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Author Topic: Flap/Elevator thickness relative to Wing/Stab thickness?  (Read 6312 times)

Offline Target

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Flap/Elevator thickness relative to Wing/Stab thickness?
« on: April 28, 2015, 06:06:11 PM »
So, when studying Les McDonald's Stiletto 660 design, I noticed that the stab trailing edge is thicker than the elevator leading edge, the stab is made from 1/2" stock and the elevator from 3/8" stock.

What does this accomplish? I have my own hunches, but would like to hear the REAL reason for this.

Thanks in advance!
« Last Edit: May 10, 2015, 09:10:07 PM by Chris Behm »
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Offline Target

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Re: Flap/Elevator thickness relative to Wing/Stab thickness?
« Reply #1 on: April 30, 2015, 11:12:13 AM »
Over fifty views and no answers/ideas? ::)
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Offline Tim Wescott

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Re: Flap/Elevator thickness relative to Wing/Stab thickness?
« Reply #2 on: April 30, 2015, 11:17:55 AM »
Here, I'll give you an answer and perhaps someone will be offended enough by it to give you a better one:

As far as I know, the idea of making a step down like that is that the elevator becomes less sensitive around neutral.  This, in turn, is supposed to make the plane groove better in level flight while still allowing for snappy turns, because changes in altitude can be accomplished by flap action alone without changing the direction of flight, rather than having to rotate the plane to make it rise or fall.
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Offline Howard Rush

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Re: Flap/Elevator thickness relative to Wing/Stab thickness?
« Reply #3 on: April 30, 2015, 12:25:22 PM »
As far as I know, the idea of making a step down like that is that the elevator becomes less sensitive around neutral.  This, in turn, is supposed to make the plane groove better in level flight while still allowing for snappy turns, because changes in altitude can be accomplished by flap action alone without changing the direction of flight, rather than having to rotate the plane to make it rise or fall.
   

That's what people think, but I haven't seen any data to back it up.  An airfoil analysis program might give some insight, but I can't get XFOIL to work with our hinge geometry.  It seems to want a smoother transition from wing to flap or stabilizer to elevator.

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Offline Target

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Re: Flap/Elevator thickness relative to Wing/Stab thickness?
« Reply #4 on: April 30, 2015, 12:36:59 PM »
Thanks for that, gents.
I'll share my thoughts, which are maybe too simple.
My thought is that at max throw on the elevator, the hinge line could be closed with no bump, bulge, and a very smooth transition occurs for an effectively tight square corner.
I think this is probably not correct, the idea Tim mentioned sounds reasonable. Has anyone asked Mr M directly what his thoughts were about it?
Thanks for the input, guys. I'm very interested in aero design and function.
Regards,
Chris
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Offline Target

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Re: Flap/Elevator thickness relative to Wing/Stab thickness?
« Reply #5 on: May 10, 2015, 06:28:39 PM »
Here, I'll give you an answer and perhaps someone will be offended enough by it to give you a better one:

As far as I know, the idea of making a step down like that is that the elevator becomes less sensitive around neutral.  This, in turn, is supposed to make the plane groove better in level flight while still allowing for snappy turns, because changes in altitude can be accomplished by flap action alone without changing the direction of flight, rather than having to rotate the plane to make it rise or fall.

Tim-
I sent a PM to Mr McDonald, and he said that your reply was pretty spot on.
He also said that he moved away from that theory in later evolution of the Stiletto design, and he wasn't sure that it was the right thing to do.
I did get some design mod info from him that sounded like it was well established.

My intent, when I build this, is to make it an electric plane, although I'm not decided 100% on that.
Any further mods some of you veterans would make to the 660 if doing an E powered rendition?
I would guess that it could be built a bit lighter for electric power.

Thanks!
Chris
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Offline Tim Wescott

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Re: Flap/Elevator thickness relative to Wing/Stab thickness?
« Reply #6 on: May 10, 2015, 06:57:02 PM »
That's what people think, but I haven't seen any data to back it up.

This may be a job for a real live wind tunnel.  I don't know if the idea is right, I was just saying that's the idea.

I wish I knew, since I'm looking at some airplane plans that call out a thinner elevator than the stab.
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Offline Steve Fitton

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Re: Flap/Elevator thickness relative to Wing/Stab thickness?
« Reply #7 on: May 12, 2015, 12:02:05 PM »
The stunt "answer" is what Tim said.

