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

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.

"Capt. Geoffrey Thorpe" The Sea Hawk at wow way d0t com wrote in message
...
snip

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...


That used to happen so frequently with Formula air racers where they chopped
the props for the O-200's to get the RPM up to get additional HP out of that
little engine that they started using a safety cable around the engine.
That way when the prop shed a blade and the engine jumped out of the engine
mount the cable held it somewhere near where it was supposed to be so you
could get the airplane on the ground in a survivable fashion!

Not something you want to play with normally.

Highflyer
Highflight Aviation Services
Pinckneyville Airport ( PJY )