The origin of whirligigs is unknown. Farmers and sailors used weather vanes and are thought to be the originators of this unique apparatus. by 400 BC the bamboo-copter of dragon butterfly, a helicopter-like rotor launched by rolling a stick was invented in China.
How to build a Football Whirligig
The Helicopter Whirligig
The Helicopter Whirligig
By Roy L. Clough Jr.
Two views of the whirligig in actual flight. Note the flexing of the rotors under power.
EXPERIMENTING CAN BE FUN WITH THIS VERY SIMPLE HELICOPTER PROJECT THIS direct-lift model utilizes a simple and foolproof counter -rotational system based on the familiar principle of the contest "whirligig."
Due to the nature of the rubber motor hook-up the two rotors are constantly and automatically in balance with each other, effectually neutralizing the torque element without gearing of any kind.
This model is the 73rd helicopter built by the writer and experience gleaned from the first 72 is incorporated in its construction. Counter -rotation is considered the best approach to the torque problem, due to the fact that as long as some power must be used to offset rotor torque that power might just as well be turned to additional lift. The length of a rubber motor is in any case limited (barring the use of gearing) in a model of this type and it is therefore essential that the best possible use be made of the power that is available. Two-bladed rotors were used on the original because they may be placed in line with the fuselage making the model a rather "flat package" and easily portable, a somewhat important factor in cramped city quarters. Three-bladed rotors would probably be okay if you wish to try them.
More power would be needed and slightly less pitch would doubtless increase the soaring qualities of the model.
Start construction with the coaxial unit. Study the plans until there is no doubt as to the action of the unit. The motor tube is 1 1/4" in diameter and is constructed of either balsa sheet or stiff drawing paper. The thrust bearing is located at the bottom of the tube and consists of a pin-washer-bead arrangement. This bearing should rotate freely but should have very little "play." Strive to get the bearing as close to the dead center of former "C" as possible. The pin-shaft is bent at this point to keep it from falling inside the tube, but it is attached to its crosspiece only after the fuselage is completed.
Cement former "B" into the other end of the tube leaving a 1/8" rim to accommodate "A." If you have chosen drawing paper for the construction of the motor tube, dope it now for additional strength. Allow plenty of time for the dope to dry before cutting holes in it for access to the rubber motor. The shaft, which turns the lower rotor, is made from either thin-wall brass or aluminum tubing. 1/8" O.D. The tubing is split two ways for about 1/2" and the split ends flattened at right angles to the tube. Now "soak" the tubing for at least five minutes in dope thinner to remove all grease and/or dirt. If this is done no difficulty will be experienced in making cement stick to it. Push the tube through the hole in former "A" (it should fit snugly), coat liberally with cement and push the whole assembly down into the "cup" formed by former "B" and the end of the motor tube. Allow a few minutes for the glue to set a little, then true up the shaft with the tube. That is, make certain the motor tube and the hollow shaft rotate in the same plane. The upper bearing is an odd piece of aluminum tubing, 1/4" long, slipped over the hollow shaft and mounted in the 1/4" by 3/8" crosspiece in the top of the fuselage.
It simplifies matters to put the crosspiece with the bearing in it on at this point, because it is impossible to slide it on after the tube-shaft is flared to take a bead bearing. The lower rotor hub is mounted next. Drill the shaft hole a little undersize and force the hub on the tube. If it feels a bit loose, wedge it tightly with short lengths of toothpick. Apply plenty of cement, let the whole thing dry for a couple of hours, and go over it with cement again. Hundreds of flights on the original model failed to loose a hub attached in this manner.
After the rear prop hub is well set, flare the end of the tube slightly to hold a glass bead, the thrust bearing for the upper rotor. Insert the propshaft through one of the access holes cut in the motor tube (tweezers are a great help) and slide a bead and washer and the upper rotor hub over the shaft. Use your favorite free-wheeler. The rotor blades are cut from 1/20" sheet and sanded over a bottle to produce a slight camber. Note that the angle of the lower rotor blades is slightly more than that of the upper. This is done because the upper blade "bites" into dead air and the air forced downward by the lower blade is already in motion. A higher blade angle on the lower blade, in theory at least, prevents compression between the rotors, and consequent in- stability.
The fuselage of the helicopter model is built up of 1/16" strip, and is quite conventional, except for the sharp curves in the forepart of the longerons. If the longerons are soaked thoroughly in hot water before any attempt is made to bend them into place, no difficulty will be encountered. Because of the light nature of the construction it will be necessary to install internal cross-braces at the points marked on the plans. Where the landing gear legs are attached on the bottom of the fuselage it is a good idea to cover in a couple of sections with 1/32" sheet balsa.
Tail fin is of 1/32" sheet. The cabin section is cellophane d in before the balance of the fuselage is covered. The original was covered with orange tissue and water -shrunk, not doped. Weight is an important factor in a model of this type; try to keep it down. The top section of the fuselage should be left uncovered until the coaxial unit is mounted.
The bottom of the tube is anchored to a 1/8" crosspiece and the upper crosspiece is mounted directly into the fuselage. Check and make certain that no part of the rotating tube binds on the inside of the fuselage.
Power will vary according to the weight of the model. Start with a double loop of 1/8" flat brown (four strands) and build it up from there. The original model flew well on this power until stepped upon and re-built, after which another loop had to be added to take care of increased weight. The tailskid, which is not shown in the photograph, was added later to avoid the necessity of holding the tail up in position in ROG flying. The model should balance 1/4" ahead of the rotor axis for straight vertical flight. Hand launching is accomplished by letting the ship take off from the hand with the nose pointed slightly downward. This model can be made to fly forward as it climbs by adding a small weight to the nose. The free-wheeler is to let it down easy.
A few pertinent facts on model helicopter design should be mentioned at this point. The first and holiest commandment of model helicopter design is: Rotor blades absolutely must balance. Lack of balance induces vibration and loss of power.
Vibration induces instability and the average helicopter is, by reason of the principles involved, not overly stable to begin with. Rotor blades should have a certain amount of "flex" to them. Rigid blades and instability are, in models of this type at least, synonymous.
The center of lateral area should be high, otherwise the flat fuselage sides will act as a fin, causing the model to spill over at the top of its flight and descend upside down. The center of gravity should be as low as possible for the same reason. About 75 percent of lateral area should be behind the axis of the rotors; otherwise the model may fly tail-first in "forward" flight. Stub-wings, elevators, flaps, and whatnot are just so much junk on helicopters. Any unnecessary object sticking out into the slipstream of the rotors is a good bid for instability. Do not expect a model helicopter to turn in performance comparable to a conventional model of similar weight.
Don't forget rubber length is limited and that the lift of the model is produced entirely by rotating vanes. Good performance, however, can be had as a result of careful construction and half-minute flights should not be uncommon with this model. If the free-wheeler works smoothly, a certain amount of soaring ability will be noticeable if the ship gets caught in a thermal. Use fresh rubber, as the model is hand-wound, or go us one better and figure out a way to use a winder on this ship. Making the bottom end of the coaxial tube and the bottom section of the fuselage under it, removable, might do this. AIR TRAILS, OCTOBER, 1946
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