Wing flaps design ?

[Larry Wong]

Has any one done a test of flap design?  1. Flaps part of wing airfoil.  2.plank( Sheet )flap not air foiled.  

[Dennis Toth]

Larry,
Search on Al Rabe - he did a series of tests comparing several different airfoils/flaps. One of them had the flap built in as the rear portion of the airfoil. In the article he stated that when he started the test he thought the design with the built in flap was going to be the super airfoil. After his tests what he found was it was very disappointing and was a poor performer. He used the Nobler airfoil/flap design as the base. His tests indicated that the simple Nobler airfoil is very good and hard to beat.

He did find an airfoil that he used on the Bearcat that used a little fatter rear section that he felt flowed into the deflected flap better than the more traditional flatter rear section. It is a very interesting series of articles.

Best,            DennisT

[Jeff Traxler]

Larry,If you can take a look a Wesley Dick's ship Velvet.It has split fowler style flaps and that plane turns very nicely.

[john e. holliday]

Larry,
Search on Al Rabe - he did a series of tests comparing several different airfoils/flaps. One of them had the flap built in as the rear portion of the airfoil. In the article he stated that when he started the test he thought the design with the built in flap was going to be the super airfoil. After his tests what he found was it was very disappointing and was a poor performer. He used the Nobler airfoil/flap design as the base. His tests indicated that the simple Nobler airfoil is very good and hard to beat.

He did find an airfoil that he used on the Bearcat that used a little fatter rear section that he felt flowed into the deflected flap better than the more traditional flatter rear section. It is a very interesting series of articles.

Best,            DennisT

You will also find that some of his airfoils were not fully semitrical(spelling again). 

[EddyR]

In the 80's there were quite a few models built with very thick airfoil trailing edges and very thick flap leading edges to try and get the flaps as part of the airfoil as opposed to adding them on to the airfoil. Maybe some who have used them will add a comment here. Dennis Adamisin had them on one of his high aspect ratio  planes.
 The thick airfoil flaps of the Juno work well on that plane but may not on others. They are draggy and need more power than flat flaps. I had both on the same plane and the airfoil ones are  better.
Ed

[Brett Buck]

You will also find that some of his airfoils were not fully semitrical(spelling again). 

  Symmetrical.

   I don't think that alters the conclusions very much if at all. Everybody who has tried this under controlled conditions has found about the same thing - making it follow the airfoil shape is not ideal if it is deflected, and has no effect if it isn't deflected. Blending it in and also using a conventional hinge is a double-whammy, because not only do you have the effect Al (and others) found, you also have a massive gaping discontinuity at the hinge line because the flap is so thick. Keith's airfoil didn't have that issue because his hinge was faired in well.

   My particular issue (and I think the same as Dennis found) is not so much with the lack of lifting ability, but the huge negative pitching moment. I had to run the CG back to ridiculous aft positions to get my airplane to turn, and I checked Denny's at a contest in Detroit in 1983, and it was also very far back. Then he pointed out that there was still a rag on the nose!

    Brett

[Kim Mortimore]

I'm curious how the Beringer-type wing with tiny flaps manages to hoist such a large fuselage.

Larry,
Do you have a particular problem you're trying to solve, such as a planned new plane likely to come out on the heavy side, etc? See you Thursday at Alameda.

[Bill Little]

  Symmetrical.

   I don't think that alters the conclusions very much if at all. Everybody who has tried this under controlled conditions has found about the same thing - making it follow the airfoil shape is not ideal if it is deflected, and has no effect if it isn't deflected. Blending it in and also using a conventional hinge is a double-whammy, because not only do you have the effect Al (and others) found, you also have a massive gaping discontinuity at the hinge line because the flap is so thick. Keith's airfoil didn't have that issue because his hinge was faired in well.

   My particular issue (and I think the same as Dennis found) is not so much with the lack of lifting ability, but the huge negative pitching moment. I had to run the CG back to ridiculous aft positions to get my airplane to turn, and I checked Denny's at a contest in Detroit in 1983, and it was also very far back. Then he pointed out that there was still a rag on the nose!

    Brett

Hi Brett,

Another question that I hope you can shed some knowledge on is when using a plank flap does it help to shape down the trailing edge or leave it constant to the rear edge and simply round the corners slightly?

