BillinIndiana

How much of a gap should be there? I followed a video on installing the CA hinges and it left me with a gap about as thick as a quarter (<coin)

Mr67Stang

That is a big gap. On a trainer that will not cause too much trouble but as you move onto faster more aerobatic planes it can cause flutter by air slipping between the gap. Some people seal the gap with clear monocoat. When I assemble control surfaces with CA type hinges, I gap them by putting a "T-pin" through the hinge and pushing the control surface to the stabalizer/wing until the gap is the size of the T-pin.

Go [link=http://www.hangar-9.com/ProdInfo/Files/HAN4250_Spitfire_60_Manual.pdf]HERE[/link] and scroll to page 8 of the manual to see what I am talking about. This is the manual for the Hangar 9 Spitfire but the technique still applies.

BillinIndiana

Here's what I read about flutter ... "Another misconception is that gaps cause flutter. This is another whole article in itself, but for the sake of this writing, just accept the fact that gaps are the cause of flutter about as much as my dog is responsible for WWII."

www.rcuniverse.com/magazine/article_display.cfm

BillinIndiana

Just found this.. www.rcuniverse.com/magazine/article_display.cfm It does say that the Gap can change the speed at which flutter will occur.

I was going to put some trim on my stabilizer , maybe I should do one of these _ _
\/

That's suppose to be the shape of the fold..Haha.. Anyways, I'll do that the full length of the stab/elevator gap... Probably wouldn't hurt, Huh?

Mr67Stang

As stated, it likely will not cause any issue with a trainer unless you are building a hotrod fast trainer.

goirish

after watching MinnFlyer in this video, just shows me how much I miss him and his knowledge. RIP

JohnBuckner

My Feedback: (1)

If you have already ca'ed your hinges and have a gap the thickness of a "Quarter" then that will work fine on your trainer and sealing the gaps with any further covering film or tape will make no differance at all as well as make the airplane look patch work.

Yes that amount of gap will reduce the response of certain higher performance types but not on your trainer. I have flown so many of the fellows trainers with gaps far worse than that and in reality it will make no differance. This come under the catagory of things that the fellows will get so wrapped up in with their first airplanes, just like the plastic pushrods that they never actually get to the field. Its sorta of modifying your first airplane to death.

Mr67Stang's (mine was a 65) suggestion to use a pin and use that thickness is a good procedure and works well.

John

jester_s1

If you're worried about it put a strip of covering or blenderm tape on the bottom of the hinge line and go fly the plane. I'll agree with others that it just doesn't matter on a trainer, but sealing gaps is a good habit to start now.

opjose

Yup completely true.

Unfortunately many of the more experience members repeat often heard erroneous advice.... much like the "fuel stays at the back of the tank, myth".

Flutter is NOT caused because of a large gap. In fact unless the gap is fairly ( or more rightly "extremely" ) large, there is almost no air traveling through the gaps. Wind tunnel videos confirm this.

Sealing hinges help to reduce flutter NOT because of air traveling through these rather thin areas, but because the material helps to stiffen the control surface and wing. In turn that changes or reduces any harmonic tendencies. So while it is not a bad idea to seal the hinge, there are many other far more effective techniques to reduce flutter.

It is not a bad idea to reduce the hinge gap as much as possible, but this should never be done to the point that it causes binding or restricted max throws.


I miss Minnflyer...

j.duncker

My Feedback: (2)

Hmm the gap in itself may not cause flutter but as any long time modeller will tell you that flying surfaces with substantial gaps WILL FLUTTER more often than those without. Maybe it is because the hinges flex or some aerodynamic slot effect .

Whatever it is hinge gapes are likely to lead to a second or so of buzzing on a high speed pass followed by bits coming off the model and a lawn dart job.

Keep those hinges tight, horns stiff and pushrods meaty to avoid flutter.

KitBuilder

Fuel stays at the bank of the tank !! LOL... I never heard that one before.

Only the plane continued to incresasingly accelerate.

opjose

Yeah, the "myth" states that "fuel stays at the back of the tank during downlines because the plane is accelerating."

Unfortunately for that to be true the plane needs a constant acceleration rate of well over 1.7g+ sustained... something our planes cannot do even when screaming towards the earth at full throttle straight down.

This "myth" was espoused by many long time members, some moderators here and such publish notables such as Clarence Lee ( Mr. Engine Clinic Himself!).

Someone here posted a video that put the kabash on this error.

Gaps CAUSING flutter is yet another such myth.

"flying surfaces with substantial gaps WILL FLUTTER more often than those without" <- this is also incorrect and sustains the flutter myth.

It is not the gap that leads to more flutter, it is resonance caused by many different factors.

Having a larger gap does not increase or exacerbate an existing problem as that statement implies nor do the surfaces flutter "more often".

Sealing a gap leads to a stiffer control surface overall, and that helps dampen the resonance. But other techniques are equally if not more effective too... e.g. a counterbalance, stiffer rods, less slop, stronger servos, guy wires or stiffening struts, even C.F. spars, stiffer/stronger wood, etc.

mike31

My Feedback: (67)

Then why do full scale aerobatic planes have minimum gaps and some have gap covers? I have seen that on Rob Hollands acro.

jester_s1

Hinge gaps are a factor in the aerodynamic performance of a control surface, so it's good to specify a minimum spec and to seal them if needed. The biggest thing that sealing the gaps does is improve the control authority at slow speeds with big throws ie, when the pressure differential is the greatest.

nute12

My Feedback: (14)

keep the linkage tight, make sure the servo gear train is tight and you will have no probs. only flutter i have had was caused by sloppy linkage.

jaka

Hi!
The gap should be kept at a minimum. An ordinary Model Magzine paper thick. That's easily done if you know what you are doing.
For a trainer it does not care as long as you keep the elevator/aileron stiff. Stiffnes is the most important thing to have in the elevator or aileron set-up.
Flutter comes from a too floppy servo/linkage/surface system!

jester_s1

Why is it so hard for people to understand that flutter is 100% the result of turbulence at the control surface hitting the two sides back and forth at the right frequency to resonate with the surface? It's not that hard to understand. For those wanting an common example, hold a piece of paper out the window while driving. The tail end of it will flop in the wind. That's flutter, and anything held in a fast moving wind can do it. For another illustration, go push a kid on a swing. If your're observant, you'll noticed that each swing back and forth takes the same amount of time no matter how high the kid goes. When you are pushing, you are adding energy at the swing's resonant frequency causing its oscillations to increase in amplitude. Good linkages can minimize it and can save your airplane when it happens, but they do nothing to prevent it. Everything can flutter. It's just a matter of at what frequency the turbulence has to hit it to make it resonate. (btw, search for "resonant frequency" on Wikipedia if you don't understand what I'm talking about) The only things that can prevent flutter are to change the resonant frequency of the control surfaces by using lighter and stiffer materials, or by mass balancing them to move their CG ahead of the hinge line to accomplish the same thing. In the swing illustration, doing either of these is like shortening the chain so that the frequency of swings gets faster. If you can only make 20 pushes a minute and the swing naturally swings 25 times a minute, you'll never get the thing going high like the kid wants. Likewise, when you raise the resonant frequency of your control surfaces beyond the speed of the turbulence that will hit them you eliminate the possibility of flutter. Good linkages and good hinges can minimize flutter's damage, but they can't do anything to prevent it from happening.