Mooney Aerodynamics

[Cargil48]

I had obviously something to say aobut what PT20J and Blue on Top said in response to my above post. And yes, I've in the meatime read the EAA site as well as two or three others, all saying the same: how the cowl flaps regulate the flow of air inside the cowling. But that we know from day one...  And I'm not speaking about after market "modifications" on the cooling flow of a given airplane, I was just and only speaking about the theory, since this thread is about aerodynamics. I will not bother you more with this subject.

[Blue on Top]

Personally, Cargil48, I like your comments and insights.  It is thought like yours that have the potential to advance our industry.

[PT20J]

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

[Blue on Top]

1 minute ago, PT20J said:

... see if it makes a speed difference. Then add them back one at a time and record what each one costs. 
Skip

It will definitely make a difference. Drag on each one will be challenging. 

[carusoam]

There are a few Long bodies with fixed steps...

Now some have two fixed steps...

Getting a retractable step would take detailed data gathering... as a first step...

Best regards,

-a-

[Cargil48]

14 hours ago, Blue on Top said:

Personally, Cargil48, I like your comments and insights.  It is thought like yours that have the potential to advance our industry.

Thank you very much, Sir!

[PT20J]

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 C_L 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 V_s1/V_so 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

 

 

Edited December 9, 2019 by PT20J

[Blue on Top]

7 hours ago, PT20J said:

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 C_L 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 flapsinces on many other airplanes. From POH data, here are the V_s1/V_so speeds (KCAS) for various airplanes at gross weight:

M20J  63/56PA24-250  62/54

A-36  62/58

C-182T  54/49

C-172S  53/48

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

@PT20J <-- I like this ; thanks!  (I've re-written this post 20+ times)  Now I'll try to answer what you asked - aileron effectiveness (or lack thereof).  The Mooney wing uses a flap to aileron ratio of  ~70/30.  This is fairly typical.  More modern designs use higher ratios (to lower stall speeds).  As a result they have smaller ailerons, smaller ailerons with roll spoilers or no ailerons with all roll spoilers (MU-2, another Mooney product).

You are correct in that hinge moment in = hinge moment out, but both ends of that equation can be tailored.  IOW, at the pilot input end, the yoke diameter can be made larger or smaller and travel more or less.  At the aileron end, the input arm can be made shorter or longer (which will change the internal system travels) and the hinge line can be moved fore and aft.  The Mooney hinge line is very close to the front (higher hinge moments), and the Cirrus hinge line is near 40% (no hinge moments - the control feel is all springs).  There are pros and cons to both.  In addition the aileron leading edge can change hinge moments dramatically (more elliptically pointy -> higher and more bread loaf -> lower hinge moments).  Also note that making internal control system travels less, makes internal control forces/load higher … and we have to account for 2 gorillas on the yokes.

Another note is that there is a single, small-diameter, very long pushrod that activates the aileron.  This pushrod goes from side of body out to the aileron bellcrank.  Personally, I don't think that this would be allowed today … especially control system buckling prevented by clearance holes in the nose ribs.  Don't get me wrong, it is simple, light and works well.  This is one reason the "Predator" wasn't the best airplane.

Bottom Line: 1) Al and Art did a great job with aircraft layout and planform.  2) The M20 has grown significantly since the early 50s, but control forces/effectiveness/harmony can be adjusted … some will take more certification work than others  

Edited December 10, 2019 by Blue on Top

[PT20J]

2 hours ago, Blue on Top said:

Another note is that there is a single, small-diameter, very long pushrod that activates the aileron.  This pushrod goes from side of body out to the aileron bellcrank.  Personally, I don't think that this would be allowed today … especially control system buckling prevented by clearance holes in the nose ribs.  Don't get me wrong, it is simple, light and works well.  This is one reason the "Predator" wasn't the best airplane.

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

[carusoam]

Possibly Need to find drawings for the plastic bearing/supports on the control systems for comparison...

Best regards,

-a-

[Cargil48]

5 hours ago, PT20J said:

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.

(...)

Skip

Why not make push/pull tubes out of stiff kevlar material and make them go through three or four precise ball bearings? These would serve as support but with minimum resistance (friction).

PS: If you want to save weight and put in smaller bearings, use needle type bearings instead of ball bearings.

Charles

 

Edited December 10, 2019 by Cargil48
Adding a PS

[Cargil48]

About that "cowling flap" debate, I have thought a lot about what has been said and finally understood what PT20J (Skip) wanted to say. But there is more to it, so if you want to debate the matter a bit more in-depth, please say so. I don't want at all to be boring, here...

Charles

Edited December 10, 2019 by Cargil48

[Blue on Top]

Ut oh, I'm getting too comfortable with MS already.  I'm going to try to move this topic to a new thread (it's not really directly aero related.

