Mooney Aerodynamics

[Cargil48]

10 hours ago, afward said:

If anything, I'd say the open end will experience slightly higher force than the closed end, primarily because it will experience more turbulence and a region of reversed flow that the closed end won't have.

Fill a bath tub (or go to a swimming pool) and do the experience with water. You'll feel the difference. Hydrodynamics or aerodynamics for this purpose is the same.

[Cargil48]

9 hours ago, afward said:

That might be beyond my skill to answer.  Yes, but real-world is messy so it's never truly stable?  Honestly I'm not sure.

What will slow down a big airplane better: a drag chute deploying or a flat panel with the same frontal area? That big amount of air stuck inside the chute plays an important role in comparison with the stream of air hitting the plate and then going around it. And I'm not referring to turbulence, I'm referring only to drag.

[Davidv]

On 12/6/2019 at 1:12 PM, Blue on Top said:

1. Others will know more, but Cessna does not completely enclose their gear … even on the M=0.93 Citation X

2. Drag delta is actually VERY small.  In fact it might actually be less drag because it gives the horizontal a little camber which the stabilizer should have.

4. The towel bar may actually be lower drag IF the blade is not at the correct angle of incidence for that flight condition.  Although the towel bar has drag all the time, if the blade separates on either surface, the drag will be higher than the towel bar.  This would be a great, simple, easy tuft test for a dedicated Mooniac to do (and video) and post here.

5. Gaps, steps (forward and aft), indentations, protrusions, etc. are all drag.  If it costs nothing but time, it's worth it.

6. BINGO!  Unless one does very dedicated, meticulous testing.  You won't know which change did what.  Looking at the airplane performance over time is a great way to see improvements.

737’s and other jets/turbo props don’t have completely enclosed gears either so I wouldn’t worry too much

[PT20J]

3 hours ago, takair said:

Did the J not have the cowl flap cove on both sides like the E and F?  My E has crude sheet metal cowl flaps, but fairly well executed recessed coves such that when the cowl flaps are “closed” there is still a calibrated amount of air flowing through.  I think this alone is quite effective in reducing drag on a Mooney vs other designs when the cowl flaps are closed.

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

[cliffy]

On 12/6/2019 at 12:14 PM, kortopates said:

For anyone wanting to reduce antenna drag, particularly with the Nav antenna towel bar or Comant  VOR/LOC/GS plate antenna on the vertical stab, you can do like I did, mount it in the wing tips. Originally my 252  had the very good Comant  CI 120-200 antennas installed in each wing tip. But when I went to the Encore conversion I had to replace them with a smaller footprint nav antenna that wouldn't interfere with the larger aileron flight control balance weights used for the increased gross weight change. LASAR came to my rescue allowing me to use their STC from their wing tip STC'd nav antenna.

I did the original flight testing for that wing tip enclosed VOR antenna at Comant for the Mooney  BUT we did it in  a C-182:-)

If anyone knows what a Smith chart is you might recall it depicts the antenna reception in a specific axis in a circular pattern.

Because we didn't have an anechoic chamber big enough to test it, we mounted the prototype in the wing tips of the company 182 and I flew it and manually cut/drew a horizontal axis chart by tuning a weak VOR station and flying a wings level 360 (to negate angle of bank influence) and watching the VOR flag dip in and out as reception failed and regained. With several tries I could see a reasonable pattern develop. We then took it to Kerrville and they put it in the Mooney wingtips. 

737 WHEEL WELL SEALING

Most folks do not remember that originally Boeing did seal the 737 wheel wells around the tires. When the 737-100 came out they had inflatable air bags that would inflate around the tire (sealing the well) after the wheels went in the wells and deflate before deployment. They were so troublesome with wear and blowout that Boeing dropped the idea and just accepted the increased drag.  They never did covered the entire wheel on the outside/bottom with any kind of door unlike their previous designs of jets. The weight and complexity evidently wasn't that much of a concern for what was then, a short range design. 

