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Jun 28, 2011, 11:37 AM
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Woodstock 1's Avatar
Quote:
Originally Posted by surfimp View Post
We have a winner

Send me your shirt size and mailing address via PM and I'll get one dispatched to you.

We'll see if it helps... hard to say.

And yep, definitely doing a Mad-tail on this one

Steve
Hey - no fair without explaining the whole thing!?!
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Jun 28, 2011, 11:57 AM
VTPR & Slope Aerobatics
surfimp's Avatar
Thread OP
Ooops sorry. Here goes:

The most glaring flaw the Le Fish has - and it certainly has flaws, and can be improved!! - is that it tends to pitch to belly when in knife edge.

The top profile of the plane already has an airfoil shape from nose to tail, and this significantly improves knife edge over designs where the fuse is a flat plate (constant thickness) from nose to tail.

However, for reasons I don't totally understand but am working on grasping, the plane has a decided tendency to pitch to belly in knife edge. There are a LOT of variables in play dictating this behavior: relative positioning of the side area volumes, relative position of the fin and rudder, shape of the rudder, rudder hinge line, etc etc etc.

I wanted to see if I could eliminate some of the pitch coupling by using a second airfoil shape, with the top of the fuse like the LE and the bottom of the fuse the TE. I spent some time visualizing the airflow around the fuse in knife edge and it seemed like this shape might help "pull" (or maybe push?) the fuse more towards the canopy as opposed to the belly.

If I'm right, great, if not... oh well! The pitch coupling still needs to be fixed, so I'll have to try something else.



Steve
Jun 28, 2011, 12:00 PM
Brett
bjaffee's Avatar
How about making the tail completely symmetrical? The plane is light enough that it shouldn't make it a big deal as far as landing goes.
Jun 28, 2011, 12:01 PM
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Woodstock 1's Avatar
Hmmm.. I reckon D Dawg was on the right track then... Because you answered in the negative, I thought it lay elsewhere...
Jun 28, 2011, 12:14 PM
VTPR & Slope Aerobatics
surfimp's Avatar
Thread OP
Well - it's not meant to affect knife edge spins, but maybe it will do something - who knows!!!

Steve
Jun 28, 2011, 12:36 PM
AMA 105167
D_Dawg's Avatar
Quote:
Originally Posted by surfimp View Post
Well - it's not meant to affect knife edge spins, but maybe it will do something - who knows!!!

Steve
And I said that in a quasi joking matter ... because from the last screenshot you posted with the fuse on its side ... it did resemble a symmetrical airfoil ... I am not going to get into a Aeronautical debate over the pure physics of fluid mechanics involving sub-sonic flight effects ... as I have not had those classes since 1987 (or was that in 85) ... so, I may have forgot a lot ...

Actually, just typing that made my brain hurt ...

Damn glad those days are over ... and Reynolds numbers ... I wont go there when it comes to models of the size we fly ...




Because everything I learned never seemed to "FULLY" apply, when it came to a model ... And the "discussion" ALWAYS leads to a "disagreement" ...

Damn, I hated Fluid Mechanics .... LOL! Sometimes I'd just rather be an Amoeba
Jun 28, 2011, 01:32 PM
VTPR & Slope Aerobatics
surfimp's Avatar
Thread OP
D_Dawg, it's still not done being shaped - got too late last night to continue and I had other stuff to do - namely a Ninja XL video to upload - but anyways, yes, it's going to have a symmetrical-ish shaped fuse from both the front profile (as shown there) and, already built into the kit, from the top view (the nose-to-tail dimension).

The airfoil-shaped fuse (nose-to-tail) and the straight leading edge were the true innovations of the original Le Fish design when released in 2006, vis-a-vis the other designs floating around at that time. It seems like it's worked out nicely and become pretty well-loved by a lot of people, that of course makes me very happy! I do not receive a royalty on these kits, my agreement with Jack is just that he sends me a kit or two when I ask for them. I want nothing more from this hobby than to fly cool planes and have good times with my friends on the slope.

