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Author Topic: Rabe rudder  (Read 10287 times)

Offline Gary Dowler

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Rabe rudder
« on: October 26, 2017, 01:21:00 AM »
Gents,
Last night I was looking at some plans for a Stunt ship called the Humbug.  Loved the look and the lines with the twin booms supporting the stab.
What I've not seen before was an adjustable rudder of some sort called a Rabe rudder on the plans.

What on earth is this, what is its function and how does it work?  Can I assume it was one of those ideas that worked better in theory than in reality? 

The plane looks like a fun project for future exploration, just curious about this rudder thing.

Gary
Profanity is the crutch of the illiterate mind

Offline Avaiojet

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Re: Rabe rudder
« Reply #1 on: October 26, 2017, 04:35:11 AM »
Trump Derangement Syndrome. TDS. 
Avaiojet Derangement Syndrome. ADS.
Amazing how ignorance can get in the way of the learning process.
If you're Trolled, you know you're doing something right.  Alpha Mike Foxtrot. "No one has ever made a difference by being like everyone else."  Marcus Cordeiro, The "Mark of Excellence," you will not be forgotten. "No amount of evidence will ever persuade an idiot."- Mark Twain. I look at the Forum as a place to contribute and make friends, some view it as a Realm where they could be King.   Proverb 11.9  "With his mouth the Godless destroys his neighbor..."  "Perhaps the greatest challenge in modeling is to build a competitive control line stunter that looks like a real airplane." David McCellan, 1980.

Offline EddyR

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Re: Rabe rudder
« Reply #2 on: October 26, 2017, 07:57:18 AM »
Use google or on here and look up Rabe Rudder. He has written a lot on it.
Locust NC 40 miles from the Huntersville field

Offline Serge_Krauss

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Re: Rabe rudder
« Reply #3 on: October 26, 2017, 07:59:53 AM »
The Rabe rudder is a creation by stunt icon Al Rabe, who might comment here. Adjusted correctly, it works quite well and has been used to advantage by top aerobatic pilots. It's purpose is to automatically counter yaw induced by gyroscopic precession in maneuvers. Again, adjusted correctly, it does that. There is much information on Al's rudder in his magazine articles and internet posts. You will find what you want to know by doing a search on "Rabe Rudder" here on Stunthangar (I just did so). Al's comments might be found here and definitely will be in the Stuka Stunt archives, searchable over on the Stuka-Stunt Forum. You can also do a search here on "AL Rabe."

Offline john e. holliday

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Re: Rabe rudder
« Reply #4 on: October 27, 2017, 11:29:26 AM »
It might be wise to get the CD's that Al has produced.   A ton of info on them.  Look in Vendors section.
John E. "DOC" Holliday
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Online Brett Buck

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Re: Rabe rudder
« Reply #5 on: October 28, 2017, 07:48:58 PM »
   When you have an engine spinning in the conventional direction, it acts like a gyroscope wheel. When you try to corner, the gyroscopic precession tends to make the airplane nose out on inside corners and nose in on outside corners. The Rabe Rudder (named for modeler Al Rabe, who invented it) move the rudder to try to compensate for this effect. In absence of other factors, it would move inboard on inside maneuvers and outboard on outboard maneuvers to try to keep the airplane from yawing due to precession. In practice, they usually end up moving outboard in both turns but a lot more on outsides than insides.

    This is a very real effect and actually works *if you can adjust it correctly*. That last is, unfortunately, a big "if" and almost no one can adjust it properly, even following Al's directions to the best of their ability, and they end up causing more problems than they would have had without it. And I don't just mean beginners, Top 5 pilots have proven time and again unable to make it work and wound up with a severe liability for having it.

     If you have enough fin and aft fuselage area, you can hold the yaw angles to something reasonable without any sort of active control like a Rabe rudder. That has been my approach and it works plenty well enough if you get everything else (like the rudder offset and leadout position) correct.

   Note that the precession also affects the pitch axis, pitching the airplane nose-up in the airplane reference frame all the time, because you are continually yawing it to the right by flying in a circle. That is handled a different way, either with *very slight* downthrust built or shimmed into the engine, *very very slight* positive incidence in the stabilizer (which is what I do), or more commonly, rigging in a bit of down elevator when the flap is neutral.

   Again, in many cases, people try this stuff and greatly overdo it, so it is safest to build everything 0-0 alignment and adjust the elevator. My stabilizer, for instance, has about 1/4 degree of positive incidence, which means I put the LE about 1/64" above the 0-0 reference line. That is just enough to know that it is not below the line. Do any more, and you might be cutting it apart later to fix it.

