SUGGESTIONS FOR BUILDING A ROTARY ENGINE MOTOR MOUNT by Paul Lamar
A motor mount is the last thing you want to build when installing a rotary engine in your aircraft. Collect all the major parts you intend to use such as rads, turbo chargers and mufflers. Build a full scale mock up of the type of installation or layout you would like to use with all the major parts that are going under the cowl. You can carve exact full scale foam models if you are not ready to buy the actual parts. This will save you a year or more from making many parts over and over again and that includes the motor mount itself. You can also use a good 3D modeling computer program such as Rhino or Solid Works. A 3D program allows you to rotate the 3D drawing to any angle and check clearances between the parts. Here are a few Rhino examples. If you don't want to spend the money for a 3D program or take the time to learn to use it contact me and I will do it for you for free as I have a lot of parts already drawn.
You can learn an amazing amount about space frames if you make a quarter scale model out of 3/16 inch square balsa sticks. Apply some typical loads and see what deflects and how stiff your model is. If you weld you can also make models out of welding rod.
All of the members of a space frame should be in pure tension or compression. Any loads applied to a space frame must be applied to a point where three or more tubes join together, A solid bolted juncture of one tube and a rigid firewall such as found on composite air planes is also acceptable. Do not assume that this is the same as feeding the load into a rubber bushing located on or in the firewall as being the same thing. This creates local bending loads in the tubing and the bolt that goes through the firewall. These bending loads must be accounted for by locating such a bolt at the juncture of at least three tubes. This distributes the bending loads over three tubes.
If these rules are violated the motor mount might still work but it is nearly impossible to predict the stresses and deflections without FEA and therefore these kinds of mounts are extremely dangerous. They may crack without warning. If you stick to the rules it will be easy to calculate the stresses using simple graphical methods and look up tables. You are unlikely to get cracking. The CG of the combination of engine prop and PSRU is where most of the loads are applied. With a 40 pound PSRU and a 20 pound prop the CG of the combination of engine prop and PSRU is right around the front end housing on a tractor air plane. That is the best place to put the main rubber motor mounts. Any number of rear mount arrangements can be used as the loads back there are very low. Less than 50 pounds.
To calculate the max loads the criteria I use is a six G pull up or a six G hard landing. This is the worst case scenario and probably exceeds the design loads of most aircraft. When you drop the air plane in there is a huge download at the engine/prop/PSRU of about 1800 pounds based on a six G de-acceleration rate. Consequently the rubber motor mounts should be located at or near this CG point. A minimum of two rubber mounts are required. This is because the prop generates about a 600 foot pound average reaction torque and one of the jobs the rubber mounts are called to perform is to keep the prop torque from twisting the engine out of the air plane. This CG rule is violated in Lycoming air cooled engine mounts because the mounting rubbers are widely separated vertically to take the cantilevered bending moment and the torque the engine/prop combination generates when a hard landing occurs. Unfortunately auto engines do not have this motor mount feature built in. If they did it would be too easy. Therefore we have to use some sort of Continental bed type mount.
The concept of load path is an interesting way to design any structure. For every action there is an equal and opposite reaction. When a load is applied to an air plane at some point, other than the CG, the load must get to the CG of the mass of the air plane through the structure of the air plane. This is called the load path. An example of this is the common practice of storing the fuel in the wings. The lift from the wing acts directly on the mass of the fuel. This reduces the bending loads on the spar and saves weight in the structure. This is also the reason the Questair Venture nose landing gear is attached to the engine and not the air plane. Every rear engine race car built for the last thirty years feeds the loads from the rear suspension directly into the engine and transmission bypassing the chassis. Since the engine is a major mass in any air plane design it pays to feed the loads generated near the engine as directly as possible into the mass of the engine.
There is a torque reaction from the prop that is reacted by the gears in the PSRU and thence into the PSRU housing through the motor mounts and then into the air plane proper.
If wide spread rubber mounts are located on the motor plate or bell housing between PSRU and engine, rather than the engine itself, that is the most direct load path. More about the wide spread later. This minimises the stress on the bolts and spacers that attach the PSRU to the engine.
