Put The Propeller On The Tail

... can to other de- signers who would like to 'Put The Propeller On The. Tail." Molt Taylor's spectacular new IMP . . . very near com- pletion. SPORT AVIATION 55.
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PUT THE

PROPELLER

THE

ByM.B. (Molt) Taylor (EAA 14794) P.O. Box 1171 Longview, Washington 98632 (Photos courtesy of author)

The Taylor Aerocar Model III with a tail mounted propeller. It surprises many people who are new to aviation to learn that Molt Taylor has had a flying car type certificated for over two decades.

w,

H OEVER HEARD OF a boat having the propeller

up in front? There are millions of boats and only a few

airplanes (probably less than 200,000 in the world). There must be some reason for this, and anyone who has investigated the problem will quickly find that even the Wright Brothers thought the propellers should PUSH. However, as those illustrious pioneers quickly found out with their

first airplane, there is much more to driving a propeller remote from the engine in an airplane than just hooking up some conventional mechanical system between the two.

The reason for this problem is quite simple. It has to do with the fact that an internal combustion engine such as we use in airplanes does not really rotate at a constant velocity. Each time a cylinder fires in the conventional piston engine there is a slight increase in the velocity of the crankshaft as each piston compresses the fuel/air in

the cylinder before it fires. This inconstant rotational (Continued on Next Page) SPORT AVIATION 51

9D F l e x m o d i f i e d back

| ENGINE is connected to drive housing, starts it turning at no load. Flow c h a r g e

9D F l e x i d y n e f r o n t

B e a r i n g cups

Bearing c a r r i e r

(thousands of fine partides of spherical steel

shot) is thrown to circumference of housing, is compressed by centrifugal force and revolves

with

Rotor

connected

Drive spacer

housing.

to

propeller is started and

accelerated by friction and wedging action of

Spacer

Nut

revolving flow charge. Slippage is zero at nor-

mal speeds, rotor slips at torsional resonance points.

Spline d r i v e shaft Spline d r i v e Flow charge^ Spacer

Rotor Housing''

PROP. ON THE TAIL ...

(Continued from Preceding Page)

speed results in what is commonly called "torsional vibration." That is, there is a relative back and forth vibration of the engine which is related to the number of cylinders that fire each revolution. The faster the engine runs, the higher the frequency of this relative vibration. Accordingly it is easy to see that if there is some mechanical shaft or other mechanism such as belts or gears between the engine crankshaft and the thing that is being driven by the engine there is bound to be some "springiness" of the connecting mechanism. The best way to visualize the problem is to consider a simple shaft between the engine and the mass it is driving. This is, of course, the propeller in an airplane drive system, but can just as well be a rotor on a generator, some compressor, or any other driven machinery. Since it is impossible to make a shaft infinitely stiff (torsionally) it becomes apparent that as the engine accelerates due to the explosion of the fuel/air mixture in a cylinder, the shaft will tend to "wrap-up" since the mass of the driven item tends to lag behind (torsionally). Thus, each time there is a power impulse from the engine there is a momentary lag (rotationally) and then the driven mass (propeller) tends to not only "catch up" with the engine, but will actually run slightly AHEAD of the engine. The propeller (or other mass) will then try to roll back to its original position before the next power pulse comes along. From this it can be seen that since an airplane engine/ propeller system encounters quite a range of different rotational speeds there are bound to be a couple of places in the "speed range" of the system where the engine is trying to turn the mass in one direction, but the mass is trying to turn backward against it. This phenomenon is, of course, only "momentary", but an investigation of the problem will quickly show that the forces involved become vir52 MARCH 1974

Flexidyne drive unit — the invention that made shaft driven, tail-mounted propellers feasible.

