Done a bit of searching and I can't find anything definitive on how to seal flaps that use Robarts hinges.  The flaps are recessed into the wing about 1/8" so there is no visible gap.  Does anybody have a technique that seals Robarts but does not severely restrict mobility.  The plane is electric and I don't want any control friction I can avoid.

I am pretty clear on the effect of leakage where the gap is exposed but not so clear when it is not exposed.

I have flown the plane unsealed and it doesn't show any signs of "needing" to have the hinges sealed but my experience is that everything flys better when they are even if they don't appear to need it.

Ken

Ken,
I have always used Robart hinges because (for me) they are easier to install. I used them for decades on R/C and have never had an issue. I seal my controls with clear Scotch wide tape. I use talcom powder in the gap after installation and deflect the control surface to the full up position during installation (tape on the bottom surface). I'm not sure this is what you were asking about, but I don't think it's any different than with other type of hinges.

Paul

Did you embed the hinge into the flap so that the pivot is behind the LE of the flap?  If you put it on the LE like you do a "DuBro" type hinge sealing is not a problem.  My pivot is 1/8" behind the LE.

Ken

Best hinge sealing tape i've found is 3M 850 clear polyester 1.9 mil thick and 3/4in wide

On RC gliders, they smear a rhin film of silicone glue into the gap to form the seal Never done that myself, but I wonder how much drag on the control system that would create. Might be worth a try on a “beater”.

   If the pivot point is 1/8" behind the flap leading edge, how much of the leading edge protrudes above the wing surface at full deflection? Would not that create some extra drag where you don't want it? And at near full deflection, that might create a gap that you would not see from the opposite side, but would allow air pressure to bleed through between the flap leading edge and the wing trailing edge? In your case, I think the over lay of tape that Keith Trostle describes is your best bet. I just wouldn't make surfaces hinge l;Ike that. No reason for it and no advantage to it, and now you have the sealing difficulties. Full scale acro airplanes have as tight a control surface as you will find, and they still seal hinge lines.
  Type at you later,
  Dan McEntee

There is another way to seal the hinge line and it is not dependent on where the pivot point is.  Ed Southwick did this on his Skylark stunters after it became apparent that sealing the hinge lines helped the turning ability of these stunt ships.

What Ed did was place the adhesive tape on the trailing edge of the stabilizer (and wing).   He affixed another piece of tape on the strip that extended over the flap (elevator) leading edge so that the piece affixed to the trailing edges would not stick to the flap (elevators).  In other words, the tape affixed to the trailing edges would lap over the hinge line and not be stuck to the leading edges of the flaps (elevators).  Ed did this on both the top and bottom of the flying surfaces.

I understand that sailplanes (model and full size) do something similar to this overlapping tape idea, but is needed on only the top surfaces.  (Those things do not do very many outside maneuvers.)

I thought is was a really neat idea and used it on one of my stunt ships for a while, both flaps and elevators.  I got the benefit of improved turning because of the taped hinge lines.

I encountered a problem with almost disastrous results.  On a flight, I started a wing over and the controls felt sluggish.  On the down side of the wingover, I was pulling hard full up and barely missed the ground and had sluggish control for the rest of the flight.  The problem was that oil from the exhaust started to collect between tape that overlayed the flaps and the flaps.  Moving the controls had to overcome the viscosity of that oil film between the two materials.  The oil could be removed but in so doing, the tape would not lay nicely over the moving surface.  New tape strips could be installed on a regular basis, but there would be no indications when the controls might freeze up again.  That idea was completely discarded after that.  But, the idea would work nicely on electrics.

Keith

Keith - How did Ed keep the tape from gradually bending away from the wing?  I would think that any tape with enough memory to lay flat would also make the controls sticky.

I believe Keith has used this type of flap which means that his recommendation may be the best way.  If memory serves me Al Rabe had the "bump" issue with the  Sea Fury and perhaps the Bearcat .  I think he fixed it on the Mustang but that was a long time ago and I might be wrong.

If you place the hinge pivot deeper than the radius of the flap LE you will get the "bump".  Sealing this type of hinge is not as critical as ones that have a visible gap.  Any air deflected through the gap will be directed aft due to the shape of the TE which produced less roll force but I want them sealed anyway if I can.