This motor plate also directly takes the thrust load as well as roughly a 1200 ft pound peak torque pulse applied to the prop from the gear box. I am not exactly sure what the exact peak value as it is mitigated somewhat by the rubber bushings in Tracy Crook's torque isolator plate between engine and gear box and the rotational inertia of the engine and gear box. Peak engine torque is roughly twice average engine torque. Average torque at the prop is up to 600 ft pounds. It is roughly three times engine average torque because of the 2.85:1 gear box. Since the rubber bushing are about two feet apart they see only a 300 foot pound pulse. The weight of the engine reduces the load on one rubber bushing and increases the load on the other. That is one reason we have gone to a non symmetrical plate. If they were only one foot apart they would see up to the full 1200 foot pound pulse and would need to have roughly three times the travel for the same pulse absorption. Jeff Doddridge is building such a motor plate at relatively low cost. He does nice work. Scroll down on the front page of this web site to see where and how you can buy such a plate.
Some motor mounts were built awhile ago by sandwiching a 1/2 inch thick aluminium plate between the oil pan and the engine. Four rubber mounts were used spaced about 14 inches apart. The front rubber mounts took 90% of the vertical load so the rear rubber mounts went along for the ride. The plate was heavy because it had to span the length of the engine. It was also subjected to the hot oil draining down to the oil pan so the rubber mounts on the exhaust side would melt in some cases.
Two wide spread rubber bushings is the main reason the Questair Venture is one of the smoothest air planes flying. It was designed by Jim Griswold of Piper Malibu fame and Ed McDonald. BTW IMHO a major breakthrough in aircraft engine motor mounts.
At one time we looked into having a sand cast bell housing made but the weight and cost made it not practical. About the thinnest wall a sand cast foundry can cast is 3/16". The best thing would be a thin wall die cast bell housing with motor mounts adapting the PSRU to the Mazda rotary engine. That too looks like it will not happen in the near future so the third best thing is to have the motor plate incorporate the rubber bushings motor mounts.
A special high capacity oil pan is welded up with a bracket for a rear motor mount. There are many ways of building the rear mount but the loads are very low so just about anything will work. More about this later. Contact me if you need help on this subject.
The primary rotary vibration mode is torsional. There are no up and down and side to side or fore and aft vibration modes like piston engines. Therefore the Lycoming dynafocal type mount is of little use to us. In addition it adds torsional loads to the engine block the stock Mazda block was not designed to absorb. The prop torque travels through the gear box housing, into the engine and all the way back to the Lycoming type mounts.
The 2 rotor pulses twice per revolution. Average p-port torque is about 200 foot pounds. Peak pulse engine torque may be twice that. With a 2.85:1 psru the prop sees 5.56 pulses per prop revolution with an average torque of about 600 foot pounds with a peak torque of up to twice that depending on how well Tracy's torque isolator plate works. At 2700 RPM that is 15,000 pulses per minute or 250 pulses per second. Well above the 12 pulses per second natural resonant frequency of the Barry rubber bushings. Therefore transmissiblity is very low. Probably less than .1. A good thing.
As I mentioned above the best way to isolate the torsional vibration is spread the motor mounts far apart and use soft rubber bushings. This is precisely what Jim Griswald did when he designed the Questair Venture front motor mount which is what we copied for the rotary. In fact we used the same type, size and brand of low cost rubber bushings as Jim used. These are under ten dollars each while Lycoming rubber bushing can cost as much as $100 each. Namely Barry Controls part number 2002.
REAR MOTOR MOUNT
The rear motor mount cannot react torsional vibration loads because it is a single joint. There fore it is totally unaffected by torsional vibration of any kind including a missing rotor or out of balance prop. If your prop is noticeable out of balance you are going to land soon anyway. The mount sees very little vertical load as the main rubber mounts are under the CG of the engine/PSRU/prop combination. Maximum load is as little as 50 pounds static and 300 pounds in a 6 G pull up. The lightly loaded rear mount also see's a reaction from prop thrust as the thrust line is off set upward from the main motor mounts. This off set under the influence of thrust tends to cancel the down load on the mount. The off set distance is 160 mm or about 6.3 inches. Down thrust angle can be adjusted by shims.
