Good points Bill and I fully agree that the Shertz Beam is infinitely preferable to the 'Mount on the adapter plate' scheme.
The only point which truly affects safety is the point about stress on the bolts. In the original configuration, the six bolts are all in a row and going on 'gut feel' stress analysis, this will result in >= 90% of the force in tension being placed on a single 6mm bolt (the one on the end). If this bolt fails, the rest of them would break in quick succession. The geometry is such that the force will increase as the next bolt in line will see an ever increasing load as the one outboard of it fails.
The fix that you proposed (tying the beam to more bolts along the side of the engine) should work well and this removes any concern I had about its safety as long as the rear mount is properly handled. This is much easier on composite structures than on 'beer can' designs.
----- Original Message -----
From: William
Sent: Sunday, May 23, 2004 11:19 AM
To: Rotary motors in aircraft
Subject: [FlyRotary] Beam Motor Mount (semi-long)
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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