Drag Reduction

For those unfamiliar, all jets come with a CDL listing (Configuration Deviation List) that shows all parts on the airplane in the slip stream that can be missing and still be flight legal AND it shows the required extra fuel burn necessary for flight calculations due to the extra drag. So any parts in the slip stream have a drag associated with it.  Anything one does to the airplane that touches the  slipstream can and will have some effect on drag. + or -   Steps down or up, airflow through the cowl, rough belly skins, etc. Its all a trade off for the engineers. Cooling drag discussions have been around here several times over the years. 

On the original 727-100 it had an electrically actuated tail skid to prevent dragging the center engine on the ground at rotation. It had such a high drag and fuel burn penalty that in corporate flying around the world many times the CB for the skid was pulled before a landing so that the skid would not deploy and have the chance of not retracting on the next over water flight where max range was required. One just made sure that the landing and rotation on take off was prefect!

[afward]

3 hours ago, Cargil48 said:

What will slow down a big airplane better: a drag chute deploying or a flat panel with the same frontal area? That big amount of air stuck inside the chute plays an important role in comparison with the stream of air hitting the plate and then going around it. And I'm not referring to turbulence, I'm referring only to drag.

A round parachute is so large (and non-rigid) that the pressure fields & gradients change moment by moment and parcel by parcel across the entirety of the captured area.  In other words, it's a really turbulent (downright chaotic) flow, which is great for generating drag.

Our planes' inlets aren't that big.  In fact, a perfect inlet would have zero turbulent flow at cruise speeds.  Real world we just try to minimize the turbulence.  One way we do that is by limiting the size of the inlet so the pressure field is relatively uniform across the inlet (at least at a given distance from the lip; obviously the center of the flow won't have the same pressure as the edge).

All that aside, having slept on it I realize your idea could be easily tested and might have some merit: The Sam James RV-10 cowl is both faster and cooler than stock, yet it does not have cowl flaps (stock does).  It would probably be pretty easy to modify to include an upper deck bypass system in the cooling plenum (I'd think it would need to re-accelerate the bypass flow and dump it as close to the cowl outlet as possible, though).

Now that I've thought of it, I'd be curious to know the results of such an experiment.  Might be worth a post to Van's Air Force to see if someone will try it.

Edited December 11, 2019 by afward

[EricJ]

4 hours ago, takair said:

Did the J not have the cowl flap cove on both sides like the E and F?  My E has crude sheet metal cowl flaps, but fairly well executed recessed coves such that when the cowl flaps are “closed” there is still a calibrated amount of air flowing through.  I think this alone is quite effective in reducing drag on a Mooney vs other designs when the cowl flaps are closed.

Yes, my J has the "flat" cowl flap on the left side and even when closed flush there's a big gap for air to get out.   The crankcase vent, fuel pump vent, sniffle valve outlet, etc., all go out there through the remaining gap. 

[GeeBee]

With respect to engine cooling, it is not all that simple. The Lycoming manuals for instance specify that the upper deck pressure needs to be 4" H2O higher than the lower deck. You can also have also have areas of too fast airflow, where you get temp rises both from airflow speed and as well as stagnation points, eddies,  and shock wave points. It seems to be part science, part art, as well as some PFM. 

I had a very high performance PA-18 and spent a lot of hours flying around with manometers pickups inside the cowling, as well as smoke generators and GoPro cameras. Just when you think you understand what is going on, you get surprised. Now nothing is bigger than the inlets of a PA-18 and nothing is more inefficient as Cub Crafters found out. I found ramping the air up over the front cylinders rather than impinging it 90 degrees upon the front made a huge difference in cooling the front as well as the rear cylinders. Not only running cooler, but temps more even. You cannot simply enlarge the exit (cowl flaps) without a serious effect upon pressure differential. Set the exit opening too high and you create back flow around the cowl flaps and change the differential dramatically. 

Fundamentally you only want enough air to cool the engine otherwise you have large cooling drag. This means you only want the airflow sufficient to create, 4 to 6 inches differential flowing smoothly. Any more and you have a rapid rise in cooling drag. How you get to that delta is variable between the inlet variations and the outlet variations. 

Consider the old Turbo Saratoga which had only one very small opening and closed upper cowl. It does not look like it makes sense, it works.

