Sealed or Unsealed hinges?

[Mark wood]

I spent a few days flying with the airshow performer Greg Koontz at his Sky Ranch flying 4/4 (full size) aerobatic airplanes. An interesting opportunity presented itself while I was there. He has two pretty much identical airplanes, American Champion Decathlons. There are a couple of differences between them. Both have Lycoming IO360 180 hp engines. One has an MT 3 blade propeller and the other has a Hartzell 2 blade propeller. One has sealed aileron hinges and the other doesn't. Most of the time we only get a subjective test on one airplane flown at different times and conditions which leads to troubles in quantifying any changes. We all suffer from confirmation bias. In this case I had to opportunity to fly two airplanes back to back in a good A:B test.

To quantify the results, I figured that if there is a positive result from sealing the hinges it could be easily done by measuring the resulting roll rate of an aileron roll. There is a lot going on but with two otherwise identical airplanes this is a good test. I was surprised by a few things. Basically, if hinge sealing makes a difference we should be able to quantify the results using a stop watch. Start the roll, simultaneously start the stop watch and check how long it takes to complete the roll. Since the flap on a model is generally a plane flap the roll rate is a direct comparison of flap effectiveness on a model.

I am going to present first the videos of the rolls in both airplanes. It honestly doesn't take a stopwatch to see the difference between the two configurations. After a bit of time, I will present the ailerons and which airplane has which configuration. The idea here is to chime in first with which one you think has sealed hinges and which one doesn't.

Test configuration A



Test configuration B

[Dan McEntee]

    I will say that Config "B" looks to be significantly faster an is the one with sealed aileron hinges. The Decathalon is a pretty good platform for this I think, just a very basic and clean planform. What do they use to seal the hinge lines with?   On some of the more serious aerobatic airplanes that I have been able to look at up close, most have had sealed hinge lines, but could not tell how they did it, just something closing the gap.
  Type at you later,
    Dan McEntee

[Paul Taylor]

I’ll take a SWAG.
The sealed hinges had the fastest roll rate due to not allowing air to bleed off.


Sent from my iPhone using Tapatalk

[Dennis Toth]

Mark,
Yes, there is a easily discernable difference between the two roll rates. For our purpose in stunt the main reason to seal the hinge line in not to get more lift from the flaps but to get even lift from the flaps.

When building with balsa/tissue and paint getting a consistent tight hinge gap is next to impossible. For our models these slight imperfections in the hinge gap can cause roll as controls are applied, sealing eliminates that.

Yes, we do get a little more efficiency with the sealed hinge line that allows less deflection for the same turn rate/size but that is a bounce. 

Best,     DennisT

[Brett Buck]

    The roll rate may or may not increase when sealed, depending on how the hinge line is configured. In particular, full-scale airplanes use a variety of methods to handle adverse yaw, some of which have the effect of a slotted flap (at least in one direction)- which might give you more roll moment than sealing it. This is particularly true if the hinge line and shape of the aileron has hooks or notches that poke up on the bottom to intentionally create drag that counters the adverse yaw caused by the differential lift.
   
   I also note that "more" lift was never really the benefit of sealing the flaps or elevator on a stunt plane, although there is no doubt* that it does on a conventional stunt hinge line. The big advantages are making it consistent from side-to-side, between "up" and "down" control motion, and over the range of motion. The original application, at least around here, was for an elevator, and had the effect of changing a "lots of up and no down" control asymmetry to nearly symmetrical. In that case there was both a huge discontinuity with sharp edges on the "top" when it was deflected down, and a gap that varied between "up" and "down". This even from an acknowledged mater modeler making it as carefully as he could.


   So, the experiment is interesting and probably tells you something, but the specifics can make it come out either way in any particular case.
     Brett

*both from flying them, and directly-on-point wind tunnel tests.

[Ken Culbertson]

Mark,
Yes, there is a easily discernable difference between the two roll rates. For our purpose in stunt the main reason to seal the hinge line in not to get more lift from the flaps but to get even lift from the flaps.

When building with balsa/tissue and paint getting a consistent tight hinge gap is next to impossible. For our models these slight imperfections in the hinge gap can cause roll as controls are applied, sealing eliminates that.

