J airfoil vs predecessors

[Shadrach]

On 6/30/2011 at 5:56 PM, 201er said:

That's interesting. I'm kind of surprised they were able to get so much more speed out of the same wing. 

And after almost 12 years the answer is.....drum roll...they weren't. They were able to get just a little more speed but they certainly made it look a lot faster.

[0TreeLemur]

Here's an interesting set of curves for the 64-412 airfoil that is used outboard on the M20 wing.  From Kindred Grey (2021). "NACA 64_1-412 Airfoil Data." CC BY 4.0. Adapted from NACA. Public domain. 

https://archive.org/details/64-412

The last figure showing drag coefficient Cd vs alpha clearly shows the magic "drag bucket", where Cd is low for section lift coeff. between about 0.2 and 0.6.   For our Mooney's with a clean wing flying with a Re>3.0E+06 (easy), that's the sweet spot.  These Cl values correspond to level flight between 180 and 110 knots, respectively, at angle of attack of approx. 0 and 3.5 degrees!

Anybody see curves like this for the inboard 63-215?

 

[PT20J]

6 hours ago, 0TreeLemur said:

Here's an interesting set of curves for the 64-412 airfoil that is used outboard on the M20 wing.  From Kindred Grey (2021). "NACA 64_1-412 Airfoil Data." CC BY 4.0. Adapted from NACA. Public domain. 

https://archive.org/details/64-412

The last figure showing drag coefficient Cd vs alpha clearly shows the magic "drag bucket", where Cd is low for section lift coeff. between about 0.2 and 0.6.   For our Mooney's with a clean wing flying with a Re>3.0E+06 (easy), that's the sweet spot.  These Cl values correspond to level flight between 180 and 110 knots, respectively, at angle of attack of approx. 0 and 3.5 degrees!

Anybody see curves like this for the inboard 63-215?

The curves are in Abbott and Von Doenhoff, Theory of Wing Sections.

Keep in mind that these curves are for highly polished wind tunnel models. True laminar flow (and the drag bucket) is very difficult to achieve in a manufactured wing. The wood wing Mooney might have come close because of it's smoothness, but the metal wing has a lot of irregularities. It doesn't take much to trip the boundary layer. Some people have installed vortex generators to improve low speed characteristics, and that certainly destroys laminar flow, and they report negligible change in cruise speed.

BTW, the venerable Piper Cherokee also has a laminar flow wing (65-415). 

Skip

[0TreeLemur]

3 hours ago, PT20J said:

The curves are in Abbott and Von Doenhoff, Theory of Wing Sections.

Keep in mind that these curves are for highly polished wind tunnel models. True laminar flow (and the drag bucket) is very difficult to achieve in a manufactured wing. The wood wing Mooney might have come close because of it's smoothness, but the metal wing has a lot of irregularities. It doesn't take much to trip the boundary layer. Some people have installed vortex generators to improve low speed characteristics, and that certainly destroys laminar flow, and they report negligible change in cruise speed.

BTW, the venerable Piper Cherokee also has a laminar flow wing (65-415). 

Skip

Do round rivet heads count as vortex generators?  

[PT20J]

5 hours ago, 0TreeLemur said:

Do round rivet heads count as vortex generators?  

No, they don’t project high enough to pull free stream air down into the boundary layer to energize it. 

All wings have some laminar flow. The wings designed to promote it have the maximum thickness point further aft along the chord line. At some point, the boundary layer always becomes turbulent because an adverse pressure gradient has to form somewhere before the flow meets the trailing edge. Mooney uses flush rivets on the portion of the wing where laminar flow may occur and round head rivets aft of where it’s expected to be turbulent. Flush rivets are more expensive to install, so there is no use, save aesthetics, to use them where there is no aerodynamic benefit.

The Cherokee likely used a laminar flow airfoil because the aft location of the maximum thickness point yields an advantageous location for the spar relative to the cabin layout.

