A Synopsis on Design - Past, Present, and Future -- I am adding past in there because I think we have come a long way in the design of sailplanes. In the past, of course, we started out with wood, plywood, and fabric. This era ended about 10 years ago when most of the people got into competition with plywood ships and found that they had trouble maintaining the surfaces and keeping them smooth. Many people such as A.J. Smith struggled for many years with ships like the LO-150. They spent all winter trying to smooth the wings and. when the weather changed in the spring they suddenly found that all the gunk they had put on the wings in the hollows had caused humps when the hollows had flattened out. This encouraged everyone to look for better materials and better methods of construction. In this country, the Schweizers went to metal and most of my work was in metal. In Europe, they went to fiberglass. There is always a question -- which is better? -- fiberglass or metal. I think that they both have their place and should be used where they work the best, One of the reasons the Europeans went to fiberglass, principally in Germany, was because they developed the fiberglass technology ahead of everyone else. They also had a very great reluctance to go to metal because they simply didn't have the technicians or the people who were familiar with metal. They have, therefore, led the world with fiberglass ships. To come to the present time, we now have less wood, more metal, and lots of fiberglass. I think that the trend in the future will be away from fiberglass. It will still be used, but I think we will go more to metal. My reason for saying this is that the prices of sailplanes are getting very high. Most of you who have sailplanes on order certainly know this., The prices are continually going up. The West German mark was re-valued, to our disadvantage in terms of dollars, and the prices have gone up additionally on these ships. The reason prices are high and the reason I say we are going to metal is that I believe we can build ships of metal in fever man-hours. I have toured all the plants in Germany and watched them build ships from fiberglass and, contrary to most opinions, the man-hours required to build the fiberglass ship are generally higher than those required to build a metal ship. Bolkow (which, in my opinion, has the best control and the best methods of all the plants I was) claimed that in building the Phoebus, they required 900 man-hours for each ship. At our rates of pay, 900 man-hours results in prohibitive cost figures. I think it is out of the question to build fiberglass ships in the United States. I am not sure that all of you are familiar with the general methods of fiberglass construction. Most of the wings have sandwich type surfaces. There is an upper panel, lower panel, front web, and a rear shear web. Quite often they have another shear web at the point of max thickness. The wing is built in two halves, the shear webs are glued, and then the two halves are assembled. The halves are built in female molds and they start out by laying the top skin into the mold, and they lay up two or three layers of fiberglass, building in more layers at the root, and then they begin fitting in small sticks of balsa wood. They run spanwise in the wing. After the balsa is in place, another inner layer of one or two sheets of glass is added. This method of construction is very, very time consuming and costly. At the Bolkow factory balsa is purchased in very large chunks and each piece is physically measured and weighed. They then calculate the density and mark it on each block of balsa. They stock these blocks in large bins and saw them up into small sticks about 3/8 inch square. The balsa is laid in accordance with the plan form of the wing. They use their heavier densities in the root and gradually change to lighter densities as they work towards the tip. You can imagine how costly it is to keep control and records on all of this balsa and to fit these pieces. When they come to the joints, they don't just butt the balsa sticks together, they scarf the ends so that they will have full strength when they move together. I think you can see that this is a very expensive way to build wings. Some of the companies over there, Schempp-Hirth for instance, I believe have gotten away from balsa and are using foam. Apparently the foam is working out all right and it is certainly a cheaper and quicker way of performing the same function. I think that Schleicher and Bolkow built a few fuselages the same way with balsa. Some of the other companies are using pure fiberglass structures, with a thicker skin. It's a little bit heavier but is cheaper and easier to work with.
There aren't very many people building sailplanes out of metal at the present time, using conventional construction. Schweizer's designs have more or less conventional construction throughout. The wings are built up with spars to carry the loads, ribs to give the shape, and aluminum covering. This conventional metal construction requires fewer man-hours than fiberglass but this type of construction still isn't good enough. I think in the future, in order to get the price of sailplanes down, we must go to even simpler methods than we have used up to this point. On the HP-15, I attempted to build a wing of much simpler construction. The wing on the HP-15 is built as follows: There is a top skin and a bottom skin. These skins are heavy at the root (1/16 inch thick). There are bent sheet metal front and rear spars. The leading edge nose cap of plastic or aluminum closes the box.
