A SOLUTION TO THE LONG SHAFT PROBLEM By Molt Taylor (EAA 14794) Box 1171 Longview, WA 98632

BEARINGS

FLOW CHARGE

A - Main Drive Shaft - 1" Tubing B - Propeller C - Coupling Mount D - Propeller Face Plate E - Propeller Mounting Flange F - Flow Charge Filler Plug G - Steel Thrust Collar ST. CROIX ULTRALIGHTS

5957 Seville St. Lake Oswego, Oregon 97034 (503) 636-4153 DRAWN BY C. WILLE 12-13-81

LNYONE WHO HAS ever tried to drive an aircraft propeller located on the end of a long shaft with an internal combustion engine can tell you it is not a very easy thing to do. If their installations have merely consisted of short shafts driven at some speed other than that of the engine via a "V" belt, they may have worked for a while. But if cog belts have been used, chances are they have experienced a lot of breakage and other problems. The reason for this is quite simple: the pulsating powerplant tries to move the propeller in a series of pulses also. Consequently, the element connecting the propeller to the engine just cannot be absolutely rigid, either in the belts or the shaft (torsional rigidity). Under power, the connecting member tends to alternately stretch and contract (or, in the case of a shaft, twist), actually storing up energy between pulses of the powerplant. The belt or shaft cannot stretch or twist indefinitely so,

This problem was present in the writer's early attempts to drive the propeller with a long shaft in the Aerocar Flying Automobile, and after much research into the problem, a simple solution was discovered. The "fix" was the well known Flexidyne Dry Fluid Drive coupling units manufactured by Dodge Manufacturing Company of Mishawaka, Indiana (no connection with Dodge automobiles). Over the years, we have incorporated the Flexidyne in a number of long propeller/drive shaft systems for homebuilts such as the IMP, Mini-IMF and, more recently, our Micro-IMF. Over 25 years ago, we thoroughly investigated the properties of these units in shaft drives and ultimately obtained full FAA Type Certification for it in the Aerocar. We have even adapted the Flexidyne in RC models - some of which have been made as small as 2 inches in diameter and run at speeds as high as 16,000 rpm.

ultimately, must rebound or release the stored energy

between pulses. At certain speeds this energy release will be out of phase with the upcoming pulses of the engine - with the result that the propeller is trying to go one way while the engine wants to go just opposite. The stress levels that ensue are usually far too high for lightweight aircraft components to endure. A "V" belt reduces this stress by slipping - much like starting a

straight geared automobile out in high by slipping the clutch. The cog belt system can't slip in this manner and thus is subject to the harmonic torsional vibration that can be so destructive. 36 MARCH 1982

This concept has been conveyed to other homebuilders and is being used in the shaft drives of a number of well known homebuilt aircraft - with complete success and absolutely no troubles. Torsional resonance problems are on the upswing these days, however, because of the growing popularity of ultralights. Many of them drive their props via a long shaft and, predictably, those using "V" belts are doing reasonably well, while those using cog belts have experienced problems. None have been very smooth.

Recently, our good friend. Chad Wille, asked about the Flexidyne. Chad, who operates St. Croix Propellers in

nearby Oswego, OR, and I came up with the idea of building the Flexidyne into the propeller, itself. The accompanying photos and drawing show the general arrangement of the components. The first test unit built to these ideas has been run extensively and exhibits completely satisfactory operation. Another friend has built a similar unit which he has flown on his Cascade Kasper Wing ultralight and he informs us that his installation is now completely smooth, easy starting and virtually vibration free, whereas it previously gave you a free massage every time you flew it. Previously, he had to use 4 "V" belts to handle the power of his engine, but now just 2 are quite sufficient. He no longer experiences the belt failures and "turnovers" he once did - or the occasions when the belts disappeared entirely. The effectiveness of the Flexidyne units has been repeatedly proven in our own work with the Mini-IMPs where we have seen sudden propeller stoppages which failed to break even wood propeller blades - or complete

stoppages where only one blade was broken, yet the

engine continued to idle.

We have made no effort in the sketch to indicate the size of the various components, because the people at St. Croix Propellers are now planning to produce propellers with the Flexidyne unit built in. These propellers are bound to be extremely smooth running and vibration free when hooked up to the various two cycle engines used in ultralights these days. The smoothness will be particularly noticeable with single cylinder engines, which are otherwise literal vibration machines. Mechanically, they will be just as beneficial in multicylinder engines, however.

Anyone who has misgivings concerning the practicability of shaft drives to aircraft propellers should be able to forget any thoughts of that kind today. We recently had an occasion to recommend the use of one to a group building a replica of the Wright Brothers Model B airplane. Problems were being encountered with the chain drive system (as did the Wrights), but the Flexidyne solved them all. If these type units had been available to the Wrights, who knows, maybe shaft driven airplanes would be as common today as shaft driven automobiles.

The operation of the Flexidyne unit is simplicity itself. The "flow charge", which consists of hundreds of thousands of tiny steel balls about 0.015 inches in diameter, is flung out centrifugally by the turning of the outer housing. The centrifugal force tends to solidify the dry mass of tiny balls around the wavy plate which is attached to the output shaft. Any torsional vibration or rotational inconsistency is damped out through the balls which slip slightly under such conditions. The result is a "fluid-like"

output rotation from the torsionally pulsating input from the internal combustion engine. There is negligible wear on the balls and some of our installations have gone as much as 2000 hours of flying. There is absolutely no lubrication used on the balls and they are quite hard, as is the wavy drive plate. Anyone desiring further information can contact either Chad Wille at St. Croix Ultralights, 5957 Seville St., Lake Oswego, OR 97034 (503/

636-4153) or the writer.

SPORT AVIATION 37