Mooney tail aerodynamics - not backwards

[PMcClure]

You can buy the 301 from Socata , the 301 was sold to Socata and produced as the TBM700 .... True story... 

I know, but the 301 didn't look like it would cost 2 million!

[201er]

Yeah. I bet ya if they started rolling out 301s back in the 80s, a nice used one could go for under $200k these days. Heck of a plane. Much better fuel to speed ratio than the TBM!

[Alan Fox]

The 301 was a miserable failure until they decided to put a turbine on it

[markm20b]

I believe another reason for the tail design was due to the pivot point of the trim system.  A swept back vertical fin would put more stress on the pivot point and require a stronger (heavier) design.

[bumper]

I believe another reason for the tail design was due to the pivot point of the trim system.  A swept back vertical fin would put more stress on the pivot point and require a stronger (heavier) design.

I respectfully disagree. In terms of dynamic loading, if the vertical fin in left the same height (length), then sweeping it back will decrease the flat plate area (e.g. if swept back 90 degrees it would theoretically have no flat plate area). So, if one wanted to have a fin/rudder combination that has the same height, sweeping it back will mean the need to extend it's length. This would obviously increase wetted area which would increase drag. I recall reading somewhere that swept surfaces really have no advantage at subsonic speeds.

 

If you are primarily considering static loading, i.e. having the weight of the fin and rudder cantilevered further aft behind the pivot, I agree that  would increase static load. However, and without any tests and being just a WAG, the added stress should be insignificant as compared to the dynamic load the pivot must be designed to withstand.

 

Bottom line, any change in loading due to sweeping the fin/rudder back would be of little consequence. I think Al did it that way as he knew there was no aerodynamic advantage in sweeping it back and there was some advantage in the flare of not doing so. 

 

bumper

[RobertE]

I honestly think we're all over thinking this. Imagine you're a small business owner trying to build a business without running out of money ( which Mooney has now done twice). If you could build a rudder that needed the same tooling and manufacturing processes as the elevator with only modest diminution in performance and could produce a distinctive profile to boot, what would you do? That is what, I believe, Mr. Mooney did. Modest diminution in performance is probably something I'll be challenged on, but the trade off in what he did, it seems to me, is accepting an elevator that has a symmetrical cross section. I THINK that requires a greater angle of attack to produce the needed lift. But it saved money.

I love my Mooney but think a few compromises were made. I don't blame Al.

[jetdriven]

Except the tail produces downforce and no airfoil shape is needed

[Robert C.]

"Everybody knows that aircraft handling tends to go critical at a high angle of attack. So I gave the vertical fin the straight up-and-down lines it still has today. Few people realize this was not a style gimmick."
-- Al Mooney, quoted by Gordon Baxter in the MAPA Log, March 1997

 

I knew I had read that somewhere and finally found it

 

Source: http://www.mooneyevents.com/quotes.htm

 

Robert

[Bob_Belville]

And here's the discussion I remember reading. It's from Larry Ball's "Those Remarkable Mooneys" chapter heading: Mooney Lore" p. 237 "Perky Tail"

 

"At least that's the way one woman described it in a letter to Mooney. Others said the tail is on backwards. In truth, it is a variable incidence tailplane with the rudder swept forward. (The Lockheed JetStar also used a variable-incidence tailplane.) Al Mooney felt the swept forward rudder would remain effective in a stall long after an aft swept fin and rudder would lose effectiveness. Here again, talk with enough aerodynamicists and you will probably find one that will agree with Al Mooney.

 

" Ralph Harmon designed the M-22 Mooney Mustang with the same distinctive Mooney tail. All production units carried this tail, but Ralph did, later on, build and flight test an M-22 with an aft swept T-tail. It's entirely possible Ralph was not convinced of the "perky tail's" value. Much later, Roy Lopresti designed and flew the Mooney 301 with a conventional swept back tail of 50 degrees. He also used conventional trim tabs on the elvator anf rudder. Although the 301 was never produced, LoPresti must have felt there was no particular advantage in staying with Mooney's trademark tail. Much later, in 1996, he told me that if he had it to do over he would have stayed with the tradition and used the Mooney tail.

