HP-19 & HP-20 UPDATEóA FREE FLIGHT INTERVIEW
Free Flight, May 1981
by Tony Burton
On April 14 , Ursula and I just happened to find ourselves in Bryan, Ohio while delivering a 2-33 to Cu-Nim [Gliding Club of Calgary] from Elmira. Since we had spoken with Henry Preiss in January on progress in Dick Schrederís Bryan Aircraft Corp. with the HP-l9 and HP-20 (see #1/81), we were naturally curious on their progress.
We spoke with Henry Press, who had given the HP 19 its first test flight only a week before hand and who produced the fuselage from his basement for our camera And we also talked with Dick Schreder about the HP-20, and inspected the work in progress.
ff: We were surprised to hear you made the first test flight of the HP-19 on the schedule you had hoped for in January. Homebuilders generally get delayed somehow. How did it go?
HENRY: The first attempt was aborted on the runway three days prior to the actual flight due to a lack of aileron response. The problem was in the effective aileron differential movement which was all wrong. In my previous gliders (RHJ-8 RHJ-9) the aileron hinge line was along the top of the surface In the HP-19, it was on the bottom side, and this made a big difference when the aileron control system geometry was taken into account. The problem was corrected easily. The first test flight took place at Bryan on 8 April with Dick towing. The elevator effectiveness on the new T-tail was very positive; too positive in fact, and the ship was touchy to control in pitch. There was a lot of porpoising on that tow. I have increased the ratio on the elevator push-pull tube bellcrank from 2:1 to 3:1. Now, the same stick movement will result in a smaller elevator deflection and make the ship more manageable in pitch.
ff: What else did you find on the flight?
HENRY: I didnít test as much as I wanted to, the lift was marginal and I only got to 3000 feet once. The stall was about 35 knots without flaps and 32-33 with positive flap. I expect that will be reduced further when the winglets are added later.
With the new airfoil, Dick didnít know what the correct (that is safe) C of G range would work out to be. A design goal was to have the ship trim out to fly in all normal flight modes with flap position changes only and little if any elevator movement. That minimizes control surface drag. Ideally, we wanted the ship to fly 40 to 50 knots with +5 to +10 degrees of flap, 60 to 70 knots with no flap, and about 90 knots with full negative flap. On this fliqht it turned out that the c of g was too far forward.
On the flap motion test I was flying about 50 to 55 knots with +10 degrees flap and when I moved the flap to -10 degrees, the ship quickly increased speed and was showing no signs of settling down as it reached 100 knots. That test had to be stopped there.
I figure that about 10 pounds added to the tail will trim the ship out properly with my weight of 200 pounds. Since the HP-20 will be similar in design, Dick has moved the wing position forward slightly to correct this problem.
By the way, the airfoil computer design work was aided by NASA, not Columbus University as I had mentioned in the last interview.
ff: Iím beginning to appreciate the value of prototype test flights! How much does the HP-l9 weigh?
HENRY: It weighs 440 pounds with radio, instruments, battery, but not counting the parachute. Remember that the spar is carbon and will hold 300 pounds of water. Iím really happy about the finished weight.
ff: That sounds great. It should do well in weak conditions with such a low dry wing loading (about 6 Ibs/ft2).
HENRY: Iím very happy with the glider it seems to have good aileron responseónoticeably better than the RS-l 5 I flew.
ff: Whatís next on the agenda?
HENRY: Well, Iím going to continue the testing of course. My spare evenings for a while will be dedicated to helping Dick get the HP-20 finished, and Iíll be looking for a buyer for my two-seater side-by side.
ff: Thanks for showing us the ship, Henry. I hope you will be able to report more test flight results before this issue of Ďfree flightí is printed.
I was snooping around Dick Schrederís glider shop, taking photos of the HP-20 project and chatting with the foreman, Deloss, when Dick walked in:
ff: Hi, Dick. Things seem to be progressing slowly but steadily since I was here last April. Now Iím spying for free flight, how are things going?
DICK: Well, as you see, the fuselage structure is complete and the stabilizer and rudder is attached. We are finishing the wing sub-structure and will be skinning them very soon.
ff: I see that the spar is different from what you have done before. The spar caps are laminated from four sheets of what looks like 060" aluminum.
