Let's say one was building an airplane and needed an engine in the 65 peak hp range. The airplane would be a slow single seater with the 1835 being a candidate for an engine. Considering the tiny weight difference between the 1835 and 2180, would there be any advantage to using a 2180 propped to peak at a lower rpm that produced around 65 HP? I would assume that you would get higher torque and lower temperatures than the 1835 but that's just a guess. Also would the longer prop be more efficient and in turn actually require less HP to produce the same results?
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Yes, to all that. ;-)
Lycs & Conts are near the far end of what you're describing; direct drive radials farther still. Cubic inches compensate for lower rpm. And slow planes need large diameter props (which force lower rpm) to increase mass flow at low speeds (think helicopter, for the extreme version of high mass flow at low speed). Accelerating a large quantity of air (large prop disc area) by a small speed increase is more efficient than accelerating a small quantity of air (small prop disc area) by a lot of speed increase.
Only thing to watch is making sure that the max pressure at high MAP & low rpm is within the safe zone of that particular engine (not likely an issue with a VW based engine, but worth discussing with the mfgr). Most likely thing would be slightly reduced engine life.
Lycs & Conts are near the far end of what you're describing; direct drive radials farther still. Cubic inches compensate for lower rpm. And slow planes need large diameter props (which force lower rpm) to increase mass flow at low speeds (think helicopter, for the extreme version of high mass flow at low speed). Accelerating a large quantity of air (large prop disc area) by a small speed increase is more efficient than accelerating a small quantity of air (small prop disc area) by a lot of speed increase.
Only thing to watch is making sure that the max pressure at high MAP & low rpm is within the safe zone of that particular engine (not likely an issue with a VW based engine, but worth discussing with the mfgr). Most likely thing would be slightly reduced engine life.
Ignition timing would need to be reviewed and possibly retarded by a few degrees (will depend on fuel & compression ratio plus other factors).
The difficulty that I have found for optimising an engine at low rpm (3,000-3,500) is cam lobe profiles. There are literally thousands of profiles and specs out there for the engine builders & tuners, but all with durations and lifts larger than OEM. Try finding one for a hopped-up low rpm engine??? The closest that I have found is the Corvair cam used in aircraft conversions. It is called the OTTO OT-10. The stock VW cam might be OK depending on what your target rpm & hp are.
The difficulty that I have found for optimising an engine at low rpm (3,000-3,500) is cam lobe profiles. There are literally thousands of profiles and specs out there for the engine builders & tuners, but all with durations and lifts larger than OEM. Try finding one for a hopped-up low rpm engine??? The closest that I have found is the Corvair cam used in aircraft conversions. It is called the OTTO OT-10. The stock VW cam might be OK depending on what your target rpm & hp are.
I can't see that operating a VW engine at a lower cruise rpm with a larger prop is likely to do any harm to the engine though obviously the prop clearance could become an issue of the change is significant.
Off the top of my head, the VP-2 specifies a 60 x 30" prop for an 1835cc engine and a 60 x 32" for a 2180cc. If those are running at a 3600 RPM redline, you're looking at a propeller tip speed of 643 mph or .84 mach. Looking at the Aerovee numbers, you only need to go down to 2700 RPM to hit 65 hp and that allows you to run a prop up to 80" in diameter for the same tip speed. That's a huge change, 10" increase in prop clearance required which would require completely new gear on the VP-2. I suppose you could go with a 3-bladed propeller at the original diameter or close to it, but then you're operating the propeller at far less than the optimum .8-.92 mach range for best efficiency.
I guess it comes down to the aircraft. If it already has the clearance for a large prop, or it's a replica or some sort that will look better with a large prop, then the 2180cc with a big, slow prop makes sense. If it's something without the ground clearance, then you're probably better off with the stock 1835cc and prop size.
Off the top of my head, the VP-2 specifies a 60 x 30" prop for an 1835cc engine and a 60 x 32" for a 2180cc. If those are running at a 3600 RPM redline, you're looking at a propeller tip speed of 643 mph or .84 mach. Looking at the Aerovee numbers, you only need to go down to 2700 RPM to hit 65 hp and that allows you to run a prop up to 80" in diameter for the same tip speed. That's a huge change, 10" increase in prop clearance required which would require completely new gear on the VP-2. I suppose you could go with a 3-bladed propeller at the original diameter or close to it, but then you're operating the propeller at far less than the optimum .8-.92 mach range for best efficiency.
I guess it comes down to the aircraft. If it already has the clearance for a large prop, or it's a replica or some sort that will look better with a large prop, then the 2180cc with a big, slow prop makes sense. If it's something without the ground clearance, then you're probably better off with the stock 1835cc and prop size.