Thanks!
Bill

[Brett Buck]

Hi Brett,

Another question that I hope you can shed some knowledge on is when using a plank flap does it help to shape down the trailing edge or leave it constant to the rear edge and simply round the corners slightly?

   I have done it both ways and couldn't tell if it made any difference or not. I sure like the looks of the tapered flaps better!

     Brett

[Howard Rush]

Another question that I hope you can shed some knowledge on is when using a plank flap does it help to shape down the trailing edge or leave it constant to the rear edge and simply round the corners slightly?

Let me know if you want to try something kinda in between.  I have a trick in mind.

[Randy Cuberly]

In the early 1980's, Bob McDonald designed and built an airplane called the Sagitta.  It had flaps that were incorporated as part of the wing airfoil with a thick trailing edge.   It was a high aspect ratio design and was published, I think, in FM.  I built one that unfortunately was short lived due to a pushrod clevis failure (don't use those things anymore).  It was an excellent flier and had a blazing corner.  It required relatively small flap deflection to overcome the previously mentioned negative pitching condition.  However I believe that was a plus and was simply because the flaps were more efficient.  Bob mentioned that in his construction article.
I have always intended to build another Sagitta, but like many things, simply never got around to it.

Randy Cuberly

[Kim Mortimore]

Let me know if you want to try something kinda in between.  I have a trick in mind.

Howard, you can be such a tease sometimes.    Are you sure there's a nerdy American engineer inside that clown suit and not a French coquette?  Of course Larry wants to hear your trick, as do other folks following this topic.

In the early 1980's, Bob McDonald designed and built an airplane called the Sagitta.  It had flaps that were incorporated as part of the wing airfoil with a thick trailing edge.   It was a high aspect ratio design and was published, I think, in FM.  I built one that unfortunately was short lived due to a pushrod clevis failure (don't use those things anymore).  It was an excellent flier and had a blazing corner.  It required relatively small flap deflection to overcome the previously mentioned negative pitching condition.  However I believe that was a plus and was simply because the flaps were more efficient.  Bob mentioned that in his construction article.
I have always intended to build another Sagitta, but like many things, simply never got around to it.

Randy Cuberly

Does the quantitative relationship between negative pitch moment and lift vary with the design, or is it fixed?

 

[Dean Pappas]

Hi All,
Everyone is discussing aerodynamics, but the thick hingeline has structural benefits completely aside from aero.
Flap twist may not be our friend.

Regards,
  Dean Pappas

[Bill Johnson]

Does the quantitative relationship between negative pitch moment and lift vary with the design, or is it fixed?

I'm going to take a shot at this: Negative pitch moment is a function of the increased camber from the flap deployment moving the center of lift of the wing aft. Flap design can have a large impact on this as evidenced by slotted Fowler flaps on large aircraft. How this effect can be mitigated on our aircraft, I have no idea. Too new to the field.

[Dennis Toth]

Guys,
Two ships come to mind that had this type of flap design - the original Kenhi Cougar (upright cowled engine) by Hi Johnson and a ship that Harold Price built the Defiant. I have the Cougar kit and it is a bear to build as it has a swept forward trailing edge too. The Defiant has part span flaps, maybe that allows this arrangement to work. On the Cougar they call for very small flap deflections.

Anyone know how Harold did with the Defiant?

Best,        DennisT

[Brett Buck]

Hi All,
Everyone is discussing aerodynamics, but the thick hingeline has structural benefits completely aside from aero.
Flap twist may not be our friend.

  I am not ignoring that, but the 1"+ wide discontinuity is nothing to sneeze at, either. You can make the flaps stiffer in other ways. To me, Al's results + the other similar experiences are persuasive - it's less lift even without the big gap, and the pitching moment + whatever else it's doing is a significant problem if there is a gap. Far more than flexibility, near as I can tell. And I am the guy regularly getting jihads declared against me for sacrificing weight for rigidity.

  Al's aerodynamic reasoning for the high aft curvature is also persuasive.

    Maybe there's something to it we all missed but someone needs to prove it wrong, because every time someone tried this, it didn't work out.

  Brett

[Howard Rush]

Everyone is discussing aerodynamics, but the thick hingeline has structural benefits completely aside from aero.
Flap twist may not be our friend.