Anyways, the most annoying flaw for me with the Le Fish so far is its pronounced tendency to pitch to belly. The prototype and every iteration I've flown so far, be it the Guppy, the Le Fish, or the Big Fish, all do this, and to me, it needs to be addressed. You can work around it by entering knife edge flight with speed and then being very moderate with the rudder, but you still have to compensate for it, and IMHO, that's unacceptable. Other designs like the Wasabi and Madslide/Slopemaster are basically neutral in knife edge, and that's what I'm going for.

So in any event, ultimately we need to revise the fuselage shape of the Le Fish, but in the meantime, since I already had a Le Fish fuse, I figured I'd try this experiment. In trying to understand why the design is pitching to belly, I've noodled a bit and believe it has to do with the relative position of the majority of rudder / fin area relative to the centerline of the fuse... I don't know if this experiment will work or not, but it's one approach. I can't wait to find out!!

Steve
Jun 28, 2011, 01:42 PM
I DS slower than I build!
Cory's Avatar
My first guess for a pitch either way in knife edge would have been decalage between the wing and horizontal tail which is really just an effect of the ballance point in relation to the NP. Are you making your planes with lifting tails? If you make a nose heavy Le Fish doesn't it pull to the "top" of the plane in knife edge?
Jun 28, 2011, 02:04 PM
VTPR & Slope Aerobatics
surfimp's Avatar
Thread OP
Decalage is 0/0 and there is no lifting tail.

I don't fly nose heavy planes so I can't say!!

Steve
Jun 28, 2011, 02:09 PM
AMA 105167
D_Dawg's Avatar
I just keep "watching" and "Reading" ... and with each New development ,,, keeps me from doing anything on my fishie ... LOL (that's my story and I'm sticking to it)

I originally though Light ... then was told to just build it normal ... but then, thinking in the mindset of the "local" slope conditions in my area, "LIGHTER" seems to be better .... but then, the "SWISS" fish showed promise ... then, the videos ... now, the NEW experiments ...

I am about to take a "SWISS CHEESE" methodology to my fishie ...

Staple it to my building board and load up my "Shot Gun" with bird shot ....

That may "Lighten it up" some ...

May need to find some "microballoons filler" for the aftermath ....

ROTFLMAO!
Jun 28, 2011, 03:23 PM
Flagstaff, AZ
dawsonh's Avatar
Steve:

It will be interesting to see if you tear-drop fuse will solve the belly-pull/KE issue. I totally forgot about that characteristic since I have always used an "elevator to rudder" mix to fix the problem... about 2-3 of up elevator for full left and right rudder. I probably have more pull than most due to my "balanced" and larger than stock rudder.

I know the power acro guys are always trying to build the perfectly balanced plane... and they always still need to have a variety of mixes. At least we don't have to deal with the motor/pro issues.

I was thinking that I might try to get a bit more rudder below the centerline on my next build.

Dawson
Jun 28, 2011, 03:48 PM
VTPR & Slope Aerobatics
surfimp's Avatar
Thread OP
Yep - I think you are onto something there, Dawson. And interestingly, when I compared the wood parts I received with my Le Fish kit versus the original prototype, they are smaller in chord and height than the original (and versus the drawing). I know Jack has done about four different revisions over the years, but I have a feeling that more rudder chord at the bottom edge is going to be helpful.

I'm sure there's a lot of different possibilities with regards to the pitch coupling and I really have *no idea* if what I've done will help, hurt or be irrelevant. It's totally just a brainstorm that I decided to try. If it works, great, if not (I am assuming it won't) then we move on to the next thing.

But in looking at the Le Fish fuse when it's on its side, and trying to envision what would cause it to pitch to belly, I notice that there's a bit of a downwards (relative to the side view) distribution of the area. This is exacerbated in the Le Fish fuses I've seen, which have a more drooped down nose than the original prototype or drawing. Assuming it's the fuse shape that is causing this pitch coupling, and not something related to decalage - and honestly, everything is on the table until the culprit is found!! - then I just try to figure out what would cause the adverse yaw -like effect we get.

