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Author Topic: Stab Aerodynamics question??  (Read 3489 times)

Offline frank williams

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Re: Stab Aerodynamics question??
« Reply #50 on: February 22, 2021, 08:52:07 PM »
Chuck
The particular data that I showed might be a little misleading in that the Re number is fairly low (20K).  Our tails see about 250K.  Howard is right that the regular facing 0012 does show the “switchback” in the 20K Re no. region, but goes away as  Re number increases.  So, maybe not the best picture but still useful I think in regard to the pointy section.
 
There is also data from Hoerner for a reversed 0012 at Re no.of 2M  and the lift slope is the same, very linear through zero.   And, even though this isn’t a conventional stab / elevator, I think it is still informative when looking at really pointy leading edges.
 
No aero guy alive would look at Paul’s le and not say, “it ain’t gonna work, it’ll stall so bad you won’t even be able to do a loop, Hell much less a hard corner”.   But it does work.  It doesn’t stall and fall out of the sky.  +/- 7-8 degrees it doesn’t show a stall like we would think.  The tail really doesn’t see the angle of attack that we think it does.  Practically I haven’t felt a stall at all.

Yes, the elevator behind moving around is going to affect the total flowfield, so its not that great a picture to use, maybe, but its real data and it is close to what we are trying to understand.

Round leading edges and pointy trailing edges are the standard for tail sections and they do work.  They have worked for a long time …… while I’m thinking about it …..have you seen the new Americas Cup AC75 boats? ….. apparently there are better ways to skin the cat …..

Offline Paul Walker

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Re: Stab Aerodynamics question??
« Reply #51 on: February 22, 2021, 09:48:17 PM »
It's times like this that I wished I had a simple wind tunnel that could work at the Re No that we use. The actual tunnel would be "easy", but the mechanisms and electronic equipment might cost a pretty penny!

Offline Chuck_Smith

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Re: Stab Aerodynamics question??
« Reply #52 on: February 23, 2021, 05:01:31 AM »
Paul, when I started using wind tunnels we had balances and manometer tubes and we did good science. No need for expensive gear.  Make sure you have one that sucks instead of blows. We used about half a million soda straws as flow straighteners and a blade to throttle the flow so we didn't even have a VFD on the fan motor!

I figure anyone who can build a nice CLPA plane has more than enough skills to built a wind tunnel. Heck, the Wright Brothers built one.



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Offline Scott Richlen

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Re: Stab Aerodynamics question??
« Reply #53 on: February 23, 2021, 06:27:04 AM »
[quoteTE of stab, 1/4 inch radius.][/quote]

So, no squared-off trailing edge on stab anymore?

Flaps TE are still squared off aren't they?  But elevator trailing edge is still tapered to sharp edge?   ???

Offline Ken Culbertson

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Re: Stab Aerodynamics question??
« Reply #54 on: February 23, 2021, 07:35:23 AM »
There has been much discussion about the LE & TE shapes.  I am convinced, through having tried them, that the sharp LE (eg a radius < 1/32") and a flat TE of about 1/8" is best for what we do.  What is left is whether the full airfoil or the flat center section is best or does it perhaps not make any difference.   Except for most profiles, I have used airfoiled sections for as long as I can remember going back to the nearly laminar ones on my first Nobler.  Flat stabs are easy to build and keep straight.  Fully airfoiled ones are not.  If there is little to be gained from the airfoil section .....

One other observation that probably has nothing to do with this.  When I was a teenager, I was involved in the development of one of the HLG classics.  What we ended up with was a "razor" sharp LE.  So much so that we had to add a spruce strip to keep the dings out from landings in the brush.

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Offline frank williams

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Re: Stab Aerodynamics question??
« Reply #55 on: February 23, 2021, 08:42:14 AM »
An easy way to test sharp leading edges is to make some "slippers",   maskiing tape them to the stab.  One inch alluminum window blind slats are a great building material in general and good for making "slippers".  Tape two strips together and then tape the glove to the stab leading edge.  Makes for a quick experiment.  It'll fit to either 1/4 in. stabs or on up to 1/2 in. stabs.

Offline Paul Walker

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Re: Stab Aerodynamics question??
« Reply #56 on: February 23, 2021, 10:10:04 AM »
[quoteTE of stab, 1/4 inch radius.]

So, no squared-off trailing edge on stab anymore?

Correct

Flaps TE are still squared off aren't they?

Yes

 But elevator trailing edge is still tapered to sharp edge?   ???

It has about a 0.06" radius



Offline Scott Richlen

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Re: Stab Aerodynamics question??
« Reply #57 on: February 23, 2021, 11:27:55 AM »
Quote
[quoteTE of stab, 1/4 inch radius.]