That's what I thought.  You'd get a different flap shape with different airspeed.  Hence I made flaps that have carbon torque tubes and are extremely stiff.  My homies think that they are too stiff.

[Howard Rush]

I'm going to take a shot at this: Negative pitch moment is a function of the increased camber from the flap deployment moving the center of lift of the wing aft. Flap design can have a large impact on this as evidenced by Fowler flaps on large aircraft. How this effect can be mitigated on our aircraft, I have no idea. Too new to the field.

Good point.  Me neither.  Kim keeps bringing up interesting stuff, doesn't he?

[Larry Wong]

WOW ! I guess my question was not so simple ,got a lot good info. Still trying to learn new trick's. ( like a old dog) keep them coming.  How about  To cover flaps or not ?  Or how many hinges per flap  ( inches apart)

[Howard Rush]

Of course Larry wants to hear your trick, as do other folks following this topic.

I was vague because I need to look up the reference again and make a drawing, and I don't have time now, because I'm girding my loins for the Naturals.  It's a flap with steep taper right at the back.  The cross section looks like a cold chisel.  I forget the name of it. It might be an easy way to reduce hinge moment.

[Bill Little]

Let me know if you want to try something kinda in between.  I have a trick in mind.

Hi Howard,

You have a PM in your message box.

Bill

[Steve Helmick]

That's what I thought.  You'd get a different flap shape with different airspeed.  Hence I made flaps that have carbon torque tubes and are extremely stiff.  My homies think that they are too stiff.  

Howard, wouldn't stiffer flaps call for a reduction in flap area, since flap area was designed/optimized based on flexible materials? Maybe also making the elevators with a CF torque tube and CF lever to increase elevator effectiveness while also keeping the iron out of the tail?   Steve

PS: Call me if you need a safety stooge anytime today or until you leave for the NATS.  

[Howard Rush]

OK, here are pictures: http://digital.library.unt.edu/ark:/67531/metadc61552/m1/16/ .  The TE shape of the upper two are the ones of interest, I think.  Disregard the hingeline stuff.  More after the Nats.

[Howard Rush]

Howard, wouldn't stiffer flaps call for a reduction in flap area, since flap area was designed/optimized based on flexible materials? Maybe also making the elevators with a CF torque tube and CF lever to increase elevator effectiveness while also keeping the iron out of the tail?

Probably so, on both counts.  It would be nice to get the iron out of the tail, even if you don't need the stiffness. 

Thanks.  I plan to be there about 5:00.

[Eric Viglione]

On a sorta related subject, I've got some very telling in-flight photo's stashed away somewhere of an flapped plane doing a square pullout.

The amazing thing in the picture is that the center of the flap closest to the horn is deflected like 35 degrees, and the last few inches out to the flap tip is at dead neutral still!!!!

I guess there was some contest balsa used on that one, heh.

The thought that occurs, is what if lucky you builds a plane that has one stiff flap and one not so stiff one?Talk about impossible to trim, especially if the flimsy one is outboard. Prolly drive someone crazy!

EricV

[Tim Wescott]

You will also find that some of his airfoils were not fully semitrical(spelling again). 

Per the article by Al that I read, he counted up all the corners in the pattern, and everything matched except that there were six more inside 120 degree tight corners than outsides -- and three of the inside ones were scary (bottom corners of the triangles and especially the hourglass).  So he made his airfoil with a bit of camber, giving up a bit of performance potential at the two top corners of the hourglass to get better performance in the triangle and the bottom of the hourglass.

People seem to manage to win big contests with symmetrical airfoils, however, so I'm not sure how big a deal it is.

[Dave_Trible]

On a sorta related subject, I've got some very telling in-flight photo's stashed away somewhere of an flapped plane doing a square pullout.

The amazing thing in the picture is that the center of the flap closest to the horn is deflected like 35 degrees, and the last few inches out to the flap tip is at dead neutral still!!!!

I guess there was some contest balsa used on that one, heh.

The thought that occurs, is what if lucky you builds a plane that has one stiff flap and one not so stiff one?Talk about impossible to trim, especially if the flimsy one is outboard. Prolly drive someone crazy!

EricV

Exactly why I cover the flaps in carbon these days.