I have a hard time believing it is decalage alone as the pitch coupling at full deflection is pretty extreme. Again, hard to say for certain, but it seems like a Le Fish with all moving tail and provision for adjustable noseweight would allow us to relatively quickly see if an incidence adjustment would fix this.

Of course, assuming we give it positive decalage and that fixes this problem, I fear it would then introduce other issues (like crappier inverted requiring more forward stick). I would rather have to do some elevator compensation for knife edge (which is what I've got setup in my own programming, BTW) rather than have a negative impact to inverted... y'know?

Anyways, this is why we test! Probably this is a fool's errand, but who knows. What's for certain is that the Wasabi does not pitch to belly appreciably, nor does the Slopemaster. The Voltij is pretty good too, though its knife edge performance is the weakest of the bunch IMHO, so a little harder to compare. So far the Slopemaster and Wasabi have the best knife edge... just roll it over, hold a touch of rudder, and off you go across the face of the slope...

AFAIK these are all setup with 0/0 decalage and non-lifting tails, and I balance all my planes as neutrally as possible, meaning I'm pretty sure I've accounted for other variables *besides* fuse shape. But who knows!!

Steve
Jun 28, 2011, 04:02 PM
vtpraerobatics.blogspot.c om
oldscooler's Avatar
Steve,
It's interesting your choosing the fuselage to try and correct the belly tuck issue. Everything I read says we should be looking more at basic set up things such as Wing incidence and CG? These seem more like the areas of trim that will have the most effect.

Not sure what Wasabi and Madslide wing incidence is specified but even just .5 degree positive WI will make a marked change as it enters KE and rudder pull. I would not be too concerned what other planes do and don't do, just what yours is doing for this experiment. All planes are different. Too far of a rearward CG can also as we know create an unstable plane. Try building in a way to quickly field adjust the wing incidence to positive. From what I read from some precision aerobatic notes is a CG set at 25-28% of MAC in connunction with positive WI beyond "0" is generally the place to be for KE perfection. May feel nose heavy but the flight surfaces will be more responsive and there will be less need for expo.

In the end i sense some area of LF's performance capabilities may be compromised a bit to attain the KE performance you are seeking. You might end up with a KE version of LF. I hope you can find the all encompassing formula in one complete version which might include this fuse mod. That would be the best result.

Eric
Jun 28, 2011, 04:54 PM
VTPR & Slope Aerobatics
surfimp's Avatar
Thread OP
The incidence is 0/0... I'm not a fantastic builder but I am 100% certain I got that part correct.

And every single iteration of the Le Fish design that I've flown - 48", 60", 66", 82" - have all exhibited exactly the same pitch to belly. Across a range of builders, CG settings, etc. So I'm pretty sure there's something going on with the design itself that causes this to happen

We fly gliders, not powered planes, and if you want to fly with your CG at 25-28% like it was a powered pattern plane, have at it! I'll pass. We have unique challenges with our gliders and the solutions that work for a motorized plane do not always translate to the distinct requirements of a glider, and specifically an aerobatics glider.

So... if we assume the CG and decalage are givens (which, for me, they are), then what are we left to play with? Fuselage shape, relative positions, etc....

Steve
Jun 28, 2011, 05:24 PM
Flagstaff, AZ
dawsonh's Avatar
So Steve... are you going to put any carbon in the fuse?

The current two carbon rods seem to be overkill.

My first idea was to use two 1mm by 3mm ribbon spars inlayed into the (inside of the) fuse halves near the outside edges... maybe give then a slight curve. This would be very stiff.

However now I am thinking a single (slightly wider) ribbon spar... sandwiched between the two fuse halves (above the wing). This should give good stiffness in the pitch axis and still get some flexibility in the yaw axis... often a good thing when crashing.

Dawson


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