Skip

[Pinecone]

9 hours ago, PT20J said:

Mooney uses flush rivets on the portion of the wing where laminar flow may occur and round head rivets aft of where it’s expected to be turbulent. Flush rivets are more expensive to install, so there is no use, save aesthetics, to use them where there is no aerodynamic benefit.

Soviet jet fighters do the same thing.

US jets are fully flush riveted because it looks cooler.

 

[cliffy]

Damn!  There are some seriously well educated, talented minds on this forum that can easily bury most of us. 

Learn something new every day. 

BTW a D converted to a C is still a D in disguise :-)  There were 254 of us at one time. 

[A64Pilot]

All I’ll add is that in theory VG’s promote laminar flow

https://www.aviationconsumer.com/accessories/vortex-generators-got-stol-think-twice/

However they add drag doing so, in my testing of a very dissimilar aircraft they do knock off a kt or two from stall, but also the same amount from high speed cruise.

There just ain’t no such thing as a free lunch.

On some aircraft that suffer from control lack of authority at low speeds they can really help with that.

In my testing I’d say they may work really well if you have an airflow separation at a particular area, but so far as putting them full span on an aircraft with no problems, in my opinion it’s not worthwhile.

Apparently most manufacturers seem to agree, because I know of very few that come from the factory with full span VG’s.

[PT20J]

VGs are advantageous in reducing flow separation at high angles of attack. We all know that the speed of the air over the top of the wing accelerates which is one of the explanations for lift. However at some point, generally after about the 25% chord point (farther aft for laminar flow wings), the air begins to slow. This is because physics demands that an adverse pressure gradient build up on the aft portion of the wing. Within the boundary layer, the adverse pressure gradient can actually cause the flow to reverse and separate from the wing. At high angles of attack, the separation progresses forward along the wing chord reducing lift and causing a stall.

Vortex generators create vortices (big surprise) -- little horizontal tornadoes -- which grab faster moving air from above and transport it down deep into the boundary layer which counteracts the reversed airflow and keeps the airflow attached to the wing. 

Originally, VG were dimensioned to be approximately as high as the boundary layer thickness. These did indeed create a small but measurable drag penalty. However it was discovered with modern CFD techniques that shorter subboundary layer vortex generators, or micro vortex generators, worked nearly as well at energizing the boundary layer and delaying stall without the drag penalty.

[A64Pilot]

Cut n pasted from the article I quoted, from my testing of two different STOL aircraft, a Maule and a turbine crop duster, I concur, there is a small speed loss, it’s not huge, but it’s real and the faster you try to cruise the greater the speed loss, of course that’s how drag works  I didn’t have an airspeed boom on the Maule so it’s stall speed is hard to quantify (at steep angles of attack A/S indicators become inaccurate) but I did have one on the Crop Duster.

“As part of adding VGs to a Cessna 182J that already had a STOL kit (see sidebar at left), we did before and after cruise speed comparisons at 65, 75 and 80 percent power. We observed a TAS loss of 1 MPH at 65 and 75 percent power (which we think is within measurement error). Going to 80 percent, well above what we think most pilots use, we saw a TAS loss of 5 MPH.”

The Maule was notorious for its small ailerons becoming ineffective at very slow speeds, and the VG’s helped greatly with that.

On both the Maule and the crop duster although the stall speed was decreased, the break was more pronounced, but as both aircraft had gentle breaks it wasn’t much of an issue, in an aircraft with a much more pronounced break, it might be.

 

 

[cliffy]

Flow separation on the aft of the wing?

Take a look at any older 20 series Lear Jet and view the vortex generators fwd of the aileron-

Had to be there to keep flow on the ailerons

What did Ol'man Lear call them?   F^&* Up fixers!

 

 

 

  

[Fly Boomer]

4 minutes ago, cliffy said:

Flow separation on the aft of the wing?

Take a look at any older 20 series Lear Jet and view the vortex generators fwd of the aileron-

Had to be there to keep flow on the ailerons

What did Ol'man Lear call them?   F^&* Up fixers!

 

 

 

  

Until reading this, the only factory job I had ever seen was on a Meridian wing (near leading edge).

