Can anybody help, please?

[Lood]

Can anyone please help with the following question posted on the popular South African aviation site, Avcom? The airplane is a M20C with just about all the available 201 modifications installed:

http://www.avcom.co.za/phpBB3/viewtopic.php?f=9&t=71246

[peter]

Log in required. Can you repost the question here?

[Lood]

Ooops - sorry about that. Quoted from Avcom:

"Riveted onto the trailing edge of my Mooney's rudder is this little tubular thing. Does anyone have an idea what its function is?
 Nope. It's not a trim tab. There's one of those near the bottom of the rudder. This thing is a little tube about 8mm in diameter running along the trailing edge. It seems to be intended to disturb the airflow leaving the rudder for some reason."

From what I can gather from the picture he posted, it's a tubular tab, about 1" wide and about a feet long. It's installed halfway down and on the back of the rudder.It looks just like the normal trim tab on a Cessna, for instance, except that this one is not flat and it is longer.

I won't mind posting my login details here for anybody to be able to take a look at the picture, but unfortunately, I can't remember my password. Login is free though.

[Ned Gravel]

It's a pencil holder

And it has sparked its share of mystery and discussion over the years.

[scottfromiowa]

It's like the fake hood scoops on cars...only different.

[tony]

I'd love to see a picture, is it possibe to post a picture or a link to one?

[Barry]

I asked someone once about that little gadget and they told me that it was to dampen flutter of the rudder. You don't want your rudder to flutter ...  )

[scottfromiowa]

Not on a Mooney, but some rudders fluttering are indeed a wonder to behold.

[DaV8or]

Because the quote you posted mentions the existance of an aftermarket rudder trim tab, my guess is the tube is to dampen potential flutter caused by the trim tab.

[tony]

Quote: DaV8or

my guess is the tube is to dampen potential flutter caused by the trim tab.

[Immelman]

I've got one on my 66E as well.... you guys have me wondering.

 

[rob]

My 65C also has it. I think it houses the afterburner assembly...

[takair]

I think it's a variation of a Gurney Flap...although that name came later.  It is basically a bandaid to alter the characteristics of an airfoil without a complete redesign.  I suspect that without this, the rudder did not return to neutral after deflection.  It essentially had a dead band.  Many aircraft simply use a 90 degree piece of sheetmetal to accomplish the same thing.  I would guess Mooney used to the tube because it was more rigid.  They probably sized it through trial an error.  Others will have a wide trailing edge rather than a sharp trailing edge that would intuitively seem more aerodynamic.  I don't beleive it has anything to do with flutter, although it was probably in place for flutter testing.  Hope this helps.  If you Google Gurney Flap, you can find some more detailed description and the various applications--including race cars.....which is where the name comes from.

Here is one explanation:  http://en.wikipedia.org/wiki/Gurney_flap

 

[tony]

Quote: JimR

 

[scottfromiowa]

Tony: Photos go to 67E and the photos are listed 6720E...

[takair]

http://www.mooneyspace.com/index.cfm?action=gallery&userGallery=2346&currentAlbum=374

[MooneyMitch]

We have the same on the '65 E model.

[carusoam]

On the 65C, that is the American Flag holder.  You just place the dowel that is used as a flag pole in the opening at the top.

Very useful for Mooney Ambassador events.  Probably works well with the SA flag also.

Best regards,

-a-

[richardheitzman]

I'll ask Bill Wheat next time I am at Dugosh

[DaV8or]

So, you have to wonder, why doesn't the F and later models have this?

[takair]

Quote: DaV8or

So, you have to wonder, why doesn't the F and later models have this?

[Barry]

I think you may notice that the placment is a little different on different planes. Mine is very near the top of the rudder.

[Lood]

Thanks for all the replies. It seems that it certainly has to do with making the rudder more effective. Here's another quote from Avcom:

"Any trailing control surface is already beyond laminar flow . The thickened boundary layer will make the middle (centre) range of control ineffective . By thickening the trailing edge by any means will increase the effective angle of attack of a control surface in both directions resulting in more immediate control response."

Sorry, I don't really know how to post the picture and my GPRS internet's speed make adding pictures a very time consuming task.

[tony]

Well I asked a few people about the tab and I did get a lot of opinions but no one said “I know what that’s for”.  The best guess I got was that the tab is functioning as a rudder wedge.  A common design solution, for having a dead band at high speed, is to incorporate a wedge.  The test pilot probably complained of a rudder dead band at high speed cruise.  This dead band is cause by flow separation around the rudder.  Putting a wedge out there to increases the rudder effectiveness when the flow detaches.  However if the Mooney design team put a wedge out there, that would increase drag.  We all know that would have been a big verboten.  So they probably came up with the circular shape to minimize the parasitic drag which in the days if slide rules was pretty ingenious. 

So then I asked, “Why is this then only on some 65, 66, and 67 models and then the factory disconnected it with later models?” I got the deer in the head lights look.  So bottom line is we are where we started…sorry

[takair]

Quote: tony

Well I asked a few people about the tab and I did get a lot of opinions but no one said “I know what that’s for”.  The best guess I got was that the tab is functioning as a rudder wedge.  A common design solution, for having a dead band at high speed, is to incorporate a wedge.  The test pilot probably complained of a rudder dead band at high speed cruise.  This dead band is cause by flow separation around the rudder.  Putting a wedge out there to increases the rudder effectiveness when the flow detaches.  However if the Mooney design team put a wedge out there, that would increase drag.  We all know that would have been a big verboten.  So they probably came up with the circular shape to minimize the parasitic drag which in the days if slide rules was pretty ingenious. 

So then I asked, “Why is this then only on some 65, 66, and 67 models and then the factory disconnected it with later models?” I got the deer in the head lights look.  So bottom line is we are where we started…sorry