Mooney Aerodynamics

[0TreeLemur]

1 hour ago, Blue on Top said:

I totally agree with all, except that 12(c) is not producing lift (may be a definition thing).  Even above stall AOA, wings (even symmetrical airfoils) produce lift (force perpendicular to the relative wind) until near an AOA of 180 degrees.  An airplane with a wing in a similar condition to 12(c) will produce ~0.80 G (Nz-stab ~= 0.80).  Great post! 

Ron, I think your point is valid, but strictly on definition of lift>0.  Is it useful lift to avoid high speed impact with the ground?  No, except in perhaps an airshow context with an immanent recovery or in the context of "gee, we're only going to hit the the ground at 0.8 of the clean terminal fall speed" .    Put a big enough engine and fast enough control system computer on it, you could make an aircraft that would fly in condition (c).   It wouldn't be fast or fun, two things I think should be priorities in flight, at least for me. 

[Blue on Top]

@0TreeLemur  The main point I wanted to make is that lift doesn't go to zero past the stall angle of attack.

I would also say that the current military fighters (F-16, F-18, etc.) use vortex lift.  In those cases, I would definitely call the wing "stalled" by the classical definition.  It is producing lift but not enough to support the airplane at 1G.  In addition, the small angle theory used to develop most of the aero equations goes out the window (and with thrust producing "lift").  But you know this.  Merry Christmas and Happy Holidays. 

[Blue on Top]

2 hours ago, PT20J said:

Ron @Blue on Top, this is not a Mooney question, but perhaps you can explain how split flaps (DC-3, B-17, C-310) do so well at increasing lift coefficient (I believe they are somewhere between a plain flap and a single-slotted flap). The whole layout seems counterintuitive. And, doesn’t it violate the Kutta condition of circulation theory?

Skip

Skip @PT20J, Ironically, it is a Mooney question; the M10 POC has split flaps.  A split flap is very similar in lift delta to a simple flap.  Data in "The Theory of Wing Sections" is almost all for split flaps.  Another characteristic is higher drag, which can be good or bad - depending if the designer wants and/or needs the drag.

It still meets the Kutta condition.  An advantage is that the upper surface remains attached at all flap deflections.  With a normal flap, the upper surface separates between roughly 25 and 35 degrees (airfoil dependent).  The bottom surface (actual split flap portion) is the same.

 Oh, the split flap on the Cessna 190/195 is to lower the pitch attitude … so the pilot can see over the nose on landing … 1 mph stall speed delta.

[Cruiser]

Ok someone explain why the paper towels in my boat fly............. seriously.

I will try to explain.

I have small rectangular windows that open at the bottom of the windshield. These let air into the cockpit when cruising. Mounted on the bulkhead just below the chart deck which the windshield mounts to is a bracket to hold a roll of paper towels. When the roll of paper towels is put on the spindle with the sheet coming off the top of the roll it is located just slightly below the stream of air rushing through the open window. 

With just the right movement of air in the cabin the trailing end of the paper towel roll will suddenly lift into the air and remain suspended horizontally from the roll in the air stream. It ungulates up and down until it finally falls out of the air stream to be lifted  again later. It can remain in the lifted state for some time and will actually pull the paper off the roll extending the length of the paper towel in the air stream. 

Is this Newton,  Bernoulli or something else 

[Blue on Top]

2 minutes ago, Cruiser said:

Ok someone explain why the paper towels in my boat fly............. seriously.

I will try to explain.

I have small rectangular windows that open at the bottom of the windshield. These let air into the cockpit when cruising. Mounted on the bulkhead just below the chart deck which the windshield mounts to is a bracket to hold a roll of paper towels. When the roll of paper towels is put on the spindle with the sheet coming off the top of the roll it is located just slightly below the stream of air rushing through the open window. 

With just the right movement of air in the cabin the trailing end of the paper towel roll will suddenly lift into the air and remain suspended horizontally from the roll in the air stream. It ungulates up and down until it finally falls out of the air stream to be lifted  again later. It can remain in the lifted state for some time and will actually pull the paper off the roll extending the length of the paper towel in the air stream. 

Is this Newton,  Bernoulli or something else 

@Cruiser  LOL!  OMG, this is funny.  It's Bernoulli.  You described it well.

The stream of air coming into the cabin has a lower static pressure (higher dynamic pressure).  The towels are being sucked up into the side of the stream of air which has lower than ambient static pressure.  The roll of paper towels will unroll when the drag on the exposed towels creates enough torque to unroll the paper towels off the spindle.

