Thanks Lynn, good info as usual
.
Here’s my observations on drive train resonance with street ported 13B (original or Renesis) configured with RWS drives (B & C) and wood props. The resonant frequency of the full drive train is around 800 rpm. This passes so quickly on startup that I don’t notice it at all. My idle is around 1600 RPM, well above the ‘rock crushing’ point. I notice the resonance only when experimenting with lower idle speeds and on shut down where it rattles for 1 or two revolutions. Not a problem since there is no more energy being fed into the system and it is quickly damped by the prop.
I expected to have different characteristics on the RD-2 drive which has no damper, just a tight spline coupling to the flywheel. There was no noticeable change at all. Apparently the resonant point and damping are mainly controlled by the gear lash and total flywheel and prop weight. On the RD-2 I used an aluminum racing flywheel with the steel clutch wear plate removed. The weight turned out to be very close to the RD-1 with flexplate setup since the damper weight was not there anymore. The no damper RD-2 is at least if not better in smoothness compared to the RD-1. Whenever I’ve felt any significant roughness, it’s always been due to prop balance rather than engine/flywheel weight & balance.
I have about 80 hours on the RD-2C now so I’ve come to believe that the damper is not a critical need on the aircraft configuration unless using a metal prop. The RD-2 is not rated for use with metal props because of this. I don’t know of anyone wanting to use a metal prop in the 20 years since I started making drives anyway. If anyone wants to experimenting with building drives I’d recommend a no-damper setup. It greatly simplified the design and looks to have no down-side other than the prop limitation. I know the no-damper sounds like blasphemy but If I ever get bored and have to make drives again that’s the way I’d go.
Tracy
Sent from Mail for Windows 10
Allow me to describe the racer setup for a Daryl Drummond built bridge ported 12-A of about 244 HP.
The builder with the RX-7 gearbox reduction system was Mr. Graham in a Varyeasy
My racer has a steel button flywheel just enough diameter to mount the 5 1/4" dual disc clutch pack. This mounts over a stock flex plate with the ring gear TIG welded to the flex plate. The flex plate is just to carry the ring gear for starting and sees no other loads except for low RPM shaking.
The Rotors are not lightened. The crank is not lightened. The gears in the trans are straight cut. The engine uses a Weber 48IDF carb and has the usual Rupp-Rupp-Rupp idle sound.
I set the idle up to 2,200 RPM because clutch management was not the drivers strong suit.
Also this got him to spend less time in the rock crushing RPM/sound from the transmission
as the car moved away from stopped.
Like a really big blender full of aircraft bolts. This around 1600 RPM. Lower with light rotors.
Best power is at 9,400 RPM. Shifting at 9,600 RPM
If you have seen the 45 pound stock flywheel you would know that there is a problem of some sort. I suggest using a steel aftermarket flywheel to drive Tracy's torque absorber or similar.
This to further damp the torque pulses and move them up the RPM band away from idle and ground operation RPM. Start with a steel flywheel or a new flex plate and check it for cracks
every preflight. The racer still has the original used flex plate, with no cracks. No driveline damage in 9 years of racing. There are straight cut gears in the quick change differential as well.
(Move the throttle off of idle slowly. The amount of power in play while passing through the problem RPM determines the amount of stress applied).