 

[EricJ]

Took these pics today while I was thinking about it.   The first is the cowl exit of a Luscombe and you can see that the bottom is bent in to a lip that creates a low pressure area behind it.   That increases the pressure differential across the engine and therefore increases cooling air flow.   The second pic is a C-150, and you can see the same thing;  a lip extends around the front of the cowl cooling air exit, to reduce the pressure behind the lip and increase cooling air flow.   You can see this sort of thing on many opposed-piston GA aircraft that don't have cowl flaps.

[carusoam]

Unfortunately,

We can’t see how the air is deflected by the ramps, or round discs, or open can....

Where this ‘disturbed’ air flow becomes visible... is flying in the rain with speed brakes deployed... and watching water go a long way around the deployed brakes...

Somebody posted a video around here once of the speed brakes in the rain...

It is similar to the affect of a flat round disc...

Lots of computer power to simulate wind flow and complex shapes.

Best regards,

-a-

[flyer338]

I wonder if the speed increase from a good wax job would be as much on a Mooney flying at 150 ktas instead of more than 200?

[GeeBee]

12 hours ago, EricJ said:

Took these pics today while I was thinking about it.   The first is the cowl exit of a Luscombe and you can see that the bottom is bent in to a lip that creates a low pressure area behind it.   That increases the pressure differential across the engine and therefore increases cooling air flow.   The second pic is a C-150, and you can see the same thing;  a lip extends around the front of the cowl cooling air exit, to reduce the pressure behind the lip and increase cooling air flow.   You can see this sort of thing on many opposed-piston GA aircraft that don't have cowl flaps.

Ah yes, the "lip". Very common. Several people with cooling problems have been helped by increasing the size of the lip. Again however you can over do it and create a back eddy. I've always wanted to experiment with vortex generators effects on engine cooling, in particular on the lower cowl.

 

[PT20J]

I'm curious about airfoils. NACA cataloged wind tunnel tests on a zillion of them (https://www.amazon.com/Theory-Wing-Sections-Aeronautical-Engineering/dp/0486605868). Richard Whitcomb  (area rule, winglets) put his name on one (GA(W)-1)). Designers have shown a proclivity to use a broad range of them (https://m-selig.ae.illinois.edu/ads/aircraft.html). Yet engineers keep inventing new ones (I believe Cirrus hired John Roncz to create a custom airfoil for them). So, what's important in  airfoil selection for a General Aviation airplane? What parameters define the optimum design? And, if creating a new one, what is the goal that cannot be achieved with an existing airfoil?

Skip

[AH-1 Cobra Pilot]

When I was studying for my BS Aero E, (the days before PCs were prevalent), I was told our department used 86% of the total VAX computer user time on campus.  That was because the senior design class was given a problem to optimize in 10 variables.  Our programs would often run for 24 hours straight.  You can easily imagine a 3-dimensional graph with the lowest point being the optimal solution.  Now imagine a 10-dimensional graph, and note that the slopes of some of the parameters are much greater than others, (in other words, you get greater change in performance from some variables compared to others).  Airfoils are a only subset of these calculations, so you can see how difficult an optimized design really is.

[EricJ]

1 hour ago, Ah-1 Cobra Pilot said:

When I was studying for my BS Aero E, (the days before PCs were prevalent), I was told our department used 86% of the total VAX computer user time on campus.  That was because the senior design class was given a problem to optimize in 10 variables.  Our programs would often run for 24 hours straight.  You can easily imagine a 3-dimensional graph with the lowest point being the optimal solution.  Now imagine a 10-dimensional graph, and note that the slopes of some of the parameters are much greater than others, (in other words, you get greater change in performance from some variables compared to others).  Airfoils are a only subset of these calculations, so you can see how difficult an optimized design really is.

These days a $35 Raspberry Pi 3 has a 4-core CPU running at over 1GHz.   Could probably do your optimization in a few minutes if you threaded the program. 

[AH-1 Cobra Pilot]

18 minutes ago, EricJ said:

These days a $35 Raspberry Pi 3 has a 4-core CPU running at over 1GHz.   Could probably do your optimization in a few minutes if you threaded the program. 

