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I welcome feedback on motor mounts, and would like to
clarify some possible misconceptions.
First of all, my butt will be on the line when I fly my
plane, therefore I truly want things to work, so if there are shortcomings in
something I am doing, I would want to hear about it, and then make my own
judgement as to whether the concern has been adequately addressed.
I subscribe to "both" lists, and try to take the best of
what each offers and reject what my engineering judgement tells me is suspect.
The evolution of the mount as best I can remember it is as
follows.
Paul Lamar is also building a KIS Cruiser, same as I am,
and he designed a motor mount that used two tripods coming forward from the
firewall to a pad for Barry mounts. He then designed a massive (in my
mind) aluminum plate that replaced the back plate of Tracy's redrive to
span from one tripod to the other. I liked the tripod idea, but had many
reservations about the redesigned back plate, including the fact that it did not
allow for the use of the two evaporator cores that I planned on using for
cooling.
Therefore, I adopted the tripod idea and searched for an
alternative way of making the connection to the engine.
I made many mockups using wood, balsa wood, and glue to
test the geometry. I had several criteria that I wanted to
meet.
1. Keep the weight down, but have adequate
strength.
2. Be compatible with Evaporator cores for water heat
rejection.
3. Provide for excellent oil cooling, with a duct feeding
into the oil cooler with a short path. I followed Ed Andersons oil cooling
trials, and wanted the oil cooler right up front like Ed found
worked.
4. Be able to use Tracy's redrive with NO modifications,
i.e. keep the starter location where it is at 6 o'clock.
With those criteria in mind, I settled on a cross beam
arrangement to span across the tripods. There were several impediments to doing
this that I had to design around.
1. I wanted to capture 6 holes in the
oil pan, but due to the curvature of the stock oil pan, that cannot be done
without making a new oil pan. So I designed and had fabricated a new oil
pan out of Aluminum. It allows the beam to capture SIX of the oil pan
bolts (not just one Tracy).
2. I needed adequate strenght -- Paul
Lamar (other list) states that you should design for a 6-G loading to simulate a
hard landing. Is that over-kill or not enough? I don't know, but it
seemed reasonable to me, therefore I used a design load to be supported by the
beam of 1800 #.
3 I needed to fabricate the beam with a
minimum of tools, I don't have easy access to metal bending brakes or mill,
therefore I wanted to use stock materials. I made up a spread sheet in
Excel, entered for formulas for stress as a function of geometry from Roark, and
explored different geometries and materials for the cross bar. I explored
round tubes, I-beams, rectangular tubes, etc. calculating the stress and the
weight of each option.
4. Since I wanted the oil cooler to fit
just below the cross-beam, I was limited in depth of the beam, in order to still
fit in my cowling. Having too deep a beam (good for stress) would
put the oil cooler too low, and the cowling would not be wide enough. I
finally settled on a 2"x1" rectangular tube made from 0.065" 4130 steel, that I
could order from Wicks or Aircraft spruce.
For the ends, I need a thickness of 1/2" to fit in the
Barry mounts, so I made some 1/2" Aluminum ends out of 2024 Aluminum plate. I
could cut and shape this material with my tools. These are held in with 4 AN-4
bolts, with metal spacers to fill the difference between the aluminum ends and
the other wall of the tubing.
Since I have bolts going through the beam into the oil pan
(6), I had to be concerned with crushing or bending the beam when tightening the
bolts. To alleviate that problem, I had small tubing welded in the holes to
bridge from the top to bottom of the beam, so I can torque down the oil pan
bolts, and the tubing takes the compressive load.
That pretty much describes my design and criteria. I
posted the pictures (I thought to both lists), and got some feedback from
Lamar's list.
Someone expressed a concern that the outer two of the six
oil pan bolts would be taking too much of the gyroscopic load and that this
represented a failure mode. They recommended having an extension on each
side of the oil pan that would tie the beam into a couple of the side oil pan
bolts. This would make the mount be attached with 10 bolts when
accomplished. I have not done that yet, but plan to when I next go to my
welder.
As this has gestated for over a year, Jerry Hey became
interested in possibly making these. He has access to a bending brake, and an
alternative method of obtaining the same stiffness is to use a 'hat section'. He
has incorporated the concept of the side wings that capture some of the side oil
pan bolts, and will fully load the beam before declaring it a product. I
personally like the closed rectangular tubing, but life is full of
compromises.
The beam motor mount requires a third mount point, at the
rear of the engine. The beam is most certainly NOT at the exact center of
gravity of the engine/redrive/propellor assembly, but I believe that it is CLOSE
to the CG. It goes under the front cast iron of the engine, not off the
redrive plate with is 3" further forward (tractor airplane). The
contention is that a single mount point for the rear end of the engine can be
used -- this was done with the Questair Venture from what I understand. I
am still evaluating the best way of making this mount, a PRELIMINARY method that
I am evaluating is to tie into the big 12mm bolt hole on the rear casting and
run to a hard point on the firewall. I can make a hardpoint anywhere I
want on a fiberglass airplane. Have I SETTLED on this rear mount -- NO, not yet,
I need to make a number of calculations, and would like to have feedback from
others.
I would like feedback on this design, with your concerns
and comments, bearing in mind that the beam is attached with 6 bolts, soon to be
10, and that the rear motor mount is still under construction and
evaluation.
Bill Schertz
KIS Cruiser # 4045
----- Original Message -----
Sent: Saturday, May 22, 2004 1:27
PM
Subject: [FlyRotary] Re: Shertz
Beam
I have examined the "Shertz Beam" Motor mount on "the other" Mail
List.
Searched and did not find a comment on this list.
Other
than it is recomended by "The Other" list, is there anything wrong
with
it?
I am looking for solid/semi solid engineering critique. I
really dont want
to talk about "feelings" or "preferance".
Thans
for your time.
I prefer the straight stuff too so to get right to the core of it:
1. Shertz Beam - Which incantation do you mean? There
is no single definition. I have seen at least 6 versions. In
it's original incantation, a SINGLE 6mm bolt in tension absorbed ALL of the
torsion forces (prop torque and vibration loads). This was
unacceptable. I have seen a few attempts to fix this, some good some
not.
2. The beam is NOT at the CG, contrary to claims otherwise.
There are significant loads on the rear mount which is not even a part of
the Shertz beam design. It has been proposed that the rear
of the engine be hung from a central point off the engine front housing from
an assembly made from 2 rod ends. The other end is supposedly attached
to a hard point near the center of the firewall. On most aircraft,
there IS NO HARD POINT at this location. In addition, on an aerobatic
plane where negative loads can occur, the "hanging rod" in compression
is 'less than ideal'.
3. This mount more or less assumes that the rad is going to be
mounted under the engine. This rad location and the Shertz beam
makes more sense on a composite canard design but on a tractor,
the mount ends up taking up space in the wrong places. The most
popular and successful cooling scheme so far use two heat exchangers in a
location that interfears with this mount. It would also not work with
what I plan to do on my -8 with a single rad.
Tracy Crook
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