A better read would be David Fitzgerald's article on the stabilizer tests he did with his Stargazer some years ago.

Whatever effect size of the elevator vs the stab has, my own (limited of course) experience is that it is far less critical than correct alignment of the flying surfaces during construction.
Steve

Eric Viglione

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Re: Flap/Elevator thickness relative to Wing/Stab thickness?
« Reply #8 on: May 12, 2015, 12:55:57 PM »
Go easy on me, I don't make any claims to having superior scientific knowledge to back this statement up, but...

I am pretty convinced that thinner material for the control surface minimizes effects from building errors *at neutral* like a high or low hinge slot at an end, or slight variation in material LE thickness or a slight warp of the control surface.

A lot of people seem to be on the hunt for the "ultimate way to do stuff", and some even can produce the theory, science and math to back it all up. I am fine with that, and think it's great. Keep pushing those boundaries, and I'll copy you when I think it's relevant and if it is easily repeatable. And that brings me to my next comment:

My goal in stunt, is to achieve something slightly different, I want "the ultimate, yet simple and repeatable way to do stuff", so I can replicate the results easily on every build. The thinner control surface should be lighter, and through finishing can be made reasonably stiff enough, it's easier on the builder for the other reasons already listed above, and, might have some advantage around neutral, if we could back that statement up for Howard.

At the wing loading we build these modern airfoils at today, it's arguable that we are nearing overkill in our flaps anyway...never mind needing super efficient ones. And... if you can build 3/8 elevators at .5 ounce each, and 1/2 elevators at .75 ounce each, I'd pick the 3/8" elevators every time and avoid the tail heavy possibility.

That's enough reason for me to keep using the thinner control surface method until there is a proven need to do otherwise.

EricV

Edit: Steve types faster and gets to the point quicker than I...heh.

Offline bob whitney

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Re: Flap/Elevator thickness relative to Wing/Stab thickness?
« Reply #9 on: May 12, 2015, 03:11:23 PM »


along that same line ,i built a  bill Simmons Scorpio with 3/8 thick stab and elevator ,i tried all kinds of balance and trims but could not get it to groove ,.i tapered the elevators to just under 1/8 in at the trailing edge and it made a whole new airplane out of it. good enough for a 1st and 2nd in advanced  nastal 30 at Brodaks
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Offline Target

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Re: Flap/Elevator thickness relative to Wing/Stab thickness?
« Reply #10 on: May 12, 2015, 07:39:29 PM »
i appreciate the discussion, gents.
Sounds also like a case of there not being a reason NOT to do it, and there are weight benefits.
Still, form my sailplane building, it normally easier to build a torsionally stiffer thicker surface at the same weight of a thinner one. Construction methods are different, however. S?P

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Chris
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Offline John Miller

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Re: Flap/Elevator thickness relative to Wing/Stab thickness?
« Reply #11 on: May 14, 2015, 09:53:54 AM »
I remember the first time the idea of thinner elevators entered into my modeling life. It was in the early 60's.  The idea was to make the elevator softer around neutral. That and sloppy controls, seemed to help reduce "hunting" and for years was the preferred method for fixing this particular problem. Up until about ten years ago, I was of the firm belief that this method was the best, and easiest way to cure the issue.

Now, I prefer a different way to cure these issues. Why would I use a different method?

I became aware of a slight hesitation at the intersections of the figure eight maneuvers. It was actually pointed out to me by my flying buddy, Gordan Delaney. The hesitation made a flat spot at the intersection. One had to anticipate, input, early to avoid this flat spot, and often resulted in the intersections that had the vertical portion either skewed to the left, or right, depending on whether the input was early or not. Timing became such an issue, and one, another problem to cure in the search for a better pattern.

Once I began to use a better cure for hunting, the hesitation at the intersections went away, and I no longer have to anticipate, or try to time my input, with a bigger chance for error.

My better method, I use 1 to 2 degrees of down thrust, which helps cure several issues our tethered planes have. I often also use a small positive incidence of the stab and elevator. I no longer use thinner elevators to soften neutral, nor do I use sloppy controls to cure hunting. These old methods make timing more of a problem to overcome while flying the pattern.

Using the down thrust, and slight positive incidence of the stab allows tight, precise controls that do not require me to anticipate and apply early input while trying to fly a better pattern.