Yes, we do get a little more efficiency with the sealed hinge line that allows less deflection for the same turn rate/size but that is a bounce. 

Best,     DennisT

I think you might find that a lot of people think it is the extra lift.  My experience is that the hinge alignment (eg. centered) has more effect on roll than does the gap.  Sealing misaligned flaps might actually cause more roll than leaving them unsealed.  A sloppy hinge line might be improved by sealing but I am not sure a truly even one would have much effect.  I have never had a plane with roll issues that was straight and unwarped, sealed of unsealed.  The only way I can see to know for sure is to have a blind test.  Have multiple pilots fly a plane back to back with hinges sealed then unsealed but don't tell them which is which.  Now if everybody has the same answer you will know.

Still, if you *think* you are flying better with sealed hinges, you probably are, even if it is having absolutely no effect on roll. 

Ken

Sorry Brett - I didn't see your post.

[Mark wood]

I knew this would spark a good conversation. My experience is that gap seals make little to no difference quantitatively meaning in a way we can measure easily. There are wind tunnel tests that show that a gap can increase the maximum CL of a flapped airfoil. The penalty is that there is often more drag at lower angles of attack. The configuration that some of the jet manufacturers use for improved roll control are done this way but they don't seal the hinges due to icing concerns. The drag impact is very hard to quantify without very careful testing. I have done A:B:A tests on many aircraft and in very few cases did I find a discernable difference between sealed and unsealed.

You can research my videos of the SV flying with the tail mounted camera where I did flight testing with tufts to observe airflow and turbulence. I cannot tell the difference in handling qualities nor in video evidence. I've done this in several full size aircraft and models as well with similar results. The one case I was able to tell the difference between sealed and unsealed was when the airplane had a gap between the stabilizer and the fuselage. Closing that gap eliminated a pitch oscillation at slow speed.

The impact of doing these kinds of things and simple flight testing often have significant confirmation bias. That doesn't really mean a lot as some times it is enough that by performing a change that may not be measurable data wise, does have an impact on our mind. That alone is sufficient to make the modification as the result is positive wether the airplane does change or not. That we believe is enough.

Okay, the red airplanes aileron has about a 3/8" gap between the leading edge and the cove. That gap closes down to about 3/16" at full deflection. By doing this, it creates a jet of higher velocity air which invigorates the boundary layer. This does two things. One is because of the shape of the surface it lowers the pressure on the portion of the "flap" in front of the hinge line which reduces the hinge moment as the hinge on this is placed at the 30% station. This pressure reduction also increases the lift of the aileron which is what drives the increased roll rate.

Both airplanes have similar overall stick force gradients although the red one has lower breakout forces. That they are similar would be expected as these airplanes are certified to FAR Part 23 which has specific stick force gradient requirement. I find flying either to be about the same but that I can fly the red one more "crisply". I'll post some detailed photos later because there are other interesting details of the implementation worth seeing and discussing.

BTW, I often seal my hinges because it seems to be the thing to do and when I do and the conversation arises where I'm asked, the positive reply belays the resulting, "well you need to seal the hinges so it will fly better".

 

[Dan McEntee]

I think you might find that a lot of people think it is the extra lift.  My experience is that the hinge alignment (eg. centered) has more effect on roll than does the gap.  Sealing misaligned flaps might actually cause more roll than leaving them unsealed.  A sloppy hinge line might be improved by sealing but I am not sure a truly even one would have much effect.  I have never had a plane with roll issues that was straight and unwarped, sealed of unsealed.  The only way I can see to know for sure is to have a blind test.  Have multiple pilots fly a plane back to back with hinges sealed then unsealed but don't tell them which is which.  Now if everybody has the same answer you will know.

Still, if you *think* you are flying better with sealed hinges, you probably are, even if it is having absolutely no effect on roll. 

Ken

Sorry Brett - I didn't see your post.

      How did roll axis on a stunt model get brought into this? The only mention of roll rate is related to the control surfaces in the test are ailerons to demonstrate the difference in efficiency between the two airplanes. Of any thing you can do to a stunt model that can have an immediate and positive effect, in my experience, it's taping up the gap in both flap and elevator hinge lines, and if no flaps, just the elevators.