So, no squared-off trailing edge on stab anymore?

Correct

Flaps TE are still squared off aren't they?

Yes

 But elevator trailing edge is still tapered to sharp edge?   ???

It has about a 0.06" radius

clop, clop, clop, clop (sound of my foot-falls running down stairs to basement shop)

"Stop Everything!  Stop Everything!  You need to round off the stab trailing edge!"  (Me yelling at building elves)

"How much?"  (Elves all yelling back in unison)

"One-quarter inch radius!"  (Me yelling back at elves)

"What's that in millimeters?  We're Scandinavian......"

Online Brett Buck

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Re: Stab Aerodynamics question??
« Reply #58 on: February 23, 2021, 11:35:16 AM »
The actual tunnel would be "easy", but the mechanisms and electronic equipment might cost a pretty penny!

     I have been wanting to build one myself since I was about 8 years old and flying rockets, but, same issue. To get any usable results you need pretty good measuring equipment because the effects we are discussing are so small.

    Brett

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Re: Stab Aerodynamics question??
« Reply #59 on: February 23, 2021, 11:56:49 AM »
clop, clop, clop, clop (sound of my foot-falls running down stairs to basement shop)

"Stop Everything!  Stop Everything!  You need to round off the stab trailing edge!"  (Me yelling at building elves)

"How much?"  (Elves all yelling back in unison)

"One-quarter inch radius!"  (Me yelling back at elves)

"What's that in millimeters?  We're Scandinavian......"

6.350 mm

Offline Chuck_Smith

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Re: Stab Aerodynamics question??
« Reply #60 on: February 23, 2021, 12:06:20 PM »
Midway in to my bourbon last night I think I had an epiphany. I'm theorizing that the sharp LE makes the stab behave like it's thinner than it really is. I vaguely remember back in the day having a conversation with some peers on Viper stability and control and how the sharp LE's on the Viper's tail behaved before they went supersonic (where sharp is great- everything gets so much easier past Mach 1). Calspan guys IIRC.

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Offline Ken Culbertson

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Re: Stab Aerodynamics question??
« Reply #61 on: February 23, 2021, 12:46:01 PM »
     I have been wanting to build one myself since I was about 8 years old and flying rockets, but, same issue. To get any usable results you need pretty good measuring equipment because the effects we are discussing are so small.

    Brett
I am trying to visualize a wind tunnel that can simulate a CL Flight.  Closest I can come up with is being able to vary the fan speed to simulate the wind constantly changing direction in 5-6 second intervals as our test subject "flies" a circular path through it, sometimes stopping to change direction 16 times.  If you find one I too have been wanting one since I was 8 and building model rockets using CO2 cartridges and crushed matches for thrust.  That was until my father made me stop with the rockets after nearly blowing my foot off and build some airplanes instead (glad he did).  He said our rockets were too dangerous - duh.

Ken
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Offline Paul Walker

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Re: Stab Aerodynamics question??
« Reply #62 on: February 23, 2021, 01:24:57 PM »
I am trying to visualize a wind tunnel that can simulate a CL Flight.  Closest I can come up with is being able to vary the fan speed to simulate the wind constantly changing direction in 5-6 second intervals as our test subject "flies" a circular path through it, sometimes stopping to change direction 16 times.  If you find one I too have been wanting one since I was 8 and building model rockets using CO2 cartridges and crushed matches for thrust.  That was until my father made me stop with the rockets after nearly blowing my foot off and build some airplanes instead (glad he did).  He said our rockets were too dangerous - duh.

Ken

The intent would be to test different airfoil cross sections with different flap configurations as well a stb/elevator combinations.
These would be at level flight AoA and at higher AoA's to simulate corners. The ability to test yaw angles is only valid for cross sections at a yaw angle an will miss the fuselage influence as well as the circulation around the tips.

The problem is test section area. Big enough for a full plane is WAY out of the scope of something at home.

But it is nice to dream about a much smaller one where a full size cross section will fit!

Online Brett Buck

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Re: Stab Aerodynamics question??
« Reply #63 on: February 23, 2021, 02:12:06 PM »
I am trying to visualize a wind tunnel that can simulate a CL Flight.  Closest I can come up with is being able to vary the fan speed to simulate the wind constantly changing direction in 5-6 second intervals as our test subject "flies" a circular path through it, sometimes stopping to change direction 16 times. 

    I don't think you would ever use it to simulate complete flights or even an entire airplane at once, just take data of various points - in this case, lift measurements at a range of extremely low AoA, and maybe flow visualization. It probably has to be big enough to allow half a stab or something like that, but trying to make a 60 mph wind tunnel large enough to contain an entire airplane, (which might have to be twice the size to avoid interaction with the tunnel) is probably beyond a reasonable home design.