 
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Offline Tim Wescott

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Re: Flap/Elevator thickness relative to Wing/Stab thickness?
« Reply #12 on: May 14, 2015, 10:25:16 AM »
Now, I prefer a different way to cure these issues.

How timely!  I'm just about to build some tail feathers; I think the elevator just got 1/8" thicker.
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Offline Steve Corbin

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Re: Flap/Elevator thickness relative to Wing/Stab thickness?
« Reply #13 on: January 28, 2017, 08:46:36 PM »

  I'm new here and haven't flown C/L for many years, most of my aeromodelling  has been free-flight and RC sloping and thermalling. But this topic caught my eye, and though it's kinda old I wanted to post.

  Way back when Jim Bede was actively designing homebuilt airplane kits, he sometimes wrote articles for airplane magazines, and one such article kinda/sorta addressed this topic.

  But his concept appears to be the opposite of what control liners want: he wanted more sensitivity near neutral, not less. He recommended that the leading edge of a control surface be slightly thicker than the trailing edge it was attached to. IIRC, it was to overcome the thickness of the slower moving air in the boundary layer that often exists on the aft portions of airfoils, or something to that effect. (I'm not an engineer).

  So it makes sense to me that having a flap or control surface with a thinner leading edge than the trailing edge it's hinged on would have an opposite effect, reducing sensitivity near neutral.

  Another way to get more sensitivity near neutral that I've seen on full-scale A/C is to add what I see as a Guerney Flap to the trailing edge, on both top and bottom surfaces. I first saw it used on a LearJet, and the factory pilot told me what they did. It seems the control surface, an aileron, without these trailing edge thickeners could deflect a few degrees without producing much effect.

  I once had a glider whose rudder had too short of a chord, and had almost no effect until it was deflected quite a bit. So there's a possible way to get the reduced effectiveness around neutral that some seem to want. Try reducing chord of the control surface, and increase travel to get the force needed to reach your goal. Easy enough to try it, or so I think. In the case of my glider, I added thin sheet foam to both sides of the rudder, using it to both increase thickness and chord slightly. Worked great, so maybe it illustrates a point. I know little, but have a great time experimenting, I guess that's one attraction of model aviation-- we can fool around with it and not likely suffer too much when we zig where we should have zagged!

Offline Mark Mc

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Re: Flap/Elevator thickness relative to Wing/Stab thickness?
« Reply #14 on: January 29, 2017, 05:04:58 PM »

along that same line ,i built a  bill Simmons Scorpio with 3/8 thick stab and elevator ,i tried all kinds of balance and trims but could not get it to groove ,.i tapered the elevators to just under 1/8 in at the trailing edge and it made a whole new airplane out of it. good enough for a 1st and 2nd in advanced  nastal 30 at Brodaks

This kind of follows the thinking from the R/C articles I read back about 30-35 years ago.  The thinking in some quarters was that, with a flat sheet elevator and stab, if the elevator was of a smaller thickness than the stab it would have the same aerodynamic qualities of an elevator that was the same thickness at the hinge line and tapered at the trailing edge.  I never really noticed the difference, but that's what I read in the mags.

Mark

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Re: Flap/Elevator thickness relative to Wing/Stab thickness?
« Reply #15 on: January 29, 2017, 07:48:03 PM »
COR , Not This Again !  :o

Bit like the opposite of the Thick Flat Flap / elevator trip ( the just round edge flat ones )

Think someone refered to the thick / thin trip as ' Dead Zone ' . As in it has to move BEFORE the Surface is Parrallel to the AIRSTREAM .
Assuming the airsteam is straight and parrallel .

A Similar Effect is obtained by ' SLOP ' in the bush . %^@ 5 m.m. ( 3/16 in. ) at the trailing Edge seems to be the Required Figure .

The THEORY is the FLAP alters the EFFECTIVE INCEDANCE , Before the Elevator alters the Angle of Attack of the Airframe .



As this old cow had millions of gallons of fuel thru it , before ( and After ) I rebushed the Elevators ( BOTH , two bushes )
and tried flying it for two weeks WEAVING everywhere , wingovers etc . Any Straight Legs .

I tried Reaming out the bushes to the previous 5 m.m. at T.E. , one was only 3.5 - as Id used bike spoke pushrods , the angle of the sit !
Now had just a tendancy to weave ( ok it weved slightly ) C.G. was 3/4 Aft of Aeromodellor ' MUSTUNT ' plan . The wing I used .
And Moments Areas .