That's too fast for a wood prop. A helical tip speed of 850fps (about .75 mach at STP) is about as fast as they should be turned in normal use.
The weight of an 1835cc engine will be about the same as a 2180, but there's a bit more work to build the 2180 (clearancing the case). Also, GPAS and many builders would highly recommend using the Force One bearing and prop hub on any stroked pulley drive Type 1 aero engine (incl a 2180), so that increases parts cost and machining costs. Using the stock pulley bearing is more common with non-stroked engines like the 1835 (but there's nothing wrong with using the Force One for an 1835, it provides a bit of insurance).
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dave wolfe
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Id be concerned using the force one hub with any crankshaft not designed to go with it. Tapering the nose of the crank I think would make the keyway area smaller and more prone to cracking. The Top Bug crank from GPAS has a shallower keyway and smaller diameter bolt to eliminate cracks. There were a few crank failures on Thatchers where I believe the force 1 hub was used but I didnt see any conclusive analysis as to what went wrong.
The '5th bearing' thing is a worthwhile consideration for any direct drive auto engine conversion; I should have mentioned it in my earlier post. People have broken VW cranks even with the smaller props typically run on them; that's why the 5th bearing setups were developed. The particular design for the added bearing is not something I've ever paid attention to for VWs, but obviously could matter. It's worth noting that there are other ways to isolate the bending loads from the crank, though they might weigh a bit more.
103
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Keen interest In this my 2276CC with a 58x34 prop climbs censna 150ish with two souls aboard at 3250 rpm. Cruise 100 at 2900 rpm with head temps around 240-260 on 95OAT day. I would like a shorter take off run. I could maintain 9" of ground clearance with current landing gear at 66" diameter. I have considered the stoker cam offered by Vaxell Camshafts - VAXELL
Claims with EFI
Possibly the cam and a larger prop would be a good upgrade to my take of or just and expensive experiment. I have attached the Polish User manual for reference it has Usefull System level diagrams for VW operators.
Claims with EFI
Possibly the cam and a larger prop would be a good upgrade to my take of or just and expensive experiment. I have attached the Polish User manual for reference it has Usefull System level diagrams for VW operators.
103
Well-Known Member
Update MS word gave me the option to translate to English. Use wisely consider Reference only!
To really know how much the engine is loaded with your prop, you really need know the MP.
When I put the flywheel drive 2180 cc engine on the SSSC I plan on using a 62"x 28" Culver prop. With the 1835 cc engine, the 60"x 26" Culver prop is the perfect combination for a climb prop for the airframe.
Don't think you will get a better cam than the stock VW cam with a direct drive engine swinging a prop. We are not after high RPM HP as used in the auto.
When I put the flywheel drive 2180 cc engine on the SSSC I plan on using a 62"x 28" Culver prop. With the 1835 cc engine, the 60"x 26" Culver prop is the perfect combination for a climb prop for the airframe.
Don't think you will get a better cam than the stock VW cam with a direct drive engine swinging a prop. We are not after high RPM HP as used in the auto.
60x26
So that chart says my speed at 3400 rpm is 60 mph ?
allonsye
Well-Known Member
I was just reading that today. Yeah, I'm not aware of any conclusive analysis either other than suspicions of the cracks originating from the keyway and cinch bolt areas.
Re: Second crank failure. A cautionary tale. (groups.io)
Yes. Is it anywhere close?
So on the one above I put in the airspeed before the rpm, I just cranked up the rpm until I got near 60hp for power.
On this one I put in 3140rpm and 60mph and it gives 45hp
On this one I put in 3140rpm and 60mph and it gives 45hp
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WOT is about 3400 rpm and 94 mph.
I think the idea of a larger displacement, lower RPM VW conversion has a lot of merit. I have given it some thought already, myself, as a potential powerplant for some of the airframes I'm interested in. I agree with Pops, the stock VW cam is great for our RPM range, with valve timing events that are pretty close to what you'd find on a Lycoming cam. If more lift is needed, you could change to 1.25 ratio rocker arms without much fuss. A couple of other good candidate camshafts are:
Eagle Racing Type 1 Camshaft, 2231 Grind (Cheater Fuel Efficient Series), 1.1 or 1.25 Rockers, CB2231 - Volkswagen Aircooled.Net VW Parts
248-54F Volkswagen Type 1 Solid Lifter Camshaft
Eagle Racing Type 1 Camshaft, 2231 Grind (Cheater Fuel Efficient Series), 1.1 or 1.25 Rockers, CB2231 - Volkswagen Aircooled.Net VW Parts
248-54F Volkswagen Type 1 Solid Lifter Camshaft
103
Well-Known Member
So I need only spring for new prop and try it! I need to finish my licenses 1st! 2276 is on the larger size !
Just varnished by Sterba 58x34 looks great!
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