Pinging MkI. A few Questions about your Prop and PSRU.

Hi MkI and Beurling;

MkI; Thanks for getting back to me about your PSRU and propeller. I was curious what level of confidence you have in these two units, overall. I guess what I revert back to is fear of the unknown, and vibration plus its effect on any mechanical unit are unknowns for me.

I also simply do not know who to trust when it comes to buying a PSRU. Company claims are simply too much like sales pitches, and often backed up with too little data to believe. Do Hy-vo chains really attenuate or dampen vibration? Not according to some manufacturers. But absolutely according to others that use these chains.

So I have been searching to find incidents of PSRU failures. I am trying to narrow my focus to spur geared units, not chains, helical gears and cogged belts, although I have found failure examples for all of those. I want to have some understanding of failure modes, and see if there is any common parts that seems to fail. So far, I have found that output shafts seem to snap and this has surprised me a bit, since I would have assumed that this would be a pretty beefy part. This would seem to suggest that it’s breaking due to tortional fatigue (don’t quote me on that) which in turn might indicate propeller vibration and power pulse issues from the engine. Perhaps these two vibrations are ganging together and making the forces bigger than predicted. You know, the propeller does its worst just as the engine does the same. I can kind of picture it, but this is pure speculation on my part. All I know is that PSRU’s are failing. Are they failing more often than props, engines???

Beurling; I’m surprised if there are belted units that handle 1000 HP. Thanks for letting me know. If you have any links on PSRU’s please share them, especially 400 HP + vendors.

I have tried to find the Camdrive vendor. Are they no longer operating?

Thanks guys, Tom.

Hi Tom, just happened upon this post, sorry to jump in but as another replica spit builder I have also researched this topic to death. I think you will find if you look around now there are several commercially available PSRU’s flying for reasonable prices which can handle high HP engine setups. The new belt drive units for experimental use are available now for 1000+ HP and feature their own oil sump system to lubricate. If I find the vendor links in my mass of favorites I will post them for you. The belt system or the planetary drive PSRU’s are definitely the way to go as they are used on many auto conversions on agcats etc. with the big block chevs and fords.

As for props depending on the scale you are building a 5bladed prop will definitely take alot of power to drive and you will most definitely end up doing some machining of the prop hub. I do not ever recall coming across a commercial 5 bladed hub.

Cheers
Beurling

Hi MkI.

I have been meaning to get back to you about the power unit in your replica Spitfire. I recall that you mentioned that it’s home made as is your propeller.

So, first off, let me say that I am not trying to voodoo hex you into having bad luck. But as you mentioned, this is a pretty significant piece of metal so I want to ask what it is that makes you inherently comfortable with the homemade approach to yours. Same for the propeller.

I know a little reading is a dangerous thing. Maybe a lot is even worse. In my attempt to come up with a lower cost solution to a $10,000 PSRU and a $20,000 MT 5 bladed propeller for a Mk XIV replica Spitfire, I have been doing a lot of searching. As I go through many websites (actually there aren’t THAT many) about propellers and gear boxes, I become more confused, partly because I truly understand very little about vibration in its many forms, and partly because there are so many contradictory claims about PSRU’s especially. Some say the Hyvo chain kills reflected vibration, or is self-damping, some say this is nonsense and wishful thinking at best. The most authoritative “looking” website I have found, with reams of info, is the EPI site. The owner has also sent me two replies to a pile of questions, including his explanation of his complete mistrust of hyvo chains.

So, here are my questions MkI:

Your Spit has around 60 hours on it, right? How do you feel about the PSRU? If vibration can be damaging, whether you can actually feel it or not, what gives you the confidence to fire up and go for a hop?

How well was the propeller built? What sort of inspections are possible to verify its airworthiness?

In short, does your power train cause you concerns above and beyond normal levels of concern, and if so, how do you alleviate those concerns in terms of believing that your gear box and propeller will stay together?

Now, having said all of that, I ask because I also believe it would be possible to home-build these two units. Your machine’s maker wasn’t the first to do so and he probably won’t be the last. In my case, with a Chevy LS2 proposed as the engine, the stakes are a bit higher, with 400 HP, as opposed to what, 250? Still significant though, in your case. And yours uses a cogged belt. How high can we go with that drive mechanism? I wouldn’t suspect that 400 HP can be belt driven, could it?

If YOU were going to design/build the PSRU and prop, how would you approach it? Or would you simply not tackle those parts of the build?

I hope this didn’t seem vague or meandering, but really, there’s a lot to ask about with these two pieces of equipment, understandably. Overall, if the $30,000 expense in just the prop and the gear box is unavoidable, that’s pretty close to a show-stopper for me (but still better than a heartbeat stopper, I guess).

Thanks for any feedback you can give. I still think your machine is a pretty terrific looking beast. I’d like be flying too!

Cheers, Tom.

Pinging MkI. A few Questions about your Prop and PSRU

Hi Tom:
Good questions re PSRU and prop. Am I inherently comfortable with my combo?….not totally.