So I concude a AFT C.G. is comesurate with the ' Dead Zone ' Elevator set up . !  mw~ S?P

Offline Target

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Re: Flap/Elevator thickness relative to Wing/Stab thickness?
« Reply #16 on: January 29, 2017, 09:00:11 PM »
What the heck did ^^^he^^^ just say??
Matt, I think you are saying that the thinner elevator and slop in the elevator horn accomplishes the same thing?

R,
Chris
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Offline EddyR

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Re: Flap/Elevator thickness relative to Wing/Stab thickness?
« Reply #17 on: March 05, 2017, 07:51:47 AM »
This is a little off topic but it is important about stab thickness. In a  article Bob G said this about his many times nats winner Nobler. He used 1/2" balsa for building the stab elevator. He tried 1/4" and 3/8" material and the Nobler did not fly as well. I talked to him and he said he had no slop in the elevator horn as it had a brass bushing in the horns.     
  In the early 1980,s many stunt planes had very thick flaps that matched the wing thick trailing edge. They did not give any advantage and were more difficult to build so they went out of favor. I tried it and it looked cool and I also used the very thick stab 1" that many were using but none of it added anything to the flight performance that was worth all the effort.  ~^
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Re: Flap/Elevator thickness relative to Wing/Stab thickness?
« Reply #18 on: March 05, 2017, 11:18:57 AM »
Hi eddy-
So, your point is that a 1/2" or thicker stab gives a plane that flies better than a thinner stab.
Also that some of the trends have fallen out of favor due to their lack of proof of flight improvement.
Is that correct?
Thanks.
R,
Chris
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Offline EddyR

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Re: Flap/Elevator thickness relative to Wing/Stab thickness?
« Reply #19 on: March 06, 2017, 06:55:51 AM »
I thought I made it pretty clear I was telling what Bob Gieseke said about his G Nobler. I had one of the fat flap elevator models that flew Ok but not worth the effort to build.
Some people will go to the effort just to get the look but it seems of little flight return for all the effort.
Model had 3/4" trailing edge with contoured flap built in. Just getting the hinges to work was a major building project. Some people love those challenges HB~> HB~> HB~>
EddyR
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Offline EddyR

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Re: Flap/Elevator thickness relative to Wing/Stab thickness?
« Reply #20 on: March 06, 2017, 07:10:25 AM »
This was one of my 3/4"+ thick trailing edges wings with very thick flaps. These flaps were not built into the trailing edge but just hung on it. The wing needed the thick Trailing edge to support the high aspect ratio wing with the big flaps. This model was one of my best flying models for many years. It had thin 3/8" stab. I built this just to use the wing in something as it was collecting dust
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Offline Brett Buck

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Re: Flap/Elevator thickness relative to Wing/Stab thickness?
« Reply #21 on: March 06, 2017, 02:51:33 PM »
   I am no fan of the "elevator thinner than stab" plan, nor of slop. For slop, in particular, I think the issue back in the good old days was that when you used bent wire pushrods, no slop=binding. So you more-or-less had to have slop to some degree or the airplane would hunt due to stiction.

    The thinner elevator idea is another idea to soften up the response around neutral, which just doesn't seem to be a good idea and has never worked out very well for me. Both the thin elevator and slop create non-linearities around neutral, which you then have to learn to deal with. All of my recent airplanes have as much effort as I can take to remove anything like aerodynamic discontinuities around the elevator, and the more I cleaned it up, the better it seemed to work. For example, the stab/elevator on my airplane are built as one piece sheeted foam, with a pretty simple "arc" from LE to TE, and about 1/2" thick at the hinge line. Then I just cut them apart. The hinge line is a conventional wedge type, with all the corners (except for the hinge line) rounded off to about 1/4" radius. The idea, stolen from Paul Walker, was to minimize flow discontinuities around the hinge line to that the flow was as consistent as possible without regard to the deflection.

   David's airplane has the flaps *thicker* than the TE of the wing, rather than thinner.

    Of course, just like anything, it might just be a coincidence that the three airplanes I built with variations on this theme all flew better, and it was something else, but I am pretty careful about experimenting and the fact that the last two flew nearly identically (aside from one little hitch that is better on the new airplane, for reasons I think I understand) suggests that I am not getting a lot of random variation.

      Brett


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