     Brett

Offline Paul Smith

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Re: Rabe rudder
« Reply #6 on: October 29, 2017, 07:26:17 AM »
I built several of them on Slow Combat and Stunt models.  They work great.  It's an over-center linkage that works from the elevator to the rudder.  It's set up so basically nothing happens on UP and LEVEL, but the rudder moves a lot on down control.   Simple harmonic motion.  The "flat" part of the circle is rigged to UP & Level and the downstroke is linked to Down elevator.

With the Slow Combat plane I could tuck under HARD with not a loss, but a gain in line pull.

I guess they went away because guys are flying big heavy powerful stunters with so much line pull that they can afford to lose a little on Down turns.
Paul Smith

Online Brett Buck

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Re: Rabe rudder
« Reply #7 on: October 29, 2017, 09:25:00 AM »

I guess they went away because guys are flying big heavy powerful stunters with so much line pull that they can afford to lose a little on Down turns.

  They haven't "gone away", it's that many people put them on the airplane, then become obsessed with adjusting them, and then think every problem they have is a rudder adjustment problem. So they screw around with it to the exclusion of all the other things that are wrong with the trim, and eventually wind uo with 10x too much movement, which ends up hurting them.  I am not talking about duffers, I mean, Top 5 pilots during the wingover where the airplane looks like a salmon jumping up a  waterfall, flop-flop-flop.

      If you get everything else exactly right, with it in a fixed (hopefully neutral) position, then only add enough movement to remove the residual yaw motion, then you have a chance. And, the movement ends up being *tiny*, not the huge 1" changes you often see. In fact, the normal mechanisms can't be adjusted for small enough offset. Keith Trostle's cam-operated system ended up working well because he could dial in the exact profile required by cut-and-try with many different cams.

    People do the same with tuned pipes, every engine run problem is a pipe length issue. Guy leaves off his uniflow plug, engine speeds up 1/2 second a lap, oh, ignore that plug, pull the pipe, back and forth everytime something acts funny and ignore everything else that might be wrong with it. The pipe length is the least likely thing to go wrong or need to be changed on a day-to-day basis, and then only when you make an intentional change somewhere else.

   Point being that there is nothing wrong with the concept, but the application seems so far beyond most people that it ends up as a net detriment rather than the minor improvement you could conceivably get from it. In any case, in stunt, people who know what they are doing don't end up with gigantic props on tiny airplanes any more, so the problem is a lot different than it was in 1970 when it was invented.

     Brett

Offline Igor Burger

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Re: Rabe rudder
« Reply #8 on: October 29, 2017, 01:07:56 PM »
people who know what they are doing don't end up with gigantic props on tiny airplanes any more

I saw nice answer for such statement ( even several times!!! ) in another thread:

"You would be quite surprised at how many people don't understand it, or actively disagree."

 VD~ VD~ VD~

Offline Air Ministry .

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Re: Rabe rudder
« Reply #9 on: October 31, 2017, 11:27:14 PM »
Theres a few places a model aeroplane can cut loose , and try and take your head off. Particulary if its blowing.
 Apparently it does, down Fort Worth way .

Theres some set up paraphanalia here . http://www.machineconcepts.co.uk/aeromodelling/rabe_rudder/rabe_rudder.html

Two Conditions ' Slamming ' corners . ( Every time Id come back from the city the agro would get a docile thing to cut loose repeatedly.
As much as 12 times in one flight . Whereas it was usuall7y flown more symathetically = or sensitively . Al fingertips , Rather than a bull whip .

Into the Clover & at the Top of the Hourglass are perhaps where its most usefull , and in the vertical outter center turn in the Sq eight .

In a ancient trove , al talks about ' p ' factor and how you know it . In a P-51 on a run way . 1 : 1 scale- pilot inside .

Under severe use ( in a corner , if a plane is running a A.o.A. ( angle of atack ) relitive to its flight path , on Insides . . .
the outside blade , is running higher effective pich ,
and the inside LESS , sometime stuff all . or None .

Putting a crude Zinger 13 x 5 on My P-51 , which works good on a fine light thin 12 x 5 ( the 13 might have twice the mass )
suddenly there was no line tension on insides . Fixed Rudder . Thus a ' toonable ' rudder , and the method of refining tune ,

Plus it can counter YAW , which is its main objective. Less Drag - Cleaner flight - less grey hairs , and you shouldnt need a crash helmet .

Offline Vitalis Pilkionis

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Re: Rabe rudder
« Reply #10 on: November 01, 2017, 11:01:20 AM »
Have anyone ever calculated or at least noticed how much does Rabe rudder increase force on a handle?