DICK: The spar is much the same as the HP-18 or RS-l 5, but wider. It is a box spar of constant 4 inch width. However, milled spar caps are no longer available at a reasonable price, so the caps are being laminated from sheet stock. The inside laminations are of decreasing length towards the tip, with only one thickness remaining outboard of the midpoint. The laminations are rivetted together along the edges and are also bonded under pressure. Furthmore, the end of each lamination is scarfed (tapered) over 6 inches to eliminate stress concentrations.
ff: It would seem that the bonding and scarfing of these laminations will be a difficult chore for the homebuilder.
DICK: The inboard half of the cap would probably be done here if these spars were to be included in a kit as itís the most highly stressed structural component and requires tools and bonding techniques not readily available to the average homebuilder.
ff: You say "if". Is there some question as to whether the HP-20 will be sold as a kit?
DICK: I wonít sell the HP-20 unless I feel that it will be a better glider than my previous kits and will fly competitively.
ff: I see that the airfoil section is quite different from the Wortmann section you used on the -18. It has a more blunt leading edge and is thicker on both sides of the spar.
DICK: I was looking for a laminar airfoil that had a higher maximum lift coefficient than the Wortmann section. I am trying to design the HP-20 to have very good climb capabilities at low speed in weak lift, and make use of the flap and considerable water ballast to go fast in strong conditions.
ff: How exactly did the section evolve during its tests?
DICK: I was able to make use of a computer program which simulates wind tunnel tests. The program accepts a drawing of the airfoil and measures and stores the airfoil coordinates. It then calculates how the airfoil would react in a wind tunnel and provides an output in the form of a graph of lift and drag coefficients at varying angles of attack. My first airfoil was just a guess at what I thought would do the job, and NASA found that although it gave about a 25% better maximllm lift coefficient, there was separation over the entire lower surface! The nose was too blunt, and other changes were suggested. After the fourth version was run through the computer, the airfoiI seemed to be giving me what I was looking for so I stopped there. I suppose the airfoil will be called the ĎSchreder Number Fourí.
ff: It would seem that with a design emphasis on good low speed performance, the flaps will be more critical in achieving reasonable penetration with your airfoil than with Wortmannís.
DICK: Yes, Henryís first flight in the HP-19 gives an indication that it will be all right. It may be that precise flap position will be fairly important to good results at various climb and cruise speeds. It will take more test flying to work that out.
ff: Is there anything else you can say about the wing?
DICK: Well, as you can see, this wing uses the same construction techniques of closely spaced foam ribs and bonded skins as I used in the last models. The HP-20 wing will have a double taper to closely approximate the ideal elliptical platform. Only comparison flights with the straight-tapered wing of the -19 will show how much advantage this will provide.
ff: An elliptical wing has rather vigorous stall characteristics since the entire wing stalls at the same time. What do you expect the stall of the double-tapered wing to be like?
DICK: The stall Is quite sudden on elliplical wings, particularly those of low aspect ratio as found on conventional aircraft. The stall will be tamer on a glider wing, and I will also be using short winglets to improve aileron effectiveness.
ff: Letís finish off with the fuselage and tail surfaces.
DIGK: Well, except for the tail, the fuselage structure is quite similar to the HP-16, and the control system is similar to the HP-18. The fuselage has a lower profile than the -16, however, and there are other minor differencesóthe canopy rail is much stiffer, for example. The vertical stabilizer contains three spars, the middle one at the point of maximum thickness to provide extra rigidity for the top-mounted elevator. The elevator has its counterweight inside the stabilizer. The horizontal stabilizer main spar is a box section formed by riveting two aluminum U-channels together.
ff: This has been a delightful tour Dick, thanks for telling us aboutthe work, and I hope that not only is the HP-20 successful, but that you also have it ready in time for the 15 Metre contest.
FOOTNOTE: The HP-20 did not get finished in time for the US 15 Metre Nationals in Minden, Nevada. Dick had to stop working on the HP-20 for a time to prepare his standby HP-18 for the 15 Metre competition. The HP-20 will debut in a Regional Contest in lonia, Michigan in early September. Henry has finished painting the HP-19 and has got some time in it but has not had an opportunity to conduct serious comparison flights.