Wing flaps design ?

[Eric Viglione]

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

Randy

I'd be more interested in displacement hulls, that self limit top speeds or will crush or overheat the engines trying... 

EricV

[Trostle]

     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.

      Brett

Several comments have been made on this thread about some material that Al Rabe wrote some time ago.  Probably most of these comments refer to Al's article "Go for Broke" in the March 73 issue of American Aircraft Modeler.  In this article, Al explains the evolutionary process that led to his Sea Fury and later Mustangs (3 first places at the Nats, and 2 National Championships and a second place at the 78 World Championships).  He shows the various airfoils he tested in a rig mounted on the hood of his car and measured the lift at various angles of attack and flap deflections.  (I will reserve comment on his "modification" of an airfoil similar to what I have used on most of my semiscale airplanes other than to say that his "modification" does not take advantage of what that airfoil can do.)  Anyway, there have also been comments here about my airfoil/flap where the airfoil is a part of the airfoil.

My flap (and elevator) hinges avoid the undesirable "gap" when the flaps are deflected because the pivot point of the flaps is near the radius of the flap LE.  I have had some success with several semiscale designs using this arrangement including one design which finished 5th two times at the Nats, the National Championship once, a 3rd place in Classic at the 2008 VSC and a first place finish with the design in the Classic event at the 2008 Nats.  Two other designs placed in or near the top ten at the Nats during the 70's with this airfoil.

The section shown below with the "thick" flap is similar to what I use.

Keith

[Trostle]

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.

(Clip)

Al's article on his airfoil tests did state that he found where the flap was a part of the airfoil to be not as effective as the more "standard" flat flap configuration.  In that article, he apparently tested only one configuration of an airfoil with the flap as part of the airfoil.  He wrote that he intentionally placed the pivot point behind the radius of the flap leading edge so that as the flap deflected, it would tend to seal the gap between the wing TE and the flap LE.  As I suggested in my previous post above, there is a problem with that.  The problem is that at some point of deflection, the flap leading edge would protrude above the wing TE, providing a disturbance to the air flow over that region.  I believe that the airflow at this region is no longer laminar, it is turbulent but is still attached to the surface of the wing.  However, if a portion of the flap LE is protruding into this airflow, I am convinced that the airflow becomes detached from the surface of the flap and any resulting lift over the flap that would otherwise be expected is significantly reduced.  If the airflow is already turbulent, it does not take mucyh of a disturbance to cause the airflow to detach.  I can go into great detail, based on my experience over the years, for me to believe this is a fact.  That explanation is a bit beyond the scope of this post of two paragraphs.


However, my next airplane for Open competition will be a semi-scale model with an airfoil somehat similar to the airfoils that Al uses, but it will have a significantly thicker flap that sort of blends into the rest of the airfoil.  And the flap pivot point will be slightly ahead of the flap LE radius point, as are all of my airplanes with blended surfaces, including my version of the Rabe Bearcat which turns just fine.

Keith

[EddyR]

Keith  What do you mean" And the flap pivot point will be slightly ahead of the flap LE radius point," How are you doing this? I must be dense as I am not sure what you mean
 Maybe a hand drawing. My Bearcat is heavy at 69=70 ounces but it has a very good turn. I did play with the airfoil quite a bit. I will never build such a heavy set of flaps again . When the plane is done I will use then as canoe paddles.
Ed

[Bill Johnson]

Keith  What do you mean" And the flap pivot point will be slightly ahead of the flap LE radius point," How are you doing this? I must be dense as I am not sure what you mean
 Maybe a hand drawing. My Bearcat is heavy at 69=70 ounces but it has a very good turn. I did play with the airfoil quite a bit. I will never build such a heavy set of flaps again . When the plane is done I will use then as canoe paddles.
Ed

If you look at the original article, http://www.iroquois.free-online.co.uk/rabefury.htm, note that the flap leading edge is a circle. In a normal configuration flap, the pivot point will be in the center of that circle. Mr. Rabe moved the pivot point aft of that. That is why he has partial LE protrusion on the opposite side of flap movement.

I believe Mr. Trostle is essentially correct in his analysis. The protruding LE does nothing to improve the effectiveness of the flap. It disrupts the boundary layer in the laminar flow without adding energy to that layer, a critical error. The only practical application for a protruding LE is on ailerons where the flight control is set up so the LE protrudes only when the aileron moves upward. The protruding LE then increases drag, reducing adverse yaw as a result of the increased lift and drag on the opposing wing/aileron.