The entire wing between the front and rear spars and the top and bottom skins is then filled with aluminum honeycomb with the cells running vertically. This type of construction simplifies the wing considerably because all but three of the normal wing ribs have been eliminated. The honeycomb next is cut and glued into the wing. This wing panel is as easy to build as the average spar in a conventional wing. The construction time is cut to about 25 percent of that required by conventions methods. Of course, the ailerons and flaps are sheet metal and hung onto the back with piano hinges. I think in the future, in order to lower the price of sailplanes, we must go to this type of construction on wings. I am talking mainly about standard class sailplanes because this class will become more popular as time goes on. There still will be some exotic ships like the Sigma, as described by Nick Goodhart, and others like the Nimbus. Such super racers will be hand built and high cost must be expected by the buyers. As Nick said, they have $100,000 invested already in the Sigma. We can't afford to spend this kind of money on standard class sailplanes or the type of sailplanes you people want to buy. Even the present prices of $8000 or $9000, I feel, are too high for most of us. We must, therefore, go to these simpler types of construction.
Fiberglass is hard to beat for fuselage nose sections. Aluminum is better for the tail boom and tail surfaces. With this type of construction, we can use a very simple fiberglass structure and add some metal to take the wing loads, the wheel loads, and tie into the tail boom. In the Toledo area there is a very good boat manufacturer who makes sailboats out of fiberglass. This company has quoted a price of about $300 to $400 for a complete pod. If you could get a pod for even $500 and just add a simple tail boom, you would have a very simple fuselage at a reasonable price. In fact, it would be the type of fuselage that, if damaged, you could afford to throw the pod away, get a new one, and hook it back up to your tail boom. It would also lend itself very well to low cost repairs.
I think this is the type of ship we are going to see in the future. This type of construction fits very well into the home builder's basement type of project. We are going to continue to see a lot of ships built by home builders, especially if we can get the man-hours down. An HP-14 can be assembled by an experienced builder in around 1000 man-hours. Even that is too much because the average fellow just can't afford to work for a couple of years to build a ship. He wants to get something finished sooner than that. I feel that with a little thought and preparation, this type of kit could be put out so that it could be assembled in about 500 man-hours, and the cost could be held below $3000. This price would fit a lot more pocketbooks. If we want scaring to grow and want to see ships built in this country, I think we are going to have to resort to this type of construction. One of the problems we are having now is getting people to use the simple flap. They have been approved by the CIVV for world competition in 1974, and by vote of the SSA Board of Directors for immediate use in the U.S. I have a list of the advantages of the simple flap and I would like to run through it. I know some of you people have flown HP-11's, HP-13's, or -14's, and you realize what some of these advantages are. Most sailplanes, of course, that meet the standard class specification now have dive brakes, and they have some pretty serious drawbacks. First, when you pull out the dive brakes on the average ship that has the DFS type, the stalling speed increases. In other words, if you are coining in to land with open dive brakes, you are landing about 10 miles an hour faster. With a simple flap, you land about 10 miles an hour slower. This means that when landing in a very bad field, you will do less damage if you run into rocks or stumps, or a very rough terrain type of field. Also, you will land shorter. By actual test in the HP-14 with no wind, we found that we could land in 88 feet from the time we touched ground until we stopped. The flaps are more effective at low speeds. Most of the DFS type dive brakes are very powerful at high speeds but as you slow up they get less effective. The DFS type of brake is very difficult to build because you have to have a slot in your wing anywhere from 5 to 8 feet long. From a structural standpoint, it is very difficult to build. As the wing flexes you get differential bending at that point. It also weakens your torque tube, your torque box, and it gives you problems when the wing flexes because the wing structure bends while the dive brake itself stays straight. Some of the European ships have solved this problem by having a small strip that is spring loaded so that it will lie on the surface as the wing bends.. Also, with all of this complicated structure right in the most critical part of the wing, cost of construction is seriously increased. The simple flap, on the other hand, is nothing but an aluminum triangular shaped box usually with no spar in it. They can be driven from one end so that no parts are required out in the wing to operate the flap. Another problem with the DFS type dive brake is with extra drag when retracted. All of you who have owned ships with a DFS type brake have tried to fill them. When you are flying in a contest and get into the air, the wing bends, thus forcing the clay to pop out. You no longer have laminar flow over that section. The simple flap is lighter in weight because the mechanism and brake boxes are eliminated.
One of the reasons the standard class requires the speed limiting dive brake is that if a student pilot gets caught in a cloud he is supposed to be able to slow up the ship and get out of the cloud. I have found, in flying in Europe, especially in England, that every time you get up in clouds you usually get icing and usually come out of the bottom of the cloud with a load of ice all over the airplane and hope that it will melt off before you get to the ground. With the DFS type dive brake sticking out, that is the first thing that the ice builds up on and I am not too sure you will always -be able to get them back in the wing. When we flew in the Internationals over in England, we iced up in every cloud above 8000 or 9000 feet.