[PT20J]

3 minutes ago, cliffy said:

Flow separation on the aft of the wing?

Take a look at any older 20 series Lear Jet and view the vortex generators fwd of the aileron-

Had to be there to keep flow on the ailerons

What did Ol'man Lear call them?   F^&* Up fixers!

 

 

 

  

Less of that these days with modern CFD programs. Used to be pretty common to have to fix stuff after flight tests. And, the more original designs get stretched and grossed up, the more fixes get applied (ask Boeing about the MAX).

https://kingairmagazine.com/article/why-do-the-ailerons-have-a-lump/

[KSMooniac]

On 4/29/2023 at 4:45 PM, A64Pilot said:

All I’ll add is that in theory VG’s promote laminar flow

https://www.aviationconsumer.com/accessories/vortex-generators-got-stol-think-twice/

However they add drag doing so, in my testing of a very dissimilar aircraft they do knock off a kt or two from stall, but also the same amount from high speed cruise.

There just ain’t no such thing as a free lunch.

On some aircraft that suffer from control lack of authority at low speeds they can really help with that.

In my testing I’d say they may work really well if you have an airflow separation at a particular area, but so far as putting them full span on an aircraft with no problems, in my opinion it’s not worthwhile.

Apparently most manufacturers seem to agree, because I know of very few that come from the factory with full span VG’s.

That linked article is substantially correct, but the author uses the wrong language in places and muddies the waters IMO.  VG's do NOT promote laminar flow, but they can delay SEPARATED flow, which is beneficial.  Many non-aerodynamically-inclined pilots/engineers/authors/sales people confuse the progression of the boundary layer over an airfoil.  Laminar flow is the holy grail in terms of performance/low-drag, but ALL airfoils cannot maintain laminar flow over the entire surface.  ALL of them transition to TURBULENT flow at some point.  This is draggier than laminar flow, but the boundary layer is still attached and everything is normal in this condition.  The boundary layer gets thicker, has more energy and thus more drag...that is natural.  As the angle of attack increases, this turbulent boundary layer can detach from the airfoil, and the result is a SEPARATED boundary layer, and this is the worst condition...highest drag, and can reduce (or eliminate) effectiveness of a control surface.  This is where VG's help by intentionally tripping the boundary layer to turbulent, because it has more energy (and drag) and is harder to separate from the airfoil. 

(Sidebar:  Poor aerodynamic design of a vehicle can also lead to separated flow in many places around a vehicle in normal flight too, leading to extra drag.  For example, at wing root/fuselage intersections, windshield/cowl intersections, under-cowl flows, landing gear intersections, etc.)

To conclude today's lesson, airfoil boundary layers progress from LAMINAR to TURBULENT, then to SEPARATED based on many factors.  ALL of them have substantial portions of turbulent flow in normal flight regimes.    SEPARATION typically comes from high angles of attack, including those "induced" from flap or aileron deflections, so VG's delaying separation over these portions of the span can be very beneficial.

IMO, VG's have no place on a Mooney that is designed for efficient high speed cruise, and not extensive low speed maneuvering or STOL missions.  VG's are a substantial safety enhancer for those missions, though, and especially Ag spraying or firefighting type flights, and for twins where Vmc can be reduced.   They're also useful for "fixing" behaviors discovered in flight test that cannot be easily corrected, such after the design is complete and the tooling is built.

Another sidebar... the SR-20 series and the Lancair/Columbia/Corvalis series of modern GA used modern (at least 1980's anyway) airfoils and computer analysis to design those wings.  They started out as clean NLF (natural laminar flow)_airfoils, but discovered in flight testing that the low speed/stall behavior could not meet FAR Part 23 requirements, so they had to resort to post-design fixes.  This led to the leading edge cuff modifications that are present today. The sharp discontinuity in the leading edge at the inboard side of the ailerons acts like a distinct VG and an aerodynamic fence of sorts, to keep the flow attached and flowing over the ailerons at higher angles of attack.  In the 90's, this wasn't really predictable with the computer tools available at the time...perhaps that could be caught in advance today but it has been a long time since I've professionally dabbled in any aero work.

