E
ekimneirbo
Like most of the people who read or discuss this thread, I am not an engineer but have prudently tried to separate facts, fiction, opinions, and myths into the proper catagories when it comes to adapting an automotive engine for airplane propulsion.
One thing to consider is that depending on the engine of choice, the facts may vary considerably. Con-verting a volkswaggen or a corvair requires different considerations than converting an LSx Chevy V8. This topic will be my thoughts on how it should be done in relation to using a Chevy LS in a High Wing, reasonably STOL type of aircraft ..........not a low wing high speed rocket where lots of hp may be desirable.
A water cooled LS engine is going to weigh more than an air cooled Lyc or Continental in the O-320/O360 range. Nothing you do will get you on an equal basis with them. That being said, the conventional wisdom these days is that builders should use a reduction drive so they can use all of the horsepower that an auto engine can make at elevated rpms.
My opinion is that there are many reasons not to use a reduction drive, and very few reasons to use one. They can be used successfully but I don't think it is the best way for a high wing STOL hopefully airplane. A reduction drive adds weight, complexity, and cost. It requires you to run your engine at an elevated rpm which places higher stress loads on engine components. This in turn shortens the engine life (TBO) of the combination. If you are running your engine at twice the rpm
level, the engine will complete the same number of cycles in half the time.
Weight is weight in an airplane. If you want quick takeoff/short landing you must try to stay as light as possible. You can reposition weight, you can counterbalance weight, but you cannot make it go away. If you want to use an automotive engine then you have already made your first compromise on weight, don't compound it with a redrive. A Geschwender will add about 80-90 lbs
to the appx 400 lbs of an LS.
There is no law that says that because an engine can produce a large amount of horsepower, you need to run the engine at that level of power. A Continental O-300 can produce higher power by being run at 3200 rpms and geared down (aka GO-300 Cessna 175), but you see where that went.
Many people are enamored by the fact that an engine can produce 400 hp. Well, I have no need for 400 hp. I only want 200-250 hp. You have to consider that the additional hp will also require you to burn more fuel to produce the hp, so you will need more fuel capacity......hence, even more weight.
Further, you have to realize that producing that much hp is also going to put a strain on the aircraft itself when you suddenly go from engine idle to full power on that botched landing. Is it possible that the sudden application of that much torque in a light aircraft might actually cause the airplane itself to begin to rotate? I don't know, but I do know that 400 hp will be of no use to me.
Another reason not to use a reduction drive is that it will cost quite a lot of money. The belted drives usually are lighter but most aren't designed for the higher hp (400) V8 setups. That leaves you with a gear or chain option for the redrive. You are probably looking at $6k or more for one of these setups, and appx 85lbs right on the nose of your airplane.
NASA did a study on the feasibility of an LS engine in an airplane and concluded that a direct drive conversion was both desirable and possible.
Reasons why you can't use direct drive on an automotive engine.
1. The engine wasn't designed for aircraft use.
OK, I can't argue that, but that doesn't mean it won't work. An engine doesn't know what its designed for, it just produces power. Its up to me to find a way to make that power useful.
2. The crankshaft will break.
If you bolt a propellor directly to a crankshaft, especially if you use an extension/adapter and do nothing else to support the propellors gyrations, it is completely possible to break a crankshaft. There are examples of people who have done so and not had any problems, but every installation is going to be different. Someone using a wooden or composite prop with a short extension will not be as likely to break a prop than someone with a heavy metal prop with lots of resonance bolted to a long extension. The devil is in the details.
To me, a properly executed direct drive would not be bolted directly to the rear crankshaft flange on an LS engine for more than one reason. First, the thrust bearing in an LS engine is in the middle of the crankshaft, not at the rear of the engine.
Secondly, although some engines rear mounted thrust bearings may be able to withstand the thrust and gyroscopic loads, some may not. I do not think its good to directly bolt a propellor on, because it gives no leeway for misalignment and applys all the loads directly to the crank like a lever.
My idea would be to isolate the propellor loads from the crankshaft as much as possible. There will always be a transfer of loads to the crank, but you can minimize their ability to cause harm.
I would make an adapter to bolt to the crankshaft. This adapter would have splines in the center.