Online Howard Rush

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Re: Rabe rudder
« Reply #11 on: November 03, 2017, 12:38:21 AM »
Have anyone ever calculated or at least noticed how much does Rabe rudder increase force on a handle?

It’s on my list of things to calculate someday.

I have a movable rudder on my current airplane. It’s been taped in place for three years. I might hook it up next season. It is wee to keep me from overdoing the movement.
The Jive Combat Team
Making combat and stunt great again

Offline Paul Smith

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Re: Rabe rudder
« Reply #12 on: November 03, 2017, 07:37:17 AM »
Have anyone ever calculated or at least noticed how much does Rabe rudder increase force on a handle?

The idea is just to NOT DECREASE the pull on down turns.
Paul Smith

Offline Igor Burger

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Re: Rabe rudder
« Reply #13 on: November 03, 2017, 07:59:12 AM »
Have anyone ever calculated or at least noticed how much does Rabe rudder increase force on a handle?

Almost nothing compared to elevator hinge moment, and elevator nothing compared to flaps, so you can only hardly see it. However rudder can have permanet up or mown momend in neutral, and that may need some handle neutral fix.

But there is another effect - if you try to pitch gyro axle, precession will convert that force to side. If you have any device which will preserve its yaw agle (rudder in our case), it will transfer to monet acting against the orriginal force, you can feel it as some kind of resistence and that is what gyroskope does - trying to keep its shaft angle in orriginal direction :- )) So that extra gyro stability with Rabe rudder can create some kind of feeling of Rabe rudder "resistance".

Offline Air Ministry .

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Re: Rabe rudder
« Reply #14 on: November 04, 2017, 09:38:55 PM »
Cor .



https://stunthanger.com/smf/open-forum/rabe-rudder-on-a-fancherized-twister/

Could be quite usefull , when your up to your arse in alligators , with the weather conditions .

Offline Vitalis Pilkionis

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Re: Rabe rudder
« Reply #15 on: November 05, 2017, 04:55:00 AM »
Almost nothing compared to elevator hinge moment, and elevator nothing compared to flaps, so you can only hardly see it. However rudder can have permanet up or mown momend in neutral, and that may need some handle neutral fix.

I see, although it's hard to believe that a plate hanging at 20O to an air stream do not add any significant load on a control system..  ::)

Offline Trostle

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Re: Rabe rudder
« Reply #16 on: November 05, 2017, 09:53:11 AM »
I see, although it's hard to believe that a plate hanging at 20O to an air stream do not add any significant load on a control system..  ::)

Where does the 20o parameter come from?  I cannot imagine any movable rudder using the Rabe concept in any kind of acceptable trim/adjustment would ever approach such extreme offset.  On my Rabe Bearcat, my rudder was set at 1/8" right offset for neutral elevator, went to an additional 1/8" right offset on down control at about 30o elevator down deflection.  Then for up elevator, the rudder went from that 1/8" right offset at neutral to about 1/16" RIGHT offset at about 30o elevator up deflection.

Gave pretty much constant line tension throughout the maneuvers.  It is very comforting to have that line tension on the outside turns of the vertical eights, hourglass and four leaf clover.  The slight right rudder on insides seemed to help smooth out the transitions from inside to outside loops in all of the round eights.

The Bearcat is sort of a different beast because it has a relatively large rudder all above the thrustline.  It required an entirely different rudder control program than other models I have had with "more conventional" placed rudders

I know this is different trim than what Al uses, but he trims his airplanes quite differently than I do with a more forward CG and uses a comparatively large handle.  I am sure that translates to different rudder trim as well.

Keith

Offline pmackenzie

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Re: Rabe rudder
« Reply #17 on: November 05, 2017, 11:01:53 AM »
Where does the 20o parameter come from?  I cannot imagine any movable rudder using the Rabe concept in any kind of acceptable trim/adjustment would ever approach such extreme offset. 

Maxbee has close to that, perhaps 15 degrees?


https://stunthanger.com/smf/open-forum/to-igor-burger/

I think I have more on my Vector 40, but it has a small moving portion. Not sure about the "acceptable trim" part in my case.  ;)

MAAC 8177

Online Brett Buck

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Re: Rabe rudder
« Reply #18 on: November 05, 2017, 11:02:16 AM »
Where does the 20o parameter come from?  I cannot imagine any movable rudder using the Rabe concept in any kind of acceptable trim/adjustment would ever approach such extreme offset.  On my Rabe Bearcat, my rudder was set at 1/8" right offset for neutral elevator, went to an additional 1/8" right offset on down control at about 30o elevator down deflection.  Then for up elevator, the rudder went from that 1/8" right offset at neutral to about 1/16" RIGHT offset at about 30o elevator up deflection.