 

[0TreeLemur]

On 12/25/2019 at 9:43 PM, Blue on Top said:

@Cruiser  LOL!  OMG, this is funny.  It's Bernoulli.  You described it well.

The stream of air coming into the cabin has a lower static pressure (higher dynamic pressure).  The towels are being sucked up into the side of the stream of air which has lower than ambient static pressure.  The roll of paper towels will unroll when the drag on the exposed towels creates enough torque to unroll the paper towels off the spindle.

 

Moving ambient air always has a lower static pressure than stationary ambient air like that in the cabin of your boat or airplane.   When we open the storm window in our Mooneys in flight, air flows out of the cabin.  

[PT20J]

Hey Ron, @Blue on Top, any thoughts on why my M20J pitches up when I lower the landing gear? I would have expected it to pitch down due to drag increment being below the CG, the airspeed stability effect of deceleration and the fact the nose wheel doesn’t weigh enough to move the CG very far forward.

Skip

[Cruiser]

so my boat acts like an airplane and has aerodynamic features. ..... cool

if I understand this, the fast moving air stream "grabs" at the stagnant air molecules in the cabin and "pulls" at them in the boundary between the moving air and the still air (friction?). As the molecules are pulled apart the air pressure is decreased and creates the unbalance which allows the "thicker" air under the towels to lift them.

close ?

[Blue on Top]

1 hour ago, 0TreeLemur said:

When we open the storm window in our Mooneys in flight, air flows out of the cabin.  

Continuing on that thread, opening the storm window should be in your POH for cabin smoke evacuation.

[Blue on Top]

1 hour ago, PT20J said:

Hey Ron, @Blue on Top, any thoughts on why my M20J pitches up when I lower the landing gear? I would have expected it to pitch down due to drag increment being below the CG, the airspeed stability effect of deceleration and the fact the nose wheel doesn’t weigh enough to move the CG very far forward.

Skip

@PT20J Skip: Tough to say and even tougher to analyze, so here's my best guesses … (and everything you have said is correct and should pitch the nose down)

1) Could have something to do with the nose bay opening.  The nose gear also destabilizes the airplane a little in directional stability.  The nose gear doors/bay may be giving a little lift.

2) (Don't shoot me here -it's happened before with very qualified test pilots, but …)  Are you doing this with your hands off the yoke?  

[Blue on Top]

1 hour ago, Cruiser said:

so my boat acts like an airplane and has aerodynamic features. ..... cool

if I understand this, the fast moving air stream "grabs" at the stagnant air molecules in the cabin and "pulls" at them in the boundary between the moving air and the still air (friction?). As the molecules are pulled apart the air pressure is decreased and creates the unbalance which allows the "thicker" air under the towels to lift them.

close ?

@Cruiser  Oh, how I want to make your day.   Your boat not only has fluid dynamics just like an airplane (air also acts like a fluid), your boat is simulating supersonic aerodynamics!  The density of water is much, much greater than >>> air.  Your boat's bow wave simulates a supersonic shock wave in air.  We use it on water tables as a 2D version of the airplane 3D shock cone, and in water tunnels, too.

Yes, the fast moving stream of air causes a lower pressure area around it.  It draws the higher pressure air (not thicker, but …) around it into the stream … pulling the paper towels up to it.  Now, because the paper towels are experiencing flow across them, drag is produced, pulling them off the roll.  Walla!

For some harmless fun … unless they catch you.  Get your leaf blower, a paint roller handle, a little duct tape and rolls of toilet paper from Sam's Club or Costco (your preference).  Then Google "toilet paper shooter".  How do you think one can TP a house in moments … and drive away before they can get to the door?

Mother Nature is a simple lady.  Modeling her characteristics on the other hand is VERY complicated.  Having it visualized (tufts, oil flows, paper towels, rain, etc.) is priceless.

Blue on Top, Ron

PS. Yes, I spent my childhood leaning over the bows of boats (all kinds) and playing billiards.  How else is a child to learn aerodynamics and physics? 

[PT20J]

8 hours ago, Blue on Top said:

@PT20J Skip: Tough to say and even tougher to analyze, so here's my best guesses … (and everything you have said is correct and should pitch the nose down)

1) Could have something to do with the nose bay opening.  The nose gear also destabilizes the airplane a little in directional stability.  The nose gear doors/bay may be giving a little lift.

2) (Don't shoot me here -it's happened before with very qualified test pilots, but …)  Are you doing this with your hands off the yoke?  