LARGER OIL PANS WITH REAR MOTOR MOUNTS
A larger aluminium oil pan adds needed capacity to the oil supply which delays the onset of oil heating on take off. A stock light duty 250 HP Mazda car engine oil pan holds only 5 quarts while a typical 230 HP six cylinder aircraft engine will hold 12 quarts. Furthermore the Mazda RX8 engine has oil breathing problems due to the shallowness of the stock oil pan. It is in dire need of a deeper oil pan for aircraft use.
At 200 mph or about 300 feet per second the typical air plane requires about 160 net HP out of the prop. Figuring 80% prop efficiency that means engine HP is around 200.
One HP is 550 pounds of drag moving at 1 foot per second so the formula is:
HP = drag x velocity.
160 HP x 550 is 88,000 pound feet per second. At 300 feet per second or 200 MPH the drag must then be 88,000 divided by 300 or 293 pounds. At steady speed of the air plane drag always equals thrust. So therefore the thrust is also 293 pounds.
LOAD ON REAR MOTOR MOUNT
Now we have a moment around the main motor mounts of 293 pounds times 6.3 inches or 675 inch pounds. The rear motor mount is located about 13.25 inches behind the front motor mount. 675 inch pounds divided by 13.25 inches gives us the up load on the rear motor mount. 675/13.25 = 51 pounds. Since the static down load on the rear mount has been measured to be 60 pounds.... at 200 MPH the load on the rear motor mount is 60 minus 51 pounds or 9 pounds. Round it out to 10 even Steven.
.049 3/4 diameter 4130 tubing weighs .3668 pounds per foot so four feet of tubing adds 1.4672 pounds to the total weight of the Questair type motor mount. Now the engine no longer need transmit the prop torque all the way back to the rear of the engine and then into the firewall as is now done with Lycoming air cooled air craft engines. A good example of load path design thinking. The wings react the prop torque so go as directly to the wings as you can through the firewall. WW II bombers and fighter planes had the motor mounts bolted directly to the spars.
BENDING MOMENTS, TUBE AND FIREWALL LOADS:
You are dealing with a 1800 ft pound bending moment and a 1800 pound shear load during a six G pull-up manoeuvre at the firewall. It is assumed the engine/PSRU/prop weighs 300 pounds and the CG of the assembly is one foot from the firewall. A hard landing will also generate these kinds of loads on the firewall due to the mass of the engine system.
Some air plane designs, mainly the early Lancairs, had only one foot between upper and lower motor mount hard points on the firewall. Consequently the high 1800 pound tensile and compressive loads on the firewall due to the bending moment can be halved or only 900 pounds if the distance between the hard points on the fire wall are two feet instead of only one foot. The vertical shear loads on the firewall will remain the same. This is advantages even if it means adding new hard points to the firewall out near the fuselage skin.
As explained above it is also best to spread the Barry rubber mounts as far apart as possible. This minimises deflection in the rubber so softer rubber can be used which maximises its ability to absorb engine vibration and torque.
HOW TO CALCULATE THE LOADS IN THE MOTOR MOUNT TUBES.
This graphical is a lot easier to do than it looks. Complete steel tube fuselages have been designed using this technique. Once you catch on things become very simple so if it does not dawn on you right away don't worry. Try it again later. See the tube load drawing below. First make scale drawings of the motor mount. Top view and side view.
Here are all the loads worked out for an RV4.
CALCULATING THE STRESS IN THE TUBES AND CHECKING FOR BUCKLING.
Written by Bill Freeman:
Once you know the force that the tube will have to support (along its axis), you can calculate the stress in the tube (which is the design limit) from:
stress = force/area where the area is the
cross sectional area of the tube. So calc the area for the outside diameter circle
area = PI * r * r or 3.1416 * outside radius squared
then subtract the area of the inside diameter circle, same equation, but use the inner radius.
You want to keep the tube stress below the yield of normalized 4130 chrome-moly steel which is about 66,000 psi. I would suggest a 10 G landing as the design point for the engine mount. Paul used six G's above but you can use ten G's if you want the motor mount to survive a minor crash or you will be doing a lot of violent aerobatics.
