# Reduction drives

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#### rv7charlie

##### Well-Known Member
Having been out of the PSRU quest for some time, (flying certified), I wondered if anyone has learned anything useful about the (usable) design of a modern PSRU unit. I used one or two on planes I built in the late 80`s to around 2000, but as was previously stated, a lot of manufacturers promised a lot of things, and somehow the produced unit always seemed to not quite work as well as promised.
If we are no closer to an answer re. a modern PSRU unit, it looks like I will have to go and drag the old IO-470 out of the shed and give it a bit of a once-over.

I realise this is a redrive forum, but another avenue that could be pursued is to just get the humangous aluminium GM or Ford engine that tickles your fancy, and invert it, and accept the fact that you will "only" get 1 hp out of every 2 ci of your engine capacity, for a normal size prop.
I am investigating this option quite seriously for an install on a BD-4 I am resurrecting, and I have a book written by a person that did just that with a ZZ4. Minimal mods, with most needed to the oiling system, but way less hassle than a redrive, imho. Your individual requirements may differ from mine.

Erik in Oz.
Direct drive V8's have been done on EZ types, where prop diameter is limited by airframe characteristics. 200-230 HP should be easy with an LS motor at ~3000 RPM. But you'd still need an external bearing (and support) to take the bending loads off the crank. And done right, it can be a significant % of the weight of a redrive. If you can accept the prop turning 'backward', an airboat reduction drive is worth a look. Relatively inexpensive (~$3k), not terribly heavy (usually around 70 lbs) for their HP handling (way more than we'd need; around 1000 HP). Charlie #### pfarber ##### Well-Known Member HBA Supporter Direct drive V8's have been done on EZ types, where prop diameter is limited by airframe characteristics. 200-230 HP should be easy with an LS motor at ~3000 RPM. But you'd still need an external bearing (and support) to take the bending loads off the crank. And done right, it can be a significant % of the weight of a redrive. If you can accept the prop turning 'backward', an airboat reduction drive is worth a look. Relatively inexpensive (~$3k), not terribly heavy (usually around 70 lbs) for their HP handling (way more than we'd need; around 1000 HP).

Charlie
An LS motor is a heavy way to make 200-250hp. You'd be better off at the V6 engine. I wouldn't think of an LS unless you needed 300+hp. Direct drive would require inverting the motor, and that's probably harder than just bolting on a PSRU.

#### 282ex

##### Member
Interesting thread and now I'm super intrigued about the possibility of running an LS direct drive to replace a 250hp IO-540. I tried finding info on a DIY PSRU/redrive using C6 but coming up empty. The direct drive option for LS, what kind of tuning or retuning was performed or should I be asking on a different thread?

#### Lendo

##### Well-Known Member
For those interested in a Planetary PSRU, there is better Planetery Gear set than the C 6, it's still a Ford and from the Process 273 Transfer case. It is the low range gear with a ratio of 2.72:1 for the 4X4 transfer case behind the Ford Power Stroke 7.3 Liter Diesel engine. It is suggested to me it will handle 3 times the torque of that engine.
I have no experience with it - just passing on the information.
George

#### rv7charlie

##### Well-Known Member
An LS motor is a heavy way to make 200-250hp. You'd be better off at the V6 engine. I wouldn't think of an LS unless you needed 300+hp. Direct drive would require inverting the motor, and that's probably harder than just bolting on a PSRU.
The request was for direct drive. Not likely you'll find a NA v6 that will do 200hp at the sub-3000rpm range you need for decent takeoff/climb performance.

#### pfarber

##### Well-Known Member
HBA Supporter
The request was for direct drive. Not likely you'll find a NA v6 that will do 200hp at the sub-3000rpm range you need for decent takeoff/climb performance.
If direct drive is a requirement then stick with Lycoming. An IO-360 will make 200HP+ with just a few mods.