 

"Probably the safest ground to stand on is to consider the Mooney tail a trademark... and leave it at that."

 

That's the end of Ball's story. Ball was a Beech guy through and through and I would not think he is completely objective in matters where Al Mooney's ideas departed conventional wisdom as understood in Wichita . 

[JohnB]

You need to read up on Al Mooney! It's an M20...you do know what that means, right?

Ok. Ill bite, M must stand for mooney, not sure what the 20 is for, FAA type designation perhaps, but what happened to M 1-19? Do you know Bob?

This is all great info everyone, very educational.

[Bob_Belville]

Ok. Ill bite, M must stand for mooney, not sure what the 20 is for, FAA type designation perhaps, but what happened to M 1-19? Do you know Bob?

This is all great info everyone, very educational.

The M20 was Al's 20th aircraft design. So... we suppose whatever he did was on purpose.

[JohnB]

Thanks Bob! at your inference, Just looked up the story of Al Mooney and is other 19 designs, very interesting, if anyone else (other than me before today) hasn't yet read this, it's interesting!

http://www.mooneyevents.com/review.htm

[PT20J]

On 8/19/2013 at 7:02 PM, JohnB said:

Ok, I thought of this and couldn't come up with an answer. Does anyone know why our tail design (straight leading edge) compared to a sloped leading edge on non-Mooney airplanes is more aerodynamic or efficient?  If it is more efficient, I wonder why havent airlines or other commercial carriers adopted it, because they think it would look funny to passengers or? If someone has a link or reference that someone has done wind tunnel tests to prove it that would be great, or is this just a signature mooney trademark? Not that Im complaining about our cool tail, but would like to know why, if anyone knows. Or if this is one of those "it's just better because someone did a test" with no physics explaination answers, I can go with that too.

I thought I'd resurrect this old thread because the original question gets to the core of a misunderstanding about the Mooney tail feathers that I see on MS from time to time. It turns out  that a vertical stabilizer with a vertical leading edge is more effective than one with a swept back leading edge, and this allows it to be of smaller area which in turn reduces the weight and drag. The Mooney tail design wasn't greatly different from other airplanes back when Al designed it - just a bit more angular. It wasn't until around 1960 I believe that Cessna started the swept fin craze making the Mooney tail appear backwards. The interesting question is why is the vertical leading edge design more efficient?

I has to do with the angle that the relative wind flows with respect to the chord line of a wing (aerodynamically, the vertical stabilizer is a vertically mounted wing with a symmetrical airfoil). The cord line is perpendicular to the leading edge of a wing. In the case of a vertical leading edge, the relative wind flows parallel to the chord line. This airflow produces the necessary aerodynamic side force  ("lift" - think Bernoulli, Newton) whenever a yaw creates an angle of attack. Now consider the swept back leading edge. The chord line is still perpendicular to the leading edge, but the relative wind is now at an angle to the chord line. Some of the relative wind will flow parallel to the chord line and some will flow spanwise. The spanwise component is "wasted" in the sense that it doesn't produce any aerodynamic force. So, the useful component of the relative wind is lessened. Aerodynamic force is a function of airspeed, angle of attack and area. To achieve the same aerodynamic force at the same airspeed and angle of attack, the swept tail will need to be larger than the straight tail. Drag is also a function of  area, so as the area increases, so will the drag. I believe that Bill Wheat said in Boots on the Ground that Al calculated that a swept tail would need to be about 20% larger.

So, if the swept tails are just for show on small planes, why do jets have them? Jets fly fast and recall that the air flowing over a wing accelerates. When the airspeed of this accelerated air gets close to the speed of sound, the drag goes way up. The rapidly increasing wave drag as the aircraft approaches the speed of sound is why it used to be caused the "sound barrier". By applying sweep, the wing sees reduced airspeed of the component of relative wind along the chord line, and this reduces the drag. Sweep is applied to all the airplane's "wings" (wing, horizontal and vertical stabilizers) for this reason.

