See What Flutter Can Do

air compressors and spray guns, write to The DeVilbiss ... booklet, "The ABC's of Spray Equipment." ... This will save much time and effort in fabricating small.
1MB taille 1 téléchargements 413 vues
See What Flutter Can Do By Bud Rinker, EAA 18158 169 El Sueno Rd. Santa Barbara, Calif. 93105 W^HEN HANK Puteck was build\V ing his Cougar he felt the horizontal stabilizer tube-spar looked a bit light, so he installed a second tube inside the first, straight through the entire tail section. Had it not been for Hank's foresight, I might not be writing this article now, or what would be far worse, I might be without a homebuilt. After purchasing the Cougar from Hank, I made several modifications, the most significant one being a change from a conventional to a tricycle gear. The windshield was showing some crazing too, so I installed a new one before putting the craft through its original test schedule— a precautionary measure in the spirit of good safety practice after modifications. Having read John Thorp's excellent article on flutter testing in the July 1964 issue of SPORT AVIATION, and Jim Dewey's report in the 1967 Spring and Summer edition of Air Progress — Homebuilt Aircraft, I felt I had at least a rudimentary idea of proper testing procedure. Two items were my main concern; windshield strength and elevator flutter since, on the Cougar, the ailerons are balanced but not the elevators. On the first test flight the curved windshield started to bow in a bit at the bottom and vibrate slightly at 180 mph; so, I postponed testing and installed a reinforcing brace. On a bright Saturday morning, I put on my helmet and chute and started out again. At 8,000 ft., I built up speed and leveled off, letting the airspeed move up in 5 mph increments, keeping an eye on the windshield and batting the stick rearward after each increase. To prevent loading the plane symmetrically, it was best always to keep it in a slight left or right turn. The Cougar is redlined at 195 and, allowing for a 5 mph indicator error, I planned to test up to 200 indicated, which would be about 210 mph true airspeed at 8,000 ft.—a good safety margin, 1 felt, for as anyone who has flown a Cougar knows, this craft builds up speed quite rapidly. Everything went fine until the increment from 195 to 200 mph was reached. Pulling up slowly and making a slight left turn, I had just mentally sighed, "Well, you made it, baby," and before I had a chance to 2*

MAY !»*•

shake the stick it shook me with about six violent fore and aft oscillations. I yanked the throttle back and continued back on the stick. Although the plane had slowed, it started on down at quite a high rate of speed. Instinctively my hand reached down and started to trim the

nose up, (which had been trimmed almost all the way down for testing). As the plane leveled off and slowed, I looked back and was astonished to see quite a severe twist in the left stabilizer. I was even more astonished to discover there was no elevator control at all. Unknown to me at the

Fig. 1

time, the elevator bellcrank at the control stick assembly had collapsed, allowing the elevators to flutter free* ly with no mechanical damping. Apparently I had just nibbled at the critical speed, because I had detected no oscillation at 195 mph.

TIP OF INSIDE TUBE

(remained with stabilizer) COLLAPSED BELLCRANK

(cut from control stick assy. I

END OF OUTSIDE TUBE

(remained in plane) THESE TWO WEDGED

With partial throttle, the plane

TOGETHER

continued level at about 100 mph;

with less throttle, it would descend, and with more it would climb. In

fear of setting something else off, 1 let the plane settle slowly, called the tower and told them of my predicament. I felt a landing could be made with the throttle but took them up on their offer to have equipment standing by. I continued down in a long, wide pattern and lined up with

the runway, making sure to clear the engine periodically. I didn't need ice now! What the ground effect would

do to my stability had me a bit concerned, but as I neared the runway, I opened the throttle slightly, the plane flared, touched and came down hard on the nose wheel. I thanked ground control for the help and they asked if I had seen my tail. "You're in for a surprise", they said. Well. Fig. 1 shows the rest. The landing drew a small crowd and I

started explaining it was my new Phantom II tail configuration (quite

stable). Someone suggested that it more closely resembled a sick gull.

All agreed that a lot could be said for the Tailwind-Cougar series' stability under adverse conditions. Oh yes, both the main spar, the forward trim connection and Hank's inside spar had broken on the right

stabilizer.

However,

the

outside

tube broke at the bushing, while the

Fig. 3

The parts that failed. Bellcrank assembly collapsed allowing free flut-

ter which generated enough force to break both outside 1'a" x .065 wall and inside 15/16 x .049 wall tube spars.

inside tube broke at a bolt hole inside the airplane. As the stabilizer came down in that last giant flutter, the two tubes wedged together and that was all that was holding it on. Thanks, Hank. If the right stabilizer

would have separated, anyone's guess as to what would have happened is probably as good as mine. Why the elevator didn't flutter during Hank's original t e s t s was probably due to the airspeed error and the fact that he flew at a lower altitude (less actual airspeed). Perhaps the different resonant frequency of th» tail due to the absence of the tailwheel or something as subtle as prop pitch caused the flutter. An

aerodynamic specialist with proper equipment could probably find the

reason, but for the time being, I will have to rely on a beefed up spar and control system, and balanced elevators. Observing that twisted steel tubing, this pilot now has a healthier respect

for the forces that can be generated from that moment of aerodynamic instability, delicately termed "flut-

ter." That goes double for testing

procedure! The moral here is that if you have

a plane with a redline over 150 mph

and don't have balanced elevators, GOOD LUCK—you could lose your tail! vi-

TWO HELPFUL BOOKLETS When the amateur aircraft builder starts making fittings of 4130 steel, he soon becomes deeply involved with files. At this time it can be very helpful to have more than a superficial knowledge of file types, use and care! One source of such specialized information is the booklet, "File Filosophy", available for the asking from Nicholson File Company, Providence, R.I. It has 50 pages and covers the subject in a manner that is both useful and interesting.