     In the large, we know what to do and more-or-less conventional analysis works OK. It's this small stuff that we are continually experimenting with.

      Brett

Offline Ken Culbertson

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Re: Stab Aerodynamics question??
« Reply #64 on: February 23, 2021, 02:29:35 PM »
The intent would be to test different airfoil cross sections with different flap configurations as well a stb/elevator combinations.
These would be at level flight AoA and at higher AoA's to simulate corners. The ability to test yaw angles is only valid for cross sections at a yaw angle an will miss the fuselage influence as well as the circulation around the tips.

The problem is test section area. Big enough for a full plane is WAY out of the scope of something at home.

But it is nice to dream about a much smaller one where a full size cross section will fit!
This would be similar to what Al did back in the 70's.  I wonder if there might be something out there in the academic world we are missing?  Surely some aspiring Aeronautical graduate student has built something we could use and probably has it sitting in a garage somewhere.  My brother is director of the Lincoln Labs flight test facility at MIT.  I am going to ask him.
NO! in only one of the possible answers.

So, from reading back through this thread I conclude that the sketch below is where we are with stab airfoils?

Ken
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Online Dan McEntee

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Re: Stab Aerodynamics question??
« Reply #65 on: February 23, 2021, 03:21:14 PM »

     As far as a wind tunnel goes, back in my sailplane days, there was a group that restored a low speed wind tunnel some where at a college in northern Illinois for the purpose of testing different air foil sections for sail planes. Clubs across the country volunteered to make the test sections of the required airfoil shapes and all the data was compiled in a book that was made available. The name Selig-Donavon is all I can remember about the compilations efforts . That is probably what you would be talking about, isn't it?  I do not know if it is still in operation or not, perhaps some one else will.
   Type at  you later,
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Online Brett Buck

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Re: Stab Aerodynamics question??
« Reply #66 on: February 23, 2021, 03:29:29 PM »
Clubs across the country volunteered to make the test sections of the required airfoil shapes and all the data was compiled in a book that was made available. The name Selig-Donavon is all I can remember about the compilations efforts . That is probably what you would be talking about, isn't it?

  Michael Selig. As i recall he did do a few stunt-relevant examples, but I don't know of anyone in stunt that knew about it and also knew what we really needed while he was doing it.

   Brett

Offline Bruce Shipp

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Re: Stab Aerodynamics question??
« Reply #67 on: February 23, 2021, 04:02:04 PM »
The Selig gang is up to I believe volume 5 of their low speed airfoil studies.  Here is a link to volume 1.

https://m-selig.ae.illinois.edu/uiuc_lsat/Low-Speed-Airfoil-Data-V1.pdf

Here’s a link to volume 5

https://m-selig.ae.illinois.edu/uiuc_lsat/Low-Speed-Airfoil-Data-V5.pdf

Just a few thousand pages in the middle.  I remember reading volume 1.  One of the main points I took away was the degradation in performance due to inaccuracies in construction.  If a giving design parameter didn’t perform in the real world the way it did in the wind tunnel or in simulation, it was most likely because it was inaccurately constructed and not because it was a bad design.

I think too often when we incorporate features into our designs that don’t work as expected we fault the idea and not the execution.  This is likely what Doug Moon experienced when he installed triangle stock on the leading edge in an attempt to replicate Paul’s leading edge shape and he recognized that after seeing the sketch of Paul’s leading edge in a previous post. 

I agree with Doug, this had been a most interesting and educational thread.

Edit: the reference to construction inaccuracies was not from volume 1 above but from the book, Airfoils at Low Speeds, 1989 by Selig, Donovan and Fraser.

5.5 Surface Waviness and Contour Accuracy

So long as an airfoil surface is smooth, that is,free of sharp edges protruding into the boundary layer, surface waviness of the type produced by Monokote over balsa appears to have no measurable impact on performance. For a 12 in chord, the Monokotejbalsa waviness has a peak-to-peak amplitude on the order of 0.02% chord. On the other hand, the warping of the covering in open-bay construction as it stretches over the cells alters the airfoil contour beyond what is normally considered waviness. In such cases, the airfoil really has no single shape and one can expect its performance to be significantly different from the nominal.

Based on our measurements, the best modern-day construction techniques used by modelers are capable of yielding contours accurate to ±0.004 in. For a 12 in chord an error of ±0.004 in is only 0.033% chord. Although at present there is no criterion on the accuracy necessary to meet the nominal performance, all indications are that errors of this order have only a small affect. On the other hand, errors two to four times this amount (which are more common) do begin to effect performance. Error in contour is undoubtedly one of the factors that explains differences in performance between two different models of the same RC sailplane.