So, @Cargil48, @carusoam, @PT20J, and those stalking in the shadows (who I appreciate, too).  Look for the new topic "Aileron Push-Pull Control Tube"

[PT20J]

5 hours ago, Cargil48 said:

About that "cowling flap" debate, I have thought a lot about what has been said and finally understood what PT20J (Skip) wanted to say. But there is more to it, so if you want to debate the matter a bit more in-depth, please say so. I don't want at all to be boring, here...

Charles

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

[PT20J]

14 hours ago, Blue on Top said:

The Mooney wing uses a flap to aileron ratio of  ~70/30.  This is fairly typical. 

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

[EricJ]

I took this pic this weekend, and it shows a little flap on top of the hood that helps draw air through the radiator.   Pressure difference makes fluid flow, and it doesn't really matter whether the pressure difference comes from increasing the input pressure or decreasing the outlet pressure, either will increase the flow.    In this case the splitter (the bottom surface extending in front of the bumper) increases the pressure in front of the radiator by reducing the amount of flow under the car.   The increase in pressure is used to produce downforce on the splitter surface and also helps push air through the radiator, brake ducts, into the intake, etc.

The little flap on top of the hood just behind the radiator helps create a little bit of low pressure behind it, which helps draw air from behind the radiator.   This increases the pressure difference from the inlet to the outlet, which increases flow.  On many cars instead of a flap there's just a sheet metal lip that extends a half-inch or so into the air flow on the leading edge of the radiator outlet in the hood.   Some cars run smooth undertrays or extend the splitter under the car (to about the front axle line) in order to reduce overall drag.   When that is done it is pretty common to exhaust the radiator outlet air out the top of the hood.    The downside there is any leak or spray from overheating winds up on the windshield right about when you need it least.

Input cowl flaps, like the Yak example posted and the barge board F1 example, decrease the pressure behind them at the inlet when they're closed or restricted, reducing flow.   On a radial engine with a large inlet, this may be all that's needed to fully control the cooling air.   Output cowl flaps, like we have on most Mooneys, controls outlet pressure by opening or closing, and can produce low pressure behind them when fully open to increase cooling air flow at the expense of some drag.   The downside of the output flap is that they extend into the airstream and create drag, but the upside is that you get controllable increased pressure differential across the engine when needed.

Augmentor tubes (like on old C310s, and some T-34s), use exhaust gas energy to decrease the pressure at the cowl outlet and increase flow at the expense of the occasional tube fire when they get oily and there's an exhaust backfire.   Everything has a tradeoff. 

 

[Cargil48]

36 minutes ago, EricJ said:

I took this pic this weekend, and it shows a little flap on top of the hood that helps draw air through the radiator.   Pressure difference makes fluid flow, and it doesn't really matter whether the pressure difference comes from increasing the input pressure or decreasing the outlet pressure, either will increase the flow.    (...)

The little flap on top of the hood just behind the radiator helps create a little bit of low pressure behind it, which helps draw air from behind the radiator.   This increases the pressure difference from the inlet to the outlet, which increases flow.  On many cars instead of a flap there's just a sheet metal lip that extends a half-inch or so into the air flow on the leading edge of the radiator outlet in the hood.  (...)

Input cowl flaps, like the Yak example posted and the barge board F1 example, decrease the pressure behind them at the inlet when they're closed or restricted, reducing flow.   (...)

 

Here one can see very clearly what you say, Eric. But this is NOT my point... This explains how to increase the outflow of air going through the radiator, by creating a low pressure area with said lip, creating a "vacuum" which in turn "sucks out" the air through a succion effect. But here, in cars, you want always maximum airflow for augmenting the cooling effect. On the airplanes the wish for that cooling effect is, as we all know, not constant, it varies, and THIS is what I will analyze next.

Edited December 10, 2019 by Cargil48
Correcting a word

[Cargil48]

"Taken from an article in an engineering internet page:

"Air Cooling: The cylinders on early opposed engines were also stuck out into the air stream to provide cooling. • However since the cylinders were directly behind each other, a thin sheet metal hood had to be installed on each side of the engine to force air down between the cylinder fins. Now all opposed engines also have cowlings that surround the entire engine. With this type of cowling, cooling air enters through forward facing openings and exits out one or more openings in the bottom rear of the cowling. The rest of the cowling is sealed with rubber strips to prevent excessive air leakage.

Because of the ram effect produced by the forward motion and prop wash, cooling air enters a cowling at a pressure above ambient. This produces what is known as pressure cooling. To facilitate the pressure cooling process, the outlet on lower cowls is flared so that when the outside air flows past the opening, an area of low pressure is created in the bottom of the cowling. This low-pressure area draws air down through the cylinders and into the lower cowl where it can exit the cowling."

This is what happens when the lower cowl flaps are opened. A continuous flow of air enters the engine compartment, cools what needs to be cooled, and exits thyrough the lower part of the cowling. This we all know and is not subject to discussion. 