A shaft (aftermarket car axle) would insert into the splines on the adapter. This would allow minor misalignment and relieve the crankshaft rear flange from gyroscopic loads, and most thrust loading. In other words, the engine would be operating on its own without much input from the propellor as far as thrust and gyrocopic loading are concerned. This would alleviate most concerns about crankshafts breaking.......in my opinion.
The shaft (axle) would have the propellor mounted to its large end. Around the axle would be a housing/adapter that would bolt to the rear of the engine block where a bellhousing was originally mounted. There would be a pair of bearing surfaces in the center of the housing that would hold 2 thrust bearings. These would be the shell type of bearings used as thrust bearings in an automotive engine...........NOT roller or ball bearings. Roller or ball may work for someone, but I don't have as much confidence in them once they begin to fail. Getting back to the housing and shaft explanation.
Picture if you will, The driven splined (axle) shaft is between the crankshaft adapter and the propellor. Around the axle shaft is another shaft which is hollow in the middle so the axle can pass thru it and connect the prop and crankshaft. This hollow shaft will have a large flange on one end .
When it is positioned around the axle, the large flange will be against the backside od the large flange on the axle. The propellor will be bolted to the axle flange with the bolts passing thru so that they also secure this hollow shaft to the assembly. In effect, the large flange of the axle would be sandwiched between the propellor and the flange on the front of the hollow shaft.
At this point, the engine would be turning the axle and driving the propellor. The hollow shaft would be be secured to the rear of the axle shaft flange and caused to rotate with the axle by the bolts holding the assembly together. The purpose of this hollow shaft is to absorb the thrust and gyroscopic loads of the prop, while allowing the axle to deal only with resonant pulsations between the engine and the prop. How does the hollow shaft do this? Two bearing surfaces are incorporated into the hollow shaft on its exterior diameter. One surface near the front, and one near the rear. There will also be some thrust flanges on this exterior surface of the hollow shaft.
What happens is that two common automotive (diesel ???) type thrust bearings are mounted around the shaft. This will provide support both rotationally and longitudenally (?) . It will resemble the support that a Lycoming provides and probably be stronger, while having a second shaft (axle) to deal with the power pulses. Naturally a housing would have to be machined to hold the components.
In my vision, for all intents and purposes the engine is now free to loaf along at 2200/2700 rpms making 200/250 hp and care less that its driving a propellor instead of a car. Sure, its not quite that simple, but I think it should work just fine........in my opinion.
Resonance wise, I'd use an aftermarket Rattler damper/balancer/absorber/whatever to deal with any issues and use a Dynavibe or something similar to fine tune the whole thing.
OK now, before all the hot shoes start telling me about the non VAR crankshaft, remember that I plan to use either a wood or composite prop which will also aid in alleviating resonance. I plan to attach a site that will be well worth reading for anyone serious about building a conversion. One thing mentioned at this site is that pre VAR crankshafts don't seem to suffer problems any more than the later model aircraft VAR crankshafts.
I have been chastised before about the fact that automotive crankshafts do not contain the same materials as aero cranks or that the same heat treatment is not used. These articles will show that it is available if someone wishes to buy a billet crank and go that route, and its not indigenous to the aircraft industry. To me, the real point is that aircraft engines may need this type of process because of the tremendous cyclic twisting caused by huge 5 inch pistons and the roughness of having only four cylinders on a 180 degree crankshaft. A smoother running V8 with more but smaller pulses swinging lighter pistons probably doesn't need it. Then couple that with the fact that you have a shaft acting as a spring between the prop and engine, and you have again lessened the effects on the crankshaft.
What VAR (vacuum arc remelt) means is that a piece of metal is placed in a vacuum furnace and heated up so that impurities in the metal will be withdrawn. It does make a stronger crankshaft, but I think it is only needed because of the mass in an aircraft engine. Think about your worse case scenario. It would be huge masses (pistons) acting at the ends of levers (crank throws) and rotating with large cyclic (power pulses). Your best scenario is lighter pistons with more/smaller pulses that ends up generating equal/better power at the same rpms.
Crankshaft Torsional Absorbers, by EPI Inc.