The book is available here:

https://m-selig.ae.illinois.edu/uiuc_lsat/Airfoils-at-Low-Speeds.pdf




« Last Edit: February 23, 2021, 04:54:01 PM by Bruce Shipp »

Online Crist Rigotti

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Re: Stab Aerodynamics question??
« Reply #68 on: February 23, 2021, 04:32:19 PM »
I'll add another question.  What about thinner elevators than their stabilizers?
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Online Brett Buck

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Re: Stab Aerodynamics question??
« Reply #69 on: February 23, 2021, 05:29:22 PM »
I'll add another question.  What about thinner elevators than their stabilizers?

  No (as a general proposition), as noted earlier.

    Brett

Online Brett Buck

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Re: Stab Aerodynamics question??
« Reply #70 on: February 23, 2021, 05:34:55 PM »
One of the main points I took away was the degradation in performance due to inaccuracies in construction.  If a giving design parameter didn’t perform in the real world the way it did in the wind tunnel or in simulation, it was most likely because it was inaccurately constructed and not because it was a bad design.

  Bingo,  and why it is so difficult to separate out effects of the intended design from many other random elements.

    Brett

Online Crist Rigotti

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Re: Stab Aerodynamics question??
« Reply #71 on: February 23, 2021, 05:45:40 PM »
  No (as a general proposition), as noted earlier.

    Brett

Brett,
Could you let me know where it was noted earlier.  Apparently I missed it.
Thanks.
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Offline frank williams

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Re: Stab Aerodynamics question??
« Reply #72 on: February 23, 2021, 09:47:04 PM »
At the PAMPA meeting at the Lubbock Nats I pointed out that the Selig tests were being done at UI and we (PAMPA) could submit models for testing.  crickets ... Don Hutchenson and Todd Lee showed interest as I recall.  I kick myself for not pushing on and getting something done.  A Nobler, a Rabe, a Fancher all would have been good tests articles.  At that time he wasn't doing sections with flaps, however they have since made fixtures to allow flap deflections.  The test articles are 12 inch chord and are two feet in span.  It still could be done.  There are instructions on how to build the models.  This is really good high zoot data.  The only applicable stunt symmetrical airfoils from the Selig data are two RC trainer sections.  I've shown data from them in other posts.  They are good info in the sense that one is blunter than the other.  The balance is three components and has pitching moment data.

So many fun things to do with our tiny airplanes.  We don't have the ultimate design yet.

Offline Dennis Toth

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Re: Stab Aerodynamics question??
« Reply #73 on: February 24, 2021, 06:30:28 AM »
Frank,
Maybe you could submit the airfoil test work done by Al Rabe with his car hood tests. With this as a starting point they could see how close he was to correct answers or what was missed (to bad he's gone, probably had that data and the test parts). Would also be interesting if Ted's Intimidation work could be tested and again see how that comes out. Last would be Dave's stab testing, be nice to have some instrumented data and see what that would predict vs. actual flying results. Interesting stuff.

Best,    DennisT 

Offline Doug Moon

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Re: Stab Aerodynamics question??
« Reply #74 on: February 24, 2021, 07:14:00 AM »


So the elevators LE doesnt start out thinner than the stab TE.  Brett said no it was noted earlier. I find this interesting as well. I have been making my stabs 1/2" thick and my elevators are 3/8" thick at the LE. That doesnt work with this newer design?

I seem to also remember Windy had a few sharp stabs on his Spitfires??
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Offline Scott Richlen

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Re: Stab Aerodynamics question??
« Reply #75 on: February 24, 2021, 07:17:51 AM »
Quote
I seem to also remember Windy had a few sharp stabs on his Spitfires??

One of the last planes he had at Brodaks had an extremely thin stab and elevator.  Maybe close to 1/8th inch thick?  Not sure what he made it of.

Offline frank williams

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Re: Stab Aerodynamics question??
« Reply #76 on: February 24, 2021, 07:34:00 AM »
Dennis -  Lots of good analysis done by the notables.  A 2D wind tunnel to look at stunt wing sections needs to be fairly tall, not so much wide, to be able to satisfactory accommodate a flapped model flowfield.  The tough part of a wind tunnel is the balance  system, either mechanical or electronic, it must measure accurately without altering the model attitude.  Lots of "stunt physics" could be run to ground.

Back to pointy stabs .... Brett hit the nail on the head in that probably the best analysis for what we are interested in right now could be done with some flow visualization of the activity around the sharp leading edge.  Working on it.  Lots of fun things to do!

