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  #1  
Old 05-03-2004, 04:08 PM
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LargePrime LargePrime is offline
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Wink Rotary Motor mount option

I know a few of you are on the ACRE Mailing List. They have been discussing a rotary motor mount called a "Shertz Beam". Here is a few pics.

Jerry Hey of j-winddesigns.com is testing out this mount and, if all goes well, should be able to provide them to the public at a subistantial savings over traditional mounts.
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Last edited by LargePrime : 05-03-2004 at 04:25 PM.
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  #2  
Old 05-03-2004, 06:48 PM
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Default Huh?

It's all Greek to me

Sorry.
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  #3  
Old 05-03-2004, 08:56 PM
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I don't remember enough of my aeronautical engineering training to provide a detailed critique of this mount, so all I say from here on is just my opinion, third hand gossip and hearsay.

Given that disclaimer, I think it sucks.

Tracy has said that this method of mounting will void his redrive warrenty, and someone who's engineering opinions I trust has said it's downright dangerous. On top of all that, it just looks wrong.

Consider the background here. This "Shertz Beam" recommendation is from a man who has said publically many times that electric water pumps on rotarys cant possibly work. He even proved it scientifically. There are now two flying with no cooling problems. He also said you cant cool a rotary on a pusher with just a NACA scoop. I am.

Here you have two people. Person 1 has been working on a plane for many years, hasn't finished it yet, but has lots of theories about what works and what doesn't. Person 2 has been flying rotaries for years, tests and flys all his products, stands by them, and has built a business around his reputation. Which of the two should you believe?

Hopefully someone with more training will jump in and give us some details.

Last edited by John Slade : 05-03-2004 at 11:03 PM.
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Old 05-03-2004, 10:56 PM
alexmadsen alexmadsen is offline
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As a (soon to be) Mechanical engineer I can say I do not like the looks of it. While I have done no structural or dynamic calculations on this design, I see a fundamental flaw in it. The engine is only supported at 2 points. This means that it is not well constrained in the rotational axis about the beam. There are 2 problems with this. One is possible rotational resonance (possibly leading to damage). The second is a large acceleration in the forward or aft direction will cause a tremendous moment (torque) on the 2 mounting points. Lets for the sake of argument say the engine is 250lb and its CG 1.5 ft above the beam. If it experiences a 10 g acceleration it there will be a 3,750 ft-lb moment on the mounting points. G lodging gets a lot higher that that in "emergency situations" and I would hate to have my engine come flying through the firewall.

F*G*L=M so 250lb*10g*1.5ft=3,750 ft-lb
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Last edited by alexmadsen : 05-03-2004 at 11:03 PM. Reason: spelling
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  #5  
Old 05-03-2004, 11:01 PM
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Quote:
I would hate to have my engine come flying through the firewall
Or depart the airplane! Without the engine weight a pusher quickly becomes a lawn dart.

There's also something about the precession giroscopic effect - or is that what you said.
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Old 05-03-2004, 11:01 PM
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PS If someone shows me that the numbers for a Shertz Beam are ok then I would consider using one.
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  #7  
Old 05-03-2004, 11:35 PM
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Quote:
If someone shows me that the numbers for a Shertz Beam are ok then I would consider using one.
I'd rather stand on the "shoulders of giants" and use something that's already been tested and flown by someone else.

When Mr. Shertz has a few hours on his beam, give me a call.

Hindmost
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  #8  
Old 05-04-2004, 01:48 AM
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Quote:
Originally Posted by LargePrime
I know a few of you are on the ACRE Mailing List. They have been discussing a rotary motor mount called a "Shertz Beam". Here is a few pics.

Jerry Hey of j-winddesigns.com is testing out this mount and, if all goes well, should be able to provide them to the public at a subistantial savings over traditional mounts.
Did John put you up to posting this just to get me going?
Please take a close look at this.
First, when people started looking at mounting this engine they came up with the pan sandwich plate mount, and people threw all sorts of red flags out because the pan SCREWS were all that was holding all the loads of the engine. It took a lot of explanation to placate peoples fears that all that was between a good day and a change in your CG were a handfull of SCREWS (not bolts, just fasteners used to hold the oil pan on, how much a load were they calculated for, Hmmmm?)
The thing is, there are enough of them and they are so distributed and they are all in sheer, and you do need to replace them with quality screws of the appropriate length, thats fine, it has worked well and reasonably proven.

So along comes this Shertz Beam idea; a bar under one edge of the pan with five small SCREWS holding the bulk of the load. As originally proposed elsewhere that was it except a simple link on the other end of the engine to handle up/down pitching loads as he believed the engine was also near its CG at this location.

