Mooney Aerodynamics

[Blue on Top]

If MS users have aerodynamics questions, I can try to answer them.  I know (and hope) others will chime in, too.  I plan on giving an Oshkosh Forum this year on this topic.  Now that I think about it, would anyone attend if I try one at Sun-N-Fun, too?

I'll start with a very controversial topic: the Mooney laminar flow wing.  

Laminar flow is a very elusive condition.  Any bug, dimple, step, etc. will trip the flow from laminar to turbulent.  The picture above (of Scott Sellmeyer's beautiful "J") shows beautiful, turbulent flow.  People misinterpret separated flow as turbulent flow.  We are really talking about the small boundary layer right at the surface.  With laminar or turbulent flow, flow velocity on the surface is 0.  Hard to believe but true.  This is why dust stays on your airplane … even after you fly.  Let's have some fun!

[carusoam]

First question...

Sort of a Schroedinger’s cat in the trunk challenge...

Are there any affects related to all the tufts placed on Scott’s wing?

Any changes to stall behavior or slow flight?

Got any other pics of high AOA of the Mooney wing where the separation moves forwards?

 

Somewhere around here somebody has a video of the affects of speed brakes using rainwater as the indicator... the disruption of airflow is quite a bit larger than the size of the speed brakes themselves...

Best regards,

-a-

[PT20J]

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

[carusoam]

Skip,

My M20C testing didn’t go that far...

I was looking to see if the plane would fly straight in the event I accidentally entered clouds as a VFR only pilot...

My plane’s wing leveler was inop... and MS was a decade away from getting started...

On perfect days it could fly straight using weight shifting inside the cabin... and using Loran to follow a non-magenta line/course...

As fuel burned off the weight imbalance between the wings can get a bit forceful... 

The turns got tighter and tighter... as the heavy wing dipped more and more...

It was trimmed to fly about 150mph...

So... if it had instrument problems like a dead vac pump and a TnB go haywire... the trip wouldn't last very long...

I have not repeated the experiment in the O... 

If flying in the clouds occurs... I will have a few AHARS some portable others panel mounted... some connected to an AP...

Best regards,

-a-

[Blue on Top]

1 hour ago, carusoam said:

1. Are there any affects related to all the tufts placed on Scott’s wing?

2. Any changes to stall behavior or slow flight?

3. Got any other pics of high AOA of the Mooney wing where the separation moves forwards?

4. Somewhere around here somebody has a video of the affects of speed brakes using rainwater as the indicator... the disruption of airflow is quite a bit larger than the size of the speed brakes themselves...

-a-

1-2. Yes.  Each tuft (piece of yarn and tape) is a miniature VG which adds energy to the boundary layer (but is taken out of the engine/propeller in the form of added drag).  The airflow will stay attached to a slightly higher AOA.  There is a Citation (during development flight testing) that stalled poorly during one of the tests, so we tufted it.  It stalled great.  So we removed all the tufts … it stalled poorly.  This is an unlikely case, but it can happen.

3. Yes.  This picture is actually a screen shot from video that we took.  His "J" stall pattern is very, very, VERY dominated by the stall strip.  In fact there is a condition where the airflow separates (leaves the airfoil surface) and reattaches about a foot aft of the leading edge … very rare.  Of course it separates again as AOA is increased.

4.  Not unusual, but it would be cool to see.  Light rain/mist on your car will show interesting patterns, too.  The flow on your rear window might actually be going forward.

-Ron Blum 

[takair]

Ron, Not too many of us in CT.  Where are you based?....Edit:  Never-mind.....I see you are at ICT......not CT.  

[Austintatious]

They aerodynamic questions I have are speed related.  When looking for efficiency, it all matters.

Does anyone KNOW how much extra speed the inner gear doors are worth?  My rocket does not have them and I am considering getting them and installing them...  but I would like to know if it is WORTH it or not. To me, worth it would be a 3+ knot gain

Is it really true that the elevator flying slightly TE up has no effect on speed?   I recently flew my aircraft with full aft CG and still my elevator was TE up in relation to the stab.  Seems to me they should be in line with one another in cruise for maximum speed, especially at aft CG.  However I have been told here that they all fly TE up and that it is of little penalty.

On a Rocket, should the cowl flaps be closing all the way?  I have seen 3 of them and on all of them the cowl flaps stay open about 1-2 inches when in the closed position... this seems like it would cause some extra drag.

Has anyone quantified the speed gain from switching from the towel bar antenna on the vertical to the plate style?

Many of my access panels below the wing have somehow become concave... I have no idea how this has occurred, but I cant help but think they are causing extra drag.  Would it be worth replacing these panels with ones that are not concave OR removing and working the existing ones so that they are flush with the wing.?

