Readers,
I switched the topic as the conversation was getting a bit lost in the 'other' thread. I copied my original post at the bottom of this post. I addressed some questions individually not realizing it wouldn't answer for everyone. So, I'll hit the highlights here:
Stability and general oddness
Unleashed is a small tail 360 that has been converted to electric trim. The trim tab extends aft of the trailing edge of the elevator, it is not a section of the elevator like most trim tabs. Combine that incorrect angle of incidence and a varying tab effectiveness and you end up with a smaller trim window than most. By that I mean, a smaller speed range where you are able to trim to a neutral stick. That range is above a full flap condition, and below top 5% of the speed range. The bottom end doesn't bother me at all, the top end gets annoying to say the least. A poor mans solution for the top end of the envelope is to pull the flaps out of reflex a few degrees. Doing this gets you back in the trim window, barely. Removing the cusp in the flaps did change these results. It now requires you to pull even more flaps out of reflex to get it trimmed. Hopefully this will be resolved if/when I
correct the horizontal angle of incidence.
Flap Cusp Removed
The common question is how much faster. Well, it is almost not measurable at this point. Maybe a few mph, but hard to tell. I have reached the limit of this propeller so it could be false indication of potential speed gains. Meaning any aero improvements may not show up just because the propeller is no longer pulling. What I do notice is the speed it accelerates has been improved. Also, it appears to bleed less
energy under G's. As Chris pointed out, this is probably due to the better lift coefficient under G's at these speeds. Thus, removing the cusp giving the wing less camber was probably an improvement. The "on track" report from Dave Morss was that it seemed to accelerate out of the corners much better than it used to. If anybody had a good view of that, he did! See video below
Although he got me on Sunday.. damn it!
Beluga Belly Fuselage Mod
First, this represents the change in shape. Rather drastic:
What started this mod
was an odd thing that would happen at high indicated airspeeds and high propeller rpm. The vertical airspeed indicator would start to swing wildly from +500 to -500 in about a 2 to 3 second interval. My static ports are in the stock location just below the rear windows, on each side. I suspected this false indication was caused by a rather drastic pressure change happening in that area. I didn't have any pre-modification tuft pictures, only word of mouth from a friend in formation, sadly not fast enough to cause the wild swing in VSI. But, from word of mouth, the tufts in that area and aft of the wing were angled up rather high. Something in the order of 30 to 45 degrees.
Now, my number 1 rule, cheat first. Plenty of super fast legacies have made a modification to this area. The idea being to keep the fuselage perpendicular to the wing from its widest point to 5% of the chord aft of the trailing edge, then blend it in. Soooo let the wing do its lift thing, before you turn it in to the fuselage. How you make that turn is rather critical, and I may have done it differently now that I know more. But, from this pic you can see the tufts for the entire side are nowhere near 45 degrees up, so I have changed the relative flow for the entire area.
Now, for the record, I was in a hurry and stuck those on in about 2 min. It lacks the resolution I would really want to see and yellow string was far from ideal. Also, the last two on the upper row were starting to peel off so the tape was screwing with their indication. I'll do it again now that I have more time and would love to compare to a "stock" 360 sometime.
Moral of the story, LOTS of work, and it may not be worth it at the slower speeds. The faster I go, the more important it may be. Again I can't give what I think is a realistic mph gain, as the propeller could be limiting the results.
For some interesting
info on the concepts check out this image. This a project out of Brazil called Anequim. The Magenta area is laminar flow. The difference in the left design and the right design is a change in the canopy! Notice the diferrence in laminar flow near the wing root.
It all works together.
Tom McNerney
Original Post
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. :)
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:
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.
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/iegd6ylVHI4Best to watch in full screen in HD. Keep in mind, objects in a wide angle lens are closer than they appear! haha
Tom McNerney