1. What was the thought process that led to a dual helical gear design for the gearbox rather than a planetary arrangement? The reason I ask is that a planetary gearbox has the nice property of containing all pressure angle forces within the planetary cluster rather than being reacted through the housing.

Satellite gears have to spin very fast. We do not have any bearings spinning faster than the engine.

2. Likewise, what was the thought process that led to using single cut helical gears rather than dual opposed helical (i.e. herringbone) gears which could eliminate the thrust loading of the shaft support bearings?

Easier manufacturing and the ball bearings at each end have no trouble with this force.

3. What type of analysis or testing has been done to determine the resonance frequency (frequencies?) of the engine / fly wheel / gear reduction / propeller system?

Strain gauge testing / accelerometer tests.

4. Does the DMF have internal damping? If not, what's the advantage of using a DMF rather than using a single mass flywheel with a tuned torsional spring (rubber coupling?) and essentially using the propeller as the secondary mass? Perhaps the propeller inertia reflected through the gearbox wouldn't be sufficient?

We use what you suggest above.

5. Can the engine output enough torque to cause the DMF to "go solid" resulting in direct transfer of engine torque to propeller?

Only if the propeller hit something.

6. Were the springs in the DMF selected specifically for your application or is this a stock DMF that's used for automotive applications? The reason I ask is that since spring life is a function of both mean and alternating stresses, even if the max deflection is identical, there can be a huge difference in spring life depending on the case. What I mean to say is that a spring with a 1% mean load and a 20% alternating load will fail far sooner than one with a 20% mean load and a 1% alternating load. Thus, I'm wondering if the mean vs. alternating load expectations of this particular application were taken into account when designing the DMF?

We tested the unit before and after the 200 hr full power test. Propeller deflection was identical before and after the test. The ultimate spring life remain to be seen. The springs are arranged in such a way that they can not fail such that the engine would disengage from the propeller. We also have 3 different spring rates in the unit.

7. What sort of FMEA was performed for the various components in the drivetrain and which component(s) have the greatest likelihood of failure?

Destructive test show the propeller to give first. Prolonged running showed slight wear on the inner race of the flywheel pilot bearing.

8. How much, if any, FEA was performed of the various components in the drivetrain and where are the peak stresses expected? What are the expected peak stress values and types of stress (primary / principal / VonMises)? OT: As a curiosity, what software do you use (if applicable)?

I do not use any stress analyses software. We test for this on completed assemblies only. We use personal judgement and automotive converted parts. Hence the term "Auto conversion engines :) Our philosophy has always been: Small displacement engines, heavy flywheels, high rpm, sturdy drive unit and light propellers. The combination has worked for many years. also, keep in mind the spring supported flywheel is not new. It has been on engines since 2003.

I know your very busy, Jan, so I appreciate any time you're willing to spend answering my questions.

No problem

A little more on your question about the dual mass. Yes, we only use the spring portion of the system, not the secondary mass. (other than the weight of our drive disk and splined shaft / reduction drive and propeller) It is defiantly the aerodynamic loading of the propeller that is loading against the springs.

Jan

<<Yes, we only use the spring portion of the system, not the secondary mass.>>

Ahhh, that answers a lot.

What Jan is saying is this particular DMF has a removable friction face. In the OEM application it is the contact surface and mass heat sink for the manual transmission's clutch as well as being the secondary flywheel mass. Jan unbolts it, throws it in the scrap heap, and bolts on his splined drive adapter.

Of course it is no longer a dual mass flywheel, which doesn't matter (other than creating name confusion). It does package a soft stiffness element into a space no greater than a standard flywheel. Very clever in that regard. Good thinking Jan!

[quote=DanH;196489Of course it is no longer a dual mass flywheel, which doesn't matter (other than creating name confusion). [/QUOTE]

This why I was confused. I did not see anything dual about the flywheel I was looking at.

Dan, what is your opinion about no testing of side loads, only thrust? Seems to me a spining prop exerts a "ton" of side force on the gearbox.

Dan, what is your opinion about no testing of side loads, only thrust? Seems to me a spining prop exerts a "ton" of side force on the gearbox.

Let me clarify. The bearing support in the G3 is 3x that of the G1-G3 with thousands of accumulated combined hours. G3 drives have a heavier output shaft, much larger spline connection and more bearings. The only item to remember regarding the front bearings and housing is to supply adequate cooling for the best possible bearing retention in the housing. The cooler the drive can operate, the more true the internals of the unit is operating. A 2 - 2.5" blast tube, right on the nose section does a lot of good.

Jan

I am wondering where the heat comes from. Is the drive simply absorbing heat from the running engine, or is the heat from drive operation. It would be interesting to calculate how much thermal energy is generated. Jan...is the drive just absorbing heat from the engine throught the big plater, or is the heat from the internals?

Second, am I reading correctly that operating temperature is mainly a concern for bearing retention...ie the bearing is a press fit, and because its an aluminum housing there is some concern or necessity to manage temperatures for that reason? What is the optimal temperature of the drive?

Hard to understand why this would be a problem-it's not a problem on turboprop engines that are geared down from around 35,000 RPM via planetary gearsets. Having lots of gear teeth in contact at all times would seem to be a more reliable way of transfering torque, while reducing speed and all in a very compact package. Am I not considering something important here?

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Planetary Gears
 

The rotational speed of the planetary gears is a potential problem and has caused some design changes in other vendor's transmisions. I believe I read about them needing a good clean supply of pressurized oil. They can obviously be made to work but there are always trade offs.

Regarding temperatures of the G3, mine has been running quite a bit cooler than the previous Gen 2 drives, 30 to 40 degrees F cooler. In the past it was thought that the reason the coolant, oil, and gearbox temps stayed so close together was conduction from the engine but I think that theory has weakened with the Gen 3 data. My Gen 3 generally runs 20 to 30 degrees F cooler than coolant and oil temps now. This is probably due to the quite large spinner gap I am have at present. As I close it up I will have to direct some cooling air to replace it.

Now I know that it is probably best to direct that cooling air right up near the front bearing support area.

Randy C

<<Dan, what is your opinion about no testing of side loads, only thrust? Seems to me a spining prop exerts a "ton" of side force on the gearbox.>>

Well, for starters you should go back and look at post #7 in this thread; I just edited it because I made an error in calculation. The prop moment isn't a ton in this case, but it is about 600 lbs.

Even so, actual measurement of side load issues would be low on my priority list. It might be interesting to check if you think (1) gearbox case fixation in relation to the engine case is weak, or (2) you're concerned with fixation of the entire engine assembly, ie, motor mount and mount plate issues. Those would be fairly easy to measure.

The real issues relating to prop moment are reliably calculated (if you check your figures at least twice!). Prop moment would be an input when calculating an L10 life for the front bearings, and even more important, a material stress value for the propshaft.