PSRU design advantages

Resonance.

God's gift of humility.



* Jim Bede, of course, with the BD-5 drive train.
Hard times and heartache...
That engine and drive ran flawlessly 24/7 for months without a single problem.
It looked goooood!

But bolted into the airframe, it would come apart in minutes.

Back on the test stand (concrete block!) it ran and ran and ran.
(the energizer bunny hadn't been born yet)

But in the airplane... pure bad news.

Now you tell me. Where's the logic in that???

As it turned out, all we needed to do to solve the drive problem on the -5
was to fill the belly of the airplane with a few inches of concrete.
(well, it's not a *great* solution, but it would fix the drive problem!)



* Molt Taylor and his AeroCar. 1950's chic!
Here is a really *good* long drive shaft problem.
I say good because it has a happy ending.

The patented "Flexodyne"(sp?) drive shaft damper.


* Tail rotor drive shafts on almost any rotary winged beast.
(Shudder)


As for engines on experimental aircraft?
My attitude parallels Peter's.

I used to see it as a way to get airborne for a few dollars less.
And in some (small) cases, VW, simple Subes, Geo, and the like, it may
still be a valid way to go.
A-65's don't grow on trees anymore.

But I too have come back to the olde timy 1930's tractor engines as the
solution for fast iron (or wood or plastic as the case may be).

Those engines evolved(?) to be the way they are because that's what does
the job best. Long stroke, slow turning, light weight, reliable.


My personal reason is weight.
Pure and simple.

Our (small experimental) airplanes are - for the most part - simply too small
to carry the extra weight *well*.

Lighter is better.
Hey!
It's a freekin airplane!
(chant mantra - lighterisbetteroooommmm...)


There is, also, that old saw about not mixing experimental engines with
experimental airframes. I wonder what nut came up with that one?



But then the topic is resonance, isn't it.
And?
With the exception of combinations which are known to have engine/prop/AIRFRAME
resonance issues (hint, hint, hint), certified engines avoid that trap entirely
(well, mostly?).


That's not my $.02,
It's my bunch of thousand bucks...
And?
In the end, my ass, and maybe yours?


Richard

for what it's worth

Big John wrote:

ADK

Look up Molt Taylor and his Aerocar system. He used a pusher
arrangement and seemed to have most of the problems solved with long
shaft back to prop.

His bird may still be flying some place to exhibitions?

Am sure someone will jump in here and give data on his bird and how he
coupled shaft to engine with a "power glide" clutch or some such. It
allowed a small amount of slippage at each power stroke to prevent the
pulse being transmitted to drive shaft and prop as I recall.

Best of luck with a difficult problem.

Big John

Ok,

I think it's also used on the Imp and Mini-Imp.


The "clutch" consists of two (wavy surfaced) plates with lead shot
loaded between them.

As the thing spun up, centrifugal force packed the shot solid, but
there was enough "give" with the shot to absorb the "jerk" (4th derivative?).


Richard

no idea why that came out in past tense...

The aircraft I am interested in is the G802 Orion, designed in France in the
80's. It had problems with a long drive shaft and so I am interested in how
others have solved a similar problem. It also incorporated a Flexidyne type
coupler but it still had problems.
I work in aviation but there isn't a lot of good suggestions (printable)
from the people and engineers I work with.


"Richard Lamb" wrote in message
ink.net...
Big John wrote:

ADK

Look up Molt Taylor and his Aerocar system. He used a pusher
arrangement and seemed to have most of the problems solved with long
shaft back to prop.

His bird may still be flying some place to exhibitions?

Am sure someone will jump in here and give data on his bird and how he
coupled shaft to engine with a "power glide" clutch or some such. It
allowed a small amount of slippage at each power stroke to prevent the
pulse being transmitted to drive shaft and prop as I recall.

Best of luck with a difficult problem.

Big John

Ok,

I think it's also used on the Imp and Mini-Imp.


The "clutch" consists of two (wavy surfaced) plates with lead shot
loaded between them.

As the thing spun up, centrifugal force packed the shot solid, but
there was enough "give" with the shot to absorb the "jerk" (4th
derivative?).


Richard

no idea why that came out in past tense...

---------------snip-----------------

The point here is
that, although a loss of power is the most obvious
---------------snip-----------------

I don't know how I inadvertantly edited out part of the point that I was
trying to make, but the sentence and paragraph should have continued to read
as follows:


The point here is that, although a loss of power is the most obvious outcome
of a torsional resonance problem, it is not the only failure mode and may
not even be the worst. For example, the same "Contact!" article also
mentioned severe flexing of a bulkhead in the fusalage due to a resonance
between the complete engine and drive system and the fusalage at critical
speed(s). That caused me to be even more afraid than the driveshaft and
redrive problems.

Peter

"Peter Dohm" wrote

The point here is that, although a loss of power is the most obvious
outcome
of a torsional resonance problem, it is not the only failure mode and may
not even be the worst. For example, the same "Contact!" article also
mentioned severe flexing of a bulkhead in the fusalage due to a resonance
between the complete engine and drive system and the fusalage at critical
speed(s). That caused me to be even more afraid than the driveshaft and
redrive problems.