 

I feel like all these things alone make little difference, but when you start to add them up, perhaps a 5-10 knot gain could be had?  I seem to be about 10 under book numbers and these items are the only things I see that might account for that ( well that or optimistic marketing)

[Blue on Top]

4 minutes ago, Austintatious said:

1. Does anyone KNOW how much extra speed the inner gear doors are worth?  My rocket does not have them and I am considering getting them and installing them...  but I would like to know if it is WORTH it or not. To me, worth it would be a 3+ knot gain

2. Is it really true that the elevator flying slightly TE up has no effect on speed?   I recently flew my aircraft with full aft CG and still my elevator was TE up in relation to the stab.  Seems to me they should be in line with one another in cruise for maximum speed, especially at aft CG.  However I have been told here that they all fly TE up and that it is of little penalty.

3. On a Rocket, should the cowl flaps be closing all the way?  I have seen 3 of them and on all of them the cowl flaps stay open about 1-2 inches when in the closed position... this seems like it would cause some extra drag.

4. Has anyone quantified the speed gain from switching from the towel bar antenna on the vertical to the plate style?

5. Many of my access panels below the wing have somehow become concave... I have no idea how this has occurred, but I cant help but think they are causing extra drag.  Would it be worth replacing these panels with ones that are not concave OR removing and working the existing ones so that they are flush with the wing.?

6. I feel like all these things alone make little difference, but when you start to add them up, perhaps a 5-10 knot gain could be had?  I seem to be about 10 under book numbers and these items are the only things I see that might account for that ( well that or optimistic marketing)

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.

[Cargil48]

35 minutes ago, Austintatious said:

On a Rocket, should the cowl flaps be closing all the way?  I have seen 3 of them and on all of them the cowl flaps stay open about 1-2 inches when in the closed position... this seems like it would cause some extra drag.

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)

[Austintatious]

7 minutes ago, 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)

EVen fully closed, there is still room for air to get out... it is just that the doors are flush with the rest of the nose.  Opening them just makes a bigger hole.

I have had that same thought... but I have never seen it said that they should not close all the way.   I am simply looking for other Rocket owners to chime in with how much their cowl flaps close.

[Austintatious]

32 minutes ago, 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.

Thanks... on the note about the step... has anyone ever quantified how much speed removing it is worth?

[Cargil48]

1 minute ago, Austintatious said:

EVen fully closed, there is still room for air to get out... it is just that the doors are flush with the rest of the nose.  Opening them just makes a bigger hole.

I have had that same thought... but I have never seen it said that they should not close all the way.   I am simply looking for other Rocket owners to chime in with how much their cowl flaps close.

Is there any gap between the engine compartment's cowling trailing edge and the beginning of the fuselage? If not, where does the hot air go out of the engine's compartment when the cowl flaps are fully closed and flush with the rest of the frame?

[kortopates]

Some years ago, I spoke with the Continental Engineer that worked with Mooney to work out the bugs with the original K model. They actually spent some serious effort researching this topic because initially they had the cowl flaps closing too far. To paraphrase perhaps a bit, he explained that closing the cowl flaps too far down actually caused a turbulent reverse airflow out of the cowling that significantly reduced airspeed by several knots. When they opened up the closed position of the cowl flaps as they have specified in the service manual they got several more knots and of course improved cooling. In sense the added drag of the cowl flaps being a bit open was optimal compared to eliminating the larger drag from the turbulent airflow caused up front at the prop from closing the cowl flaps too tight. 

Edited December 6, 2019 by kortopates

[EricJ]

41 minutes ago, Blue on Top said:

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.

So a cool thing to have would be a plate antenna on a swivel axle mounted such that the antenna doubles as an angle of attack indicator.    It minimizes the drag of the antenna and provides an AOA estimate from relatively clear air (I'm always suspicious of the ones mounted near or under the wing).

[kortopates]

28 minutes ago, Austintatious said:

Thanks... on the note about the step... has anyone ever quantified how much speed removing it is worth?

I didn't qualify it with any data, but after adding one it wasn't noticeable. If I was flying a Cafe Mooney for speed and efficiency trials of course it wouldn't have been added. But it just took one time to disembark with snow/ice on cold ramp to make me feel like a idiot for not having it. Although I didn't get hurt, it was clear to me how easy one could slip falling back on to the flaps (forget about the bruised ass of the pilot - it'll heal! but possibly not my flaps  Anyway supposedly its in 1-2 knots penalty and I think 2 is even exaggerated but for an avid skier it was really worth it to me and my wife. Its not very hard to remove it and collect your own data but Mooney has said the drag reducing cover is very effective. 

Edited December 6, 2019 by kortopates

[kortopates]

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.