Offline Chuck_Smith

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Re: Stab Aerodynamics question??
« Reply #77 on: February 24, 2021, 07:49:19 AM »
The problem with most small wind tunnels is they only do a 2D analysis of a section, so you need to transform the data based upon aspect ration. If you make it big enough to mount a 3D model inside the model needs to clear the sides by at least 1/2 a wingspan. So you scale down the model, and now you need to either increase the flow velocity or pressurize the wind tunnel to achieve Reynolds numbers with dynamic similarity for analysis. Finally it will be quasi steady-state data, you won't get data on control per se- you'll only be determining the stability coefficients - which are fun math-wise because they have multiple subscripts. That will only give you aero data, you'll still need to know the plane's inertia tensor to get he control response. Then you need to know the apparent mass effects of the air too.

Might be more fun to build a plane with changeable parts.

In the end,  I believe what we're really talking about here is "handling" and it's so subjective and personal that there's not going to be a one-size-fits-all solution.  I've had people fly my ships and ask me how I can fly them so sensitive and rearward CG, I've flown other great flyer's planes and thought they were annoyingly nose heavy. I fly with a very light hand, others want to give harder inputs.

I can tell you empirically that sealing the elevator gap reduces the required force for a given deflection and that can make a ship seem to turn better when it's not the case, but it "feels" a lot more responsive.




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Online Crist Rigotti

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Re: Stab Aerodynamics question??
« Reply #78 on: February 24, 2021, 08:05:49 AM »
Here is what the Max Bee plans show for a stab/elevator side view.  Seems that the elevator is much thinner than the stab.

I have no idea why it wants to show upside down!
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Offline frank williams

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Re: Stab Aerodynamics question??
« Reply #79 on: February 24, 2021, 08:16:40 AM »
Doug - I feel that the elevators size and shape is a bit secondary to the stab leading edge.  I personally don't disapprove of the thinner elevators generally.  The flow around the stab / elevator intersection generally begins to "let go"  at about 15 degrees of deflection.  Its tough for the flow to make the turn.  The best thing to do is to seal the hingeline for sure.  There are "slats" and "thingies" that can be treat the hingeline, .. but that later.

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Re: Stab Aerodynamics question??
« Reply #80 on: February 24, 2021, 09:24:36 PM »




LJ / BJ ? ? sweeper 900. Sorry its a bit rough. But is correct .

Thats a classic ' dead zone ' stab. airfoil . A derivitive of the Greenaway ' Tadpole ' stab./ elevator . ?

The EXPLANATION , particularly in regard to Igors Geared Flaps - Is that FLAP Movement alters Airfoil A.o.A. without altering aircraft A.o.A. . IF there is no initial aerodynamic funtion at the Stabilizer .

We have found a 3/4 inch rearward C. G. with 5 m.m. slack at elevator T.Edge. repeatedly though there are exceptions where the Zero Slack may be benificial .

Typically corrections at the handle for hunting/ porpoiseing exacervate it , without the dead zone set up , wherby theres a small Wing Lift initial control effect rather than a aircraft pitch response .

The Fore & aft lift thing is a bit like under & overster ( pushing & pulling, to Mr Petty  ;)) on a car or motorcyle . A well balanced one you can get it to do both, simultaeneously . Throttle & steeringwheel
Rear wheel drive, of course .

Supose the high lift wing IS a bit like a front wheel drive , where the wings lifting it through the corner , and the ( small ) tailplan follows / is neutral . MB 3 . With enough elevator travel ,
or the tail slewed around . See the " .60 Nobler " . same thing .

Online Brett Buck

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Re: Stab Aerodynamics question??
« Reply #81 on: February 24, 2021, 09:42:48 PM »
Thats a classic ' dead zone ' stab. airfoil . A derivitive of the Greenaway ' Tadpole ' stab./ elevator . ?

    That existed long before Big Jim was doing it. The theory is that it runs in the "shadow" of the elevator. It makes it less effective around zero, although the fact that it at least partially fairs into it greatly reduces the effect over a squared-off hinge line. I expect in Igor's case it tends to increase the effect of the logarithmic flap horn.

     Brett

       

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Re: Stab Aerodynamics question??
« Reply #82 on: February 25, 2021, 08:43:06 PM »
Yep , as regards dead zone . But wondered if Igor & the Tadpole were familiar .



Typical drawing .

Most plans shoe a ' double wedge ' . 1/2 Stab. 3/8 Elev. or similar . , Etc . Even if photos of the model dont .many appear airfoiled . Out with the magnifying glass .  !  S?P


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