Skip

[carusoam]

+1 for quoting Bill Wheat and Boots... 

Best regards,

-a-

[0TreeLemur]

On 8/20/2013 at 12:29 PM, kmyfm20s said:

I agree the tail will always look like a Mooney tail but it also changes position. Take a look at this article and look at the tail with this plane only 90 kts. I didn't intend to overstate the sweep portion of the tail but those slight changes have a impact on the aerodynamics. http://www.mooney.org.au/files/Up_to_Speed_Mooney.pdf

Broken link.   The google machine doesn't find it.  Does someone having a copy of this file upload it to our files section, if not copyrighted?  THX.

[0TreeLemur]

10 hours ago, PT20J said:

I has to do with the angle that the relative wind flows with respect to the chord line of a wing. The cord line is perpendicular to the leading edge of a wing....

I  I believe that Bill Wheat said in Boots on the Ground that Al calculated that a swept tail would need to be about 20% larger.

Skip

This is a great point- well said.    The wing sweep angle of a surface is defined as that angle formed by the longitudinal axis of the aircraft and the 25% chord line of the surface.   By that definition, both the wings and vertical stabilizer on a M20 are swept forward.

[Shadrach]

15 hours ago, PT20J said:

I thought I'd resurrect this old thread because the original question gets to the core of a misunderstanding about the Mooney tail feathers that I see on MS from time to time. It turns out  that a vertical stabilizer with a vertical leading edge is more effective than one with a swept back leading edge, and this allows it to be of smaller area which in turn reduces the weight and drag. The Mooney tail design wasn't greatly different from other airplanes back when Al designed it - just a bit more angular. It wasn't until around 1960 I believe that Cessna started the swept fin craze making the Mooney tail appear backwards. The interesting question is why is the vertical leading edge design more efficient?

I has to do with the angle that the relative wind flows with respect to the chord line of a wing (aerodynamically, the vertical stabilizer is a vertically mounted wing with a symmetrical airfoil). The cord line is perpendicular to the leading edge of a wing. In the case of a vertical leading edge, the relative wind flows parallel to the chord line. This airflow produces the necessary aerodynamic side force  ("lift" - think Bernoulli, Newton) whenever a yaw creates an angle of attack. Now consider the swept back leading edge. The chord line is still perpendicular to the leading edge, but the relative wind is now at an angle to the chord line. Some of the relative wind will flow parallel to the chord line and some will flow spanwise. The spanwise component is "wasted" in the sense that it doesn't produce any aerodynamic force. So, the useful component of the relative wind is lessened. Aerodynamic force is a function of airspeed, angle of attack and area. To achieve the same aerodynamic force at the same airspeed and angle of attack, the swept tail will need to be larger than the straight tail. Drag is also a function of  area, so as the area increases, so will the drag. I believe that Bill Wheat said in Boots on the Ground that Al calculated that a swept tail would need to be about 20% larger.

So, if the swept tails are just for show on small planes, why do jets have them? Jets fly fast and recall that the air flowing over a wing accelerates. When the airspeed of this accelerated air gets close to the speed of sound, the drag goes way up. The rapidly increasing wave drag as the aircraft approaches the speed of sound is why it used to be caused the "sound barrier". By applying sweep, the wing sees reduced airspeed of the component of relative wind along the chord line, and this reduces the drag. Sweep is applied to all the airplane's "wings" (wing, horizontal and vertical stabilizers) for this reason.

Skip

Excellent explanation. The only thing I would empathize is that the control advantage of the vertical tail and the disadvantage of the swept tail are both multiplied as the aircrafts AOA increases and airspeed decreases.