[Avaiojet]

I drew and posted this diagram better than a year ago in The International's build Thread. It shows the elevator cross section configeration.

Clearly I'll have the elevator LE pined/hinged so the LE isn't proud at all anywhere in the movement. The Elevator LE is .375" in diameter.

The same drawing can be used for the flap LE also. The flaps will not be proud anywhere in the movement also.

The flap LE diameter is .675. There will be a TE "overhang" at the TE of both the wing and stab.

The wing TE is a constant, but the stab TE tapers in thickness.

Charles 

[Chris Wilson]

Anything is possible on paper Charles but I do wonder how those drawings would translate in real life.

For example, how would the wing handle flex at the joint without binding on the hinge cover plates?

And the exact building needed would be a nightmare.

If I recall correctly though Claus Maikus used a similar system once but varied it with a matching concave on the fixed part of the tailplane's trailing edge that closely followed the convex leading edge of the elevator.

[Tim Wescott]

And the exact building needed would be a nightmare.

Only if you're in the habit of actually completing airplanes and flying them.

[Avaiojet]

Only if you're in the habit of actually completing airplanes and flying them.

Anything is possible on paper Charles but I do wonder how those drawings would translate in real life.

For example, how would the wing handle flex at the joint without binding on the hinge cover plates?

And the exact building needed would be a nightmare.

If I recall correctly though Claus Maikus used a similar system once but varied it with a matching concave on the fixed part of the tailplane's trailing edge that closely followed the convex leading edge of the elevator.

Chris,

I'll respond to that and bring something to the table.

This type of construction is quite common in R/C scale. Really common in fact. Seen it plenty of times and I've done it a couple of times. The drawing was just something quick I threw together.

What I've used, as other modelers have, is triangle stock for closing off the hinge area of the "ailerons" at the wing TE. Really a common and typical method of construction used in R/C scale. I didn't invent it, this can be seen on plenty of scale R/C plans.

No, I've never done long flaps, but the ailerons on the first 33% 90" Gee Bee Z I built, a Don Neill design, had to be close to .75" thick at the aileron root. Don used this method of construction on a few scale model Racers he designed in the late 80's. Don's Gee Bee Z models were used in the movie "The Rocketeer." Don was a talented modeler who gave us plenty, but unfortunately is no longer with us.

Yes, as you mentioned, "nightmare" is a good word but not impossible.

I do believe this form of construction can be used anywhere desired, including on CL models.

The International I designed, and have that modified Ares wing for, would or could be a good candidate for this. Unfortunately, as I mentioned a while back, I'm not delighted with the weight of the wing. I may not use it.

Certainly I cannot comment on what advantages or disadventages this would have. We have members who have the ability to do this.

I do know it would have a good appearance. IMHO.

In spite of the fact that I may not use the wing I started, I have made headway with the stab and elevator design.

I decided on using a tube in a tube. Here's part of the "nightmare."

Charles

Edited: Decided to add something.

[Curt D Contrata]

...and there is always the Jolly Flapper, by Larry Scarinzi.

http://www.rcgroups.com/forums/showthread.php?t=1288656&page=13

Curt

[Trostle]

Keith  What do you mean" And the flap pivot point will be slightly ahead of the flap LE radius point," How are you doing this? I must be dense as I am not sure what you mean
 Maybe a hand drawing. My Bearcat is heavy at 69=70 ounces but it has a very good turn. I did play with the airfoil quite a bit. I will never build such a heavy set of flaps again . When the plane is done I will use then as canoe paddles.
Ed

This is the flap and elevator hinging arrangements that I use on my stunt ships.  What I tried to explain previously but did not do a very good job with it is that the LE's of the flap and elevators are round.  The TE's of the wing and horizontal tail are concave.  The pivot point is just forward of the center of that radius by about 1/32" for the flaps as well as the elevators.  This avoids any hint that the leading edge of the flap or elevator will rise into the airflow over the deflected surface.

I could go into a lengthy discussion regarding why I am convinced this is important.  (I even offset the hinge line on the 1/4" flaps of my Rabe Bearcat with the concave wing TE similar to what is shown below.  I still set the pivot point just forward of the center of that radius.)

The hinges that are used to do this is another story.  And yes, I tape the hinge lines and that is another story also.

BTW, one of my favorite quotes from Al Rabe is that it is impossible to have a flap that is too stiff.  He is willing to sacrifice weight for flap stiffness.

Keith