"Oh, by the way, honey . . . there IS something I haven't told you!"

And when he starts shopping around for an air compressor, he becomes about the most confused man in captivity! Equipment catalogs and stores are as full of compressors as the Pentagon is of brass hats, and in a short time our would-be compressor purchaser is as confused as he could be over such matters as displacement, free air, cut-in and cut-out pressures, single-stage and twostage pumps and so on. To find out what is what about air compressors and spray guns, write to The DeVilbiss Company, Toledo, Ohio, for a copy of their free 64-page booklet, "The ABC's of Spray Equipment." Both the addresses given above are adequate and will reach the companies; they are both big firms in smaller cities. ® SPORT AVIATION

29

(Color Photo By Charles E. Wendt, Jr.)

DAVENPORT'S "GOLDEN NUGGET" The Davenport "Golden Nugget", N-5131, is an original design fully aerobatic biplane by Bradley Davenport, EAA 12267, of 11930 Reed St., Broomfield, Colo. The engine is a centerlined 125 hp Lycoming 0-290-G with a dummied radial cowl for aesthetic effect. The cruise is 120 mph, and rate of climb is 800-1000 fpm. The cockpit is fully enclosed with a sliding canopy which, along with a small heater and electrical system, assures all-year operation of the craft.

SOME RANDOM THOUGHTS By Kenneth B. Grayson, EAA 12509 40 Don St., East Patchogue, L.I., N.Y. USING PLIOBOND CEMENT Use a plastic squeeze bottle for Pliobond; cut the nozzle to supply the amount needed and squeeze it out as required. Any excess can be sucked up by squeezing the bottle when upright and controlling the sucking up of the cement. Dried Pliobond can be peeled off as it does not stick to the plastic. Keep the cover over the nozzle when not in use, and when filling the bottle merely remove the nozzle cap and squeeze the empty bottle fully and invert into Pliobond can, sucking the cement into the bottle. Wipe the excess off and re-cap. Using this technique has resulted in no waste or accidental spillage in over two years. The nozzle can also be used for spreading the Pliobond when necessary. USING MYLAR DRAFTING MATERIALS Mylar is used by loftsmen for laying out exact dimensions and for scale work. Don't use scissors on mylar unless they are tight, but if used to start a cut hold the blades half open and guide the scissors into the cutting line without any blade movement. Most drafting and en30

MAY 1961

gineering supply stores can provide special pencils and erasers for mylar. Always use the frosted side for work. INSTALLING PLEXIGLAS When installing plexiglas, always make the holes for screws at least 1/16 in. oversize and use the standard 1 in. by Vs in. gummy electrician's tape as a combination gasket and anti-cracking material between the plexiglas and any metal. This tape can be purchased at electrical supply stores and is a rubber type tape. HAND NIBBLER A nibbler operated by hand can be purchased at most electrical supply houses and at all electronic supply stores for about $4.00. It cuts a 1/16 in. by V4 in. chunk in any metal up to about 1/16 in., except the hardest of steels. This will save much time and effort in fabricating small parts. WHEELS Balance those wheels and especially the nose wheel

to reduce shimmy and tire wear.

,*:

1928

CURTISS ROBIN

This beautiful 1928 Curtiss Robin was 1961 winner of the AAA Sweepstakes Trophy. It is owned by EAAer Dave Blanton, owner of Javelin Aircraft Co., 9175 E. Douglas, Wichita, Kans. It is complete with original engine, prop, wheels, brakes and instruments. Dave has also restored a Travelaire 2000 powered with an OX-5. He is well known for his design, manufacture and installation of long range fuel tanks for all types of aircraft. He com-

pleted the fuel tanks for Jim Bede's "around the world aircraft" which just recently

completed a 40 hour test flight.

NEWS NOTE The Academy of Model Aeronautics has scheduled its annual National Championships for August 3-8, 1968, at the Olathe Naval Air Station in Kansas. EAA participation in the form of flight demonstrations by builders of their homebuilt aircraft is invited. Those who are interested and willing to help round out this program are invited to contact William Ong of Ong Aircraft Corp., P. O. Box 214, Kansas City, Mo. ®

WACO INFORMATION By Ray Brandly, EAA 38963 Original factory decals, manuals, drawings, pictures, prints, 3-view drawings, Waco parts, authentic information, etc. Decals are free to any Waco owner who sends in his Waco serial number and N number. Streamline flying wires, cadmium and stainless steel, immediate delivery, most sizes. Contact: National Waco Club, 2650 W. Alex. - Bellbrook Rd., Dayton, Ohio 45459. ® SPORT AVIATION

31