Another advantage of the trailing edge simple flap is, when making an approach to land, you can come in at a steeper angle than you can with any of the ships I have ever flown with the DFS type dive brakes. Also, when you put the flap down, it puts the nose of the ship down so you have very good visibility over the nose to see the field in which you are landing.
A further feature that you get with the trailing edge flap when you put the flap down is that the decalage angle between the wing and the horizontal tail is increased. This tends to increase the longitudinal stability. In most of the HP's, when you are making an approach with the flaps down, you can let go of the stick and be very stable. When the stick is pushed forward and then released the nose will pop right back up to the trim speed.
An additional flap advantage is realized on takeoff. You can get off the ground much quicker because down flaps have the effect of increasing the angle of attack of the wing and gives a higher lift coefficient.
Therefore, if you are taking off in a crosswind or under adverse conditions, you can get into the air much quicker than you can without a flap. I have mentioned that this type of flap does not require any special bracing in the wing as would be required with the DFS type.
When you are flying at relatively high speeds in gusty air with the trailing edge flap, you actually strengthen the wing when you put the flap down because it moves the center of pressure inboard towards the fuselage. If you are flying at high speed you can lock out at your wings and they will actually bend down because at the higher speeds the angle of attack of your wing outboard of the flaps is negative and you are getting negative lift on that portion of the wing. The reverse is usually true on the DFS type dive brakes because you kill off your lift in the area that has the brakes, and then the wing from the brake to the tip has to carry the weight of the ship. They had some severe problems with this phenomenon on the Dart. In some of the early tests, the wings bent up very sharply. Subsequent calculations revealed that the spars were below the necessary strength in this condition. The spars had to be reinforced before flight tests could be continued. There is also a possibility with a trailing edge flap of getting some improvement in performance at low speed and at high speeds by varying flap position settings. There is a definite increase in performance when you are running at high speed and can put your flaps up slightly because it lines up the fuselage better with the airflow and instead of flying along with a nose down, the nose comes up and the ship trims in a more level attitude. At low speed there are flap advantages because you can fly a little slower, which allows a smaller turn radius and allows the sailplane to circle a little closer to the core of the thermal. On approach, you have much better visibility as you are coming into the field.
I think that in the future, the material used in sailplane construction is going to be determined 'by economics. If we can save money by using fiberglass over metal, then certainly fiberglass should be used. If metal will work out better than fiberglass, then it should be used. Of course, you have some other aspects to consider and that is pilot protection. I personally believe that metal in the fuselage area is a little bit better because when it buckles it still retains some strength. When fiberglass lets go you don't have much left.
This is about the extent of my discussion on types of materials and the trends in design. I think Nick gave a very good presentation of the trends in open class ships. About the only way this class can go from now on is to higher aspect ratios and greater spans. The increase in span and the resulting increase in chord will give a better Reynolds number. Almost anybody can sit down with a pencil and paper and find that if you can increase the span and increase the Reynolds number, improvement of the glide ratio is automatic. Of course, as has been said earlier, glide ratio isn't everything, and as the ships get bigger, the problems get bigger. There are very real problems of being able to trailer the ship, tear it down, and assemble it. Longer wings introduce other problems. Increased droop in the wings at rest is one. Since it is quite impractical to keep making the ship higher (although in the case of the Sigma they have done it by extending the landing gear farther), long flexible wings will about touch the ground at rest. If you land in a field with high grass or a high crop, one of those wings is going to catch and you are going to do a vicious ground loop. With such a large ship, the chances of damage are much greater than with a smaller standard class ship. I think-that most of us are afraid of these bigger ships and feel that the open class is going to turn into a giant class. Maybe these practical disadvantages will limit the size of the ships, but certainly there is no limit to the cost and the complexity. This is one reason why I have become more interested in the standard class. Only time will tell where we are going, but as of now, it looks to me like the standard class is going to become much more important and much more attractive to the average pilot. This is the reason why I built the HP-15 and I intend in the future to do most of my work and experimentation in the standard class. I do wish that we could get more pressure to get across our feelings on this flap situation because I feel that it is a definite improvement. I think it would make all of our ships safer and less prone to damage. The people in Europe, of course, have had very little experience with flaps and they are most reluctant to make the change. I think if anybody has any questions they would like to ask, this might be the time.
Schreder: I flew the HP-12 in England in the Internationals in 1963. We took the outboard half of the flap and hinged it at the top so that when the bottom of the inboard section came down the outboard section went up. It fulfilled the requirement of the rules in that it didn't really change the camber of the wing but I didn't feel that it was a very good solution to the problem because here we were trying to comply with the letter of the rules which say that you should have no camber changing device. We were actually making the construction more complicated and more costly. We. of course, lost most of the advantages that flaps normally have and they were not as effective. For the same speed or the same flap setting, the speeds were higher.