   I picked the sealed hinge to be airplane B because of the increased roll rate, basically 3 seconds versus 5 seconds. In relation to a stunt model, I have had my own models and others I have helped with get a more responsive turn , simply by adding a few pieces of tape. The seal makes the control surface more efficient and positive.

  Type at you later,
   Dan McEntee

[Mark wood]

      How did roll axis on a stunt model get brought into this? The only mention of roll rate is related to the control surfaces in the test are ailerons to demonstrate the difference in efficiency between the two airplanes. Of any thing you can do to a stunt model that can have an immediate and positive effect, in my experience, it's taping up the gap in both flap and elevator hinge lines, and if no flaps, just the elevators.

   I picked the sealed hinge to be airplane B because of the increased roll rate, basically 3 seconds versus 5 seconds. In relation to a stunt model, I have had my own models and others I have helped with get a more responsive turn , simply by adding a few pieces of tape. The seal makes the control surface more efficient and positive.

  Type at you later,
   Dan McEntee

The flap on a stunt model is no different that the flap on the end of the wing. Both are plain flaps. Roll rate on the later is a direct comparison and quantifiable while the stunt model is not measurable therefore not quantifiable. The results are interesting because what make the performance better is transferable. Simple as that.

[Perry Rose]

Are the aileron deflections the same on both planes with the stick to the stops? Airspeeds the same?

[Mark wood]

Are the aileron deflections the same on both planes with the stick to the stops? Airspeeds the same?

Yes, both are basically the same aircraft. The white one has sealed hinges and spades to reduce the stick force gradient and the red one uses an offset hinge at 30% of the surface to reduce the force gradient. Reducing the stick force is imperative in order to achieve full deflection. Many of the present plain hinge models likely never truly see full max deflection due to the hinge moment of the combined flap and elevator. Reduction of these is one of the most major improvements made in the full size aerobatic arena. There is a huge resistance to look at and follow these new developments within the CL community here which is ok as what is present is the same as it ever was and has been working as such for a very long time. I have been systematically working on this avenue for a few years now and learn something new every day. These particular test flights were super awesome as they gave me an opportunity to perform back to back comparisons. Here's a couple photos of both airplanes. I will post some better details of the ailerons later.

[Mark wood]

another view.

[Mark wood]

Here are a few photos. Unfortunately I don't have a good photo of the white airplane aileron from the top. It is tighter than the bottom for a reason I don't know.

[Mark wood]

A couple more of the red plane

[Mark wood]

Red aileron in neutral to show the contour. The contour and shape are what make the flap work better. The gap acts to pump energy into the boundary layer. Howard posted a phot of Russian made model with a hinge line of 12.5% and the nose of the flap had a bulbus shape which come from a Whitcombe report from the 1940's. What you see here is the 200's + approach. This is modern day technology...

[Mark wood]

One more before I head out to the shop and go fly toy airplanes. This is the view from the top showing the gap. It is quite large and very much contrary to the 3x5 card of popular opinion but I like to base my opinion upon quantifiable data.

[Brett Buck]

The flap on a stunt model is no different that the flap on the end of the wing. Both are plain flaps. Roll rate on the later is a direct comparison and quantifiable while the stunt model is not measurable therefore not quantifiable. The results are interesting because what make the performance better is transferable. Simple as that.

    Try sealing the flaps on the inside, and leave the outside unsealed. Then get back to me about confirmation bias. Or, if you are *really sure*, do it the other way!

   I would note that it is such a strong effect that there is a detectable difference between sealing between the hinges (leaving about a 1/32 gap at each hinge) and sealing over the hinge.

     BTW, you are generally right about hinge moment, as a general proposition you cannot get a lot of deflection due to hinge moments. Trying to get more tension might get you more deflection in some cases, but most of the methods to "get more line tension" cause bigger problems than the one you set out to solve. In some corner cases, *adding weight* to get more tension for a given speed greatly improves your margin over the Netzeband Wall. The limiting factors is making it so much heavier that the wing loading considerations or vertical climb is so impacted that it is a net negative. This was almost always the case with classic engines, so making it lighter always made it better. But not any more.

