features of various designs and ponder why the designer made certain choices. The Mooney flaps are a bit of a puzzlement to me. The J has a clean stall speed of 63 KCAS which is above the CAR 3 max of 61, so clearly a high lift device is called for. The choice of a slotted flap seems reasonable as it gives the best increase in CL with a simple hinge mechanism. The thing I don't understand is the span. Flaps and ailerons compete for wingspan. By allocating so much to the flaps, the ailerons had to be made short and of greater chord (to get the area required for roll authority) which increases the aileron hinge moments. Coupled with the short throw of the Mooney control wheel (stick forces are governed by hinge moment and stick lever arm), the aileron design makes the stick forces high in roll -- so much so that the aileron trailing edges were eventually beveled to improve the situation (I've heard claim this was done in order to reduce forces to accommodate the addition of the PC system or maybe an autopilot). I wouldn't call the M20 roll rate sprightly, but adding more aileron chord to increase the area would have made the stick force worse. So, it seems that the choice of flap span came at a cost of roll rate and stick force. The Mooney flaps do more effectively reduce stall speed than flaps on many other airplanes. From POH data, here are the Vs1/Vso speeds (KCAS) for various airplanes at gross weight: M20J 63/56 PA24-250 62/54 A-36 62/58 C-182T 49/54 C-172S 48/53 PA28-181 59/53 The Bonanza and Cherokee flaps are not terribly effective at reducing stall speed. The Cessna flaps with shorter span but greater chord and some rearward travel (although apparently not enough that Cessna claimed them to be Fowlers) are better, but still less effective than the Mooney flaps. So, apparently span is more important in flap design than chord. The Comanche flaps appear similar to the Mooney flaps, though shorter, and provide the same or perhaps slightly better stall speed reduction while leaving more room for ailerons with (presumably) lower roll axis stick forces. Ron @Blue on Top any thoughts on this? Did Al make the flaps too long, or am I missing something? Skip

OK, so now I have to ask: who makes the oil?

You know, I’m comparing a 25 year old original lens with the new LASAR lens. Who knows what Mooney is currently shipping??? Next time I see a newer plane on the ramp, I’m going to look at the wingtips

Maybe, Paul, but my LASAR lens was noticeably lesser quality than the Mooney lens as I noted. Also, if the drawing in the IPC is accurate, the Mooney lens is already trimmed. But, the LASAR lens was good enough for me and in stock so I didn’t even inquire about the cost of the Mooney part because my mother taught me never to ask a question if you can’t stand to hear the answer Skip

This would be a fun experiment if you had the interior out. Measure TAS by 3 vector GPS test. Remove all the antennas and tape over the holes and see if it makes a speed difference. Then add them back one at a time and record what each one costs. Skip

Did you install one of these? I’m curious how it compares to the LASAR part. Does it require trimming? Also, I found the radius at the extreme outboard point where the lens meets up with the wingtip to be off just slightly on the left side LASAR part and it also isn’t as distortion free as the original. How’s the fit and finish of Great Lakes part. The LASAR part is fine unless you are a perfectionist, but maybe the Great Lakes part is better for the extra cost. Skip

I’m curious what experience others have with the Aspen synthesized AOA indicator. Does it seem accurate? Do you use it? I have a EFD 1000 (PFD only) with AOA installed by the previous owner. Mine seems to be out of calibration as I’m deep into the crosshatch at 65 KIAS, 2400 lb., full flaps in a M20J which seems to provide a reasonable compromise between too much float and enough reserve for a good flare. Also the display is so small that I find it hard to read at a glance. Skip

@donkaye Just type the @ followed by a the first few letters of the username and a drop down list will appear with the closest matches to select from. What approach speeds do you teach for the J? Skip

LASAR has a PMA one that is much cheaper than the Mooney part. You’ll have to trim it to size and drill the mounting holes. There’s a thread somewhere on MS with recommendations on various ways to do that. I used a Dremel cutoff wheel and step drill. Sand the edges smooth after trimming to avoid any stress risers - acrylic loves to crack. Skip

If you really want to get into the details of airplane cooling design, check out resources at EAA. There’s lots of good information there. Homebuilders have two great advantages: engineering time is “free”, and they can optimize a design without worrying about production cost.

The Yak-52 has that setup. Not sure it makes much difference where you put the throttle to control the airflow. Skip