I won't shoot -- it's always good to check. But, yes I did this stick free with only minor rudder inputs to keep the wings level. Here is a very rough time history I did years ago with my '78 J. It appears that the '94 has similar behavior. Just another mystery of the universe 

Skip

[Blue on Top]

Skip @PT20J  The gear has excited the longitudinal long period (phugoid) mode.  There are two longitudinal, oscillatory modes in aircraft.  A short period (<2 seconds) has to be heavily damped, and a long period (phugoid) has to be controllable.    From your great graph, it looks like it has a 14-15 second period.  Very controllable.

Interesting to note is the large amplitude of pitch change (+/- 10 degrees, initially).  Ironically, the oscillation (entire maneuver) is all at a constant AOA.  During this maneuver, the airplane is trading altitude and airspeed … all at the same AOA.

Cool data!

[PT20J]

On 12/5/2019 at 10:07 PM, PT20J said:

Many years ago I had an interesting conversation with flight test and handling qualities engineer Roger Hoh (whose most recent achievement is the Heli-SAS for the R-44) about spiral stability. He made a point that the classic graveyard spiral (ever steepening bank and ever increasing airspeed until the wings come off or the ground intercedes) is somewhat mythical and if the airplane has sufficient longitudinal stability, such behavior is more caused by mis-rigging or lateral imbalance than the inherent dynamics of the airplane.  At the time I owned a '78 M20J and one day I took it up to about 5000' and trimmed it for 90 KIAS in level flight and let go of the controls. This airplane was rigged properly and it flew pretty straight for long enough that I got impatient and gave it a small poke (impulse I believe the test pilots say) on the right rudder. It slowly rolled off to the right and the nose went down and the airspeed increased pretty rapidly. But then, just as Roger said it would, the longitudinal stability (phugoid) kicked in (just about at the point where the airspeed was getting close enough to redline that I was about to end the exercise) and the nose rose and the airspeed decreased and the roll rate also decreased. The airspeed dropped off until somewhere above stall and then the nose started down again. As I recall, the airplane completed two and a half cycles, each of decreasing amplitude, and ended up in a steady 45-deg banked descending turn at around 90 KIAS. I wish I had tried this in other airplanes, but I never have. So, I'm wondering if Roger was right and this is common behavior, or if there's something about the Mooney that makes this work out.

Skip

Ron, @Blue on Top since we are discussing phugoids ....this early post got lost in other discussions, but I’m still curious if this behavior is common in other airplanes. 
Skip

P.S. I'm wondering if the Mooney might have a more stable spiral than some other airplanes due to the aileron-rudder interconnect and the trim bungees. The former might affect stick-free lateral stability while the latter should increase stick-free longitudinal stability. (I think).

Edited December 28, 2019 by PT20J
Added comment about aileron-rudder interconnect and trim bungees

[Blue on Top]

On 12/27/2019 at 11:07 PM, PT20J said:

Ron, @Blue on Top since we are discussing phugoids ....this early post got lost in other discussions, but I’m still curious if this behavior is common in other airplanes. 
Skip

P.S. I'm wondering if the Mooney might have a more stable spiral than some other airplanes due to the aileron-rudder interconnect and the trim bungees. The former might affect stick-free lateral stability while the latter should increase stick-free longitudinal stability. (I think).

Skip @PT20J  Yes, all airplanes have longitudinal short period and long period (phugoid) oscillations.  What may be different between airplanes is how much they are damped.  For example, the short periods have to be heavily damped (there is an FAA definition for that), but they also might be deadbeat (they return to the initial condition without any oscillation.  The phugoid on the other hand is only noted in certification as to not cause undue pilot workload.  I'd have to look at the regulations to know the exact wording.

Trim bungees will only effect longitudinal characteristics.

My guess is that the aileron-rudder interconnect doesn't change the spiral stability of the airplane because the interconnect doesn't move if either of the control surfaces don't move.  Spiral stability is the opposite of Dutch roll stability.  Typically, light GA airplanes will have a negative spiral stability.  The contrary would be a continual Dutch roll oscillation … which would get annoying.  The is why most larger airplanes have yaw dampers.

 

[PT20J]

23 hours ago, Blue on Top said:

Skip @PT20J  Yes, all airplanes have longitudinal short period and long period (phugoid) oscillations.  What may be different between airplanes is how much they are damped.  For example, the short periods have to be heavily damped (there is an FAA definition for that), but they also might be deadbeat (they return to the initial condition without any oscillation.  The phugoid on the other hand is only noted in certification as to not cause undue pilot workload.  I'd have to look at the regulations to know the exact wording.