Crankshaft Design, Materials, Loads and Manufacturing, by EPI Inc.
Take the time to read these attachments whether you agree with me or not.............
One thing to consider is that depending on the engine of choice, the facts may vary considerably. Con-verting a volkswaggen or a corvair requires different considerations than converting an LSx Chevy V8. This topic will be my thoughts on how it should be done in relation to using a Chevy LS in a High Wing, reasonably STOL type of aircraft ..........not a low wing high speed rocket where lots of hp may be desirable.
A water cooled LS engine is going to weigh more than an air cooled Lyc or Continental in the O-320/O360 range. Nothing you do will get you on an equal basis with them. That being said, the conventional wisdom these days is that builders should use a reduction drive so they can use all of the horsepower that an auto engine can make at elevated rpms.
My opinion is that there are many reasons not to use a reduction drive, and very few reasons to use one. They can be used successfully but I don't think it is the best way for a high wing STOL hopefully airplane. A reduction drive adds weight, complexity, and cost. It requires you to run your engine at an elevated rpm which places higher stress loads on engine components. This in turn shortens the engine life (TBO) of the combination. If you are running your engine at twice the rpm
level, the engine will complete the same number of cycles in half the time.
Weight is weight in an airplane. If you want quick takeoff/short landing you must try to stay as light as possible. You can reposition weight, you can counterbalance weight, but you cannot make it go away. If you want to use an automotive engine then you have already made your first compromise on weight, don't compound it with a redrive. A Geschwender will add about 80-90 lbs
to the appx 400 lbs of an LS.
There is no law that says that because an engine can produce a large amount of horsepower, you need to run the engine at that level of power. A Continental O-300 can produce higher power by being run at 3200 rpms and geared down (aka GO-300 Cessna 175), but you see where that went.
Many people are enamored by the fact that an engine can produce 400 hp. Well, I have no need for 400 hp. I only want 200-250 hp. You have to consider that the additional hp will also require you to burn more fuel to produce the hp, so you will need more fuel capacity......hence, even more weight.
Further, you have to realize that producing that much hp is also going to put a strain on the aircraft itself when you suddenly go from engine idle to full power on that botched landing. Is it possible that the sudden application of that much torque in a light aircraft might actually cause the airplane itself to begin to rotate? I don't know, but I do know that 400 hp will be of no use to me.
Another reason not to use a reduction drive is that it will cost quite a lot of money. The belted drives usually are lighter but most aren't designed for the higher hp (400) V8 setups. That leaves you with a gear or chain option for the redrive. You are probably looking at $6k or more for one of these setups, and appx 85lbs right on the nose of your airplane.
NASA did a study on the feasibility of an LS engine in an airplane and concluded that a direct drive conversion was both desirable and possible.
Reasons why you can't use direct drive on an automotive engine.
1. The engine wasn't designed for aircraft use.
OK, I can't argue that, but that doesn't mean it won't work. An engine doesn't know what its designed for, it just produces power. Its up to me to find a way to make that power useful.
2. The crankshaft will break.
If you bolt a propellor directly to a crankshaft, especially if you use an extension/adapter and do nothing else to support the propellors gyrations, it is completely possible to break a crankshaft. There are examples of people who have done so and not had any problems, but every installation is going to be different. Someone using a wooden or composite prop with a short extension will not be as likely to break a prop than someone with a heavy metal prop with lots of resonance bolted to a long extension. The devil is in the details.
To me, a properly executed direct drive would not be bolted directly to the rear crankshaft flange on an LS engine for more than one reason. First, the thrust bearing in an LS engine is in the middle of the crankshaft, not at the rear of the engine.
Secondly, although some engines rear mounted thrust bearings may be able to withstand the thrust and gyroscopic loads, some may not. I do not think its good to directly bolt a propellor on, because it gives no leeway for misalignment and applys all the loads directly to the crank like a lever.
My idea would be to isolate the propellor loads from the crankshaft as much as possible. There will always be a transfer of loads to the crank, but you can minimize their ability to cause harm.
I would make an adapter to bolt to the crankshaft. This adapter would have splines in the center.