Question: I understand you have a very unusual root attachment on the wing on the HP-15, and I wonder if you could briefly describe that to us and tell us how it worked out.
Schreder: On the original design of the HP-15 wing, there was quite a problem in how to join the two wing panels together, since all of the loads were being carried by the skins. You have the two wing panels coming into the fuselage like this, and the first attempt to make the wings was to build stub spars in the roots similar to the European fiberglass designs. So, in one wing, we found in order to carry all of the bending loads here at the root, that these spars had to be made of solid 7075-T6 aluminum. They were screwed to the skin. There are two of these on one side and one on the other side. The single spar was 1-1/2 inches thick and about 4-1/4 inches deep. The two straddling spars in the other wing were each 3/4 inch thick and of identical shape. These two spars weighed 44 pounds. This was a tremendous amount of weight, and every time I lifted them I said to myself, "There must be a better way." In working on the problem I came up with this kind of solution. We brought the wings in to about 1-1/2 inches apart, then took straps of 7075-T6, 1/2 inch thick and 3/4 inch wide, and riveted them to the skins. These straps are riveted to the skins with 3/16 inch rivets and there are 10-32 bolts through the ribs. In other words, there are 11 straps on one side, and another 10 over on the other side, top and bottom, and as you put the wings together, they interlock like a piano hinge. The pin holes are 1/4 inch. The two pins are curved to fit both the top and bottom contours of the wing -with handles on them. To put the two wings together, the fingers mesh. You put the pin in and shove. The pins are curved and you wouldn't think they'd ever go through but they do and it's a very efficient and lightweight connection for the wing. The total weight of all of these 42 straps and the two pins, is 11 pounds. The net saving on this type of construction is 33 pounds or about 10 percent on the wing weight.
Question: I was curious as to the bonding method of the honeycomb and the skin. What do you use to bond your honeycomb to your skin?
Schreder: When I first proposed this type of construction to the Honey-comb Corporation of America, they said that it was impossible to do without an autoclave and without very sophisticated equipment; but we went ahead and finally got our honeycomb from Hexcel.
I got the blocks of honeycomb from Hexcel Corporation. When you see the block, it looks just like a solid block of aluminum. They are two inches thick -- 2000 sheets of aluminum foil 0.001 inch thick, laminated together to make a 2-inch block. We cut the blocks by band sawing the contour into the block. When you pull the block apart, it will open up like a Japanese Christmas bell and form hexagonal cells. But when you pull them apart to form the cells, the block shrinks about 20 percent. So, in developing the contour, you have to allow for this shrinkage so that when you pull it apart you will wind up with a width that will just fit. The overall depth is about 1/16th greater than the cavity in the wing so that there will be a bit to trim. The block is stretched out to a 12-foot length. The spars and the bottom skin are coated with epoxy cement. You must etch the skins very carefully first to assure a good bond. The stretched honeycomb is laid into the cavity (with the top skin off). The wing rests on a supporting structure mounted on a table to maintain the lower skin contour. You push the aluminum honeycomb down into the cavity and put weights on it to hold it in place so that it is touching all the way along the bottom. Then let it set until the glue cures. You then take a light above the core, look down into the core through the cells, and if there's even one single cell that hasn't bonded to the lower skin, it Is very apparent because you get a different reflective pattern. If there are several cells, there will be a bright area where the glue hasn't touched the cells. In other words, if you didn't have good contact in one spot (if the honeycomb was up a little) then you wouldn't get any contact with the bottom skin. All you do then is take a hypodermic syringe with epoxy resin mixed in it and inject it down in one of the cells in the center of that area. We only had a few small areas, maybe as big as a half dollar, where we didn't get contact. The glue would run down the cell and spread out where there was no contact. When the cement rises to touch the honeycomb it forms fillets around the base of the honeycomb cells
After the voids were filled and the cement cured, the bottom half of the wing was finished. Templates are then laid from the front spar to the rear spar to check the proper contour of the wing. Special templates are then fitted every 18 inches. Since we made the honeycomb blocks a little bit higher than it had to be, the templates would be about 1/16th of an inch above the spar. A block of wood with sandpaper glued to the surface is used to sand the honeycomb. This honeycomb material is only 0.0007 inch thick and it sands very rapidly -- much faster than balsa wood. You must sand carefully and keep fitting the template. You sand the high spots until the template touches both the front and rear spars and contacts the honeycomb all the way between. This is done at each 18-inch station and then a longer sandpaper covered board is used to bring the honeycomb down to contour between stations. Just run it back and forth in between until you can lay the straightedge spanwise and you have no high spots. Then you're all ready to put your top skin on.