The time to run the program is now quick, but the months to build the equations and analyze the results is still there.

[Jim F]

On ‎12‎/‎10‎/‎2019 at 9:23 PM, PT20J said:

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

Hi Skip,

Just to add some air race history.  It is my understanding that Pete Law (Skunk works) and Dave Zeuschel first added a spray bar to the P-51 radiator scoop in 1971
Read more at https://www.airspacemag.com/flight-today/how-reno-racers-keep-their-cool-16828199/#4rY6g4xyOSQFBr5t.99

The idea was to reduce drag by making the radiator intake smaller and add ADI spray bar to lower the inlet air temp(evaporative cooling). 

I love this thread.....

[Igor_U]

Well,

Supermarine S6 racing plane used wings and floats as radiator for it's liquid cooled engine. but's quite impractical in real life... 

[Blue on Top]

4 hours ago, PT20J said:

I'm curious about airfoils. NACA cataloged wind tunnel tests on a zillion of them (https://www.amazon.com/Theory-Wing-Sections-Aeronautical-Engineering/dp/0486605868). Richard Whitcomb  (area rule, winglets) put his name on one (GA(W)-1)). Designers have shown a proclivity to use a broad range of them (https://m-selig.ae.illinois.edu/ads/aircraft.html). Yet engineers keep inventing new ones (I believe Cirrus hired John Roncz to create a custom airfoil for them). So, what's important in  airfoil selection for a General Aviation airplane? What parameters define the optimum design? And, if creating a new one, what is the goal that cannot be achieved with an existing airfoil?

Skip

The optimum airfoil is all in the eye of the designer.  Everything in design is a compromise.  It would be like asking 100 pilots what is the best airplane?  You'll get at least 152 answers.  It all depends on your end goals.  Airfoils are design tradeoffs of: laminar vs. turbulent boundary layer flow; tolerance to contamination (bugs, dirt, rivet heads, etc.), aft vs. forward loading (pressure distributions); forward, mid or aft camber; ice vs no ice; spar depth (structures); spar placement (structures); flap configuration; flap system mechanism complexity; cruise Cl (and the associated drag); maximum Cl, pitching moment allowed, etc.  That is just for the 2D airfoil.

Then, add the complexity of 3D … the wing, and one adds: the stall pattern (the 2D stall characteristics have little to nothing to do with how the airplane will stall).  What about wing twist (wash out or wash in)?  Do we twist geometrically (physically) or aerodynamically (by using a different airfoil root to tip) or both.  Taper ratio changes everything.  Sweep (or forward sweep in the case of all Mooney surfaces)?  Dihedral amount?  Low or high wing (and there are A LOT of pros and Cons here)?  The list goes on and on.  The very, very tip of the iceberg has been exposed .

[Blue on Top]

15 hours ago, carusoam said:

 … We can’t see how the air is deflected by the ramps, or round discs, or open can....

-a-

There is a great book out there "Illustrated Guide to Aerodynamics" by Hubert "Skip" Smith.  I highly recommend it.  eBay for $5, including shipping, I'm guessing.

As mentioned, rain is a great visualization tool.  So is dirt, oil (aero folk use this for flow tests, laminar flow, etc.), yarn/thread tufts, etc.  I love to watch vehicles in a light rain (or just with a wet surface.  Any movement of the water is visual drag.  Similarly, any movement of the air after an airplane has passed is energy that the airplane imparted on the air (drag being a major portion).  So way cool.

[Blue on Top]

2 hours ago, Ah-1 Cobra Pilot said:

The time to run the program is now quick, but the months to build the equations and analyze the results is still there.

Actually, everything is getting much, much faster.  One can farm out processing, using hundreds of cores if desired.  Autonomous VB within Excel can parameterize variables.  We once optimized a winglet in less than a day: shape, airfoil thickness, height, sweep, rise, % chord of rise, dihedral angle, etc.  Technology is just so way cool.  And the next generation will laugh at us on how could we even get anything accomplished with todays current technologies. 