That is along the lines of what I wrote. The more a person reads about
torsional resonance, the more one understands how much there is that can go
wrong.

I am convinced not to use a long driveshaft, for my personal use. g
--
Jim in NC

Richard Lamb wrote:

But then the topic is resonance, isn't it.
And?
With the exception of combinations which are known to have
engine/prop/AIRFRAME
resonance issues (hint, hint, hint), certified engines avoid that trap
entirely
(well, mostly?).


I'm glad to add the "(well, mostly?)", because I could read that as "not
at all". What you do have with a certified engine is a situation where
the prop manufacturer has tested their prop with that engine. It's a
well studied combination that everyone knows how to work with.

For instance, you can't cut a metal prop down but so far. Why? Because
everyone knows where that resonant point is, and cutting the prop down
to 70" will land you smack-dab in the middle of patooky when one of the
blades comes off.

But build a new prop of your own design, sufficiently different from the
typical prop...then it doesn't matter if you put it on a certified
powerplant or the front of your Oldsmobile. Unless you have the
facilities to test it, its longevity is just guesswork.


--
This is by far the hardest lesson about freedom. It goes against
instinct, and morality, to just sit back and watch people make
mistakes. We want to help them, which means control them and their
decisions, but in doing so we actually hurt them (and ourselves)."

Richard Lamb wrote:

But then the topic is resonance, isn't it.
And?
With the exception of combinations which are known to have
engine/prop/AIRFRAME
resonance issues (hint, hint, hint), certified engines avoid that trap
entirely
(well, mostly?).


"Ernest Christley" wrote in message
...

I'm glad to add the "(well, mostly?)", because I could read that as "not
at all". What you do have with a certified engine is a situation where
the prop manufacturer has tested their prop with that engine. It's a
well studied combination that everyone knows how to work with.

For instance, you can't cut a metal prop down but so far. Why? Because
everyone knows where that resonant point is, and cutting the prop down
to 70" will land you smack-dab in the middle of patooky when one of the
blades comes off.

But build a new prop of your own design, sufficiently different from the
typical prop...then it doesn't matter if you put it on a certified
powerplant or the front of your Oldsmobile. Unless you have the
facilities to test it, its longevity is just guesswork.


--
This is by far the hardest lesson about freedom. It goes against
instinct, and morality, to just sit back and watch people make
mistakes. We want to help them, which means control them and their
decisions, but in doing so we actually hurt them (and ourselves)."

I think that the point of "(well mostly)" may have been that even the tried
and true combinations, such as certain Lycoming engines with approved
constant speed props, failures can occur long after the design appears to be
mature, and some of the failures may yet prove to be related to resonance.
The moral may be that everything is experimental, but some things are less
experimental than others.

At least, that would have been my point...

Peter

"Peter Dohm" wrote

I think that the point of "(well mostly)" may have been that even the
tried
and true combinations, such as certain Lycoming engines with approved
constant speed props, failures can occur long after the design appears to
be
mature, and some of the failures may yet prove to be related to resonance.
The moral may be that everything is experimental, but some things are less
experimental than others.

It was noted (I think it was here) that when engine mount bushings wear out,
the properties of the mount/engine/airframe combination may have changed
enough that the resonance may become a factor again, where it was not - when
the mount bushings were fresh.
--
Jim in NC

"Ernest Christley" wrote in message
...
...
I'm glad to add the "(well, mostly?)", because I could read that as "not
at all". What you do have with a certified engine is a situation where
the prop manufacturer has tested their prop with that engine. It's a well
studied combination that everyone knows how to work with.

For instance, you can't cut a metal prop down but so far. Why? Because
everyone knows where that resonant point is, and cutting the prop down to
70" will land you smack-dab in the middle of patooky when one of the
blades comes off.

But build a new prop of your own design, sufficiently different from the
typical prop...then it doesn't matter if you put it on a certified
powerplant or the front of your Oldsmobile. Unless you have the
facilities to test it, its longevity is just guesswork.

That was a problem on some of the early T-18's that were using cut down
propellers - resonance was fatiguing the propeller which would then shed a
blade, which would then encourage the engine to attempt to part company with
the airframe...

--
Geoff
The Sea Hawk at Wow Way d0t Com
remove spaces and make the obvious substitutions to reply by mail
Spell checking is left as an excercise for the reader.

IF you had to design a PSRU, to drive a pusher propellor via shaft, what
would your experience dictate? Thinking along the lines of a gearbelt, chain
or gear. Please, I would appreciate the collective experience available on
this group. I have decided on the aircraft, but want to make it the most
reliable and safest it can be.

"ADK" wrote in message
news:X6TXf.28774$%H.11944@clgrps13...
This is probably going to open old wounds. What I would like is
experienced input on the advantages, for economic, efficiency and
longevity etc. of different types of redrives.

I am leaning towards a cog-belt reducer in a 6 cylinder, liquid cooled,
configuration driving a long drive shaft to the prop.