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  • September 19, 2021, 11:02:29 PM

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Author Topic: That pesky stab leading edge…again  (Read 514 times)

Online Matt Colan

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That pesky stab leading edge…again
« on: September 13, 2021, 08:34:30 PM »
I recently got to fly Bob Gieseke’s 2003 Bear at a contest. It was an incredible airplane and blew everything else I’ve flown out of the water. Anyone who flies it should have to take a 15 point handicap!! That airplane has a round leading edge, I believe it’s a molded 1/2” unit. The airplane tracked just as well as my plane which sports the current day sharp leading edge stab. I’ve read all the posts and thinking of the sharp leading edge and understand all the logic behind it, the last two planes have a super sharp leading edge and have tracked better than airplanes that didn’t have as sharp a leading edge.

After flying that plane, I’ve been asking myself, is there a particular reason why it tracked just as well compared to the popular thinking of today? The thoughts I’ve had that may be why is the stab is mounted much higher than the wing, and the leading edge is perfectly round. Or are we all grasping at straws and a good old fashioned, straight, well built airplane by a master going to be better in general?
Matt Colan

Online Ken Culbertson

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Re: That pesky stab leading edge…again
« Reply #1 on: September 13, 2021, 10:22:00 PM »
Or are we all grasping at straws and a good old fashioned, straight, well built airplane by a master going to be better in general?
The answer to your question is simply "If it was built by Bob and it still exists then it was perfect."  Perfection in building was his starting point.  I am glad to see his legacy still being flown.

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Offline Brett Buck

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Re: That pesky stab leading edge…again
« Reply #2 on: September 14, 2021, 05:25:52 AM »
I recently got to fly Bob Gieseke’s 2003 Bear at a contest. It was an incredible airplane and blew everything else I’ve flown out of the water. Anyone who flies it should have to take a 15 point handicap!! That airplane has a round leading edge, I believe it’s a molded 1/2” unit. The airplane tracked just as well as my plane which sports the current day sharp leading edge stab. I’ve read all the posts and thinking of the sharp leading edge and understand all the logic behind it, the last two planes have a super sharp leading edge and have tracked better than airplanes that didn’t have as sharp a leading edge.

    The very quick answer is that there a *a lot of other things* that you are not considering and that we don't even necessarily know about that *also* affect the tracking, it's not just one factor that makes or breaks it. We went 1/4 century building airplanes with 1/2" thick flat stabs with 1/4" LE radius, and a fair percentage or even most of those tracked just fine - and some of them didn't. How it tracks is not solely determined by the stab airfoil.

     This is a classic example of the "killer feature" school of design - if you have a "killer feature", you build an airplane with it, and then assume anything it does, good or bad, is because the "feature" - and not one of the thousands of other factors that might be affecting it, that you aren't paying any attention to, or you don't even know about. Or, you note that some airplane does not have the "killer feature" and therefore it can't be any good.

   Rabe Rudders are the classic example - put on a Rabe Rudder, and, immediately, all trim functions are the result of the rudder settings, if it is great, then, the rudder fixed everything, if it is bad, obviously the problem is that the rudder is not adjusted correctly. Never mind the tipweight, flap differential, leadout sweep, and the dozens of other things that might be right or wrong about it whether you have a Rabe Rudder or not. The idea behind the Rabe Rudder is about right, or close as you can get with a mechanical system, but the effect it is intended to correct for is typically very small and usually handled perfectly well with other trim settings. If all it was doing was correcting for precession,  should require only *tiny* movements, and they should be symmetrical. What do you see? +- 1/2" and usually wildly cocked off to the right, on both insides and outsides. 

  Sealing hinge lines is another example - "so and so's airplane won the 58 East Podunk Stunt Criterium with 1/8" gaps, therefore, it's not important {you stupid eggheads}".

     All that having various "features" right does is *raise the odds* and eliminate or reduce, typically, *one possible* issue or mode of performance shortfall - it *doesn't ensure that every single airplane with that feature is magically fixed*, or that not having the feature is certain to result in a dog*. As you find.