Dave

[Tim Wescott]

On a sorta related subject, I've got some very telling in-flight photo's stashed away somewhere of an flapped plane doing a square pullout.

The amazing thing in the picture is that the center of the flap closest to the horn is deflected like 35 degrees, and the last few inches out to the flap tip is at dead neutral still!!!!

I've seen that picture, or one quite like it.  I think of it when I'm matching my flaps from side to side.

[Howard Rush]

On a sorta related subject, I've got some very telling in-flight photo's stashed away somewhere of an flapped plane doing a square pullout.

The amazing thing in the picture is that the center of the flap closest to the horn is deflected like 35 degrees, and the last few inches out to the flap tip is at dead neutral still!!!!

I guess there was some contest balsa used on that one, heh.

The thought that occurs, is what if lucky you builds a plane that has one stiff flap and one not so stiff one?Talk about impossible to trim, especially if the flimsy one is outboard. Prolly drive someone crazy!

I would like a copy of that picture, please, if you can find it.  I thought Steve took it.

[Randy Cuberly]

It would seem the smallest possible flap deflection to produce the desired lift would be the best solution.  What I experienced on the Sagitta was a flap deflection of only about 15 to 16 degrees necessary for about 30 degrees of elevator deflection to produce an excellent hard corner with a flat recovery.
This means that the interruption in the wing surface would actually be relatively small, and easy to slightly inset into the trailing edge.

Certainly there are many other factors in this situation...for instance Al's experimentation was aimed at producing maximum lift with his airfoils, not producing the most efficient lift in terms of lift vs drag.
Higher aspect ratios with lighter wing loadings do not need to sacrafice with higher drag ratios to achieve the DESIRED lift.
Al's Semi-Scale airplanes had relatively high wing loadings and needed large flap deflections to produce maximum lift for a given speed.  My 50 oz 650 sq in Sagitta did not.
My point here (if there is one ) is that the flap design is not an independent factor but should probably be well considered in the overall requirements of the airplane design.
Based on personal experience with "wimpy" flaps I think flap and wing stiffness is one of the most important considerations in making an airplane that is predictable to trim, especially if a hard corner is desirable...

Randy Cuberly

[Dennis Adamisin]

BEST thing about thick flaps: what Dean said (very stiff in torsion, every degree of deflection COUNTS). I also capped the TE with spruce so I could make the TE as thin as possible. 

WORST thing about thick flaps: what Brett said.  (goofy hinge moments)  I used a textbook 0018 at the root and a 0021 tip with the highpoint moved forward to 25%.  With this set of 'foils, when the flaps deflected there was a big time discontinuity at the hingeline.  I have always thought it significant that my first blended flap was also the poster child for sealed hinge lines.  The next three airplanes took advantage of it with slower controls & ever aft CG.  I also beveled the LE of the flap as little as possible - just barely enough

Also was using thick Ted Fancher inspired tail airfoils that resulted in very stiff elevators.


I honestly do not remember the "balance rag" incident, but that sounds pretty funny...

[Chris Wilson]

I think that flat plate flaps with a standard convex airfoil is the most effective because flap deflection 'corrects' the total shape somewhat.

Just lay a straight edge between the flaps trailing edge (at 0º) and the high point of the wing and witness a slight concavity described between the wing and that straight edge - that is the condition in level flight with a slight reflex shape ruining the section.

Now deflect the flap so that the concave area is at its smallest - to my thinking there should be a slight improvement in efficiency between these two conditions for very little extra drag.

Further deflection of course adds drag like crazy.

These are just my thoughts but to me the flat plate flap would seem to get a high percentage lift vs drag increase around neutral for no extra loss due to design as opposed to the flap being part of the section.

Hope that is explained OK - flak jacket on!

[Geoff Goodworth]

As interesting as this discussion is, I think it has moved away a bit from the original question.

If I may summarise.

Al Rabe's experiments were pretty conclusive that having the flap as part of the airfoil was not as effective as a flap added to the back of a truncated airfoil.

In my limited experience, a flat flap with the TE rounded off seems to work about as well as a tapered sheet flap but does not look as good.

But then we have the success enjoyed by the Beringer designs with the flap as part of the airfoil. Also, Pat Johnston has used this style of flap on some of his latest designs and reports outstanding turning ability.

