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« Last post by Bob Hunt on Today at 07:50:29 AM »
I have a ton to say about this subject, and sorry to be tardy in coming to the party on this, but I had to cut out a block of time to properly explain my findings/beliefs/theories/feelings/hunches/revelations on the subject.
Disclaimer: I have absolutely no schooling or training in any aspect of aerodynamics. All my findings/theories are from personal observations and experience. I’ve thrown a lot of “mud” at the wall and a great deal of it has stuck there over the years; and a lot has fallen on the ground as well. The mud that relates to swept-forward trailing edge hinge lines for me has not only stuck to the wall, it has formed a sort of cornerstone
I guess my first real introduction to swept forward trailing edges (or at least the one that stands out in my mind) was watching Billy Werwage fly his Juno at the 1979 Nats in Lincoln, Nebraska. I missed the Top-5 fly-off that year due to a stupid mistake (I finished in 6th…), but in retrospect it may have been a blessing. I don’t usually watch the competition when I’m flying at contests; it tends to make me nervous. Being a spectator at that Nats allowed me to critically watch the Top-5 flights. Those flights were flown in one of the worst winds ever encountered at a Nats. And the wind severely affected all the flights flown that day; but they affected one flier’s performance noticeably less than the others. Billy’s Juno just appeared to be easier to fly that day, and it handled the wind in my opinion far better than any of the other planes flying that day. Even though he ended up a close second to the old master, Bob Gieseke on that occasion, it was Billy’s flights that stood out to me. I’m going to insert here a section of my Genesis Saga book that tells that story:
"The infamous Nebraska wind had awakened overnight and the conditions were deteriorating rapidly. By the time the Fly-Off began, the wind was at least a steady 20 MPH with much higher gusts. This made me even madder at myself; my Genesis had handled the high winds in England so well, and I knew that I would have had an edge in those conditions.
I didn’t realize it as I sat there fuming at my stupidity, but I was about to learn a great and valuable lesson. The five Open fliers who were in the fly-off were extremely gifted pilots, and they each had models that fit their very diverse personalities. Bob Gieseke was flying his legendary Gieseke Nobler, Ted Fancher was flying his gorgeous Excitation, Les McDonald had his incredible and super-sleek I-Beam Stiletto, Al Rabe was flying the P-51 Mustang that he had used to place second at the 1978 World Championships, and Bill Werwage brought his highly innovative and exquisitely beautiful Juno.
"The Fly-Off turned out to be pretty much an exercise in survival. The high winds made clean flying virtually impossible, and most of the planes were getting blown off track in each maneuver.The one exception was Billy’s Juno. It just seemed to cruise through the wind effortlessly. It also appeared to turn easily. In high winds the tendency is for the stick pressure to build up, making sharp cornering difficult. The Juno seemed to turn as if it were on ball bearings. It also appeared to be less affected in the maneuver shapes than the other planes. The illusion made me sit up and take notice. I wanted a ship that flew like the Juno! I doubt that I would have come to that conclusion at that point had I been flying that day. I don’t like to watch the other pilots fly when I’m competing.
"Just for the record, Bob Gieseke won that Nats with Bill Werwage in a very close second place.
"I called Billy soon after that Nats and asked him about the design features of the Juno. The main thing that set it apart was the dramatically swept forward trailing edge. It had 2 ½ inches of forward sweep per panel! Other features included ribbed flaps that had what can best be described as a “humped” airfoil. They were built-up units, and they looked very exotic. I learned from Bill that the uniquely airfoiled flaps were not what made that ship perform so well in the wind. It was the swept forward trailing edge according to him.
In fact, Billy had designed and flown many other models that featured swept-forward trailing edges. He got the idea to do that from his mentors in the Ohio/Detroit area who had also designed many models featuring swept-forward trailing edges." (end)
Why does this feature enhance the performance in the wind? Well, the following is an excerpt from the Vulcan article that Billy wrote for Model Aviation magazine many years later.
“The most significant thing that was different about the Vulcan was the high amount of forward sweep in the trailing edge. This was done with two thoughts in mind. I wanted to keep the center of pressure (CP) from moving aft on the wing when the flaps were deployed. With the forward swept hinge line, the average flap position is ahead of the point where it would be on a straight hinge line model, and therefore when deployed the flaps would not move the CP as far aft. The effect this has is to not over stabilize the model by moving the CP too far back from the center of gravity. In other words the model would not become as nose heavy with the flaps deployed. One of the major benefits of this arrangement is that the stick pressure felt in the handle, especially in high wind conditions, is dramatically reduced. Remember, that when the flaps are moved either up or down, the airfoil is changed to an undercamber type, which increases the wing’s lift. But it also changes the point of the center of the lift, which is also known as the center of pressure.
“On a straight trailing edge model, when the flaps are deployed, the CP moves a much greater distance, and over stabilization occurs. The results are more stick pressure, and a model that requires more input to achieve directional change. In calm conditions this is almost imperceivable, but in heavy wind it can be dramatic!
