Windshields And Buffeting By Bill Skipper, EAA 17258 P. O. Box 177, Evans, Colo. HIS WAS the day! A day to be long remembered . . . T for several reasons! This was the day which occurred only once in the lifetime of a homebuilder, the day you make the first flight in your first homebuilt airplane! Sure, you may build many other airplanes and fly them for the first time, but you fly your first airplane for the first time only once! I went to Denver's Stapleton International Airport and personally acquired my test area from G. G. Pettibone, Air Traffic Area Supervisor, a very cooperative FAA man. James Stevenson of the FAA General Aviation District Office 3 in Broomfield, Colo., had assured me by phone that he or someone from his office would be up to Greeley to inspect my Stits SA3B "Skip-Craft", N-133WS, and to watch it fly as soon as I got my test area assigned and the necessary papers made up ... description, threeview, weight and balance, etc. Since I have made my living in the cockpit of a 450 hp Stearman dusting and spraying for the past 18 years, and this was mid-summer and my busy season, I was trying to arrange to be finished spraying weeds and get to Greeley airport in time to meet whoever came from the FAA. I operate in an area some 30 miles away from Greeley and it isn't easy to get home at a predetermined time. However, I made it at 11:00 A.M. and there was Mr. Henderson, pugnacious chin and all, waiting for me. We drove over to the new row of metal T-hangars where I am fortunate to have room to store both the Stits and my other rare bird, a Bucker "Jungmann", one of only two original "Jungmanns" left in the world. Man, was it hot! About 97 deg. in the open shade and probably 125 inside the metal hangar. I opened the doors and we rolled the shiny red and white bird out where he had room to circulate around it and pick it to shreds. He looked at the papers, then looked at the airplane, asked questions, and then looked some more. I was very lucky

in that he found only one serious oversight and that was the fact that I had neglected to tighten the rear strut adjustment locking nut after rigging. I did so immediately . . . both of them!

Finally, he did some typing on a white sheet with blue printing on it, then did the same to both sides of a yellow card, signed them, then wrote some more in the brand-new Aircraft Log Book, signed that and said . . . "Now, let's see if it will fly." Another homebuilder, John Dalton, who built and flies a beautiful single-place Stits with an 0-290-G4, was

there all the time, so we rolled the new job out into the

This windshield design was the final solution to the buffeting problems encountered on the "Playboy."

dazzling sunlight and I crawled into the roomy office while he spun the new wood prop. The Lysol caught on the third pull and I slowly taxied clear down to the east end of the mile-long asphalt runway to take advantage of a slight west breeze. After running through my check list, I headed her down the white stripe and opened it up. I had done some high-speed taxiing and the Stits was docile, what with the fuselage lengthened 13 in. over the

Stits plans, the cockpit widened to 40 in. and the rudder area increased a bit. The tail lifted easily and, after gaining speed rapidly, it lifted off, bobbing gently in the noon turbulence. Right away, I saw that the wood prop was no good on this bird. The recording tachometer showed 2900 rpm and the airspeed was indicating 65 mph. I thought of shutting her down and landing but, by this time, I was too far down the runway to do this safely, so decided to take her around and land. But it wouldn't climb and I didn't want to over-rev the engine which is red-lined at 2600 rpm, so at 200 ft. I settled on a medium of 2800 rpm and 75 mph which is a little skinny on a hot day, with the test-flight weight at 1,200 lbs. and the Poudre River to fly down on the downwind. We made it okay and Mr. Henderson agreed with me that another prop must be found, and left. John and I rolled the Stits back into the hangar and looked it over. We both received quite an unpleasant shock when we found the new wood prop had developed a long, slanting crack which began a third of the way out from the hub and angled raggedly across the blade toward the tip from the trailing edge, stopping some five inches short of the tip but definitely headed toward throwing about half a blade. How soon this would have happened I don't know, but I would hazard a guess of another minute or so at the most! I removed the cracked prop and searched everywhere for something else to put on the Stits. Finally, I received the loan of a Sensenich 74-53 metal prop, which gives plenty of clearance as long as you three-point it. Getting this installed, torqued and safetied, I chocked her up to see what I had. Not bad . . . 2300 rpm static! By now it was 5:00 P.M. and starting to cool down some. I taxied out

onto the runway, checked her out and again headed down the white stripe, this time to the east into a 10-12 knot breeze. The Stits broke ground at 65 mph and there was

quite a bit of torque, as expected with such a largediameter prop on such a small airplane. Airspeed immediately increased to 70 mph and the tach showed 2500 rpm. I let her climb to 400 ft., dropped the nose, checked be(Continued on next poge) SPORT AVIATION

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WINDSHIELDS AND BUFFETING . . .