Edited April 27, 2019 by Shadrach

[Andy95W]

Before he died, Gordon Baxter had a great story about this concept.  When he was interviewing Al Mooney in preparation for writing The Al Mooney Story, Al sketched the tail of the M18 and M20 at high angles of attack.

The important concept wasn't so much the leading edge of the stabilizer, but the swept forward hinge line of the control surface.  As Skip described, the swept tail is largely wasted because what's important is the perpendicular component of the relative wind.

Gordon said that Al sketched something like this:

 

[Bob_Belville]

Before he died, Gordon Baxter had a great story about this concept.  When he was interviewing Al Mooney in preparation for writing The Al Mooney Story, Al sketched the tail of the M18 and M20 at high angles of attack.
The important concept wasn't so much the leading edge of the stabilizer, but the swept forward hinge line of the control surface.  As Skip described, the swept tail is largely wasted because what's important is the perpendicular component of the relative wind.
Gordon said that Al sketched something like this:
 

Right or wrong, that’s the explanation I am familiar with. It gives the most rudder effect per square foot.


Sent from my iPad using Tapatalk

[Alan Fox]

Funny , I see LOTS of modern Vee Tail designs flying , but not a single modern design with a straight vertical tail..... Wonder why....

 

[Alan Fox]

Too add insult to injury.....https://en.wikipedia.org/wiki/Category:V-tail_aircraft

[EricJ]

2 hours ago, Alan Fox said:

Funny , I see LOTS of modern Vee Tail designs flying , but not a single modern design with a straight vertical tail..... Wonder why....

 

Trolling, trolling, trolling.   Keep them chickens trolling, RAWHIDE!    *crack*

 

[JohnB]

On 4/26/2019 at 8:34 PM, PT20J said:

 To achieve the same aerodynamic force at the same airspeed and angle of attack, the swept tail will need to be larger than the straight tail. Drag is also a function of  area, so as the believe that Bill Wheat said in Boots on the Ground that Al calculated that a swept tail would need to be about 20% larger.

So, if the swept tails are just for show on small planes, why do jets have them? Jets fly fast and recall that the air flowing over a wing accelerates. When the airspeed of this accelerated air gets close to the speed of sound, the drag goes way up. The rapidly increasing wave drag as the aircraft approaches the speed of sound is why it used to be caused the "sound barrier". By applying sweep, the wing sees reduced airspeed of the component of relative wind along the chord line, and this reduces the drag. Sweep is applied to all the airplane's "wings" (wing, horizontal and vertical stabilizers) for this reason.

Skip

Well thought out explanation, makes perfect sense. Thanks Skip! 

[PT20J]

On 4/27/2019 at 6:51 PM, Bob_Belville said:

The important concept wasn't so much the leading edge of the stabilizer, but the swept forward hinge line of the control surface.

This is an interesting idea which I believe has merit, but I don't think the important parameter is the angle of the rudder hinge line. The hinge line is set at an angle of 25-deg relative to the leading edge of the fin and that's much more than any attainable angle of attack. But the shape of the stabilizer creates an effective forward sweep of 12.5 degrees measured at the mean chord line. I was simplifying when I earlier stated that the chord line is perpendicular to the leading edge -- it's more accurately perpendicular to the mean chord line. Taking the sweep into account, the optimum airflow occurs at an angle of 12.5 degrees nose up which is likely somewhere near the stall angle of attack. The required area of the stabilizer is set by the stability and control requirements at low speeds. It makes good sense to optimize the efficiency at the low speed portion of the envelope because this will minimize the area required and at cruise speeds, where a much smaller stabilizer would be required, any extra area just adds drag.

Skip

[Aerodon]

My 2c.    I don't think it has much to do with aerodynamics, but all to do with structure and weight.  It starts with the decision to have an all moving vertical and horizontal stabilizer.  Then to keep the loads on the hinge as low as possible, one needs to have the horizontal and vertical aerodynamic forces as close to the hinge as possible.  Best way to do this is keep most of the structural weight and aerodynamic forces as far forward as possible = straight leading edge.

Aerodon