A shaft (aftermarket car axle) would insert into the splines on the adapter. This would allow minor misalignment and relieve the crankshaft rear flange from gyroscopic loads, and most thrust loading. In other words, the engine would be operating on its own without much input from the propellor as far as thrust and gyrocopic loading are concerned. This would alleviate most concerns about crankshafts breaking.......in my opinion.
The shaft (axle) would have the propellor mounted to its large end. Around the axle would be a housing/adapter that would bolt to the rear of the engine block where a bellhousing was originally mounted. There would be a pair of bearing surfaces in the center of the housing that would hold 2 thrust bearings. These would be the shell type of bearings used as thrust bearings in an automotive engine...........NOT roller or ball bearings. Roller or ball may work for someone, but I don't have as much confidence in them once they begin to fail. Getting back to the housing and shaft explanation.
Picture if you will, The driven splined (axle) shaft is between the crankshaft adapter and the propellor. Around the axle shaft is another shaft which is hollow in the middle so the axle can pass thru it and connect the prop and crankshaft. This hollow shaft will have a large flange on one end .
When it is positioned around the axle, the large flange will be against the backside od the large flange on the axle. The propellor will be bolted to the axle flange with the bolts passing thru so that they also secure this hollow shaft to the assembly. In effect, the large flange of the axle would be sandwiched between the propellor and the flange on the front of the hollow shaft.
At this point, the engine would be turning the axle and driving the propellor. The hollow shaft would be be secured to the rear of the axle shaft flange and caused to rotate with the axle by the bolts holding the assembly together. The purpose of this hollow shaft is to absorb the thrust and gyroscopic loads of the prop, while allowing the axle to deal only with resonant pulsations between the engine and the prop. How does the hollow shaft do this? Two bearing surfaces are incorporated into the hollow shaft on its exterior diameter. One surface near the front, and one near the rear. There will also be some thrust flanges on this exterior surface of the hollow shaft.
What happens is that two common automotive (diesel ???) type thrust bearings are mounted around the shaft. This will provide support both rotationally and longitudenally (?) . It will resemble the support that a Lycoming provides and probably be stronger, while having a second shaft (axle) to deal with the power pulses. Naturally a housing would have to be machined to hold the components.
In my vision, for all intents and purposes the engine is now free to loaf along at 2200/2700 rpms making 200/250 hp and care less that its driving a propellor instead of a car. Sure, its not quite that simple, but I think it should work just fine........in my opinion.
Resonance wise, I'd use an aftermarket Rattler damper/balancer/absorber/whatever to deal with any issues and use a Dynavibe or something similar to fine tune the whole thing.
OK now, before all the hot shoes start telling me about the non VAR crankshaft, remember that I plan to use either a wood or composite prop which will also aid in alleviating resonance. I plan to attach a site that will be well worth reading for anyone serious about building a conversion. One thing mentioned at this site is that pre VAR crankshafts don't seem to suffer problems any more than the later model aircraft VAR crankshafts.
I have been chastised before about the fact that automotive crankshafts do not contain the same materials as aero cranks or that the same heat treatment is not used. These articles will show that it is available if someone wishes to buy a billet crank and go that route, and its not indigenous to the aircraft industry. To me, the real point is that aircraft engines may need this type of process because of the tremendous cyclic twisting caused by huge 5 inch pistons and the roughness of having only four cylinders on a 180 degree crankshaft. A smoother running V8 with more but smaller pulses swinging lighter pistons probably doesn't need it. Then couple that with the fact that you have a shaft acting as a spring between the prop and engine, and you have again lessened the effects on the crankshaft.
What VAR (vacuum arc remelt) means is that a piece of metal is placed in a vacuum furnace and heated up so that impurities in the metal will be withdrawn. It does make a stronger crankshaft, but I think it is only needed because of the mass in an aircraft engine. Think about your worse case scenario. It would be huge masses (pistons) acting at the ends of levers (crank throws) and rotating with large cyclic (power pulses). Your best scenario is lighter pistons with more/smaller pulses that ends up generating equal/better power at the same rpms.
Crankshaft Torsional Absorbers, by EPI Inc.
Crankshaft Design, Materials, Loads and Manufacturing, by EPI Inc.
Take the time to read these attachments whether you agree with me or not.............