   That's why it takes careful experimentation over many years to figure out if some "feature" is a net benefit in a general case. Your "feature" might make one airplane fly better, but the next one might be only ordinary (for completely unrelated reasons).  If you do it strictly by experiment, you are doing it statistically. You have one data point right now - what is the standard deviation of your sample?

   Jumping to conclusions about design/trim changes is very easy and very common.

      Brett

Online Matt Colan

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Re: That pesky stab leading edge…again
« Reply #3 on: September 14, 2021, 08:27:21 AM »
   That's why it takes careful experimentation over many years to figure out if some "feature" is a net benefit in a general case. Your "feature" might make one airplane fly better, but the next one might be only ordinary (for completely unrelated reasons).  If you do it strictly by experiment, you are doing it statistically. You have one data point right now - what is the standard deviation of your sample?

   Jumping to conclusions about design/trim changes is very easy and very common.

      Brett

Yep, and I think I’m probably as guilty as anyone else in jumping to conclusions. Anytime I see a trick aerodynamic device (elevator wedges, trip tape etc.) I want to get out to the field ASAP to try it out. So far I’ve only tried these on one airplane and the improvement was very noticeable. Flying the bear turned my thinking on the leading edge of the stab on its head and has me rethinking other areas of the plane or the trim that I would want to try and see what happens.

It’s very hard to test apples to apples unless you had a take apart plane and could do the necessary tests much like David did with his tail experiments.

So much to learn and it’s all fascinating!!
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Online Ken Culbertson

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Re: That pesky stab leading edge…again
« Reply #4 on: September 14, 2021, 08:30:12 AM »
It’s very hard to test apples to apples unless you had a take apart plane and could do the necessary tests much like David did with his tail experiments.
Adjustable? LL~

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Offline Avaiojet

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Re: That pesky stab leading edge…again
« Reply #5 on: September 14, 2021, 09:31:51 AM »
When I was flying R/C Pattern, the concern was having a defined airfoil with the stab and elevator combined. A healthy airfoil, even on the small 55" or so pattern ships.

I'm trying to remember, but I don't remember conversations about the shape of the stab's leading edge?

It was always the airfoil.



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Online Tim Wescott

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Re: That pesky stab leading edge…again
« Reply #6 on: September 14, 2021, 10:30:44 AM »
It’s very hard to test apples to apples unless you had a take apart plane and could do the necessary tests much like David did with his tail experiments.

Yes, and even then you run the risk of comparing a stab with a round leading edge and a super-shiny finish against one with a sharp leading edge and an 80-grit-and-Rustoleum finish, and concluding that it's all about the leading edge and not the finish.

I think Brett has mentioned taping music wire to a stab leading edge to put a point on it, and getting positive results.  That's at least reducing the variation, but unless he did naked stab, stab + tape, and stab + tape + wire, there's still a chance that it was just the tape that made the difference.

(Edit: lately I've been doing a lot shop work, and noticing that since I'm right handed there's a definite handedness to how I work on the plane -- i.e. the top of the inboard wing gets the same treatment as the bottom of the outboard wing does, and visa-versa.  I've been wondering if a really skilled appearance judge could tell if a plane was built by a righty or a lefty -- now I'm wondering if one tends to build a bit of roll into one's airframe as well).
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Online Ken Culbertson

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Re: That pesky stab leading edge…again
« Reply #7 on: September 14, 2021, 10:47:37 AM »
This is all starting to remind me of a joke we passed around when I was a Machinist in my youth.

Specifications - mill to .0005 thickness +-.25

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Online Brent Williams

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Re: That pesky stab leading edge…again
« Reply #8 on: September 14, 2021, 11:03:09 AM »
Bob Gieseke sure was not shy with the total side area and side area aft of the wing on this plane.  "Passive yaw control", I think was the term Brett applied to this deep section method.