     But, the previous comment illustrates the issue - people are routinely using 20-30-40 degrees of movement on huge rudders. And some of the common linkages won't permit much smaller. Your cam setup is really slick and lets you dial in the right values, but most people don't have anything like the necessary adjustability.

     My point was that while the concept behind the Rabe rudder is generally correct (and about as good as you can do with a simple mechanical system), people have NO IDEA how to properly adjust them, and almost always wind up with far too much movement which ends up making it worse rather than better. I walked up to one contender's airplane at a NATs, moved the controls, saw that the rudder moved about 45 degrees, and said, "well, I guess I don't need to worry about this guy". Of course, when the airplane flew, it fishtailed all over the place.

     My recommended procedure to adjust it is different from Al's - fix it in place (preferably with no offset), then trim the airplane as well as it can be trimmed without it. This might take weeks/months. Then, and only then, add just enough movement to remove the very slight residual yaw movement. If the residual yaw movement is substantial, you probably don't have it trimmed well enough to even start in with the rudder. Every time you change something, start over. DON'T immediately assume that the rudder adjustment is the problem and it just needs to be tweaked some more.

   When you move large rudders 30 degrees, it's definitely going to require substantial hinge moment.

      Brett

Offline Igor Burger

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Re: Rabe rudder
« Reply #19 on: November 05, 2017, 12:03:37 PM »
I see, although it's hard to believe that a plate hanging at 20O to an air stream do not add any significant load on a control system..  ::)

It DOES have, but it is nothing compared to much larger elevator deflected 2x more and that is nothing compared to flaps hinge moment which can be may be 10x more than elevator moment.

Offline Igor Burger

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Re: Rabe rudder
« Reply #20 on: November 05, 2017, 12:06:39 PM »
Maxbee has close to that, perhaps 15 degrees?


Yes, but remember that fuselage is 1/ yawed little bit (so the rudder is efectively deflected less) and 2/ most of that is in downwash of long fuselage. That is one of the reasons why it needs asymetric deflection.

Offline Igor Burger

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Re: Rabe rudder
« Reply #21 on: November 05, 2017, 12:12:17 PM »

Under severe use ( in a corner , if a plane is running a A.o.A. ( angle of atack ) relitive to its flight path , on Insides . . .
the outside blade , is running higher effective pich ,
and the inside LESS , sometime stuff all . or None .


Modern stunters have usually 0 AoA during almost all maneuvers, so it usually does not have too much influence on yaw.

Offline Ted Fancher

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Re: Rabe rudder
« Reply #22 on: November 05, 2017, 03:46:38 PM »
Yes, but remember that fuselage is 1/ yawed little bit (so the rudder is efectively deflected less) and 2/ most of that is in downwash of long fuselage. That is one of the reasons why it needs asymetric deflection.

Hi Igor,

Also, If I recall correctly, the Max Bee rudder is very low aspect ratio and thus requires greater angles of attack (sideways rudder offset in this case) to achieve the same lift--yaw input--as a surface with a higher aspect ratio.  Have to admit, I had one of those patented Brett Buck imitations of Jack Benny expressing shock...an "O.M.G., what was he thinking" moments...when I saw the amount of Rabe rudder input utilized on your ship in Gyula in 2010.  Also have to admit the airplane flew great notwithstanding my dropped jaw moment!

Re your comment about fuselage "yaw"; are you aligned with the group who favor more forward CGs and aft leadout locations on aircraft with Star Wars vintage powertrains here in the states, Igor?  If so, how do you rationalize the divergence from IC powered flight trim from an aerodynamic point of view?

Ted

Online Brett Buck

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Re: Rabe rudder
« Reply #23 on: November 05, 2017, 03:51:44 PM »
It DOES have, but it is nothing compared to much larger elevator deflected 2x more and that is nothing compared to flaps hinge moment which can be may be 10x more than elevator moment.

   That all sounds about right, but with some of the systems I have seen, it's not negligible.