     Brett

p.s. As always, I admit with I rip off/"learn from" others. Bill Werewage seems to have been the first person to grasp the effect, comparing cloth-hinge models with pinned-hinge models. As per usual, he did not exactly go out of his way to share that information.  Denny Adamison was the first person that mentioned it in print, howevet, and it was one those "D'oh! <slaps forehead>" moments for a lot of us. Frank Williams did the wind tunnel test where he got 40% more lift and a much more linear response.

https://stunthanger.com/smf/stunt-design/some-videos-of-a-tufted-sv11/msg625169/#msg625169

Paul Walker was the one who first noticed it mattered if you sealed over the hinges or between (maybe the 2005 TT?), and for fans of confirmation bias, I thought he was exhibiting Howard's accused "princess and the pea" phenomenon. Then I tried it, it was not subtle.  Since Denny mentioned it, Ted and I solved lots of problems with both stalling, and with massive inside/outside turn differences on various airplanes by sealing the hinge lines. The original Imitation-type stabilizers were massively thick and had very deep, sharp-edged hinge lines and that was where it made the biggest difference, like, from almost unflyable (to me, Ted can fly anything...) to our current standard of performance.

   P. P. S. BTW, on the topic of confirmation bias, I had it in reverse for several things - the hard-point handle, which sat in my toolbox for months until Ted cajoled me into trying it -and I am still using the same handle today!  The most irritating was the elevator wedges, I took off, flew 1/2 a lap and yelled a bad word out to my flying buddies. It was the worst day of my competition life - because David was right about it. Dennis Nunes and Jim Aron can confirm my reaction.

[Mark wood]

    Try sealing the flaps on the inside, and leave the outside unsealed. Then get back to me about confirmation bias. Or, if you are *really sure*, do it the other way!

   I would note that it is such a strong effect that there is a detectable difference between sealing between the hinges (leaving about a 1/32 gap at each hinge) and sealing over the hinge.

     BTW, you are generally right about hinge moment, as a general proposition you cannot get a lot of deflection due to hinge moments. Trying to get more tension might get you more deflection in some cases, but most of the methods to "get more line tension" cause bigger problems than the one you set out to solve. In some corner cases, *adding weight* to get more tension for a given speed greatly improves your margin over the Netzeband Wall. The limiting factors is making it so much heavier that the wing loading considerations or vertical climb is so impacted that it is a net negative. This was almost always the case with classic engines, so making it lighter always made it better. But not any more.

     Brett

Yes sealing on the inside is something we do and try. So, yes, my observation covers that as well. Your model and experience may be different. Good quantifiable data as presented is undeniable. What you miss is that the interaction of the gap with the flap airfoil increases the total CL more that the sealed surface does. And very much measurably so. The red airplane with the same size ailerons roll 30% faster than the other one. Even on the very best circumstances changing the gap seal to a web in the center would not make that much change. And keep in mind that sealed version of this airplane ahs both top and bottom seals. There are two thing going on. One is the increase if total lift and the other is reduction of the hinge moment. Both configurations have moment reduction application. The white airplane has spades and the end result is both are similar enough in force gradient.

Reducing the hinge moment to zero is 100% possible and I have gone beyond zero hinge moment into force gradient reversal which is an interesting experience. So, again reaching CL max with a flapped model is 100% doable. It takes looking beyond the how its always been done this way before because of the limitation you cite. Take a very close look at the profile of the flap on the Decathlon and then work your fluid dynamics brain through the pressure response and the thickness increases.  Then realize that this reduction in pressure is occurring in front the hinge point which reduces the hinge moment. The reason the Decathlon force gradient is what it is due to Part 23 requirements not because it couldn't be lighter. The pilots ability to push the stick over to the stop is a function of how much force there is on the stick. 

Here's come statistics of this implementation. The  airfoil of the flap is more or less a slab sided cone section with what appears to be a NACA 0024 series but could be a simple ellipse forward section. The maximum thickness is at 25% chord and the hinge point is at about 29.5% chord. This would imply that the aileron would be very nearly neutrally stable if it were in free stream and might even be negative. At the station the section is about 1" thicker than the wing at that location. The thickness ratio is 24%.

[Mark wood]

Here are some photos of the flap in the full down and full up positions. Notice how the gap closes down. This creates a high velocity airstream that invigorates the boundary layer and reduces the pressure over the surface. This is a significant part of how more lift is generated in both directions. It is also part of how the hinge moment is reduced.