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Hi Chris,
Since I have upgraded from 320 to 360 my airplane is now on the
forward side of its CG range. It's nice now when I throw a few things in
the back. Since this discussion all came up I looked at my elevator in
flight and it is clearly is flying in an up elevator condition even though the
trim spring is trying to push the stick forward to keep level flight.
That is why I was thinking that a little reduced down thrust may not have the
tail fighting so much to keep everything in balance for level flight. Don’t
have room to do much but wondering it that would help put the elevator in more of
a neutral position.
Craig Schulze
N73S
From: Chris Zavatson
[mailto:chris_zavatson@yahoo.com]
Sent: Tuesday, October 08, 2013 4:55 AM
To: lml@lancaironline.net
Subject: Re: [LML] 320/360 performance and stability
"I was thinking about reducing engine
downthrust. Any thoughts on this Chris?"
Less downthrust is destabilizing.
If you are going to change downthrust, I would love to
record your neutral point before and after. It is rare to get stability
data on a propeller changes like this.
Here is paper that discusses the stability changes to the
Hawkeye E-2C simply by increasing prop disc solidity. Four blade to eight
blade prop.
From: Craig Schulze [mailto:craig@skybolt.net]
Sent: Thursday, October 03, 2013 1:04
PM
To: 'dudewanarace@yahoo.com'
Subject: RE: 320/360 performance and
stability
What type of odd experiences are you having with
yours? I have been noticing that the faster I go the more pitch sensitive
it is. I have the small tail too and my elevator position is about the
same as yours in cruise. I was thinking about reducing engine
downthrust. Any thoughts on this Chris?
What were the results of flattening the bottom of the
flaps? Any speed gain?
From: dudewanarace@yahoo.com [mailto:dudewanarace@yahoo.com]
Sent: Wednesday, October 02, 2013 4:50
AM
To: lml@lancaironline.net
Subject: 320/360 performance and
stability
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
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