You make some excellent arguments but I think maybe some of what I said was misconstrued or looked at in the wrong light. I'll respond to them individually.
Your idea and reasoning behind it are good and in line with my own thoughts on the process, except that in my case I still want the baseline amount of power and ideally, I'd like to run the engine upside down in order for it to fit within a fairly normal cowl. My own reasoning for this is based on the need for a simpler system and the fact that in my opinion, many of the manufacturers out there don't have sufficient engineering behind their products to cover all the bases of the redrive's design requirements. Some of these drives you can just glance at and see that they would be unable to sufficiently cope with the flight environment - not for any significant length of time anyway.
The LS series is a good choice in that it is a reliable configuration with a wide after-market for a variety of improvements and configurations. But making those choices, especially for direct drive, requires a specific bit of knowledge and experience since not all aspects of the engine relate directly to this application.
This seems to be one of those situations where everyone follows the same lead and assumes it must be the only/best way.....
Depends on what you're trying to achieve. The bottom line of the choice is simply that if you're going to put 500 pounds of engine on your airplane, ideally you'd like to see more than 150 to 200 horsepower out of it. With that power-to-weight you might as well go out and get a moderate timed Lyc or Continental since those engines will deliver a better ratio. True, the aircraft engines might set you back a bit more but in the end you'll know that you have an engine specifically designed for flying, not something that had top be modified to work in the particular situation.
As you say, it depends on what you are trying to achieve. I would like to achieve 250 hp at 2700 rpms. I would be satisfied if I can attain 210 hp or more. I believe that to be easily attainable. I don't see the need to produce
400 hp because it will be unusable most of the time (in a 4 seat high wing with 160 mph max speed 2500 lb gross) A Lancair or some very fast airplane might make use of this, but when you throw all that horsepower at the airframe to save you from a situation,I don't think its necessary or advisable.
The reason I want to use something other than a Lyc is the high cost of parts and maintainance,thermal shock concerns, rusty cam problems,and disbelief that all those log books for used engines are accurate.
If you're like one other member of this board who is installing the V-8 in a vintage biplane design, then the amount of power is not that critical since the old engines of the orignal airplanes had relatively low power ratios to start with. As such, the V-8 will come close to providing the airplane with the proper amount of power and mass balance. But if you're looking for at least some level of efficiency, then the redrive will allow you to operate the engine somewhat higher on the power and torque curves, and within more normal operating envelopes so that you can still use an off the shelf prop.
I'm not sure what you mean by "normal operating range" and "some level of efficiency" but just because an engine is capable of operating at a higher rpm level doesn't mean it should be, or that it is inefficient when operated at a less demanding rpm.My S10 with a 4.3 V6 turns about 1800 rpm when I am driving 65 mph. It virtually never operates at 4000 rpm for other than a momentary burst into highway traffic. So to me operating at 2200 wouldn't be much different as a normal operating range.
To produce horsepower requires burning a specific amount of fuel. If you run an engine at a higher rpm range and produce more horsepower you will burn more fuel. My son had an LT1 Camaro/6 speed. In 6th gear (2 overdrives) it would go about 70/75 mph at 1500 rpm and get high 20s mpg (27/28).
1. You gain the ability to run the engine at a higher RPM and create more HP
for takeoff. You will also need more HP to carry the weight.
At this power level, the weight of the redrive is relatively minor as compared to the performance you might gain. Furthermore, the power gain is not just for takeoff - while you yourself may want to cruise at 100 hp, the more common applications of this power and weight class would barely stay in the air with the throttle set that low.
As far as the weight is concerned, the 70 pounds or so that the redrive might cost you is fairly negligible where airplane performance is concerned, especially given the torque and power benefit its use will generate.
The weight of a redrive is very consequential in many airplane designs, especially when placed way out on the nose of the airplane. In many instances that additional weight may be the difference in whether a conversion is even possible. On the
2. You can run the engine at a somewhat higher RPM level and increase cruising speed. This along with the added weight will reduce the distance you can travel before landing, and may be greater than the airplanes maximum speed. You'll get to the gas pump quicker....
Nothing says you have to run with the throttle full open. But the increased power provides the airframe with much more flexibility. Also, if you encounter a situation where you might suddenly need the power, it's certainly nice if it's there in the first place. In a general sense, limiting your available power when you don't need to is a compromise in safety and efficiency. In the end, you'll pay a much greater weight-based penalty in installing this heavy engine configuration without taking advantage of the performance it is capable of providing you.
Doesn't matter whether you are running wide open or not, if you are producing more horsepower you are burning more fuel.I don't feel that you are "limiting" your available power,only deciding what you need and then building to that need.Lycoming proved that an O320 would produce more power at a higher rpm by gearing the propellor down, but they went back to direct drive. So, while its possible to produce more HP, its it needed?
3. You increase the cost and complexity of building the airplane and reduce the TBO.
Huh? If your use of a redrive reduces your TBO and so significantly increases the cost of the airplane, you've chosen the wrong product or approach. But engines and airframes need to be matched. This is not something to be simply eyeballed.