To be followed.

[Cargil48]

Now let's close the lower cowl flaps with the intention to stop the cooling of the engine. The incoming air has nowhere to go, right? It builds up pressure inside the engine department and when that pressure surpasses the pressure of the incoming air, blocks it and this is where PT20J/Skip says "No problem, the air then goes around the exterior of the cowling."  So far, so good, but I ask: At what cost? At the cost of "a wall" stemming against the airflow wanting to get through the front openings and blocked by high pressure air... And this meanhs drag, a lot of drag!

Let's go back to the example Skip mentioned before, the one of the cylinder shaped tin can with a small hole at the bottom: As I said already, take that tin can and hold i8t outside a car's window at 60 mph. What tremendous force you feel in your hand? The pressure of the air wanting to go in and not getting into... The air then goes around the tin can, but you feel always that tremendous aerodynamic induced force in your hand!

To be continued

Edited December 10, 2019 by Cargil48
Correcting a word

[Cargil48]

Now, with those movable front cowl opening flaps mentioned before, as the Yak has as you say, the air flow is going to be smooth! It enters, is deflected down at wish (partially or fully) and exits the engine department again, WITHOUT any high pressure area blocking it. The upper cowl flap can be actuated manually or automatically, driven by a thermostat, just like in our everyday cars. Now tell me, by substituying a thick amount of compressed air with a steady air flow with much less interfearing stuff, how many knots will you gain??

End

[afward]

I'm no (aerospace-) engineer, but I don't see how merely putting a flap downstream from the intake is going to address the pressure spillage problem around the intakes.  To really have an effect, it would have to be at the intake itself such that the intake doesn't swallow any more air than the system intends to allow through.

A movable spike in a round inlet is an obvious solution, but for our planes that would be problematic due to the closeness of the back of the prop.  We could in theory build a circumferential intake with movable ends (it would look like the CAFE Mooney's intake), but that has weight and complexity concerns.

I've thought on many occasions that a large NACA inlet on each cowl cheek would probably perform better than what we have now, and would be throttle-able with a reduced drag impact, but surely people far more knowledgeable and capable of testing it have already thought of it and rejected it for some really good reason I'm not aware of...

[Cargil48]

2 hours ago, afward said:

I'm no (aerospace-) engineer, but I don't see how merely putting a flap downstream from the intake is going to address the pressure spillage problem around the intakes.  To really have an effect, it would have to be at the intake itself such that the intake doesn't swallow any more air than the system intends to allow through.

That's not the point, not to "swallow any more air than the system intends to allow through." My point is, as repetedly said, to direct the incoming flow to where it has to go. Straight in, if full cooling is wanted. Hlafway down, if only partial cooling (of the cylinder heads) is intended. or fully down, if we want the cylinders to get any cooling air.

PS: I have to make an edit because the last part is not correctly written. It must be as follows:

"My point is, as repetedly said, to direct the incoming flow to where it has to go. Straight in, if full cooling is wanted. Halfway down, if only partial cooling (of the cylinder heads) is intended. or fully down, if we don't want the cylinders to get any cooling air.

Edited December 10, 2019 by Cargil48

[Cargil48]

I've finally come upon a design that suits my thoughts. This one shows a forward baffle which directs the incoming air upwards, making the air then go down the cylinder heads. Now, imagine a system that would make this baffle movable, fixed to a axis, so as to make it swing up 45º then to neutral and wanting to divert the air down, swinging fully up, to 45º . Now, in the shown diagram, we see it at down 45º.  This to prove that it nis possible to install a baffle in the space fore of the cylinders.

[afward]

Wow, I missed what you were saying there.  Makes sense, but I still see issues:

1) It doesn't take a lot of bypass air to completely wreck cooling flow from upper to lower deck.  I'd think you'd want to duct the bypass air down to the outlet so this isn't an issue.  That has drag implications.

2) You're always going to flow the "same" amount of air at a given airspeed, which means the flowrate at cruise will be quite a bit higher than necessary (because sizing is controlled by Vx climb cooling requirements).  That also has drag implications.

3) Both the above items mean substantial drag at higher speeds, with spillage being a possibility if conditions are right.

In theory, cowl flaps sidestep all of that by allowing the inlet & outlet to be sized for cruise, while still allowing for enough flow to cool during a Vx climb.

Now, someone with a CFD model can probably work through all this and come up with a working design.  What I don't know is if it'll actually be any better than what we have today.

[Cargil48]

10 minutes ago, afward said:

What I don't know is if it'll actually be any better than what we have today.

That is exactly the aim of the entire subject I am posting here. Why? Because of the effecti I am constantly pointing at, of the air inside the engine department having nowhere to go when the cowl flaps are closed and the associated drag that implies. I must again point to Skip's example of the tin can with a small opening at the bottom and what I have been saying about that...For me that represents a lot of drag, but...