An airboat motor is more than good enough after an inspection and costs $4k-ish #### rv7charlie ##### Well-Known Member If direct drive is a requirement then stick with Lycoming. An IO-360 will make 200HP+ with just a few mods. An airboat motor is more than good enough after an inspection and costs$4k-ish
An angle valve IO360 will do it with zero mods.
On the subject of $4kish airboat motors, your supplier is much more generous than than the (known-reputable) one I checked with, about a decade ago. ($12K, with a red tagged crankshaft, due to a slight scratch on the fillet to the crank flange)

Charlie

#### rv7charlie

##### Well-Known Member
For those interested in a Planetary PSRU, there is better Planetery Gear set than the C 6, it's still a Ford and from the Process 273 Transfer case. It is the low range gear with a ratio of 2.72:1 for the 4X4 transfer case behind the Ford Power Stroke 7.3 Liter Diesel engine. It is suggested to me it will handle 3 times the torque of that engine.
I have no experience with it - just passing on the information.
George
I like the ratio better than the 2.85; does the prop turn right at that ratio selection? Any chance there's one closer to 2.4? That would allow a rotary to turn 6500 with the prop at 2700. Makes it a lot easier for the prop carver to get it right.

#### wsimpso1

##### Super Moderator
Staff member
Log Member
Sure, the C6 is drag raced, so there are aftermarket parts for them and they are still current parts in the aftermarket - we may expect that they will be around a while yet. Using a gear set originally designed for 1960's big engine cars and pickups and carried forward in to the E4OD, 4R100, 5R100 and 5R110 with better shafts/bearings/gears and then more gears and better carriers like that set is the ultimate piece of hardware for PSRU's seems a little silly to me. Cramming more parts and more expensive parts in the same diameter and width package has limits...

Since the 5R110 went out of production in 2009, the automatic transmission makers and transfer case makers of the world have designed and built a bunch of different automatic transmissions with planetary gearsets for heavy duty trucks with higher speeds, higher dutycycle, much higher torque capacities and much, much higher reliability and life requirements. Somehow I suspect that one of those gearsets might have about the right gear ratio, be way sturdier, and be only a little heavier. Getting to know the lead guy who rebuilds heavy duty pickup transmissions in your area might pay the PSRU designer big dividends.

Billski

#### pictsidhe

##### Well-Known Member
I like the ratio better than the 2.85; does the prop turn right at that ratio selection? Any chance there's one closer to 2.4? That would allow a rotary to turn 6500 with the prop at 2700. Makes it a lot easier for the prop carver to get it right.
or a given speed, hp and prop diameter, there will be an optimum RPM. It's worth working it what that is before picking your redrive ratio. Or, ask your prop guy what rpm he recommends you shoot for.

#### rv7charlie

##### Well-Known Member
A friend who's a mech engineer (with a strong aviation 'lean') is telling me that the most sensible thing to do is buy off-the-shelf straight cut individual gears and have the ring gear custom cut/treated, to pick the exact ratio desired. That removes the thrust issue of helical cut gears at the expense of minimal weight & noise penalty. Of course, he has the training, plus access to CNC mills, etc that I don't. 'A man's got to know his limitations.' I'm fairly confident that the primary reason for the relatively low success rate of alternative engines is that many of us (I include myself) often don't.

#### Bill Welter

##### Member
A direct drive auto conversion always scares me, I keep wondering when the prop gyrations are going to finally break the crank or chew up the thrust bearing. Besides, auto engines are don't make enough power at 2700 RPM. I'd go with a redrive just to keep from breaking the crank. Folks have used airboat redrives like Rotator that have spur gears in order to increase the RPM and raise the prop shaft

#### cheapracer

##### Well-Known Member
Log Member
Besides, auto engines are don't make enough power at 2700 RPM.
Just absolute nonsense.

What you probably meant to say was even though "auto" engines make the same hp per cubic inch or more at any rpm you care to choose, usually auto engines continue to make far superior power at a much higher rpm and suffer a penalty of weight for block and component strength, and water cooling to handle those power levels.

Hence we turn to a smaller "auto" engine to reduce the weight, and make use of those higher rpms by way of a PSRU.

.. and hence we end up with threads like this ...

Here is a 3 dyno graphs of an LS3 chev V8 with similar cubic inches to a Lycoming 0-360, basic engine model is rated at 180hp at 2700 rpms, note the LS3 here is not an engine setup for 2700 rpm, in fact these 3 go on to make 460 to 500hp eventually. A LS3 setup for 2700 rpm will probably add 10 to 15 hp on these.

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#### GESchwarz

##### Well-Known Member
Garry, re your inquiry re the RWS reduction drive. Price is \$2750 USD Currently parts have been loaned out, so the complete unit will not be available for a month or so.
I am very interested Neil. Can you send me the details on this unit, that being the RWS specs, and the history of this particular unit?

#### GESchwarz

##### Well-Known Member
I would bet that you will get way better results with engine side flywheels and springs downstream, like the other successful units are doing.