tually infinite, and anyone who has experimented with a long shaft driven prop installation can tell you that at the "resonance" point, the weight of the required shaft gets so great that the airplane couldn't lift it, much less carry people, too. With very small engines this problem is not so great, and it is sometimes possible to use a sufficiently stiff shaft that is strong enough to operate for some time. However, each time the engine is accelerated through the resonance point, the shaft will quickly become fatigued due to the high loads being imposed. At other times the problem shows up by it just being impossible to accelerate the engine through the resonance speed. This point of resonance (or points) can be changed by the engineer who designs the system by either of two things. He can make the shaft stiffer (or more flexible, torsionally) or he can change the weight of the mass (propeller) being driven. It is easy to see that the use of belts in the "system" tends to lower the frequency of the resonance point while the use of a heavy metal propeller would tend to increase the frequency of the resonance. In the past some designers have tried to develop systems where the resonance will occur at an engine speed somewhere below the normal idling or low speed of the normal operation of the system. This, of course, does reduce the number of cycles of vibration that the system encounters, but the system still has to go through this frequency each time the engine is stopped or started. You can demonstrate this whole process to yourself easily with any conventional automobile that has a manual gear shift and a manual clutch by merely letting the engine idle as slow as it will

The installation of a modified Beech electric prop on the IMP prototype. The prop hub will soon be fitted with special fiber-glass blades having the radical new supercritical airfoil.

The FLEXIDYNE unit as installed in the IMP prototype. Note the mirror taped in place so as to align the location of the rear thrust bearing to get the shaft as straight as possible.

go in high gear. Usually, such a manual transmission car will idle eventually slow enough so that it will start to "buck." This is the point of "resonance" of the system, torsionally. Further, if you then press on the accelerator you can find that the car will usually not accelerate up to speed again and will either "kill" the engine or just continue to "buck" that much harder. The only way to stop the "bucking" is to "slip the clutch" a bit. This lets the engine speed up and bring the car drive line up past the resonance point again. In the Wright Brother's airplanes, they had chain drive to their propellers, and since these chains could not be infinitely tight, they had slack on one side as the engine accelerated. The other side of the chain drive became taut. This resulted in fantastic vibration or "whipping" of the chain drives. They solved this problem by merely putting the slack side of the chain in a tube but the ensuing racket of the chain whipping in that tube must have been something else! You can confirm this by looking at the photos

resonance problem. A couple that come to mind are the Bell P-39 and P-63. These WW II fighters had the engine behind the pilot and the prop on the nose. This permitted the designer to install a heavy cannon to fire through the prop hub on the centerline of the airplane. It also permitted the aerodynamicist to develop a nice clean nose profile for the airplane. However, even with 12 cylinders firing into the long drive shaft (so that the impulses were closer

of early chain driven pusher prop installations and you will note that many of them put the chains in tubes on both

sides of the drive system. These tubes had another incidental benefit since they permitted the twisted chain (which they did to get counter-rotation propellers) to pass each other without getting fouled up. There have been a lot of experimental airplanes built in the past in which the designers tried to put the propeller either back on the tail, out on the wings, or otherwise remote from the engine. Some of these have had notable success through very ingenious mechanical and devices which were used to overcome the torsional

Midship bearing and flex coupling and rear shaft section of the IMP prototype ... showing how all thrust loads and

prop loads are taken into the tail cone and not into the engine or front shaft.

together — 6 per revolution), there were still vibration

problems and those airplanes used a system of tuned torsional vibration dampers to eliminate the otherwise destructive torsional vibrations. Many light aircraft have this same torsional vibration in their crankshafts. The Continental 0-300 is a good example. In those engines there are a couple of "tuned pendulus vibration dampers" on the

crankshaft. Theseare little weights which swing torsionally in relation to the crankshaft. They have appropriate pendulum lengths and weight so that they will vibrate torsionally in opposition to the resonance of the crankshaft at its resonance peaks. Thus, they tend to lag behind as the crankshaft accelerates, and then swing ahead and pull the crankshaft along with them. This "damping" reduces the resonance forces sufficiently so that allowable stress levels are not exceeded. Other engines have crankshafts which overcome this problem by simple brute force. They just make the shaft so stiff and strong that it will not vibrate torsionally enough to exceed allowable stress levels. Putting the propeller aft on the tail of the airplane has a lot of advantages if it can be made to operate satisfactor(Continued on Next Page)

Cooling fan, FLEXIDYNE and shaft with flex coupling

similar to those used in helicopters — a stack of steel discs between two flanges.

