Hinge line fairing

[Luis Strufaldi]

I don't know if this topic should be in the Open Forum or Engineering board, but someone can sort that out later.

With a recent "influx" of Yatsenko Sharks here in Brazil, a discussion sprang at the field regarding hinge lines fairings vs. tape (I am using the term "fairing" because that seems to be the full size plane nomenclature for the hangs above and bellow the trailing edge of wing and stab that hide the hinge line gap).

The concensus of the "experts" here seems to be that, by moving the pivot point back, creating a round leading edge at the control surfaces and adding the "fairings", the resulting hinge line acts like a "sealed" hinge line.

Does it effectively create a sealed hinge line? Is it worth the extra effort involved in the making of it? Is there any other benefits and how does it compare to a tape seal.

I'm interested in your opinions on this.

[RC Storick]

On my Youtube channel I show how to make this. There are 3 videos but here is one of them that may explain the process.

I will try to make a video later today showing the detailed finished product.

(I am using the term "fairing" because that seems to be the full size plane nomenclature for the hangs above and bellow the trailing edge of wing and stab that hide the hinge line gap).

The concensus of the "experts" here seems to be that, by moving the pivot point back, creating a round leading edge at the control surfaces and adding the "fairings", the resulting hinge line acts like a "sealed" hinge line.

Does it effectively create a sealed hinge line? Yes Is it worth the extra effort involved in the making of it? It is to me Is there any other benefits and how does it compare to a tape seal. Super free movement

[RC Storick]

[Brett Buck]

I don't know if this topic should be in the Open Forum or Engineering board, but someone can sort that out later.

With a recent "influx" of Yatsenko Sharks here in Brazil, a discussion sprang at the field regarding hinge lines fairings vs. tape (I am using the term "fairing" because that seems to be the full size plane nomenclature for the hangs above and bellow the trailing edge of wing and stab that hide the hinge line gap).

The concensus of the "experts" here seems to be that, by moving the pivot point back, creating a round leading edge at the control surfaces and adding the "fairings", the resulting hinge line acts like a "sealed" hinge line.

Does it effectively create a sealed hinge line? Is it worth the extra effort involved in the making of it? Is there any other benefits and how does it compare to a tape seal.

I'm interested in your opinions on this.

   It does not act as a seal. That's not necessarily an issue if you can control the fits well enough to not get a lot of variation from side to side and from airplane to airplane. Depending on how it is configured, the fairing might help or it might hurt. Most experiment have found that this style of hinge to be a net negative as far as Cl goes, but you would have to try this particular variation to make any conclusions about it.

      Brett

[Randy Powell]

I've done this before. Tricky to get it to seal and have free movement of control, but it can be done.

[RC Storick]

It works great! Try it and you won't do it any other way again.

[Tim Wescott]

This is kind of a distillation of a number of discussions I've seen, plus what makes sense to me:

The problem with hinge lines isn't so much the drag, or the losses as air flows from one surface to another, or any of that.  The problem is that, on a model, it's hard to get it to do what it does consistently without sealing the gap.  So having a model with nice close gaps and an unsealed hinge line can mean that it's not acting the same from side to side, or when it's upright vs. inverted, etc.  This, in turn, gives you trim-fits.

Sealing the hinge lines fixes that not because it necessarily make things the most efficient (for some reason people often fall into the trap of making stunters efficient -- why?  The whole goal is to fly pretty!).  Sealing the hinge lines fixes that because it makes the airplane consistent.

An alternative that seems to work for some people is to intentionally make the hinge gaps huge -- like 1/8" or more.  This gets you consistency because small variations represent less of a percentage change in the gap.  However, it goes against the grain for most people, and some judges may feel that it looks butt-ugly.

I can't comment on the Yatsenko-style fairings, or on Robert's video, because I'm not sure exactly what's being done.  Watch, and use your judgment.  I'll probably still use tape.

[RC Storick]

Well there you have it,consistency I am consistently bad. This must be the reason. 

[Perry Rose]

I use clear tape with half the sticky covered and then it's put on the fixed surface and overhangs the movable surface sealing the gap with a frictionless flap. Monokote trimkote works and matches it's color monokote.

[NED-088]

Interesting.... Been doing my hinge lines in this style since 1985.

[Luis Strufaldi]

My engineer mind agrees with Tim on this in that your hinge line must be either really tight or really loose and that consistency is the name of the game (and that also explains why my models fly the way they do! Poorly ).

I remember that one of the features that really caught my eye on Dave Fitzgerald's TG (and there are many!) was exactly his hinge line. He explained to me that he removes the wire from the hinges prior to installing and runs a thin wire "connecting" all the barrels. In doing this, he achieves at least three things: He can disassemble the flaps by removing the wire, he guarantees that all the barrels are aligned and the hinge line is constant because he uses the wire to guarantee that there is no gap (Please correct me if I'm wrong, Dave). This leads to a "consistent" hinge line!

It is undeniable that the so called "gapless hinges" look amazing, but the extra work is killer. Perhaps if there were ready made hinges things would be different.