Trim bungees will only effect longitudinal characteristics.

My guess is that the aileron-rudder interconnect doesn't change the spiral stability of the airplane because the interconnect doesn't move if either of the control surfaces don't move.  Spiral stability is the opposite of Dutch roll stability.  Typically, light GA airplanes will have a negative spiral stability.  The contrary would be a continual Dutch roll oscillation … which would get annoying.  The is why most larger airplanes have yaw dampers.

 

I guess my question is really if it’s typical behavior for most airplanes to stabilize in a descending turn at constant airspeed and bank angle after the spiral begins?

Regarding the aileron-rudder interconnect, my thought was that the rudder would tend to streamline and perhaps the springs would act as aileron centering springs. I seem to recall reading an old NACA report years ago where it was found that centering springs improved spiral stability.

Skip

[EricJ]

40 minutes ago, PT20J said:

I guess my question is really if it’s typical behavior for most airplanes to stabilize in a descending turn at constant airspeed and bank angle after the spiral begins?

This has been bugging me lately, for certain configurations, especially since in the past I've seen advice and vids on using low-speed stable spiral characteristics to descend through clouds.   Apparently with some airplanes if you apply full flaps and full up trim and power off, you will get a stable, low-speed spiral descent.   I can't find it now, but somebody had made a vid doing this in a C-150 or something similar and it was impressive.   The idea was that if you need to descend through a cloud layer, you'll come out in a predictable attitude (without having to resort to spinning it down and hoping you have enough room for recovery once you break out).

Still looking for the vid, but I can't find it.   

[Ibra]

30 minutes ago, EricJ said:

Apparently with some airplanes if you apply full flaps and full up trim and power off, you will get a stable, low-speed spiral descent.   I can't find it now, but somebody had made a vid doing this in a C-150 or something similar and it was impressive

That was one technique to get through clouds in gliders, we were allowed to fly them in IMC (more "cloud flying" than IFR flying) as long as you have an ASI and Turn Indicator but on the case of a dead battery you would loose the Turn Indicator (say after a long wave flight at -16C with no engine), you trim for 1.2*VS0 open air-breaks (if they are not stuck with ice ) and let go the controls, the glider descends nicely in a low-speed spiral dive, although, I could fly on instruments, this was my preferred way to get through the layer, Gliders have very clean wings and in a steep bank without air-breaks they just go wild in clouds faster than my skills or the accuracy of non-certified electrical instruments

But you have to get a dry run in VMC on the day with and without air-breaks, as experience does vary with type, weight and GC position

I have done the same experience in the M20J (no speed breaks) on idle and flaps, for some reason it does not like it (as does an Archer or DA40) but you can keep it under control at 1.2*VS0-1.4*VS0 with some rudder input out of the turn (assuming you know which way )

Why doing it? I believe you can still fly an aircraft with no visual references, electric or engine using only elevator trim + rudder + ASI

But, I don't believe you can make a success out of it unless the ceiling is 1500ft agl

Edited December 30, 2019 by Ibra

[EricJ]

46 minutes ago, EricJ said:

This has been bugging me lately, for certain configurations, especially since in the past I've seen advice and vids on using low-speed stable spiral characteristics to descend through clouds.   Apparently with some airplanes if you apply full flaps and full up trim and power off, you will get a stable, low-speed spiral descent.   I can't find it now, but somebody had made a vid doing this in a C-150 or something similar and it was impressive.   The idea was that if you need to descend through a cloud layer, you'll come out in a predictable attitude (without having to resort to spinning it down and hoping you have enough room for recovery once you break out).

Still looking for the vid, but I can't find it.   

Well, wth, it was a thread here:
 

 

[Blue on Top]

 Wow, I love how all y'all are going after the wives tales.  Sorry for the delay, I had to go through all the other thread, too (love this stuff).

@PT20J Skip, no it is not common for an airplane to enter a stabilized spiral and stay there.  There are so many factors that influence that flight characteristic: (aircraft specific) rigging, control surface shape (or bent), wing shape (or bent/twisted), fuel imbalance (and total amount), etc.  The Cessna singles have very, very, very high spiral stability.  IOW, the airplane has to be forcibly controlled to Dutch roll (the opposite mode from the spiral mode).

As for recovering from a VFR into IMC situation, use your instruments to tell you what is going on.  Note: Most gliders (that are not used in IFR operation) don't have attitude indicators.  The odds of losing those instruments at the same time are very, very low … that is with a 6-pack.  For those reading this thread that have glass panels and no round dial backups.  Get to know the failure modes of your instruments!