     Brett

Online Dan McEntee

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Re: Rabe rudder
« Reply #24 on: November 05, 2017, 08:14:31 PM »
  Is there anywhere we can view Keith's cam operated system? I don't ever remembering hearing or reading about that anywhere and would like to check that out. I've never had a stunt model with a Rabe rudder system but I'm interested in learning more about it.
   Type at you later,
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Offline Trostle

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Re: Rabe rudder
« Reply #25 on: November 05, 2017, 11:24:48 PM »
  Is there anywhere we can view Keith's cam operated system? I don't ever remembering hearing or reading about that anywhere and would like to check that out. I've never had a stunt model with a Rabe rudder system but I'm interested in learning more about it.
   Type at you later,
  Dan McEntee

Hi Dan,

I had a 3-page article on my system in the April 2008 issue of Stunt News.  I will try to post a copy of that article.

One aspect of this approach is that all of the system is internal at the rear end of the fuselage.  There are no external linkages.  Also, this allows essentially unlimited variability on rudder travel including direction and rate of deflection with regard to elevator movement.

Keith

Keith


Offline Igor Burger

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Re: Rabe rudder
« Reply #26 on: November 06, 2017, 12:54:23 AM »
Also, If I recall correctly, the Max Bee rudder is very low aspect ratio and thus requires greater angles of attack (sideways rudder offset in this case) to achieve the same lift--yaw input--as a surface with a higher aspect ratio.

Yes, it was my Max II built for World champ in Sebnitz 2002. It has orriginally OS Max 46LA for power. It was second of two twins differing only in logarithmic flaps.

http://www.netax.sk/hexoft/stunt/the_max_ii.htm

the other one is here:
http://www.netax.sk/hexoft/stunt/the_max.htm

Later I used it for my first and so far the only rework for electric power train, And that was used in Hungary 2010 wher you took that picture of rudder for Brett as you said :- )).

So yes it has shorter rudder as visible on picture taken on Eoro champ Bulgaria 2015 where that model was used by young Chinese boy Letong after he crashed his model in training. The newer Max Be has wider rudder of larger area, because I simply needed more rudder effect. I was also on doubt if to do it wide and vertically short or narrow and tall. Wider and shorter solution has stronger feedback to controlls as the hinge moment is larger. However knowing forces from flaps I decined to do it as it is and it aparentlu works well :- ))  My target with that vertically short solution was to keep rudder in line of fuselage, not to induce rollin forces in maneuvers.





Re your comment about fuselage "yaw"; are you aligned with the group who favor more forward CGs and aft leadout locations on aircraft with Star Wars vintage powertrains here in the states, Igor?  If so, how do you rationalize the divergence from IC powered flight trim from an aerodynamic point of view?

Those links above shows probable answer for that question. You can see that I moved CG to 15% on model with log. flaps (it is written in text). Since then I moved to actual aproximately 17 or 18 % (on both Max II and also Max Bee) as I learned flying with sensitive models. So yes I have CG front of that I used with IC engines in past, but I moved it back after rework to electric on that particular model :- )))

So in my case the CG did not move with electric and thus I do not have anything supporting idea that electric needs CG front of CG for IC :- ))

Offline Air Ministry .

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Re: Rabe rudder
« Reply #27 on: December 09, 2017, 01:25:25 AM »
its easy if you know how . :-X various stabs in the dark sometimes achives results .  :P

Offline Air Ministry .

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Re: Rabe rudder
« Reply #28 on: December 09, 2017, 01:30:09 AM »











« Last Edit: December 09, 2017, 01:57:38 AM by Matt Spencer »

Offline Air Ministry .

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Re: Rabe rudder
« Reply #29 on: December 09, 2017, 02:02:32 AM »
 >:( >:( >:( >:(  :-\









Quote
Quote
In the Mustang article "Evolution of a Thoroughbred" there is a photo of what became my standard stab and elevator construction.  My stabs are typically built using a built up spar consisting of a 1/4" x 1/8" balsa web with 1/16" x 3/16" spruce top and bottom spar caps.  The ribs are aerodynamically contoured, about 1/4" thicker than the trailing edge and about 5/8" at the root .  This gave the sheeted stab about 3/4" thickness at the root with 3/8" elevators..  This was a 1973 design, and the configuration I still use today.

This stab/elevator configuration wasn't detailed in the articles as I didn't see anything that unusual in them.  It was a full scale aircraft practice and only seemed worth incorporating on stunt ships.

The intent, of course was to soften elevator response around neutral using this configuration instead of the then more common practice of using "slop" in the elevator horn bushing. 
« Last Edit: December 09, 2017, 10:33:20 PM by Matt Spencer »

Offline Igor Burger

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Re: Rabe rudder
« Reply #30 on: December 09, 2017, 02:08:06 AM »
Those articles are theoretically OK, but they missed one fact, AoA at prop and at wing not need to be equivalent :- ))


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