“Another benefit of the forward sweep in the trailing edge hinge line is the effectively longer tail moment. The moment is now measured from the average of the flap’s position in relation to the stabilizer/elevator hinge line.”
I made plans to begin my experiments with the swept-forward TE concept during the 1979-1980 building season. There was no urgency to have a competition ready ship for my title defense because my 1978 WC ship was still in great shape. (end).
And I did just that, but I didn’t realize that I’d need to use the results of those experiments the very next year. I lost my World Championship winning Genesis 46 MkIII in a freak accident at the 1979 FAITeam Trials, where I was flying the warm up flights for the judges (hit the back end of a car that was parked on the edge of the circle!). As defending champ I didn’t have to try out for the team; I would fly as an individual, not a team member in 1980 as Bob Gieseke had done in 1976, and Les McDonald in 1978. That gave us effectively four chances to win the title as a country.
As soon as I returned home I put a clean piece of vellum on the drafting table and designed a brand new (smaller - 630 square inches - same as the Juno…) Genesis with 1 7/16-inch per panel sweep forward on each wing panel (I just could not bring myself to include the extreme 2 1/2 inch per panel forward sweep that the Juno employed…) I designed a new set of airfoils that “looked right” to me. I made the tip airfoil quite a bit thinner than the root, and moved the high point of the root airfoil back a bit more than normal (as Billy had done on most of his early I-Beam ships and the current Juno design). I moved the high point at the tips slightly forward of normal. As mentioned above, I don’t have any aerodynamic training; I just do what looks and feels right to me.
I built two new ships that winter. I did have a few initial bumps in the road trimming the one that I intended to use at the 1980 World Championships, but they proved not to be because of the swept forward TE. I’ll insert here a bit of that story that is also a part of the Genesis Saga book:
“The one thing I wasn’t too sure about was the control linkage at the flap horn. Not having constructed a ship with a swept-forward trailing edge before, I just naturally assumed that I would need to use two flap horns and make a yoked pushrod to connect them and then use a single pushrod off one of the flap horns to the elevator horn. I had seen this setup in Bob Gialdini’s Sting Ray article in American Modeler magazine, as he used a swept-forward trailing edge on that design.
"The builds for the two new ships went fairly smoothly (remember, I built two of these at the same time), and I really stayed on them because I didn’t have a ship that I felt was good enough to fly at the World Championships. If these ships didn’t fly well, I would be cooked.
"For some reason I decided to test fly the shorter span version first and get that one trimmed out. The first flight was so traumatic for me that I almost packed it in and quit. First off the engine would go lean just after the loops and not return to its initial setting. I had provided a large cooling air inlet so I could not fathom why I was getting what appeared to be thermal runaway. To add to my misery the ship had a pronounced roll at the intersection points of the figure eights. It was too touchy as well for my taste, but I already had the static CG a bit farther forward of where I normally put it.
"Okay, on the ride home from that disaster I tried to figure out the problems one-by-one. Why was the engine overheating? That was a poser. There was a huge oval air inlet in the nose of that ship and the cylinder was right in line with the air blast. I called Bill Simons about this problem and he suggested that I install a Rabe-style baffle around the engine fins. This baffle was made from sheet tin stock and had a fairly small opening in the front and a somewhat larger opening at the rear. I wasn’t too hopeful that this would solve the problem… but it did! Apparently there was a dead spot behind the cowl air inlet and the stagnant air just overheated the engine. Once the baffle was fitted, the OS .40 FSR ran flawlessly. One problem solved. Thank you Bill Simons and Al Rabe!
"The CG problem was easy to fix; I just added nose weight until it felt “groovy.” The reason that I had to move the CG forward was because of the swept-forward trailing edge and the position of the high point adjacent to the quarter chord. This bird just had too long a tail moment because of the sweep, and it was tail-heavy!
"Okay, now I was flying patterns, but the roll problem through transition points in the figure eight maneuvers just would not go away. I tried adding and subtracting tip weight, adding and subtracting flap area via taped on area tabs, leadout positioning and just about everything else I could think of, but the roll persisted.
"I had been going flying with my next door neighbor who was a very elderly man. He was in his late 80’s when I met him in 1979, and we instantly became very good friends. It turns out that Mr. Sullivan was an extremely gifted engineer who had been involved heavily in the initial development of television.