[Cargil48]

1 hour ago, kortopates said:

Some years ago, I spoke with the Continental Engineer that worked with Mooney to work out the bugs with the original K model. They actually spent some serious effort researching this topic because initially they had the cowl flaps closing too far. To paraphrase perhaps a bit, he explained that closing the cowl flaps too far down actually caused a turbulent reverse airflow out of the cowling that significantly reduced airspeed by several knots. When they opened up the closed position of the cowl flaps as they have specified in the service manual they got several more knots and of course improved cooling. In sense the added drag of the cowl flaps being a bit open was optimal compared to eliminating the larger drag from the turbulent airflow caused up front at the prop from closing the cowl flaps too tight. 

This what you mention is called "heat build-up" inside the engine compartment. Plus a huge aumount of air pressure inside it since the air comes in up front to cool the cylinder heads and has afterwards nowhere to go... Hence that turbulence you mention which causes a drop of several knots. No wonder, I'd say.

PS: That's why in any reciprocating engine equipped plane there is the need to be somewhere a gap to let that air out. And more: Since heated air expands, it can be used to the benefit of the speed. I remeber reading many years ago that on the P-51 the cooling effect of that big belly radiatior was good for a 50lb of thrust...

Edited December 6, 2019 by Cargil48
Adding a PS

[PT20J]

1 hour ago, Cargil48 said:

It would accumulate inside the engine compartment, right? What goes in must come out...

At the speeds we fly, air is (to good approximation) incompressible, so it won't accumulate. It won't go in if it can't get out.

[Cargil48]

54 minutes ago, PT20J said:

At the speeds we fly, air is (to good approximation) incompressible, so it won't accumulate. It won't go in if it can't get out.

Maybe... so how do you keep your engine cool if the air won't go in?...

[Austintatious]

I would very much like to see the maintenance manual information on cowl flaps if anyone has it.

FWIW, if the cowl flaps did close all the way, there is a large area around the exhaust pipes that air could still flow out of.  they do not seal 100% even when fully closed.

[Cargil48]

https://www.avweb.com/features/leading-edge-21-remember-your-cowl-flaps/

[PT20J]

15 minutes ago, Cargil48 said:

Maybe... so how do you keep your engine cool if the air won't go in?...

The air that flows out through the cowl flaps obviously came in through the inlets. The cowl flaps regulate the air flow. “Extra” air does not flow into the cowling and accumulate. 

Take an empty tin can and punch a hole in the bottom. Hold the open end under a faucet and turn the water on. Water flows into the can and some flow out through the hole. When the can gets full, what happens to the extra water? Air works the same way.

Edited December 6, 2019 by PT20J

[Cargil48]

4 minutes ago, Austintatious said:

I would very much like to see the maintenance manual information on cowl flaps if anyone has it.

FWIW, if the cowl flaps did close all the way, there is a large area around the exhaust pipes that air could still flow out of.  they do not seal 100% even when fully closed.

Look, saying again that I'm, "squawking" as a layman (from the pilot's point of view), the system is this: Your have a four to six cylinder engine that needs a lot of air to cool down all the cylinders if you want to keep to the predicted TBO. You reguloate this either through the mixture (cooling with fuel) or through the cowling flaps aperture (more or less, keep an eye on your CHT instrument). For me it is logic that if you have air available (which is there for free...) you won't use fuel to cool the engine, right? Way too expensive. Now, where does the incoming air go against, after travelling all the way through the cylinder heads? Correct, against the firewall. And this means drag, a lot of drag... Or, as it is done for quite some time, you deflect the air travel downwards to avoid it hittiung the firewall which as we all know is a wall sitting there full way against the slipstream. If as you say the openings for the exhaust pipes (if you have two of them, of course) are wide enough, the cooling air can be directed there. And in this case one uses the cowl flaps only when the engine is reved up for full power, or if the surrounding air is warm. But one thing is sure: If you get more air inside the engine department as can get out, you have that loss of speed referred in the post above. 

Regards,

CarlosG.

[Cargil48]

12 minutes ago, PT20J said:

The air that flows out through the cowl flaps obviously came in through the inlets. The cowl flaps regulate the air flow. “Extra” air does not flow into the cowling and accumulate. 

Take an empty tin can and punch a hole in the bottom. Hold the open end under a faucet and turn the water on. Water flows into the can and some flow out through the hole. When the can gets full, what happens to the extra water? Air works the same way.

Except for one thing, my friend: The extra pressure you create which makes the excess water not going in and turning around the tin can translated to aviation is called one horrible word: drag...

[Hank]

Regardless of how far the cowl flaps close, the open area needs to be slightly larger than the intake area on the cowl, else you won't have good flow. Bad flow creates drag . . . High power operations require more flow, so a larger exit. In low power cruise, the exit can be reduced but must still be larger than the intake.