SPORT AVIATION 53

Another angle of the rear prop shaft installation. The small, dark colored rod is the drive for changing pitch of the prop.

This is a miniature engine/shaft/prop system that will be installed in a radio controlled model of the IMP. It even has a tiny two inch, fully workable FLEXIDYNE drive.

Dynamic test stand built and used by Molt Taylor to test and evaluate the vibration and stress levels of hi engine/shaft/prop combination that will be used in the prototype IMP. This sort of test work is so esoteric that the FAA has asked Molt to allow them to observe.

PROP. ON THE TAIL ...

(Continued from Preceding Page)

ily. First, the airplane then does not have to fly in its own slipstream and turbulence from the propeller. It is well known that the air flow back from the propeller is. anything but smooth. There are impulses of air due to the passage of the blades which make the windshield vibrate each time a blade of the prop passes in front of the cabin. Further,

the very speed of the slipstream is such that when the airplane may be flying at 100 miles per hour in the air mass that it is flying in, the actual velocity of the air passing

over the fuselage (and any other part of the airplane in the prop wash) may be up to 150 mph. Thus, the major part of the airplane may be developing drag at an apparent 150 mph while the machine itself is only flying 100 miles per hour. From a more theoretical standpoint it can be demonstrated that the propeller up in front is destabilizing both in pitch and yaw so that the tractor airplane needs larger tail surfaces than one with the prop behind the tail. The tail propeller is also an assist to the dynamic 54 MARCH 1974

(motion) stability of the airplane so that the tail pusher airplane will tend to fly straight and keep flying straight, whereas the tractor type airplane tends to keep on turning once it starts to turn. The obvious advantages of these characteristics are apparent and add up to less power being required for the same speed, better flight characteristics and easier operation.

The writer experienced this torsional resonance prob-

lem early in the development of the Aerocar "Flying Automobiles." In that design it was desirable to put the propeller on the tail for still a different reason since who wanted to have to take the prop off the "car" each time he wanted to drive it? So, the prop was put on the removable "tail-trailer" part of the Aerocars so that when the tailwings part of the machine are removed, the prop comes with it. The hook-up of the shaft in such a way that it automatically disconnects and reconnects was easily resolved so the operator never has to think about the

removal or reconnection of the prop when he goes from flight to road operation or vice versa. When the torsional vibration and resonance problem was first encountered in the Aerocars, it looked like we had run into an almost insurmountable problem. First solutions were to try to use some sort of manual clutch to engage

and disengage the prop shaft so that the prop could be engaged after the engine was running and before takeoff. This posed quite a problem since it was found that the natural torsional resonance peak of the practical, lightweight shaft/propeller installation occurred at about

1000 rpm. This would have meant that the engine would have to idle at over 1000 rpm (at least as an airplane) so that the prop would be engaged at some speed higher than that. This is what some helicopters do to overcome this

resonance problem and accounts for the relatively high idling speed of some piston driven light helicopters. This also meant an additional operation would be necessary to fly, shut down, etc. What was needed was some sort of

plate (called the impeller) is partially filled with little tiny steel balls. There are about 5 pounds of them in the

Aerocar drive. When the engine starts, the housing whirls

the little balls out centrifugally so that they form a ring of balls which has a density which is more or less proportional to the velocity of the engine. Since the wavy edge of the

impeller is immersed in this ring of balls, it is dragged around by them. The force with which the impeller is driven is dependent on the speed of rotation of the housing

with the ring of balls in it. The faster it turns, the more torque the impeller will impart to the driven shaft. Thus,

it can be seen that the torque capacity of the clutch is dependent on the speed with which it turns. Further, it can be seen that if there are any impulses in the torsional drive such as during torsional vibration peaks, the balls merely slip by the impeller (wavy plate). Thus, the clutch will "drift" slightly to dissipate the vibratory peaks. This