If the stress is too high, use a thicker wall or larger diameter tube to increase the area which will reduce the stress. This is all there is to it for a tube in tension. NOT SO SIMPLE for a tube in compression since it will possibly fail due to buckling.
Take a plastic soda straw and pull on it as hard as you can with your two hands. Can't break it, can you? Now push on the ends with your thumbs - pretty easy to crumple it up, huh? This is buckling, which is a lateral (sideways) instability in a long skinny member under compression. Long skinny structural members can fail in buckling WAY below the yield stress tensile load.
The simplest thing to do about buckling is to use a relatively large diameter tube to give a stronger lateral stiffness. You can go look in std eng tables, or if there is enough interest I'll do a short ' how to' on basic buckling calcs. But, if you use a relatively large diameter tube and decrease the wall thickness to keep the cross sectional area in bounds i.e. not too heavy, you will be more buckle resistant. Also, a center brace, even with a *very* small brace tube, will dramatically increase the buckling resistance, because it cuts the effective length of the tube in half. Remember, buckling only happens in tubes in compression.
Written by Paul Lamar:
BTW the bible for this sort of thing is Analysis & Design of Flight Vehicle Structures by E.F. Bruhn et. al. Tri State Offset Company 817 Main Street Cincinnati Ohio 45202 Published in January 1965.
I am sure Dick Van G keeps a copy along side his drawing table or computer. I paid $14.75 for my copy 30 years ago but I'll bet it is well over $100 now. Well worth it however. Plenty of buckling tables for both 4130 and aluminium tubing both round and streamline. Here is a good example. This is for 4130 tubing with a .049 wall thickness. About right for motor mount tubes. C2 means the tube is welded on both ends. Therefore it is more stable in compression and less likely to buckle. C1 means the tube has rod ends or ball joints on both ends and more likely to buckle at a give compressive load. The weight of the tubing is also shown.
There is also a very good chapter on aircraft loads with lots of worked examples. The book reflects the distillation of 65 years of experience with aircraft structures.
FABRICATING YOUR MOTOR MOUNT USING AND EXISTING MOTOR MOUNT AS A GUIDE.
Locate the motor plate or in the proper position relative to the fire wall. Remove any tubes that need to be removed, grind the cut areas smooth, tack weld on the new tubes. Remove the mount from the fire wall and finish welding. Ideally send it out for normalising and re-heat treat but that is not entirely required. Then re-ream the fire wall bolt holes.
One can also build a motor mount welding fixture out of large steel tubing or wood and ply wood.
STATIC TESTING YOUR MOTOR MOUNT DESIGN.
Static test your motor mount by using the air plane as a lever to pick up the front of a front engine car. The average front engine car weighs about 3000 to 3600 pounds so half to two thirds of that is 1500 to 2000 pounds. Your static weight target load for the complete motor mount is 1800 pounds. It is hard to come up with that much dead weight other than a car. You would have to buy 36... 50 pound bags of cement and then find enough room to hang those bags off your motor mount so use your car.
Don't worry about the download on the horizontal stabiliser. If it won't take about 700 pounds down force it won't survive a six G pull out either. Apply the force mainly to the stabiliser spar near the point where it intersects the fuselage. You can distribute some load along the spar if you need help. BTW this in effect is static load testing the entire fuselage and landing gear. Don't try to static load test the motor mount to ten G landing loads. You will probably fail something in the fuselage or the stabiliser.
If you decide to design your own motor mount after reading all this I admire your sense of adventure. The rotary engine generates a lot of heat both in the exhaust and in the oil. I suggest you keep the rubber bushing as far from the heat as you are able. Some have used thick metal plates sandwiched between engine and oil pan. This is not a wise thing to do as number one it is a second large source for oil leaks. Number two it conducts a lot of oil heat into the rubber bushings. Number three it is hard to get the rubber bushing away from the exhaust heat. Number four it is hard to design a motor mount where all the tubes are in either compression or tension and to clear the exhaust manifold or under-the-cowl muffler if used. As you should know after reading all this article motor mount tubes subject to bending loads leads to metal fatigue and cracks. Yes it has been done. Notably certain Cessna models use a similar design but these have had AD's after years of use to beef them up because of cracks. I know as I have a C182.