Ken

Al Rabe, to my knowledge, did not incorporate a recessed hinge line on either of his flaps or his elevators.   You may be familiar with the "tests" he performed with different wing sections mounted on the hood of his car, then run up and down a runway and was able to measure the lift created by different airfoils, angles of attack and flap deflections.  That is where the airfoil that he used on his Sea Fury and later Mustangs came from.   In those tests, he did a "variation" of the airfoil that I used on my the Focke Wulf Ta 152 that placed several times at the Nats and was published in MAN.  He intentionally place the pivot point rearward so that the flap LE would rise above the wing TE (yes - a "bump") with the idea that it would at least partially seal the gap when the flap was fully deflected.  His findings were that this section did not improve what he was getting with Sea Fury section.

I know for a fact that the "bump" created by the rearward position of the pivot point is definitely NOT a good thing.  As the air flows over the wing, by the time it gets to the flap hinge line, that air flow is probably turbulent.  When the air flow sees (feels) that "bump", I think the air flow separates and any lift that might be created over the flap is negated, though there is still some resultant lift created from the bottom of the deflected flap.

Why do I believe this?  I will try to explain.  I built a Rabe Bearcat, essentially from the magazine plans, using the published airfoil for the wing but the wing was built using Bob Hunt's Lost Foam process (meaning it had a molded LE, no LE strip)  The stab which was essentially a flat plate with a rounded leading edge.  The stab is 3/8" thick and was formed with molded sheet (so there was no LE strip) -- It is just a 1/16" shell with only a few ribs.   Based on my experience with these thicker sections where I recess the pivot points for the elevators (and flaps), but that pivot point is about 1/32" forward of the radius center of the LE of the elevators.  This means that as the elevator is deflected, the LE of the elevator drops away slightly from the surface of the stabilizer.  So far, this is fine.  That Bearcat performed very well, particularly when I taped the hinge line with the tape applied as I explain earlier in this thread where the tape is attached to the elevator LE as the LE rotates inside the stab TE.  Things are still fine so far and the taped hinge line definitely improved the turning performance of the airplane.  The tape is place on the bottom of the stab/elevators hinge line.  However, a problem raises its ugly head when oil gets between the tape and the elevator LE.  After a number of flights, the tape will loose its adhesion to the bottom TE of the stab and as up elevator is applied, the tape will bulge, creating a "bump", probably no more  1/32" to 1/16" for a few inches extending from the root of the elevators.  The turning ability of the Bearcat was definitely impacted.  Removing the tape, cleaning the surfaces from oil, replacing the tape and the turning ability returns.  I have experienced this with every one of my models where I have recessed the pivot point.  Some of these airplanes have flaps and elevators that are as much as 1" thick at the root sections.  There is no visible gap between the elevators and stab or the flaps and wing.

That "Bump" is not a good thing and can easily be avoided by careful placement of the pivot points.  Placing the hinges on these thick flaps and elevators gets kind of tricky when the flaps and elevators are tapered in thickness.

Hope this makes sense.  If your would like to talk about it, send me a message with your phone number.

Keith 

Before I go that route, it would be nice to know if sealing an embedded hinge line has the same effect as it does on "normal" flaps.  I would think it would be less.

Ken

Taping the hinge lines on the flaps and elevators that employ recessed pivot points still improves the turning ability of the airplane.   I would not know how to measure the improvement for taping the hinge lines comparing airplanes with recessed pivot points to those which do not. 

When I use the recessed pivot points, I keep the same area behind the hinge line as the surface would have had without the recessed pivot points.  (This essentially widens the flaps/elevators by those measurements of the recessed hinge lines.)   I do believe that when you recess the pivot point, the effective deflected area is decreased by the amount the hinge lines are recessed.

I cannot claim that this system is a real improvement over the "standard" hinging methods.  I do know that my airplanes have had an "acceptable" turning ability.  The experienced pilots who flew my Bearcat agreed.  My Focke Wulf Ta 152 did OK at three Nats.  Several other semiscale designs also flew well at other Nats including a derivative of my Focke Wulf which had Rabe influence for a Fw 190D (ST .46), a semiscale Little Mike Goodyear with one of the first rear exhaust .40's (custom made - a long story), and a P-40Q with OS .25.

Keith

This was published several years back.  The really interesting thing to me was that the "open" gap was very very small.  Not an 1/8 inch gap as you see on some local planes, but a very tight "front-row" gap of less than 1/32 inch.  It was just barely big enough to slip and IBM card into the gap. 

The hinge line was a "standard"  45 degree elevator leading edge against a flat stab surface.  There was no "cove" joint in the test article.  It might be worth redoing the tests.   I've been using the recessed cove hinge line for a good while on current planes.  I like the look of the joint, but it is difficult to easily seal.   

It is possible to get the hinge line positioned (off the perfect center-line) such that at full flap deflection the flap does seal (bumps into) the wing/stab trailing edge.  This should boost the lift at the full flap deflection.