The top skin is predrilled before epoxy is applied. A drill guide with 1-inch spacing is used to run a series of holes from the root out to the tip through the skin into the spar. All holes Are countersunk so that the rivets are all ready to put in. The skin is now ready to go on. You lift the wing up, turn it over and shake cut all of the shavings you got when you sanded the honeycomb. You put epoxy cement on the inside of this top skin and put the wing back on the form blocks that are mounted on the table. This takes all the twist out of the wing and holds the wing straight and level so that when you put the top skin on, your wing will have no twist and will be perfectly straight. Bricks are placed on the wing to hold the skin against the honeycomb.
The skin is then clecoed to the front and rear spars. Rivets are installed and covered with strips of tape so that riveting can be expedited from one end to the other. A more sophisticated arrangement would be to enclose the wing in a plastic bag after it was riveted and evacuate the air. I didn't bother to go that far.
The next question you're going to ask is what happens if the skin does not touch perfectly at some place. After the glue has had a chance to cure, you take the weights off and take a piece of metal and go along the top and click the skin. If there is any place where the glue did not make contact with the skin you will get a very flat hollow sound. On one wing we had absolutely no hollow spots. On the other wing we had two spots right about in the area where the two 12-foot skins came together and had a doubler plate on the inside. At the edges of the doubler plate there was an area several inches wide on each side of the doubler plate right at the top of the airfoil section. By tapping the wing I could easily outline the area that had not made contact. You take a grease pencil and mark off the shape of the void on a piece of plastic. Then you turn the wing over and lay the piece of plastic on the bottom and transfer those voids to the bottom skin. Drill a 0.040 inch diameter hole in the center of each one of these voids -- just big enough for the hypodermic needle. You inject the epoxy through these holes. It goes down in the cell, runs underneath until it contacts all of the free cells. We drilled two holes. One that we injected in and another over in the corner of the void. When all the cells were wetted, the excess came up in the other hole and popped out the top, We then knew we had the voids filled. We let it cure and went back with a piece of metal and tapped on the top where we had the void and it sounded exactly like the rest of the wing. We then had a sound wing.
It seems like a lot of work but each wing took about one hour to put all of the honeycomb in place and to get it weighted down. It took three hours on each wing panel to sand the top down to contour, and it took about two hours to rivet the top skin and place the weights. I would say that each one of these wing panels could be assembled by two men in a day. In other words, you could assemble one wing and if you had the other one ready to go, you could get one riveted together and then go over to the next one and rivet it together. In two days you could put the two wings together. That is, you could put all of the honeycomb in the wing. This is a tremendous saving in time because only two ribs at the root and one at the tip were used in the whole wing. The leading edge was formed out of a thermoplastic high-impact material. The flaps and ailerons are simply bent up out of sheet metal.
Question: How large were the spaces in the honeycomb?
Schreder: The honeycomb that I used had 3/16th inch cells, and the foil itself was 0.001 inch thick.
Question: How about the weight?
Schreder: It weighed 2.0 pounds per cubic foot. That is about equal in weight to the lightest foam you can get. And incidentally, at the beginning of this project, we had intended to use urethane foam. We bought the very best we could get and made a test panel. This test section had the same dimensions as the root and was about four feet long. We had each end mounted to a 6-inch I-beam. One I-beam was tied down to the floor and the other was above the floor with a platform on the end so we could add weights. We tested this section and got up to four G's. We had the equivalent of shear and bending forces that we would get at four GIs when it began to show signs of failure. While we stood there and watched (it was upside down and what would normally be the top skin was on the bottom) it began to wrinkle inward. As that wrinkle progressed, it went faster and faster and all of a sudden it went in and the wing failed. We, therefore, figured the 2-pound urethane foam was too light so we got 4-pound foam. We duplicated the test and the 4-pound foam was getting to be an appreciable amount of weight. It was amounting to about 30 pounds of weight per wing and we didn't figure we could afford that much weight, especially when it wouldn't do the job.
We then immediately went over to aluminum honeycomb and the aluminum Honeycomb is roughly ten times as strong as the foam in compression for the sane weight per square foot. We ran the aluminum honeycomb lip to nine G's and broke our fixture, so we gave up at that point figuring that was good enough. Actually, the strength of this honeycomb is 50,000 pounds per square foot. You could lay one of these wing panels on the ground and drive the biggest truck, loaded with anything, up that wing panel and you would not crush the wing.