An engine has only so many cycles in its lifetime before it has to be overhauled. If an engine has a 2000 hr TBO and runs at an average of 2500 rpms, it will complete 300,000,000 reveloutions.At 4000 rpm it will only take the engine 1250 hours to comple that many revolutions....so the lifespan of the engine has been effectively shortened even though it has completed the same number of revolutions. Also, the stresses placed upon the crankshaft,con rods, bearings, pistons, has more than doubled. Couple this with higher operating temps within the cylinder and their possible effect on the exhaust valve and spark plug and you have lessened your safety factor
even further. You are right that running an engine at less than peak hp is limiting it, but I prefer to look at it as increasing the safety factor by insuring that the engine isn't stressed. Its really just the way either of us wish to view the situation.
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4. You may have to compensate to get your weight and balance to offset the added weight. You lose cargo capacity, and gliding distance, increase stall speed.
To a point, true. But again, the added weight of the redrive as compared to the airframe's operational or gross weight is fairly small. All you'll most likely need to do is shift the battery or configure the cooling system other than in front of the firewall.
Yes, I agree that other items can often be shifted to compensate for the added weight in relation to your Center of Gravity....but it is still weight that can't be compensated for in the overall weight of the airplane.Weight is weight.......
My opinion (and its just that...an opinion) is that trying to mimic an airplane engine is the best way to go. If 160/200 hp is sufficient to fly the airplane, then what good is having 300 hp?
True, but if all you need is that small amount of power, why would you install a 450 pound engine on the nose? Talk about inefficiency and weight penalty.
Despite claims otherwise, most airplane owners fly at 75% power or higher. If all you need is 200 hp max, there certainly are more efficient choices.
I think the term "75% power is kind of arbitrary. RPM wise you may reduce your rpm from 2700 to 2200, but the horsepower being produced is not exactly linear. As rpm increases hp climbs more radically than at lower rpms.
So I think when you cut back to 75% you are refering to rpms not HP.
If I'm correct in that assumption, cutting back a 180 hp Lycoming to 75%
would not mean you are operating at 135hp, but at something less than that.
The 100 HP statement may have been a little low, but not falling from the sky unreasonable.....at least in the type of plane I'm interested in.
How do you get there? My opinion again...cubic inches. Everyone wants to purchase a ready made factory engine and plug and play. A 427 cu in LS7 doesn't weigh anymore than a 346 cu in LS1...but it sure costs a lot more.
Excellent point and quite true. But cubes is not all there's to it. You can also gain tremendous benefit by employing boost (turbocharging, not super charging - lighter), improving the timing through a more optimal cam, and of course, better breathing (intake and exhaust design).
You are correct that there are other ways, but what I'm looking for is simplicity and reliability...not complexity and additional weight. But it is a fact that those are other ways to do it.
Everyone is throwing up their hands and saying the engine is not designed to deal with the thrust, the harmonics, or the prop twisting and flexing.
These are not opinions - these are simply the base physics of the situation. Ignore them at your own peril. I'd equate this to something like making light of designing your wing structure. You do it wrong and things might go crunch at the worst possible time. And for an airplane, the worst possible time is each time you get more than ten feet off the ground.
Every propellor powered airplane deals with these factors. Do you have any hard evidence that shows that the engines will not perform satisfactorily?
If not, then it is an opinion....just like mine. If the crankshafts were totally unsuited to this, then virtually every one of them would fail. Since there are examples out there flying successfully I must assume (don't say it) that they will work if done properly. The question is"what is "prop erly" (pun intended).
I just think that well intended people scare people away with worries about unquantified unknowns..........
OK, how many of you have ever looked at or measured a crankshaft in a Lycoming? The crank has a 2.375 diameter crankshaft main journal and they use it on O320/O360/O540......even a 540.
Comparing an aircraft crank to a mass produced automotive product is downright foolish. The manufacturing processes for aircraft products are substantially different than for automotive ones, resulting is significantly different structural properties, especially when considering issues of fatigue, notch sensitivity, and yes, even strength. Just because it's steel and measures about the same is no comparison.
Thrust face...the Chevy won't handle the thrust. Well there is where the "rub" comes in. The LS1/LS7 engine has the thrust face in the middle of the crank instead of at the rear.
There's much more than thrust to worry about, and certainly more than just the crank. First off, the loads imposed by the prop are not only axial (thrust) but also radial (out of plane shear) and moment based (gyroscopic). These can be quite substantial, especially the latter, imposing loads on the structure possibly orders of magnitude higher than anything an automotive product is designed to handle. Keep in mind that the automotive crank only sees pure torsion, nothing else. Introduce even a small variation to that and all the original design assumptions go out the window.
If both crankshafts are made from the same composition of metal and the automotive one is thicker, it would seem that it can handle the same loading even if an engineer didn't say so........