If anyone wants to math model these systems, we can talk about that too, including arc springs and all of the effects on spring rates.

Billski
Thank you Billski, you're my man on this. I have a Mazda Renesis Rotary engine and will have a Tracy Crook PSRU. I can use your help on coming up with a damping device bolted right to the flywheel that will in-turn drive the PSRU.

Who are these other guys who have come up with successful dampers?

#### Lendo

##### Well-Known Member
GES,
Tracy's damper was successful for his PSRU. If you haven't got one with the PSRU, perhaps you can copy one.
George

#### wsimpso1

##### Super Moderator
Staff member
Log Member
Thank you Billski, you're my man on this. I have a Mazda Renesis Rotary engine and will have a Tracy Crook PSRU. I can use your help on coming up with a damping device bolted right to the flywheel that will in-turn drive the PSRU.

Who are these other guys who have come up with successful dampers?
IIRC, RWS uses a flywheel attached to the eccentric shaft flange, with several elastomer bushings around the periphery. Then the gearbox input shaft has pins that mate with holes in the elastomer bushings. Similar arrangements have been done with with other systems.

Tracey Crook appeared to have a reliable system. He showed up at AirVenture with complete regularity... I have never heard numbers on fleet size, fleet hours, and failure rates. Using his inertia and bushings is probably a pretty good starting point.

I will not be selecting your equipment for you. I am planning an O360 for my bird.

BIllski

#### rv7charlie

##### Well-Known Member
or a given speed, hp and prop diameter, there will be an optimum RPM. It's worth working it what that is before picking your redrive ratio. Or, ask your prop guy what rpm he recommends you shoot for.
For the planes most of us fly for fun, prop max diameter is limited by ground clearance, and blade count is forced higher by HP. Tip speed limits rarely come into significant play at our speeds and diameters, with 'conventional' aircraft engines.

I know what the prop carver will recommend: 2700 rpm at the typical safe dia for the airframe, at WOT, at 75% power altitude (around 8K feet), because that's what the engine and airframe expect, and what he's designed for, his whole career. I've also seen what happens when a highly respected prop carver is forced out of his comfort zone, designing for larger than expected diameter & lower rpm at the same HP & airspeed. (It wasn't pretty; it caused the purchaser a TON of grief because he thought the prop carver knew what he was doing).

Charlie

#### rv7charlie

##### Well-Known Member
GE,

With the RWS drive, you should have received a roughly square aluminum damper plate with a rubber 'donut' in each of the four corners. Looks like the attached image, which shows the damper plate attached to the RX-7 car 'flex plate'. The damper plate bolts to an auto-transmission 'flex plate' and accepts the drive's splined input shaft. The Mazda flex plates have been known to crack, even in the car, so at least one flier has replaced the flex plate with a light weight steel racing flywheel (requires some mods).

I know quite a few guys who have been flying the drive for many years (a couple of decades, in some cases). The only serious problems with the damper plate I've heard about were on a heavily boosted turbo'd 13B, and on a peripheral ported 20B (3 rotor) that had serious tuning issues (messed up ignition timing, etc) early in its life. Other than that, the soft 'damper' elements in the plates seem to have a life in the hundreds of hours. One of the fliers on the Flyrotary list recently had a run of replacement soft dampers manufactured, but I believe all have been spoken for.

Anyway, it's going to be tough to make the drive work without the damper plate.

Charlie

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#### aeromomentum

##### Well-Known Member
A direct drive auto conversion always scares me, I keep wondering when the prop gyrations are going to finally break the crank or chew up the thrust bearing. Besides, auto engines are don't make enough power at 2700 RPM. I'd go with a redrive just to keep from breaking the crank. Folks have used airboat redrives like Rotator that have spur gears in order to increase the RPM and raise the prop shaft
While I design and make redrives many direct drive auto engine conversions have been done. A simple place to start in your calculations is to use a prop with the same or less moment of inertia as the flywheel used on the same engine in marine applications. Many airboats have been built with direct drive V8 engines. An LS series motor can be bored and stroked to about to almost 430 ci and should be able to put out about 260hp at 2700 rpm. This could make a very cost and fairly weight effective aircraft power plant. I actually worked on this back in the late 1990's but went on to other things. And don't forget about the direct drive inverted Whitman Buick/Rover V8 conversions. A friend built and flew a direct drive V8 but with a bearing supported prop shaft and turned the engine at 3400 rpm with a smaller prop.

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