Question: Do you roll the skins?
Schreder: No. We preform the skins right in the shop at Bryan. We found that the nearest place to get skins like that rolled would be the West coast. We had gone through that once before. It was very expensive and we didn't get too good a job. We developed a method for bending them right in our shop and we had the exact contour prerolled into the skin. In other words, we tapered the roll from one end to the other.
Question: What contour was it?
Schreder: The contour we used was a constant radius, however, we could put any contour in that we wanted.
Question: Dick, how many joints do you have on the length of each wing panel?
Schreder: There are two joints. We had two 12-foot panels and then we added about five inches out at the tip just to make a standard class 15-meter span.
Question: Dick, you've had this out now for about a year or so. Have you found the epoxy still holding on all of your cells? Are there any voids?
Schreder: There are no voids. Of course, I am not absolutely sure that you have to be free of voids, unless you had a large void. The minute the skin starts to go down it touches the core and I don't think it would fail. I have clicked the wings several times since they were built and can find no voids.
Question: Would you care to elaborate on the HP-10, the basic philosophy, and how you ended up doing this?
Schreder: The HP-10 had a different type or construction. On the HP-10 we had wing panels that had double surfaces much like the Libelle wings only they were aluminum instead of fiberglass. We used a "Z" section at each spar and then another skin on the inside, and had a slice of constant thickness honeycomb --I believe it was 3/8ths of an inch thick. The trouble with this system is that in order to form these honeycomb panels, you must have very elaborate female forms. Building the forms was a much bigger job than building the wing. The forms had to have the contour. There was a built-up box of aluminum and it had to be made of 3/16th inch thick aluminum. Each rivet coming through the surface had to have an O-ring around it before you drove it from the bottom side so the air couldn't leak through. We had Goodyear Aircraft in Akron, Ohio, make these panels for us. They wanted $5000 for one form 12 feet long and the width of the wing. On the HP-10 we used a rectangular wing so that we would only have to have one form for the top surface and one for the bottom. If you made a tapered wing, that means you would have to have eight forms. This HP-l0 type of construction is, about, pound for pound, the same for the size wings we are talking about as filling the wing completely with honeycomb. With this system you have to have forms for the panels and this would be almost impossible for a home builder. You'd have to make the forms.
Question: Could you build a little larger chord than this and get away with it?
Schreder: Yes. There is no restriction on the size except that as you increase the volume of the wing, you increase the weight of the honeycomb, and if you get a very thick wing and a very wide chord, I am sure that the honeycomb would weigh more than the conventional ribs and spar. But for the wing that was built on the HP-15, this was by far the lightest type of construction that could have been used. The wing was only 4.312 inches thick at the root. The top panel carries the compression in normal flight and the bottom panel carries the tension. Since the wing is only four inches deep, in flight you get quite a noticeable amount of deflection. You soon get used to this deflection.
Question: How much did the wing weigh?
Schreder: These wing panels weighed, after they were painted, exactly 100 pounds each. This was a pretty low weight considering we had a 33 to 1 aspect ratio and a 12-G safety factor.
Question: (Byars) Would you review the design philosophy on the -15 and discuss how you coped with what you feel might have been the trouble that you had in Marfa?
Schreder: Well, the design philosophy on the HP-15 was to go to a much higher aspect ratio than anyone had used before. We had a standard 15meter span. The wing chord at the root was 24 inches and at the tip it was 12 inches. The thickness of the wing was 15 percent and the airfoil section was NACA 662/618. We found that with this particular airfoil we could lay constant radius curves over 60 percent of the chord. In other words, the part that we would have to make and fill with honeycomb had a constant radius -- very little deviation from the basic airfoil. So this is one reason why this airfoil was chosen.
The problem I ran into at Marfa! We made one test flight before leaving for Marfa. On that test flight it was an unsoarable day and all we could do was make straight runs. The ship had a great glide ratio. This was borne out at Marfa. I don't think I ever flew with anybody at Marfa that I couldn't pull away from except the BJ-4. We made runs side by side at speeds up to about 150 mph, and our performance was almost identical until we got up to the very high speeds and then the BJ-4 would slowly pull away from me. I could pull away from most of the other ships. I didn't get a chance to fly with the ASW-12, so I can't speak for that.
My problem was when I got into a thermal. I couldn't climb and this was very embarrassing. You have no idea what kind of a feeling this gives you if you find a nice thermal and start working it and a bunch of characters like A.J. Smith and George Moffat come tearing over there way down below and shortly thereafter they climb above you!
Comment: (Steve du Pont) You'll get used to that!