And of course the loads have to go somewhere. The next area then to investigate is that associated with the crankcase since that will be the primary stress transfer medium that will transfer the flight loads into the airframe. Now you need to consider the strength and durability of the block, which if it's cast aluminum, will be very poor in fatigue, will be notch sensitive (sensitive to stresses in areas of abrupt geometry change or left over machine marks created during manufacturing), and will have generally low structural properties to start with. Again, keep in mind that the basic engine is designed to output only torque - introduce flight loads and you're out of the original design envelope. And yes, it is very significant and not to be taken lightly.
The case of an LS1/LS7 is much stronger than previous Chevy alum blocks and has cross bolted mains. The LS7 has steel main caps while the others have powdered metal. To my knowledge no one has broken any engine cases but I'm not going to say its impossible....but not probable. As far as the flight loads go, I would think that all the torture engines go through in offroad racing and airboat racing would demonstrate their ruggedness.
The thrust of a Lycoming is applied directly against the face of the aluminum engine case. Look at one and you will see that its not an extremely large contact area....and its directly against the aluminum....hmmmm. So while I don't know if the Chevy will handle the thrust with no problem, I've never seen any factual information showing it can't.
The Lyc case is designed to carry flight loads associated with the props it is certified for. The Chevy crankcase is designed to carry a stationary bell housing and a drive system that transfers no axial and/or no bending moments into the crank. The trust bearings in the automotive engine are there to only carry positioning loads that the engine sees as a result of expansion due to heating - that's why the thrust bearings are in the middle of the crank.
I don't know why they put it in the middle, but most thrust bearing failures in automobiles are caused by ballooning of the torque converter which forces the crank into the thrust bearing further than its mechanical limit.
The Lycoming does have an elongated snout which contains the equivalent of two rod bearing surfaces with a space in between.This purportedly provides the strength to resist the prop side loading. It does a good job. It would seem that if a small housing were made that could support a set of those bearings and an oil supply and return routed to it, then one would have a virtual equivalent of the Lycoming . A short shaft would have to be made to bolt to the Chevy and protrude thru the housing for mounting the prop.
It's not that simple but in essence you're right. The key to this is to make sure that you isolate the prop loads from the engine and an externally supported shaft in a proper housing is pretty much the only solution.
Lycoming doesn't isolate prop loads from the engine...........
Oh, I almost forgot...the crank will break. Well maybe it will and maybe it won't. They break off airplane engines so some will probably break off some auto engine conversions...if they aren't done properly.
The only aircraft crank failures that I know of happened either due to ground impact or due to a manufacturing flaw. If you mount a prop directly to an automotive crank you can almost bank on it failing.
And you know this for a fact..........how?
Both types of crankshafts can be bought made from the same materials, so I don't see a problem there.
No! Different processes of manufacturing, heat treat, machining, and finishing.
Do you know what the differences in manufacturing are? I think that you are making an assumption without intimate knowledge of the available products.
What manufacturing step makes a crankshaft resist torsional twisting? Can you make a blanket statement that all crankshafts produced by aftermarket manufacturers are unworthy of consideration? I have a maching background and have investigated some of the newer processes for crank manufacture. I was pretty impressed. So the crankshafts and metalurgy of Lycoming crankshafts designed and produced 60 years ago are better than what any automotive crank maker can do today.............. I don't agree at all.
As for harmonics...some airplane engines are not supposed to be run in certain rpm ranges...or they may break.
This is not solely an engine issue - the engines are specifically tested and certified for particular props. It is that combination of engine and prop that sets the green arc. Outside of that arc the combination can encounter harmonics that can be damaging. Considering that a relatively wide range of harmonics off of the primary natural frequency can damage the system, this is also something that needs to be addressed in any new development, either through testing or through proper isolation design.
Also you can put a harmonic damper on an auto engine. Everyone calls them "balancers" but the truth is that they are dampers that are designed to absorb harmonics.
Inapplicable - these small harmonic dampers are designed to only take care of the engine's internal vibrations caused by the physical characteristics of the engine's rotating components - these have absolutely nothing to do and no ability to deal with any external components you install.
Isn't the purpose of the 5th order weights that Continental places inside their
engines to combat internal harmonics? And riding or reducing these harmonics is what prevents you from destroying your engine.When an engine destroys itself harmonically its because a vibration has been allowed to reach a resonance that built until it became catastropic. At least thats my understanding. A dampner continually absorbs harmonic vibration and prevents it from building.
Ever see one for an airplane engine? I haven't, but there probably is one somewhere.
Airplane engines are designed so that they don't need them, although there is some evidence that their use might be beneficial when using a wood prop. I have seen one made for the four cylinder Lycs that incorporates into the ring gear.
Again I'll refer you to the counterweights that Continental installs...same principal.
I've rationalized a lot of things here because its what I want to make happen. If you disagree or have other ideas, feel free to state them. I'm always willing to learn...like I said, this is just my opinion.
You're on the right track but I'd guess that you'll need to take your considerations deeper than just this simplistic analysis - after all, your life depends on it.
We seem to be at opposite ends of the spectrum on this subject, but I enjoy the give and take .........
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