[A64Pilot]

VG’s on Ag planes are a somewhat mixed issue.

You see a fully loaded Ag plane has a very high stall speed, an Ag plane often has a useful load that exceeds it’s empty weight and the stall speed requirement doesn’t include the hopper, so in a normal turn when heavy they are nibbling at stall. Almost all Ag operators either bend the stall warning to shut the thing up or otherwise disable the thing as it gets obnoxious listening to it in nearly every turn

Some aircraft like the Thrush enjoy a good safety record largely because they have very gentle stalls, and in the case of the Thrush the pre-stall buffet is very pronounced and comes well before the actual stall. If you have any Piper 140 time they stall just like a 140, that is power off level flight they don’t really even stall, just mush but you have full control in all axis.

So as VG’s do in fact lower the stall speed by slightly increasing the critical angle of attack one would think they be the best thing going for an Ag plane and in fact as they are so easy to apply and very cheap would come from the factory already installed. ( You see there are many Countries in the world that do not accept US STC’s) but of course do the TC.

Well here’s the problem, while VG’s do in fact lower the stall speed slightly, they do make the stall break more abrupt, which in the case of the Thrush is OK, but only because it has such a benign stall, not all Ag planes do.

So I had Annie Brogan (Microaerodynamics VG’s) spend some time at the factory and we installed some kits on different aircraft with her DER doing the flights, got the STC’s and incorporated them into our Type Certificate as optional, so people in Countries that won’t accept a US STC can install them, plus it makes their installation easier as it’s not a supplement, they are part of the TC. By the way I got very attractive pricing from her so that I could put them on at the factory if a buyer wanted them, they were popular in Central America, not so much in the US.

If anyone who has them installed is going to re-paint she will sell you a re-paint kit at a fraction of what a kit with STC cost. It’s the same kit, just no STC

Best way to install them is on a bare metal wing, they are installed with Loctite Depend adhesive, which is an astonishingly strong adhesive, but if installed onto a painted wing the paint bond is what will let go when a refueler drags the hose on the wing or a brush hits one in washing the airplane etc. But if glued directly to the metal they will bend before the glue lets go.

However I agree that VG’s on a Mooney is a strange thing, I won’t argue that they do slightly decrease stall, but as a Mooney’s mission almost always is from one long paved runway to another it makes you wonder why they are needed as there IS a slight decrease in high speed cruise and as so many spend big bucks decreasing drag even slightly it would seem curious as to why they are on a Mooney.

[cliffy]

On 4/30/2023 at 9:04 PM, Fly Boomer said:

Until reading this, the only factory job I had ever seen was on a Meridian wing (near leading edge).

Those who forget history are bound to repeat it!

Take a look at all the perturbances on a latte model Beech 1900!  F Fixers everywhere. 

The thing is a pig without them.

[mike20papa]

I use to take my A model out after annual when the back seat and passenger seat were still out.   It was a "hoot" to climb out after take off in shallow left climbing turns above the airport to about 7,000 ft roll out above the clouds and then watch the clouds & ground pass below and remember the words in the POH intro when Al says, "this airplane gives you keys to the aerial kingdom"  

Early Model Mooney Performance Table.pdf

[carusoam]

Mooneys with VGs…

1) Short field landings… my favorite airport is about 2.2k’ long…

Approaching slowly to carry the minimum excess energy to stop comfortably….

VGs add a comfort buffer…

 

2) Cost of additional drag… 

Less of an issue at Mooney cruise altitudes…?

More comfortable in the FLs..

 

Kinda sounds like the added drag of a Fiki system…. 

Best regards,

-a-

Edited May 3, 2023 by carusoam

[carusoam]

I love the threads from 2011…

1) My posts didn’t get as many bullet points…

2) They were kinda written in whole sentences…

3) @201er is still addicted to his Mooney, more today than yesterday… but not as much as tomorrow…

Best regards,

-a-