My prop is a very simple and finely crafted ground adjustable unit that is relatively easy to inspect (albeit after un-doing about 2 miles of lockwire!!!) and I do have a great deal of confidence in it.

The cog-belt PSRU, being homebuilt, with no specific anti-harmonic design considerations/features, is the part that gives me a bit of angst. It again is quite well built (out of welded aluminum plate) however it uses a combination of sealed and “greasable” bearings with no oil lube system. The cog belt is quite heavy (@ 5″ wide) for the power it handles (only @ 150 h.p. I estimate) however it is now close to 20 years old and I will be challenged to find an exact replacement. To mitigate the “inherrent” risks somewhat, I will be employing a “frequent grease injection” program (ie. a shot every 8-10 hrs) and a corresponding high frequency of belt and bearing inspections. Given that the combo does run extremely smooth (to the pilot’s feel at any rate), likely due in part to the considerable nose ballast (mounted directly on the PSRU) and my relatively low power loading, I am confident to operate the drive combo in a careful and conservative fashion. I will be flying the Spit with a chute regardless.

As for the Hyvo chain units, they are probably fine. A lot of them are flying in front of relatively high-powered engines. I have checked into one for my Buick V8 and I could source a custom-machined unit for approximately $14K. The problem is; all my cowls, coolant header tank, spinner etc. would likely have to change. The matching 3-blade controllable pitch prop is @ $6K therefore I would be adding another $20K to the overall tab and then have to completely re-engineer the front of the airplane. These potential mods won’t be in the cards for a while as my available time with a young family is almost nil and I am still trying to finish the induction system mods and re-paint of the airplane in a BoB scheme.

As for how I would tackle a higher powered unit like yours, I would likely go the route Terry Wilshire has chosen and buy a purpose built, adequately tested vendor PSRU. Terry has incorporated a certified (with mods) propellor on his 75% Mk.1 which undoubtedly willl serve him well when he gets it flying again. On the other hand, if machining is your thing and you have access to all the right equipment, you could essentially copy a vendor unit and build one yourself, likely saving close to $20K.

Airplanes unfortunately, are not cheap now matter how you approach them. If you were to drop the 5-bladed prop idea (ie. buy a more common species of prop like a 3 or 4 blade unit) you could save close to $10K). If you are cost constrained (as most of us are), one other factor you might want to reconsider is that 5 blades are going to require an awful lot of power to turn, requiring a big, heavy, gas-guzzling engine that like you also noted, may be outside the comfort zone for a cog-belt PSRU. Sorry, I can’t offer much more insight on these aspects.

That about covers it methinks.
Regards, Mk1
——————–

Hi MkI.

I have been meaning to get back to you about the power unit in your replica Spitfire. I recall that you mentioned that it’s home made as is your propeller.

So, first off, let me say that I am not trying to voodoo hex you into having bad luck. But as you mentioned, this is a pretty significant piece of metal so I want to ask what it is that makes you inherently comfortable with the homemade approach to yours. Same for the propeller.

I know a little reading is a dangerous thing. Maybe a lot is even worse. In my attempt to come up with a lower cost solution to a $10,000 PSRU and a $20,000 MT 5 bladed propeller for a Mk XIV replica Spitfire, I have been doing a lot of searching. As I go through many websites (actually there aren’t THAT many) about propellers and gear boxes, I become more confused, partly because I truly understand very little about vibration in its many forms, and partly because there are so many contradictory claims about PSRU’s especially. Some say the Hyvo chain kills reflected vibration, or is self-damping, some say this is nonsense and wishful thinking at best. The most authoritative “looking” website I have found, with reams of info, is the EPI site. The owner has also sent me two replies to a pile of questions, including his explanation of his complete mistrust of hyvo chains.

So, here are my questions MkI:

Your Spit has around 60 hours on it, right? How do you feel about the PSRU? If vibration can be damaging, whether you can actually feel it or not, what gives you the confidence to fire up and go for a hop?

How well was the propeller built? What sort of inspections are possible to verify its airworthiness?

In short, does your power train cause you concerns above and beyond normal levels of concern, and if so, how do you alleviate those concerns in terms of believing that your gear box and propeller will stay together?

Now, having said all of that, I ask because I also believe it would be possible to home-build these two units. Your machine’s maker wasn’t the first to do so and he probably won’t be the last. In my case, with a Chevy LS2 proposed as the engine, the stakes are a bit higher, with 400 HP, as opposed to what, 250? Still significant though, in your case. And yours uses a cogged belt. How high can we go with that drive mechanism? I wouldn’t suspect that 400 HP can be belt driven, could it?

If YOU were going to design/build the PSRU and prop, how would you approach it? Or would you simply not tackle those parts of the build?

I hope this didn’t seem vague or meandering, but really, there’s a lot to ask about with these two pieces of equipment, understandably. Overall, if the $30,000 expense in just the prop and the gear box is unavoidable, that’s pretty close to a show-stopper for me (but still better than a heartbeat stopper, I guess).

Thanks for any feedback you can give. I still think your machine is a pretty terrific looking beast. I’d like be flying too!

Cheers, Tom.