is exactly like slipping the clutch on the car when it starts to "buck" when it is driven too slowly in high gear. The

automatic system to merely "bypass" the point of resonance and the prohibitive peak stress loads of the system. There was also the problem of engine backfire during starting or stopping which is common with magneto ignition systems. Accordingly, in 1949 we just accidentally ran across a magazine story about a little mechanical clutch device which had just then been invented in France. We contacted the people who had acquired rights to this device for the U.S. and found that they called it a "Powdermatic" drive

whole thing is as simple as that. The application of this device now makes it possible to design light aircraft with

quite sketchy, particularly since it had been originally devised to install on small electric motors to let them

prop no longer has to pull the airplane through its own

or "Dry Fluid Clutch." Technical data on the device was come up to speed before they picked up the load. This

eliminated the need for starting brushes in some electric motor installations (a very common system in many AC motors). Fortunately, we were able to get one of the early models of the device and installed it in the prototype Aerocar drive system. After some experimentation we found that not only was it the ideal solution to the problem of driving an airplane propeller on the end of a long shaft, but it also solved a lot of other problems such as quick stoppages of the prop as well as a very significant reduction in the vibratory stress levels in the propeller itself. The thing that really dictates the structural design of a propeller is usually the excitation forces on the prop from the

explosions that occur in a single cylinder of the engine. When the propeller is attached directly to the crankshaft flange as in most tractor lightplanes, the propeller really takes a beating since it has to literally hold the engine back against the forces of each cylinder and then it has to carry the engine along until the next driving cylinder fires. This "excites" the propeller and the stress levels developed are merely controlled through putting enough "beef (material) into the prop to take the loads. It is this

the propeller on the tail without worrying about torsional

resonance, quick starts, or engine backfiring. It also lets the the designer use lighter propellers for a given engine power with safety, and greatly protects the propeller itself from excessive vibration-induced loads into the bearings of controllable propellers, etc. While the FLEXIDYNE for the Aerocar (and the IMP) weighs about 14 pounds, it permits quickly regaining this slight weight penalty through performance from the tail prop. (Remember, the slipstream).

Aerocars have accumulated thousands of hours of flight operations with FLEXIDYNE drives to their propellers without the slightest mechanical problems or service difficulties. Examination of the "Flow-charge" of little steel balls (they are about 0.020 dia.) shows no detectable wear of the balls after the thousands of hours. I am sure that other designers will find this simple device an easy solution to the previously almost insurmountable problem of putting the propeller on the tail. Our new IMP "homebuilt" design will, of course, employ this device for its tail prop, and we have recently developed a tiny little "dry fluid drive" clutch which we have installed on a radio controlled model of the EMP. This installation will turn

15,000 rpm and drive a 30 inch long drive shaft to the

model propeller without any destructive vibration. We

would be pleased to convey any help we can to other designers who would like to 'Put The Propeller On The Tail."

excitation which is causing some of our homebuilt lightplanes to loose their props, particularly with the larger

four cylinder engine installations. The application of the "Dry Fluid Drive" type clutch system not only eliminates any mechanical operations for the pilot, but also greatly reduces the stress levels in the prop since no torsional forces greater than what the clutch will pass can get to the prop from the engine. The "Dry Fluid Clutch" is now being manufactured in the U.S. by the Dodge Mfg. Co. of Mishawaka, Ind. and

is sold under their trade name as their "FLEXIDYNE."

These units are now used in many commercial installations and are to be found in many common pieces of equipment such as some types of clothes dryers, washing machines,

etc. The FLEXIDYNE is a simple housing which is driven

by the engine. The one in the Aerocars (and our new IMP) is 9 inches ID. It is about 2 inches thick. The housing is made of cast aluminum. Inside the housing is a simple circular steel plate which has its edges scalloped (or wrinkled to a wavy shape). The space around this wavy

Molt Taylor's spectacular new IMP . . . very near completion. SPORT AVIATION 55