(Continued from poge 31)

hind and laid into a left turn. That prop is rough, I

thought! Probably turn it 180 deg. and take care of it. The instruments shimmied. Airspeed went on up to 90 mph and I started a climb. The roughness increased and I then realized that it wasn't the prop but somewhere behind me there was something going on. I love open-cockpit airplanes, so my Stits is an opencockpit airplane. I had Plasticrafts in Denver heat and bend my windshield from 3/16 in. Plexiglas and the cockpit is very cozy. With all of this visibility, it was no problem to rotate the cranium and roll the eyeballs over the tail end of the little runt, and what I saw pleased me not at all. The leading edges of the stabilizer toward the outer ends were doing a whappety-flap, whipping up and down in no particular rhythm, but viciously trying to twist themselves off about where the brace wires tie in at the rear near the outer elevator hinge. The upper bay of the vertical fin was even in on the act, see-sawing back and forth rapidly and erratically. Strangely, hardly any of this wild and frantic abuse was transmitted to the stick

and rudder pedals.

Reflex action reduced power and airspeed, but the buffeting went right on, almost with no appreciable let-up. I knew it was buffeting and not flutter . . . there is a vast difference! I had experienced flutter only once before and that was aileron flutter on a Szekely-powered "Bull Pup" I used to fly often back in the late thirties. This, however, had no rhythm like a flutter, but was wild and erratic.

In holding the nose up at minimum speed, I rolled

the trim tab back to relieve the pressure, but the tab, located on the left elevator, had no effect whatever and this told me that there was no air-flow over at least the inboard half of the horizontal surfaces. At this point I began to wonder just how good my welding was on the tail surfaces and how much abuse the 4130 tubing would stand, all the while edging back into a position for a quick landing. Meanwhile, I discovered that if I dropped the nose the buffeting lessened. But this was not too good, since I hadn't the altitude to spare and, when I added power, the buffeting increased. The only route was to get on the runway and this I did without losing the tail assembly. I took it back to the hangar and put it away. My first thought was to add front brace-wires to the tail, but this wouldn't remove the buffeting, but only make it stand more of it.

So I got hold of another long-time friend who operates a very successful rebuilding shop and we discussed the buffeting problem. Ed Beegles, who would like to build an airplane if he had the time, thought it was the windshield and suggested that I remove it entirely and fly it to see if the buffeting was still there. Having flown a Stearman duster thousands of hours without a windshield, this was no problem, so the next morning I came back out to the field and gave the empennage a thorough eyeballing. The only damage I could find was to the front stabilizer mount. This mount consists of a length of 13/16 in. by .049 wall 4130 which is turned on a lathe to slipfit inside a piece of 7s by .035 wall tubing. The machined section passes through the % in. section, protruding three inches from either end and the leading edge of the stabilizer then slips over this. A 3/16 in. hole is drilled through both tubes and a 10-32 aircraft bolt is installed with an elastic stop-nut to prevent the stabilizer from coming off. The piece of 13/16 in. is rosette-welded to the % in. tubing. The Stits plans call for the front

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JANUARY 1966

The oversize metal propeller put the airplane back into the air after the original wood propeller was found to be badly cracked.

mount to be attached to a "Cub" screw-jack for trim, but I used an adjustable tab on the elevator and mounted the front stabilizer attachment with % in. by .049 bushing stock welded to the top longerons and stabilizer mount. These are then assembled with ¥4 in. bolts, with washers added or deleted to obtain the proper degree of incidence at gross load and cruising speed. The trim tab then handles all other slight trim problems. The point that I am trying to make is this: When it came time to make this front stabilizer mount, I was short of enough 13/16 in. stock to pass all the way through the center mounting tube. I only had one foot left, so I sawed it in two and used half on either side with three inches rosette-welded inside each end of the % in. tube, leaving three inches protruding to slide the stabilizer leading edge over. The severe buffeting had been enough to loosen the rosette welds on the left side, which could have been pretty serious had the front of the stabilizer come loose. I drilled 3/16 in. holes through the mount on both sides and placed 10-32 bolts and stop-nuts here too. That took care of that! I believe that I will have no more trouble there. However, if and when the time ever comes to re-cover the Stits, I will add the front brace wires and make a new stabilizer mount with the inside tube going all the way through. After making sure there was no other damage to the rear end or, for that matter, anywhere else, I removed the windshield. It is mounted with 10-32 truss-head machine screws into rivnuts. As soon as the Stits lifted off, I knew that the buffeting was solved. I flew it for nearly half an hour, the longest flight yet, and nothing could have been smoother. I came back and, John and I, using one of those felt marking pens, designed a new outline on the windshield and used a handsaw to trim it to the "new look." One short test hop with the new windshield (and a parachute) revealed that Ed Beegles was right . . . the buffeting was gone! The modified windshield now looks like the old Mercer "Runabout", being low and rising in front of each seat and swooping downward to a nice radius in the center. Just be careful about using too big a windshield on these open-cockpit, short-coupled airplanes! ®

INFORMATION WANTED If anyone has any knowledge of the current status of the design and/or aircraft itself . . . the Aero "Streak" high performance two-place all-metal aircraft designed

around 1947 . . . Bob Stinson of 7292 Peaceful Valley Rd.,

Acme, Mien., would like to hear from them. Apparently the only one of its kind, little has been mentioned about this seemingly fine aircraft since that period. ®