[john e. holliday]

There was at one time a gapless hinge.   It was a heavy cloth that was stitched down the center.   It was an iron on and you had to make sure the seam was centered.   I had one of the sets and wound up removing the thread and cutting the cloth into separate hinge pieces.  My self I'm not that good at flying the pattern, but I try to make the hinge gap as close as I can get it.

[Doug Moon]

Robert,
Do you have a vidoe somewhere showing installing the pocket hinge into the elevator?

Thanks
Doug

[Brett Buck]

There was at one time a gapless hinge.   It was a heavy cloth that was stitched down the center.   It was an iron on and you had to make sure the seam was centered.   I had one of the sets and wound up removing the thread and cutting the cloth into separate hinge pieces.  My self I'm not that good at flying the pattern, but I try to make the hinge gap as close as I can get it.

   One of the things it took a while to realize is that switching from cloth hinges to pinned hinges changed the way the airplanes flew. A continuous set of over/under cloth hinges seals the gap pretty well. Pinned hinges tend to leave a gap, and unless you make the shape exactly right, the gap changes through the travel. Most plans using pinned hinges show them incorrectly, where they are inset into the flap or elevator, and not inset at all into the fixed surface. That guarantees the gap changes as you move it. It's absolutely the worst way imaginable to make it  - you would do better just rounding off the edges and leaving the entire barrel width in the gap. I think Billy was one of the first to make the connection on that one.

     A large gap is better than a small one, to first approximation. That's why some people probably don't see the issue - although it may have led people to think they needed really thick or other specially shaped airfoils. The  consistency is aided by having a large gap, but the difference in performance between a nearly impossibly small gap and a sealed gap is dramatic, with test data to prove it. That's *why* sealing it aids in predictability, because the difference between a .005 gap and an .010 gap is huge in terms of effectiveness. You can live without extra lift capability (since no one in the modern era is very close to running out of lift capability in the first place, with a few exceptions for pointy airfoils), but you can't tolerate 25% more on the inboard wing than the outboard, with it changing from 25% to 0 to -25% as you move the controls.

     The various knuckle-type hinge deals are a different way to go about it. Al's car-hood experiments showed the "blended-flap" type of flap and knuckle hinge (like Keith Trostle's NATs winner), to everyone's surprise, gave less lift than a conventional hinge and flap. That may have been for the reason Al thought -that the upper surface discontinuity caused separation. His solution (much like Igor's) was to move the high point aft to that the fixed and moving sections faired in better with the very large deflections used at the time.

     The blended flap with the conventional "wedge" hinge line is even worse, I and many other people ran across that one in the early 80's. Almost certainly because of the massive sharp-edged gap it creates. It also seemed to have the effect of dramatically increasing the pitching moment. I can recall Larry Robertson checking the CG on Dennis Adamisin's "blended flap" airplanes at a contest in Detroit, and just like mine, the CG had to be radically aft to get it to turn. Then he point out it was even further aft once you took the rag off the nose! Denny of course was the first one to point out the value of sealing the hinge line in print, too, as far as I know.  I am not aware of any experiments with conventional thin flap shapes and the knuckle hinge, or as Bob describes it, the flap fairing. I would expect it to be generally better than a thin flap with an unsealed wedge hinge line, some better than a blended flap and knuckle hinge, and a lot better than a blended flap with a conventional hinge line. It's likely not at good as a sealed hinge line, but it might be good enough that other things matter more.

    To tell you how critical the degree of sealing is, Paul Walker found that sealing in-between hinges (leaving 1/16-1/8" gaps) was substantially different than sealing over them. I think he discovered that trimming his airplane  at the 2005 NATS.  I went home that evening and changed mine, and sure enough, in my case it made a small but distinct change, and I had to take out some tip weight to get it right again. That was the 3-4 flight before I crashed it after I stepped into a hole on the 600x600 pad and fell down. Subsequent experiments have confirmed the effect.

    This was all discussed extensively on SSW about 15 years ago, but the really good threads about it appear to have gone missing along with the earlier RCO forum (which is completely gone).

     Brett

[Trostle]

(Snip)
  
     The various knuckle-type hinge deals are a different way to go about it. Al's car-hood experiments showed the "blended-flap" type of flap and knuckle hinge (like Keith Trostle's NATs winner), to everyone's surprise, gave less lift than a conventional hinge and flap. That may have been for the reason Al thought -that the upper surface discontinuity caused separation. His solution (much like Igor's) was to move the high point aft to that the fixed and moving sections faired in better with the very large deflections used at the time.

(Snip)

       Brett

Interesting question.  Interesting comments.  And a pertinent comment about Al's "car hood" experiments.  All relevant.

First.  All of my stunt models use the recessed hinge line for the flaps, elevators and on my rudders.  (I use a variation of the Rabe Rudder to offset gyroscopic recession, but that is another subject.)  I feel that my stunt ships over the years have turned as well as most.

Yes, Al's experiments showed the flap as part of the airfoil with the recessed hinge line based on the airfoil on my Ta 152 that won the '70 Nats did not produce as much lift as the airfoils that he developed for his Super Semi Scale Stunt Ships (S⁵).  His experiment was not flawed, but the test sample he made was and I will try to explain.