Visualize this: a sudden pitch up or down as in a pullup or dive. Swinging a large 3 blade prop to absorb all the HP this engine can generate.
Gyroscopic precession is such that any force applied will be realised 90 degrees out of sync. In other words, the up or down pitch will result in a huge sideways twist or rotation about the CG of the engine. If you have 5 little screws holding this on, in reality the center 3 are conveinient pivot points and the outer ones are going to take the whole load.
How confident do you feel about securing your whole engine on with two oil pan screws?
After the two outer ones shear off the rest will follow quickly. Did I mention it would be a good idea in any new engine configuration to tether the engine to the firewall with a steel cable.. just in case.
I mentioned this gyroscopic load oversight to another list owner and he responded with the appropriate OOPS and added two small tabs to incorporate 3 more screws on each side of the pan.
Please don't do this.
At this time I feel there are two concepts that do work, one is the Conversion Concepts mounts. He has improved and upgraded his design and it looks sound.
The other is a hybrid that you have to do yourself. Tracy Crook is adamant about not using the redrive plate as a mount because of the gyro loads.. that is a solid 1/2" aluminum plate and he is very concerned (think about those weenie oil pan screws). But if you use the redrive plate (you have to make a larger one) as the main mount and the lower edge is extended down a bit more, you can put a bottom plate perpendicular to it that goes to the 5 pan screws to help stabilize it. This is the route we will probably go ourselves. While Tracy does not approve, he feels it is a step in a positive direction.
Think critically and be careful of those with an agenda to get people on board and for what motives.
The rotary engine is a wonderful thing. It is not a turnkey package yet but is becoming a buffet of ideas.
Be safe and take care, Chrissi
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  #9  
Old 05-04-2004, 01:22 PM
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Please tell me that a large portion of this mount not shown on those pictures. If not, I must conclude that it is a bad idea.

As Alex and Crissi have already mentioned, the thrust, inertial, and gyroscopic loads result in a large moment about the axis of the beam. There is no wide couple to withstand this moment as with all other engine mounts I've studied. Not in the connection of the beam to the engine, as pointed out by Crissi. Nor at the connection of the beam to the mount frame.

Instead, the moment appears to be transmitted through a pair of rubber bushings that are positioned along the axis of the moment. Rubber bushings are not fixed moment connections! They are intended to be used as a "couple" so that moments are resolved into compression in the axis of the through bolt.

If the goal is a wildly variable thrust line angle, then I say "kudos!". Otherwise, I can't see how you could make the structure rigid enough without paying a stiff penalty with regards to weight. That sort of defeats the purpose.

This design appears very similar to that I've seen on some automotive engine mounts. What is missing is that the transmission mounts in those automotive installations provide the other half of the couple. The engine and transmission act as one unit.

Matt
Structural Engineer
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  #10  
Old 05-04-2004, 07:40 PM
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Matt,
You are right, missing from the pictures is the fwd mount I mentioned. The individual who drew these frequently leaves it out because he thinks it is not of much importance other than to establish thrust angle, in fact it is often shown sort of like a turnbuckle arrangement. My feeling is that it is a significant part of the mount and is needed to establish the thrust line as well as counter the torsional loads... assuming the main mounting point is doing the job of handling support and thrust loads. Scarey huh?
...Chrissi
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  #11  
Old 05-04-2004, 09:50 PM
alexmadsen alexmadsen is offline
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Default important detail

That’s kind of an important detail that I have never seen on any of the drawings. It completly changes the mount !!!!(making incomplete drawing is a very bad habit to get into)
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  #12  
Old 05-22-2004, 12:24 PM
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Here are a few of my comments on this.
1. Of course any engine mount needs a minimum of 3 mounting points.
2. The Schertz beam is supposedly mounted near the cg of the combined engine/psru, so the loads on the single front mount (as in a pusher) are indeed comparatively low.
3. I have the 1/4" aluminum plate between the engine, since I built my installation in 1997 and the only resource was Tracy's conversion book. It was the easiest way to get something going. I don't think it's necessary to use a plate, and I don't like it because I get oil leaks. Eventually I will make a 3-point mount, probably using the 3rd gen. rear side housing - it has the stock mount points on this housing instead of the center housing as in 2nd. gen. I started to build a mount using the 2nd gen hardpoints, but the distance was too short between the front mount and the two rear mounts - it just appeared to be too short-coupled.
4. With only 5 bolts in the Schertz beam into the rear oil pan bolts - this does >seem< a little marginal. At least the bolts are going into the steel rear housing and not aluminum as in the rotor housings. They are 6mm bolts, nearly equivalent to 1/4" bolts. Your canard is held on with only two 1/4" bolts. The bolts in the canard are in shear, while the forces on the bolts in the Schertz beam might be higher in tension. I'll let mechanical engineers debate that further.

The Eccentric
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  #13  
Old 08-30-2004, 05:45 PM
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LargePrime LargePrime is offline
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Default Thread, back from the dead

more pics.
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