"Mr. Sullivan liked all things technical and he developed a keen interest in my building and flying program. He loved to watch me fly, and I took him out to the field to show him the problem with the new Genesis. He said that it appeared to him that the model was rolling about the same amount in each direction as I did inside and outside maneuvers. He gripped the flaps from the rear and moved them in opposite directions. I could hear a noticeable “click” as he moved the flaps one way and the same click as he moved them in the opposite direction. I realized that the pushrod ends that went through the flap horns were not cinched up tight against the horns, and when pressure was applied, the ends were sliding through the horn bearings and allowing the flaps to move opposite to each other under load. ‘Okay,’ I thought, 'There are no appearance points to worry about in FAI,’ so I bit the bullet and carefully cut small hatches out of each fuselage side adjacent to where the pushrods ends were soldered. Sure enough, there was considerable in and out pushrod movement evident at the horns when the flaps were moved in opposition. Mr. Sullivan grabbed the end of one of the pushrods with a pair of pliers as I heated the solder on the retaining washer, and then he pulled the slop out of the system and I let the solder harden. We did this on both sides and then checked the flaps for differential movement. There was none!
The next day we went to the field again and flew the ship. It was like a brand new model and it didn’t have any roll issues at all through the maneuvers!
I called my team mate, Bill Werwage, and related this whole story to him and he just laughed. He told me that there was no need to use the separate flap horns in the first place. He had always used a straight horn on his swept-forward trailing edge models with no ill effects. He did tell me that it was imperative to install the flap horn so that the legs of the horn were absolutely square to the center line of the wing, and that the legs that go into the flaps be installed as close to the inboard edge of each flap as possible. It’s also necessary to sand a “flat” at the trailing edge of the wing to allow the horn wire to seat against the back of the wing. This flat has to be 90 degrees to the top view center line of the fuselage.
I also later discovered that putting the first hinge out about five inches from the fuselage on either side allows the flaps to ride up and down by a few thousandths of an inch and further smooth out the control motion. I have since made several models with swept-forward hinge lines and have used a straight horn in each of them with no problems whatsoever.” (end)
"That airplane went on to capture a Silver Medal and a Nats second place in less than a month and a half. I just couldn’t get by that pesky Stiletto guy on each occasion… And, it turned out to be the very best maneuver “scriber” I’ve ever owned… Except when the wind blew very hard. Again, the issue here was not the forward sweep in the TE’s, it was the result of me trying to get too cute in making a very thin aft fuselage. The fuselage twisted whenever the wind got too severe and caused all sorts of trouble. But, in calm to moderate conditions, that plane was a killer." (end)
Some years later - and after a long sabbatical into RC Pattern - I used the exact same wing, tail and moments to design the first Crossfire. But this time I made sure the aft fuselage was plenty stiff. And I again reprised all those numbers in building the Crossfire I designed in 2010. That model was the victim of a rogue dust-devil at the 2011 Nats, but was re-built over the ensuing winter and paid off by capturing a 3rd place finish at the 2012 Nats. It is still performing extremely well, and is one of the very best wind airplanes I’ve ever owned.
Because of the great wind flying characteristics of the swept forward TE hinge line in the 1980 Genesis and the subsequent Crossfire variants, I continue to incorporate some degree of forward sweep on almost all my designs; especially my series of twins (except for the Second Wind twin, which was the first of my multi-motor designs).
In speaking with my good buddy, Dean Pappas - who does have aerodynamic schooling - I found out that the forward sweep affords what may be even a more valuable performance increase in windy conditions. I’ll insert what Dean had to say here:
“First off, just to point out the happy hopelessness of this discussion, a stunt ship flying maneuvers on the surface of the hemisphere in steady runway wind is a horrifically complicated system to describe completely (Don’t even get me started about flying one of these in turbulence…).
“From an aerodynamic standpoint, wing sweep is measured along the quarter chord line. An un-swept, tapered wing has a trailing edge rake that is three times the leading edge rake angle. So, recognize that all stunt ships, including Bill’s and Bob’s, have swept wings. Airplanes with straight flap hinge lines actually have lots of sweep. If you take a swept wing and fly it yawed to incoming air (like a CL Stunt ship always does…) then a rolling moment will be produced. In the maneuvering area the airplane is always flying with the relative wind coming from the left side of the nose. The airplane is yawed to the right. Every time the airplane is G loaded in a maneuver the rolling tendency will ‘bang’ the outboard tip. Side note: If you’ve ever taken tip weight out to trim for a runway type wind, now you know why! This also explains why some of those older, constant chord designs, such as the Chief, fly much better than you’d expect in wind.
“The second thing that happens with a swept wing, especially if you have half-span flaps, or if you have full-span flaps that interfere with the wing tip vortices, is that the center of lift of the wing will move inboard and outboard with changes in flap deflection and G loading. On a swept wing this also means that the center of lift will also move fore and aft.
“If all you did was fly in heavy wind, you would still be stuck with a compromise between control system geometry and the undesirable effect of wing sweep. And, bad control system geometry and/or construction can trash all of this. Say what you will, but a straight flap hinge line is easier to construct properly. The Crossfire has .75 degree of sweep along the quarter chord line, including the flap. Compare that to what you are flying today.” (end)
Wow! This stunt design stuff is complicated! I had no idea that I’d incorporated any of that stuff into my designs. So, the bottom line is this: If it works better for YOU, then it is a good thing. Some love it, and some hate it. I’ll continue to use it…
Bob Hunt
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