Schreder: Thank you, Steve. I feel better already. Anyhow, meanwhile in the thermal... First of all, I could fly alongside other ships and could slow right down until I reached the HP-15's stall speed. The problem was when I got into a thermal and would start cranking around at lower speeds, I could feel this separation. I could feel the aileron shaking and I could feel the flaps shuddering -- which indicated that the air was leaving the upper wing surface. It wasn't following over the top surfaces of the flaps and the ailerons. I felt that the angle of airfoil in the rear was just too great and I was getting about the same type of problem that Dick Johnson had with his mahogany bomber, where he had used an Eppler airfoil. It had a very similar shape. The maximum thickness was pretty far aft and the airfoil closed so rapidly back there that the air just wouldn't follow the upper surface at higher angles, intact.
After I got back home, I increased the root chord of my flap from five inches to eight inches. Now, this apparently has solved my problem. I've only flown the ship twice since doing this. We haven't had any soaring weather all winter, so I can't tell what's going to happen when we are out soaring. The two times I flew it, we had a K-7 up for comparison. We had very weak thermals where we could just about hold our altitude for 10 or 15 minutes at a time, and I was able to stay right with the K-7. So, I'm hoping that I've solved the problem; but I'll find out soon whether it is corrected or not,
Question: What is the wing loading now?
Schreder: The wing loading now is down to about 7-1/2 pounds. It was over eight pounds in Marfa.
Question: Aspect ratio?
Schreder: Down to 30 to 1.
Question: What's your indicated stall speed now, Dick?
Schreder- Indicated stall speed now is about 43 mph. It was 47 in Marfa with flaps in neutral.
Comment: (Moffat) It might interest people to know that at Marfa, I was doing a test with Dick the first day he flew, and I discovered that he wasn't climbing very well. I followed him around at the same speed to see what speed he was thermaling and it turned out to be 78 mph.
Schreder: You have no idea how I suffered.
Question: The arguments that have frequently been heard of dive brakes over flaps is that in the landing approach, when you get slowed down with the flap configuration, what do you do when you get in sink and need to dump the flaps in order to make the field? The argument is that, well, if you've got dive brakes, you turn them off and that improves the flying characteristics so that you can get the extra penetration to make it in to the field.
Schreder: This is really the only valid question against flaps but it really doesn't have any bearing and I'll tell you why. In the first place, when you make an approach with any sailplane, it is my theory that you should have 15 or 20 mph above your stalling speed, according to how gusty it is. You use either your flaps or your dive brakes, not to control your air speed, but to control your glide path as you approach. In other words, you always use the same pattern around the field and then when it looks like you are too high you begin cranking in your dive brakes or your flaps; and you maintain that same speed all the way down to your flare out. Now, this, I'm sure, is the best method to make an approach. In the case of using dive brakes, as soon as you see you are a little low you immediately retract but you still maintain that same speed unless you have to slow down to reach the field. But, if you are coming in on a normal approach angle and you can see you are getting a little below it you retract your dive brakes, You do the same thing with flaps. If you are coming around on final and it looks like you are a little too low, you immediately retract your flaps and, of course, your glide flattens right out. You continue until you get back on your glide path where you crank them down again. Now. the one thing you don't ever do is leave your flaps down and just keep pulling the stick back trying to zoom in to the field, any more than you would leave your dive brakes out and keep pulling your nose up trying to hold altitude to get to the field. So, I really don't think it is a valid question. Now, it's true, if you left them out and kept slowing down until you got to your stall speed with the flaps down and saw you weren't going to reach the field, there is nothing you could do.
Question; The question is, what if you slowed down too much for a flaps up configuration and, in reality, you're saying that this should never happen.
Schreder: That's right. They don't know how to use the flaps. And, the only people who ask this question are the people who have never used them. Comment: I've been flying HP's a lot since '66 and the situation he describes will never happen to a fellow who has made two or three or four landings with them. One of the advantages that Dick mentioned was a steeper approach. That immediately takes the situation and wipes it out. On final approach I'm anywhere from 200 to 400 feet higher than I would come in with a conventional glider and I could either overshoot the field flaps up or put both flaps on and undershoot the field. With these type of flaps you've got such a wide angle that you will never find yourself doing it short.
Comment: The advantage, then, is the slower speed on actual touchdown?
Schreder: That's right. If you did get yourself into a position where you couldn't make the field, you would be much better off with flaps than you would with dive brakes because if you are going to run into a fence or you are going to go into the boondocks across the road from the airport, you are much better off with flaps because when you see you are going to go in there, you just crank them down all the way, and, if you have a 10 mph wind you touch down at about 25 or 30 mph, whereas if you had dive brakes, you'd be going maybe 10 mph faster, and if you put the dive brakes out, you'd be going even faster. So, this is the extra safety feature of the flaps.