In the diagnosis of the combination of design factors that makes this plane turn and track so well, does the higher placement of the stab above the wing centerline contribute to these positive attributes at all?


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Online Matt Colan

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Re: That pesky stab leading edge…again
« Reply #9 on: September 14, 2021, 11:10:33 AM »
Bob Gieseke sure was not shy with the total side area and side area aft of the wing on this plane.  "Passive yaw control", I think was the term Brett applied to this deep section method.

In the diagnosis of the combination of design factors that makes this plane turn and track so well, does the higher placement of the stab above the wing centerline contribute to these positive attributes at all?




That’s an awesome picture!!!

That’s another idea and thought I’ve been having that I think is much easier to compare. The side area of my plane aft of the wing is much smaller, more slender, longer and a giant rudder. Doug and I had the same motor and same prop and my plane has a very noticeable yaw on hard outside corners, and the Bear had zero, none, nada yaw. It could be I don’t have my leadouts in the perfect spot yet but that plane has always had it no matter where my leadouts have been, just less yaw in other areas
Matt Colan

Online Brent Williams

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Re: That pesky stab leading edge…again
« Reply #10 on: September 14, 2021, 11:28:08 AM »
Here is a picture of Brett's Infinity which shows the planes significant aft fuselage area.
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Offline Brett Buck

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Re: That pesky stab leading edge…again
« Reply #11 on: September 14, 2021, 11:35:19 AM »
Yep, and I think I’m probably as guilty as anyone else in jumping to conclusions.

  You, me, and everybody else! Drawing conclusions/interpreting experimental results is a tricky business no matter what the topic.

    Brett

Offline Brett Buck

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Re: That pesky stab leading edge…again
« Reply #12 on: September 14, 2021, 11:36:37 AM »
Adjustable?

   Removeable/replaceable. You can do the same thing with "slippers" over a fixed stabilizer, should you be inclined.

     Brett

Offline Brett Buck

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Re: That pesky stab leading edge…again
« Reply #13 on: September 14, 2021, 01:02:40 PM »
Here is a picture of Brett's Infinity which shows the planes significant aft fuselage area.

   Also note that this one has an airfoiled tail. I have similar pictures from 2004 with a 5/8" thick flat stab with a 5/16 LE radius. Both track identically, and nearly perfectly, I routinely got 40 points on takeoff and inverted flight because I could just park it and watch it fly around.

    The side area may or may not have something to do with this - it was not *intended* to have any effect on tracking, just to increase the passive yaw stability against the otherwise uncompensated precession effects.

        Brett

Online Matt Colan

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Re: That pesky stab leading edge…again
« Reply #14 on: September 14, 2021, 02:32:08 PM »
The side area may or may not have something to do with this - it was not *intended* to have any effect on tracking, just to increase the passive yaw stability against the otherwise uncompensated precession effects.

        Brett

Out of curiosity, roughly how deep is your fuselage aft of the wing?
Matt Colan

Offline Bob Hunt

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Re: That pesky stab leading edge…again
« Reply #15 on: September 14, 2021, 05:29:05 PM »
Here's yet another deep aft fuselage, with a lot of fuse area in front of the wing as well. The absence of a vertical fin also attributed to an airplane that was (still is...) very stable down wind. I liked it so much that I'm building a significantly update version around the Crossfire wing (which, by the way, was originally the 1980 Genesis wing. Hey, it's all in the family...).

Later - Bob Hunt

 

Offline Mike Ferguson

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Re: That pesky stab leading edge…again
« Reply #16 on: September 15, 2021, 11:19:39 AM »
Here's yet another deep aft fuselage, with a lot of fuse area in front of the wing as well. The absence of a vertical fin also attributed to an airplane that was (still is...) very stable down wind. I liked it so much that I'm building a significantly update version around the Crossfire wing (which, by the way, was originally the 1980 Genesis wing. Hey, it's all in the family...).

Later - Bob Hunt

I like that it's got a little Oosa-Amma rudder influence too! Looks great.


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