So who knows! I sure don't, but for the time being, I'm using tapered flaps attached to a truncated airfoil like almost everybody else.

As for flap deflection, years ago two Australians, Brian Hampton and Brian Gardner conducted experiments to see how flap much deflection was used/required.

Brian Hampton used his very clever adjustable bellcrank to dial in from zero, the flap deflection required to produce a good pattern; Brian Gardner measured the amount of flap deflection he used to fly a good pattern. Coming from almost opposite sides of the exercise, both arrived at a figure around 15°.

[Brian Hampton]

It would seem the smallest possible flap deflection to produce the desired lift would be the best solution.

I quite agree. The model I designed had the flaps as part of the airfoil section but also had fully adjustable (independent) movement from zero to around +/-30 degrees. With flaps set at zero there was a trace of stall in the last corners of both triangle and hourglass even with a very light wing loading of 10.25 ounce/sq foot. Flaps set at +/-5 degrees eliminated the stall and kept added drag to a minimum to minimise loss of airspeed in hard turns. Higher wing loading would have required a bit more flap movement on this model.

[Howard Rush]

I think that flat plate flaps with a standard convex airfoil is the most effective because flap deflection 'corrects' the total shape somewhat.

Just lay a straight edge between the flaps trailing edge (at 0º) and the high point of the wing and witness a slight concavity described between the wing and that straight edge - that is the condition in level flight with a slight reflex shape ruining the section.

Now deflect the flap so that the concave area is at its smallest - to my thinking there should be a slight improvement in efficiency between these two conditions for very little extra drag.

Further deflection of course adds drag like crazy.

These are just my thoughts but to me the flat plate flap would seem to get a high percentage lift vs drag increase around neutral for no extra loss due to design as opposed to the flap being part of the section.

Hope that is explained OK - flak jacket on!

Sounds like the flaps used by the current World Champion.

[Bill Johnson]

While the subject is out there, I was wondering why, if aerodynamic twisting and stiffness is an issue, why are C/L flaps full span? It would seem to me that a half-span flap would be significantly stiffer so would have the same effect while just slightly more deployment angle.

[Brett Buck]

While the subject is out there, I was wondering why, if aerodynamic twisting and stiffness is an issue, why are C/L flaps full span? It would seem to me that a half-span flap would be significantly stiffer so would have the same effect while just slightly more deployment angle.

    The issue is two-fold - first, the more of the wing that is flapped, the more effective it is. Putting a narrow flap on the entire wing is much more effective in terms of producing Cl than a large flap over only part of the wing. The second issue is hinge moment, if you put only a half-span flap, to sufficient lift, you need to make the chord very large, and that increases the hinge moment far more than a narrow full-span flap of the same effectiveness. Both of these were adequately documented in the Imitiation article.

    There's a third issue that may or may not be compelling - if you have partial-span flaps, at the end of the flap you will have lot of turbulence generated where the deflected flap ends and the fixed part begins. That not only reduces the effectiveness further, it also creates the possibility that the drag or lift associated with the transition if this turbulence is not the same on both wings. This appears to have been demonstrated sufficiently to my satisfaction by Paul Walker and his "lightweight" Impact (I forget which one, it was largely orange) - weird trim issues with the flaps at maybe 75% span and no weird trim issues with fixed portions permitted to move with the rest of the flap.

    Making the flaps more rigid is desirable but hinge moment in particular is a compelling problem. My thinking right now is that I will do whatever it takes to make the flaps stiff, and make them THE SAME on each side (probably more important), and use relatively narrow-chord nearly full-span flaps.

    Brett

[Bill Johnson]

Thanks, Brett. That all makes perfect sense. It'll take some time to wrap my mind around the aerodynamic issues encountered in C/L flying.

[Brett Buck]

Thanks, Brett. That all makes perfect sense. It'll take some time to wrap my mind around the aerodynamic issues encountered in C/L flying.

   I have been on that trail for 40 years+!   Excellence is a journey, not a destination

    Brett

[Howard Rush]

Thanks, Brett. That all makes perfect sense. It'll take some time to wrap my mind around the aerodynamic issues encountered in C/L flying.

It's sure taking lots of time for me.  

Brett's piece above makes sense to me, too. 