For example, know that with the new electric AIs, a loss of pitot, static, GPS, etc. will degrade your AI.  Know what happens with the loss of any accelerometer.  IOW, failure modes of the electronic versions of the round dials are not the same as failures of the round dials.  As an example for certification today, loss of attitude is considered catastrophic (assumed electronic).  Everyone (until recently) had to fly home partial panel.  That is one advantage of the 6-pack.

[carusoam]

I used to try to dispel the OWTs I learned in flight school...

The what to dos when this fails...

My M20C got to do a few experiments...

When you let go of the yoke...it starts banking...

Which way, How fast and how deep depends on the difference of fuel in the tanks... 

No matter what level of fuel you have... it won’t stay that way... and the bank steepens quickly...

Fortunately, we live in a modern world with multiple methods of having a BU AI. Way more than a TC...

TCs wear out without telling anyone... you only find out in bumpy weather... then they are un-followable...

 

So....

If you fly in IMC... get yourself a fancy BU AI so you aren’t dependent on everything working like they are brand new...

Briefly, TCs suck. They are only slightly better than TnBs. And that is when they are not worn...

 

PP thoughts only... stuff I remember from the IR training... where the TCs were un-followable...

Best regards,

-a-

[Ibra]

Same observation in M20J, hard to stop it going fast dives handoff but can do with little of rudder control but not something I felt you could bet on your life on for flying in convective IMC

My point is limited pannel flying is easy when the goal is to keep wing level and speed under control (not losing the wings), it can be done handoff rudder only on a well trimmed Mooney 

Of course, completely different from accurate navigation or shooting a precision approach or missed segment from minima (never done any of these on TC only )

Nothing against Mooneys, by comparaison I flew DA40 that stalls wing level hand and feets off all the way from 8000ft to 2000ft (1500fpm on VSI and 50kts on ASI), but I would still take the Mooney 

[PT20J]

Somewhere around the mid-1960s, I believe, Mooney beveled the trailing edges of the ailerons. I've heard it said, but do not know for certain, that this was done at the time of introduction of Positive Control to lighten the control forces for the roll servos. I've found lots of references describing beveling the trailing edges as being a method of aerodynamic balance used since the 1940s to reduce control forces, but I've yet to find a good analysis of exactly how it works. My understanding comes from some fairly cryptic comments in an old NACA report that I can no longer lay my hands on. The general idea is that increased hinge moments are caused by flow separation and thickening and beveling the trailing edge increases the camber at the trailing edge which reduces the adverse pressure gradient so that the flow can remain attached. Ron @Blue on Top, did I understand this correctly, or is there a better explanation?

Skip

[cliffy]

You are stating what I remember also. The early Mooneys have ailerons that have an undercambered airfoil shape and the later ones have a straight taper (that looks equal on both top and bottom surfaces)  to the trailing edge.   I also read somewhere that this was done to lessen the control forces for the PC system so that would make the change around 1975 or so. My 64 has an "after market" PC  but I think the 65s had them from the factory. I could be off a year or so though. My PC system works just fine also. 

[Blue on Top]

@PT20J and @cliffy  This is interesting.  I should think about this more, but off the top of my head the trailing edges should be as small/sharp as possible for minimum drag (at these speeds).  Thicker trailing edges make the surface more stable … at the cost of drag … similar to adding a T-strip.  Both will have a tendency to center the surface when the controls are let go (stick free).  The more centering, the more force would be required to move the surface.  Changing the TE to more pointed will cause the lateral-directional stability to not be as good.  IOW, the aileron will not return to neutral when let go.

Aft camber (of the wing but showing up as a cusp on the aileron) will aft load the airfoil.  An NLF airfoil is typically/already aft loaded.  This will not increase/decrease the aileron forces as the opposing aileron is helping it, and the ailerons act as a loop (yes, the system is push-pull tubes, but …).  Aft loading will cause the ailerons to "float" more, but in a Mooney that just means that all the aileron control system free-play is taken out in that direction.  So … when roll control is input is made, the down-going aileron will move first.  This is totally irrelevant as we typically only care about what is called "total aileron".  IOW (for roll control), a degree of UP travel equals a degree of DOWN travel; we add the two for calculations.  IOW, if the left aileron goes up 10 degrees and the right goes down 5 degrees, we use 15 degrees of aileron travel.

Hope this helps, but I'm guessing I have added confusion.

Blue on Top, Ron