On that test, Al offset the hinge line behind the radius of the flap leading edge so that when the surface starts to deflect, that leading edge starts to seal the gap between the flap.  The result was that the flap LE slightly raises above the surface of the wing TE.  This acts as a span wise spoiler to the airflow over that portion of the wing.  In all probability, that airflow is already turbulent before it reaches the flap.  But when the airflow encounters that spoiler, it will separate, thus reducing the effectiveness of the flaps.  As Brett mentioned in his paragraph above, this "surface discontinuity" may have caused separation.   I know for a fact that any kind of spoiler at that wing/flap area absolutely reduces lift.  I could go into a long explanation about how I know this for a fact, but just suffice it to say here that when I use tape to seal the hinge lines (even on these recessed hinge line configurations - yes, it can be done), the method of applying that tape will sometimes result in a slight ridge forming at the hinge line (due to oil causing the tape adhesive to not properly stick to the surface - another long explanation), that the turning ability of my ships is significantly reduced.  So, in other words, Al's tests, impressive as they were, did not give a true measure of the effectiveness of that flap hinging arrangement if the hinge line would have been properly placed.

What I do with all of my stunt ships is to place the hinge line slightly in front of the LE radius (even works on tapered thickness flaps/elevators - another long story) so that the flap (elevator) LE edges starts to recede from the surface of the wing (stabilizer) TE.  This give the effect of having the flap (elevator) thickness being slightly less than the LE preceding it.  And, in my opinion, as well as others, having control surfaces slightly thinner that the wing (stabilizer) TE is desirable because the models seem less sensitive around neutral to have a better groove with little or no effect on turning ability/responsiveness.

So, my recommendation to whoever wants to play with this business of recessed hinge lines on flaps and elevators, do so. But I would strongly recommend the four following points.

1.  Place the pivot point slightly ahead of the flap and elevator hinge lines.  I have found that 1/32" is sufficient for the difference on a full size stunt ship.

2.  Contour the TE of the wing/stabilizer to the contour of the flap and elevator LE to minimize any gap throughout the flap elevator movembent.  In this thread, this is called fairings at the TE.

3.  There is a way to tape these hinge lines.  I have found on all of my stunt ships for the past 20 years or more that it is effective to tape the elevators, less effective to tape the flaps with these kind of hinge lines. When fabricating the TE fairing, allowances should be made to be able to tape the hinge line.  For those that are interested, I will explain.

4.  Keep in mind that when the hinge line is recessed into the flap and elevator leading edges, that the "effective area" of the flaps and elevators is being reduced by the distance of that hinge line to the flap  and elevator hinge lines.  The chord of the flap and elevators should be increased by that amount to maintain the same "effective area".  (What I am calling "effective area"  is the area behind the hinge lines.)

Now, for a disclaimer:

I am in no way a golden arm in the world of serious competition stunt.  Over the years, my airplanes have performed well in competition using this recessed hinge line idea and have had adequate turning ability, and some observers have commented after flying my models  that they turn better than most.  My designs often have wing flaps that are as munch as 1" thick at the root tapering to 5/8" at the tip with an elevator that is almost 3/4" thick at the root to 1/2" at the tip.

What I have written here is not to argue or disagree with anyone.  Instead, what I have written is based on my observations and experiments over more than a few years.

(That Focke Wulf that won the 70 Nats was limited in its turning ability because it had 15% flaps and a 15% tail which was not uncommon in the designs from that period.  By the way, another Focke Wulf, same design, from the RSM kit won the Classic event at the 2008 Nats.)

Keith

[Tim Wescott]

It is undeniable that the so called "gapless hinges" look amazing, but the extra work is killer. Perhaps if there were ready made hinges things would be different.

That's how I do it and it's hardly any extra work.  You just need to grind off the heads of the hinge pins with a dremel tool, and make a music wire pin.

[Howard Rush]

There is a way to tape these hinge lines.  I have found on all of my stunt ships for the past 20 years or more that it is effective to tape the elevators, less effective to tape the flaps with these kind of hinge lines. When fabricating the TE fairing, allowances should be made to be able to tape the hinge line.  For those that are interested, I will explain.

Please.  I can't figure out how to do it.

[REX1945]

I can't figure out what's so wrong with cloth (Dacron) hinges :

[Dean Pappas]

There is absolutely NOTHING wrong with Dacron hinges ... but you knew that already.
Regards,
  Dean Pappas

[Jim Kraft]

I have been using Tom Morris's cloth hinges for years without problems. I use them with no gap between them and full span. They pretty much disappear under silk or silk span. They are easy to do when applied with thinned dope. A couple of pics of my Magician that have them on both the elevator and flaps. You have to look really close in person to see them at all. And they work butter smooth with no drag at all. There is a slight tendency for them to return to neutral for the first couple of flights, but after that they are completely free.

[fred cesquim]

i really love the looks of a faired hinge line, but is really time consuming and weight a bit more.
still stick with the flat du-bro hinges, no gap if done properly, so i see no reason to use tape