Lower rpm 2180

Homebuilt Aircraft & Kit Plane Forum

Help Support Homebuilt Aircraft & Kit Plane Forum:

Saskguy

New Member
Joined
Mar 18, 2019
Messages
4
Location
Canada
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?
 

rv7charlie

Well-Known Member
Joined
Nov 17, 2014
Messages
2,823
Location
Pocahontas MS
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.
 

TiPi

Well-Known Member
Log Member
Joined
Aug 25, 2019
Messages
580
Location
Peeramon (AUS)
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.
 

cluttonfred

Well-Known Member
Joined
Feb 13, 2010
Messages
9,328
Location
World traveler
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.

1643274429118.png

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.
 

Vigilant1

Well-Known Member
Joined
Jan 24, 2011
Messages
7,628
Location
US
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. ...
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.
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).
 
Last edited:

dave wolfe

Well-Known Member
Joined
Dec 7, 2019
Messages
137
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.
 

rv7charlie

Well-Known Member
Joined
Nov 17, 2014
Messages
2,823
Location
Pocahontas MS
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

Well-Known Member
Joined
May 2, 2015
Messages
445
Location
Wauwatosa WI
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
1643292804525.png

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.
 

Attachments

blane.c

Well-Known Member
Joined
Jun 27, 2015
Messages
5,915
Location
capital district NY
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
View attachment 121199

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.
couple first at 152 kg thrust at 43mph (Guessing rotation/initial climb) second 100mph cruise, both 66 inch prop.
1.jpg
2.jpg
 

Pops

Well-Known Member
Log Member
Joined
Jan 1, 2013
Messages
10,649
Location
USA.
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.
 

allonsye

Well-Known Member
Joined
May 28, 2013
Messages
51
Location
McGaheysville, VA
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.
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)
 

blane.c

Well-Known Member
Joined
Jun 27, 2015
Messages
5,915
Location
capital district NY
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

4.jpg
 
Last edited:

fly2kads

Well-Known Member
Joined
Jan 2, 2010
Messages
1,913
Location
Justin, TX
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
 

103

Well-Known Member
Joined
May 2, 2015
Messages
445
Location
Wauwatosa WI
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
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!
 
Last edited:
Top