[cliffy]

Just a note on P-51 racing. Back in the LA Air Racing days (mid 60s) at Fox Field there was a purple P-51 that I was a crew member on and we had the Bardahl race team (race boats, hydroplanes) mechanics work on the 51. In those days 90 inches of mercury was about all the engines would take before hand grenading. The Bardahl boys took a hand drill (on the ramp at Fox) and drilled a hole into the engine blower case between the pilots legs and screwed in an AN fitting. A hose ran from there to a standard water gate valve by the pilots leg and then to an old 28V fuel pump and  a big tank behind the pilot seat. Bardahl put in what they called "bug juice" and told the pilot to go to 90 inches, turn on the pump and open the gate valve until the engine shook and turn it down enough to take away the shake and then go to 95 inches.  It worked. BTW, I was just the grunt on the crew. Basically the wax and polish, clean up guy, young and impressionable. 

No fancy computers or data sheets, knock sensors or multiport injectors, just eyeball engineering and a pressure carb and a big a&* engine. BTW, my Dad used to wrench on race cars in Los Angeles when they had banked wooden board tracks. Look that one up.  

Edited December 13, 2019 by cliffy

[skykrawler]

On 12/6/2019 at 1:35 PM, Cargil48 said:

Exscuse me "squawking" here as a layman. But since the Rocket's big engine produces much more heat than the normal engine, if one could shut completely the cowl faps, where would the hot air go? It would accumulate inside the engine compartment, right? What goes in must come out...

(Pic taken from AOPA's website)

 

Cargil48 ......your original response/post, as a layman,  misunderstood the nature of 'fully closing the cowl' to which you refer.    This is because when fully closed there is still an area for airflow ... we hope which has been carefully calibrated by the designer to provide enough cooling airflow for cruise at higher than standard day temperature.  This must be the case because you continually refer to a fully closed position having no airflow.

Your argument regarding having doors to control the flow at the front cowl opening has regressed to begging the question.   Perhaps there is a list of aircraft with this type design that the group can evaluate and compare to the Mooney, Cessna, Piper designs.

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19980214918.pdf

[John Mininger]

Welcome to Mooney Space Ron.

I wanted to chime in and say that I enjoyed your forums at Oshkosh last year very much. Two of my take aways were:

• I knew that swept tails originated out of aircraft manufacturers marketing departments, as opposed to the engineering departments. But I did not know that straight tails like those on the Mooney were so much more efficient that they could actually be designed and built smaller then their swept tail counterparts.
• Because Al Mooney had such a sour relationship with the Mooney Company at the time the company wanted to convert the wing from wood to metal. The company actually hired Ralph Harmon, the lead designer of the Beech Bonanza to design that famously strong Mooney metal wing.

Again, welcome Ron. I'm going to comment on AOA's on the other string.

PS I'm the guy who retrieved your mouse after your first forum last year.

[0TreeLemur]

On 12/12/2019 at 10:37 AM, PT20J said:

I'm curious about airfoils. NACA cataloged wind tunnel tests on a zillion of them (https://www.amazon.com/Theory-Wing-Sections-Aeronautical-Engineering/dp/0486605868). Richard Whitcomb  (area rule, winglets) put his name on one (GA(W)-1)). Designers have shown a proclivity to use a broad range of them (https://m-selig.ae.illinois.edu/ads/aircraft.html). Yet engineers keep inventing new ones (I believe Cirrus hired John Roncz to create a custom airfoil for them). So, what's important in  airfoil selection for a General Aviation airplane? What parameters define the optimum design? And, if creating a new one, what is the goal that cannot be achieved with an existing airfoil?

Skip

Skip,

This paper by Selig, Maughmer, and Somers (1995) is really interesting.  It compares the root aerofoil on our Mooneys, the NACA 63₂215 against a more modern design NLF(1)-0015, showing about 25% less drag in cruise at Re~~9.0E+06.  This more modern airfoil does indeed have an extensive region of laminar flow for a narrow range of alpha around cruise that they refer to as  "laminar bucket" in the polar diagram.  Wow- it also apparently is landable without flaps.   I think it is a very interesting read.

Fred

SeligMaughmerSomers-1995-JofAC-NLF-Airfoil-Design.pdf