Question: Could you elaborate a little bit on the difference between tile tails of different sailplanes?
Schreder: Yes. I favor V-tails for several reasons. First of all, and most important, I feel that the V-tail is less likely to be damaged in landing. About the same can be said for a T-tail, although I think you could go into a higher crop with a V-tail than you could with a T-tail. The next point is that the V-tail is stronger than a T-tail. A T-tail has the mass up so high that if you do a ground loop and hit something with the tail, your horizontal stabilizer is probably going to keep going; and also, the T-tail (I've built both) is much more complicated to build and it just isn't possible, without adding a lot of weight, to get the same strength into it. And then, with the T-tail, it is harder to counterbalance the rudder. So, I prefer the V-tail because it's lighter, it's stronger, and it's simpler to make. In the air, I don't feel that there is a whole lot of difference between the V-tail and a T-tail, or a conventional tail. Sometimes the English don't quite agree with us on that. I will say this, I don't think the V-tail is quite as effective on the ground, but I don't believe you can depend on your vertical rudder to steer on the ground anyway because if you land in a crosswind you are not going to have any rudder control after you have slowed down below a certain point. We try to solve the problem on the HP's by having a steerable tail wheel; and I think that anyone who has flown an HP with a good steerable tail wheel can bear me out when I say that you can land even in a moderate crosswind and taxi right up and stop on your tiedown area.
Question: Is the V-tail cheaper to build than the conventional?
Schreder: Yes. You really only have two pieces to build In a V-tail, where you have essentially three with a conventional or a T. And, of course, in the V-tail, the two pieces are identical but are right and left.
Question: Dick, you mentioned that economics and safety were going to be primary criteria for design of sailplanes of the future. It would seem to me that this would be very true if the performance could be comparable to the fiberglass. Is there anything inherently better suited for performance than fiberglass or metal?
Schreder: I don't think so. I think that from a weight standpoint, the fiberglass and metal ships are about the same. I think it's a little easier to get heavy with fiberglass because it's easier to add more material. More resin might be needed in some places; and one of the problems of fiberglass is the difficulty of getting local stiffness. You can have a fiberglass panel that is more than adequate in strength, but, if somebody leans on it, they are apt to crack it,
Question: Do you wind up having to fill and sand on the wing like you do it on the others?
Schreder: No. On this wing we had absolutely perfect contours on the top and bottom and the only place where we really had to do any filling was where we had this plastic leading edge coming in. Even though it was the same thickness as our skin, you do get a little irregularity and you must spray it and sand it to get it smooth.
Question: Are you going to make an HP-16 anytime soon
Schreder: Oh, I wouldn't make any rash statements today one way or the other. I probably will, just judging from past experience.
Question: Are you aware of the efforts that have been made to use vacuum forming plastics in sailplanes, and, if so, would you comment on that?
Schreder: To use vacuum forming plastics? I'm not really aware of any efforts that have been made. Vacuum forming is a beautiful way of getting contours but I'm in the plastics business and I have yet to see a thermal plastic material that has any appreciable strength where you could use it for a structural member. It's very good for fairings. We used it for fairings on the HP-14 and the HP-11, but I haven't heard of any structural applications. I hope, someday, that there will be that type of material which can be used. I really don't see any reason why you couldn't imbed fiberglass in a thermoplastic material. The trouble is that it would be very hard to form it to make a compound curve where you couldn't stretch the fibers.
Question: You mentioned, in discussing this fiberglass pod, that it did not give you as much pilot protection as metal. Have you given any thoughts possibly to laying a metal mesh inside this fiberglass pod for strengthening and protection?
Schreder: No. I really hadn't considered that. The way we intend to get around that problem is to possibly use aluminum rails for reinforcing around the cockpit.
Question: Will you comment about the FK-3, the new German all-metal aircraft? I think you said you saw it and flew it-
Schreder: No, I didn't fly it, I saw it down at Marfa and it's a beautiful looking ship. It had a different type of construction. I'm not exactly sure what it locked like inside but I was told that it had a conventional spar and very thin skin but that they used urethane ribs. I don't know exactly how thick they were but they were spaced about every three inches. It was a very smooth wing and, of course, they got away from using any rivets where the ribs were attached.
Question: It was glued on?
Schreder: It was glued on. If it's been adequately tested, it looks like a good system. I'm a little bit afraid of urethane foam since we ran a test on it.
Copyright Soaring Symposia All rights reserved. Permission to copy this article is granted for non-commercial use, in its entirety, and with this copyright notice attached.