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