One thing that's not obvious is that flaps affect the pressures of the whole wing; they're not something independent added to the back. Here you can see how streamlines and pressures of the whole section change when flaps are deflected.  Pressures go from high (red) to low (blue).

[Brett Buck]

One thing that's not obvious is that flaps affect the pressures of the whole wing; they're not something independent added to the back.

   That was a significant misapprehension that plagued some of the earlier attempts at engineering analysis by "trial and error". I think most people get it now.

    BTW, can you tell us how Hans the air molecule knows to speed up as he goes over the flap, so it can catch up with his buddy Fritz by the trailing edge and thus create low pressure on the top via the Bernoulli principle?  That's one is a far greater misapprehension that is very much still with us.

     Brett

   

[Howard Rush]

That's one is a far greater misapprehension that is very much still with us.

Not to mention that a propeller is a screw going through cheese, rather than an aerodynamic device.

[Bill Ervin]

Thanks for that diagram Howard.  It was interesting to see the high pressure under the wing taking a dramatic shift aft while the low pressure on top remaining much more evenly distributed over the chord.  Also interesting to see the high pressure localized right under the flap hinge line.  Seal those hinge lines.

Bill

[Serge_Krauss]

I thought Hans won.

[RandySmith]

I would like a copy of that picture, please, if you can find it.  I thought Steve took it.

I have pictures of that from the 70s, and a video, I will try to dig them up, that was one of the reasons I kept the larger area at the root of the center part of the wing and the flaps got smaller as they went to the tip, Imagine if you made the tip of the flap as wide as the root.. you would have a lot of force trying to twist them. Howards, torque  tubes  would need to be called into action. It was also a small part of the reason for non full span flaps, of about 7/8 span .  My near full span flap have no trim issues, and contrary to some beliefs the same plane goes thru corners cleaner than with full span.

Randy

[RandySmith]

"BTW, can you tell us how Hans the air molecule knows to speed up as he goes over the flap, so it can catch up with his buddy Fritz by the trailing edge and thus create low pressure on the top via the Bernoulli principle?  That's one is a far greater misapprehension that is very much still with us.

     Brett"


Anyone know the theory of why a boat planes out and rises to the top of the water when accelerated?   

Randy

[RandySmith]

I have pictures of that from the 70s, and a video, I will try to dig them up, that was one of the reasons I kept the larger area at the root of the center part of the wing and the flaps got smaller as they went to the tip, Imagine if you made the tip of the flap as wide as the root.. you would have a lot of force trying to twist them. Howards, torque  tubes  would need to be called into action. It was also a small part of the reason for non full span flaps, of about 7/8 span .  My near full span flap have no trim issues, and contrary to some beliefs the same plane goes thru corners cleaner than with full span.

Randy

The same video I have also shows very clearly the flaps being forced open between the hinges when going thru maneuvers. You can see this clearly as it is against a bright sky background. After seeing that it is very clear.. seal the hinge lines.
That was the main reason I use many , 7 or so, small hinges per flap, instead of a few large ones, I found that out by trying the 4 1/4 scale hinges back in the early 80s... lots of small light hinges are better than a few 1/4 scale hinges.
I think the tube method that McDonald used, and the Howard tube flaps are probably the best way to do this.

Randy

[Brett Buck]

Anyone know the theory of why a boat planes out and rises to the top of the water when accelerated?   

Randy

   I always assumed it was witchcraft. Witches float like ducks, if you are not a witch, you sink. Everybody knows that.

    Brett

[RandySmith]

  I always assumed it was witchcraft. Witches float like ducks, if you are not a witch, you sink. Everybody knows that.

    Brett

LOL     I thought it may have been that M  word..  ya know (magic)   weird a boat bottom is sorta shaped like an airfoil on a plane... hmmmm..

Randy

[Peter Germann]

Has any one done a test of flap design?  1. Flaps part of wing airfoil.  2.plank( Sheet )flap not air foiled.  

While I have not done such testing myself, it is interesting to note that there is a substantial fleet of rather successful Yatsenko Shark airplanes out there, regulary doing pretty good at all level contests.  With the flaps being part of the wing airfoil, the flap / elevator deflection ratio is approx. 30° flaps at 45° elevator.

rgds, Peter Germann