lml@lancaironline.net Mailing Lijsten Archief

Bill,

The intersection on the upper wing is crucial to the performance of the wing in reflex. Laminar flow on the properly smooth upper wing surface is over 60% of the chord. The reflexed portion of the flap allows for re-attachment of the airflow and a reduction in drag.

I have seen results of the laminar flow by flying through smokey air over the central states in September (some years ago) with some bugs on the leading edge. A stop in Boze on my way to the Labor Day factory fly in had me looking down the wing into the setting sun whilst I was tying down. I observed smoke particle marked V's emanating from bugs obtained on takeoff in Illinois and those V's easily extended back more than 60% of the chord before there was separation. If only I had taken pictures (uh, this was before digital cameras in everyone's smart-ass phones)

See the "Aerodynamics for Naval Aviators" manual for the consequence of wing dirtying particles being closer together than 1" resulting in loss of laminar flow.

In another aircraft I had obtained a "dog-bone" cross section shaped (along the chord) wooden prop that was more efficient and quieter than a more normal airfoil shaped prop. Why? Because the separated air reattached itself to the rise in the surface somewhat after the mid point.

I believe the reflexed wing accomplishes a similar reattachment on the upper surface.

Some builders may not treat the lower wing surface with proper respect in the area under the main spar where the D-section and the bottom skin are joined. It is crucial that this area be smooth (no waviness) because the flow attachment remains much further back than 60%. Indeed, the concave shape of the flap bottom may be important to the performance of the wing when at 0-reflex. Those of us that were trying to achieve max speed didn't really care whether the wing performed efficiently at moderate speeds ( I only considered two speeds - max cruise and approach to landing). Remember that one would prefer that the re-joined air at the TE meet at the same speed so no additional TE drag is introduced.

The aileron flat shape and squared off 1/4" deep TE is similar to those seen on jets. I believe this is for increased control surface effectiveness regardless of flap position. (BTW, I had mine both rigged slightly higher than the -7 degrees reflex - a separate discussion).

Anyway, perhaps flattening the bottom of the reflexed flap may reduce drag - it would be interesting to see comparable before and after data.

In the early days some people tried zig-zag tape to try to re-attach air to the lower wing surface. I don't think it was very successful. Such tape was used by glider folk for that purpose, but we are talking about significantly different air speeds and probably different thickness boundary layers.

The question would be if the drag induced by vortex generators exciting separated air would be less than the reduction in drag created by an such an attempt to re-attach the air flow? This as opposed to the change in the air flow at reflex by flattening the bottom of the flap.

Hmmmmmm ............

Back to my rocking chair now .....

Grayhawk

In a message dated 10/6/2013 7:45:10 A.M. Central Daylight Time, wfhannahan@yahoo.com writes:

Tom, thanks for sharing your work.

Just wondering if you, or anyone else, tried vortex generators in the concave wing/fuselage intersection behind the spar. The idea being to reduce drag by maintaining attached airflow in that region without the big mod.

Regards,

Bill Hannahan

wfhannahan@yahoo.com

From: "dudewanarace@yahoo.com" <dudewanarace@yahoo.com>
To: lml@lancaironline.net
Sent: Wednesday, October 2, 2013 5:49 AM
Subject: [LML] 320/360 performance and stability

Readers,

I have been watching this conversation with interest. I have some odd experiences with stability in my small tail 360 that was improperly built (not by me). Geez, that is going to hurt resale.. haha

Anyway, I have been waiting to comment as it will just fog the data that has been presented given my totally odd arrangement and lack of any real data. All of my 'data' is seat of the pants, thus not worth mentioning. But, figured I could expand on an envelope probably few have visited just for interesting reading.

This is my elevator angle with the airplane in a forward C.G. condition (header fuel only, single pilot), flaps in reflex and, well, going as fast as an RV-7 will go in formation. :)

http://www.n54sg.com/images/tuft_test_08.jpg

So, this problem poses a few issues.

First, this is obviously drag, probably a measurable amount thus for me the driving force behind correcting it one day. Second is the available elevator travel. If you read the manual, I have the correct amount of up / down elevator travel. But, if the photo is my starting point, it means I have much less up, and way too much down available to use. The important part being the elevator up while in the flare. Given a forward C.G. and a huge amount of flaps, this can be an issue. (ask me how I know...)

Next I would like to mention that not all small tails trim the same. Some use a spring system to bias the entire elevator. Others use trim tabs. I have a tab that due to its placement and odd elevator deflection has a limited functional envelope. Another driving force to change incidence.

Now the often obvious question people ask is why haven't I fixed it yet. Well, because the job of fixing it is going to totally suck, and I wanted it to be the last thing I do as my other aerodynamic changes may affect the angle of incidence. This brings me to the next subject, what I have changed.

So I have this airplane going faster than most and figured why stop now.. I made a rather drastic change that some call the beluga belly. It has been done to a few Legacys that race with varied applications of the same idea. Those familiar with the 320/360 fuselage will notice it in this picture:

http://www.n54sg.com/images/tuft_test_04.jpg

I'm working on a write up for my website detailing the project and its purpose and will have that posted sometime soon. But, I will report that this did change the downwash on my horiztonal and did change my required angle of incidence. Hence, I'm glad I waited to change that. It actually requires less up elevator than it did before so less negative incidence. The general theory is I have corrected some flow around the fuselage thus making the entire horizontal a bit more effective. Some modified Legacys experienced something similar. I only wish it would have corrected it more! I now know more about this mod and maybe would have applied it differently. Just not sure I'm willing to do the work again for unknown gains.

The other aerodynamic change I made (that relates to the original stability post) is I removed the cusp from the bottom surface of my flaps as suggested in a book about GA airfoils by Harry Riblett. Below is a simplified version of his drawing. (Not accurate, just for explanation purposes) The solid black is the modification.

http://www.n54sg.com/images/Flap_Drawing.jpg

So, what I have done to the camber of the wing is a bit odd I suppose, but it was odd to start with. Keep in mind, the 320 / 360 ailerons already have this modification. I didn't get the 10 kts I thought I would. (Aren't all mods worth 10 kts? haha) But, it is a different airfoil. Stall was no different, but the pitch force did increase with flaps extended. Not a bad thing in my opinion. Overall it is hard to explain, it is a different wing, just can't pinpoint how.

In the end I think I have made the airplane aerodynamically better, but I have moved the problem. It seems with just a little bit cleaner airplane I ran in to the limit of the propeller. Previously more rpm always netted more speed. Now the top 250ish rpm doesn't do much at all. Total bummer! Having to learn a lot more about propellers than I ever thought I would now...

Results of my airplane at Reno this year:

Qualifying: 268.272 mph
Sport Medallion: 1st 261.906 mph (only 2600 rpm!)
Heat 1C: 3rd 268.300 mph
Heat 2C: 2nd 265.030 mph
Heat 3C: 2nd 266.717 mph
Bronze Race: 2nd 266..944 mph

I have some really cool video from my helmet/dash cameras, just trying to get it all edited. Hear is a teaser of some VERY close racing with Dave Morss in his Legacy: http://youtu.be/iegd6ylVHI4

Best to watch in full screen in HD. Keep in mind, objects in a wide angle lens are closer than they appear! haha

Tom McNerney

www.N54SG.com